OSI/TS Configuration and Management Manual

Introduction
OSI/TS Configuration and Management Manual—424831-001
1-7
Wide Area Networks
Wide Area Networks
For communicating over WANs, the OSI/TS subsystem uses the connection-mode
services of the X25AM access method. Since X25AM does not support connectionless
data transfer, the OSI/TS subsystem also implements the SNDCF to bridge the
connectionless network service supported by the internet protocol (IP) and the
connection-mode service supported by X25AM. SNDCF provides the subnetwork
services required by the IP and, in effect, extends the use of IP to wide area
subnetworks.
X25AM implements the lower three layers of the OSI model:
•
Network Layer: CCITT X.25 1980 (enhanced to support the 1984 extended
addressing facility)
•
Data Link Layer: CCITT LAPB
•
Physical Layer: RS-449, RS-232C, X.21, V.35
Figure 1-4. Data Flow Between OSI/TS and Underlying Subsystems
104CDT .CDD
Transport
Network
Data Link
Physical
Layer
TSP
IP
ES-IS
RS-232C,
RS-449, X.21,
V.35
WAN
LAPB
Controllers
802.3 CSMA/CD
Controllers
50-ohm
Baseband
LAN
NSP
NSP