Specifications

3-2
Cisco AS5800 Operations, Administration, Maintenance, and Provisioning Guide
DOC-7810814=
Chapter 3 Operations
Verifying Modem Performance
Async DataComm Model
Figure 3-1 shows how traditional DTE-to-DCE relationships map to a Cisco network access server
(NAS). Data terminal equipment (DTE) uses data communication equipment (DCE) to send data over
the PSTN.
In the context of EIA/TIA-232 and Cisco IOS software:
• The DTE is the client PC and the Cisco IOS TTY lines.
• The DCE is the client modem and the modem inside the NAS.
• The dashed line between the DCEs is the modem carrier running on top of the voiceband circuit
through the PSTN. EIA/TIA-232 (whether physical or logical) is used on the DTE lines, not on the
DCE link.
• The PSTN circuit runs through the circuit-switched half of the NAS.
Figure 3-1 A Standard Dialup Connection
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). EIA/TIA-232 signaling on the line is
displayed by the show line command and debug modem command. Figure 3-2 shows the modem access
connectivity path.
DTE
(Cisco IOS line)
33177
DTE
(Client PC)
DCE
(Client modem)
DCE link (circuit)
PSTN
DTE link
DTE link
Physical
RS-232 cable
Virtual
RS-232 interface
IOS line (DTE)
DCE
(NAS modem)
Inside a Cisco
network access server