Variable-Length Dial Peers
When considering how to implement your Voice over network, you need to
think about the number of digits the router will be dealing with. If you only
have fixed-length dialing where users apply four or five-digit dialing to connect
to other office phones, the creation of dial plans is really quite simple.You need
to know the destination patterns used and build the dial peers based on destination
patterns.
On the other hand, some users will need to have full dialing privileges for all
their calling needs.When this is the case, you need to implement variable-length
dialing plans, something which is bit more complicated.When unsure about the
dialing habits of office users, you are generally left with two options:
You could create a dial plan that includes all possible prefixes and wild
cards to ensure all calls are routed (not fun).
You could implement variable-length dial peers.The router or Cisco
CallManager will then collect the dialed string digits and route them
based on pattern matching (highly recommended).
Remember that fixed-length peers are exactly that, fixed length.They will
always have the same number of digits associated with them whether they are
wildcard digits or just dialed digits. For example, if you only configure your
router or Cisco CallManager for fixed-length dial plans, the digits received by the
router (or Cisco CallManager) must have the appropriate number of dialed digits.
If you set up the router to accept ten-digit calls, the router will only connect
once all ten digits are dialed. If in this scenario you set up a static area code along
with seven digits, and the user doesn’t dial that area code, the call will not be able
to complete because it does not match the dial peer.
Variable-length dial plans allow for the router or Cisco CallManager to
receive inconsistent dialed digits and compare them to its routing table. It can do
this through the configuration of several options. For example there can be the
inclusion of the command destination-pattern with its options.The following
configuration of a variable-length dial peer will hopefully give you some idea of
what we’re talking about, and the explanation that follows should illuminate the
configuration.
dial-peer voice 1 voip \\Sets the dial peer as VoIP
destination-pattern 9T \\dial peer must be matched when the
\\router receives the Number 9 + any
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304 Chapter 9 • AVVID Dial Plans
\\digits, or the call will terminate
session target ip4:10.1.100.1 \\When the dial peer is matched, it will
\\ setup a call to 10.1.100.1
Several characters, used as switches, can be inserted into the destinationpattern
command.The preceding configuration uses the “T” switch, which is a
timeout character.You could also configure a termination character defined with
the command dial peer terminator
for termination, but you can choose other characters. Keep in mind, though, that
this command can only be found on routers that are voice-enabled.To enable the
termination character, you can do any of the following:
Use the “#” as termination character that can be sent from the
Telephony device, thus making it like a Cell phone send key. So the
router would receive the “#” character and know that it need to send all
of the characters that were dialed before the “#” key.
When configuring the voice-port, you can add the command “timeouts
interdigit” and define the amount of time that router or Cisco
CallManager will wait between dialed digits (normally set to 10 seconds
by default) before sending the digits.You may want to configure this for
a smaller interval, as many times users will become inpatient with this
long a wait.
When you install Cisco CallManager within North America, you can use the
“@” character with the creation of route patterns to create variable length dial
plans.This way the user can dial a seven-digit local number, or ten- (area code +
number) and eleven- (1 + area code + number) digit numbers to call long distance.
When the number reaches the last dialed digit, the call will immediately be
placed.The “@” character will not work outside North America, though.
In order to construct variable-length dial plans in the past, you needed to
configure the Cisco CallManager with a router pattern that consisted of 0.!
within the setup. By setting up the wildcard, the Cisco CallManager would then
be able to utilize variable-length dial plans, but it also needed to use the timeout
after the last digit before it would place a call to the gateway.The alternative was
to create variable-length dial plans for the entire national calling-number scheme.
A lot of support issues needed to be addressed to make this feasible, but it
allowed a myriad of calling features and offered users a minimal wait.
For international calls, you will need to implement the wildcard setup, as
North American systems are not designed to match foreign exchanges.