Wednesday, 3 February 2016


MGCP TGW with PRI interfaces introduced new concept which is MGCP Backhaul. Another connection will be established between MGCP GW and CUCM over port TCP 2428. This connection will pass Q931 messages received over PRI to CUCM for processing. MGCP GW won't process Q931 messages locally. It will only terminate Q921 messages from PRI.

  1. The gateway and the PSTN switch exchange ISDN Q.921 Layer 2 information. This registers the gateway with the ISDN switch and ensures that the ISDN connection remains active. Q.921 information stops at the gateway.

mgcp-gateway#sh isdn status
Global ISDN Switchtype = primary-ni

%Q.931 is backhauled to CCM MANAGER 0x0003 on DSL 0. Layer 3 output may not apply

ISDN Serial0/0/0:15 interface
        dsl 0, interface ISDN Switchtype = primary-net5
        L2 Protocol = Q.921 0x0000  L3 Protocol(s) = CCM MANAGER 0x0003
    Layer 1 Status:
    Layer 2 Status:
        TEI = 0, Ces = 1, SAPI = 0, State = MULTIPLE_FRAME_ESTABLISHED
    Layer 3 Status:
        0 Active Layer 3 Call(s)
    Active dsl 0 CCBs = 0
    The Free Channel Mask:  0xFFFF7FFF
    Number of L2 Discards = 0, L2 Session ID = 14
    Total Allocated ISDN CCBs = 0

  1. An ISDN Q.931 Setup call-control message arrives. The gateway puts it in a packet addressed to TCP port 2428 and sends it to the CallManager using PRI Backhaul. The CallManager responds by instructing the gateway to create a connection. The gateway sends an acknowledgement that contains SDP information about its IP, Port, Codec, DTMF Relay, etc

Jan 27 21:48:21 MST: MGCP Packet received from>
CRCX 945 S0/SU0/DS1-0/ MGCP 0.1
C: D000000002924996000000F500000001      !!! … Unique Call ID
X: 1     !!! … Unique RequestIdentifier
L: p:20, a:PCMU, s:off, t:00     !!! … Local Connection Options
M: recvonly     !!! … Call Mode
R: D/[0-9ABCD*#]     !!! … RequestedEvents
Q: process,loop
<--- p="">

Jan 27 21:48:21 MST: MGCP Packet sent to>
200 945 OK
I: 6     !!! … MGCP GW will provide CUCM with unique ConnectionId which will be created between MGCP GW and Phone

c=IN IP4
m=audio 16408 RTP/AVP 0 100
a=rtpmap:100 X-NSE/8000
a=fmtp:100 192-194

  1. The CallManager then sends three Q.931 messages: Call Proceeding, Alerting, and Connect through the gateway to the PSTN. These messages let the other side of the connection know that CallManager is placing the call, and then that the call has been answered.

We are assuming that Ringback will be provided by Telco to caller and CUCM will not cut-through MGCP gateway to get Ringback from CUCM. If Ringback will be provided by CUCM, CUCM will send MDCX message to MGCP Gateway to provide the Details of ANN Server. MGCP Gateway will connect to ANN Server to get Ringback and send it across to caller over PSTN

  1. After the call is brought up, the CallManager sends the gateway an MGCP MDCX message providing the details of the phone (IP, Port, Codec, etc)

Jan 27 21:48:22 MST: MGCP Packet received from>
MDCX 947 S0/SU0/DS1-0/ MGCP 0.1
C: D000000002924996000000F500000001
I: 6
X: 1
L: p:20, a:G.729, s:off, t:b8
M: sendrecv
R: D/[0-9ABCD*#]
Q: process,loop

o=- 6 0 IN EPN S0/SU0/DS1-0/
s=Cisco SDP 0
t=0 0
m=audio 19160 RTP/AVP 18
c=IN IP4
<--- p="">

Jan 27 21:48:22 MST: MGCP Packet sent to>
200 947 OK
<--- p="">

  1. The CallManager gets Q931 acknowledgement of the connection from the other side of the call and RTP stream will be flowing between endpoints

a. Backhaul messages are having DSCP value AF31 by default (use the command 'show mgcp | i dscp')
b. In case CUCM fails, MGCP GW ISDN PRI D-Channel won't come up (it will always show status=TEI ASSIGNED). This is because backhaul connection is down; therefore; Q931 messages are failing.

When one of the parties terminates the call:

  1. CUCM signals MGCP GW to terminate the connection using DLCX message

Jan 27 21:32:06 MST: MGCP Packet received from>
DLCX 729 S0/SU0/DS1-0/ MGCP 0.1
C: D0000000029246b2000000F500000004
I: 5
X: 1f
<--- p="">

  1. MGCP Gateway will acknowledge the message using 250OK and which includes call statistics report sent to CUCM

Jan 27 21:32:06 MST: MGCP Packet sent to>
250 729 OK
P: PS=11674, OS=233480, PR=11679, OR=233448, PL=3, JI=10, LA=55
<--- p="">

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