Call Control

Mar 15, 2001 12:00 PM, Zeeshan Khan

New signaling protocols form the basis for next-generation converged networks, allowing them to communicate.

Networks convergence is now considered the “Holy Grail” of the telecom world. This convergence can be categorized in several ways and will have significant effect within various network domains. At the end of the day, every network operator’s goal will be to provide seamless connectivity between different domains and to carry calls end-to-end — packaged with a suite of services across those domains.

The principles behind converged networking not only recognize circuit- and packet-switched domains, but also embrace wireless networks and seek to extend services to all of the above — regardless of protocol or physical media. This calls for separation of transmission, media, gateways and geographical presence from the service logic that activates, controls and manages a particular service.

Signaling protocols lie at the heart of the next-generation wireless and converged networks because they provide the language through which devices can communicate and interact. Next-generation signaling solutions theoretically make services and applications easier to implement, deliver and manage since they have built-in capability to interoperate among different network domains.

SS7 is a protocol that is designed for PSTN signaling. With its circuit-switched orientation and limitations for integrating data services, it is not suitable for IP advanced integrated voice and data services. Today, IP-based signaling protocols contend to open up the brave new world of service offerings in wireless, wireline as well as converged packet- and circuit-switched networks.

Next-Gen Signaling

With the emergence of advanced and intelligent signaling protocols, wireless networks convergence and services delivery is close to becoming a reality. The next-generation signaling network will provide session-based call models. New IP-based signaling protocols such as session-initiation protocol (SIP) treat the individual legs of a call as a session. Therefore, it is possible to set up sessions between users, network elements and services. For instance, when a call involves sessions between a number of end users, call-control functions could then initiate new sessions to alert applications such as billing or multimedia applications. By placing intelligent-service engines at strategic points within the wireless network, you can deploy session-based services across the network. To describe SIP in the context of the wireless world, consider how the 3G Partnership Project (3GPP) positions it (www.3gpp.org).

3GPP is a global partnership/working group comprised of major telecom and wireless standards bodies that cooperate in the production of globally applicable technical specifications and technical reports for a 3G mobile system. A 3GPP sub-group, Technical Specifications Group - Services and System Aspects, has identified that “for multimedia type services delivered via the packet-switch (PS) domain within a proposed architecture, SIP will be used between the user equipment (UE) and the call state control function (CSCF) as the single call-control protocol.

Mobile-station-application-execution-environment terminals would benefit by being SIP compatible. An API between the application and session management for multimedia applications could do this” (Multimedia includes voice over PS domain.)

A new IN element in the network, the CSCF server (see Figure 1), is a registry for IP multimedia services. It contains a register of available services within the network and corresponding sets of parameters for each. It is in the IP multimedia domain and communicates with other multimedia elements above the IP layer. SIP communicates on a client-server model using a structure similar to e-mail headers.

SIP Operation/Service Scenario

The following is one classic example of a SIP operations/service scenario:

• UE sends a SIP command to the CSCF. (SIP has a basic command set including invite, bye, cancel, options register.)

• CSCF looks up which service has been requested and returns a set of predefined packet-data protocol (PDP) context parameters for the UE.

• UE uses the returned PDP context parameters for establishing/controlling a PDP context.

Without SIP and a CSCF, the UE would have to decide which PDP context parameters to use (for example, QoS). With SIP, the PDP context parameters are predefined for a range of services (such as multimedia voice, multimedia video, etc.). The services are registered on the CSCF.

SIP Advantages

There are many advantages when implementing SIP in a network:

• Only a high-level description of the service is required (for example, multimedia voice).

• New service descriptions can be introduced as the network develops.

• Network parameters for multimedia service can be optimized for the network (for example, realistic QoS parameters stored and updated as the network develops).

• The UE does not need to translate service requests into specific network parameters (allows simple low-power terminals).

• The UE obtains the PDP context parameters and is still in control of the lower-layer signaling.

• Quick roll out of new services can be achieved if UE can determine the network (PDP) parameters by itself.

If the network cannot support the request, then signaling and control is slower (UE must negotiate parameters with the network several times before success).

Emerging Solutions

A plethora of next-generation services soon will be introduced into wireless and voice-over-IP networks, which will undoubtedly increase both the capabilities and the complexities of the core network infrastructure. As a result, the carriers must build networks that are capable of delivering sophisticated services while facing a reduction in revenue attributed to pure voice calls.

Solutions founded on the deployment of SIP-based proxy servers, to route calls to switches and services, are emerging to enable these offerings. The next-generation signaling networks will provide levels of abstraction from the underlying network architecture. Their signaling protocols and service-creation environments will allow third-party developers and ASPs to write applications — regardless of the underlying transport and physical layers — that carriers can effortlessly access and dynamically deliver.

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Khan was formerly with SS8 Networks (www.ss8networks.com).