UNIT IV

Transmission Media

Transmission media is a pathway that carries the information from sender to receiver. We use

different types of cables or waves to transmit data. Data is transmitted normally through

electrical or electromagnetic signals.

An electrical signal is in the form of current. An electromagnetic signal is series of

electromagnetic energy pulses at various frequencies. These signals can be transmitted through

copper wires, optical fibers, atmosphere, water and vacuum Different Medias have different

properties like bandwidth, delay, cost and ease of installation and maintenance. Transmission

media is also called Communication channel.

Types of Transmission Media

Transmission media is broadly classified into two groups.

1. Wired or Guided Media: This type of media are the cables that are tangible or have

physical existence and are limited by the physical geography. Unguided media in use are

twisted pair cable, co-axial cable and fiber optical cable. Each of them has its own

characteristics like transmission speed, effect of noise, physical appearance, cost etc.

2. Wireless or Unguided Media: This type of media are the ways of transmitting data

without using any cables. These media are not bounded by physical geography. This type

of transmission is called Wireless communication. Nowadays wireless communication is

becoming popular. Wireless LANs are being installed in office and college campuses.

This transmission uses Microwave, Radio wave, Infra red are some of popular unbound

transmission media.

Guided Media

1. Twisted Pair Cable

Copper wires are the most common wires used for transmitting signals because of good

performance at low costs. They are most commonly used in telephone lines. However, if two or

more wires are lying together, they can interfere with each other’s signals. To reduce this

electromagnetic interference, pair of copper wires are twisted together in helical shape like a

DNA molecule. Such twisted copper wires are called twisted pair. To reduce interference

between nearby twisted pairs, the twist rates are different for each pair.

Advantages of twisted pair cable:

Twisted pair cable are the oldest and most popular cables all over the world. This is due to the

many advantages that they offer

Trained personnel easily available due to shallow learning curve

Can be used for both analog and digital transmissions

Least expensive for short distances

Entire network does not go down if a part of network is damaged

Disadvantages of twisted pair cable

Disadvantages of twisted pair cable:

Twisted pair cables offer some disadvantages too −

Signal cannot travel long distances without repeaters

High error rate for distances greater than 100m

Very thin and hence breaks easily

Not suitable for broadband connections

Types of twisted pair cable

Unshielded twisted pair cable

Shielded twisted pair cable

(i) Unshielded Twisted Pair (UTP)

UTP is the copper media, inherited from telephony, which is being used for increasingly higher

data rates, and is rapidly becoming the de facto standard for horizontal wiring, the connection

between, and including, the outlet and the termination in the communication closet.

UTP is a very flexible, low cost media, and can be used for either voice or data communications.

Its greatest disadvantage is the limited bandwidth, which restricts long distance transmission

with low error rates.

(ii) Shielded Twisted Pair Cable (STP)

STP is heavier and more difficult to manufacture, but it can greatly improve the signaling rate in

a given transmission scheme Twisting provides cancellation of magnetically induced fields and

currents on a pair of conductors. Magnetic fields arise around other heavy current-carrying

conductors and around large electric motors. Various grades of copper cables are available, with

Grade 5 being the best and most expensive. Grade 5 copper, appropriate for use in 100-Mbps

applications, has more twists per inch than lower grades. More twists per inch means more linear

feet of copper wire used to make up a cable run, and more copper means more money.

2. Coaxial Cable

Coaxial cable is a two-conductor cable in which one conductor forms an electromagnetic shield

around the other. The two conductors are separated by insulation. It is a constant impedance

transmission cable. This media is used in base band and broadband transmission. Coaxial cables

do not produce external electric and magnetic fields and are not affected by them. This makes

them ideally suited, although more expensive, for transmitting signals.

3. Fiber Optic Cable

The Fiber Optic Cable contains one or many fibers, each of them wrapped with a plastic tube

and an external coating. Its construction includes

CORE - made of glass.

CLADDING - made of glass.

Plastic coating

Fiber optics is being used more often as user applications demand higher and higher bandwidths.

The term “bandwidth” technically means the difference between the highest and lowest

frequencies of a transmission channel, in hertz (Hz). More commonly, it means the capacity or

amount of data that can be sent through a given circuit.

A bandwidth of 100 Mbps is standard using fiber optic cables. When first introduced, fiber was

considered only for special applications because it was expensive and difficult to work with. In

recent years, the quest for greater bandwidth combined with easier-to-use fiber has made it more

common. Tools and training for installing and troubleshooting fiber are readily available. There

are three basic fiber optic cables available: multimode step index, multimode graded index, and

single mode. Multimode fibers usually are driven by LEDs at each end of the cable, while single

mode lasers usually drive fibers. Single mode fibers can achieve much higher bandwidths than

multimode fibers, but are thinner (10 microns) and physically weaker than multimode.

