5. Multiplexing and Switching – Express Learning: Data Communications and Computer Networks


Multiplexing and Switching

1. What is meant by bandwidth utilization?

Ans: Bandwidth utilization refers to the wise use of the available bandwidth in order to achieve the utmost efficiency, privacy and anti-jamming. Bandwidth can be used optimally using two techniques, namely, multiplexing and spreading. Multiplexing allows sharing the available bandwidth of a communication link among a number of communicating stations, thus achieving efficiency. On the other hand, spreading allows expanding the bandwidth of link for achieving privacy and anti-jamming.

2. What is multiplexing? In what situations, it can be used?

Ans: Multiplexing is a technique used for transmitting several signals simultaneously over a single communication link. An analogy of multiplexing can be made with a multilane highway. Just as a multilane highway can carry increased volumes of traffic in multiple lanes at higher speeds and at relatively low incremental cost per lane, the higher-capacity circuit can carry multiple conversations in multiple channels at relatively low incremental cost per channel. Multiplexing is done to utilize the available bandwidth properly and to improve the efficiency during a transmission. In a multiplexed system (Figure 5.1), several devices share a communication link called common medium. Each part of the communication link being used for carrying transmission between an individual pair of input and an output line is referred to as a channel.

At the sender's end, the N input lines are combined into a single stream by a communication device, called multiplexer (MUX). At the receiver's end, another communication device, called demultiplexer (DEMUX), completes the communication process by separating multiplexed signals from a communication link and distributing them to corresponding N output lines.

Figure 5.1 A Multiplexed System

Multiplexing can be used in situations where the signals to be transmitted through the transmission medium have lower bandwidth than that of the medium. This is because in such situations, it is possible to combine the several low-bandwidth signals and transmitting them simultaneously through the transmission medium of larger bandwidth.

3. Why we need multiplexing in a communication channel?

Ans: Multiplexing is needed in a communication channel because of the following reasons:

To send several signals simultaneously over a single communication channel.
To reduce the cost of transmission.
To effectively utilize the available bandwidth of the communication channel.

4. Explain the different multiplexing techniques.

Ans: Multiplexing can be done using three techniques: frequency-division multiplexing (FDM), wavelength-division multiplexing (WDM) and time-division multiplexing (TDM).

Frequency-Division Multiplexing

FDM is used when the bandwidth of the transmission medium is greater than the total bandwidth requirement of the signals to be transmitted. It is often used for baseband analog signals. At the sender's end, the signals generated by each sending device are of similar frequency range. Within the multiplexer, these similar signals modulate carrier waves of different frequencies and then the modulated signals are merged into a single composite signal, which is sent over the transmission medium. The carrier frequencies are kept well separated by assigning a different range of bandwidth (channel) that is sufficient to hold the modulated signal. There may also be some unused bandwidth between successive channels called guard bands, in order to avoid interference of signals between those channels.

At the receiver's end, the multiplexed signal is applied to a series of bandpass filters which breaks it into component signals. These component signals are then passed to a demodulator that separates the signals from their carriers and distributes them to different output lines (Figure 5.2).

Figure 5.2 Frequency-division Multiplexing

Though FDM is an analog multiplexing technique, it can also be used to multiplex the digital signals. However, before multiplexing, the digital signals must be converted to analog signals. Some common applications of FDM include radio broadcasting and TV networks.

Wavelength-Division Multiplexing

WDM is an analog multiplexing technique designed to utilize the high data rate capability of fibre-optic cables. A fibre-optic cable has a much higher data rate than coaxial and twisted-pair cables and using it as a single link wastes a lot of precious bandwidth. Using WDM, we can merge many signals into a single one and hence, utilize the available bandwidth efficiently. Conceptually, FDM and WDM are same; that is, both combine several signals of different frequencies into one, but the major difference is that the latter involves fibre-optic cables and optical signals as well as the signals are of very high frequency.

In WDM, the multiplexing and demultiplexing are done with the help of a prism (Figure 5.3), which bends the light beams by different amounts depending on their angle of incidence and wavelength. The prism used at the sender's end combines the multiple light beams with narrow frequency bands from different sources to form a single wider band of light which is then passed through fibre-optic cable. At the receiver's end, the prism splits the composite signal into individual signals. An application of WDM is the SONET network.

Figure 5.3 Wavelength-division Multiplexing

Time-Division Multiplexing

TDM is a digital multiplexing technique that allows the high bandwidth of a link to be shared amongst several signals. Unlike FDM and WDM, in which signals operate at the same time but with different frequencies, in TDM, signals operate at the same frequency but at different times. In other words, the link is time-shared instead of sharing parts of bandwidth among several signals.

Figure 5.4 gives a conceptual view of TDM. At the sender's end, the time-division multiplexer allocates each input signal a period of time or time slot. Each sending device is assigned the transmission path for a predefined time slot. Three sending signals, Signals 1, 2 and 3, occupy the transmission sequentially. As shown in the figure, time slots A, B, P, Q, X and Y follow one after the other to carry signals from the three sources, which upon reaching the demultiplexer, are sent to the intended receiver.

Though TDM is a digital multiplexing technique, it can also multiplex analog signals. However, before multiplexing, analog signals must be converted to DSs.

5. Explain different schemes of TDM.

Ans: TDM can be divided into two different schemes, namely, synchronous TDM and statistical TDM.

Figure 5.4 Time-division Multiplexing

Synchronous TDM

In this technique, data flow of each input source is divided into units where a unit can be a bit, byte or a combination of bytes. Each input unit is allotted one input time slot. A cycle of input units from each input source is grouped into a frame. Each frame is divided into time slots and one slot is dedicated for a unit from each input source. That is, for n connections, we have n time slots in a frame and every slot has its own respective sender. The duration of input unit and the duration of each frame are same unless some other information is carried by the frame. However, the duration of each slot is n times shorter where n is the number of input lines. For example, if duration of input unit is t seconds, then the duration of each slot will be t/n seconds for n input lines. The data transmission rate for the output link must be n times the data transmission rate of an input line to ensure the transmission of data. In addition, one or more synchronization bits are to be included in the beginning of each frame to ensure the data synchronization between the multiplexer and the demultiplexer.

