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4.1.1.5
 
The Breakout Gateway Control Function (BGCF) 
This node is basically a SIP server that handles routing functionality based on telephone 
numbers. It is only used in sessions that are initiated by an IMS UE and addressed to a 
user in a CS network. The main functions are to select an appropriate network where 
inter-working with the CS domains is to occur or to select an appropriate Public 
Switched Telephone Network/ CS (PSTN/CS) gateway, if inter-working is to occur in 
the same network where the BGCF is located.

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4.1 TMMS Service based on IMS: : Overview 
Page 46 of 93 
4.1.1.6
 
The PSTN/CS Gateway 
This gateway makes it possible for IMS UEs to make and receive calls to and from a CS 
network. The PSTN/CS gateway can be decomposed into several functions: 
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Signaling Gateway (SGW) 
The SGW interfaces the signaling plane in the CS network and performs 
lower layer protocol conversion. 
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Media Gateway Control Function (MGCF) 
This node is the central node of the PSTN/CS gateway, and controls one 
or many Media Gateways (MGW) which is described below. It 
implements a state machine that does protocol conversion and maps SIP 
into either ISDN User Part (ISUP) over IP or Bearer-Independent Call 
Control (BICC) over IP. It is responsible for transferring and distributing 
media between participants in multimedia sessions.
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Media Gateway
This gateway interfaces the media plane of the PSTN or the CS network. 
On one side the MGW can send and receive IMS media over RTP, and 
on the other side it uses one or more Pulse Code Modulation (PCM) time 
slots to connect to the CS network. It converts media from one format to 
another. It is important that the MGW performs its functions as quickly 
as possible so that delay is not added to transmission of the information.