Equipment costs for transmitting and receiving single mode fiber signals are much higher (at

least four times) than for multimode signals. One distinct advantage of fiber optic cables is noise

immunity. Fiber optic cables can be routed indiscriminately through high should be rated for

heating/ ventilation/air conditioning HVAC) plenums where they can withstand fires per

National Fire Protection Association (NFPA) requirements

Transmission Impairment

Signals travel through transmission media, which are not perfect. The imperfection causes signal

impairment. This means that the signal at the beginning of the medium is not the same as the

signal at the end of the medium. What is sent is not what is received. Three causes of impairment

are attenuation, distortion, and noise.

1. Attenuation

The strength of a signal decrease with the increase in distance travelled over a medium.

Attenuation means loss of energy. When any signal travels over a medium or channel, it loses

some of its energy in the form of heat in the resistance of the medium. Attenuation decides the

signal to noise ratio hence the quality of received signal.

2. Distortion

Another meaning of distortion is change in shape of the signal. This type of distortion is

observed for the composite signals made by different frequencies. If the medium is not perfect,

then all the frequency components present at the input will not only be equally attenuated and

will not be proportionally delayed.

3. Noise

When the data travels over a transmission medium, noise gets added to it. Noise is a major

limiting factor in communication system performance.

Noise can be categorized into four types as follows:

Thermal noise

Intermodulation noise

Crosstalk

Impulse noise

MULTIPLEXING

Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals

across a single data link.

Frequency-Division Multiplexing

Frequency-division multiplexing (FDM) is an analog technique that can be applied when the

bandwidth of a link (in hertz) is greater than the combined bandwidths of the signals to be

transmitted. In FOM, signals generated by each sending device modulate different carrier

frequencies. These modulated signals are then combined into a single composite signal that can be

transported by the link. Carrier frequencies are separated by sufficient bandwidth to accommodate

the modulated signal. These bandwidth ranges are the channels through which the various signals

travel. Channels can be separated by strips of unused bandwidth-guard bands-to prevent signals

from overlapping. In addition, carrier frequencies must not interfere with the original data

frequencies.

Wavelength-Division Multiplexing

Wavelength-division multiplexing (WDM) is designed to use the high-data-rate capability of

fiber-optic cable. The optical fiber data rate is higher than the data rate of metallic transmission

cable. Using a fiber-optic cable for one single line wastes the available bandwidth. Multiplexing

allows us to combine several lines into one.

WDM is conceptually the same as FDM, except that the multiplexing and demultiplexing involve

optical signals transmitted through fiber-optic channels. The idea is the same: We are combining

different signals of different frequencies. The difference is that the frequencies are very high.

Although WDM technology is very complex, the basic idea is very simple. We want to combine

multiple light sources into one single light at the multiplexer and do the reverse at the

demultiplexer. The combining and splitting of light sources are easily handled by a prism. Recall

from basic physics that a prism bends a beam of light based on the angle of incidence and the

frequency. Using this technique, a multiplexer can be made to combine several input beams of

light, each containing a narrow band of frequencies, into one output beam of a wider band of

frequencies. A demultiplexer can also be made to reverse the process.

Time Division Multiplexing

Time division multiplexing is a technique used to transmit a signal over a single communication

channel by dividing the time frame into slots – one slot for each message signal. Time-division

multiplexing is primarily applied to digital signals as well as analog signals, wherein several low

speed channels are multiplexed into high-speed channels for transmission. Based on the time,

each low-speed channel is allocated to a specific position, where it works in synchronized mode.

At both the ends, i.e., the multiplexer and demultiplexer are timely synchronized and

simultaneously switched to the next channel.

Time division multiplexing is classifieds into four types:

Synchronous time-division multiplexing

Asynchronous time-division multiplexing

Interleaving time-division multiplexing

Statistical time-division multiplexing

Synchronous Time Division Multiplexing

Synchronous time division multiplexing can be used for both analog and digital signals. In

synchronous TDM, the connection of input is connected to a frame. If there are ‘n’ connections,

then a frame is divided into ‘n’ time slots – and, for each unit, one slot is allocated – one for each

input line. In this synchronous TDM sampling, the rate is same for all the signals, and this

sampling requires a common clock signal at both the sender and receiver end. In synchronous

TDM, the multiplexer allocates the same slot to each device at all times.

