Call Control Protocol

The call control protocols used when transporting fax, modem, and text with Cisco voice gateways in IP networks are H.323, Session Initiation Protocol (SIP), Media Gateway Control Protocol (MGCP), and Skinny Client Control Protocol (SCCP). However, not all of these call control or voice signaling protocols support all the various passthrough and relay transport methods. Table 7-4 provides a quick overview of the transport methods supported by the H.323, SIP, MGCP, and SCCP voice signaling protocols for Cisco IOS voice gateways.

Table 7-4 IOS Gateway Passthrough and Relay Support for H.323, SIP, MGCP, and SCCP Call Control Protocols

Transport Method

H.323

SIP

MGCP

SCCP

Cisco Fax Relay

Yes*

Yes*

Yes*

Yes*

NSE-based T.38 Fax Relay

Yes

Yes

Yes

Yes

Protocol-based T.38 Fax Relay

Yes

Yes

Yes

No

Fax pass-through

Yes

Yes

No

No

Modem passthrough

Yes

Yes

Yes

Yes

Cisco modem relay

Yes

Yes

Yes

Yes

Secure modem relay

No

No

Yes

Yes

Cisco text relay

Yes

Yes

Yes

Yes

Text over G.711

Yes

Yes

Yes

Yes

IOS platforms such as the 5350, 5400, and 5850 using the NextPort DSP cards do not support Cisco fax relay or the SCCP voice signaling protocol.

IOS platforms such as the 5350, 5400, and 5850 using the NextPort DSP cards do not support Cisco fax relay or the SCCP voice signaling protocol.

A call agent, such as Unified CM, must also support protocol-based T.38 fax relay.

The information in Table 7-4 shows that H.323 and SIP are the two call control protocols that provide you with the most options for transporting fax, modem, and text calls. The only exception to this is in the case of secure modem relay, which is supported only by MGCP and SCCP. However, secure modem relay is a niche application that is not widely implemented. You can find more information on secure modem relay in the section "Secure Modem Relay" later in this chapter.

The drawbacks of MGCP are the lack of support for fax pass-through and the requirement for a compatible call agent (CA) with protocol-based T.38 fax relay. Currently, only certain versions of Unified CM possess full interoperability with an IOS voice gateway configured for protocol-based T.38 and MGCP. See the section "Unified CM Integration" later in this chapter for more information about Unified CM support of T.38 fax relay.

The SCCP call control protocol lacks support for any transport method that uses a protocol-based switchover. Therefore, SCCP does not support fax pass-through or protocol-based T.38 fax relay. The transport methods in Table 7-4 that use alternative switchover methods, such as Named Signaling Events (NSE), are compatible with SCCP.

Of course, many other factors exist when choosing a call control protocol for a VoIP network, and fax, modem, and text support is typically not the major, deciding factor. However, if fax, modem, and text support is taken into consideration when selecting a voice signaling protocol, H.323 and SIP provide more options and flexibility as compared to MGCP or SCCP.

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