datacommchapter 3

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FACULTY OF ENGINEERING,

CYBERJAYA

YEAR 2008

ECP 2056 :DATACOMMUNICATIONS AND COMPUTERNETWORKING

Saiful Jumaat Osman, SUPELEC France

3.1 Transmission media guided & unguided

3.2 Bandwidth utilization: multiplexing

FDM, TDM & WDM, SPREAD SPECTRUM

CHAPTER 3 : Characteristics of Data CommunicationNetworks


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ECP 2056 :DATACOMMUNICATIONS AND COMPUTER NETWORKING


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Figure 1 Transmission medium and physical


Signals travel along the media, directed and contained by thephysical limits of the medium


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Figure 2 Classes of transmission



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GUIDED MEDIAGUIDED MEDIA

Guided media, which are those that provide aGuided media, which are those that provide aconduit from one device to another, include twisted-conduit from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable.pair cable, coaxial cable, and fiber-optic cable.

Topics discussed in this section:Topics discussed in this section:


3 main types of transmission medium used forwired LANs:- Twisted pair- Coaxial cable [coax]

- Optical Fiber


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Figure 3 Twisted-pair


Two type: shielded and unshieldedUsed primarily in star and tree/hub networkUnshielded twisted pair [UTP]:

- does not include any extra shielding around the wire pair- ordinary telephone line and commonly used for LAN- least expensive, easy to work [less rigid], simple to install- subject to external electromagnetic interference- limited length


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Figure 4 UTP and STP cables


Shielded twisted pair [STP]:- Covered with foil shield [polyester covered with aluminumon both sides] to reduced interference and crosstalk- Better performance, but more expensive and difficult towork then UTP [heavy and bulky]


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Figure 5 UTP connector



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Figure 6 Coaxial



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Table 2 Categories of coaxial cables


Coaxial Cable

Used primarily in bus networksOperating with either baseband or broadbandBaseband:

- all available bandwidth is used to derive a singletransmission channel

Broadband:

- available bandwidth is divided to derived a number oflower bandwidth subchannels on one cable


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Figure 7 BNC



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Figure 8 Bending of light


OPTICAL FIBER : Provides a medium for signals using lightinstead of electricity


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Figure 9 Optical



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Figure 10 Propagation


Basically two modes of transmission in fiber:(a) Single mode fiber - a light ray in one direction only(b) Multi-mode fiber - a number of path in which light raymay travel


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Figure 11 Modes


Further classified byreflective index profile oftheir core

They can be either stepindex or graded index

Three main types of fiberare:(a) Single mode fiber(b)Multimode stepped index

(c)Multimode graded index


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Figure 12 Fiber



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Figure 13 Fiber-optic cable


d d d d d


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UNGUIDED MEDIA: WIRELESSUNGUIDED MEDIA: WIRELESS

Unguided media transport electromagnetic wavesUnguided media transport electromagnetic waveswithout using a physical conductor. This type ofwithout using a physical conductor. This type of

communication is often referred to as wirelesscommunication is often referred to as wirelesscommunication.communication.

Radio Waves

MicrowavesInfrared



3 1 T i i di id d & id d


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Figure 14 Propagation


Signals travel from source to destination in severalways:i. Ground propagationii. Sky propagationiii. line-of-sight propagation



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Table 3 Bands



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3 1 Transmission media guided & unguided


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Radio waves are used for multicastcommunications, such as radio and

television, and paging systems.

Note




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Figure 16 Unidirectional




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Microwaves are used for unicastcommunication such as cellular telephones,satellite networks,and wireless LANs.

Note




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Infrared signals can be used for short-rangecommunication in a closed area using line-

of-sight propagation.

Note

3. a s ss o ed a gu ded & u gu ded


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3.2 Bandwidth utilization: multiplexing FDM, TDM &


3.2 Bandwidth utilization:

lti l i FDM TDM & WDM


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Multiplexing allows several transmission sources to share

a larger transmission capacity to make efficient use of highspeed telecommunications lines

MUX combines (multiplexes) data from the n input lines and

transmits over a higher-capacity data link

DEMUX accepts the multiplexed data stream, separates

(demultiplexes) the data according to channel, and delivers

them to the appropriate output lines

multiplexing FDM, TDM & WDM


lti l i FDM TDM & WDM


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Whenever the bandwidth of a medium linking twoWhenever the bandwidth of a medium linking twodevices is greater than the bandwidth needs of thedevices is greater than the bandwidth needs of thedevices, the link can be shared. Multiplexing is thedevices, the link can be shared. Multiplexing is theset of techniques that allows the simultaneousset of techniques that allows the simultaneoustransmission of multiple signals across a singletransmission of multiple signals across a single

data link. As data and telecommunications usedata link. As data and telecommunications useincreases, so does traffic.increases, so does traffic.

