Cisco ONS 15454 SDH Manual page 19

Chapter 11 Circuits and Tunnels
11.12 Path Signal Label, C2 Byte
One of the overhead bytes in the SDH frame is the C2 byte. The SDH standard defines the C2 byte as
the path signal label. The purpose of this byte is to communicate the payload type being encapsulated
by the high-order path overhead (HO-POH). The C2 byte functions similarly to EtherType and Logical
Link Control (LLC)/Subnetwork Access Protocol (SNAP) header fields on an Ethernet network; it
allows a single interface to transport multiple payload types simultaneously.
byte hex values.
Table 11-8
Hex Code
0x00
0x01
0x02
0x03
0x04
0x12
0x13
0x14
0x15
0xFE
0xFF
If a circuit is provisioned using a terminating card, the terminating card provides the C2 byte. A
low-order path circuit is terminated at the cross-connect card and the cross-connect card generates the
C2 byte (0x02) downstream to the VC terminating cards. The cross-connect generates the C2 value
(0x02) to the terminating card. If an STM-N circuit is created with no terminating cards, the test
equipment must supply the path overhead in terminating mode. If the test equipment is in pass-through
mode, the C2 values usually change rapidly between 0x00 and 0xFF. Adding a terminating card to an
STM-N circuit usually fixes a circuit having C2 byte problems.
11.13 Automatic Circuit Routing
If you select automatic routing during circuit creation, CTC routes the circuit by dividing the entire
circuit route into segments based on protection domains. For unprotected segments of circuits
provisioned as fully protected, CTC finds an alternate route to protect the segment, creating a virtual
SNCP. Each segment of a circuit path is a separate protection domain. Each protection domain is
protected in a specific protection scheme including card protection (1+1, 1:1, etc.) or SDH topology
(SNCP, MS-SPRing, etc.).
The following list provides principles and characteristics of automatic circuit routing:
•
78-18119-01
STM Path Signal Label Assignments for Signals
Content of the STM Synchronous Payload Envelope (SPE)
Unequipped
Equipped—nonspecific payload
Tributary unit group (TUG) structure
Locked tributary unit (TU-n)
Asynchronous mapping of 34,368 kbps or 44,736 kbps into container-3 (C-3)
Asynchronous mapping of 139,264 kbps into container-4 (C-4)
Mapping for asynchronous transfer mode (ATM)
Mapping for Distributed Queue Dual Bus (DQDB)
Asynchronous mapping for Fiber Distributed Data Interface (FDDI)
0.181 Test signal (TSS1 to TSS3) mapping SDH network (see ITU-T G.707)
Virtual container-alarm indication signal (VC-AIS)
Circuit routing tries to use the shortest path within the user-specified or network-specified
constraints. Low-order tunnels are preferable for low-order circuits because low-order tunnels are
considered shortcuts when CTC calculates a circuit path in path-protected mesh networks.
11.12 Path Signal Label, C2 Byte
Table 11-8
Cisco ONS 15454 SDH Reference Manual, R8.5
provides the C2
11-19