Figure 5.5 shows a conceptual view of synchronous TDM in which data from the three input sources has been divided into units (P1, P2, P3), (Q1, Q2) and (R1, R2, R3). As the total number of input lines is three, each frame also has three time slots. Now, if the duration of an input unit is t seconds, then after every t seconds, a unit is collected from each source and kept into the corresponding time slot in the frame. For example, Frame 1 contains (P1, Q1, R1). In our example, the data rate for the transmission link must be three times the connection rate to ensure the flow of data. In addition, the duration of each frame is three time slots whereas the duration of each time slot is t/3.

Figure 5.5 Synchronous TDM

A major drawback of synchronous TDM is that it is not highly efficient. Since each slot in the frame is pre-assigned to a fixed source, the empty slots are transmitted in case one or more sources do not have any data to send. As a result, the capacity of a channel is wasted. For example, in Figure 5.5, an empty slot is transmitted in Frame 3 because the corresponding input line has no data to send during that time period.

Statistical TDM

Statistical TDM, also called asynchronous TDM or intelligent TDM, overcomes the disadvantage of synchronous TDM by assigning the slots in a frame dynamically to input sources. A slot is assigned to an input source only when the source has some data to send. Thus, no empty slots are transmitted which in turn result in an efficient utilization of bandwidth. In statistical TDM, generally each frame has less number of slots than the number of input lines and the capacity of link is also less than the combined capacity of channels.

Figure 5.6 shows a conceptual view of statistical TDM. At the sender's end, the multiplexer scans the input sources one by one in a circular manner and assigns a slot if the source has some data to send; otherwise, it moves on to the next input source. Hence, slots are filled up as long as an input source has some data to send. At the receiver's end, the demultiplexer receives the frame and distributes data in slots of frame to appropriate output lines.

Figure 5.6 Statistical TDM

Unlike synchronous TDM, there is no fixed relationship between the inputs and the outputs in statistical TDM because here slots in a frame are not pre-assigned or reserved. Thus, to ensure the delivery of data to appropriate output lines, each slot in the frame stores the destination address along with data to indicate where the data has to be delivered. For n output lines, an m-bit address is required to define each output line where m = log2n. Though the inclusion of address information in a slot ensures the proper delivery, it incurs more overhead per slot. Another difference between synchronous and statistical TDMs is that the latter technique does not require the synchronization among the frames thereby eliminating the need of including synchronization bits within each frame.

6. Distinguish multilevel, multiple-slot and pulse-stuffed TDMs.

Ans: Multilevel multiplexing, multiple-slot allocation and pulse stuffing are the strategies used in TDM to handle the different input data rates of the input lines.

Multilevel Multiplexing

Multiple multiplexing is used in situations where the data rate of an input line is an integral multiple of other input lines. As the name suggests, several levels of multiplexing are used in this technique. To understand, consider Figure 5.7 where we have three input lines; the first two input lines have a data rate of 80 kbps each and the last input line has a data rate of 160 kbps. As the data rate of third input line (160 kbps) is a multiple of that of other two (80 kbps), the first two input lines can be multiplexed to produce a data rate of 160 kbps that is equal to that of the third input line. Then, another level of multiplexing can be used to combine the output of first two input lines and the third input line thereby generating an output line with a data rate of 320 kbps.

Figure 5.7 Multilevel Multiplexing

Multiple-Slot Allocation

Like multilevel multiplexing, multiple-slot allocation technique is also used in situations where the data rate of an input line is an integral multiple of other input lines. Generally, one slot per each input source is reserved in the frame being transmitted. However, sometimes, it is more useful to allocate multiple slots corresponding to a single input line in the frame. For example, again consider Figure 5.7 where we can divide one 160-kbps input line into two (each of 80 kbps) with the help of serial-to-parallel converter and then multiplex the four input lines of 80 kbps each to create an output line of 320 kbps. However, now there will be total four slots in the transmitting frame with two slots corresponding to input line with originally 160 kbps data rate (Figure 5.8).

Figure 5.8 Multiple-slot Allocation

Pulse Stuffing

Pulse stuffing, also known as bit padding or bit stuffing, technique is used in situations where the data rates of input lines are not the integral multiples of each other. In this technique, the highest input data rate is determined and then dummy bits are added to input lines with lower data rates to make the data rate of all the lines equal. For example, consider Figure 5.9 where the first input line has the highest data rate equal to 80 kbps and other two input lines have data rate of 60 and 70 kbps, respectively. Hence, the data rates of second and third input lines are pulse-stuffed to increase the rate to 80 kbps.

Figure 5.9 Pulse Stuffing

7. What is the purpose of framing bit? Is it used in FDM or TDM?

Ans: Framing bits are used in TDM to ensure the synchronization between multiplexer and demultiplexer. Without synchronization, there may be some error in delivery of bits; that is, a bit may be delivered to the wrong output line. Thus, one or more synchronization bits, called framing bits, are generally added at the beginning of each transmitting frame. The framing bits follow a specific pattern in each frame, which allows the demultiplexer to get in synchronization with the input data and divide the time slots with accuracy.

8. What is the frame format in statistical TDM?

Ans: In statistical TDM, there are two possible frame structures that a multiplexer can use. These structures are described as follows:

One Data Source Per Frame: This frame structure permits to include data from one source per each frame. The frame consists of two parts: address, which is used to identify the source, and data, which is of variable length [Figure 5.10(a)]. The end of data field is indicated by the end of overall frame. This type of frame structure is suitable in environments where the load is less. However, as the load increases, this frame structure becomes inefficient.
Multiple Data Sources Per Frame: This frame structure permits to include data from multiple sources in a single frame. For each source, the frame contains address to identify the source, length of data field and data [see Figure 5.10(b)]. This frame structure helps to improve efficiency.