Frequency-Division MultiplexingWavelength-Division MultiplexingSynchronous Time-Division MultiplexingStatistical Time-Division Multiplexing




lti l i g FDM TDM & WDM


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Figure 1 Dividing a link into



multiplexing FDM TDM & WDM


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Figure 2 Categories of





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Figure 3 Frequency-division


Possible when useful bandwidth of medium exceedsrequired bandwidth(BW) of channel

Each signal is modulated to a different carrierfrequency

Carrier frequencies separated so signals do not overlap(guard bands)

e.g. broadcast radio

Channel allocated even if no data


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Figure 5 FDM demultiplexing





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FDM System

In Figure b, the

spectrum of signal mi is

shifted to centered at fi.fi must be chosen so

that the BWs of various

signals do not

significantly overlap.


Example


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Assume that a voice channel occupies a bandwidth of

4 kHz. We need to combine three voice channels into alink with a bandwidth of 12 kHz, from 20 to 32 kHz.Show the configuration, using the frequency domain.Assume there are no guard bands.

SolutionWe shift (modulate) each of the three voice channels to adifferent bandwidth, as shown in Figure 6.6. We use the 20- to24-kHz bandwidth for the first channel, the 24- to 28-kHz

bandwidth for the second channel, and the 28- to 32-kHzbandwidth for the third one. Then we combine them as shownin Figure 6.6.


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Example

Example


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Five channels, each with a 100-KHz bandwidth, are to bemultiplexed together. What is the minimum bandwidth ofthe link if there is a need for a guard band of 10 KHz

between the channels to prevent interference?

SolutionSolution

For five channels, we need at least four guard bands. Thismeans that the required bandwidth is at least

5 x 100 + 4 x 10= 540 KHz,

as shown in Figure

Example 2


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Figure 6 Analog

3.2 Bandwidth utilization:multiplexing FDM, TDM & WDM


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Wavelength Division Multiplexing

Wave-Division multiplexing(WDM) is designed to use the high datarate capability of fiber-optic cable.

The optical fiber data rate is higher than the data rate of metallictransmission cable. Using fiber-optic cable for one single linewastes the available bandwidth.

WDM is conceptually the same as FDM, except that themultiplexing and demultiplexing involve multiple beams of light atdifferent frequency

p g ,


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In a TDM, the data rate of the link is n times faster, andthe unit duration is n times shorter.

Multiple digital signals can be carried on a singletransmission path interleaved in time

Interleaving- may be at bit level of blocks. Time slots pre-

assigned to sources and fixed. Time slots are allocatedeven if no data. And time slots do not have to be evenlydistributed amongst sources

Synchronous Time Division Multiplexing



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Multiple digital signals (or analogsignals carrying digital data) arecarried out on a signaltransmission path by interleavingportions of each in time



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TDM SystemFigure a) Each buffer is

typically one bit or onecharacter in length

Figure b)- Format of

transmitted data: frames

Channel: the sequenceof slots dedicated to one

source, from frame to

frame

Figure c)- The interleaved

data are demultiplexed and

routed to the appropriate

destination buffer.


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Example 3


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b. The duration of each output time slot is one-third of theinput time slot. This means that the duration of the outputtime slot is 1/3 ms.

c. Each frame carries three output time slots. So the duration ofa frame is 3 1/3 ms, or 1 ms. The duration of a frame is the

same as the duration of an input unit.


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Example 4


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c. The output bit rate is the inverse of the output bit duration or

1/(4s) or 4 Mbps. This can also be deduced from the factthat the output rate is 4 times as fast as any input rate; so theoutput rate = 4 1 Mbps = 4 Mbps.

d. The frame rate is always the same as any input rate. So theframe rate is 1,000,000 frames per second. Because we aresending 4 bits in each frame, we can verify the result of theprevious question by multiplying the frame rate by thenumber of bits per frame.

