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Feature Article
September 2001

Call Control In The New Public Network: An Overview


The monolithic circuit switches of the traditional PSTN are well known. There are over 19,000 Class 5 switches deployed in central offices and over 500 Class 4 tandem switches deployed in regional offices across the United States alone. While the annual growth rate of these voice switches has recently been slow and steady, the traditional voice switching architecture will soon undergo an evolution similar to that of the computing architecture in the early 1980s, in which advanced PC workstations running third-party software replaced vertically integrated proprietary mainframes.

Carriers have begun to migrate to next-generation packet-switched voice networks because of intensifying competition, significant cost efficiencies, and new service revenue opportunities. Multiple vendors, both new and incumbent, who specialize in switching hardware, call control software, and new applications software are beginning to offer a dizzying array of different solutions. Nevertheless, astute choices can be made to optimize the performance of the packet voice solution and the return on investment of the purchase decisions.

In an environment of intensifying competition for customers among incumbent and competitive carriers, next-generation packet-switched voice networks offer several fundamental benefits: Significantly reduced capital and operation costs, less reliance on overlay networks, simplified service provisioning, and new services creating additional revenue and differentiation.

The four key elements in the softswitch architecture are:

  • Media Gateway;
  • Softswitch (or Media Gateway Controller);
  • Signaling Gateway (functionality may be integrated in softswitch); and
  • Application Server.

The media gateway is an access/trunking switch that converts and switches TDM voice traffic and packet voice traffic, and provides interworking between the two types of interfaces. The packet voice traffic can be either IP or ATM depending on the capabilities of the media gateway and the carrier requirements. The media gateway should be capable of providing interworking between multiple access protocols: Any pair of TDM, ATM, and IP, for example.

Media gateways must accommodate current traffic as well as future growth. Media gateways currently range from 2 to 20 Gbps switching capacity. Port capacity for a Class 4/5 switch typically ranges from 10,000 to 100,000 DS0s.

Media gateways must be highly reliable, usually exceeding five nines availability to be considered carrier-class. This type of reliability requires superior components, quality manufacturing processes, and rigorous product testing. Full fault-tolerance and redundancy are critical to service availability assurance. NEBS Level 3 Certification is required and OSMINE compliance may be required depending on the carrier.

Interworking and Interoperability
Carriers may use media gateways and softswitches from multiple vendors; as a result, interoperable MGCP/Megaco/SIP protocols are necessary. Bearer interfaces (TDM, ATM, IP) and signaling protocols (PNNI, BICC) interworking are also essential.

Media gateway footprint drives occupancy costs. More scalable products will accommodate denser modules and typically reduce occupancy costs.

Carriers deploying media gateways will always consider a cost-per-port comparison. Density is typically an important factor in lowering cost-per-port.

Most media gateways have been developed initially to transport voice using either ATM cells or IP packets. While many industry analysts think that carriers will migrate to IP/MPLS, for now most carriers rely on ATM to provide the carrier-class quality of service required to deliver packet voice services. Either way, a media gateway that meets industry standards is critical.

The softswitch, or media gateway controller, provides the intelligent call control for the voice network. The softswitch also provides another essential function in the context of packet networks; it provides the interworking between the different signaling on the TDM and on the packet portions of the network, e.g., ISUP and MGCP protocols. The softswitch may have a Class 4 and/or Class 5 feature set depending on its role in the network.

Important factors for softswitches are:

Feature Set
The number and type of Class 4 and/or Class 5 features provided.

System Capacity
Voice processing capability, typically measured as voice calls per second or busy hour call attempts (BHCAs), per system, is a critical factor.

Softswitches must support and interwork among multiple signaling protocols to provide more flexibility to the carrier deploying its system of choice.

Softswitches must provide carrier-class reliability. Full fault-tolerance and redundancy is critical to service availability assurance. Fault tolerance between the softswitch and the media gateway is also essential.

Softswitches must interoperate with a wide range of other network elements, especially other softswitches, because their role is central to the functionality of the packet voice infrastructure. Carriers view interoperability as the top concern in the purchasing decision for new equipment (Figure 1).

Figure 1

The softswitch can be either physically separate from or physically integrated with the media gateway. In the former, a general-purpose server platform provides call control, and in the latter, an optimized module fits within a standard slot on the media gateway chassis.

