SMDS or Switched Multimegabit Data Service is another service being offered by telephone companies in many countries. This service, which had become popular in the USA in the 90s, was developed by Bellcore in 1980. Figure 6.6 indicates how normal leased line connections can be established between some LANs.
Telephones: Other Issues:
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Other telephone services include items, such as Synchronous Optical Networks (SONET) and Distributed Queue Dual Bus (DQDB). We shall not concern ourselves with SONET and the associated CCITT recommendation SDH (Synchronous Digital Hierarchy), since these are purely technical issues and unlikely to ever form part of the strategic decision making in Information Technology.
Below we discuss these briefly:
1. T1 carrier was a developed while working out a method for digitizing analogue signals. Frequency- division multiplexing does not work well with digital signals. Time-division multiplexing works only with digital signals, which the telephone world is gradually converting to.
This required a device to convert analogue signals to sampled signals. The standard sampling rate was determined as 125 µs, since this was the lowest rate at which no information would be lost. The method used to do this sampling was Pulse Code Modulation (PCM).
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T1 carrier provides a method to do this sampling. In a T1 carrier 24 voice channels are multiplexed. This multiplexing is done by sampling each of the 24 channels in sequence and creating a new code that is sent through the T1 carrier. With a sampling rate of 125 µs per channel, plus one extra bit per channel, over the 24 channels, the data transfer rate works out to 1.544 Mbps.
This can be worked out as follows:
Since the sampling rate is 125 µs, and 8 bits are used from each channel (7 data bits and 1 bit for signalling) and since there are 24 channels, the frame size should consist of 24 x 8 or 192 bits. However, 1 bit is added for frame synchronization, giving a frame size of 193 bits. Thus a frame of 193 bits will be generated every 125 µs.
Therefore, a T1 carrier will have a data transmission rate of 193 x 106/125 bits per second or a rate of 1.544 mbps. Incidentally, when the T1 carrier is used to transmit only data, only 23 of the 24 channels can be used, the 24th being reserved for internal synchronization. While T1 is not yet the accepted standard for PCM, it is widely used.
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2. T2, T3 and T4 carriers are further developments of the T1 carrier in as much as the T2 carrier is obtained by multiplexing multiple T1 carriers (which is possible in Time-Division Multiplexing). 4 T1 streams are multiplexed into one T2 stream. The data rate for the T2 stream is 6.312 Mbps, which is slightly more than 4 times the T1 stream. This because extra bits are used for internal control purposes.
Similarly, 6 T2 streams are combined to create a T3 stream in which the data rate is 44.736 Mbps and 7 T3 streams are multiplexed to form a T4 stream for a data rate of 274.176 Mbps. Therefore, starting with a data stream of 64 Kbps in one channel of a T1 carrier we can end up with nearly 275 Mbps in a T4 carrier.
These schemes were adopted by the US and Japan and the format used are technically known as DS1. CCITT recommendations are, however, different from these two. Data transmission between countries is invariably done using the CCITT recommended procedures.
3. E1 carriers conform to CCITT recommendations. This carrier has 32 8-bit data sample channels and, therefore, a transmission rate of 2.048 Mbps. This standard is used widely throughout the rest of the world including India and Europe.
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E2, E3 and E4 carriers are configured in a manner similar to T2, T3 and T4 carriers. In India, carriers that conform to the CCITT recommendations are used, that is, E1, E2, E3 and E4 carriers are used, whereas T1, T2, T3 and T4 are used in the US.