UNIT II: TRANSISTORS AND AMPLIFIERS

Bipolar junction transistor – CB, CE, CC configuration and characteristics – Biasing circuits – Class A, B and C amplifiers – Field effect transistor –Configuration and characteristic of FET amplifier –SCR, diac, triac, UJT – Characteristics and simple applications – Switching transistors –Concept of feedback –Negative feedback – Application in temperature and motor speed control.

BIPOLAR TRANSISTOR BASICS:

OBJECTIVE:

To know about the basic concept of transistors, types of transistor and amplifiers. At the end of session the learners will able to understand the types of transistor

and application of transistor.

In the Diode tutorials we saw that simple diodes are made up from two pieces of semiconductor material, either Silicon or Germanium to form a simple PN-junction and we also learnt about their properties and characteristics. If we now join together two individual diodes end to end giving two PN-junctions connected together in series, we now have a three layer, two junction, three terminal device forming the basis of a Bipolar Junction Transistor, or BJT for short. This type of transistor is generally known as a Bipolar Transistor, because its basic construction consists of two PN-junctions with each terminal or connection being given a name to identify it and these are known as the Emitter, Base and Collector respectively.

The word Transistor is an acronym, and is a combination of the words Transfer Resistor used to describe their mode of operation way back in their early days of development. There are two basic types of bipolar transistor construction, NPN and PNP, which basically describes the physical arrangement of the P-type and N-type semiconductor materials from which they are made. Bipolar Transistors are "CURRENT" Amplifying or current regulating devices that control the amount of current flowing through them in proportion to the amount of biasing current applied to their base terminal. The principle of operation of the two transistors types NPN and PNP, is exactly the same the only difference being in the biasing (base current) and the polarity of the power supply for each type.

Bipolar Transistor Construction

The construction and circuit symbols for both the NPN and PNP bipolar transistor are shown above with the arrow in the circuit symbol always showing the direction of conventional current flow between the base terminal and its emitter terminal, with the direction of the arrow pointing from the positive P-type region to the negative N-type region, exactly the same as for the standard diode symbol.

There are basically three possible ways to connect a Bipolar Transistor within an electronic circuit with each method of connection responding differently to its input signal as the static characteristics of the transistor vary with each circuit arrangement.

CB, CE, CC CONFIGURATION AND CHARACTERISTICS:

OBJECTIVE:

To know about the basic concept of CB, CE and CC configuration and characteristics.

At the end of session the learners will able to understand the collector, emitter and base of characteristics.

1. Common Base Configuration - has Voltage Gain but no Current Gain.2. Common Emitter Configuration - has both Current and Voltage Gain.3. Common Collector Configuration - has Current Gain but no Voltage Gain.

The Common Base Configuration.

As its name suggests, in the Common Base or Grounded Base configuration, the BASE connection is common to both the input signal AND the output signal with the input signal being applied between the base and the emitter terminals. The corresponding output signal is taken from between the base and the collector terminals as shown with the base terminal grounded or connected to a fixed reference voltage point. The input current flowing into the emitter is quite large as its the sum of both the base current and collector current respectively therefore, the collector current output is less than the emitter current input resulting in a Current Gain for this type of circuit of less than "1", or in other words it "Attenuates" the signal.

The Common Base Amplifier Circuit

This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signal voltages Vin and Vout are in-Phase. This type of arrangement is not very

common due to its unusually high voltage gain characteristics. Its Output characteristics represent that of a forward biased diode while the Input characteristics represent that of an illuminated photo- diode. Also this type of configuration has a high ratio of Output to Input resistance or more importantly "Load" resistance (RL) to "Input" resistance (Rin) giving it a value of "Resistance Gain". Then the Voltage Gain for a common base can therefore be given as:

Common Base Voltage Gain

The Common Base circuit is generally only used in single stage amplifier circuits such as microphone pre-amplifier or RF radio amplifiers due to its very good high frequency response.

The Common Emitter Configuration.

In the Common Emitter or Grounded Emitter configuration, the input signal is applied between the base, while the output is taken from between the collector and the emitter as shown. This type of configuration is the most commonly used circuit for transistor based amplifiers and which represents the "normal" method of connection. The common emitter amplifier configuration produces the highest current and power gain of all the three bipolar transistor configurations. This is mainly because the input impedance is LOW as it is connected to a forward-biased junction, while the output impedance is HIGH as it is taken from a reverse-biased junction.The Common Emitter Amplifier Circuit

In this type of configuration, the current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as Ie = Ic + Ib. Also, as the load resistance (RL) is connected in series with the collector, the Current gain of the Common Emitter Transistor Amplifier is quite large as it is the ratio of Ic/Ib and is given the symbol of Beta, (β). Since the relationship between these three currents is determined by the transistor itself, any small change in the base current will result in a large change in the collector current. Then, small changes in base current will thus control the current in the Emitter/Collector circuit.

By combining the expressions for both Alpha, α and Beta, β the mathematical relationship

between these parameters and therefore the current gain of the amplifier can be given as:

Where: "Ic" is the current flowing into the collector terminal, "Ib" is the current flowing into the base terminal and "Ie" is the current flowing out of the emitter terminal.Then to summarise, this type of bipolar transistor configuration has a greater input impedance, Current and Power gain than that of the common Base configuration but its Voltage gain is much lower. The common emitter is an inverting amplifier circuit resulting in the output signal being 180o out of phase with the input voltage signal.

The Common Collector Configuration:

In the Common Collector or Grounded Collector configuration, the collector is now common and the input signal is connected to the Base, while the output is taken from the Emitter load as shown. This type of configuration is commonly known as a Voltage Follower or Emitter Follower circuit. The Emitter follower configuration is very useful for impedance matching applications because of the very high input impedance, in the region of hundreds of thousands of Ohms, and it has relatively low output impedance.The Common Collector Amplifier Circuit

The Common Emitter configuration has a current gain equal to the β value of the transistor itself. In the common collector configuration the load resistance is situated in series with the emitter so its current is equal to that of the emitter current. As the emitter current is the combination of the collector AND base currents combined, the load resistance in this type of amplifier configuration also has both the collector current and the input current of the base flowing through it. Then the current gain of the circuit is given as:

This type of bipolar transistor configuration is a non-inverting amplifier circuit in that the signal voltages of Vin and Vout are "In-Phase". It has a voltage gain that is always less than "1" (unity). The load resistance of the common collector amplifier configuration receives both the base and collector currents giving a large current gain (as with the Common Emitter configuration) therefore, providing good current amplification with very little voltage gain.

Bipolar Transistor Summary.

The behavior of the bipolar transistor in each one of the above circuit configurations is very different and produces different circuit characteristics with regards to Input impedance, Output impedance and Gain and this is summarized in the table below.

Transistor Characteristics

The static characteristics for Bipolar Transistor amplifiers can be divided into the following main groups.

Input Characteristics:- Common Base - IE ÷ VEB

Common Emitter - IB ÷ VBE

Output Characteristics:- Common Base - IC ÷ VC

Common Emitter - IC ÷ VC

Transfer Characteristics:- Common Base - IE ÷ IC

Common Emitter - IB ÷ IC

With the characteristics of the different transistor configurations given in the following table:

Characteristic CommonBase

CommonEmitter

CommonCollector

Input impedance Low Medium High

Output impedance Very High High Low

Phase Angle 0o 180o 0o

Voltage Gain High Medium Low

Current Gain Low Medium High

Power Gain Low Very High

BIASING CIRCUITS

OBJECTIVE:

To know about the basic concept biasing circuits. At the end of session the learners will able to understand the biasing circuits

Transistor as a switch

BJT used as an electronic switch, in grounded-emitter configuration.

Transistors are commonly used as electronic switches, for both high power applications including switched-mode power supplies and low power applications such as logic gates.

In a grounded-emitter transistor circuit, such as the light-switch circuit shown, as the base voltage rises the base and collector current rise exponentially, and the collector voltage drops because of the collector load resistor. The relevant equations:

VRC = ICE × RC, the voltage across the load (the lamp with resistance RC)

VRC + VCE = VCC, the supply voltage shown as 6V

If VCE could fall to 0 (perfect closed switch) then Ic could go no higher than VCC / RC, even with higher base voltage and current. The transistor is then said to be saturated. Hence, values of input voltage can be chosen such that the output is either completely off,[13] or completely on. The transistor is acting as a switch, and this type of operation is common in digital circuits where only "on" and "off" values are relevant.

Transistor as an amplifier

Amplifier circuit, standard common-emitter configuration.

The common-emitter amplifier is designed so that a small change in voltage in (Vin) changes the small current through the base of the transistor and the transistor's current amplification combined with the properties of the circuit mean that small swings in Vin produce large changes in Vout.

Various configurations of single transistor amplifier are possible, with some providing current gain, some voltage gain, and some both.

From mobile phones to televisions, vast numbers of products include amplifiers for sound reproduction, radio transmission, and signal processing. The first discrete transistor audio amplifiers barely supplied a few hundred milliwatts, but power and audio fidelity gradually increased as better transistors became available and amplifier architecture evolved.

Modern transistor audio amplifiers of up to a few hundred watts are common and relatively inexpensive

Voltage divider bias

Voltage divider bias

The voltage divider is formed using external resistors R1 and R2. The voltage across R2 forward biases the emitter junction. By proper selection of resistors R1 and R2, the operating point of the transistor can be made independent of β. In this circuit, the voltage divider holds the base voltage fixed independent of base current provided the divider current is large compared to the base current. However, even with a fixed base voltage, collector current varies with temperature (for example) so an emitter resistor is added to stabilize the Q-point, similar to the above circuits with emitter resistor.