Asynchronous Time-Division Multiplexing

In asynchronous time-division multiplexing, the sampling rate is different for different signals,

and it doesn’t require a common clock. If the devices have nothing to transmit, then their time

slot is allocated to another device. Designing of a commutator or de-commutator is difficult and

the bandwidth is less for time-division multiplexing. This type of time-division multiplexing is

used in asynchronous transfer mode networks.

Interleaving

Time-division multiplexing can be visualized as two fast rotating switches on the multiplexing

and demultiplexing side. At the same speed these switches rotate and synchronize, but in

opposite directions. When the switch opens at the multiplexer side in front of a connection, it has

the opportunity to send a unit into the path. In the same way, when the switch opens on the

demultiplexer side in front of a connection that has the opportunity to receive a unit from the

path. This process is called interleaving.

Statistical Time-Division Multiplexing

Statistical time-division multiplexing is used to transmit several types of data concurrently across

a single transmission cable. This is often used for managing data being transmitted via LAN or

WAN. The data is simultaneously transmitted from the input devices that are connected to the

network including printers, fax machines, and computers. This type of multiplexing is also used

in telephone switch board settings to manage the calls. Statistical TDM is similar to dynamic

bandwidth allocation, an in this type of time-division multiplexing, a communication channel is

divided into an arbitrary number of data streams.

Error Detection and Correction

Error is a condition when the output information does not match with the input information.

During transmission, digital signals suffer from noise that can introduce errors in the binary bits

travelling from one system to other. That means a 0 bit may change to 1 or a 1 bit may change to

0.

Main types of errors are

Single-Bit Errors

Burst Errors

1. Single-Bit errors

As name suggest single-bit errors occur when a single bit gets changed during transmission of

data due to interference in network communication. The Term Single-Bit error means that only 1

bit of a given data unit (such as a byte, character, or packet) is changed from 1 to 0 or from 0 to

1.

Single-bit errors are least likely type of error because their duration or noise is normally longer

than duration of 1 bit.

2. Burst Errors

When more than a single bit of data unit gets corrupted it is known as Burst error. In comparison

of single-bit errors, burst errors are more likely to occur. Because as we know that the duration

of noise is generally longer than the duration of transferring 1bit, that means with longer duration

noise can corrupt more than 1 bit easily. Number of bit affected depends on the data rate and

duration of noise.

Detection

The central concept in detecting or correcting errors is redundancy. To be able to detect or

correct errors, we need to send some extra bits with our data. These redundant bits are added by

the sender and removed by the receiver. Their presence allows the receiver to detect or correct

corrupted bits.

Redundancy

Redundancy is achieved through various coding schemes. The sender adds redundant bits

through a process that creates a relationship between the redundant bits and the actual data bits.

The receiver checks the relationships between the two sets of bits to detect or correct the errors.

Error Detecting Techniques

1. Vertical Redundancy Check (VRC)

This error detection scheme is quite popular in telecommunications and computer networking.

The scheme works as follows: the message to be transmitted is split into nibbles (4 bits) and with

each nibble a parity bit is associated before data transmission. Note that with even parity bit

scheme, the number of 1’s in the nibble including the parity bit must be even. For example, for

the message 1 0 0 1 0 0 0 1 1 1 1 1 with even parity bit scheme, the message to be transmitted is

The bit inside the ’box’ at the end of the nibble represents the even parity bit corresponding to

that nibble.

1. This scheme detects all single bit errors. Further, it detects all multiple bit errors as along

as the number of bits corrupted is odd (referred to as odd bit errors). Suppose the message

to be transmitted is

and the message received at the receiver is

2. 0 represents that this bit is in error. For the above example, when the receiver performs

the parity check, it detects that there was an error in the first nibble during transmission

as there is a mismatch between the parity bit and the data in the nibble. However, the

receiver does not know which bit in that nibble is in error. Similarly, the following error

is also detected by the receiver.

the message to be transmitted is

and the message received at the receiver is

Three bits are corrupted by the transmission medium and this is detected by the receiver

as there is a mismatch between the 2nd nibble and the 2nd parity bit. It is important to note

that the error in the first nibble is unnoticed as there is no mismatch between the data and

the parity bit.

3. The above example also suggests that not all even bit errors (multiple bit errors with the

number of bits corrupted being even) are detected by this scheme. If even bit error is such

that the even number of bits are corrupted in each nibble, then such error is unnoticed at

the receiver. However, if even bit error is such that at least one of the nibble has odd

number of bits in error, then such error is detected by VRC.

