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Feature Article
March 2001


The Role Of The Softswitch In The Next-Generation Network


Tremendous growth in data services is forcing public carriers to take a serious look at voice and data convergence. Packet-based networking technologies such as ATM and IP are reasonably mature to address this task. In addition, several of the standards organizations including ITU, IETF, Multi-Service Switching Forum, ATM Forum, and International Softswitch Consortium are promoting the cause of separation of call control from bearer control. Therefore, with the availability of packet-based bearer technologies and the decoupling of call control from bearer control, the stage is being set for public carriers to move from the current "one network -- one service" paradigm to a "one network -- multiple services" paradigm.

Figure 1 (below) provides a comparison between the existing switch architecture and the proposed next generation switch architecture.

Figure 1

The current switches are "monolithic" in nature, using vendor proprietary protocols for communications between different subsystems. A major advance brought forth by the next-generation network (NGN) architecture is the "opening" of these protocols. IETF and ITU have taken the lead in specifying standards for such protocols. For example, MGCP and Megaco (H.248) define the interface between a softswitch and the subtending media gateways. Similarly, JAIN/Parley group has defined standards-based APIs between the softswitch and feature servers. These feature servers are analogous to the Signal Control Points (SCPs) in the AIN and provide repositories for the services and their associated databases. With the distributed switch approach for the NGN, carriers shall be able to procure "best of breed" network elements in a multi-vendor environment and hopefully, at a lower cost.

Referring again to Figure 1, the NGN switching architecture is based on three logically separate layers. These layers can change independently with the technology evolution without affecting the overall functions of the network.

The transport layer is responsible for the transmission of the bearer traffic. The transmission technology may be TDM, ATM, or IP based. However, due to the bandwidth efficiency inherent in the packet networks, next-generation networks will likely be based on packet technologies such as ATM and IP.

The switching layer provides the control logic required for call processing and routing of the bearer traffic. The "softswitch" as a product resides in this layer.

The service layer provides the call logic and the databases for supporting telephony services, e.g., 800 numbers, local number portability, etc.

The softswitch provides the call processing intelligence. It communicates with the subtending media gateways in the transport layer with standards-based control protocols such as MGCP and Megaco (H.248). Using these protocols it determines the two end points that need to participate in a call (or a session). Once identified, it then commands the two end points to establish a bearer path between themselves. The end points then use appropriate mechanisms (ATM SVCs/PVCs and/or IP-based) for the establishment of the bearer path. This separation of the two signaling mechanisms allows the same softswitch and the associated service logic to be shared across different types of bearer networks (TDM or ATM or IP). Another advantage of this approach is that subscribers or media types (data, video) can be added/dropped relatively easily to an existing call or session, thereby facilitating "multimedia" communications.

Eventually the transport layer will become packet-based end to end. However, the current PSTN is circuit-switch-based. Therefore, in order to connect two end points that may reside in networks based on different technologies, one needs a media gateway. A media gateway provides bearer level interworking and may also provide signaling level interworking to establish calls that traverse networks built with different media types such as ATM and IP.

Media Gateways
There are several different media gateways that are available from different vendors. These media gateways have different names depending upon where they are used in the network.

Residential Gateway: This media gateway is used in a residential/SOHO environment. It may have two to four analog voice ports and an Ethernet LAN port. The network side interface is typically ADSL-based. Using the LAN, multiple PCs in a residence can simultaneously access the Internet or corporate VPN. The voice ports provide regular RJ-11 interfaces for the telephones. The unit provides interworking between the circuit-switched voice to VoATM or VoIP on the network side.

Business Access Gateway: This media gateway is used in small to medium sized businesses. It typically provides two to 16 analog voice ports or a digital T1 interface for on-site connectivity to a Key system or a PBX. The unit may also have one or more data interfaces -- an Ethernet interface for LAN connectivity and a V.35 interface for an onsite router. The network side interface is typically SDSL-based or a T1.

Voice Gateway Or Trunking Gateway: This media gateway is designed for the central office environment. It provides a large number of voice trunks -- typically a few thousand. It also provides interworking between an ATM- or IP-based packet network on one side and the circuit-switched PSTN on the other.

Distributed Class 5 Switch Application
A Class 5-type network based on a softswitch will interact with subtending residential gateways, business access gateways, and voice gateways. Using this network, a subscriber connected to a residential media gateway can make a voice call to other residential or business subscribers connected to their respective media gateways with end-to-end connectivity for the call over the packet network. For calls for which one end is connected to the current PSTN, the call will traverse the voice gateway with associated signaling carried over the SS7 network.

Tandem Switch Replacement Application
Due to falling rates for long-distance telephony, the Inter-Exchange Carriers (IXCs) are looking for ways to reduce costs for carrying voice traffic. One obvious choice is to packetize the long-distance voice network. This has several advantages. First, due to the inherent dynamic bandwidth utilization capability of the packet network, coupled with advances in voice compression technologies, carriers can carry a significantly large (two to three times more, depending on the technologies used) number of voice calls over their existing transmission networks. Secondly, the same transmission network can also be utilized for carrying the data traffic. This reduces the operations overhead for managing two different and separate networks.

The role that softswitches and media gateways play in next-generation network architectures is very significant. Based on the intelligence contained in the call agents, a variety of existing and new voice related services, including those listed below, can be supported over a packet-based network:

  • Local calling based on 7-digit routing
  • Domestic long-distance calling based on 1+10 digit routing
  • International calling per ITU's E.164 numbering plan
  • Multi-line Hunting
  • DID/DoD
  • DNIS/ANI delivery
  • Caller-ID
  • Call waiting with Caller ID
  • Cancel call waiting
  • Call forwarding -- all, on busy, on no answer
  • Three-party conference calling
  • Account codes/Authorization codes
  • Distinctive ringing
  • E-911
  • Local number portability
  • Pre-carrier selection (FG-D)
  • Casual calling (10-10-XXX calls)
  • Operator service
  • 800 toll-free calls

Sudhir Gupta is director of strategic marketing at Accelerated Networks. Accelerated Networks is a market leader in multiservice broadband access (MSBA) products that enable telecommunications service providers to bundle voice and data services over a single broadband access network. For additional information, please visit

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