Although most vendors use external server platforms, a few vendors use the optimized softswitch modules. Physically integrated softswitches provide multiple advantages over these general-purpose platforms: Simpler interoperability, higher call capacity, smaller footprint, better redundancy, lower cost, and more
efficient operations.

Another important feature of certain softswitches is the capability to provide call control for multiple media gateways distributed across the packet voice network. While solutions from many vendors require collocation of a softswitch with every media gateway, a more efficient and cost-effective approach is for remote softswitches to control bearer voice traffic on multiple media gateways located in separate central offices.

The signaling gateway function communicates directly with the signaling transfer point (STP) in an SS7 network via SS7 signaling messages in order to provide authorized services and route voice traffic through the bearer traffic network. In some vendor solutions, the signaling gateway is a separate network element, but ideally is integrated into the softswitch, which offers lower cost, simpler operations, and a smaller footprint.

The application server is a product that provides enhanced services that may include "follow me" roaming, automatic language translation services, and data-driven voice services. Until now, carriers have relied heavily on the feature set of the traditional circuit switch. In a softswitch environment, however, carriers will have the flexibility to offer new services without replacing or updating the entire packet voice platform.

How the primary network elements in a softswitch architecture communicate with each other is not an easy discussion, because there are a plethora of standards-based and proprietary protocols. Nevertheless, the total number of used protocols is shrinking, and can be segmented based on the specific interfaces (Table 1).

Table 1

Network Elements Signaling Protocol Elements
Media Gateway-Media Gateway PNNI, BICC
Media Gateway-Softswitch MGCP, Megaco (H.248)
Softswitch-SS7 Network ISUP, TCAP
Softswitch-Softswitch or Application Server SIP


PNNI (private network-network interface) is the standards-based protocol used by media gateways and ATM switches to share routing and signaling information to establish and maintain real-time voice and data connections. The BICC (bearer independent call control) signaling protocol, which is a modified SS7 ISUP protocol, correlated with ATM and PNNI signaling, is used as a signaling interworking function.

MGCP (Media Gateway Controller Protocol) is the signaling protocol between the media gateway and softswitch used until now for carrier deployments. Most vendors' MGCP implementations are proprietary. Megaco (H.248) is the compromise ITU-T communications protocol that will eventually replace MGCP. Megaco is still under development by most vendors and has not been widely tested for interoperability. Consequently, Megaco is still too new for carrier deployments.

ISUP is an SS7 protocol for signaling the parameters and procedures to set up and tear down circuit-switched voice calls between a softswitch/signaling gateway and an STP. TCAP is used to exchange control-related messages between a softswitch/signaling gateway and application databases (SCP, or signal control point).

SIP (Session Initiation Protocol) is an IETF protocol for transporting call control, authentication and other signaling messages among softswitches and other devices through an IP network.

The migration to packet switched voice networks will be an evolution, not a revolution. The most likely scenario is for carriers to pursue a "cap and replace" strategy for Class 4 tandem switches, followed later this decade by Class 5 switch replacement. With so many emerging network elements and communications protocols, the vision of the New Public Network can be somewhat daunting. But similar to the revolution from mainframes to PC workstations, significant capital cost and operational savings will prove to spur carriers and service providers to make the migration. With the range of packet voice infrastructure products currently being developed and launched, this segment is definitely one of the most exciting in telecommunications.

Purchase Recommendations

Purchase recommendation: Select a media gateway that offers scalable port density, has a high initial non-blocking switching capacity, offers multiple physical interfaces, has proven reliability, has proven interoperability with external softswitches, occupies a small footprint, and begins with ATM while supporting IP/MPLS longer-term.

Purchase recommendation: Select a softswitch that can meet call processing capacity, has desired feature set, has proven reliability, has proven interoperability with media gateways, other softswitches, and application servers, and incorporates the signaling gateway functionality. A physically integrated softswitch capable of supporting call control with multiple distributed media gateways offers distinct performance and value advantages.

Purchase recommendation: Select an application server that provides attractive new services, scales to meet customer needs, and has proven interoperability with other softswitches and application servers.

Andrew Hendry is product marketing manager at Oresis Communications. Oresis Communications is a developer of next-generation switching systems that provide integrated multiservice data switching functionality and media gateway/tandem softswitches.

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