Figure B-3. Network Using Dlci Assignments - ADTRAN Atlas 550 User Manual

Data Link Connection Identifier (DLCI)
User-to-Network Interface
Local Management Interface (LMI)
61200305L1-1
Appendix B. OSI Model and Frame Relay Technology Overview
An address called a Data Link Connection Identifier (DLCI) uniquely iden-
tifies each of the virtual circuits in the frame relay network. A DLCI does not
address the equipment at the far end of the virtual circuit, but addresses the
next piece of frame relay equipment within the network. The next piece of
frame relay equipment now becomes responsible for transporting all frames
from the incoming port to the appropriate outgoing port.
Figure B-3 illustrates a network using DLCI assignments. In this example,
the router at site A sends a frame packet to site B, by placing the data on
DLCI 100. Knowing that all packets coming in DLCI 100 must go out DLCI
225, Frame Relay Switch A places the packets on DLCI 225 and sends them
out to Frame Relay Switch B. Frame Relay Switch B then takes the frame
packets from DLCI 225 and places them on DLCI 35 for delivery to the site
B router. From this example, you can see that each piece of frame relay
equipment only knows about the DLCIs local to it. Hence, you will hear
"DLCIs only have local significance."
A
DLCI 100
DLCI 200
Frame Relay/Router
The interface between the customer and the frame relay switch is called the
User-to-Network Interface (UNI). Three different types of signaling can
transmit across this interface: LMI (Group of Four), Annex A
(ITU-T Q.933-A), and Annex D (ANSI T1.617-D). Unfortunately, due to sig-
naling differences among the three types, they are incompatible with one an-
other, and DLCI assignments vary among the three types. Table B-2 on page
B-6 and Table B-3 on page B-6 give the assignments for the three types.
LMI is the standard published by the Frame Relay Consortium in 1990 to
create a defined interface on the UNI. The Consortium, composed of Cisco
ATLAS 550 User Manual
DLCI 225
DLCI 650
Frame Relay
Frame Relay
Switch B
Switch A

Figure B-3. Network Using DLCI Assignments

B
Frame Relay/Router
DLCI 35
DLCI 501
C
Frame Relay/Router
B-5