DATA COMMUNICATION Principles The TDM Switch The TDM

DATA COMMUNICATION Principles : The TDM Switch

The TDM Switch A TDM switch consists of a multiplexer and demultiplexer pair. The demultiplexer performs the inverse operation of a multiplexer. For this purpose, it needs the information about slot allocation. It extracts data back from slots and forwards bits to the assigned recipients. A multiplexer / demultiplexer pair is shown in Figure 7 -3. The data leaving a demultiplexer in a TDM switch may consist of TDM frames as well. In this case, the frames arriving at the demultiplexer does not have the exact same format as the frames leaving the demultiplexer. The switch may be getting input from many lines and making new multiplex frames according to the destination of each slot. In addition to a pair of multiplexer/demultiplexer, the TDM switch also has the intelligence of knowing where each slot is headed for. It will then switch the slots individually into frames on the output links. See for example Figure 7 -4.

Thus, a TDM switch performs both multiplexing and demultiplexing. In Figure 7 -4, there are two input links and three output links of the TDM switch. Slots from different traffic sources are labeled with different patterns. The switch stores a table that has the detailed information about which slot on which input link is supposed to be switched to which outgoing frame and slot. It demultiplexes the incoming frames and stores the information in the slots to wait for the appropriate output frame and slot. At the output, it performs multiplexing of the stored slots according to the information in a lookup table. Thus, when there is no data for a particular output slot, the slot will be transmitted empty (shown as white) in Figure 7 -4. In statistical TDM, the switch is a router that performs switching based on the source and destination addresses contained within the data block from each user.

In general, the statistical TDM switch performs the following tasks. 1. Framing: In order to identify the beginning and end of a user data block. 2. Demultiplexing: In order to be able to switch each slot to the required output link. 3. Storage of slots: In order to wait for the designated output slots on designated outgoing links. 4. Multiplexing: On each output link.

REFERENCES • Ahmad A. - Data Communication Principles. For Fixed and Wireless Networks • Cornelius T. Leondes - Database and Data Communication Network Systems, Three-Volume Set_. . -Academic Press