Fig. 10.19. VT2 mapped into STS-1 frame.

4. VT 6 is a 108 byte frame that is made up of 12 columns and 9 rows. It is used for transmission of a DS-2 frame with a data rate of 6.312 Mbps.

Within an STS-1 frame, each VT occupies a number of columns. Within the STS-1, VT groups can be mixed together to form an STS-1 payload. When two or more tributaries are inserted into a single STS-1 frame, they are interleaved column by column. SONET provides mechanisms for identifying each VT and separating them without demultiplexing the entire stream. For example a VT group may contain one VT-6, 2, VT-2s or 4 VT 1.5s. To synchronize the various low speed signals to a common rate before multiplexing, bit stuffing is used. All services below DS-3 rate are transported in the VT structure.

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