In this circuit the base voltage is given by:

voltage across

provided .

Also

For the given circuit,

Merits:

Unlike above circuits, only one dc supply is necessary. Operating point is almost independent of β variation. Operating point stabilized against shift in temperature.

Demerits:

In this circuit, to keep IC independent of β the following condition must be met:

which is approximately the case if

where R1 || R2 denotes the equivalent resistance of R1 and R2 connected in parallel.

As β-value is fixed for a given transistor, this relation can be satisfied either by keeping RE fairly large, or making R1||R2 very low.

If RE is of large value, high VCC is necessary. This increases cost as well as precautions necessary while handling.

If R1 || R2 is low, either R1 is low, or R2 is low, or both are low. A low R1 raises VB closer to VC, reducing the available swing in collector voltage, and limiting how large RC can be made without driving the transistor out of active mode. A low R2 lowers Vbe, reducing the allowed collector current. Lowering both resistor values draws more current from the power supply and lowers the input resistance of the amplifier as seen from the base.

AC as well as DC feedback is caused by RE, which reduces the AC voltage gain of the amplifier. A method to avoid AC feedback while retaining DC feedback is discussed below.

B ase Bias

The simplest biasing applies a base-bias resistor between the base and a base battery VBB. It is convenient to use the existing VCC supply instead of a new bias supply. An example of an audio amplifier stage using base-biasing is ―Crystal radio with one transistor . . . ‖ crystal radio, Ch 9 . Note the resistor from the base to the battery terminal. A similar circuit is shown in Figure below.

Write a KVL (Krichhoff's voltage law) equation about the loop containing the battery, RB, and the VBE diode drop on the transistor in Figure below. Note that we use VBB for the base supply, even though it is actually VCC. If β is large we can make the approximation that IC =IE.For silicon transistors VBE≅0.7V.

Base-bias

Silicon small signal transistors typically have a β in the range of 100-300. Assuming that we have a β=100 transistor, what value of base-bias resistor is required to yield an emitter current of 1mA?

Solving the IE base-bias equation for RB and substituting β, VBB, VBE, and IE yields 930kΩ.The closest standard value is 910kΩ.

What is the the emitter current with a 910kΩ resistor? What is the emitter current if we randomly get a β=300 transistor?

The emitter current is little changed in using the standard value 910kΩ resistor. However, with a change in β from 100 to 300, the emitter current has tripled. This is not acceptable in a power amplifier if we expect the collector voltage to swing from near VCC to near ground. However, for low level signals from micro-volts to a about a volt, the bias point can be centered for a β of square root of (100·300)=173. The bias point will still drift by a considerable amount . However, low level signals will not be clipped.

Base-bias by its self is not suitable for high emitter currents, as used in power amplifiers. The base-biased emitter current is not temperature stable. Thermal run away is the result of high emitter current causing a temperature increase which causes an increase in emitter current, which further increases temperature.

Collector-feedback bias

Variations in bias due to temperature and beta may be reduced by moving the VBB end of the base-bias resistor to the collector as in Figure below. If the emitter current were to increase, the voltage drop across RC increases, decreasing VC, decreasing IB fed back to the base. This, in turn, decreases the emitter current, correcting the original increase.

Write a KVL equation about the loop containing the battery, RC , RB , and the VBE drop.Substitute IC≅IE and IB≅IE/β. Solving for IE yields the IE CFB-bias equation. Solving for

IByields the IB CFB-bias equation.

Collector-feedback bias.

Find the required collector feedback bias resistor for an emitter current of 1 mA, a 4.7K collector load resistor, and a transistor with β=100 . Find the collector voltage VC. It should be approximately midway between VCC and ground.

The closest standard value to the 460k collector feedback bias resistor is 470k. Find the emitter current IE with the 470 K resistor. Recalculate the emitter current for a transistor with β=100 and β=300.

We see that as beta changes from 100 to 300, the emitter current increases from 0.989mA to1.48mA. This is an improvement over the previous base-bias circuit which had an increase from 1.02mA to 3.07mA. Collector feedback bias is twice as stable as base-bias with respect to beta variation.

Emitter-bias

Inserting a resistor RE in the emitter circuit as in Figure b e low causes degeneration, also known as negative feedback. This opposes a change in emitter current IE due to temperature changes, resistor tolerances, beta variation, or power supply tolerance. Typical tolerances are as follows: resistor— 5%, beta— 100-300, power supply— 5%. Why might the emitter resistor stabilize a change in current? The polarity of the voltage drop across RE is due to the collector battery VCC. The end of the resistor closest to the (-) battery terminal is (-), the end closest to the (+) terminal it (+). Note that the (-) end of RE is connected via VBB battery and RB to the base. Any increase in current flow through RE will increase the magnitude of negative voltage applied to the base circuit, decreasing the base current, decreasing the emitter current. This decreasing emitter current partially compensates the original increase.

Emitter-bias

Note that base-bias battery VBB is used instead of VCC to bias the base in Figure above. Later we will show that the emitter-bias is more effective with a lower base bias battery. Meanwhile, we write the KVL equation for the loop through the base-emitter circuit, paying

attention to the polarities on the components. We substitute IB≅IE/β and solve for emittercurrent IE. This equation can be solved for RB , equation: RB emitter-bias, Figure above.

Before applying the equations: RB emitter-bias and IE emitter-bias, Figure above, we need to choose values for RC and RE . RC is related to the collector supply VCC and the desired collector current IC which we assume is approximately the emitter current IE. Normally the bias point for VC is set to half of VCC. Though, it could be set higher to compensate for the voltage drop across the emitter resistor RE. The collector current is whatever we require or choose. It could range from micro-Amps to Amps depending on the application and transistor rating. We choose IC = 1mA, typical of a small-signal transistor circuit. We calculate a value for RC and choose a close standard value. An emitter resistor which is 10-50% of the collector load resistor usually works well.

Collector-to-base biased bipolar amplifier

Figure 2: Left - small-signal circuit corresponding to Figure 1; center - inserting independent source and marking leads to be cut; right - cutting the dependent source free and short- circuiting broken leads

Figure 1 (top right) shows a bipolar amplifier with feedback bias resistor Rf driven by a Norton signal source. Figure 2 (left panel) shows the corresponding small-signal circuit obtained by replacing the transistor with its hybrid-pi model. The objective is to find the return ratio of the dependent current source in this amplifier.[9] To reach the objective, the steps outlined above are followed. Figure 2 (center panel) shows the application of these steps up to Step 4, with the dependent source moved to the left of the inserted source of value it, and the leads targeted for cutting marked with an x. Figure 2 (right panel) shows the circuit set up for calculation of the return ratio T, which is

The return current is

The feedback current in Rf is found by current division to be:

The base-emitter voltage vπ is then, from Ohm's law:

Consequently,

Application in asymptotic gain model

The overall transresistance gain of this amplifier can be shown to be:

with R1 = RS || rπ and R2 = RD || rO.

This expression can be rewritten in the form used by the asymptotic gain model, which expresses the overall gain of a feedback amplifier in terms of several independent factors that are often more easily derived separately than the overall gain itself, and that often provide insight into the circuit. This form is:

where the so-called asymptotic gain G∞ is the gain at infinite gm, namely:

and the so-called feed forward or direct feedthrough G0 is the gain for zero gm, namely:

For additional applications of this method, see asymptotic gain model.

1. DC Biasing Circuits2. The ac operation of an amplifier depends on the initial dc values of IB, IC, and VCE.3. By varying IB around an initial dc value, IC and VCE are made to vary around their

initial dc values.

4. DC biasing is a static operation since it deals with setting a fixed (steady) level of current (through the device) with a desired fixed voltage drop across the device.

+VCC

RC

RBv out

v in vceib

ic

Purpose of the DC biasing circuit

1. To turn the device ―ON‖2. To place it in operation in the region of its characteristic where the device operates

most linearly, i.e. to set up the initial dc values of IB, IC, and VCE

Voltage-Divider Bias

• The voltage – divider (or potentiometer) bias circuit is by far the most commonly used.• RB1, RB2

• C3

voltage-divider to set the value of VB , IB +VCC

to short circuit ac signals to ground, while not effect the DC operating (or biasing)of a circuit

RC

R1

cm

RR

IIEVIC

2 I

(RE stabilizes the ac signals)

Bypass Capacitor

+VCC

Graphical DC Bias Analysis

IC RCR1

VCC

IC

RC

for IC

1

CE

E

VCE

RE

RERE 0

VCC

RC E RC E

Point - slope form of straight line equation :y x IC(sat) = VCC/(RC+RE)

IC(mA)

DC Load Line

VCE(off) = VCC

• The straight line is know as the DC load line• Its significance is that regardless of the behavior of the transistor, the collector current

IC and the collector-emitter voltage VCE must always lie on the load line, depends ONLY on the VCC, RC and RE

• (i.e. The dc load line is a graph that represents all the possible combinations of IC and VCE for a given amplifier. For every possible value of IC, and amplifier will have a corresponding value of VCE.)

• It must be true at the same time as the transistor characteristic. Solve two condition using simultaneous equation

graphically Q-point !!

