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70 Chapter 3

Path Overhead

The final piece of overhead is the path overhead contained inside the
SPE. Path overhead contains 9 bytes of information starting at the
first byte of the STS SPE. The path overhead provides for communi-
cation between two pieces of PTE. The PTE is where the actual mul-
tiplexing and demultiplexing functions take place as the services are
mapped into the SONET frame.The functions supported by the path
overhead are as follows:

Performance monitoring of the STS SPE
Signal labeling of the individually mapped payloads
Path status
Path trace

Virtual Tributaries

SONET is more than the STS frame. It also defines the sub-STS
levels of payload that can be carried. The STS-1 payload can be sub-
divided into virtual tributaries (VTs), synchronous signals to trans-
port lower-speed signals. The normal sizes defined by SONET are
shown Table 3-3.

Table 3-3
VT Bit Rate Equivalent DS-n Level Required Bytes
The Values of the (rows columns)
Virtual Tributaries
1.5 1.728 Mbps DS-1 27 (9 3)
(VTs) Defined for
SONET 2 2.304 Mbps E-1 36 (9 4)

3 3.456 Mbps DS-1C 54 (9 6)
6 6.912 Mbps DS-2 108 (9 12)