3Com MSR 50 Series Configuration Manual page 543

Overview 543
involved, however, the sending and receiving devices at network borders must use
the same J0 byte. With the j0 byte, operators can detect and troubleshoot faults in
advance or use less time to recover networks.
POH
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The payload of an STM-N frame includes path overhead (POH), which monitors
low-speed tributary signals.
While SOH monitors the section layer, POH monitors the path layer. POH is divided
into higher-order path overhead and lower-order path overhead.
Higher-order path overhead monitors paths at the VC-4/VC-3 level.
Similar to the J0 byte, the higher-order VC-N path trace byte J1 is included in the
higher-order path overhead to send the higher-order path access point identifier
repeatedly. Based on the identifier, the receiving end of the path can make sure
that it is in continuous connection with the specified sending end. The J1 byte at
the receiving and transmission ends should be matched.
The path signal label byte C2 is also included in the higher-order path overhead to
indicate the multiplexing structure of VC frames and the property of payload, for
instance, whether the path is carrying services, what type of services are carried,
and how they are mapped. The sender and receiver must use the same C2 byte.
CPOS Interface At present, some government agencies and enterprises use low-end and
Application Scenario mid-range devices to access transmission networks through E1/T1 leased lines.
Users who require bandwidth between E1 and T3 (44 Mbps), data centers for
example lease multiple E1/T1 lines.
The bandwidth of all these users is aggregated to one or more CPOS interfaces
through a transmission network, and then connected to a high-end device where
the low-end devices are uniquely identified by timeslots.
In actual applications, the connection between these low-end devices and the
CPOS interfaces likely involves more than one transmission networks and as such,
may require relay. This is similar to the scenario where low-end devices are
connected to a high-end device through one or multiple E1/T1 leased lines.