Example


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Four 1-kbps connections are multiplexed together. A

unit is 1 bit. Find (a ) the duration of 1 bit beforemultiplexing, (b ) the transmission rate of the link, (c)the duration of a time slot, and (d ) the duration of aframe.

SolutionWe can answer the questions as follows:a . The duration of 1 bit before multiplexing is 1 / 1 kbps, or

0.001 s (1 ms).

b. The rate of the link is 4 times the rate of a connection, or 4kbps.

Example 5


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c. The duration of each time slot is one-fourth of the durationof each bit before multiplexing, or 1/4 ms or 250 s. Notethat we can also calculate this from the data rate of the link,4 kbps. The bit duration is the inverse of the data rate, or 1/4kbps or 250 s.

d. The duration of a frame is always the same as the durationof a unit before multiplexing, or 1 ms. We can also calculatethis in another way. Each frame in this case has four time

slots. So the duration of a frame is 4 times 250 s, or 1 ms.



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Figure 9

Example


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Four channels are multiplexed using TDM. If each channelsends 100 bytes /s and we multiplex 1 byte per channel, showthe frame traveling on the link, the size of the frame, theduration of a frame, the frame rate, and the bit rate for the link.

SolutionThe multiplexer is shown in Figure 6.16. Each frame carries 1

byte from each channel; the size of each frame, therefore, is 4bytes, or 32 bits. Because each channel is sending 100 bytes/sand a frame carries 1 byte from each channel, the frame ratemust be 100 frames per second. The bit rate is 100 32, or3200 bps.

Figure10Example8

Example


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A multiplexer combines four 100-kbps channels using a time

slot of 2 bits. Show the output with four arbitrary inputs. What isthe frame rate? What is the frame duration? What is the bitrate? What is the bit duration?

Solution

Figure 6.17 shows the output for four arbitrary inputs. The linkcarries 50,000 frames per second. The frame duration is therefore1/50,000 s or 20 s. The frame rate is 50,000 frames per second, andeach frame carries 8 bits; the bit rate is 50,000 8 = 400,000 bits or400 kbps. The bit duration is 1/400,000 s, or 2.5 s.

Fi ure 11 Example 9

Pulse Stuffing


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Figure 12 Pulse stuffing

Problem - Synchronizing data sources If each source has a separate clock, any variation among clock could

cause loss of synchronization Data rates from different sources not related by simple rational

number Solution - Pulse Stuffing

Outgoing data rate (excluding framing bits) higher than sum of incoming rates Stuff extra dummy bits or pulses into each incoming signal until it matches local

clock Stuffed pulses inserted at fixed locations in frame and removed at demultiplexer


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Example 8


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b. Each source sends 250 characters per second; therefore, the

duration of a character is 1/250 s, or4 ms.

c. Each frame has one character from each source, whichmeans the link needs to send 250 frames per second to keepthe transmission rate of each source.

d. The duration of each frame is 1/250 s, or 4 ms. Note that theduration of each frame is the same as the duration of eachcharacter coming from each source.

e. Each frame carries 4 characters and 1 extra synchronizing

bit. This means that each frame is4 8 + 1 = 33 bits.


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Figure 14 Digital



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Table 1 DS and T line rates



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Figure 15 T-1 line for multiplexing telephone lines



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Figure 16 T-1 frame structure



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Table 2 E line rates



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Figure 17 TDM slot



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Statistical TDM In Synchronous TDM many slots are wasted

Statistical TDM allocates time slots dynamicallybased on demand

Multiplexer scans input lines and collects data untilframe full

Data rate on the multiplexed line is less than thesum of the data rates of the attached devices

3.2 Bandwidth utilization:multiplexing SPREAD SPECTRUM


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SPREADSPREADIn spread spectrum (SS), we combine signals fromIn spread spectrum (SS), we combine signals fromdifferent sources to fit into a larger bandwidth, butdifferent sources to fit into a larger bandwidth, butour goals are to prevent eavesdropping andour goals are to prevent eavesdropping and

jamming. To achieve these goals, spread spectrum jamming. To achieve these goals, spread spectrumtechniques add redundancy.techniques add redundancy.

