Media Gateway Control Protocol Gateways

MGCP is a plain-text protocol used by call-control devices to manage IP telephony gateways.

MGCP (IETF RFC 2705) is a master/slave protocol that allows a call-control device (CUCM) to take control of a specific port on a gateway. MGCP has the advantage of centralized gateway administration and provides for scalable IP telephony solutions. CUCM controls the MGCP state of the configured ports of the gateway. MGCP allows centralized administration of the dial plan and gives CUCM

per-port control of all gateway interfaces. The gateway interfaces are traditional time-division multiplexing (TDM) interfaces, including analog and digital interfaces, which can interconnect PBXs, voice-mail systems, plain old telephone service (POTS) phones, and other traditional device connections. MGCP is implemented with the use of a series of plaintext commands (Session Description Protocol [SDP]) sent over User Datagram Protocol (UDP) port 2427 from CUCM to the gateway.

It is also important to note that for an MGCP interaction to take place with CUCM, the gateway and gateway interface must have CUCM support. Use the Feature Navigator tool (http://www.cisco.com/go/fn) to make sure that the router or switch platform version of Cisco IOS Catalyst Operating System (CatOS) software is compatible with the CUCM version MGCP features.

Endpoint Identifiers

The MGCP call agent (CUCM) directs its commands to the gateway to manage an endpoint or a group of endpoints on the device. Endpoint identifiers address individual endpoint interfaces.

Endpoint identifiers consist of two logical parts. The first part is the local name of the endpoint (hostname of fully qualified domain name). The two parts are separated by an at symbol (@). If the local part represents a hierarchy, the subparts of the hierarchy are separated by a slash (/). The local ID may be representative of a particular gateway/circuit number, and the circuit number may in turn be representative of a circuit ID/channel number.

Figure 10-1 shows a gateway with two endpoint interfaces. The show voice port summary Cisco IOS command would display the hardware T1 or E1 voice WAN interface card (VWIC) in slot 1, subslot 1, port 1 (1/1/1) and the Foreign Exchange Station (FXS) analog interface in slot 2, subslot 1, port 1 (2/1/1). The T1 or E1 card provides the ability to trunk 24 (T1-CAS), 23 (T1-PRI), or 30 (E1) calls to the PSTN. The configuration of the T1 or E1 interface is done primarily in CUCM for MGCP gateways. The first of CUCM's two configured endpoints (S1/SU1/[email protected]) indicates that the T1 interface was configured as an ISDN PRI using the DS1 (digital signal level 1) signaling specification. The second interface is an analog access line number (AALN) that indicates the interface is an FXS or Foreign Exchange Office (FXO) port. An FXS port generates dial tone to the device plugged into the RJ-11 port. End-user devices such as fax machines and analog phones are normally plugged into FXS ports. FXO RJ-11 ports accept dial tone and are normally connected to the PSTN to make and receive phone calls. MGCP gateways do not support caller ID services on FXO ports. If caller ID over analog FXO interfaces is a requirement, H.323 gateways should be used. H.323 is covered in a later chapter.

Figure 10-1 MGCP Endpoint Nomenclature

Slot 2 Subunit 1 Hostname

S1/SU1/[email protected] AALN/S2/SU1/[email protected]

Slot 2 Subunit 1 Hostname

MGCP

AALN/S2/SU1/[email protected]

Endpoint Type (Analog Line)

Port 1

FXS VWIC 2/1/1

Both MGCP and Skinny Client Control Protocol (SCCP) are master/slave protocols; the CUCM is the master server for both protocols. SCCP and MGCP devices cannot operate without CUCM:

■ SCCP IP phones communicate directly with CUCM for all call setup signaling.

■ MGCP gateways communicate directly with CUCM for all call setup signaling.

Figure 10-2 displays a call that was received over an MGCP gateway to an SCCP IP phone. The MGCP communication from the gateway is sent to the CUCM for call processing. CUCM performs digit analysis on the received digits, and the lookup results in an SCCP endpoint. CUCM sends information to the gateway, allowing the gateway endpoint to play a ringback tone to the calling party. CUCM also sends SCCP signaling to the Cisco IP Phone, indicating that there is an incoming call. When the destination Cisco IP Phone end user answers the phone, CUCM coordinates the communication of the media path between the Cisco IP Phone and the MGCP gateway. The voice bearer traffic from the gateway to the IP phone uses Real-Time Transfer Protocol (RTP). RTP uses an even port number in the UDP port range of 16,384 through 32,767.

Figure 10-2 MGCP Call Flow

CUCM

Figure 10-2 MGCP Call Flow

CUCM

MGCP Gateway Support

MGCP support in CUCM includes a wide range of analog and digital interfaces that can be used on several Cisco router and switch platforms. Tables 10-1, 10-2, and 10-3 list MGCP-supported analog and digital features.