Q-Point (Static Operation Point)

• When a transistor does not have an ac input, it will have specific dc values of IC andVCE.

• These values correspond to a specific point on the dc load line. This point is called theQ-point.

• The letter Q corresponds to the word (Latent) quiescent, meaning at rest.• A quiescent amplifier is one that has no ac signal applied and therefore has constant dc

values of IC and VCE.• The intersection of the dc bias value of IB with the dc load line determines the Q-

point.• It is desirable to have the Q-point centered on the load line. Why?• When a circuit is designed to have a centered Q-point, the amplifier is said to be

midpoint biased.• Midpoint biasing allows optimum ac operation of the amplifier.

DC Biasing + AC signal

• When an ac signal is applied to the base of the transistor, IC and VCE will both vary around their Q-point values.

• When the Q-point is centered, IC and VCE can both make the maximum possible transitions above and below their initial dc values.

• When the Q-point is above the center on the load line, the input signal may cause the transistor to saturate. When this happens, a part of the output signal will be clipped off.

• When the Q-point is below midpoint on the load line, the input signal may cause the transistor to cutoff. This can also cause a portion of the output signal to be clipped.

DC and AC Equivalent Circuits

+VCC

+VCC

RCR

1R

R1

CIC R

vin

R1//R2

rCvce

Bias Circuit DC equivalent circuitAC equ ckt

rC = RC//RL

• The ac load line of a given amplifier will not follow the plot of the dc load line.• This is due to the dc load of an amplifier is different from the ac load.

ac load line

ICQ - point

dc load line

VCE

What does the ac load line tell you?

• The ac load line is used to tell you the maximum possible output voltage swing for a given common-emitter amplifier.

• In other words, the ac load line will tell you the maximum possible peak-to-peak output voltage (Vpp ) from a given amplifier.

• This maximum Vpp is referred to as the compliance of the amplifier. (AC Saturation Current Ic(sat) , AC Cutoff Voltage VCE(off) )

Bias stabilization

The establishment of an operating point on the transistor volt-ampere characteristics by means of direct voltages and currents.

Since the transistor is a three-terminal device, any one of the three terminals may be used as a common terminal to both input and output. In most transistor circuits the emitter is used as the common terminal, and this common emitter, or grounded emitter, is indicated in illus. a. If the transistor is to used as a linear device, such as an audio amplifier, it must be biased to operate in the active region. In this region the collector is biased in the reverse direction and the emitter in the forward direction. The area in the common-emitter transistor characteristics to the right of the ordinate VCE = 0 and above IC = 0 is the active region. Two more biasing regions are of special interest for those cases in which the transistor is intended to operate as a switch. These are the saturation and cutoffregions. The saturation region may be defined as the region where the collector current is independent of base current for given values of VCC and RL. Thus, the onset of saturation can be considered to take place at the knee of the common-emitter transistor curves. See also Amplifier; Transistor.

Translator circuits. (a) Fixed-bias. (b) Collector-to-base bias. (c) Self-bias.

In saturation, the transistor current IC is nominally VCC/RL. Since RL is small, it may be necessary to keep VCCcorrespondingly small in order to stay within the limitations imposed by the transistor on maximum-current and collector-power dissipation. In the cutoff region it is required that the emitter current IE be zero, and to accomplish this it is necessary to reverse- bias the emitter junction so that the collector current is approximately equal to thereverse saturation current ICO. A reverse-biasing voltage of the order of 0.1 V across the emitter junction willordinarily be adequate to cut off either a germanium or silicon transistor.

The particular method to be used in establishing an operating point on the transistor characteristics depends on whether the transistor is to operate in the active, saturation or cutoff regions; on the application under consideration; on the thermal stability of the circuit; and on other factors.

In a fixed-bias circuit, the operating point for the circuit of illus. a can be established by noting that the required current IB is constant, independent of the quiescent collector current IC, which is why this circuit is called the fixed-bias circuit. Transistor biasing circuits are frequently compared in terms of the value of the stability factor S = ∂IC/∂ICO, which is the rate of change of collector current with respect to reverse saturation current. The smaller the value of S, the less likely the circuit will exhibit thermal runaway. S, as defined here, cannot be smaller than unity. Other stability factors are defined in terms of dc current gain hFE as∂IC/∂hFE, and in terms of base-to-emitter voltage as ∂IC/∂VBE. However, bias circuits with small values of S will also perform satisfactorily for transistors that have large variations of hFE and VBE. For the fixed-bias circuit it can be shown that S = hFE + 1, and if hFE = 50, thenS = 51. Such a large value of S makes thermal runaway a definite possibility with this circuit.

In collector-to-base bias, an improvement in stability is obtained if the resistor RB in illus. a is returned to the collector junction rather than to the battery terminal. Such a connection is shown in illus. b. In this bias circuit, if ICtends to increase (either because of a rise in temperature or because the transistor has been replaced by another), then VCE decreases. Hence IB also decreases and, as a consequence of this lowered bias current, the collector current is not allowed to increase as much as it would if fixed bias were used. The stability factor S is shown in Eq. (1).

1. This value is smaller than hFE + 1, which is the value obtained for the fixed-bias case.

If the load resistance RL is very small, as in a transformer-coupled circuit, then the previous expression for S shows that there would be no improvement in the stabilization in the collector-to-base bias circuit over the fixed-bias circuit. A circuit that can be used even if there is zero dc resistance in series with the collector terminal is the self-biasing configuration of illus. c. The current in the resistance RE in the emitter lead causes a voltage drop which is in the direction to reverse-bias the emitter junction. Since this junction must be forward-biased (for active region bias), the ble e d e r R1-R2 has been added to the circuit.

If IC tends to increase, the current in RE increases. As a consequence of the increase in voltage drop across RE, the base current is decreased. Hence IC will increase less than it would have had there been no self-biasing resistor RE. The stabilization factor for the self-bias circuit is shown by Eq. (2), where RB = R1R2/(R1 + R2). The smaller the value of RB, the

2. better the stabilization. Even if RB approaches zero, the value of S cannot be reduced below unity.

In order to avoid the loss of signal gain because of the degeneration caused by RE, this resistor is often bypassed by a very large capacitance, so that its reactance at the frequencies under consideration is very small.

The selection of an appropriate operating point (ID, VGS, VDS) for a field-effect transistor (FET) amplifier stage is determined by considerations similar to those given to transistors, as discussed previously. These considerations are output-voltage swing, distortion, power dissipation, voltage gain, and drift of drain current. In most cases it is not possible to satisfy all desired specifications simultaneously.

EC1451 – MOBILE AND WIRELESS COMMUNICATION

UNIT II WIRELESS PROTOCOLS

Issues and challenges of wireless networks – Location management – Resource management – Routing – Power management – Security – Wireless media access techniques – ALOHA – CSMA – Wireless LAN – MAN – IEEE 802.11 (a–b–e–f–g–h–i) – Bluetooth. Wireless routing protocols – Mobile IP – IPv4 – IPv6 – Wireless TCP. Protocols for 3G & 4G cellular networks – IMT – 2000 – UMTS – CDMA2000 – Mobility management and handover technologies – All-IP based cellular network

Issues and challenges of wireless networksObjective:

To know about the practical difficulties involved in wireless networksTo understand the remedies of the above problems

Wireless local area networks (LANs) are playing a major role in the information technology revolution. They are finding their way into a wide variety of markets including financial sectors, corporations, health care, and education. For example, wireless devices are used in New York Stock Exchange for trade reporting. Employees in a company can initiate a wireless video conference instantaneously without having to go through the tedious procedure of connecting the communicating devices using wires. Indeed, according to a research study by Frost and Sullivan, the wireless LAN market is set to reach $697.7 million in 2003.

One of the factors that could have had an adverse impact on the market for wireless devices, is the interoperability issue between products developed by different vendors. However, the IEEE has developed the 802.11 standard, compliance with which should alleviate this issue. Other factors that will impact the long-term success of wireless LANs largely depends on improving the technology, reducing installation costs, and predicting the market and customer needs. While initial costs to install a wireless LAN infrastructure may be greater than its wired counter-part, in the long-term, benefits due to the wireless network can be significantly higher when the users are constantly mobile.Critical Challenges.

24

Since wireless devices need to be small and wireless networks are bandwidth limited, some of the key challenges in wireless networks are:

data rate enhancements minimizing size and cost low power networking user security

Enhancing Data Rate.

Improving the current data rates to support future high speed applications is essential, especially, if multimedia service are to be provided. Data rate is a function of various factors such as the data compression algorithm, interference mitigation through error-resilient coding, power control, and the data transfer protocol. Therefore, it is imperative that manufacturers implement a well thought out design that considers these factors in order to achieve higher data rates.

Data compression plays a major role when multimedia applications such as video conferencing is to be supported by a wireless network. Currently, compression standards such as MPEG-4 produce compression ratios of the order of 75 to 100. The challenge now is to improve these data compression algorithms to produce high quality audio and video even at these compression rates. Unfortunately, highly compressed multimedia data is more sensitive to network errors and interference and this necessitates the use of algorithms to protect sensitive data from being corrupted. Efficient error control algorithms with low overhead must be explored. Another way to enhance the data rates would be to employ intelligent data transfer protocols that adapt to the time-varying network and traffic characteristics.

Low Power Design.

