METRODATA LTD
WCM1400/WCM1800/WCM1820
Quick Start Guide
76-02-087 Rev D
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3 Theory of Operation
3.1 Frame Encapsulation
The WCM1800 uses the Generic Framing Procedure (GFP) to encapsulate the Ethernet
frames for transmission over the WAN link(s). GFP is a byte aligned procedure and hence
is suitable for use with multi link bonding. GFP does not use bit stuffing as with HDLC and
is therefore a deterministic protocol. The diagram below compares HDLC and GFP for
encapsulation of an Ethernet Frame
The native Ethernet frame is encapsulated under GFP as shown.
The GFP core header is four bytes long and is used for packet delineation. To decouple
the line rate from the packet rate, idle frames consisting of the core header only are sent.
The GFP payload header provides information about the payload type and also whether
the payload includes an FCS. The GFP payload header uses 4 bytes.
The GFP payload area contains the Ethernet Frame payload. Since the receiver uses the
core header pattern to delineate packets, scrambling the payload helps to prevent aliasing
of the header and improves reliability.
