Class 5 Switch-based architecture for IP Centrex

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1. Ordinary analog telephone sets and fax machines may be directly served by a Class 5 switch or may be served through a customer gateway. On behalf of the analog devices, the customer gateway performs voice digitization and packetization. It also converts analog signaling into IP telephony messages.
2. Digital telephone sets, using ISDN's Q.931 signaling messages, may be directly served by a Class 5 switch or may be served through a customer gateway. On behalf of the digital telephone set, the customer gateway performs voice packetization and converts Q.931 messages into IP telephony messages.
3. The customer gateway has an Ethernet interface for connecting to the LAN. It also provides jacks to connect analog devices (telephones, fax machines) and/or ISDN telephone sets. The customer gateway sends packetized voice and IP telephony messages across the LAN, over the broadband access facility, and through the managed backbone network to the network gateway.
4. The IP Phone combines the customer gateway functionality and telephone set functionality into one device. The IP Phone has an Ethernet interface to the LAN. The IP Phone may use the analog call model or ISDN call model when formulating IP telephony messages. Based on the call model used, the Class 5 switch offers analog-like or ISDN-like features.
5. The network gateway converts between the IP telephony messages and packetized voice traffic (that the customer gateway and IP Phone use) and the GR-303 messages and circuit-switch voice traffic (that the Class 5 switch uses). The GR-303 message set accommodates ISDN as well as POTS signals.