6.1 Dialer Rotary
6.1.3 Dealing with dialer timers
Because DDR links are considered temporary, some timers are necessary to regulate the connection for operations, such as the length of time a connection stays up if there is contention for using the same line.

After placing a call, the router must be told how long to keep the connection up when it has not seen an interesting packet for a certain amount of time. The dialer idle-timeout command specifies the idle time (in seconds) before the line is disconnected. Every time the router processes an interesting packet, it resets this timer. The default is 120 seconds. This command, which is used on lines for which there is no contention, applies to inbound and outbound calls. This is an inactivity timer. The syntax for the dialer idle-timeout command follows:

Router(config-if)#dialer idle-timeout seconds

Intricacies of Idle Timers

The answering router also starts an idle-timeout timer as soon as it answers a call, and can break your connection after 120 seconds (default timer is for 120 seconds). You can fix this by configuring a very high idle-timeout value on the answering router, defining interesting traffic with a dialer-list statement, and configuring the corresponding dialer group on the answering interface (so the answering router knows what is interesting traffic and can reset its timer).

The dialer fast-idle command specifies the amount of time that a connected line remains idle before it is disconnected to allow a second call, which is destined for a second location over this same line, to be placed. This command, which is used on lines for which there is contention, applies to inbound and outbound calls. The line is considered idle when no interesting packets are being sent across it. If the line becomes idle for the configured length of time, the current call is disconnected immediately and the line is available for new calls. The default fast-idle time is 20 seconds. This is an inactivity timer for contended interfaces. The syntax for the dialer fast-idle command is as follows:

Router(config-if)#dialer fast-idle seconds

On local calls, analog modems can take 20 to 30 seconds to synchronize to each other, including the time to dial and answer. Also, international calls take longer than local calls to connect. The dialer wait-for-carrier-time command (as follows) specifies how long (in seconds) to wait for carrier tone. On asynchronous interfaces, this command sets the total time allowed for the chat-script to run. The default time is 30 seconds. For asynchronous lines, it is better to increase the value of this parameter to 60 seconds to compensate for the possible delay in the telephone network.

Router(config-if)#dialer wait-for-carrier-time seconds

As you saw in the previous chapter, some configurations are required on only some category of interfaces. An example of this is the SPID configuration, required only for ISDN BRI interfaces. Similarly, specific configuration is required when performing DDR with async interfaces.

The dialer in-band command enables DDR and V.25bis dialing on the dialer or async interface. V.25bis is an ITU-T standard for in-band signaling to bit synchronous data communications equipment (DCE) devices. A variety of devices support V.25bis, ranging from analog V.32 modems to ISDN terminal adapters to inverse multiplexers.

The syntax for the dialer in-band command is as follows:

Router(config-if)#dialer in-band

Other examples of peculiar commands are isdn incoming-voice modem and interface group-async.

The isdn incoming-voice modem command is used to configure the D channel to switch incoming analog calls to the internal modems. The syntax for the isdn incoming-voice modem command is as follows:

Router(config)#interface serial 1/0:23
Router(config-if)#isdn incoming-voice modem

The interface group-async command is used to create an asynchronous group interface, which can be associated with other asynchronous interfaces. This association allows you to configure the group interface and all interfaces of its members with a single command entered at the asynchronous group interface command line. Although you can have more than one group interface on a router, a member interface can be associated with only one group. The syntax of commands for creating a group-async, interface group-async, and to associate members, group-range, are as follows:

Router(config)#interface group-async 1
Router(config-if)#group-range 65 70