ADTRAN Atlas 550 - Manual (Page 218)

Appendix B. OSI Model and Frame Relay Technology Overview
B-6 ATLAS 550 User Manual 61200305L1-1
Systems, DEC, Nortel, and StrataCom, is commonly referred to as the Group
of Four.
Annex A and Annex D
The International Telecommunications Union Telecommunication Stan-
dardization Sector (ITU-T) adopted Annex A as the interface standard for in-
ternational frame relay applications. The American National Standards
Institute (ANSI) modified the Frame Relay Consortium’s interface specifica-
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 bandwidth the 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.
The CIR can be thought of as the size of the virtual connection from end to
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
from site A to the frame relay network and a 56-kbps DDS line from site B to
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.