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The Call Agent can ask the gateway to collect digits dialed by the
user. This facility is intended to be used with residential gateways
to collect the numbers that a user dials; it can also be used with
trunking gateways and access gateways alike, to collect access codes,
credit card numbers and other numbers requested by call control
services.
One procedure is for the gateway to notify the Call Agent of each
individual dialed digit, as soon as they are dialed. However, such a
procedure generates a large number of interactions. It is preferable
to accumulate the dialed numbers in a buffer, and to transmit them in
a single message.
The problem with this accumulation approach, however, is that it is
hard for the gateway to predict how many numbers it needs to
accumulate before transmission. For example, using the phone on our
desk, we can dial the following numbers:
------------------------------------------------------
| 0 | Local operator |
| 00 | Long distance operator |
| xxxx | Local extension number |
| 8xxxxxxx | Local number |
| #xxxxxxx | Shortcut to local number at|
| | other corporate sites |
| *xx | Star services |
| 91xxxxxxxxxx | Long distance number |
| 9011 + up to 15 digits| International number |
------------------------------------------------------
The solution to this problem is to have the Call Agent load the
gateway with a digit map that may correspond to the dial plan. This
digit map is expressed using a syntax derived from the Unix system
command, egrep. For example, the dial plan described above results
in the following digit map:
(0T|00T|\[1-7]xxx|8xxxxxxx|#xxxxxxx|*xx|91xxxxxxxxxx|9011x.T)
The formal syntax of the digit map is described by the DigitMap rule
in the formal syntax description of the protocol (see Appendix A) -
support for basic digit map letters is REQUIRED while support for
extension digit map letters is OPTIONAL. A gateway receiving a digit
map with an extension digit map letter not supported SHOULD return
error code 537 (unknown digit map extension).
A digit map, according to this syntax, is defined either by a (case
insensitive) "string" or by a list of strings. Each string in the
list is an alternative numbering scheme, specified either as a set of
digits or timers, or as an expression over which the gateway will
attempt to find a shortest possible match. The following constructs
can be used in each numbering scheme:
* Digit: A digit from "0" to "9".
* Timer: The symbol "T" matching a timer expiry.
* DTMF: A digit, a timer, or one of the symbols "A", "B", "C",
"D", "#", or "*". Extensions may be defined.
* Wildcard: The symbol "x" which matches any digit ("0" to "9").
* Range: One or more DTMF symbols enclosed between square brackets
("\[" and "]").
* Subrange: Two digits separated by hyphen ("-") which matches any
digit between and including the two. The subrange
construct can only be used inside a range construct,
i.e., between "\[" and "]".
* Position: A period (".") which matches an arbitrary number,
including zero, of occurrences of the preceding
construct.
A gateway that detects events to be matched against a digit map MUST
do the following:
1) Add the event code as a token to the end of an internal state
variable for the endpoint called the "current dial string".
2) Apply the current dial string to the digit map table, attempting a
match to each expression in the digit map.
3) If the result is under-qualified (partially matches at least one
entry in the digit map and doesn't completely match another
entry), do nothing further.
If the result matches an entry, or is over-qualified (i.e., no
further digits could possibly produce a match), send the list of
accumulated events to the Call Agent. A match, in this
specification, can be either a "perfect match," exactly matching one
of the specified alternatives, or an impossible match, which occurs
when the dial string does not match any of the alternatives.
Unexpected timers, for example, can cause "impossible matches". Both
perfect matches and impossible matches trigger notification of the
accumulated digits (which may include other events - see Section
2.3.3).
The following example illustrates the above. Assume we have the
digit map:
(xxxxxxx|x11)
and a current dial string of "41". Given the input "1" the current
dial string becomes "411". We have a partial match with "xxxxxxx",
but a complete match with "x11", and hence we send "411" to the Call
Agent.
The following digit map example is more subtle:
(0\[12].|00|1\[12].1|2x.#)
Given the input "0", a match will occur immediately since position
(".") allows for zero occurrences of the preceding construct. The
input "00" can thus never be produced in this digit map.
Given the input "1", only a partial match exists. The input "12" is
also only a partial match, however both "11" and "121" are a match.
Given the input "2", a partial match exists. A partial match also
exists for the input "23", "234", "2345", etc. A full match does not
occur here until a "#" is generated, e.g., "2345#". The input "2#"
would also have been a match.
Note that digit maps simply define a way of matching sequences of
event codes against a grammar. Although digit maps as defined here
are for DTMF input, extension packages can also be defined so that
digit maps can be used for other types of input represented by event
codes that adhere to the digit map syntax already defined for these
event codes (e.g., "1" or "T"). Where such usage is envisioned, the
definition of the particular event(s) SHOULD explicitly state that in
the package definition.
Since digit maps are not bounded in size, it is RECOMMENDED that
gateways support digit maps up to at least 2048 bytes per endpoint.
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