Class 5 Switches with IP Trunks

The number 5 in reference to telecommunications switches remains from predivestiture times when the U.S. telephone network had a hierarchical model in which five classes (according to their respective functions) of switches were employed. Specifically, class 5 was reserved for the central office switches. Although the name endures, the actual capabilities of some of today’s large class 5 switches allow them to be configured for tandem, government, and other specialized applications. Furthermore, those class 5 switches that are equipped with appropriate signaling gateways are sometimes used as gateways to foreign networks. On the other end of the spectrum are smaller class 5 switches that are sometimes employed by large enterprises as PBXs.

Large class 5 switches (with about 200,000 access lines and 100,000 trunks) support metropolitan and urban areas; smaller ones (with about 40,000 access lines and 20,000 trunks) support rural areas; and the smallest ones (with about 20,000 access lines and 10,000 trunks) typically support small towns. Advanced switches provide ISDN (PRI and BRI) and ADSL access, and they invariably support SS No. 7 signaling and its applications (such as Intelligent Network) as well as the interfaces to telecommunications network traffic management platforms, billing systems, and other operations systems.

There are very few vendors of telecommunications switches; their products are so complex and so dissimilar—insofar as their architecture, hardware, and software are concerned—that each warrants a separate book. We do not attempt to review the existing hardware and software solutions, nor do we describe the resulting IP services or mechanisms that support them. Our review of VoIP gateways, gatekeepers, remote access servers, and SS7 gateways, whose functions are combined in the new switches, covers the IP services and mechanisms. We do report on the emerging solutions for IP connectivity (often combined with support for sending voice over ATM and frame relay networks—thus justifying the name multiservice switching used in the industry to describe these capabilities), which effectively make the telephony switches into packet switches.

Such switching solutions unite traditional call processing with the support of modem pooling and remote access, thus allowing ISPs (or enterprises) to terminate calls directly at the multiservice switch. All the features of remote access servers described in the preceding text are supported. In addition, the VoIP gateways and gatekeepers are also part of the multiservice switch, so the PSTN voice service is converted to voice over IP as close to the source as possible. Note that because class 5 switches already support SS No. 7 and its applications, support of the PSTN signaling and, in particular, use of Intelligent Network—which immediately solves the issues of supporting services and number portability, as well as the use of the end-to-end PSTN operations and management—come automatically with the product. Again, all the relevant IP telephony and remote access capabilities (including out-of-band signaling over IP) described earlier in this chapter are supported, but since they are supported at the switch itself, no front-end boxes for performing interconnections and conversions are needed. Telephone companies or enterprises that plan to replace their existing switches should replace them with the emerging multiservice switches; in many cases the costs of upgrade and integration of all the products needed to provide such functions are below the cost of an appropriate multiservice switch.

Typically, the features supported by the multiservice switches (in addition to over 3000 voice-related features inherent from the PSTN) come in feature groups called bundles in PSTN parlance. To date, the following bundles are available:

§  Residential

§  SOHO

§  Large enterprise

§  Multienterprise

§  Virtual telco

Bundling eliminates the need to buy unnecessary features; each bundle costs less than an all-or-nothing package.

One more application of IP trunking is offloading the signaling network. Messages between the SS No. 7 endpoints (network elements) can be tunneled through the IP network. The IETF sigtran working group is presently developing the transport layer standard protocol for such tunneling; for the moment, there are several proprietary solutions. Here again an important and interesting duality can be observed: While the IP endpoints (such as SS7 gateways) adapt to the PSTN in order to connect to its signaling network, the PSTN switches use IP to tunnel the PSTN signaling.

This concludes the description of the available types of products. We are ready to proceed with the criteria for choosing those that best suit your applications.