Figure 5.10 Statistical TDM Frame Structures

9. Describe the DS hierarchy.

Ans: The DS hierarchy, also called digital signal service, is a hierarchy of digital signals which the telephone companies use for implementing TDM. This hierarchy consists of several levels with each level supporting a different data rate. The digital signals at lower levels in hierarchy are multiplexed into signals at higher levels in hierarchy (Figure 5.11). Various levels of DS hierarchy are described as follows:

Figure 5.11 DS Hierarchy

DS-0: This service is at the lowest level of hierarchy. It is a single digital channel of just 64 kbps.
DS-1: This service has a capacity of 1.544 Mbps which is 24 times of a DS-0 channel with an additional overhead of 8 kbps. It can be utilized for multiplexing 24 DS-0 channels, as a single service for transmissions of 1.544 Mbps or can even be used in different combinations to utilize capacity up to 1.544 Mbps.
DS-2: This service has a capacity of 6.312 Mbps which is 96 times of DS-0 with an additional overhead of 168 kbps. It can be utilized for multiplexing 96 DS-0 channels, four DS-1 channels or can also be utilized as a single service for transmission up to 6.312 Mbps.
DS-3: This service has a capacity of 44.376 Mbps which is seven times the data rate of DS-2 channels with an additional overhead of 1.368 Mbps. It can be utilized for multiplexing 672 DS-0 channels, 28 DS-1 channels or seven DS-2 channels. In addition, DS-3 can be utilized as a single transmission line or as a combination of its previous hierarchies.
DS-4: This service is at the highest level of the DS hierarchy and has a capacity of 274.176 Mbps. It can be utilized for multiplexing six DS-3 channels, 42 DS-2 channels, 168 DS-1 channels or 4,032 DS-0 channels. It can also be used as a combination of service types at lower levels of hierarchy.

10. Explain the analog hierarchy used by telephone networks with an example.

Ans: Analog hierarchy is used by telephone companies for maximizing the efficiency of their infrastructure such as switches and other transmission equipments. In analog hierarchy, the analog signals from the lower bandwidth channels are multiplexed onto the higher bandwidth lines at different levels. For multiplexing of analog signals at all levels of hierarchy, the FDM technique is used. The standards for multiplexing vary in countries but the basic principle remains the same. One of such analog hierarchies is used by AT&T in the United States. This analog hierarchy comprises voice channels, groups, super groups, master groups and jumbo groups (Figure 5.12).

Figure 5.12 Analog Hierarchy

At the lowest level of hierarchy, 12 voice channels, each having a bandwidth of 4 kHz, are multiplexed onto a higher bandwidth line thereby forming a group of 48 kHz (12 × 4 kHz). At the next level of hierarchy, five groups (that is, 60 voice channels) are multiplexed to form a super group that has a bandwidth of 240 kHz (5 × 48 kHz). Further, 10 super groups (that is, 600 voice channels) are multiplexed to form a master group. A master group must have a bandwidth of 2.4 MHz (10 × 240 kHz). However, to avoid interference between the multiplexed signals, the requirement of guard bands increases the total bandwidth of a master group to 2.52 MHz. Finally, six master groups are multiplexed to form a jumbo group that must have a bandwidth of 15.12 MHz (6 × 2.52 MHz). However, the requirement of guard bands to separate the master groups increases the total bandwidth of a jumbo group to 16.984 MHz.

11. What is inverse multiplexing?

Ans: Inverse multiplexing is the opposite of the multiplexing technique. In this technique, the data from a single high-speed line or source is split into chunks of data that can be transmitted over low-speed lines at the same time and without any data loss in the combined data rate.

12. What do you understand by spread spectrum?

Ans: Spread spectrum is a technique used to expand the available bandwidth of a communication link in order to achieve goals such as privacy and anti-jamming. For achieving these goals, the spread spectrum techniques expand the original bandwidth required for every system (adding redundancy) such that the signals from different sources can together fit into larger bandwidth. For example, if the bandwidth requirement of each system is B, then the spread spectrum techniques increase it to B' such that B' >> B. To spread the bandwidth, a spreading process is used which is independent of the original signal. The spreading process uses a spreading code—a series of numbers following the specific pattern—and results in expanded bandwidth.

The spread spectrum technique is generally used in wireless LANs and WANs where the systems sharing the air transmission medium must be able to communicate without any interception from intruders, blocking of message during transmission and so on.

13. List the principles to be followed by the spread spectrum technique?

Ans: The spread spectrum technique uses the following principles to achieve its goals.

The spreading process must be independent of the original signal. This implies that spreading process is used after the source has generated the signal.
Each source station is assigned a bandwidth much greater than what it actually needs thereby allowing redundancy.

14. Compare frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS) technique?

Ans: FHSS and DSSS are the techniques to expand the bandwidth of the digital signal. However, both techniques follow a different process to achieve their goal.

Frequency Hopping Spread Spectrum

In this technique, a number of different carrier frequencies (say, N) are used. The input signal modulates one of N carrier frequencies in each hopping period (Thop). As shown in Figure 5.13, FHSS uses a pseudorandom code generator, also known as pseudorandom noise (PN), which generates an m-bit pattern (where m = log2N) for every hopping period Thop. For example, if an application uses 16 hopping frequencies, then the pseudorandom code generator creates 16 different four-bit patterns one per each hopping period. During each hopping period, the m-bit pattern generated by pseudorandom code generator is used by the frequency table to determine the hopping frequency to be used for that hopping period. The frequency selected from the frequency table is passed to the frequency synthesizer which generates a carrier signal of the selected frequency. The input signal modulates the carrier signal generated by the frequency synthesizer and as a result, spread signal with bandwidth (BFHSS) much greater than the original bandwidth (B) is produced.

Figure 5.13 Frequency Hopping Spread Spectrum

Since one out of N hopping frequencies is used by a source station in each hopping period, the remaining N-1 frequencies can be used by other N-1 stations. Thus, N hopping frequencies (N channels) can be multiplexed into one using the same bandwidth BFHSS. That is, N source stations can share the same bandwidth BFHSS. In addition, due to randomization in the frequency and the frequent frequency hops, the intruders may be able to intercept the signal or send noise to jam the signal possibly for one hopping period but not for the entire period thus, achieving privacy and anti-jamming.

Direct Sequence Spread Spectrum

In this technique, each bit of the original signal is replaced by n bits using a spreading code. As shown in Figure 5.14, DSSS uses a chip generator that generates a spreading code of n bits (called chips) for each signal bit. To expand the bandwidth of the original signal, each bit of the original signal is multiplied by the code generated by the chip generator. After multiplication, the resultant spread signal with a bandwidth n times of that of the original signal is produced. DSSS provides privacy between the sender and the receiver if the intruder does not know the spreading code. It also prevents interference if a different spreading code is used for each source station.

