Chapter 3 Time Division Multiplexing The concept of T ime Division Multiplexing TDM Examples

1. Chapter 3 Time Division Multiplexing • The concept of Time Division Multiplexing • TDM Examples • Frame Synchronization • TDM Hierarchy • Packet Transmission Huseyin Bilgekul Eeng360 Communication Systems I Department of Electrical and Electronic Engineering Eastern Mediterranean University

2. Multiplexing is the transmission of information from more than one source • to more than one destination over the same transmission medium. • Although transmission occurs in the same medium, they do not necessarily • occur at the same time • Transmission medium can be: • Metallic wire pair • Coaxial cable • PCS mobile phone • Microwave radio system • Optical fiber cable

3. To make efficient use of high-speed telecommunications lines, some form of multiplexing is used. The two common forms of multiplexing are 1) frequency division multiplexing (FDM) and 2) time division multiplexing (TDM). Frequency division multiplexing can be used with analog signals. A number of signals are carried simultaneously on the same medium by allocating to each signal a different frequency band. Modulation equipment is needed to move each signal to the required frequency band, and multiplexing equipment is needed to combine the modulated signals.

4. Synchronous time division multiplexing can be used with digital signals or analog signals carrying digital data. In this form of multiplexing, data from various sources are carried in repetitive frames. Each frame consists of a set of time slots, and each source is assigned one or more time slots per frame. The effect is to interleave bits of data from the various sources.

5. Figure 01 depicts the multiplexing function in its simplest form.There are n inputs to a multiplexer. The multiplexer is connected by a single data link to a demultiplexer. The link is able to carry n separate channels of data. The multiplexer combines (multiplexes) data from the n input lines and transmits over a higher-capacity data link.The demultiplexer accepts the multiplexed data stream, separates (demultiplexes) the data according to channel, and delivers data to the appropriate output lines. Figure 01: Multiplexing

7. Time Division Multiplex • Channel gets the whole spectrum for a certain amount of time • Advantages: • only one carrier in themedium at any time • throughput high even for many users • Disadvantages: • precise synchronization necessary Channels ki k1 k2 k3 k4 k5 k6 c f t

8. Frequency Division Multiplex • Separation of spectrum into smaller frequency bands • Channel gets band of the spectrum for the whole time • Advantages: • no dynamic coordination needed • works also for analog signals • Disadvantages: • waste of bandwidth if traffic distributed unevenly • inflexible • guard spaces Channels ki k3 k4 k5 k6 c f t

9. Multiplexing Two basic forms of multiplexing. (a) Frequency-division multiplexing (FDM) (with guardbands).(b) Time-division multiplexing(TDM); no provision is made here for synchronizing pulses. FDM TDM

10. FDM System

11. A number of analog or digital signals are to be multiplexed onto the same transmission medium. Each signal is modulated onto a carrier ; because multiple carriers are to be used, each is referred to as a subcarrier. Any type of modulation may be used. The resulting analog, modulated signals are then summed to produce a composite baseband signal.The spectrum of signal is shifted to be centered on fi For this scheme to work, must be chosen so that the bandwidths of the various signals do not significantly overlap. Otherwise, it will be impossible to recover the original signals. The composite signal may then be shifted as a whole to another carrier frequency by an additional modulation step.

12. The long-distance carrier system provided in the United States and throughout the world is designed to transmit voiceband signals over high-capacity transmission links, such as coaxial cable and microwave systems. The earliest, and still a very common, technique for utilizing high-capacity links is FDM. In the United States,AT&T has designated a hierarchy of FDM schemes to accommodate transmission systems of various capacities. A similar, but unfortunately not identical, system has beenadopted internationally under the auspices of ITU-T (Table 8.1).

13. Example 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. Assume there are no guard bands. Solution We shift (modulate) each of the three voice channels to a different bandwidth, as shown in Figure 6.6. We use the 20- to 24-kHz bandwidth for the first channel, the 24- to 28-kHz bandwidth for the second channel, and the 28- to 32-kHz bandwidth for the third one. Then we combine them as shown in Figure 6.6.

15. Analog Hierarchy

16. The Advanced Mobile Phone System (AMPS) uses two bands. The first band of 824 to 849 MHz is used for sending, and 869 to 894 MHz is used for receiving. Each user has a bandwidth of 30 kHz in each direction. How many people can use their cellular phones simultaneously? Solution Each band is 25 MHz. If we divide 25 MHz by 30 kHz, we get 833.33. In reality, the band is divided into 832 channels. Of these, 42 channels are used for control, which means only 790 channels are available for cellular phone users.

17. Synchronous time division multiplexing is possible when the achievable data rate (sometimes, unfortunately, called bandwidth) of the medium exceeds the data rate of digital signals to be transmitted. Multiple digital signals (or analog signals carrying digital data) can be carried on a single transmission path by interleaving portions of each signal in time. The interleaving can be at the bit level or in blocks of bytes or larger quantities.

18. Time Division Multiplexing Definition: Time Division Multiplexing (TDM) is the time interleaving of samples from several sources so that the information from these sources can be transmitted serially over a single communication channel. At the Transmitter • Simultaneous transmission of several signals on a time-sharing basis. • Each signal occupies its own distinct time slot, using all frequencies, for the duration of the transmission. • Slots may be permanently assigned on demand. At the Receiver • Decommutator (sampler) has to be synchronized with the incoming waveform  Frame Synchronization • Low pass filter • ISI – poor channel filtering • Feedthrough of one channel's signal into another channel -- Crosstalk Applications of TDM: Digital Telephony, Data communications, Satellite Access, Cellular radio.

19. Time Division Multiplexing Conceptual diagram of multiplexing-demultiplexing. PAM TDM System

20. Illustrating 4-Channel PAM TDM Multiplexing

21. Digital Time Division Multiplexing • Time Division Multiplexing (TDM) can be accomplished at bit or byte (word) level. • Channhels having different data rates can also be TDM multiplexed but must be interleaved accordingly. Digit Interleaving Interleaving channel with different bit rates WORD or Byte Interleaving Interleaving channel with different bit rates using two multiplexers

22. Time Division Multiplexing Pulse width of TDM PAM: Pulse width of TDM PCM:

24. TDM Example (Multiplexing Analog and Digital) • Source 1: 2 kHz bandwidth. • Source 2: 4 kHz bandwidth. • Source 3: 2 kHz bandwidth. • Source 4-11: Digital 7200 bits/sec. 16 ksam/s 64 kb/s 8x7.2=57.6 kb/sUse stuff bits to complete 7.2 to 8 kb/s.Now 8 and 64 rates are complete multıples 128 kb/s

25. Digital Hierarchy for US, Canada and Japan DS0 64 kbit/s

26. DS-1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is the primary digital telephone standard used in the United States, Canada and Japan and is able to transmit up to 24 multiplexed voice and data calls over telephone lines. E-carrier is used in place of T-carrier outside the United States, Canada, Japan, and South Korea.

27. T-1 line for multiplexing telephone lines

28. T-1 frame structure

29. CCITT Digital TDM Hierarchy

30. E line rates

31. Packet Transmission System • TDM is Synchronous Transfer Mode (STM) technology - Data source is assigned a specific time slot – fixed data rate - More efficient when sources have a fixed data rate - Inefficient to accommodate bursty data source • Packet Transmission System - Partitions source data into data packets (destination address, header) - Efficiently assigns network resources when the sources have bursty data - Examples : Internet TCP/IP technology and the Asynchronous Transfer Mode (ATM) technology.