61200.130L1-1 FSU User Manual 1Chapter 1. IntroductionChapter 1IntroductionUNDERSTANDING FRAME RELAYFrame relay is a wide area network (WAN) service designed tominimize physical connections. This is accomplished by usingvirtual connections within the frame relay cloud and accessingthese virtual circuits with normally one physical connection ateach location to the frame relay service. Virtual circuits areaddressed using header information at the beginning of eachframe. These frames are formatted by the user's CPE equipmentsuch as the ADTRAN FSU.ANSI standards describe how each frame must be constructed toprovide interoperability between CPE equipment and framerelay switching equipment. Each frame must contain a header,at least one byte of information data, two bytes of CRC16, and atrailing flag 0x7E.This header information contains a virtual circuit address knownas 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 patheach time a user protocol session is established. The networkservice provider assigns these DLCIs at subscription time. SVCs,on the other hand, have DLCIs dynamically assigned each time auser protocol session is established. The CPE equipment mustrequest a call and the DLCI is assigned by the network switching
