METRODATA LTD
WCM1400/WCM1800/WCM1820
Quick Start Guide
76-02-087 Rev D
14 of 61
The GFP frame contains an optional FCS, Frame Check Sequence. Packets received
with an FCS error will be discarded. Since the GFP frame is protected from bit errors by
the GFP payload FCS, the MAC FCS may be stripped and recalculated at the remote end
to save bandwidth.
3.2 Virtual Concatenation and LCAS
Virtual Concatenation (VCAT) is a method of bonding multiple E1 circuits together to form
a single connection with increased capacity. LCAS, Link Capacity Adjustment Procedure,
is a protocol which will enable a failed link to be removed from the VCAT group to allow
traffic to pass over a reduced capacity link.
VCAT inserts a single overhead byte immediately following FAS pattern in the first frame
of a CRC-4 multiframe. The VCAT control byte is shown below:
The VCAT control byte forms a 16 frame multi frame to pass all the required information.
Each link is given a sequence number, and a status bit (MST). Through the use of the
VCAT control word, the multiple links in the group can be aligned with upto 250ms of delay
variation compensated for.
LCAS operates using the VCAT control word and uses the MST status bits and
CTRL/LCAS control to monitor the status of all members of the group, and remove any
that fail or add in any new links in a hitless manner.
