Softswitches and Trunk Gateways

Softswitches and trunk gateways are an emerging force on the VoDSL landscape,and they offer serviceproviders a lower-cost alternative to traditional Class 4/5 TDM switches.Softswitches run on standardserver hardware or fault-tolerant computers and provide call control,administration, and custom callingfeatures for packet voice network devices. Trunk gateways are central office(CO) hardware platformsfor internetworking between the packet voice and PSTN networks. Used togetheras a distributedswitching system, Softswitches and trunk gateways perform call control androuting for subscribertelephony traffic placed between packet voice endpoints or between packet andcircuit-switched subscribers.

The Softswitch software (also known as a call agent or media gatewaycontroller) communicates withthe subscriber’s IAD, which contains signaling intelligence. The call agentand IAD use a signalingprotocol such as MGCP to provision the individual voice ports, dial numbers,indicate progress tones,and initiate features like caller ID and call transfer.

For calls to PSTN destinations (off-net), voice traffic is forwarded to thetrunk gateway, where it is converted to circuit-switched voice and sent to the PSTN via TDM links. Forcalls to other VoDSLsubscribers that are connected to the packet-voice-switch data network(on-net), the voice traffic isrouted in the data domain and bypasses the PSTN.

MGCP

MGCP is a combination of Internet Protocol Device Control (IPDC) and SimpleGateway ControlProtocol (SGCP). MGCP enables distributed control and management of mediagateways at the edgeof multi-service packet networks through the use of Softswitch call agents ormedia gateway controllers.

A telephony media gateway or trunk gateway is a network element that providesconversion betweenPSTN circuit-switched voice traffic and packets carried over a data network.Media gateways can beVoice over IP (VoIP) gateways, voice over ATM gateways, MGCP-capable IADs(access and residentialgateways), and IP PBXs.

Distributed Trunk Gateway and Softswitch Networks

Legacy voicenetworks and GR-303 gateways are dependent on the Class 5 switch for callingfeatures, switching, dialtone, and interconnection to the PSTN. Softswitches and media gateways take thefeatures and functionalityof the gateway and Class 5 switch and partition these functions into multipledevices that may bedistributed throughout the carrier network at optimum points.

The distributed VoDSL network has a client-server architecture, in which IADsand media gateways areintelligent MGCP clients delivering line-side features like call waiting andcall transfer, while SoftswitchMGCP call agents are servers, controlling all signaling, featureadministration, and call routing for bothline-side features and network trunk interactions. A single call agent may beused to control multiple trunkgateways. This is a significant departure from the centralized approach,where all calls must pass througha large voice switch, and signaling intelligence is replicated at every endoffice. In the distributed model,the trunk gateways can be dispersed with a single Softswitch performing Class5 signaling and SS7 mediation.

Trunk gateways convert between circuit and packet voice, and mux packet voiceto and from PSTN TDMtrunks under direction of the Softswitch call agent. With the distributedmodel, off-net traffic may behanded off to the PSTN at multiple locations, with the call agent determiningthe optimum trunkgateway and routing for the call. On-net voice bearer traffic between packetvoice endpoints is forwardedby distributed routing elements, including the IAD, IP-aware DSL AccessMultiplexer (DSLAM), androuters located in the core of the network.

The net result of this distribution of gateway and signaling functions is theability to avoid the toll charges,interconnect fees, and backhaul charges that are unavoidable with thecentralized approach. Becauseresources are shared and may be added as needed, distributed VoDSL networksare extremely scalable,and can be used by carriers to handle enterprise, regional, national, orglobal deployments.