Figure 5.14 DSSS Process

Figure 5.15 illustrates the DSSS process. The spreading code used in this example is eight chips with the pattern 10110100. Each bit of the original signal is multiplied by the eight-bit spreading code to obtain the new spread signal. Now, if the original signal rate is S, then the new spread signal rate will be 8S. This implies that the required bandwidth for the spread signal would be eight times of the original bandwidth.

Figure 5.15 DSSS Example

15. What is meant by the term switching?

Ans: On a network, switching means routing traffic by setting up temporary connections between two or more network points. This is done by devices located at different locations on the network, called switches (or exchanges). Switches create temporary connections amongst two or more devices connected to them. In a switched network, some switches are directly connected to the communicating devices, while others are used for routing or forwarding information.

Figure 5.16 Switched Network

Figure 5.16 depicts a switched network in which communicating devices are labelled A, B, C and so on and switches are labelled I, II, III, IV and so on. Each switch is connected either to a communicating device or to any other switch for forwarding information. Notice that multiple switches are used to complete the connection between any two communicating devices at a time, hence saving the extra links required in case of a point-to-point connection.

16. Explain different types of switching techniques along with their advantages and disadvantages.

Ans: There are three different types of switching techniques; namely, circuit switching, message switching and packet switching.

Circuit Switching

When a device wants to communicate with another device, circuit switching technique creates a fixed-bandwidth channel, called a circuit, between the source and the destination. This circuit is a physical path that is reserved exclusively for a particular information flow, and no other flow can use it. Other circuits are isolated from each other, and thus their environment is well controlled. For example, in Figure 5.17, if device A wants to communicate with device D, sets of resources (switches I, II and III) are allocated which act as a circuit for the communication to take place. The path taken by data between its source and destination is determined by the circuit on which it is flowing, and does not change during the lifetime of the connection. The circuit is terminated when the connection is closed. Therefore, this method is called circuit switching.

Figure 5.17 Circuit Switching

Some advantages of circuit switching are as follows:

It is a simple method and does not require the use of any special facilities.
After a circuit is determined and established, data is transmitted with just a minimal propagation delay and without congestion.
It is the preferred choice for the transmission of real-time data and voice.

Some disadvantages of circuit switching are as follows:

The time required to establish a dedicated circuit between two stations is 10 s approximately and this time duration increases more depending upon the distance between the stations. For many applications, this is unsuitable and undesirable.
The resources are not efficiently utilized during circuit switching in a computer network.
For communication amongst stations that use costly and high-speed transmission lines, circuit switching is not cost effective and economical, as communication between stations occurs generally in fast and small time gaps.

Packet Switching

Packet switching introduces the idea of breaking data into packets, which are discrete units of potentially variable length blocks of data. Apart from data, these packets also contain a header with control information such as the destination address and the priority of the message. These packets are passed by the source point to their local packet switching exchange (PSE). When the PSE receives a packet, it inspects the destination address contained in the packet. Each PSE contains a navigation directory specifying the outgoing links to be used for each network address. On receipt of each packet, the PSE examines the packet header information and then either removes the header or forwards the packet to another system. If the communication channel is not free, then the packet is placed in a queue until the channel becomes free. As each packet is received at each transitional PSE along the route, it is forwarded on the appropriate link mixed with other packets. At the destination PSE, the packet is finally passed to its destination. Note that not all packets of the same message, travelling between the same two points, will necessarily follow the same route. Therefore, after reaching their destination, each packet is put into order by a packet assembler and disassembler (PAD).

For example, in Figure 5.18, four packets (1, 2, 3 and 4) once divided on machine A are transmitted via various routes, which arrive on the destination machine D in an unordered manner. The destination machine then assembles the arrived packets in order and retrieves the information.

Figure 5.18 Packet Switching

Some advantages of packet switching are as follows:

It is a fault-tolerant technique.
It provides a much fairer and cost-efficient sharing of the resources. In addition, if no data is available to the sender at some point during a communication, then no packet is transmitted over the network and no resources are wasted.
It requires lesser storage capacity for stations.
The transmission rate in packet switching is fast.

Some disadvantages of packet switching are as follows:

As packets do not follow a specific path, they may arrive out of order at the destination.
The cost incurred per packet is large.

Message Switching

A message is a unit of information which can be of varying length. Message switching is one of the earliest types of switching techniques, which was common in the 1960s and 1970s. This switching technique employs two mechanisms; they are store-and-forward mechanism and broadcast mechanism. In store-and-forward mechanism (Figure 5.19), a special device (usually, a computer system with large storage capacity) in the network receives the message from a communicating device and stores it into its memory. Then, it finds a free route and sends the stored information to the intended receiver. In such kind of switching, a message is always delivered to one intermediate device where it is stored and then rerouted to its final destination.

Figure 5.19 Store-and-Forward Mechanism of Message Switching

In broadcast switching mechanism, the message is broadcasted over a broadcast channel as shown in Figure 5.20. As the messages pass by the broadcast channel, every station connected to the channel checks the destination address of each message. A station accepts only the message that is addressed to it.

Some advantages of message switching are as follows:

In message switching technique, no physical path is established in advance.
The transmission channels are used very effectively in message switching, as they are allotted only when they are required.

Some disadvantages of message switching are as follows:

It is a slow process.
It involves propagation and queuing delay; in addition, a large capacity of data storage is required.
With unlimited message length, the switching nodes needed to have a larger storage space to buffer the message.

Figure 5.20 Broadcast Mechanism of Message Switching

17. Compare circuit switching, packet switching and message switching techniques.

Ans: The comparison of circuit switching, packet switching and message switching is summarized in Table 5.1.

Table 5.1 Comparison of Circuit Switching, Packet Switching and Message Switching

Circuit switching   Packet switching   Message switching
  • A physical circuit is established before transmission begins.
  • No physical circuit is established before transmission.
  • No physical circuit is established before transmission.
  • Message to be transmitted is in the form of packets.
  • Message to be transmitted is in the form of packets.
  • Message to be transmitted is in the form of blocks.
  • Store-and-forward technique is not used.
  • Store-and-forward technique is used.
  • Store-and-forward technique is used.
  • It can be used with real-time applications.
  • It can be used with real-time applications.
  • It cannot be used with real-time applications.
  • It is used in telephone network for bi-directional, fast and real-time data transfer.
  • It is used for the Internet.
  • It was used in the transmission of voice signals and messages.


