ADTRAN 1204006L1 Manual do Utilizador descarregar pdf (Página 33)

Chapter 1. Introduction

61200275L1-1 TSU IQ+ User Manual 1-3

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 ateach

location to the frame relay service. Virtual circuits are addressed

using header information at the beginning of each frame. These

frames are formattedby the user's customer premises equipment

(CPE)suchastheADTRANTSUIQ+.

ANSI (American National Standards Institute) 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 avirtualcircuit addressknown as

a DLCI (data link connection identifier). The header information

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 isassigned bythe network switching equipment.

This DLCI is valid until the call is disconnected and may be

assigned a different value each time a call is requested.

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ADTRAN 1204006L1 Manual do Utilizador Página 33