METRODATA LTD
WCM1100 / WCM3100 / WCM-Serial
User Manual
76-02-082 Rev J
88 of 140
5.3.6.3 Encapsulation
The WCM product range supports bit synchronous HDLC encapsulation for unframed, or
framed interfaces, or alternatively X.86 LAPS or GFP byte synchronous modes for Framed
interfaces.
5.3.6.3.1 HDLC
HDLC mode is the only option supported on the WCM-Serial, and on WCM1100/3100
when unframed interfaces are selected. HDLC uses flags (0x7E) and bit stuffing to
transmit packets over the WAN interface. When operating in HDLC mode, there are
several parameters which can be changed
5.3.6.3.1.1 HDLC FCS
When a packet is encapsulated a frame check sequence is added to the end of the packet
to enable transmission error detection. The options here are
16 bit
A 16 bit FCS is calculated and added to the packet
32 bit
A 32 bit FCS is calculated and added to the packet
Both ends of the link MUST match otherwise traffic will not flow, For operation with an
unmanaged WC product, 16 bit mode must be used.
5.3.6.3.1.2 Strip MAC FCS
A LAN packet is protected by a 32 bit FCS. Since the encapsulator will add an HDLC FCS
it is possible to save bandwidth by removing the Ethernet FCS and recalculate at the far
end.
Both ends of the link MUST match otherwise traffic will not flow, For operation with an
unmanaged WC product, or previous software version MAC FCS Stripping mode must be
used.
For greatest protection against errors, MAC stripping should be disabled and the frame
FCS sent end to end to protect against any internal errors. This requires extra overhead,
so it is a trade off between error protection and bandwidth.