18. Describe the major components of a telephone network?

Ans: The traditional telephone network, referred to as plain old telephone system (POTS), was purely an analog system used to transmit voice. However, with advancement in computing technologies, they were used to transmit other data along with voice. Three major components of a telephone network are local loops, trunks and switching offices (Figure 5.21). These components are described as follows:

Local Loops: A twisted-pair cable connection between each subscriber's telephone and the nearest end office of telephone company (also called local central office) is referred to as a local loop. Generally, the distance between the subscribers' telephone and the nearest end office is 1–10 km. Also referred to as the last mile, the local loops follow analog technology and can extend up to several miles. When a local loop utilized for voice, its bandwidth is 4 kHz. Earlier, uninsulated copper wires were commonly used for local loops. However, these days, category 3 twisted pairs are used for local loops.
Trunks: The transmission media used for communication between switching offices is referred to as trunk. Generally, fibre-optic cable or satellite transmission is used for trunks. A single trunk can carry multiple conversations over a single path by using a multiplexing technique.
Switching Offices: In telephone network, no two subscribers are connected by a permanent physical link. Instead, switches are used to provide a connection between the different subscribers. These switches are located in the switching offices of the telephone company and connect many local loops or trunks. In telephone network, there are several levels of switching offices including end offices, toll offices, tandem offices and regional offices.

Figure 5.21 Components of a Telephone Network

When a subscriber calls to another subscriber connected to same end office, a direct electrical connection is established between the local loops with the help of switching mechanism within the end office. However, if the called subscriber's telephone is connected to a different end office, the connection is established by the toll offices or tandem offices. Several end offices are connected to their nearby switching centre called toll office or tandem office (if end offices are in the same local area) through toll connecting trunks. If the caller and called subscriber do not have a common toll office, the connection between them is established by regional offices to which toll offices are connected via high bandwidth intertoll trunks.

19. Explain dial-up modems along with their standards in detail.

Ans: The word modem is an acronym for modulator-demodulator. The modulator converts digital data into analog signals (that is, modulation) and the demodulator converts the analog signal back into digital data (that is, demodulation). A dial-up modem uses the traditional telephone line as the transmission media and enables to send digital data over analog telephone lines.

Figure 5.22 depicts the modulation/demodulation process during communication between machines A and B. At the sender' side (that is, machine A), the digital signals from machine A are sent to the modem, which converts them into analog signals. These analog signals are transmitted over the telephone lines and received by the modem at the receiver's end (that is, machine B). The modem at machine B converts analog signals back into digital signals and sends them to computer B.

ITU-T gave V-series standards for effective working of modems. Some modems of this series are as follows:

V.22: This modem transmits data at 1,200 bps and works over two-wire leased line. It provides full-duplex synchronous transmission.
V.32: This modem transmits data at 9,600 bps and designed for full-duplex synchronous transmission. This modem works on two-wire leased line or telephone network. This standard uses the technique called trellis coding that detects error which gets introduced during transmission. In this coding, data stream is divided into four-bit sections and one extra bit is added to each quadbit (four-bit pattern) during data transmission for error detection.
V.32bis: This modem transmits data at 14,400 bps and is the enhancement of V.32 standard. V.32bis includes feature of fall-back and fall-forward which help the modem to adjust upstream and downstream speed automatically. This adjustment of speed depends on the quality of the signal.
V.34: This modem transmits data up to 33.6 kbps and works on a two-wire leased line. It is designed for full-duplex synchronous and asynchronous transmissions and also supports error correcting feature.
V.90: This modem is designed for full-duplex synchronous and asynchronous transmission over two-wire connection. The V.90 series modems are also called 56-K modems because their data transmission rate is 56 kbps. They are asymmetric in nature as the speed of upstream and downstream varies. They support downloading data rate up to 56 kbps and uploading up to 33.6 kbps. The main reason for the difference in the speed between uploading and downloading is that in uploading, signal gets sampled at various switching stations and gets affected by noise which is introduced here but in downloading, there is no such sampling of signals.
V.92: This modem has the capability to upload the data at the rate of 48 kbps while its downloading data rate is same as that of V.90 standard, that is, 56 kbps. This modem has the advanced feature of call-waiting service.

Figure 5.22 Modulation/Demodulation Process

20. Write a short note on DSL and its different technologies.

Ans: DSL (stands for digital subscriber line) was developed by the telephone companies to fulfil the requirement of high-speed data access and the efficient utilization of the bandwidth of the local loops. DSL is a very promising technology, as it provides the customers with a telephone connection and high-speed Internet access simultaneously over the existing local loops. To provide a simultaneous telephone connection and Internet access, DSL systems are installed between the telephone exchange and the customers' site.

The DSL technology comprises many different technologies including ADSL, VDSL, HDSL and SDSL. Often, this group of technologies is referred to as xDSL where x represents A, V, H or S. These technologies are described as follows:

ADSL: It stands for asymmetric digital subscriber line, also called asymmetric DSL. ADSL uses the existing local loops and provides a higher downstream rate from the telephone exchange to the subscriber than in the reverse direction. This asymmetry in data rates is suitable for the applications such as video-on-demand and Internet surfing, as the users of these applications need to download more data in comparison to uploading and that too at a higher speed. Along with the Internet service, ADSL also provides simultaneous telephone connection over the same copper cable. By using FDM, the voice signals are separated from data signals (upstream and downstream). ADSL technology is useful for residential customers and not for the business customers, as business customers often require larger bandwidth for both downloading and uploading.
ADSL Lite: It is a newer version of the ADSL technology. Also known as the spliterless ADSL, this technology can provide a maximum upstream data rate of 512 kbps and a maximum downstream data rate of 1.5 Mbps. This technology differs from ADSL technology in the sense that ADSL technology requires installing a splitter at the subscribers' home or business location to split voice and data, while if ADSL Lite modem is used, no such splitter is required at the premises of the subscriber and all the splitting is done at the telephone exchange. The ADSL Lite modem is directly plugged into the telephone's jack at the subscribers' premises and connected to the computer.
HDSL: It stands for high-bit-rate digital subscriber line, also called high-bit-rate DSL. It was developed by Bellcore as a substitute for the T-1 (1.544 Mbps) line. In T-1 lines, alternate mark inversion (AMI) encoding is used. This encoding is subject to attenuation at high frequencies and repeaters are to be used for covering longer distance. Thus, it was quite expensive. On the other hand, in HDSL, 2B1Q encoding is used which is less susceptible to attenuation at higher frequencies. Also, a very high data rate of 1.544 Mbps can be achieved without using repeaters to a distance of approximately 4 km. To achieve full-duplex transmission, HDSL uses two twisted pairs with one pair for each direction. HDSL2, a variant of HDSL, uses single pair of twisted cables and is under development.
SDSL: It stands for symmetric digital subscriber line, also called symmetric DSL. It was developed as a single copper pair version of HDSL. SDSL uses 2B1Q line coding and provides full-duplex transmission on a single pair of wires. It provides same data rate of 768 kbps for both upstream and downstream, as it supports symmetric communication.
VDSL: It stands for very high-bit-rate digital subscriber line, also called very high-bit-rate DSL. It is similar to ADSL and offers very high data transfer rates over small distances using coaxial, fibre-optic and twisted-pair cables. The downstream data rate for VDSL is 25–55 Mbps and the upstream data rate is generally 3.2 Mbps.

