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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.
� A �
4
IP
Asynchronous
network
interfaces
Packet-
Routing and
3
switched
switching engine
Packet interface
half
(Ethernet, frame
relay,etc.)
Asynchronous
Characters
TTY lines
2
Modems
Circuit-
switched
64K DS0
half
TDM CSM bus
1
T1 controllers
PSTN interface
(T1, E1, BRI)
Circuit network
Client
(PSTN)
Client PC modem
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To understand the general call-processing sequence, match the following numbered list with the
numbers shown in Figure 4-2:
64K DS0 circuits extend from the NAS modems, through the internal TDM CSM bus, and through
the circuit network (PSTN).
The NAS modems demodulate digital streams into analog-voiceband modulation. The virtual
RS-232 interface connects the modems (DCE) to the TTY lines.
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.
The packets are delivered into the IP network.
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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.
� l S S
Inside a Cisco
network access server
TxD
RxD
RTS
CTS
DTR
DSR DCD
RI
DCE DTE
(NAS modem) (IOS line)
i 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.
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Table 4-1 describes how Cisco uses it s RS-232 pins. The signal direction in the table is from the
perspective of the DTE (IOS line):
" Data signals (TxD, RxD)
" Hardware flow control signals (RTS, CTS)
" Modem signals (DTR, DSR, DCD, RI)
I S S I S �
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Transmit Data (TxD)   > DTE transmits data to DCE.
(Output)
Receive Data (RxD)
(Input)
Request To Send (RTS)   > DTE uses the RTS output signal to indicate if it can
(Output) receive characters into the Rx input buffer1.
The DCE should not send data to the DTE when DTR
input is low (no RTS).
Clear To Send (CTS)
(Input) into its buffers.
DCE asserts CTS only if the DCE is able to accept
data.
Data Terminal Ready   > DTE signals to DCE that it can continue to accept data
(DTR) (Output) into its buffers.
DTE asserts RTS only if the DTE is able to accept
data.
Data Carrier Detect
(DCD) (Input) remote modem. Dropping DCD terminates the
session.
DCD will be up on the DCE only if the DCE has
achieved data mode with its peer DCE (client
modem).
1. The name RTS is illogical with the function (able to receive) due to historical reasons.
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To display the current modem-hardware states applied to a specific Cisco IOS line, enter the show line
tty number command. The states of each logical RS-232 pin change according to line conditions and
modem events.
The following shows a line-side inspection of the idle state for TTY line 1:
5300-NAS#show line tty 1
Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns Int
I 1 TTY - inout - - - 2 0 0/0 -
Line 1, Location:"", Type:""
Length:24 lines, Width:80 columns
Status:No Exit Banner
Capabilities:Hardware Flowcontrol In, Hardware Flowcontrol Out
Modem Callout, Modem RI is CD, Line usable as async interface
Integrated Modem
Modem state:Idle
modem(slot/port)=1/0, state=IDLE [ Pobierz całość w formacie PDF ]