The size and battery power limitation of wireless mobile devices place a limit on the range and throughput that can be supported by a wireless LAN. Bottlenecks in the wired portion of a LAN also affect the throughput.

The complexity and hence the power consumption of wireless devices vary significantly depending on the kind of spread spectrum technology being used to implement the wireless LAN. Normally, direct sequence spread spectrum (DSSS) based implementations require large and power-hungry hardware compared to frequency hopped spread spectrum (FHSS). They tend to consume about two to three times the power of an equivalent FHSS system. But, the complex circuitry provides better error recovery capability to DSSS systems compared to FHSS. FHSS is generally less tolerant to multipath and other interference. In fact, there is a constant debate going on both in academia and the wireless industry regarding the pros and cons of DSSS versus FHSS. It is the right time for researchers and developers to approach these issues in wireless LAN technologies together and from a global perspective. This may also enable different vendors to develop wireless LAN technologies that can co-exist and operate together.

Security Issues.

25

Security is a big concern in wireless networking, especially in m-commerce and e-commerce applications. Mobility of users increases the security concerns in a wireless network. Current wireless networks employ authentication and data encryption techniques on the air interface to provide security to its users. The IEEE 801.11 standard describes wired equivalent privacy (WEP) that defines a method to authenticate users and encrypt data between the PC card and the wireless LAN access point. In large enterprises, an IP network level security solution could ensure that the corporate network and proprietary data are safe. Virtual private network (VPN) is an option to make access to fixed access networks reliable. Since hackers are getting smarter, it is imperative that wireless security features must be updated constantly.

Short questions:

1. Give the issues in wireless networks.2. What is network security?

Mobility Management

Objective:

To understand the various concepts of mobility management techniquesTo know the key terms of mobility management

Mobility Management is the set of procedures used to support user mobility in wireless network. Mobility management ensures an user remain connected while he is moving from one place to other place. Mobility management generally consists of two parts. Location Management refers to maintain the location of a mobile terminal and page the calls towards that specific mobile terminal. Handoff Management on the other hand involves maintaining an users connection as he moves, changing his network access point to the network [1]. Location management can be divide into two parts, first one is location registration or location update and the final one is, paging or location finding. The general approach of location management is to maintain a database either central or distributed and keep the locations of the mobile terminals either periodically or measuring some parameter. This database is then used to find a mobile terminal and forward calls to them. Location Update refers to the action needed to maintain a consistent database with the locations of the mobile terminals. Paging, on the other hand, means locating the mobile phone actually and forwards the incoming call to that specific mobile terminal [1]. Plenty of messages are transmitted for these purposes and called signaling messages.

Compared to the wired networks, wireless network has some serious drawbacks. Bandwidth in wireless network is extremely scarce. Interference in wireless network is several times higher than wired network. A 1MB link in wired network exerts almost same capacity independent of external factors. A wireless link, however, does not exert same capacity as it is affected by external factors such as rain, fog, humidity, temperature, presence of other wireless

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channel and so on. Thus any service in wireless network needs extreme care as resource is very low and variable. Thus, goal of an efficient location management scheme is to maintain and provide locations of mobile terminals at low cost. Low cost in this case refers to using less number of signalling messages and using less time.

Location Management Basic

A wireless communication system must keep track of the users to forward the incoming calls a mobile terminal. To specify the location of a mobile terminal a wireless network usually divided into some Location Areas. Location Area (LA) is a part of a wireless network within which no location update has to be done. In case of a cellular network, one Location Area generally consists of several cells. A mobile unit can freely move within on Location Area but has to notify the network when it moves to a new Location Area. The network then probes every cells in a Location Area to locate certain mobile terminal [7]. Effect of large Location Area is low frequency of location registration. However, large Location Area means more cells within one Location Area and needs more paging messages while forwarding an incoming call to a specific mobile terminal. Thus size of an Location Area determines the trade off between location update cost versus location finding cost.

In this report, we have documented the taxonomy and philosophy of various Location Management schemes. describes some preliminary terms about wireless networks that are necessary to understand these report. focuses on taxonomy and parts of Location Management schemes. describes different Static Location Update strategies. we discuss about Dynamic Location Update strategies. has different paging mechanisms. Finally, concludes with summary and future research on Location Management.

A Network is a set of connected systems to facilitate information and computation sharing. In a Computer Network, the connected systems are computers. A computer network is said to be Wired Network if all the connections among the computers are done using some wires. When wireless links are used, the network is called Wireless Network. An wireless network is said to be Mobile Network if the connected devices can change their positions and communicate from different location. In a Cellular Network the total network is divided into some cells.

The device that is used by an user to be connected to mobile network and continue his communication is called a Mobile Terminal (MT). Mobile terminal and mobile device have the same meaning and used interchangeably throughout the report. Base Station (BS) is a tower or antenna that is used to transmit or receive signal in a single cell [7]. Base Station Controller (BSC) coordinates several Base Stations. It handles channel allocation,

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controls handoffs, performs paging and works as an interface between Base Station and central network. Mobile Switching Center (MSC) coordinates several BSCs. MSC handles basic call setup, call switching, call routing, Billing and mobility management. It also manages communication with other networks.

As mobile devices move around different cells in a network, they must register their new location to enable the network to forward to calls towards the devices. Location Management is a two-stage process. Details operations of Location Management is shown in Fig2.1. Location update is the process where mobile terminal periodically notifies the network of its new attachment point. Network will then authenticate and store mobile terminal’s location. In the second stage called paging, network is queried about the user location and this

information is used to forward the incoming call towards the mobile terminal. General technique of Location Management involves using a database. The location of each mobile terminal is tracked and stored in this database. Various signaling messages are transmitted during location update and paging among different network components. As the number of mobile users increases, amount of signaling messages also increases. So, new and improved scheme is needed for location management for efficient working of wireless network.

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Location Update

Location update is used to inform the network about the uncertainty of a mobile terminal. Each mobile device has to update its location from time to time. The exact procedure however differs among different location update schemes. Location update procedure begins with an update message sent the mobile device itself. Then the location database is updated after some signaling messages. A key issue in location update is, when should a mobile terminal send an update message? If the updating process is less frequent than required, the network may clog into paging phase and send paging message to huge number of cells. The result would be large paging delay. Location update schemes can be divided into two main classes: Static Location Update and Dynamic Location Update. Static Location Update is dictated by network topology. On the other hand, Dynamic Location Update is done based on the user’s call and mobility pattern [29]. Each class has various number of sub-schemes. Fig. 3.1 shows a details view of location update strategies.

Static Location Update

Static Location Update schemes have their location update frequency dependent on network topology which is independent of user mobility or calling behavior. Static schemes offer lower computational cost but they are less efficient than dynamic schemes [7]. Various Static Location Update Strategies are discussed below.

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Dynamic Location Update

Dynamic Location Update schemes rely on different strategies for different users. This difference is dictated by some parameter. Dynamic schemes results in better performance compared to static schemes at the cost of higher computational complexity. If any dynamic location update strategy is followed, a user may send location update message from any cells at any time. Dynamic Location Update has been area of interesting research for the last few years. Various Dynamic Location Update schemes are summarized below.

Short questions:1. Define mobility management2. Write the types of mobility management3. What is static location update?

Resource Management and Power Management:

Objective:

To understand the concept of resource sharing and utilizationTo know about the techniques of power management

Cellular radio systems rely on a subsequent allocation and reuse of channels throughout acoverage region. Each cell is allocated a group of radio channels. Neighboring cells are given channel groups that contain completely different channels. By limiting the coverage area within the boundaries of a cell, the same group of channels may be used to cover different cells that are separated from one another by some distance.

Cellular mobile communication systems are characterized by their high degree of capacity. Consequently they have to serve the maximum possible

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number of calls, though the number of channels per cell is limited. On the other hand, cells in the same cluster must not use the same channel because of the increased possibility of various kinds of interference that appear mainly during the busy hours of the system. Hence the use of techniques that are capable of ensuring that the spectrum assigned for use in mobile communications will be optimally utilized is gaining ever-increasing importance. This makes the tasks of resource management more and more crucial. Some of the important objectives of resource management are the minimization of the interference level and handoffs as well as the adaptation to varying traffic and interference scenarios. Due to the time- and space-varying nature of the cellular system, the radio resource management tasks need to adapt to factors such as interference, traffic, and propagation environment. Some of the radio resource management tasks performed by cellular systems include admission control, power control, handoff, and channel assignment.

Frequency management and channel assignment:

The proper management of frequencies is very important in the development of a good communications plan because the available electromagnetic spectrum is highly congested. During the planning stage, if proper care is not taken in selecting frequencies, the frequencies chosen may interfere with each other. Channel assignment is the process that allocates calls to the channels of a cellular system. The main focus on research concerning channel assignment is to find strategies that give maximal channel reuse without violating the interference constraints so that blocking is minimal.

Handoff:

Handoff is the mechanism that transfers an ongoing call from one base station (BS) to another as a user moves through the coverage area of a cellular system. Therefore, it must be fast and efficient to prevent the quality of service from degenerating to an unacceptable level. This is probably the most sensitive aspect of the mobility provision and is an essential element of cellular communications, since the process chosen for handoff management will affect other mobility issues.

Admission control:

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Whenever a new call arrives (or a request for service or a handoff), the radio resource management system has to decide if this particular call may be allowed into the system. An algorithm making these decisions is called an admission control algorithm and prevents the system from being overloaded. New and continuing calls can be treated differently. For example, handoffs may be prioritized, new calls may be queued, etc.