Table 5.2 summarizes all the DSL technologies along with their characteristics.

Table 5.2 Summary of the DSL Technologies

21. What are the two types of switch technologies used in circuit switching? Compare them.

Ans: In circuit switching, two types of switch technologies are used, namely, space-division switch and time-division switch.

Space-Division Switch

In space-division switching, many different connections in the circuit that are active at the same time use different switching paths which are separated spatially. Initially, this technology was developed for the analog signals; however, presently, it is being used for both analog signals and DSs. Space-division switches may be cross-bar switches or multistage switches.

Cross-bar Switch: This switch connects p input lines with q output lines in the form of a matrix of the order p×q; the value of p and q may be equal. Each input line is connected to output line with the help of a transistor at each cross-point—the intersection point of input and output lines. Each cross-point is basically an electric switch that can be opened or closed by a control unit depending on whether or not communication is required between the connecting input and output lines. Figure 5.23 shows a schematic diagram of a 4 × 4 cross-bar switch.

A cross-bar switch has certain limitations, which are as follows:

  • To connect m inputs with m outputs, m2 number of cross-points is required. In addition, the number of cross-points required increases with the increase in number of lines to be connected. This makes the cross-bar switches expensive and more complex.
  • The cross-points are not utilized properly. Even when all the devices are connected, only a few cross-points are active.
  • If a cross-point fails, the devices connected via that cross-point cannot connect to each other.
Multistage Switch: This switch overcomes the limitations of the cross-bar switch. A multistage space-division switch consists of several stages, each containing (p×q) cross-bar switches. These switches can be connected in several ways to build a large multistage network (usually, three-stage network). In general, for n inputs and n outputs, m = log2n stages are required. Figure 5.24 shows a three-stage space-division switch. Here, each cross-point in the middle stage can be accessed through several cross-points in the first or third stage.

Figure 5.23 A 4 × 4 Cross-bar Switch

Figure 5.24 A Three-stage Space-division Switch

The main advantage of multistage switches is that the cost of having an m × m multistage network is much lower than that of an m × m cross-bar switch because the former network requires a lesser number of cross-points as compared to the latter network. However, in case of heavy traffic, it suffers from blocking problem due to which it cannot process every set of requests simultaneously.

Time-division Switch

In time-division switch, TDM is implemented inside a switch. Various active connections inside the switch can utilize the same switching path in an interleaved manner. One of the commonly used methods of time-division switching is time slot interchange (TSI), which alters the sequencing of slots depending on the connection desired. TSI has a random access memory (RAM) and a control unit. The data is stored in RAM sequentially, however, retrieved selectively based on the information in control unit. The control unit stores the control information such as which input line to connect to which output.

Figure 5.25 depicts the operation of TSI. Suppose the input lines want to send data to output lines in the order 1 to 4, 2 to 3, 3 to 2 and 4 to 1. This information is stored in control unit. At the sender's end, the input unit from each of the four input lines is put serially into time slots to build a frame. The data in the time slots is stored on the RAM in the exact order in which it is received, that is, A, B, C and D. The data is retrieved from RAM and then filled up in the time slots of the output frame in the order as determined by the control unit, that is, D, C, B and A.

Figure 5.25 Time-division Switch

A disadvantage of time-division switching is that it requires a sufficient amount of RAM always to be able to store and forward data from the incoming frames to the outgoing frames.

22. Explain in brief about the generations of cable TV networks.

Ans: Cable TV networks provide a wide variety of services ranging from distributing video signals to Internet access. There are mainly two generations of cable networks: traditional cable networks and hybrid-fibre-coaxial networks.

Traditional Cable Networks

Cable TV network started providing video services in the late 1940s. An antenna mounted on the top of a building or tower was used to receive the signals from the TV stations and to distribute them to subscribers with the help of coaxial cables. That's why it was also referred to as community antenna TV (CATV).

Figure 5.26 shows a conceptual view of traditional cable networks. The broadcasting stations send video signals to cable TV office called head end. The head end transmits these video signals to subscribers' houses with the help of coaxial cable. To deal with the attenuation problem, amplifiers were used. At the subscribers' end, splitters were used to split the cable and taps and drop cables were used to make the connection to the subscriber's houses. Communication in these cable networks was unidirectional; signals could be transmitted only in one direction from cable TV office to subscriber's premises.

Figure 5.26 Traditional Cable TV Network

Hybrid Fibre-coaxial (HFC) Network

In this generation of cable TV network, cable operators decided to provide Internet services. This cable network is so called because it uses both the fibre-optic cable and the coaxial cable, the fibre-optic cable for long haul runs while coaxial cable to the houses. The interface between the electrical and the optical parts of the system is provided through electro-optical converters known as fibre nodes.

Figure 5.27 shows an HFC network. The signals transmitted from cable TV office, referred to as regional cable head (RCH), are passed to the distribution hubs that act as subordinate to RCH. The distribution hubs perform modulation of signals and distribute them to fibre nodes over the fibre-optic cable. The fibre nodes split the signal to send same signal to each coaxial cable.

Figure 5.27 HFC Network

In comparison to traditional cable TV network, HFC network requires less number of amplifiers as well as offers bidirectional communication.

