Specifications

4-3
Cisco AS5x00 Case Study for Basic IP Modem Services
11/24/1999
Section 4 Verifying Modem Performance
Background on Asynchronous Data Communications
Logical Packet and Circuit Components of a NAS
The NAS functions as a gateway between two different networks:

A circuit-switched network (for example, the PSTN)

A packet-switched network (for example, the Internet)
The NAS is half a circuit switch and half a packet switch (router). RS-232 signaling on the line is
displayed by the
show line
command and
debug modem
command. Figure 4-2 shows the modem
access connectivity path.
Figure 4-2 Modem Access Connectivity Path
29654
Asynchronous
interfaces
Asynchronous
Characters
Modems
TTY lines
64K DS0
TDM CSM bus
T1 controllers
Packet-
switched
half
Circuit-
switched
half
Circuit network
(PSTN)
Client PC
Client
modem
PSTN interface
(T1, E1, BRI)
Packet interface
(Ethernet, frame
relay,etc.)
Routing and
switching engine
IP
network
3
4
2
1