| 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
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