23. Which devices are needed for using cable networks for data transmission?

Ans: Traditional cable TV network was used only for providing video signals but nowadays, HFC network—an extension of traditional cable TV network—is being used to provide high-speed access to the Internet. However, to use cable TV network for data transmission, two devices are needed, namely, cable modem (CM) and CM transmission system (CMTS).

Cable Modem

This is an external device that is installed at the end user's premises where the Internet has to be accessed. It is connected to the user's home PC via Ethernet port and it works similar to an ADSL modem. CM divides the HFC network into two channels, upstream and downstream channels, with downstream channel having a higher data transmission rate than the upstream channel. The upstream channel is used for transmission from the home PC to the head end and the downstream channel is used to handle transmission in the opposite direction. Since HFC is a shared broadcast network, each packet from the head end is broadcasted on every link to every home; however, vice versa does not happen. As a result, when several users are downloading contents simultaneously through the downstream channel, the transmission rate received by each user is much lesser than the actual rate of downstream channel. In contrast, when only some users are surfing the web, each user will receive a full downstream rate because it is very rare that two users request the same page at the same time.

Cable Modem Transmission System

This device is installed inside the distribution hub of HFC network by the cable company. The data is received by the CMTS from both the Internet and the user. When CMTS receives data from Internet, it passes the data to the combiner. The combiner combines the data with the video signals received from head end and passes the resultant information to the intended user. On the other hand, the data received by the CMTS from the end user is simply forwarded to the Internet.

24. Write a short note on ISDN.

Ans: ISDN (stands for integrated services digital network) was developed by ITU-T in 1976. The idea behind ISDN is to digitize the telephone network, so that all the data including audio, video and text could be transmitted over existing telephone lines. The main objective of ISDN is to create a wide area network (WAN) for providing global end-to-end connectivity over digital media. For this, it integrates different transmission services with no addition of new links or subscriber lines. ISDN provides higher data rates usually 2 Mbps on a local link and 64 kbps or 128 kbps on a wide area link.

ISDN caters to the need of both voice and non-voice applications of the same network. It also supports a variety of applications which involve switched (circuit switched or packet switched) as well as non-switched connections. The new services can be added to ISDN provided they are compatible with 64 kbps switched digital connections. ISDN can use X.25 standards and can be used as local access link in frame relay and X.25 networks.

Though ISDN is a digital service and offers much more bandwidth than standard telephone service, still like telephone service, the ISDN users also need to dial an ISDN number (different from standard telephone number) for establishing a connection and connecting to other sites. A user can establish a digital connection by dialling another ISDN number from his/her own ISDN number. However, unlike leased lines which provide permanent connection, the ISDN users can disconnect ISDN WAN link whenever desired.

25. What are the two types of ISDN?

Ans: Based on the transmission and switching capabilities, there are two types of ISDN, namely, narrowband ISDN and broadband ISDN.

Narrowband ISDN (N-ISDN): This is the first generation of ISDN and uses circuit-switching technique. It has smaller bandwidth and supports low bit rates (usually up to 64 kbps). Due to lower bit rates, N-ISDN cannot cater to the needs of certain applications such as multimedia applications. Four-wire twisted pairs are used for transmission in N-ISDN thereby resulting in poor quality of service. N-ISDN follows the concept of frame relay.
Broadband ISDN (B-ISDN): This is the second generation of ISDN and uses packet-switching technology. It supports high-resolution video and multimedia applications. It can support data rates of hundreds of Mbps (usually up to 622 Mbps) and thus, is suitable to be used for high-resolution video and multimedia applications. Optical fibre is used as the transmission media for B-ISDN thereby resulting in better quality of service as compared to N-ISDN. B-ISDN follows the concept of cell relay.

26. What are the services provided by ISDN?

Ans: ISDN provides various services including data applications, existing voice applications, facsimile, videotext and teletext services. These services are grouped under the following three categories.

Bearer or Carrier Services: These services allow the sender and the receiver to exchange information such as video, voice and text in real time. A message from the sender can be communicated to the receiver without any modification in the original content of the message. Bearer services are provided by ISDN using packet switching, circuit switching, cell relay and frame relay. These services correspond to the physical, data link and network layers of the OSI model.
Teleservices: These services not only permit to transport information but include information-processing function also. To transport information, teleservices make use of bearer services while to process the information, a set of higher-level functions that correspond to the transport, session, presentation and application layers of the OSI model are used. Some examples of teleservices are videotex, teletex and telefax.
Supplementary Services: These services are used in combination with at least one teleservice or bearer service but cannot be used alone. For example, reverse charging, call waiting and message handling are some of the supplementary services that are provided with the bearer and teleservices.

27. Describe the type of channels provided by ISDN along with their purpose.

Ans: The ISDN standard has defined three types of channels, namely, bearer (B) channel, data (D) channel and hybrid (H) channel, with each channel having a specific data rate.

B Channel: This is the basic channel used to carry only the user traffic in digital form such as digitized voice and digital data at the rate of 64 kbps. The transmission through B channel is full-duplex as long as the required data transmission rate is up to 64 kbps. Multiple conversations destined for a single receiver can be carried over a single B channel with the use of multiplexing. Since a B channel provides end-to-end transmission, signals can be demultiplexed only at the receiver's end and not in the mid way.
D Channel: This channel is mainly used to carry the control information that is required for establishing and terminating the switched connections on the B channels. For example, the dialled digits while establishing a telephone connection are passed over the D channel. Under certain circumstances, the D channel can be used to carry user data also. The D channel provides data rates of 16 or 64 kbps depending on the requirements of the user.
H Channel: This channel is used for applications having higher data requirements such as video-conferencing and teleconferencing. There are certain types of H channels including H0 channel with a data rate of 384 kbps, H11 channel with a data rate of 1,536 kbps and H12 channel with a data rate of 1,920 kbps.

28. Distinguish BRI and PRI in ISDN.

Ans: In ISDN network, each user is connected to the central office via digital pipes called digital subscriber loops. The digital subscriber loops are of two types, namely, basic rate interface (BRI) and primary rate interface (PRI), with each type catering to a different level of customers' needs. Table 5.3 lists some differences between these two types.

