11.4 TRUNKING SYSTEMS

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11.4 TRUNKING SYSTEMS

Two switches are connected together through trunks. The trunks are of different types:

1. Two-wire analog trunks, which are used to interconnect small switches.

2. Four-wire trunks, which are also used to interconnect small switches.

3. T1 carriers which are digital trunks. Each T1 carrier carries 24 voice channels. In Europe, the equivalent standard is referred to as E1 trunk. Each E1 trunk supports 30 voice channels.

Generally, the switches are connected through T1 carriers. Data corresponding to 24 voice channels is multiplexed to form the T1 carrier. For every 125 microseconds, the bit stream from each voice channel consists of 8 bits out of which 7 bits are data and one bit is control information. Hence, for each voice channel, the total data consist of 7 X 8000 = 56, 000 bps of voice and 1 × 8000 bps = 8000 bps of control information.

Small switches are interconnected using 2-wire or 4-wire analog trunks or digital T1 carriers. T1 carrier supports 24 voice channels using Time Division Multiplexing (TDM) technique.

Note 

In T1 carrier, a frame consists of 193 bits—192 bits corresponding to 24 voice channels' data and one additional bit for framing. The frame duration is 125 microseconds. Hence, the gross data rate of T1 carrier is 1.544 Mbps.

Figure 11.7: Carrier Frame Format.

In major towns and cities, because of high traffic, T1 carriers will not suffice. In such a case, T2, T3, and T4 carriers are used. Higher capacity trunks are obtained by multiplexing T1 carriers. The standard for this digital hierarchy is shown in Figure 11.8. Four T1 carriers are multiplexed to obtain T2 carrier. Seven T2 carriers are multiplexed to obtain T3 carrier. Six T3 carriers are multiplexed to obtain T4 carrier.

Figure 11.8: Trunks with Higher Capacities.

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