CLASS 5 SWITCH-BASED ARCHITECTURE FOR HOSTED IP
Hosted IP's configurations usually fit into two broad architectural categories, which center on the system in the hosted location. The first is the Class 5 Switch Architecture (source: www.ip-centrex.org, see Figure 1 below) in which an existent Class 5 switch, e.g., 5ESS, DMS-100, or GTD-5, supports IP Centrex service in addition to traditional plain old telephone service (POTS) and ISDN lines.
Two new components are necessary for this architecture. They are the Network Gateway (NG), which connects to the Class 5 switch, and the Customer Gateway (CG), housed on the customer premises. The NG connects with the Class 5 and translates signaling information it receives from the CG into a protocol that the Class 5 understands. The NG also de-packetizes 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 CG.
The CG communicates with the NG via packets in IP telephony protocol, e.g., H.323 or Session Initiation Protocol (SIP), and supports traditional device interfaces for analog and ISDN devices. As a result, the NG, CG, and connecting packet network appear to the Class 5 switch as an ordinary digital loop carrier (DLC) system. The telephone sets connected to the CG appear as ordinary phone lines to the Class 5. Consequently, the Class 5 can deliver the same features to IP Centrex, analog, and ISDN Centrex users without need for an upgrade by the service provider.
FIGURE 1 Class 5 Switch-Based Architecture for IP Centrex
A. Ordinary analog telephone sets and fax machines may be directly served by a Class 5 switch or may be served through the CG. On behalf of the analog devices, the CG performs voice digitization and packetization. It also converts analog signaling into IP telephony messages.
B. The Class 5 switch can serve digital ISDN telephone sets directly or through the CG. On behalf of the digital telephone set, the CG performs voice packetization and converts ISDN signaling messages into IP telephony messages.
C. The CG has an Ethernet interface for connecting to the customer LAN. It also provides jacks to connect analog devices (telephones, fax machines) and/or ISDN telephone sets. The CG sends packetized voice and IP telephony messages across the LAN, over the broadband access facility, and through the managed backbone network to the NG.
D. The IP Phone combines the CG 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.
E. The NG "translates" digital and packetized messages and voice traffic between the CG and Class 5 as necessary.
source: www.ip-centrex.org
SOFTSWITCH ARCHITECTURE
The second Hosted IP architectural configuration features a softswitch (Broadsoft, Deutsche Telekom, or Cisco) in place of the Class 5 CO switch. The 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, but the softswitch is not involved in transport or switching of the packetized voice stream. The softswitch, CG, and IP telephones signal one another over a packet network using an IP telephony protocol, such as H.323 or SIP.
Upon receipt of call setup information, the softswitch determines the called party’s location. If the called party is a member of the Centrex group, then the softswitch instructs the originating CG or IP device and the terminating CG or IP device to route the packetized voice streams directly to one another. As a result, the voice stream never leaves the LAN/WAN. If the called party is served by the Public Switched Telephone Network (PSTN), then the softswitch instructs the originating CG or IP device to route the packetized voice stream to a trunk gateway (TG).
The TG has traditional interoffice facilities to Class 4 or Class 5 switches in the PSTN. The TG packetizes/de-packetizes the voice stream so that it can be transmitted over circuit-switched, i.e., PSTN, facilities. The TG works in conjunction with a signaling gateway. The signaling gateway is used to exchange SS7 messages with the PSTN. The softswitch provides signaling instructions to the trunk and signaling gateways. Figure 2 below depicts the softswitch architecture for Hosted IP.
FIGURE 2 Softswitch-Based Architecture for IP Centrex
A. The CG can serve ordinary analog telephone sets and fax machines. On behalf of the analog devices, the Customer Gateway digitizes and packetizes voice transmissions. It also converts analog signaling into IP telephony messages.
B. The CG connects to the customer LAN. It also provides jacks to connect analog devices (telephones, fax machines) and/or ISDN telephone sets. The CG sends IP telephony messages across the LAN, over the broadband access facility, and through the managed backbone network to the Softswitch. The Softswitch provides the CG with the IP address of the other party. The CG uses this information to send packetized voice streams directly to the destination. The destination may be another CG or IP device on the same LAN, a CG or IP device on a different LAN across the backbone network, or the TG (for parties served by traditional lines on the PSTN).
C. The IP device combines the CG functionality and telephone set functionality into a single device. The IP device connects directly to the LAN and sends IP telephony messages across the LAN, over the broadband access facility, and through the managed backbone network to the Softswitch. The Softswitch provides the IP device with the IP address of the other party. The IP device uses this information to send packetized voice streams directly to the destination. The destination may be another IP device or CG on the same LAN, an IP device or CG on a different LAN across the backbone network, or the TG for parties served by traditional lines on the PSTN.
D. The Softswitch provides the call control and service logic for the Hosted IP service. The Softswitch exchanges IP Telephony messages with CGs, IP devices, Signaling Gateways, TGs, and other Softswitches.
E. When a call must pass between the backbone packet network and the PSTN, e.g., a Hosted IP customer calls a traditional analog customer, the Softswitch uses the Signaling Gateway to exchange SS7 messages with PSTN Class 4 and Class 5 switches through Signal Transfer Points.
F. When a call must pass between the backbone packet network and the PSTN, e.g., an IP Centrex customer calls a traditional analog customer, the Softswitch directs the call to a TG. The TG packetizes/depacketizes the voice stream so that it can be transmitted over a DS1 trunk to PSTN Class 4 and Class 5 switches.
source:www.ip-centrex.org
A unique twist on these architectures is offered by Star2Star Communications, which President/CTO Joe Rhem describes as a "distributed" configuration. In this scenario, a solid state PBX is located at the customer’s site. The PBX, which supports up to 500 users, handles all on site traffic, while calls among distributed locations are handled by a "mirrored" system in the data center. In this way Star2Star manages traffic more efficiently and reduces packet latency.
