Network Architectures

IP Centrex solutions are being developed on a number of platforms, including Class 5 switches and softswitches.

• Class 5 Switch Architecture
• Softswitch Architecture

Class 5 Switch Architecture

In this platform, existing Class 5 switches (without any special upgrades!) support IP Centrex service in addition to traditional Plain Old Telephone Service (POTS) and ISDN lines. This is accomplished through the introduction of a new network element--the Network Gateway--and a new type of CPE--the Customer Gateway. (As an alternative to using a Customer Gateway, a customer may use new IP Phones, which are described on the IP Centrex CPE page of this site.) The Network Gateway and Customer Gateway signal each other over a packet network using an IP telephony protocol, such as H.323 or SIP. They also each support traditional interfaces to communicate with existing equipment. The Network Gateway connects to the switch as if it were a digital loop carrier system. (Digital loop carriers use protocols like GR-303 to deliver POTS and ISDN signaling information to switches for longer-than-average loops.) The Network Gateway translates any signaling information it receives from the Customer Gateway into a protocol that the switch understands; it also depacketizes the voice stream for delivery to the switch. Similarly, it translates signaling messages from the switch into the IP telephony protocol and packetizes the voice stream for transmission to the Customer Gateway. The Customer Gateway performs comparable functions for the standard telephone sets that it supports. As a result, the Network Gateway, Customer Gateway, and packet network connecting them appear to the Class 5 switch as an ordinary digital loop carrier (DLC) system, and the telephone sets connected to the Customer Gateway appear to the switch as ordinary phone lines. Because the IP Centrex solution is treated as a DLC system by the Class 5 switch, the switch is able to deliver the same features to IP Centrex users that it delivers to analog and ISDN Centrex users. Consequently, an extensive set of features is immediately available to IP Centrex users without needing to upgrade the Class 5 switch.

Softswitch Architecture

In a different approach to IP Centrex, the Class 5 switch is replaced by a softswitch. A softswitch is a telephony application running on an industrial grade server in the network. Like the Class 5 switch, the softswitch provides call control and service logic. Unlike the Class 5 switch, the softswitch is not involved in transport or switching of the packetized voice stream. The softswitch and the IP Centrex CPE (Customer Gateways and IP Phones) signal one another over a packet network using an IP telephony protocol, such as H.323 or SIP. After it receives call setup information, the softswitch determines where the called party resides. If the called party is a member of the Centrex group, then the softswitch instructs the originating Customer Gateway (or IP Phone) and terminating Customer Gateway (or IP Phone) to route the packetized voice streams directly to one another; consequently the voice stream never leaves the LAN/WAN. If the called party is served by the Public Switched Telephony Network, then the softswitch instructs the originating Customer Gateway (or IP Phone) to route the packetized voice stream to a trunk gateway. The trunk gateway has traditional interoffice facilities to Class 4 or Class 5 switches in the PSTN. The trunk gateway packetizes/depacketizes the voice stream so that it can be transmitted over these circuit-switched facilities. The trunk gateway works in conjunction with a signaling gateway. The signaling gateway is used to exchange SS7 messages with the PSTN. Both the trunk and signaling gateways receive their instructions from the softswitch.

Softswitches have only recently become available, and though feature development is proceeding rapidly, today's softswitches do not yet support a rich set of features. Consequently, the trials currently being planned are largely aimed at user's with more modest feature requirements.

The softswitch architecture for IP Centrex is illustrated the network diagram below. The diagram also explains the function of each component.