Cisco WS-X4604-GWY - VoIP Gateway Especificaciones descargar pdf (Pagina 102)

Chapter 4 Gateway Selection

Supplementary Services

4-8

Cisco IP Telephony Network Design Guide

78-11103-03

IOS H.323 Gateways

Only H.323v1 was supported prior to Cisco CallManager Release 3.0. The

inability to modify the destination of an Real-Time Transport Protocol (RTP)

stream after H.323v1 call setup prohibited supplementary services such as hold,

forward, and transfer. Because H.323v1 provides no capability to move an RTP

stream from one destination to another after original call setup, the software MTP

tool was used to provide supplementary service support on the Cisco IOS

gateways.

MTP, which runs as a software process on either the Cisco CallManager or on a

separate Windows NT 4.0 server, terminates the RTP stream from the Cisco IOS

gateway and the RTP stream from an IP phone. This workaround enables an IP

phone to support supplementary services when using a Cisco IOS VoIP gateway

because the RTP stream from the MTP to the Cisco IOS gateway is never modified

until call completion. All RTP stream changes are made on the Skinny Station side

of the MTP connection. An additional major caveat for using the software MTP

is that it supports only G.711 voice streams; no compressed voice calls are

supported. This greatly limits WAN systems.

The use of H.323v2 in Cisco IOS Release 12.0(7)T and above (specifically the

OpenLogicalChannel, CloseLogicalChannel, and emptyCapabiliySet features) by

Cisco IOS gateways and Cisco CallManager Release 3.0(5) eliminates the

requirement for MTP to provide supplementary services. Because MTP is no

longer needed to terminate the G.711 RTP streams from both the IP phones and

the Cisco IOS gateway, compressed voice calls (G.723.1 and G.729a) are now

supported between Cisco IOS gateways and Cisco CallManager endpoints.

Once an H.323v2 call is set up between an Cisco IOS gateway and an IP phone,

using the Cisco CallManager as an H.323 proxy, the IP phone can request to

modify the bearer connection. Because the RTP stream is directly connected to the

IP phone from the Cisco IOS gateway, a supported voice codec can be negotiated.

The following steps illustrate the process that occurs if IP phone 1 wants to

transfer the call from the Cisco IOS gateway to IP phone 2:

1. IP phone 1 issues a transfer request to Cisco CallManager using the Skinny

Station Protocol.

2. Cisco CallManager translates this request into an H.323v2

CloseLogicalChannel request to the Cisco IOS gateway for the appropriate

SessionID.

3. The Cisco IOS gateway closes the RTP channel to IP phone 1.

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Cisco WS-X4604-GWY - VoIP Gateway Especificaciones Pagina 102