61200.130L1-1 FSU User Manual 1
Chapter 1. Introduction
Chapter 1Introduction

UNDERSTANDING FRAME RELAY

Frame relay is a wide area network (WAN) service designed to
minimize physical connections. This is accomplished by using
virtual connections within the frame relay cloud and accessing
these virtual circuits with normally one physical connection at
each location to the frame relay service. Virtual circuits are
addressed using header information at the beginning of each
frame. These frames are formatted by the user's CPE equipment
such as the ADTRAN FSU.
ANSI standards describe how each frame must be constructed to
provide interoperability between CPE equipment and frame
relay switching equipment. Each frame must contain a header,
at least one byte of information data, two bytes of CRC16, and a
trailing flag 0x7E.
This header information contains a virtual circuit address known
as a DLCI (data link connection identifier). The header informa-
tion also contains bits used for network congestion control.
Frame relay virtual circuits may be defined as permanent (PVC)
or switched (SVC). PVCs have the same DLCI for a given path
each time a user protocol session is established. The network
service provider assigns these DLCIs at subscription time. SVCs,
on the other hand, have DLCIs dynamically assigned each time a
user protocol session is established. The CPE equipment must
request a call and the DLCI is assigned by the network switching