Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

“softswitch” in quotes, since it is mostly a marketing concept to designate the

combination of the call agent and media converter in IP-PSTN gateways, but also

for the central control of MGCP phones or adapters for end users.

A close look at the telephony-oriented approaches will reveal that they provide

little more than voice-only features ad nauseam, such as call forwarding, and

so on, inspired by the by the PBX, since the telephone companies were hoping

to replace private PBX voice networks with carrier-based Centrex voice services.

Master/Slave VoIP Systems

Device control protocols can be found in proprietary IP PBX designs and also

in various approaches for VoIP such as Media Gateway Control Protocol

(MGCP), Media Gateway Control (MEGACO), and H.248. The decomposition

of an IP telephony gateway using a device control protocol between the gate-

way controller (GC) and the media gateway (MG) is shown in Figure 19.1a.

Figure 19.1 Decomposition using master/slave protocols (a) IP telephony gateway, (b)

application server, (c) IP telephony gateway network with central call agent (CA), and (d)

residential gateway (RG) for telephony

GC

SIP

RTP

MGCP

CAS, Q.931, SS7

PCM

PSTN

PSTN PSTN

Internet

MG

CA

GC GC

RG

CAS, Q.931, SS7

MGCP

MGCP, etc.

Internet

“packet”

MG MG

MG

GC

MG

a

cd

b

SIP

API’s

Packages

Devices

API’s MEGACO

RTP

API’s

318 Chapter 19