Circuit Emulation (Cpipe) Services
Page 130 7210 SAS M Services Guide
For n *64 kb/s structured circuit emulation with CAS, the implementation is almost identical to
that of CES without CAS. When CAS operation is enabled, timeslot 16 cannot be included in the
channel group on E1 carriers. The CAS option is enabled or disabled at the port level; therefore, it
applies to all channel groups on that E1 port.
The packet size is based on 16 frames per packet for E1 when CAS is enabled and is not user-
configurable. For example, if the number of timeslots is 4, then the payload size is 64 octets. This
16-frame fixed configuration is logical because an E1 multi-frame contains 16 frames; therefore,
proper bit positioning for the A, B, C, and D CAS signaling bits can be ensured at each end of the
pseudo wire. Table shows the payload sizes based on the number of timeslots.
For CAS, the signaling portion adds (n/2) bytes (n is an even integer) or ((n+1)/2) bytes (n is odd)
to the packet, where n is the number of timeslots in the channel group. Note that you do not
include the additional signaling bytes in the configuration setting of the TDM payload size.
However, the operating system includes the additional bytes in the total packet payload, and the
total payload must be accounted for when setting the service-mtu size. Continuing the example
above, since n = 4, the total payload is 64 octets plus (4/2 = 2) CAS octets, or 66 octets. Refer to
Figure 18 to see the structure of the CES with CAS payload.
CES fragmentation is not supported.
Note: If you configure the service-mtu size to be smaller than the total payload size (payload plus
CAS bytes), then the Cpipe will not become operational. This must be considered if you change
the service-mtu from its default value.
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