Lecture 2.4. Multiplexing

Learning Outcomes Discuss the concept of Multiplexing Explain & calculate frequency-division multiplexing. Explain & calculate time-division multiplexing. Differentiate between SDM,FDM and TDM.

22 November Multiplexing

22 November Needs Multiplexing – Process of transmitting two or more signals simultaneously Multiplexing Principles

Multiplexing vs. No Multiplexing

Multiplexing : Applications Four communication applications that would be prohibitively expensive or impossible without multiplexing are: 1. Telephone systems 2. Telemetry 3. Satellites 4. Broadcasting (radio and TV)

Types of Multiplexing Categories of Multiplexing AnalogDigital

Types of Multiplexing The two most common types of multiplexing are 1. Frequency-division multiplexing (FDM) Generally used for analog information. Individual signals to be transmitted are assigned a different frequency within a common bandwidth. 2. Time-division multiplexing (TDM) Generally used for digital information. Multiple signals are transmitted in different time slots on a single channel.

These two basic methods are illustrated below. 4 TDM: messages occupy all the channel bandwidth but for short time intervals of time FDM: all signals are transmitted at the same time (all the time) but in different frequency bands

Frequency Division Multiplexing FDM: all signals are transmitted at the same time (all the time) but in different frequency bands

Frequency Division Multiplexing 22 November

FDM FDM(Frequency-Division Multiplexing) is an analog technique that can be applied when the bandwidth of a link (useful bandwidth of the medium excess) is greater than the combined bandwidths of the signals to be transmitted BW signal << BW medium

FDM signal generation FDM process each telephone generates a signal of a similar frequency range these signals are modulated onto different carrier frequencies(f1, f2, f3)

FDM signal generation modulated onto different carrier frequencies Requires its own carrier frequency modulated onto different carrier frequencies Requires its own carrier frequency Composite signal FDM multiplexing process, time-domain

FDM signal generation FDM multiplexing process, frequency-domain

FDM signal generation Demultiplexing separates the individual signals from their carries and passes them to the waiting receivers.

FDM signal generation FDM demultiplexing process, time-domain

FDM signal generation FDM demultiplexing, frequency-domain

FDM: Composite signal spectrum 22 November WHY???? BW signal << BW medium

FDM: Composite signal spectrum For telephony, the physical line is divided (notionally) into 4kHz bands or channels, i.e. the channel spacing is 4kHz. Thus we now have: guard bands - to reduce adjacent channel crosstalk. 9

Frequency Division Multiplex Advantages: no dynamic coordination needed Disadvantages: waste of bandwidth if traffic distributed unevenly guard spaces k3k3 k4k4 k5k5 k6k6 f t c Channels k i

Frequency Division Multiplexing 22 November

Frequency Division Multiplexing Example : Cable Television coaxial cable has a bandwidth of approximately 500Mhz individual television channel require about 6Mhz of bandwidth for transmission How many channels it will carry?? can carry 83 channels theoretically

Frequency Division Multiplexing Each broadcast stations carries an information signal (voice & music ) which occupies bandwidth between 0Hz ~5kHz Impossible to differentiate or separate one station’s transmission from another

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Example 1 Assume that a voice channel occupies a bandwidth of 4 KHz. We need to combine three voice channels into a link with a bandwidth of 12 KHz, from 20 to 32 KHz. Show the configuration using the frequency domain without the use of guard bands. Solution Shift (modulate) each of the three voice channels to a different bandwidth, as shown in Figure 6.6.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure 6.6 Example 1

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Example 2 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? Solution For five channels, we need at least four guard bands. This means that the required bandwidth is at least 5 x x 10 = 540 KHz, as shown in Figure 6.7.

McGraw-Hill©The McGraw-Hill Companies, Inc., 2004 Figure 6.7 Example 2

Multiplexing Hierarchy 22 November

Example: analogue carrier system for telephony 22 November

Time Division Multiplexing 22 November

TDM

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Time-Division Multiplexing Figure : The basic TDM concept. Time slot, t s

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TDM 22 November

22 November Transmission Line.. How to calculate transmission rate???

PCM Line Speed / Transmission rate Data rate at which serial PCM bits are clocked out of the PCM encoder onto the transmission line. 22 November Line speed = sample rate (f s ) X number of bits in the compressed PCM code Line speed = sample X bits second sample

TDM Example : For a single PCM system with a sample rate = 6000 per second and a seven –bit compressed PCM code, determine the line speed. 22 November Line speed = 6000 sample X 7 bits second sample = 42, 000 bps

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22 November Statistical TDM is useful for applications in which the low-bit-rate streams have speeds that vary in time.

Asynchronous TDM 22 November

Asynchronous TDM 22 November

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TDM(cont ’ d) Examples of asynchronous TDM frames a. Case 1: Only three lines sending data

TDM(cont ’ d) b. Case 2: Only four lines sending data

TDM(cont ’ d) c. Case 3: All five lines sending data

TDM(cont ’ d) Inverse Multiplexing takes the data stream from one high-speed line and breaks it into portion that can be sent across several lower speed lines simultaneously, with no loss in the collective data rate

TDM(cont ’ d) Multiplexing and inverse multiplexing high-speed breaks it into portion

TDM(cont ’ d) Why do we need inverse multiplexing ? wants to send data, voice, and video each of which requires a different data rate. [example] voice - 64 Kbps link data Kbps link video - 1,544 Mbps link

Statistical TDM Frame Formats

Review questions Why is multiplexing so cost effective? Explain how FDM works. How is interference avoided by using FDM Explain how synchronous TDM works. Why is statistical TDM more efficient than a synchronous TDM multiplexer?

Group Activity Explain about : FDM Hierarchy / Analog Hierarchy SDM (Space Division Multiplexing) Digital Carriers System / Digital Signal Service / Digital T Carriers Synchronous Digital Hierarchy (SDH) Plesiochronous digital hierarchy (PDH) 55

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