SONET

Figure: SONET Hierarchy at STS-12 shows the SONET hierarchy at STS-12 (OC-12).

Figure: SONET Hierarchy at STS-12

A SONET multiplexing structure allows several combinations of signal transportation. For example, at the STS-3 (OC-3) level:

• STS-3 is achieved by interleaving three STS-1s byte by byte

• an STS-1 payload can be subdivided into virtual tributary groups (VTGs) and virtual tributaries (VTs). Each STS-1 may contain seven VTGs, which in turn carry sub-STS traffic in VTs. There are four VT sizes:

  • VT1.5 (1.728 Mb/s) (typically used for DS1, indicated in the CLI as vt15)

  • VT2 (2.304 Mb/s) (typically transports one E1)

  • VT3 (3.456 Mb/s) (not shown in Figure: SONET Hierarchy at STS-12 )

  • VT6 (6.912 Mb/s)

• each VTG can contain four VT1.5s, three VT2s, two VT3s, or one VT6

• each VTG can carry only VTs of the same size