[cisco-voip] Modem Passthrough Dramas

[Drewbles]

Hi,

I have a client that is currently planning a big bang cut over from an
Ericsson MD110 to CUCM 9.1. I have recently found out that they have
hundreds of modems that are connected to analog lines hanging off the
MD110. These are dialled from other internal modems and also external
parties calling into the modems directly via DID. During and after the
migration to CUCM they will be leaving the modems on the analog lines
connected to the MD110 and will be slowly moved over later on.

I have configured the gateways for modem passthrough and it seems to be
working. I can see some calls switchover (using 'debug voip rtp session
named-event' and seeing event 192 & 193) but some other will not negotiate.
At the moment calls to and from the MD110 are via two 3925 gateways using
multiple E1s to the MD110 and SIP to CUCM. These gateways also provide
connectivity to the PSTN. The problems calls are dialling the modems from a
modem connected to a VG2XX. The VG224/VG204s are running SCCP.

I can all calls negotiate modem passthrough by looking at the output of
'show call active voice brief' while the modems are negotiating. This is
from the terminating gateway (3925) of a failed call:

0    : 179 -1510115396ms.1 (16:31:39.074 AEST Mon Mar 3 2014) +0 pid:0
Originate  connecting

dur 00:00:28 tx:1409/224504 rx:1408/224344 dscp:0 media:0 audio tos:0xB8
video tos:0x0

IP 172.28.100.1:24898 SRTP: off rtt:2ms pl:25190/0ms lost:0/0/0
delay:65/65/65ms g711ulaw TextRelay: off Transcoded: No

media inactive detected:n media contrl rcvd:n/a timestamp:n/a

long duration call detected:n long duration call duration:n/a timestamp:n/a

LostPacketRate:0.00 OutOfOrderRate:0.00

  MODEMPASS nse buf:0/0 loss 0% 0/0  last 2519s dur:0/0s

For example a call that works is:
[PSTN modem] --E1--> [3925] --E1--> [MD110] --> [modem] - 100% success rate

If I then dial from a modem connected to a VG204 to the same modem
connected to the MD110 it fails. Call flow is:
[VG204 modem] --SCCP--> [CUCM] --SIP--> [3925] --E1--> [MD110] --> [modem]
- 100% failure

However if I dial the PSTN modem in the first example from the VG204 modem
the call is successfully:
[VG204 modem] --SCCP--> [CUCM] --SIP--> [3925] --E1--> [PSTN modem]

I don't have specific dial peers configured on the VG204 or the 3925
gateway configured for fax/modem but I do have the following global
commands:

*VG204*
voice service voip
 modem passthrough nee codec g711ulaw
!
stcapp register capability 0/0 modem-passthrough

*3925*
voice service voip
 fax protocol t38 nse version 0 ls-redundancy 0 hs-redundancy 0 fallback
pass-through g711ulaw
 modem passthrough nee codec g711ulaw

I do have fax relay configured on the 3925 but I have also tried changing
it to fax passthrough with no change in the behaviour of the modems calls.

I know it's a big long winded but if anyone can provide feedback or
suggestions that would be much appreciated. Happy to provide debugs.


Cheers,

Drew.
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