Troubleshooting MGCP and Related Protocol Interfaces to the IP Network
Media gateway control protocol (MGCP) bases its call control and intelligence in centralized call agents, also called media gateway controllers. The call agents issue commands to simple, low-cost endpoints, which are housed in media gateways (MGs), and they also receive event reports from the gateways. MGCP messages between call agents and media gateways are sent over IP/UDP.
For information about configuring MGCP, refer to the MGCP and Related Protocols Configuration Guide.
This chapter contains the following sections:
MGCP Overview
Two basic MGCP constructs are endpoints and connections. An endpoint is a source or sink for call data (RTP/IP) that is flowing through the gateway. A common type of endpoint is found at the physical interface between the POTS or PSTN service and the gateway; this type of endpoint might be an analog voice port or a digital DS0 group. There are other types of endpoints as well, and some are logical rather than physical. An endpoint is identified by a two-part endpoint name that contains the name of the entity on which it exists (for example, an access server or router) and the local name by which it is known (for example, a port identifier).
Call agents manage call flow using standard MGCP commands that are sent to the endpoints under their control. The commands are delivered in standard ASCII text, and can contain session descriptions transmitted in Session Description Protocol (SDP), a text-based protocol. These messages are sent over IP/UDP.
Call agents keep track of endpoint and connection status through the gateway's reporting of standard events that are detected from endpoints and connections. Call agents also direct gateways to apply certain standard signals when a POTS/PSTN connection expects them. For example, when someone picks up a telephone handset, an off-hook event is detected on an endpoint on the residential gateway to which the telephone is connected. The gateway reports the event to a call agent, which orders the gateway to apply the dial-tone signal to the endpoint reporting the off-hook event. The person picking up the handset hears dial tone.
Figure 41 shows a hypothetical MGCP network with both residential and trunking gateways. The residential gateway has telephone sets connected to the gateway's FXS voice ports. MGCP or NCS over IP/UDP is used for call control and reporting to the call agent, and Real-time Transmission Protocol (RTP) is used to transmit the actual voice data.
Figure 41 also shows two trunking gateways with T1 (or E1) connections to the PSTN. Incoming time-division multiplexing (TDM) data is sent through the gateway into the packet network through the use of RTP. MGCP or TGCP over IP/UDP is used for call control and reporting to the call agent. Signaling System 7 (SS7) data travels a different route, however, bypassing the trunking gateway entirely in favor of a specialized signaling gateway, where the signaling data is transformed to ISUP/IP format and relayed to the call agent. Communication between two signaling gateways in the same packet network can be done with ISUP/IP, H.323, or Session Initiation Protocol (SIP).
Figure 41 MGCP Network Model
MGCP Gateway Call Flow
The following example shows an MGCP call flow. The network topology is shown in Figure 42 . In this example, the FXS port is registered as an endpoint to Cisco CallManager.
Figure 42 MGCP Call Flow Topology
User Access Verification
In this example, the show log command is used to show the MGCP packets sent between the Cisco gateway and Cisco CallManager.
Router#
Router#show log
Syslog logging: enabled (0 messages dropped, 1 messages rate-limited, 0 flushes,
0 overruns, xml disabled)
Console logging: level debugging, 280 messages logged, xml disabled
Monitor logging: level debugging, 109 messages logged, xml disabled
Buffer logging: level debugging, 69 messages logged, xml disabled
Logging Exception size (4096 bytes)
Count and timestamp logging messages: disabled
Trap logging: level informational, 39 message lines logged
Log Buffer (10000000 bytes):
The FXS port goes off hook and notifies the CallAgent of the status through observed event. Here, the MGCP packet is sent by the gateway.
*Mar 1 02:16:29.399: send_mgcp_msg, MGCP Packet sent to 172.6.104.54
*Mar 1 02:16:29.399: NTFY 186 aaln/S1/SU0/0@Router MGCP 0.1
X: 1b
O: L/hd
Here, the gateway receives the response from the CallAgent.
*Mar 1 02:16:29.399: MGCP Packet received from 172.6.104.54-
200 186
The CallAgent directs the port to provide the dial tone and requests a notification if there is any change of events such as a port disconnect or digits received.
*Mar 1 02:16:29.411: MGCP Packet received from 172.6.104.54-
RQNT 40 AALN/S1/SU0/0@Router MGCP 0.1
X: 1c
R: L/hu, D/[0-9ABCD*#]
S: L/dl
Q: process,loop
*Mar 1 02:16:29.419: send_mgcp_msg, MGCP Packet sent to 172.6.104.54
The FXS port (endpoint) notifies the Call Agent about the digits that it received.
