Softswitch – An Overview
Softswitch is a ubiquitous term that is used in the VoIP world to represent the switching point for VoIP calls. Usually the Softswitch does not handle the media; the Media Gateways in turn handle the Media Conversion from IP to any Circuit Switched Networks like PSTN, PLMN (GSM/CDMA), etc. Before we get on to the deployment scenarios of Softswitches, we can have a very brief look at the Protocols involved the VoIP framework. There are two types of protocols that are involved in VoIP. H.323 and SIP are peer-to-peer protocols, while MGCP and MEGACO are master-slave protocols. Other than these, there is a SIGTRAN protocol suite for taking SS7 signaling over IP. H.323 is an umbrella recommendation from ITU-T for multimedia communication over packet networks. It encompasses the following main recommendations:
It encompasses many other standard protocols from various International Standard bodies. H.323 has a wide installed base and is defined comprehensively. SIP (Session Initiation Protocol) is an RFC from IETF which is used to establish any kind of session between two entities. The exact type of Session that is needed to be established is exchanged using SDP (Session Description Protocol) embedded in the SIP Protocol.
The Audio/Video Codecs, Fax and Data Transmissions and the carrier protocol for these (RTP/RTCP) remain the same as in H.323. SIP is a fast growing protocol, which is defined flexibly.
MGCP (Media Gateway Control Protocol) is an RFC from IETF in a Decomposed Media Gateway model. The earlier Media Gateways were monolithic with the complete Call Signaling / Control handling in both the networks (say IP and PSTN), Media Packetization/De-Packetization, Compression/De-Compression and RTP Encoding/Decoding.
Slowly the Media Gateway Decomposition architectures started coming up with a Media Gateway Controller (MGC) acting as a Call Agent while only the media related activities being done on the Media Gateway (MG). This is a Master Slave protocol between MGC and MG.This protocol has a wide installed base for Residential Gateways and also in Carrier Networks. MEGACO (MEdia GAteway COntrol Protocol) / H.248 is a joint recommendation from ITU-T and IETF. It moves the edge-centric focus of MGCP to the core. MEGACO is the future way to go for the Carrier Media Gateway networks.
The current support available in various products for MEGACO itself is low, leave alone its extensive deployment. But slowly the support for MEGACO is growing and would be deployed in Carrier networks where SS7 interfacing would also be needed.
SIGTRAN is a protocol suite for SS7 signaling over IP. It uses the SCTP (Stream Control Transfer Protocol) as the Transport Level Protocol instead of TCP/UDP. The various components in SIGTRAN are (M2PA, M2UA,M3UA,SUA) over which normal SCCP,ISUP,TCAP signaling is done. This enables the existing SS7 network infrastructure to be retained and is neatly interfaced into the VoIP world.
Softswitches come in various flavors intended for use in various situations. We will take the following more popular deployment scenarios:
The TeNeT Group is working with Nexge to develop an Softswitch that incorporates the above technologies and caters to a wide variety of deployment scenarios.