Power control: In cellular networks, it is desirable to maintain bit error rates above a chosen minimum.

This would require the carrier to interference ratio of the radio links be maintained above a corresponding minimum value for the network. Power control is a specific resource management process that performs this task. It is evident that an integrated radio resource management scheme can make necessary trade-offs between the individual goals of these tasks to obtain better performance and increase system capacity within specified quality constraints. However, a combination of individual radio resource management tasks is also possible. For example, handoff and channel assignment tasks, or power control assisted admission schemes can be combined to provide interesting results.

Short Questions:1. Define power management2. Define resource management3. Explain the issues of power and resource managements.

Bluetooth:

Objective:

At the end of the session the learner will be able to know about the basic concepts of Bluetooth.To understand the techniques involved in the Bluetooth technology.

INTRODUCTION: Bluetooth IS an open specification for short range wireless voice and data communications. In 1998, Erickson, Nokia, IBM, Toshiba and Intel formed a Special Interest Group to expand the wireless concepts under PAN and develop a standard under IEEE 802.15 WPAN. In 1999 first specification was accepted as the IEEE 802.15 WPAN standard. This standard is also studying the interference between Bluetooth and 802.11 products.

It is the technology for short range ad hoc networking that is designed for an integrated voice and data

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applications. Unlike WLAN, it has lower data rate but it has an embedded mechanism to support voice applications. It is inexpensive personal area ad hoc network operating in unlicensed bands and owned by the user. The application scenarios are shown below:

Figure (a) is the wire replacement to connect a PC or laptop to its keyboard, mouse, microphone and notepad. It avoids wiring surrounding PC. The second scenario is ad hoc networking of several different users at very short range in an area such as a conference room. The third scenario is to use Bluetooth as a AP to the wide area voice and data services provided by the cellular networks, wired connection or satellite links. Although, it may be similar to 802.11, the AP in Bluetooth is used in an integrated manner to connect to both voice and data backbone infrastructure.Overall Architecture.

The topology of Bluetooth is referred to as Scattered Ad Hoc Topology as which shown below.

A number of small networks support a few terminals to co exits or possibly interoperate with one another. This requires plug and play environment which means the network must be self configurable to form a new small network and a procedure for participation in an existing one.

To implement such environment, the system must be capable of providing different states for connecting to the network. That is the terminals must have the option to associate with multiple network at the same time. The access methods should allow formation of small

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independent ad hoc cells. As well as the possibility of interacting with large voice and data network considered by Bluetooth.

Towards achieving this, Bluetooth defines a small cell as a Piconet and identifies four states for each node. These are Master ‘M’, Slave ‘S‘, Stand BY ‘SB’ and Parked\Hold ‘P’. Each terminals can be and ‘M’ or ‘S’. In this Bluetooth allows an S terminal to participate in more than one Piconet. An M terminal in Bluetooth can handle seven simultaneous and upto 200 active slaves in a piconet. If access is not available, a terminal can enter the ‘SB mode waiting to join the Piconet later. A MS can also be in a Parked \ Hold, ‘P’ state in a low power connection. In parked mode, the terminal releases its MAC address while in ‘SB’ state it keeps its MAC address. Upto 10 piconet can operate in one area. Bluetooth operate in ISM bands at 2.4 GHz . The advantage is the world wide availability and disadvantage is the interference due to other systems operating in the same band.

Protocol Stack:The protocol stack of Bluetooth is shown below which facilitates voice, data

communication and control signaling.

RF Layer specifies the radio modem used for transmission and reception of the information. The Baseband layer specifies the link control at bit and packet level. It specifies coding and

encryption for packet assembly and frequency hopping operation. LMP- Link Management Protocol configures the link to other devices by providing for

authentication and encryption, state of units in the piconet, power modes, traffic scheduling and packet format.

L2CAP – Logical Link control and adaptation protocol provides connection–oriented and connectionless data services to the upper layer protocol. These services include protocol multiplexing, segmentation and reassembly and group abstraction for data packet upto 64 kb in length.

Audio signal is directly transferred form the application to the Baseband. Also LMP and the application exchange control messages interact to prepare the physical transport to the application.

Different application may use different protocol stacks but nevertheless all of them share the same physical and data link control mechanism.

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For example the protocol stack for implementation of cordless telephone over Bluetooth, vCard an application for credit card verification and Wireless Applications are shown below:

TCP (Telephony control Protocol) over Bluetooth : Above figure shows that audio stack for implementation of the cordless telephone. The audio signal is directly transferred to the Baseband layer while SDC(Service Discovery Protocol ) and TCP (Telephone control Protocol) operate over L2CAP. LMP handles signaling and connection management.

vCard (Credit Card Verification) over Bluetooth :In case of credit card verification application, OBEX is meant for Object Exchange protocol that is accommodated by the RFCOMM protocol. The sequence for implementation of credit card verification over Bluetooth is vCard – OBEX – RFCOMM – L2CAP – Baseband – RF.

WAP over Bluetooth : Wireless Application Environment (WAE) protocol defines applications over the Wireless Access Protocol (WAP). WAP uses TCP / UDP protocols for Internet Access on top of the PPP that runs over the RFCOMM.

FTP over Bluetooth : In this OBEX and RFCOMM manage the data transfer. SDP provide the connection.

RFCOMM is a “cable replacement” protocol that emulates the standard RS 232 control and data signals over Bluetooth Baseband. Using RFCOMM a number of non- Bluetooth specific protocols can be implemented on the Bluetooth devices to support legacy applications.

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Overall Bluetooth protocol can be divided into three classes. Bluetooth specific protocols for Baseband, LMP, L2CAP and SDPBluetooth protocols that included existing protocols such as TCP and RFCOMM.Other protocols that are adapted by Bluetooth such as PPP, UDP/TCP, OBEX,WAP,vCard, cCal, WAE etc

Physical Connection: PHY layer of Bluetooth is embedded in the RF and Baseband layer of Bluetooth protocol stack. It normally uses 0 dbm (1 mw) for coverage from few centimeters to 10 meters range and 20 dbm, (100 mw) for coverage upto 100 m. This restrictions is imposed as it operates in the ISM band which is open to all as well as 802.11 also operates in the same frequency. The power output level restrictions will reduce the interferences if not eliminate the same.Modulation Characteristics:  The Bluetooth radio module uses GFSK (Gaussian Frequency Shift Keying) where a binary one is represented by a positive frequency deviation and a binary zero by a negative frequency deviation. BT is set to 0.5 and the modulation index must be between 0.28 and 0.35.

* Transmitter Characteristics

Power Classes:  Each device is classified into 3 power classes, Power Class 1, 2 & 3.

Power Class 1: is designed for long range (~100m) devices, with a max output power of 20 dBm, Power Class 2: for ordinary range devices (~10m) devices, with a max output power of 4 dBm, Power Class 3: for short range devices (~10cm) devices, with a max output power of 0 dBm.

* Radio Frequency Tolerance: The transmitted initial center frequency accuracy must be ±75 kHz from Fc. The initial frequency accuracy is defined as being the frequency accuracy before any information is transmitted. Note that the frequency drift requirement is not included in the ±75 kHz.

* Receiver Characteristics

* Sensitivity Level:    The receiver must have a sensitivity level for which the bit error rate (BER) 0.1% is met. For Bluetooth this means an actual sensitivity level of -70dBm or better.

* RSSI: Receiver Signal Strength Indicator (Optional):    A transceiver that wishes to take part in a power-controlled link must be able to measure its own receiver signal strength and determine if the transmitter on the other side of the link should increase or decrease its output power level. A Receiver Signal Strength Indicator (RSSI) makes this possible

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Frequency band of operation is 2.402 GHz to 2.480 GHz. It uses two GFSK modem with a transmission rate of 1 Mbps that hops over 79 channels in the given band. The Bluetooth hopping rate is 1600 hops per sec, that is 625 micro sec dwell time.

Bluetooth assigns a specific hopping pattern for each piconet. The pattern is decided by the piconet identity and the master clock phase residing in the Master terminal in the Piconet. (This gives an distinct identity for each piconet as well as for each of mobile station which can act as a Master as the phase of the clock will be different). Frequency hopping strategy is shown below:

The channel is divided into time slots (1600 slots per second or 625 micro seconds per slot called dwell time)where each slot corresponds to an RF hop frequency. Consecutive hops correspond to different RF hop frequencies

The 79 frequency hops at the given band are arranged in odd and even groups. Overall hopping pattern is divided into 32 hop segments. The 32 hop pseudorandom hopping pattern segment is generated based on the master identity of a piconet (Bluetooth device address (BD_ADDR) of the master) and clock phase of a Mobile station. This facilitates each piconet to have a seperate identity as well as each MS to become either a Master or be a part of two or more piconet in which case it can be a Master in one and Slave in rest.. Each 32 hop sequence starts at a point in the spectrum and hops over the pattern that covers 64 MHz as it hops either on odd or even frequencies. After completion of each segment, the sequence is altered and the segment is shifted 16 frequencies to the forward direction. This way the entire frequencies are used at an equal probability. Change of the clock or identity of the piconets will change the sequence and the segment mapping, that allows different piconet to operate with different set of random codes.