*Mar 1 02:16:29.419: 200 40 OK
*Mar 1 02:16:33.595: send_mgcp_msg, MGCP Packet sent to 172.6.104.54
*Mar 1 02:16:33.595: NTFY 187 AALN/S1/SU0/0@Router MGCP 0.1
X: 1c
O: D/1
*Mar 1 02:16:33.599: MGCP Packet received from 172.6.104.54-
200 187
*Mar 1 02:16:33.603: MGCP Packet received from 172.6.104.54-
RQNT 41 AALN/S1/SU0/0@Router MGCP 0.1
X: 1d
R: L/hu, D/[0-9ABCD*#], L/hf
S:
Q: process,loop
*Mar 1 02:16:33.607: send_mgcp_msg, MGCP Packet sent to 172.6.104.54
*Mar 1 02:16:33.607: 200 41 OK
*Mar 1 02:16:35.655: send_mgcp_msg, MGCP Packet sent to 172.6.104.54
*Mar 1 02:16:35.655: NTFY 188 AALN/S1/SU0/0@Router MGCP 0.1
X: 1d
O: D/0
*Mar 1 02:16:35.659: MGCP Packet received from 172.6.104.54-
200 188
*Mar 1 02:16:37.275: send_mgcp_msg, MGCP Packet sent to 172.6.104.54
*Mar 1 02:16:37.275: NTFY 189 AALN/S1/SU0/0@Router MGCP 0.1
X: 1d
O: D/0
*Mar 1 02:16:37.279: MGCP Packet received from 172.6.104.54-
200 189
*Mar 1 02:16:38.815: send_mgcp_msg, MGCP Packet sent to 172.6.104.54
*Mar 1 02:16:38.815: NTFY 190 AALN/S1/SU0/0@Router MGCP 0.1
X: 1d
O: D/1
<---
*Mar 1 02:16:38.819: MGCP Packet received from 172.6.104.54-
200 190
*Mar 1 02:16:38.839: MGCP Packet received from 172.6.104.54-
CRCX 42 AALN/S1/SU0/0@Router MGCP 0.1
C: A00000000200001d
X: 1e
M: inactive
R: L/hu
Q: process,loop
*Mar 1 02:16:38.851: send_mgcp_msg, MGCP Packet sent to 172.6.104.54
*Mar 1 02:16:38.851: 200 42 OK
I: 4
v=0
c=IN IP4 172.16.13.41
m=audio 19156 RTP/AVP 0 8 99 101 102 2 15 103 4 104 105 106 107 18 100
a=rtpmap:99 G.729a/8000
a=rtpmap:101 G.726-16/8000
a=rtpmap:102 G.726-24/8000
a=rtpmap:103 G.723.1-H/8000
a=rtpmap:104 G.723.1-L/8000
a=rtpmap:105 G.729b/8000
a=rtpmap:106 G.723.1a-H/8000
a=rtpmap:107 G.723.1a-L/8000
a=rtpmap:100 X-NSE/8000
a=fmtp:100 200-202
a=X-sqn:0
a=X-cap: 1 audio RTP/AVP 100
a=X-cpar: a=rtpmap:100 X-NSE/8000
a=X-cpar: a=fmtp:100 200-202
a=X-cap: 2 image udptl t38
<---
*Mar 1 02:16:38.855: MGCP Packet received from 172.6.104.54-
RQNT 43 AALN/S1/SU0/0@Router MGCP 0.1
X: 1f
R: L/hu
S: G/rt
Q: process,loop
At this time the call is sent to the IP Phone. The CA directs the IP Phone to start playing ringbacks.
*Mar 1 02:16:38.859: send_mgcp_msg, MGCP Packet sent to 172.6.104.54
*Mar 1 02:16:38.859: 200 43 OK
<---
*Mar 1 02:16:46.159: MGCP Packet received from 172.6.104.54-
MDCX 44 AALN/S1/SU0/0@Router MGCP 0.1
C: A00000000200001d
I: 4
X: 20
L: p:20, a:PCMU, s:off
M: recvonly
R: L/hu
Q: process,loop
*Mar 1 02:16:46.167: send_mgcp_msg, MGCP Packet sent to 172.6.104.54
*Mar 1 02:16:46.167: 200 44 OK
<---
*Mar 1 02:16:46.171: MGCP Packet received from 172.6.104.54-
RQNT 45 AALN/S1/SU0/0@Router MGCP 0.1
X: 21
R: L/hu, D/[0-9ABCD*#], L/hf
S:
Q: process,loop
*Mar 1 02:16:46.175: send_mgcp_msg, MGCP Packet sent to 172.6.104.54 --->
*Mar 1 02:16:46.175: 200 45 OK
*Mar 1 02:16:46.179: MGCP Packet received from 172.6.104.54-
MDCX 46 AALN/S1/SU0/0@Router MGCP 0.1
C: A00000000200001d
I: 4
X: 22
L: p:20, a:PCMU, s:off
M: sendrecv
R: L/hu, L/hf, D/[0-9ABCD*#]
S:
Q: process,loop
v=0
o=- 4 0 IN EPN AALN/S1/SU0/0@Router
s=Cisco SDP 0
t=0 0
c=IN IP4 10.17.179.33
m=audio 28390 RTP/AVP 96
a=rtpmap:96 PCMU
*Mar 1 02:16:46.187: send_mgcp_msg, MGCP Packet sent to 172.6.104.54
*Mar 1 02:16:46.191: 200 46 OK
Now the call is answered. Note that media is cut in both directions.After about 12 seconds the called party on the IP Phone drops the call.
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