4. If nibble based VRC scheme is followed, then the overhead is 25%, i.e., if the size of the

message is m, the number of even parity bits is m/4 , which is 25% overhead. In general,

if the size of the split is k, then the number of even parity bits is m/k and the percentage

of overhead is 100/k . It is easy to see that the higher the value of k, the lesser the

percentage of overhead. However, there is a trade-off between k and the error detection

efficiency. The error detection efficiency refers to the percentage of errors being detected

by VRC when simulated on a large number of messages. In general, the lower the value

of k, the higher the error detection efficiency.

2. Longitudinal Redundancy Check (LRC)

In contrast to VRC, LRC assigns a parity byte (nibble) along with the message to be

transmitted. Suppose the message to be transmitted is

then, we compute the even parity nibble as follows:

Note that in this scheme, the number of 1’s in each column including the bit in the parity

nibble must be even.

1. Similar to VRC, LRC detects all single bit and odd bit errors. Some even bit errors are

detected and the rest is unnoticed by the receiver.

2. The following error is detected by LRC but not by VRC. For the message 1 0 1 1 1 0 0 0

1 0 0 1 , suppose the received message is

3. In the above example, there is mismatch between the number of 1’s and the parity bit in

Columns 2 and 3.

4. The following error is detected by VRC but not by LRC. For the message 1 0 1 1 1 0 0 0

1 0 0 1 , suppose the received message is

The above error is unnoticed by the receiver if we follow VRC scheme whereas it is

detected by LRC as illustrated below

5. Here again, not all even bit errors are detected by this scheme. If the error is such that

each column has even number of bits in error, then such error is undetected. However, if

the distribution is such that at least one column contains an odd number of bits in error,

then such errors are always detected at the receiver.

3. Cyclic Redundancy Check (CRC)

CRC performs mod 2 arithmetic (exclusive-OR) on the message using a divisor

polynomial. Firstly, the message to be transmitted is appended with CRC bits and the

number of such bits is the degree of the divisor polynomial. The divisor polynomial 1 1 0

1 corresponds to the polynomial x3 + x2 + 1. For example, for the message 1 0 0 1 0 0

with the divisor polynomial 1 1 0 1, the message after appending CRC bits is 1 0 0 1 0 0

0 0 0. We compute CRC on the modified message M as follows

1. To perform xor arithmetic, the leading bit of M must be ’1’. If leading bit is not ’1’,

then choose the first bit of M for which the bit value is ’1’. Assuming the leading bit

is ’1’, we perform xor arithmetic with the first d + 1 bits of M. This results in a value

with the leading bit ’0’. If suppose the next bit of the leading bit is ’1’, then we bring

the next bit ((d + 2)nd bit) from M so that we will have d + 1 bits with a leading bit

’1’ for the next xor operation. Otherwise,

we identify the first bit in the result which is ’1’ and bring appropriate number of bits

from M so that we will have d + 1 bits with a leading bit ’1’ for the next xor operation. At

each step of computation, we ensure the leading bit is ’1’ and perform xor arithmetic on d

+ 1 bits of M with the divisor polynomial

2. We see that the remainder is 0 0 1 which is the CRC bits. Further, CRC bits is

appended with the message to be transmitted (equivalent to performing xor between

CRC and the modified message) before data transmission. Let M0 be the message to

be transmitted by the sender.

3. At the receiver, on receiving the message M0, the receiver performs CRC check

similar to the sender. That is, M0 xor divisor polynomial is performed to check

whether there is an error in data transmission. If the remainder of the CRC check is

zero, then the receiver declares that there is no error, otherwise, it declares that there

is an error in transmission

4. Checksum

This scheme is a peculiar one wherein we perform 1’s complement arithmetic on the data to be

transmitted. For the message 1 0 1 1 1 0 0 0 1 0 0 1 , we compute checksum in two steps. The

first step performs 1’s complement addition on the data and the second step performs the

complement on the result of Step 1. The result of Step 2 is the checksum which would be

augmented along with the data to be transmitted.

1. Perform 1’s complement addition

2. Compute Checksum which is the 1’s complement of the result of addition

For the above example, the checksum is 0 0 1 0. Along with the message, we append the

checksum of the message. i.e., the message to be transmitted is

At the receiver, we perform 1’s complement addition on the received data (message plus

checksum), if the result is 1 1 1 1, then the receiver declares that there is no error in the

transmission. Otherwise, it declares that there is an error in the transmission.

This error is unnoticed by LRC scheme as even number of identical bits are corrupted in

acolumn and this change has no impact on the even parity bit. However, checksum performs 1's

complement addition and hence there will be a change in the final sum if even number of

identical bits are corrupted.