To protect the integrity of the transmitted data, Bluetooth uses two error correction schemes in the Baseband controller. An FEC code is applied to header information and if required it can be applied to payload data for the voice oriented synchronous packets. An unnumbered ARQ scheme is also applied by the Baseband layer for the asynchronous data oriented information in which the recipient acknowledges data transmitted.

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MAC Mechanism: The medium access mechanism of Bluetooth is FH-TDMA/TDD (Frequency Hopping – Time Division Multiple Access / Time Division Duplex) system that employs polling to establish the link. The fast hopping of 1600 hops per second (625 micro sec dwell time) allows one packet of 625 bits to be transmitted which provides a data rate of 1 Mbps. In each piconet, “M” decides and polls a “S” Polling instead of contention provides less overheads for the short pulse (625 bits).

Frame Format: In this one packet is constructed for one hop, that is for 625 micro sec. This basic slot can be extended to combine three (1875 micro sec )or five slots (3125 micro sec). This feature along with FH/TDMA/TDD allows a M terminal to poll multiple S at different data rates for voice and data communication. This is shown below:

In this a time gap of 200 micro sec is specified between adjacent slots to switch from transmitter to receive mode for the TDD operation. The different slot combination for Transmission and reception provides different data rate to be achieved by the M as mostly the large volume of data needs to be down loaded from M.

Each of the slot is a frame whose format is given below:

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It contains 72 bit access code, 54 bit header and 0-2744 bits of for different payloads that can be as long as five slots Access code field consists of a four bit preamble and a four bit trailer with 64 bit synchronization PN sequence. The 48 bit unique MAC address of Bluetooth device is used as seed to derive the PN sequence.

Short Questions:1. Explain about the concept of Bluetooth.

Mobile IP

Objective:

To understand the concepts of mobile internet protocolsTo know about the basic idea about IP

Mobile IP: The purpose of mobile IP is to provide TCP / IP support to the wireless nodes such as laptops and notebooks and even mobile routers (as in the case of airborne routers). Wireless nodes have the inherent problem of compatibility, transparency, scalability and efficiency. That is current TCP / IP network structure should not be modified as it has spread allover the globe. Transparency means that mobility should remain invisible to higher application layers. That is in spite of lower wireless bandwidth and more discontinuances, the higher layers should work. Enhancing IP for mobility should not generate message flooding as mobile IP provision is likely to lead to with every device implementing mobile IP protocol and hence it needs to be efficient and scalable.

Following diagram indicates the layout of Mobile IP and brief descriptions of the these entities is given below.

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Mobile Node: MN is the mobile node that can change its point of attachment to the internet using mobile IP. The MN keeps its IP address and can continuously communicate with any other system in the internet as long as link layer connectivity is given.

Correspondent Node (CN): CN is the node that corresponds with the mobile node. It can be fixed or even mobile. IN the above diagram, it is assumed to be fixed.

Home Network: It is the subnet the MN belongs to with respect to its IP address. NO mobile IP support is needed within its home network.

Foreign Network: It is the current subnet the MN visits and which is not the home network.

Foreign Agent (FA): It is the wired network end point from which the wireless region begins. FAs are routers and form the default routers for the MN. FA can provide the following services:

a. FA address forms the COA (Care Of Address) for the MN node.b. FA can provide security services to the visiting node.

Care Of Address (COA) COA defines the current location of the MN from an IP point of view. All packets sent to the MN are delivered to the COA, not directly to the IP address of the MN. Packet delivery to the MN from Home Agent (HA) to Foreign Agent (FA) is done using tunneling. COA is the tunnel end point.

Home Agent (HA) The HA provides several services for the MN and is located in the home network. The tunnel for packets towards the MN starts at the HA. HA maintains location registry that is the current location of MN by the current COA. The HA can be implemented normally on a router.

Mobility Agent : Either a HA or FA are known as mobility agents.

Tunneling:

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As can be seen in the network, a CN, is connected via a router to the internet as are the home network and the foreign networks. HA and FA are implemented on the router that are connected to the network. The MN is currently in the foreign network. The packets are transferred from the HA to FA in tunneled format and the FA de tunnels the packets and transmit the same to the MN.

Short questions:1. Explain the various parts of Mobile IP.2. Define tunneling.3. Define mobile node.

IPV6:

Objective:To know key features about IPv6

The network layer that is present in use in commonly referred to as IPv4. Although IPv4 is well designed and has helped the internet to grow rapidly, it has some deficiencies, These deficiencies has made it unsuitable for the fast growing internet. To overcome these deficiencies, Internet Protocol, Version 6 protocol has been proposed and it has evolved into a standard. Important features of IPv6 are highlighted below:

IPv6 uses 128-bit address instead of 32-bit address to provide larger address spaceUses more flexible header format, which simplifies and speeds up the routing process Basic header followed by extended header Resource Allocation options, which was not present in IPv4 Provision of new/future protocol options Support for security with the help of encryption and authentication Support for fragmentation at source

TCP:

Objective:

At the end of the session the learner will be able to know about the concepts of transmission control protocol

To understand the working mechanism of the TCP

Introduction

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Transport protocols typically designed for Fixed end-systems Fixed, wired networks

TCP congestion control Packet loss in fixed networks typically due to (temporary) overload situations Routers discard packets as soon as the buffers are full TCP recognizes congestion only indirectly via missing acknowledgements Retransmissions unwise, they would only contribute to the congestion and make it even

worse Slow-start algorithm as reaction

TCP Slow Start

Sender calculates a congestion window for a receiver Start with a congestion window size equal to one segment Exponential increase of the congestion window up to the congestion threshold, then

linear increase Missing acknowledgement causes the reduction of the congestion threshold to one half of

the current congestion window Congestion window starts again with one segment

TCP Fast Retransmit/Recovery

TCP sends an acknowledgement only after receiving a packet If a sender receives several acknowledgements for the same packet, this is due to a gap in

received packets at the receiver However, the receiver got all packets up to the gap and is actually receiving packets Therefore, packet loss is not due to congestion, continue with current congestion window

(do not use slow-start)Influences of mobility on TCP

TCP assumes congestion if packets are dropped Typically wrong in wireless networks, here we often have packet loss due to

transmission errors Furthermore, mobility itself can cause packet loss, if e.g. a mobile node roams from

one access point (e.g. foreign agent in Mobile IP) to another while there are still packets in transit to the wrong access point and forwarding is not possible

The performance of an unchanged TCP degrades severely However, TCP cannot be changed fundamentally due to the large base of installation

in the fixed network, TCP for mobility has to remain compatible The basic TCP mechanisms keep the whole Internet together

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access point (foreign agent) wired Internet

“wireless” TCP standard TCP

Internet

socket migrationand state transfer

Indirect TCP I

Indirect TCP or I-TCP segments the connection No changes to the TCP protocol for hosts connected to the wired Internet, millions of

computers use (variants of) this protocol Optimized TCP protocol for mobile hosts Splitting of the TCP connection at, e.g., the foreign agent into 2 TCP connections, no

real end-to-end connection any longer Hosts in the fixed part of the net do not notice the characteristics of the wireless part

I-TCP socket and state migration

Advantages

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„wired“ Internet

end-to-end TCP connection

foreignagent

No changes in the fixed network necessary, no changes for the hosts (TCP protocol) necessary, all current optimizations to TCP still work

Transmission errors on the wireless link do not propagate into the fixed network Simple to control, mobile TCP is used only for one hop between, e.g., a foreign agent and

mobile host Therefore, a very fast retransmission of packets is possible, the short delay on the mobile

hop is knownDisadvantages

Loss of end-to-end semantics, an acknowledgement to a sender does not any longer mean that a receiver really got a packet, foreign agents might crash

Higher latency possible due to buffering of data within the foreign agent and forwarding to a new foreign agent

Snooping TCP

Transparent extension of TCP within the foreign agent Buffering of packets sent to the mobile host lst packets on the wireless link (both directions!) will be retransmitted immediately by the

mobile host or foreign agent, respectively (so called “local” retransmission) The foreign agent therefore “snoops” the packet flow and recognizes acknowledgements

in both directions, it also filters ACKs Changes of TCP only within the foreign agent (+min. MH change)

Data transfer to the mobile host

FA buffers data until it receives ACK of the MH, FA detects packet loss via duplicated ACKs or time-out

Fast retransmission possible, transparent for the fixed network Data transfer from the mobile host

FA detects packet loss on the wireless link via sequence numbers, FA answers directly with a NACK to the MH

MH can now retransmit data with only a very short delay

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Advantages:

Maintain end-to-end semantics No change to correspondent node No major state transfer during handover

Problems

Snooping TCP does not isolate the wireless link well May need change to MH to handle NACKs Snooping might be useless depending on encryption schemes

Mobile TCP

Special handling of lengthy and/or frequent disconnections M-TCP splits as I-TCP does

Unmodified TCP fixed network to supervisory host (SH) Optimized TCP SH to MH

Supervisory host No caching, no retransmission Monitors all packets, if disconnection detected

set sender window size to 0 sender automatically goes into persistent mode

Old or new SH reopen the windowAdvantages

Maintains semantics, supports disconnection, no buffer forwardingDisadvantages

Loss on wireless link propagated into fixed network Adapted TCP on wireless link

Fast retransmit/fast recovery

Change of foreign agent often results in packet loss TCP reacts with slow-start although there is no congestion

