ZyXEL Communications 681 User Manual

Frames are “relayed” through switches in the network, creating basically a virtual private network (VPN)
between linked sites. This logical point-to-point circuit between customer sites is called a VC (virtual
circuit) of which there are two types. Permanent VCs (PVCs) have constant end points and need to be set up
in advance while switched VCs (SVCs) are set up on the fly as needed. The Prestige 681 supports PVCs
only.

The frame format for frame relay is as follows.

1

2-4

0-4,000

2

1

Flag

Link Info

Data

FCS

Flag

Upper DLCI (6 bits)

C/R

EA 0

Lower DLCI (4 bits)

FECN

BECN

DE

EA 1