Specifications

4-4
Cisco AS5x00 Case Study for Basic IP Modem Services
11/24/1999
Section 4 Verifying Modem Performance
Background on Asynchronous Data Communications
To understand the general call-processing sequence, match the following numbered list with the
numbers shown in Figure 4-2:
1.
64K DS0 circuits extend from the NAS modems, through the internal TDM CSM bus, and through
the circuit network (PSTN).
2.
The NAS modems demodulate digital streams into analog-voiceband modulation. The virtual
RS-232 interface connects the modems (DCE) to the TTY lines.
3.
The TTY lines are mapped into asynchronous interfaces. Interfaces are Cisco IOS objects that move
packets. TTY lines function at Layer 1. Interfaces function at Layer 2 and Layer 3.
4.
The packets are delivered into the IP network.
RS-232 in Cisco IOS
The Cisco IOS variation of asynchronous RS-232 is shown in Figure 4-3. The variation exists between
the Cisco IOS line (DTE) and the NAS modem (DCE).

Six RS-232 pins exist between each NAS modem and Cisco IOS line. One or more grounding wires
also exist on physical RS-232 lines; however, these wires do not convey signaling.

Each pin controls a different RS-232 signal.

The arrows in Figure 4-3 indicate the signal transmission direction.
Figure 4-3 Cisco IOS RS-232
Tech Tip
In Figure 4-3, notice that the DSR signal is the DCD signal for the modem. In the scheme
of Cisco IOS, the DCD pin on the DCE is strapped to the DSR pin on the Cisco IOS DTE
side. What the Cisco IOS calls DSR is not DSR; it is DCD. The DCE’s actual DSR pin and
ring ignore (RI) pin are ignored by the Cisco IOS.
28230
TxD
RxD
RTS
CTS
DTR
DCDDSR
RI
DCE
(NAS modem)
DTE
(IOS line)
Inside a Cisco
network access server