Forced fast retransmit As soon as the mobile host has registered with a new foreign agent, the MH sends

duplicated acknowledgements on purpose This forces the fast retransmit mode at the communication partners Additionally, the TCP on the MH is forced to continue sending with the actual

window size and not to go into slow-start after registrationAdvantage

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Simple changes result in significant higher performance Disadvantage

Further mix of IP and TCP (to know when there is a new registration), no transparent approach

Transmission/time-out freezing

Mobile hosts can be disconnected for a longer time No packet exchange possible, e.g., in a tunnel, disconnection due to overloaded cells

or mux. with higher priority traffic TCP disconnects after time-out completely

TCP freezing MAC layer is often able to detect interruption in advance MAC can inform TCP layer of upcoming loss of connection TCP stops sending, but does not assume a congested link MAC layer signals again if reconnected

Advantage

Scheme is independent of data and TCP mechanisms (Ack,SN) => works even with IPsecDisadvantage

TCP on mobile host has to be changed, mechanism depends on MAC layerSelective retransmission

TCP acknowledgements are often cumulative ACK n acknowledges correct and in-sequence receipt of packets up to n If single packets are missing quite often a whole packet sequence beginning at the gap

has to be retransmitted (go-back-n), thus wasting bandwidth Selective retransmission as one solution

RFC2018 allows for acknowledgements of single packets, not only acknowledgements of in-sequence packet streams without gaps Sender can now retransmit only the missing packetsAdvantage:

Much higher efficiencyDisadvantage

More complex software in a receiver, more buffer needed at the receiverTransaction oriented TCP

TCP phases Connection setup, data transmission, connection release Using 3-way-handshake needs 3 packets for setup and release, respectively

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Thus, even short messages need a minimum of 7 packets! Transaction oriented TCP

RFC1644, T-TCP, describes a TCP version to avoid this overhead Connection setup, data transfer and connection release can be combined Thus, only 2 or 3 packets are needed

Advantage

EfficiencyDisadvantage

Requires changed TC Mobility no longer transparent

IEEE 802.11

Objective: To understand the basic concepts of IEEE 802.11 To know the various types of IEEE 802.11

IEEE 802. x specifies a number of standards, like Ethernet, token ring etc. Wireless is also clubbed along with these as the protocol structure is similar. In all these, only the physical layer and Data Link Control layer are different leaving all other upper layers same. The data link layer is divided into Logical Link Control Layer and Medium Access Control Layer as shown below:

General functionality of Physical layer is to provide encoding / decoding of signals, synchronization and bit transmission and reception. Similarly the functionality of MAC layer is - on transmission, to assemble data into frame with address and error detection fields, on reception , to disassemble frame and perform address recognition and error detection. Govern access to the LAN transmission medium and to provide interface to higher layers and perform flow and error control.

IN addition to the above functionality, WLAN inlcude the support of power management, handling hidden nodes ability to operate world wide. To create world wide operability, the ISM bands at 900MHz and 2.4 MHz are selected in addition to IR transmission reception.System Architecture: Following is the figure of infrastructure based network.

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Logical Link Control

Medium Access Control

Physical

Upper Layer Protocol

Nodes – known as stations – associate are wirelessly connected to access points (AP) that are within the radio coverage. The radio coverage area of the Access Point is known as Base Service Set (BSS). The association between the station and a BSS is dynamic. Station may turn off, come within range and go out of range etcTwo or more BSS are connected via a Distribution System (DS). This process extends the reachability of the nodes in a BSS. This network is called Extended Service Set (ESS). The ESS appears a single logical LAN to the logical control level (LLC). The DS connects the BSS / ESS AP with a portal - which is implemented in a device such as a bridge or router that is part of a wired LAN - which forms the Interworking units to other LANs. IEEE 802.11 also allows to build Ad Hoc Networks as shown in the following figure:

IN this case a BSS comprises a group of stations that use the same frequency. There is connectivity between the station within a BSS but not to nodes of other BSS. However if the radio frequency do not overlap, there can be more number of BSS in the same geographical area Also 802.11 does not specify any special nodes that support routing, forwarding of data or exchange of topology information.

Protocol Architecture: It is intended that IEEE 802.11 protocol architecture fits seamlessly into the other 802.x standards. Following figure depicts the 802.11 integrated to Ethernet, that is 802.3 protocol via abridge.

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As it can be seen, higher layers look the same for wired as well as wireless node. Also, applications should not notice any difference apart from the lower bandwidth and higher access time from the wireless LAN. The upper part of the Data Link Control Layer - the LLC – covers the differences of the medium access control layers needed for different media (IR, Radio- FHSS, DSSS CCK).

The PHYSICAL layer is subdivided into a PLCP – Physical Layer Convergence Protocol and the PMD – Physical Medium dependent Sub layer. As shown below:

The basic task of MAC layer comprises of medium access, fragmentation of user data and encryption.

The PLCP sub layer provides a carrier sense signal called Clear Channel Assessment(CCA) and provides a common PHY Service Access Point (SAP)

PMD sub layer handles modulation and encoding / decoding of signals.

The MAC Management layer supports, association and re association of a station to an access point and roaming between different access points. It also controls, authentication mechanism , encryption, synchronization of a station with regard to an access point and power management.

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MAC Management also maintains MAC Management Information Base.

PHY Management includes channel tuning and PHY MIB maintenance.

Station management interacts with both the management layers and is responsible for additional layer functions such as - control of bridging and interaction with the distribution system in case of access point.

Physical Layer:

IEEE 802. 11 supports different type of physical layer. One layer based on Infra red and two layers based on the radio transmission (primarily in the ISM bands at 2.4 GHz). These were introduced in the year 1997. In addition , two more variants with higher data rates were introduced in the year 1999. These were IEEE 802.11 a and 802.11b . The details are shown below:

App PHY variants include the provision of the clear channel assessment (CCA). The purpose of this signal is to provide medium access mechanism by indicating if the medium is busy or idle. The transmission technology determines exactly how this signal is obtained.

Also, the physical layers offers a service access point (SAP) with a 1 or 2 Mbps transfer rate to the MAC layer.

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Following chart gives the details of the PHYSICAL Layer specifications:

Direct Sequence Spread Spectrum:

In this up to seven channels, each with a bandwidth of 5 MHz can a be used Number of channel available in each country depends on the spectrum allocated by that country. The encoding schemes that is used is DBPSK for the 1 Mbps and DQPSK for 2 Mbps rate. DSSS makes use of the chipping code or pseudo noise sequence, to spread the data rate and hence the bandwidth of the signal. For IEEE 802.11, a 11 bit barker sequence is used. Maximum

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transmit powr is lmited to 100 mw EIRP(Equivalent Isotropically Radiated Power) All bits are

transmitted by the DSSS PHY layer are scrambled with the polynomial s(z) = z7 + z4 + 1.

Following figures shows a frame of the physical layer using DSSS:

The frame consists of two parts, the PLCP part (preamble and header) and the payload part. PLCP part is always transmitted at 1 Mbps where as the pay load, that is the MAC data can use either 1 Mbps or 2 Mbps. The details of the field is given below:

Synchronisation : The first 128 bits are for synchronization, gain setting, energy detection (for the CCA) and frequency offset compensation. These are scrambled 1 bits.

Start Frame Delimiter (SFD) : This 16 bit files is used for synchronization at the beginning of a frame and consists of pattern 1111001110100000.

Signal : As on now only two values have been specified . 0x 0A indicates 1 Mbps and Ox 14 indicates 2 Mbps. Other values have been reserved for future use.

Service : This fields is reserved for futures use. 0x 00 indicates an IEEE 802.11 compliant frame.

Length: 16 bits are used for length indication. Header Error Check : Signal, service and length fields are protected by this checksum

using CRC 16 polynomial.Frequency Hopping Spread Spectrum:

Frequency hopping spread spectrum technique allows for coexistence of multiple networks in the same area by separating different networks using different hoping sequences. The standard defines 79 hopping channels for NA and 23 hopping channels for Japan (each with a bandwidth of 1 MHz in the 2.4 MHz ISM band.) For modulation, FHSS schemes uses two level Gaussian FSK for the 1 Mbps system and for four level GFSK for 2 Mbps system. In this bits one and zero are encoded as deviations from the current carrier frequency. In case of four level, four different deviations form the carrier frequency. While sending and receiving is mandatory at 1 Mbps for all devices, operation at 2 Mbps is optional. Following diagram shows the frame of the physical layer used with FHSS.

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In this also there two parts - PLCP part and Payload part. Functions of various fields are described below:

Synchronisation: The PLCP preamble starts with 80 bit synchronization which is 010101 .. bit pattern. This is used for synchronization and for CCA.

Start Frame Delimiter (SFD): These 16 bits indicate that start of the frame and this provide frame synchronization. The SFD pattern is 0000110010111101

PLCP _PDU Length Word (PLW): This fields indicates the length of the payload in bytes including the 32 bit CRC at the end of the payload. PLW can range between 0 – 4095.

PLCP Signaling Files (PSF) : This four bit field indicates thje data rate of the payload following 1 or 2 Mbps.

Header error check : The PLCP header is protected by a 16 bit checksum with the standard ITU_ T generator Polynomial .