Table 5.3 Differences Between BRI and PRI

  • It comprises two B channels and one D channel, thus, referred to as 2B + D channel.
  • It comprises up to 23 B channels and one D channel in North America and Japan. In countries such as Europe, Australia and other parts of the world, it comprises 30 B channels and one D channel.
  • The BRI-D channel operates at 16 kbps.
  • The PRI-D channel operates at 64 kbps.
  • It provides total bit rate of 192 kbps.
  • In North America and Japan, it provides total bit rate of 1.544 Mbps and in other parts of the world, it provides total bit rate of 2.048 Mbps.
  • It is primarily used at home for connecting to Internet or business networks.
  • It is primarily used in business replacing the leased lines that can provide the same bandwidth and signal quality but with more flexibility.


29. Explain the architecture of ISDN.

Ans: In ISDN network, to enable the users to access the services of BRI and PRI, certain devices are used. The main devices used in the architecture of ISDN (Figure 5.28) include terminal equipment (types 1 and 2), network termination devices (types 1 and 2) and terminal adapters. All these devices are used in the subscriber's premises. In addition, several reference points are used to determine the interfaces between any two elements of ISDN installation. The description of these equipments is as follows:

Terminal Equipment 1 (TE1): It refers to a device that supports ISDN standards. For example, computer or ISDN telephone. To connect TE1s to the ISDN network, a four-wire twisted-pair cable is used.
Terminal Equipment 2 (TE2): It refers to a device that is non-specialized ISDN. For example, the standard telephone is a non-ISDN device.
Terminal Adapters: It refers to a device that translates the information in a non-ISDN format into a form usable by an ISDN device. It is generally used with TE2s devices.
Network Termination 1 (NT1): It refers to a device that connects the internal system at the subscriber's end to digital subscriber loop. It organizes the data received from the subscriber's end into frames that are to be sent over the network. It also translates the frames received from the network into a format which can be understood by the subscriber's devices. It interleaves the bytes of data from subscriber's devices but is not a multiplexer; the multiplexing occurs automatically, as NTI provides synchronization between the data stream and the process of creating a frame.
Network Termination 2 (NT2): It refers to a device that performs multiplexing, flow control and packetizing at the physical, data link and network layers of OSI model, respectively. It acts as an intermediate device between data-producing devices and NT1. An example of NT2 is private branch exchange (PBX) that coordinates transmissions from telephone lines and multiplexes them, so that they can be transmitted by NT1.
Reference Points: It refers to a tag that is used to define logical interfaces between various termination devices. The reference points used in ISDN architecture include R, S, T and U. Reference point R specifies the link between TE1 and TA. Reference point S specifies the link between TE1 or TA and NT1 or NT2. Reference point T specifies the link between NT1 and NT2. Reference point U specifies the link between NT1 and ISDN office.

Figure 5.28 Architecture of ISDN

To exchange the control information between end user and the network, protocols are used. ISDN uses more than one twisted pair to provide the full-duplex communication link between the end user and the central office. The operation performed by central office includes accommodating multiplexed access to provide high-speed interface using digital PBX and LAN. With the help of central office, subscriber can access circuit switched network and packet switched network.

30. Five 1-kbps connections are multiplexed together and each unit represents 1 bit. Find:

(a) the duration of 1 bit before multiplexing,

(b) the transmission rate of the link,

(c) the duration of a time slot and

(d) the duration of a frame.

Ans:  (a) The duration of one bit before multiplexing is 1/1 kbps, that is, 0.001 s or 1 ms.

(b)  The transmission rate of the link will be five times the data rate of the connection, that is, 5 kbps.

(c)  The duration of a time slot will be one-fifth of the duration of each bit before multiplexing, that is, 1/5 ms or 200 μs.

(d)  Since there are five slots in a frame, the duration of a frame will be equal to five times the duration of a time slot, that is, 5 × 200 μs or 1 ms.

31. Five channels, each with a 100-KHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 KHz between the channels to prevent interference?

Ans: To multiplex five channels, four guard bands are needed. Thus, the minimum bandwidth (B) required will be calculated as follows:

B = 5 × (100 KHz) + 4 × (10 KHz) = 540 KHz

Multiple Choice Questions

  1. A device that combines n input lines into a single stream is known as__________.

    (a) Demultiplexer

    (b) Serial to parallel convertor

    (c) Prism

    (d) Multiplexer

  2. Multiplexing can be used in situations where the signals to be transmitted through the transmission medium have__________bandwidth than that of the medium.

    (a) Higher

    (b) Lower

    (c) Equal

    (d) None of these

  3. Which of the following multiplexing techniques involves signals composed of light beams?

    (a) FDM

    (b) TDM

    (c) WDM

    (d) All of these

  4. Multilevel multiplexing and multiple-slot allocation are the strategies used in

    (a) TDM

    (b) WDM

    (c) Both (a) and (b)

    (d) FDM

  5. The digital signal service has__________levels.

    (a) Five

    (b) Seven

    (c) Six

    (d) Eight

  6. Which of the following is not a group in the analog hierarchy?

    (a) Voice channels

    (b) Master groups

    (c) Slave groups

    (d) Jumbo groups

  7. A technique used to expand the bandwidth of a communication link to achieve goals such as privacy and anti-jamming is called__________.

    (a) Spread spectrum

    (b) Multiplexing

    (c) Switching

    (d) Pulse stuffing

  8. Which of the following is not a switching technique?

    (a) Circuit switching

    (b) Data switching

    (c) Message switching

    (d) Packet switching

  9. Store-and-forward technique is used in

    (a) Message switching

    (b) Packet switching

    (c) Time-division multiplexing

    (d) Both (a) and (b)

  10. The transmission media used for communication between switching offices is referred to as

    (a) Switch

    (b) Local loop

    (c) Trunk

    (d) None of these

  11. Which of the following is a DSL technology?

    (a) ADSL Lite

    (b) VDSL

    (c) SDSL

    (d) All of these

  12. The broadcasting stations send video signals to cable TV office, called__________.

    (a) Fibre node

    (b) Head end

    (c) Toll office

    (d) CATV

  13. Narrow band ISDN supports bandwidth up to

    (a) 128 kbps

    (b) 112 kbps

    (c) 96 kbps

    (d) 64 kbps


1. (d)

2. (b)

3. (c)

4. (a)

5. (a)

6. (c)

7. (a)

8. (b)

9. (d)

10. (c)

11. (d)

12. (b)

13. (d)