LCAS messages are continuously exchanged. Changes in the link capacity are sent in
advance so that the receiver can switch to the new configuration as soon as it arrives.
This allows for dynamic resizing of the concatenated channel or the temporary removal
of a failed member link. LCAS utilizes the same H4 path overhead bytes that virtual
concatenation uses. Because the H4 bytes are in the SONET path overhead, only the
path terminating elements are involved in maintaining the circuit. In order to utilize
this concatenated channel, a method to distribute client data across the STS-1s and
recover them at the far end is required.
Figure 10.3 SONET virtual concatenation
3:1 3:1
16:1
STS-48
STS-3
STS-3
STS-1
STS-1
STS-1
STS-1
STS-1
STS-1 3:1
STS-3
STS-1
STS-1
STS-1
3:1
STS-3
STS-1
STS-1
STS-1
STS-1-3v
STS-1-5v
TDM: Circuit Bonding 271
Generic Framing Procedure (GFP) The Generic Framing Procedure (GFP) standard
was created by ITU to standardize framing capability for upper layer protocols to run
over transmission systems. There are two favors of GFP: Frame-MappedMode and
Transparent Mode.
A Frame-Mapped GFP is a Protocol Data Unit (PDU)- oriented adaptation mode.
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