CUCM allows the Cisco IOS MGCP gateway to pull its MGCP-related configuration from the Cisco TFTP server. This eliminates the need for manual MGCP gateway configuration.

CUCM also supports Q.931 backhauling. Q.931 backhauling is supported on ISDN PRI, BRI, and E1 using common channel signaling (CCS). The MGCP call agent (CUCM) takes control of the ISDN D channel. The gateway sends CUCM all Q.931 call setup and teardown traffic over TCP port 2428.

Table 10-1 lists supported MGCP hardware.

Table 10-1 MGCP Gateway Hardware Support

Gateway

Supported Voice Hardware

Remarks

Cisco 3800

Analog FXS/FXO, T1 CAS (E&M Wink Start; Delay Dial only), T1/E1 PRI

Beginning with Cisco IOS Release 12.3.11T

Cisco 2800

Same as Cisco 3800

Beginning with Cisco IOS Release 12.3.8T4

Table 10-1 MGCP Gateway Hardware Support (Continued)

Gateway

Supported Voice Hardware

Remarks

Cisco 3700

Same as Cisco 3800

Cisco 3640 and 3660

Same as Cisco 3800

Cisco 2600/2600XM/V G200

Same as Cisco 3800

Cisco 1751 and 1760

Same as Cisco 3800

WS-X4604-GWY Module

Same as Cisco 3800

Comm. Media Module (CMM)

T1 CAS FXS, T1/E1 PRI, FXS

WS-X6608-x1 Module and FXS Module WS-X6624

T1 CAS E&M, T1 CAS FXS, T1/E1 PRI, FXS with WS-6624

VG224

FXS only

Cisco ATA 188

FXS only

Table 10-2 lists supported MGCP analog features. Table 10-2 MGCP Analog Features

Gateway

FXS

Battery

Reversal

Analog DID

CAMA 911

Cisco 3800

Yes

Yes

No

Yes

No

No

Cisco 2800

Yes

Yes

No

Yes

No

No

Cisco 3700

Yes

Yes

No

Yes

No

No

Cisco 3640 and 3660

Yes

Yes

No

Yes

No

No

Cisco 2600 and 2600XM

Yes

Yes

No

Yes

No

No

VG200

Yes

Yes

No

Yes

No

No

Cisco 1751 and 1760

Yes

Yes

No

Yes

No

No

WS-X4604-GWY Module

Yes

Yes

No

No

No

No

Communication Media Module (CMM) 24FXS

Yes

N/A

N/A

N/A

N/A

N/A

FXS Module WS-X6624

Yes

N/A

N/A

N/A

N/A

N/A

VG224

Yes

N/A

N/A

N/A

N/A

N/A

Cisco ATA 188

Yes

N/A

N/A

N/A

N/A

N/A

Table 10-3 lists supported MGCP digital features.

Table 10-3 MGCP Digital Features

Gateway

BRI*

TI CAS (E&M)

T1 PRI

T1 QSIG

E1 PRI

E1 QSIG

Cisco 3800

12.4(2)T

Yest

Yest

Yest

Yest

Yest

Cisco 2800

12.4(2)T

Yest

Yest

Yest

Yest

Yest

Cisco 3700

12.4(2)T

Yest

Yest

Yest

Yest

Yest

Cisco 3640 and 3660

12.4(2)T

Yest

Yest

Yest

Yest

Yest

Cisco 2600 and 2600XM

12.4(2)T

Yest

Yest

Yest

Yest

Yest

VG200

No

Yes

Yes

Yes

Yes

Yes

Cisco 1751 and 1760

12.3(14)T

Yes

Yes

Yes

Yes

Yes

WS-X4604-GWY Module

No

Yes

Yes

Yes

Yes

Yes

Communication Media Module (CMM) 6T1/E1

N/A

Yes

Yes

Yes

Yes

Yes

WS-X6608-T1/E1

N/A

Yes

Yes

Yes

Yes

Yes

*Cisco IOS 12.4(2)T supports BRI MGCP with the following hardware: NM-HDV2, NM-HD-XX, and on-board HWIC slots. BRI MGCP is also supported on older Cisco IOS releases with NM-1V/2V hardware.

*Cisco IOS 12.4(2)T supports BRI MGCP with the following hardware: NM-HDV2, NM-HD-XX, and on-board HWIC slots. BRI MGCP is also supported on older Cisco IOS releases with NM-1V/2V hardware.

^AIM-VOICE-30 modules require Cisco IOS Release 12.2.13T.

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