Infrared:

The IEEE 802.11 infrared scheme is omni directional rather than point to point. A range of up to 20 m is possible. The modulation scheme for the 1 Mbps data rate is known as 16 PPM (Pulse Position Modulation). In this scheme group of 4 data bits is mapped into one of the 16 PPM symbols, each symbol is a string of 16 bits. Each 16 bit string consists of fifteen 0 and one binary 1. For the 2 Mbps data rate, each group of 2 data bits is mapped into one of four 4 bit sequences. Each sequence consists of a three 0 and one binary 1. The actual transmission uses an intensity modulation schemes in which the presence of a signal corresponds to a binary 1 and the absence of a signal corresponds to a binary 0.

IEEE 802.11 a:

This makes use of 5 GHz band. In this orthogonal frequency division multiplexing (OFDM) is used. This is also called multi carrier modulation. Uses multiple carrier signals at different frequencies, sending some of the bits on each channel. This is similar to FDM. But in the case of OFDM, all of the sub channels are dedicated to a single data source. The possible data rates are 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. The systems uses up to 52 sub carriers that are modulated using BPSK, QPSK 16 QAM or 64 QAM, depending on the data rate required.

IEEE 802.11b:

It is an extension of the IEEE 802.11 DS-SS scheme, providing rates of 5.5 and 11 Mb[s. The chipping rate is 11 MHz, which is the sme as the original scheme, thereby occupying the same bandwidth. To achieve higher data rate in the same bandwidth at the same chipping rate, a modulation scheme known as Complementary Code Keying (CCK) is used.

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In CCK, input data are treated in blocks of 8 bits at a rate of 1.375 MHz ( 8 bits / symbol * 1.375 MHz = 11 Mbps). Six of these bits are mapped into one of the 64 codes sequences based on the use of 8 x 8 Walsh matrix. The out put of the mapping and two additional bits forms the input to a QPSK modulator

MAC FRAME FORMAT: The frame transmission in case of WLAN can be categorised into three types: Data, Management and Control. These are indicated in the MAC frame. Following figure shows the MAC frame format. This is a general format : (a) in octets while (b) in bits

Overall frame structure is shown in the(a). Details are as follows:FC : Frame Control indicates the type of frame : Data/Management / controlD/I : Duration / connection ID: When used as a duration field, indicates the time in microseconds the channel will be used by the source. In some cases, it may indicate connection identifier.Addresses: These are Source/ Destination/ Sender and Receiver addresses.SC: Sequence Control: Four bits are used for numbering fragmentation of a message while the 12 bits are used sequence number sent between a transmitter and a receiver.Frame Body: These are MAC SDU limited to a length of 2312 bytes.Frame Check Sequence: a 32 bit Cyclical Redundancy Check.

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Details of the frame control fields are shown in part (b) of the above figure.:Protocol Version: 802.11 version. Currently it is set to 0Type: Identifies the frame as control, management or data frame.Sub Type: Further identifies various function under each type. Details of Type and Sub type are shown below:

1. Management Type (00) 0000 / 0001 : Station requesting association with a AP (BSS) and AP’s response 0010 / 0011 : Re association request and response. Sent by a station when moves out

of a BSS and moves into a different BSS. 0100 / 0101 : Probe Request and Response Used to obtain information from another

station or AP 1000 : Beacon 1001 : Announcement Traffic Indication Map : ATIM: A station making an

announcement to other station that it has buffered data to transmit and its power low. Disassociation: Used by a station to terminate Association. 1011 / 1100 : Authentication and De authentication. Indicates using secure

communication.2. Control Type : 01

1010 : Power Save Poll: Request the AP or other station to transmit any buffered data as the station is in power saving mode.

1011 / 1100 : RTS and CTS; 1101 : ACK 1110 / 1111 : CF End / CF end with ACK.: Announces the end of contention free

period (PCF) / ACK the CF end.3. Data Frames : 10:

0000 / 0001 : Data / Data with CF ACK. Simple data transfer / Sends ACK for previously received data along with Data

0010 / 0011 : Data + CF – Poll : Used by PCF to deliver data a mobile station and also to request that the mobile station send a data frame that it may have buffered.

0100 / 0101 : No data / CF ACK no Data indicates that the frame carries no data, polls or ACK, but used to carry power management bit int the frame fields to the AP to indicate the AP that is changing to low power operation mode.

0110 / 0111 : CF Poll (no data)/ CF Poll ACK (no data) : These fames with no data.

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MAC Management Sub Layer: This sub layer handles various management functions such as registration, handoff,

power management, and security. These are explained briefly:Registration: . The beacon is a management frame that is transmitted quasi periodically by the AP to establish the timing synchronisation function (TSF). It contains information such as the BSS – ID, Time stamp (for synchronisation), power management , and roaming. Received signal strength (RSS) measurements are made on the beacon message. Association is a process by which a MS registers with an AP which is carried out with the exchange of association request and response frames. Only on registration, the distribution system will know to which BSS an MS is attached.

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Hand Off: There are three types of mobility in WLAN .- No transition : Implies the MS is within or is moving within a BSA.- BSS Transition: Indicates that the MS moves from one BSS to another in the same ESS.- ESS Transition: This is the movement of MS from one BSS to another BSS that is part of

new ESS. In this case connection may break unless it has higher layer IP connection.

The handoff procedure is a WLAN is shown in the following figure:

The AP broadcast a beacon signal periodically ( typically once in 100 ms). An Ms that scans the beacon signal and associates itself with the AP with the strongest beacon. The beacon contains information corresponding to the AP such as Time Stamp, beacon interval, capability, ESS Id and traffic indication Map (TIM). The MS uses this information in the beacon to distinguish between different APs.

The MS keeps tack of the RSS of the beacon of the beacon of the AP with which is it is associated, and when the RSS becomes weak, it starts to scan for stronger beacons from the neighbouring APs. The scanning process can be either active or passive. In passive scanning, the MS simply listens to available beacons. In active scanning, the MS sends a probe request to a targeted set of APs that are capable of receiving its probe. Each AP that receives the probes responds with a probe response that contains the same information that is available in the beacon except for the TIM. The probe response thus serves the MS to select the AP with strongest beacon and sends reassociation requests to the new AP. In response, the new AP sends re association which contains information of MS and that of old AP. In Response the new AP sends the re association response that has the information about the supported bit rates, station ID and so on needed for communication. The old AP is not informed by the MS about the

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change of location. The hand off is intimated by using IAPP (Inter Access point protocol) standard that intimates the old AP about handoff through wired network.

Power Management: Power Management is carried out by putting the station in ‘awake’ position during

reception of inbound data and putting them to ‘sleep’ position during idle period. This explained based on the following figure:

This is achieved as follows: All the station are time synchronized with the help of TSF (Time Synchronisation Frame). After synchronisation, all the station synchronise the awake / sleep period with the beacon of AP. Now any data that needs to be delivered to any node is transmitted by the source station by accessing the medium by following medium access control mechanism. The AP buffers all such data and transmits during beacon period in the form of TIM (Traffic Indication Map) the details regarding which station has data waiting. The station that come on periodically during beacon period, listen to this information and if they have any data waiting for them, will stay active to receive the same. This saves the power of the stations. Security: IEEE 802.11 provides both privacy and authentication mechanism. The mechanism is known as

Wired Equivalent Privacy (WEP). To provide privacy and data integrity, WEP uses an encryption algorithm based on the RC4 algorithm. Following figure shows the encryption process.

Above figure shows the encryption process. The integrity algorithm is the 32 bit CRC that is appended to the end of MAC frame. For encryption process, a 40 bit secred key is shared by two participants in the exchange. An Initialisation Vector I(IV ) is I concatenated to the

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secret key. The resulting block form the seed that is input to the pseudorandom number generator (PRNG) defined in RC4. The PRNG generates a bit sequence of the same length as that of MAC frame plus its CRC. A bit by bit exclusive OR between the MAC frame and the PRNG sequence produces the cipher text. The IV is changed periodically (as often as every transmission). Every time the IV is changed, the PRNG sequence is changed, which provides protection against eavesdropper.

At the receiving end, the receiver retrieves the IV from the data block and concatenates this with the shared secret key to generate the same key sequence used by sender. This key sequence is then XORed with the incoming block to recover the plaintext. This technique makes use of the property AB B = A. Finally the receiver compares the incoming CRC with the CRC calculated at the receiver to validate integrity.

Authentication:There are two types of authentication : Open System and Shared Key.

Open system authentication simply provides a way for two parties to agree to exchange data and provides no security benefits. IN this one party sends a MAC control frame, known as authentication frame to other party. The frame indicates that this is an open system authentication type. The other party responds with its own authentication frame and the process is complete.

Shared Key Authentication: This requires that the two parties share a secret key not shared by any other party. This key is used to assure that both sides are authenticated to each other. The procedure is as follows:a. A sends a MAC authentication frame with an authentication algorithm identification of

“Shared Key” and with station identifier that identifies the sending stationb. B responds with an authentication frame that includes a 128 octet challenge text which

is generated using WEP PRNG. c. A transmits an authentication frame that includes the challenge text just received from B.

The entire frame is encrypted using WEP.d. B Receives the encrypted frame and decrypts it using WEP and the secret key shared

with A. If decryption is successful (matching CRC), then B compares the inkling challenge text with the challenge text that it sent in the second message. B then sends an authentication message to A with a status code indicating success or failure.

Short questions:1. Briefly describe handoff procedure in 802.11 2. Briefly describe MAC sub layer of 802.11 and indicate how it provides both

contention based and contention free access mechanism3. Explain detail the MAC frame format of 802.11.

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