Configuring the Router

To use the modem, the router must be properly configured to access it. There are two phases of configuration necessary when preparing the router to use the modem. The first is the logical configuration of the asynchronous interface. The second phase is the configuration of the physical characteristics of the asynchronous line.

Once you prepare the router to physically see the modem, you must configure it to use the specific modem attached to it. This final phase can be manual, automatic, or a combination of the two.

Logical Router Configuration

Much confusion stems from the two separate phases of asynchronous configuration on a router. Simply put, the asynchronous interface configuration of the router is the logical information about a connection, such as the IP address, encapsulation mechanism, and authentication.

Before the logical configuration can begin, however, you must have access to at least one asynchronous interface. Depending on the router, you might have to put a serial interface into asynchronous mode. The command to place a serial interface into asynchronous mode is physical-layer {sync | async} issued from interface configuration mode:

 Router(config-if)# physical-layer async 

This command configures the interface to interact with the asynchronous modem, as opposed to waiting for or providing clocking for a synchronous connection.

To verify that an interface is in asynchronous mode, display the current status of the router's lines from privileged exec mode, using the show line command:

 Router#show line    Tty Typ     Tx/Rx    A   Modem  Roty AccO AccI   Uses   Noise  Overruns *    0 CTY               -    -      -    -    -      0       0     0/0      1 TTY 115200/115200 -  inout    -    -    -      0       0     0/0     65 AUX   9600/9600   -    -      -    -    -      0       0     0/0 *   66 VTY               -    -      -    -    -      8       0     0/0     67 VTY               -    -      -    -    -      3       0     0/0     68 VTY               -    -      -    -    -      1       0     0/0     69 VTY               -    -      -    -    -      0       0     0/0     70 VTY               -    -      -    -    -      0       0     0/0 

You must have at least one line showing as TTY to successfully configure a modem.

One important aspect of the logical configuration is the encapsulation. You configure encapsulation with the encapsulation {slip | ppp} command:

 Router(config-if)#encapsulation ppp 

The command configures the router to attempt either Point-to-Point Protocol (PPP) or Serial Line Internet Protocol (SLIP) encapsulation. Cisco routers support only SLIP or PPP encapsulation. The logical configuration is the same you use with ISDN and dial-on-demand routing (DDR).

The importance of putting the serial interface into asynchronous mode cannot be stressed enough. If the serial interface is not in asynchronous mode, it will not be configurable as a line and will be unavailable for use with a modem.

Physical Interface Configuration

The physical configuration of the router interface, which you perform in line config mode, consists of the physical characteristics of the connection. At this point, we define the speed of the connection, flow control, whether the modem is to be used for incoming only or incoming and outgoing calls, and where the database for authentication resides:

 login (local,tacacs) flowcontrol (none,software,hardware) speed (0-4294967295) modem (dialin,inout) 

A basic physical configuration would be as follows:

 Router(config-line)# login local Router(config-line)# flowcontrol hardware Router(config-line)# speed 115200 Router(config-line)# modem inout 

The first line, login local, specifies that the local router database will be used for authentication, as opposed to a Terminal Access Controller Access Control System + (TACACS+) server. Flowcontrol hardware tells the port that the flow control mechanism will be built-in hardware, as opposed to software. The speed 115200 command determines the speed at which the router will send data to the modem. The last command, modem inout, establishes that the modem can be used for both incoming and outgoing calls.

In addition, you can define data bits, stop bits, parity, and which protocols will be passed through the line.

Know the minimum configuration requirements for configuring access to a modem. It must be in asynchronous mode, and it must have one of the modem commands (inout or dialin).

Attaching the Modem

This entire configuration process does us absolutely no good unless the modem is attached to the router. Although it might seem a rather simple task, it is important to know which mechanisms are available to make the connection between devices.

Depending on the interfaces available to you, you can use several cable sets and combinations of cable and adapter to connect the modem and router. The simplest connection uses a serial cable, with the appropriate connector on one end for the router and a 25-pin EIA/TIA-232 male connector on the other end. The male connector attaches to the 25-pin female port on the modem. This configuration works well when you have a small number of modems (one or two) to attach to a single router.

Routers that are explicitly designed to connect to multiple modems are called access servers, and they employ one of two techniques to attach the modems. When attaching more than two modems to a router, you typically use an 8-port "octal" or "octopus" cable. An octal cable has a wide connector, similar to an SCSI3 interface, which plugs into the corresponding port on the router. The other end of the cable has eight RJ-45 connectors that you can either connect directly to specialized modems or adapt with an RJ-45-to-DB-25 male adapter. The alternative to an octal cable and external modems is a bank of internal modems, referred to as modem ISDN channel aggregation (MICA) modems.

The knowledge of the existence of MICA modems should be sufficient to answer most questions relating to them on the exam.

You can make the final connection between a router and modem through the aux port. When connecting the modem through the aux port, you plug a console cable (rollover cable) directly into the aux port. You plug the other end of the cable directly into the modem or into an RJ-45-to-DB-25 male adapter that you plug into the modem.

Although the majority of modems use a 25-pin connector, only 8 pins are used to communicate between the DTE and DCE devices. Table 6.3 shows the pins used on either end of the connection and what each is used for.

Table 6.3. Modem Signaling Pinout

DTE Device	Signal Direction			DCE Device
TxD (Transmit)	2	->	2	TxD
RxD (Receive)	3	<-	3	RxD
Ground	7	<->	7	Ground
RTS	4	->	4	RTS
CTS	5	<-	5	CTS
DTR	20	->	20	DTR
CD	8	<-	8	CD
DSR	6	<-	6	DSR

It is also important to note that on a null-modem cable, the DSR and CD pins are combined into one lead on the cable.

Using the Modem

The final task in configuring an asynchronous connection via a modem is to define which modem the router is connected to. To pass information through the modem, the router must be able to communicate correctly with the modem. The standard by which the router initializes and configures the modem is the Hayes AT command set. The router uses standard AT or ATTENTION commands to interact with most modems; Table 6.4 lists the most common commands. Additionally, you use some nonstandard commands with specific modems to perform specific tasks.

Table 6.4. Standard AT Commands

Command	Result
AT&F	Reloads the factory defaults in the modem.
ATS0=x	Sets S register 0 (auto-answer) to pick up on ring x.
ATE0	Turns ECHO off when typing commands.
ATM0	Turns off the modem speaker.
ATS2=255	Sets S register 2 (in-band signaling) to ignore +++, which would break the connection.
AT&C1	Forces CD to reflect the true line state.
AT&D2	Hangs up the modem at DTR low.

The best resource for any nonstandard commands supported by your modem is the modem documentation.