Subject: RE: [Megaco] Megaco Vs. MGCP digitmap questions

From: "Kevin Boyle" <[email-masked]>
Date: Wed, 9 Jan 2008 10:32:28 -0500

Comments inline. [KJBII]
 
Kevin

________________________________

From: Ramesh Babu Kuppili [mailto:[email-masked]] 
Sent: Thursday, December 20, 2007 3:14 PM
To: [email-masked]
Cc: Steven Schofield
Subject: [Megaco] Megaco Vs. MGCP digitmap questions

Hello,
 
I have a question about the difference between MGCP and Megaco
Digitmaps.
 
Having read about MGCP digitmap (RFC 3435) and Megaco digitmap (RFC
3525), there are major differences in the way the digitmaps are
processed in these two
procotols.  I would like the MGCP/Megaco gurus to confirm if my
understanding of the RFC is correct.
[KJBII] Note that the Megaco RFC is a number of years out of date.  I
suggest referring to ITU-T Recommendation H.248.1. 
 
1.  MGCP requires that an exact match completes the dialing, even if the
rule is ambiguous.  For example, a digitmap like (0|00) will match '0'
and never get to '00'.  For
            '00' to match the digitmap must be (0T|00). 
    Megaco requires that an exact match to wait if it is ambiguous.  For
example, a digitmap like (0|00) will not match and go on '0'.  On the
first '0', it starts the inter digit
            timer to see if '00' will be dialed.  In megaco it seems
that we cannot have a fast match in the real world.  For example, how
would we create a digit map that can
            fast match for 911 and also be able to do seven digit
dialing for numbers that start with 912, 921 etc.
[KJBII] This can be achieved through careful DigitMap construction.
Note that H.248.16 contains some alternative DigitMap matching rules
that allow the specification of "shortest possible match", rather than
the "longest possible match" rules that are defined in the base spec. 
 
2. MGCP can only specify T timer.  Megaco can specify S (short timer)
and L (Long timer).
[KJBII] This is incomplete.  H.248 has four digit timers: T is the start
timer, S is the short interdigit timer, L is the long interdigit timer
and Z is the long digit duration timer. 
 
3. In Megaco given the digitmap
(1xx|9xx|0[2-9]xxx|00[1-9]xxxx|[2-8]xxxxxx|0[2-9]xxxxxxx|05xxxxxxxx|x.F|
E|F|[0-9].L) and the user dials *26#.  
        My understanding is that we will
        have a unambiguous match as soon as the user dials '*'
(asterisk) as the fist digit.  So we send a notify with '*' (asterisk)
and clear the dialled digit buffer.  Then
        when the user dials "26#" we have an unambigous match for x.F
and we send a notify for 26#.  Please let me know if my understanding is
correct.
[KJBII] Actually, this is not exactly correct.  The '*' is reported
immediately as you say.  The remaining digits are buffered pending the
re-arming of the DigitMap completion event or the request for individual
digits.  If the one of these two things do not occur, the digits are
discarded at the end of the buffering time specified in the previous
DigitMap completion event or upon receipt of an Events Descriptor that
does not contain digits or a DigitMap completion event.  See H.248.1
Clause 7.1.14.4.

- ramesh
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