EEE 360: Communication Systems I

Lecture Notes Prepared by Dr. Erhan A. Ince ©2003

Time Division Multiplexing (TDM) 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. Figure 3-35 illustrates TDM applied to three analog sources that are multiplexed over a PCM system.

At the receiver the decommutator (sampler) has to be synchronized with the incoming waveform so that the PAM samples corresponding to source-1 will appear on the channel-1 output.

EEE 360: Communication Systems I

Lecture Notes Prepared by Dr. Erhan A. Ince ©2003

Frame Synchronization Frame synchronization is needed at the TDM receiver so that the received multiplexed data can be sorted and directed to appropriate output channel. Frame sync is provided to the receiver in two different ways: provided to the de-multiplexer circuit by sending a frame sync signal from the transmitter over a separate channel derive the frame sync from the TDM signal itself Figure 3.36 show that frame sync can be multiplexed along with the information words in an N channel TDM system.

Figure 3-37 shows that the frame sync is recovered from the corrupted TDM signal by using a frame synchronizer circuit which crosscorrelates the regenerated TDM signal with the expected unique sync word vector s = (s1,s2,…,sK).

EEE 360: Communication Systems I

Lecture Notes Prepared by Dr. Erhan A. Ince ©2003

Synchronous and Asynchronous Lines

For bit sync, data transmission systems are designed to operate with either synchronous or asynchronous serial data lines.

In a synchronous system, each device is designed so that its internal clock is relatively stable for a long period of time and it is synchronized to the master clock

In an asynchronous system, the timing is precise only for the bits within each character (word). Also called start-stop signaling. Show Example 3-6 : Design of a Time-Division Multiplexer (pages 201-202)

EEE 360: Communication Systems I

Lecture Notes Prepared by Dr. Erhan A. Ince ©2003

TDM Hierarchy In practice TDM(s) may be grouped into two:

1. multiplexers used in conjunction with digital computer systems to merge digital signals from several sources

EEE 360: Communication Systems I

Lecture Notes Prepared by Dr. Erhan A. Ince ©2003

2. used by common carriers to combine different sources into high-speed digital TDM signal for transmission.

Standards adopted by North America and Japan are different from those that have been adopted in other parts of the world. North America/Japan standards were first adopted by AT&T. Later an other set of standards were adopted by CCITT under the auspieces of ITU. North America TDM hierarchy is as shown in Fig. 3-40.

The corresponding CCITT TDM standard that is used elsewhere in the world is as shown in Fig. 3-41.

EEE 360: Communication Systems I

Lecture Notes Prepared by Dr. Erhan A. Ince ©2003

With the development of high-bit rate fiber optic systems, it has become apparent that the original TDM standards are not adequate. A new TDM standard called SONET (Synchronous Optical Network) was proposed by Belcore and has evolved into an international standard. This SONET standard is shown in Table 3-10.

EEE 360: Communication Systems I

Lecture Notes Prepared by Dr. Erhan A. Ince ©2003

The telephone industry is moving towards an all-digital network that integrates voice and data over a single telephone line from each user to the telephone company equipment. This approach is called the Integrated Service Digital Network (ISDN). The standard implementation of ISDN services uses an overall data rate of 144 k bits/s which is broken down into two B-channels (64 k bits/s) and one D-channel (16 k bits/s). The two B-channels carry PCM-encoded telephone conversation or data and the D-channel is used for signaling to set up calls, disconnect calls, and route data.