Frequency Hopping Spread Spectrum (FHSS)Direct Sequence Spread Spectrum Synchronous (DSSS)




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Figure 18 Spread



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Figure 19 Frequency hopping spreadspectrum (FHSS)

Signals are sent on

different carrier frequenciesusing apseudorandom sequenceknown to both sender and

receiver

Unauthorized person whotunes his/her receiver to onecarrier frequency (subband)may only receive part of the



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Figure 20 Frequency selection in



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Figure 21 FHSS



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Figure 22 Bandwidth


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Figure 23 DSSS


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FACULTY OF ENGINEERING,

CYBERJAYA

YEAR 2008

ECP 2056 :DATACOMMUNICATIONS AND COMPUTERNETWORKING

Saiful Jumaat Osman, SUPELEC France

3.3 Circuit switched data networks

3.4 Packet-switched data networks

CHAPTER 3 : Characteristics of Data CommunicationNetworks


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3.3) Packet-switched datanetworks

3.4) Circuit switched datanetworks


CHAPTER 3 Characteristics of Data Communication Networks:Packet-switched data networks & Circuit switched datanetworks

What is WAN?


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Covers large geographical areas (towns, cities, states,countries,..) or usually across an area of multiple km radius.

LAN depends on their own hardware or equipment but WANs mayuse public, leased, or private communication equipments(combined).

Usually consists of several interconnected switching nodesThe nodes provide a switching facility that will move the data from onenode to another until they reach their destination

What is WAN?


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CHAPTER 3 Characteristics of Data Communication Networks:Packet-switched data networks & Circuit switched datanetworks


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Figure 1 Switched

CHAPTER 3 Characteristics of Data Communication Networks:Packet-switched data networks & Circuit switched data

networks

N d


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Nodes Nodes may connect to other nodes only (e.g. node 5), or

to stations and other nodes (e.g. node 6) Node-to-node links usually multiplexed (FDM or TDM) Some redundant connections (alternative paths) are

desirable for reliability Two general types of switching technologies

Circuit switchingPacket switching


networks


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Figure 2 Taxonomy of switched

CIRCUIT-SWITCHED NETWORKSCIRCUIT-SWITCHED NETWORKS


networks


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CIRCUIT-SWITCHED NETWORKSCIRCUIT SWITCHED NETWORKS

A circuit-switched network consists of a set of switches A circuit-switched network consists of a set of switchesconnected by physical links. A connection between twoconnected by physical links. A connection between twostations is a dedicated path made of one or more links.stations is a dedicated path made of one or more links.However, each connection uses only one dedicated channeHowever, each connection uses only one dedicated channelon each link. Each link is normally divided into n channels bon each link. Each link is normally divided into n channels b

using FDM or TDM.using FDM or TDM.

Three Phases

EfficiencyDelayCircuit-Switched Technology in TelephoneNetworks



networks

D di d i i h b


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Dedicated communication path between two

stationsThe path is a connected sequence of links betweennetwork nodesOn each physical link, a logical channel isdedicated to the connection

The nodes must have switching capacity andchannel capacity to establish connection

The nodes must have intelligence to work outrouting


networks

A circuit switched network is made of a set of


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A circuit-switched network is made of a set ofswitches connected by physical links, in which each

link is divided into n channels.

Figure 3 A trivial circuit-switched network


networks

In circuit switching, the resources need to be reserved during


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In circuit switching, the resources need to be reserved duringthe setup phase;

the resources remain dedicated for the entire duration of datatransfer until the teardown phase.

Circuit switching involves 3 phases:Circuit switching involves 3 phases: I) Circuit Establishment

Before any signal can be transmitted, an endto end (station-to-station) circuit must beestablished

II) Data Transfer Information is transferred Generally full duplex and digital nowadays

III) Circuit Disconnect The connection can be terminated by either

one of the two stations If requested by node A, signal for deallocating

dedicated resources is propagated to nodes4,5 and 6


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As a trivial example, let us use a circuit-switched network to connect eight

Example


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telephones in a small area. Communication is through 4-kHz voice channels.

We assume that each link uses FDM to connect a maximum of two voicechannels. The bandwidth of each link is then 8 kHz. Figure 4 shows thesituation. Telephone 1 is connected to telephone 7; 2 to 5; 3 to 8; and 4 to 6.Of course the situation may change when new connections are made. Theswitch controls the connections.

