Specifications

4-2
Cisco AS5x00 Case Study for Basic IP Modem Services
11/24/1999
Section 4 Verifying Modem Performance
Background on Asynchronous Data Communications
Async DataComm Model
Figure 4-1 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 RS-232 and Cisco IOS:

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. RS-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 4-1 A Standard Dial-Up Connection
DTE
(Cisco IOS line)
28074
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