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Fig. 10.18. Virtual tributories.

There are four types of VTs that map into the STS-1 payload.

1. VT 1.5 is a frame of 27 bytes that is made up of 3 columns and 9 rows. The data rate of VT 1.5 is calculated as follows.

Data rate VT 1.5 = 27 bytes x 8 bits x 8000 frames/sec = 1.728 Mbps.

The VT 1.5 is used for transmission of DS-1 with a data rate of 1.54 Mbps. The STS-1 payload can transmit 28 VT 1.55.

2. VT 2 is a frame of 36 bytes that is made up of 4 columns and 9 rows. It is used for transmission of an European E-1 line (can E-line can carry 30 voice channels). The data rate of VT 2 is

= 36 bytes x 8 bits x 8000 frames/sec = 2.304 Mbps.

Fig. 10.19 shows VT 2 framed into STS-1 frame.

3. VT 3 is a 54 byte frame (made up of 6 columns and 9 rows) used for transmission of a DS-1 C frame with a data rate of 3.152 Mbps.

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