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Appendix B. OSI Model and Frame Relay Technology Overview

B-6 ATLAS 550 User Manual 61200305L1-1

Systems,DEC,Nortel,andStrataCom,iscommonlyreferredtoastheGroup

of Four.

Annex A and Annex D

The International Telecommunications Union Telecommunication Stan-

dardizationSector(ITU-T)adoptedAnnexAastheinterfacestandardforin-

ternational frame relay applications. The American National Standards

Institute (ANSI)modifiedtheFrameRelayConsortium’s interfacespecifica-

tion and ratified it as Annex D—an interface standard for the United States.

Committed Information Rate (CIR)

Customers can order a circuit with a guaranteed amount of bandwidth for

their virtual connections. This amount is called the Committed Information

Rate (CIR), and it defines how much bandwidththe customer is guaranteed

during normal network operation. Any data transmitted above this pur-

chased rate is discard eligible (DE) by the network. That is, this data can be

discarded in the event of network congestion.

TheCIRcanbethoughtofasthesizeofthevirtualconnectionfromendto

end. The CIR can be purchased in different increments up to the wire speed

of the slowest link. For example, if the circuit in Figure B-3 had T1 access

fromsiteAtotheframerelaynetworkanda56-kbpsDDSlinefromsiteBto

Table B-2. LMI (Group of Four) DLCI Assignments

DLCI Use

0 Call control signaling channel.

1-15 Reserved for future use.

16-1007 Available for customer data.

1008-1022 Reserved for future use.

1023 LMI channel.

Table B-3. Annex A and Annex D DLCI Assignments

DLCI Use

0 Carries frame relay signaling (LMI channel).

1-15 Reserved for future use.

16-991 Available for customer data.

992-1007 Management DLCIs for layer 2.

1008-1022 Reserved for future use.

1023 Higher layer protocol communication channel.

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ADTRAN ATLAS 550 Manual de usuario Pagina 218