The new access-centered network

The workhorse of the public network, the Class 5 switch is groaning under the weight of customer demand for more services. Is open access architecture its knight in shining armor?

Two thousand years ago, all roads led to Rome, the congested center of an empire. Today, all access paths in the telecommunications network lead to increasingly congested Class 5 switches located in central offices (COs).

When Rome was the center of the world, a centralized node of communications was a great idea, congestion or no. In today’s telecommunications network, the expensive Class 5 switch has become a choke point, and the industry is looking for ways to solve the problem.

With the emergence of next generation technologies, the demand is greater than ever to deliver more services, faster and cheaper, to customers over the public switched telephone network (PSTN). This is difficult to do, however, when even new services such as digital subscriber line (DSL), voice over IP (VoIP), voice over DSL (VoDSL), and voice over ATM (VoATM) must connect to the local loop via the circuit-based Class 5 switch. Until now, only the Class 5 switch could provide dial tone for voice calls, connect calls to the appropriate network and handle connections for circuits destined for other networks.

In addition, value-added features such as call forwarding, call waiting and caller ID, are located on the Class 5 switch. Users dial into other features such as conference bridges, through the switch. Operations, administration, maintenance, and provisioning (OAM&P) software is dependent on it as well, because it is the only network element that has information about local loop use and configuration.

The solution to Class 5 switch congestion is the next generation switch, or softswitch, which decouples physical switching hardware from logical software control, making it easier to introduce scalable services and features.

Enhancing the Network 

The solution to Class 5 switch congestion is the next generation switch, or softswitch, which decouples physical switching hardware from logical software control, making it easier to introduce scalable services and features. Hardware-independent softswitches are designed with an emphasis on selling software-based services.

Backbone switches, for example, provide media gateway call control, replacing legacy Class 4 switch functions between local and long-haul networks. Such backbone switch replacements are often termed “hardswitches.” Software-based, these hardswitches are closed in by an architecture that is dependent upon specific hardware platforms and proprietary media gateway protocols.

Another segment uses the “softswitch” banner to deliver hardswitches using standard protocols between the application and hardware layers, but has limited openness because its feature sets are optimized to run on particular hardware platforms. These fall into two PSTN switching categories: Class 4 and Class 5 replacement.

Concentration of Functions

How have so many functions come to depend on the Class 5 switch? Historically, the PSTN served individual telephones over separate copper loops directly tied into local COs, where human operators connected calls. COs eventually evolved into today’s sophisticated, automated Class 5 switches. COs serve fairly large areas, and many customers are located more than 15,000 feet away from them.

The digital loop carrier (DLC) emerged as a way to reduce the cost of deploying the local loop network over long distances by aggregating individual local phone circuits in outlying areas and passing them to the Class 5 switch over a high-capacity DS-3 line.

Next generation DLCs (NGDLCs), which emerged in the early 1990s, are larger, more sophisticated devices that actually concentrate traffic. Built-in ATM switching capability enables NGDLC platforms to multiplex calls onto different logical connections with the switch, increasing service capacity by a factor of four to six. 

A high-speed ATM backplane anticipated future demand for broadband access services such as video on demand and DSL. Many NGDLCs are fed by fiber and thus offer a direct connection to Sonet-level bandwidth, allowing them to handle very large amounts of subscriber voice and data traffic. The location and design of today’s NGDLCs thus make them an ideal place from which to deliver broadband services.

The Access-Centered Network

As currently configured, a Class 5 switch at a CO controls each NGDLC, acting as the master to NGDLC slaves. A new, software-based open access architecture (OAA), however, breaks the stranglehold of the Class 5 switch, unlocking the latent capabilities of network access devices to deliver revenue-generating services directly at the subscriber access point. The OAA leverages existing, widely deployed access devices—such as NGDLCs—as media gateways and can deliver services through them from Class 5 switches, softswitches or feature servers located on any downstream network. In addition, the OAA allows carriers to allocate network capacity at the subscriber’s first entry point to the network, thereby optimally matching network resources to subscriber needs.

The key to the OAA is a software-based access switch that enables services for users connected to NGDLCs and other access devices. The OAA thus provides virtual (logical) control of network resources. Access switches move call control and signaling intelligence to the edge of telecom networks, providing what might be considered a Class 6 layer.

Because the access switch is software-based, it can run on an off-the-shelf workstation located in a CO or data center and connect to an access platform via a very narrow bandwidth signaling connection on the existing transport link. Using standard protocols, the access switch relays service requests between customers and carriers via the local access network, enabling subscribers to access any available service on any available network. As new media and protocols are deployed in the network, the access switch quickly mediates connections throughout the network, enabling carriers to more quickly and easily match bandwidth requirements to services.

By deploying access switches at the network edge, carriers gain full visibility and control of network resources, enabling them to more quickly and easily meet customer needs with dynamic assignment of service capabilities.

By deploying access switches at the network edge, carriers gain full visibility and control of network resources, enabling them to more quickly and easily meet customer needs with dynamic assignment of service capabilities.

The Three Layers of Switch Functions

The OAA arranges network elements into three logically separate layers, with the access switch operating at the middle, or control, layer.

• The bottom, or media, layer defines service delivery—that is, TDM, ATM and IP—at the hardware level. This layer consists of access devices such as NGDLCs, DSLAMs, voice–over-DSL gateways, broadband remote access switches and other platforms. The access switch supplies call control and intelligence to these access devices, matching each service with the appropriate resources.

• The middle, or control, layer handles call control and signaling functions. It also mediates between the other two layers. The access switch, the signaling system 7 (SS7) network and softswitches all operate within this layer. The access switch makes converged services available through an open set of APIs that connect it with feature or application servers, other softswitches, and a shared OAM&P database. The access switch also connects calls to or from other parts of the PSTN via the SS7 network. Service requests originating in one network under the access switch’s control can be transparently directed to and terminated in any other network.

• The top, or applications, layer supports feature servers, application servers and application services providers (ASPs) via standard session initiation protocol (SIP), JAIN™ and parlay application interfaces. Working through these interfaces, the access switch provides transparent control of carrier feature servers and directs the flow of services to each subscriber. A shared OAM&P database synchronizes the multicarrier delivery of services to subscribers served through a common CO.

Using the access switch to virtually move intelligence to the edge offers a number of advantages. The access edge is the logical place to offer broadband services because of its proximity to users. Intelligence projected to the NGDLC gives providers resource control and scalability at the point closest to the customer, making the NGDLCs the logical place to support services.

From Class 5 to the Edge

By using the inherent switching capabilities of the NGDLC, coupled with the intelligence of an access switch, the OAA relieves congestion at the switch and allows operators to expand services for a smaller investment. Carriers can deploy new services from their existing network assets, enabling them to evolve more quickly to next generation networks. Because the access switch is software-based, operators can bring services to customers quickly and easily.

Rome may have fallen from within, but the OAA—and its emphasis on strengthening the network edge—will ensure the vitality of the telecommunications network for years to come…even should the center crumble.
Cyril Matthews is Director of Product Management for Westwave Communications, Santa Rosa, CA.  He can be reached at Cyril.Matthews@westwave.com.

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