Figure 4 Circuit-switched network used in Example 8.1

As another example, consider a circuit-switched network that connectscomputers in two remote offices of a private company The offices are

Example


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computers in two remote offices of a private company. The offices areconnected using a T-1 line leased from a communication service provider.There are two 4 8 (4 inputs and 8 outputs) switches in this network. Foreach switch, four output ports are folded into the input ports to allowcommunication between computers in the same office. Four other output

ports allow communication between the two offices. Figure 5 shows the

situation.

Figure 5 Circuit-switched network used in Example 8.2

Figure 6 Delay in a circuit switched


networks


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Figure 6 Delay in a circuit-switched


networks

Limitations of Circuit Switchin


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Limitations of Circuit Switchin Circuit switching is originally designed to handle voice

trafficDedicated resources (e.g. channel/connection) is allocated to

a particular call As the networks began to handle more and more data-

type traffic, two shortcomings became apparent:Resources are wasted

In a typical user/host data connection, much of the time the line isidle.

Data rate is fixed This limits the utility of the network in interconnecting a variety of

host computers and terminals


networks


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DATAGRAM NETWORKS


networks


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In data communications, we need to send messagesIn data communications, we need to send messagesfrom one end system to another. If the message isfrom one end system to another. If the message isgoing to pass through a packet-switched network, igoing to pass through a packet-switched network, itneeds to be divided into packets of fixed or variableneeds to be divided into packets of fixed or variable

size. The size of the packet is determined by thesize. The size of the packet is determined by thenetwork and the governing protocol.network and the governing protocol.

Routing Table

EfficiencyDelayDatagram Networks in the Internet


CHAPTER 3 Characteristics of Data Communication Networks:

Packet-switched data networks & Circuit switched datanetworks

Packet-switchin Princi les


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Packet switchin Princi les Data are transmitted in small packets

Typically 1000 octetsLonger messages are split into a series of packetsEach packet contains a portion of user data plus some control info

for routing purpose

Packets are received, stored briefly (buffered) and past on to thenext node and finally to the destination

Store and forward

Advantages over Circuit Switch




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g Greater line efficiency Single node to node link can be shared by many packets over time

Packets queued and transmitted as fast as possible

Data rate conversion Different stations connect to the local node at their own data rates

Nodes buffer data if required to equalize rates

No blocking Packets are accepted even when network is busy but need to tolerate

delay

Prioritization can be applied Packets with high priority will experience less delay than lower-priority

packets. Can have different queues with different priorities


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Figure 7 A datagram network with four switches




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Figure 7 A datagram network with four switches

Figure 8 Routing table in a datagram




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Figure 8 Routing table in a datagram

A switch in a datagram network uses a routing




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table that is based on the destination address.

The destination address in the header of a packetin a datagram network

remains the same during the entire journey of thepacket.

Switching in the Internet is done by using thedatagram approach

to packet switching at the network layer.

Figure 9 Delay in a datagram




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VIRTUAL-CIRCUIT NETWORKSVIRTUAL-CIRCUIT NETWORKS




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A virtual-circuit network is a cross between a A virtual-circuit network is a cross between acircuit-switched network and a datagram network. Itcircuit-switched network and a datagram network. Ithas some characteristics of both.has some characteristics of both.

AddressingThree Phases

EfficiencyDelayCircuit-Switched Technology in WANs


Figure 10 Virtual-circuit




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Figure 11 Virtual-circuit identifier In virtual circuitnetworks a virtual




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networks, a virtual

circuit identifier (VCI)is used as theidentifier for datatransfer.

It is used by a frame

between twoswitches. When a frame arrives

at a switch, it has oneVCI; when it leaves, it

has another.

Virtual Circuit Approach (cont.)




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Virtual Circuit Approach (cont.)

To communicate, a source and destinationneed to go through three phases:

Setup

Data transfer

Teardown

Setup Phase




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Preplanned route established before any packetssent.

All switches need to have a table entry for this

virtual circuit.

These switching tables contain information such asincoming port/VCI and outgoing port/VCI.

This phase is implemented in two approaches: Permanent virtual circuit (PVC)

Switched virtual circuit (SVC)

Permanent Virtual Circuit




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The corresponding table entry is recordedfor all switches by the administrator.

An outgoing VCI is given to the source, andan incoming VCI is given to the destination.

The source always uses this VCI to send

frames to that particular destination.

The PVC is like a leased telephone line.


