SIP MWI Mechanisms used by CUE: “Outcalling”

Cisco Unity Express uses three type of Message Waiting Indication mechanisms when integrated with a CME or SRST router. All 3 mechanism used are based upon the SIP protocol. Contrast this with the way CUE updates the MWI status on phones registered to UCM- in this instance JTAPI is used.

The focus on this particular article is a technique known as outcalling. As the name suggests the CUE module simply makes a call to the CME router using SIP. An INVITE message is sent to the CME gateway. A copy of this INVITE message is shown below (debug ccsip messages).

In this instance we have the MWI numbers created as ephone-dn’s within the CME configuration, as shown below.

ephone-dn 3
 mwi on
 number 3999….
!
ephone-dn 4
 mwi off
 number 3998….

This information is imported from the CME and into the CUE module when the CUE is being set up. The picture above shows CUE informing the CME that extension 3006 has a message waiting and that the CME should light the MWI lamp on the phone which has extension 3006 configured.

The CUE configuration related to MWI is shown below:

Note: We are focusing on the outcalling method in this particular article but there will also be a secondary notification using the NOTIFY message.

The outcalling procedure is reserved for SCCP phones. The CME router needs to receive the incoming MWI INVITE message from the CUE and pass this onto the ephone-dn which contains the mwi on statement. Remember ephone-dn’s are merely branded POTS dial-peers. Taking a look at our ephone-dn configuration from earlier, you will see the following dial-peers after issuing a show telephony-service dial-peer command:

dial-peer voice 40002 pots
destination-pattern 3999….$
port 50/0/2
!
dial-peer voice 40003 pots
destination-pattern 3998….$
port 50/0/3

If we use the debug voip dialpeer command we should see the outgoing dial-peer in this example being set to 40002. Once the outgoing dial-peer has been correctly matched, the mwi on procedure is invoked and the light for 3006 will be lit. However the following dial-peer shown below will cause problems:

dial-peer voice 1 voip
incoming called-number .
 codec g729r8
dtmf-relay h245-alphanumeric

This dial-peer is very much required in order for the CME to receive calls from over the WAN (using gatekeeper or from UCM). Also the codec here is shown for information purposes since this is the default codec within a dial-peer.

One of the major problems is that the INVITE being sent by the CUE module contains Session Description information (SDP). A codec is specified within the SDP portion of the INVITE message (although logically speaking no codec is required since this is an MWI update and there is no resultant active call). The codec specified from CUE will always be g711ulaw. The inbound dial-peer on the CME is dial-peer 1 which has the codec set to g729r8. Consequently there is a codec mismatch and the call will fail (dial-peer 40002 will never be selected because the call/mwi update is failing on the inbound call leg and there is no subsequent outbound call leg which is housing the mwi on/off procedures).

The solution is not to change the codec in dial-peer 1 since this will have an impact on calls being received from the WAN. Instead you can create another dial-peer which is more specific for the MWI #’s as shown below:

dial-peer voice 2 voip
incoming called-number 399[89]….
codec g711u

Note: the inbound dial-peers support either the SIP or H323 protocol and does not need to be defined.

Going back to the picture of the INVITE message, a debug voip dialpeer will show the CME gateway match dial-peer 2 as the inbound dial-peer and 40002/3 as the outbound dial-peer whenever there is a MWI update being sent from the CUE using the outcalling method.

–
Vik Malhi – CCIE #13890
Instructor – IPexpert, Inc.
Mailto: vmalhi@ipexpert.com

SIP MWI Mechanisms used by CUE: "Outcalling", 4.3 out of 5 based on 3 ratings

Tags: ccie voice, cue model, sip mwi, sip protocol