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Figure 12 Switch and tables in a virtual-circuit network




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Figure 13Source-to-destination data transfer in a virtual-circuit network


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Setup requestA setup request frame is sent from the source to




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114Figure 14 Setup request in a virtual-circuit

the destination.

AcknowledgmentAn acknowledgment frame can complete the




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Figure 15 Setup acknowledgment in a virtual-circuit

entries in the switching tables.

Data Transfer Phase




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Once established, all the packets will follow thesame route.

Fixed for the duration of the logical connection.

Teardown Phase




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Source A, after sending all frames to B,sends a special frame called a teardownrequest.

Destination B responds with a teardownconfirmation frame.

All switches erase the corresponding entryfrom their tables.

Virtual Circuits v Datagram




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Virtual Circuits Network can provide sequencing and error control

Packets are forwarded more quickly No routing decisions to make

Less reliable as loss of that node affect all the circuits

over that node Datagram

No need call setup phase Better if few packets

More flexible Routing can be used to avoid congested parts of the network More reliable

If a node fails, subsequent packets may find an alternativeroute that bypasses that node.




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In virtual-circuit switching, all packetsbelonging to the same source anddestination travel the same path;

but the packets may arrive at thedestination with different delays

if resource allocation is on demand.

Figure 16 Delay in a virtual-circuit




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Circuit Switching Concept




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Circuit switching

elements

Elements of a Circuit-Switch Node Digital Switch




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Provide transparent signal path between devices

Full duplex transmission

Network Interface

Functions and hardware needed to connect digital devicesto the network

e.g. data processing devices and digital telephones to thenetwork.

Control Unit Carry out three general tasks

1. Establish connections

2. Maintain connection

3. Disconnect or tear down connection

Characteristics of a Circuit Switching Devicel k




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Blocking Blocking can happen when a network is unable to

connect stations because all paths are in use

Acceptable for voice systems since short durationcalls, only a fraction of the telephone will be engagedat any time

Non-blocking Permits all stations to connect (in pairs) at once

Suited for data connections as a terminal can beconnected for long hours at a time

Space Division Switching




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The signal paths are physically separated from oneanother (divided in space)

Crossbar switch

Limitations: Number of crosspoints grows as square of number ofstations costly

Loss of crosspoint prevents connection

Inefficient use of crosspoints as even when all stations

connected, only a few crosspoints are engaged To overcome these limitations Multiple-Stage

switches

Crossbar Switch




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125Outputlines

Inputl

ines


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Time Division Switching




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1291133 2244 3311 4411

Ti Sl t I t h (TSI)




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Time-Slot Interchange (TSI) A TSI consists of random access memory

(RAM) with several memory location

The RAM fills up with incoming data fromtime slots in the order received

Slots are then sent out in an order basedon the decision of a control unit

TSI (cont.)




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TDM Bus




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The input and output lines are connectedto a high-speed bus through input andoutput gates (microswitches)

Only one pair of input/output gates isclosed for each time slot

This pair of gates allows data to betransferred using the bus

The control unit opens and closes the gates

TDM Bus (cont.)


networks


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Space- and Time-Division Switch Combinations

Space division


networks


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Space-division Fast

The number of crosspoint required

Time-division

Needs no crosspointProcessing delays

Combining the two results in switches that areoptimized both physically and temporally

Multistage switches of this sort can be designed astime-space-time (TST), time-space-space-time(TSST), space-time-time-space (STTS), or otherpossible combinations

TST Switch


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The delay is mainly caused by Call

Performance Analysis Circuit Switching


networks


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The delay is mainly caused by CallRequest.

A processing delay is incurred at each nodeduring the call request for setting theroute.

There is no delay for Call Accept signal asthe connection is already set up.

The delay is similar to circuit switching

Performance Analysis VC Packet Switching


networks


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The delay is similar to circuit switching Call Request incurs a delay at each node

Call Accept experiences node delaysbecause this packet is queued at eachnode and must wait its turn fortransmission.

Data packets also queued and delayed.

Does not require a call setup hence faster

Performance Analysis DatagramPacket Switching


networks


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Does not require a call setup hence fasterfor short messages.

Because each individual datagram is routedindependently, the processing for eachdatagram at each node may be longer thanfor VC.

For long messages VC is better.


networks


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networks


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datacommchapter 3

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