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IP telephone system

As we know PBXs are local exchanges within the office premises. They are the pool of telephone trunks or lines connecting with local telephone company. High capacity path between telephone company and central office is referred as trunk. In this way PBXs help hundreds of users share the common pool of just about 10 trunks. This way organizations cut down cost of calls between users of office premises as those will be free and also they need to pay for few of the trunks lines to the telephone company.

PBXs avoid the separate wirings to individual telephones as all the users are interfaced with PBXs located within the organization. Telephone systems based on voice over IP protocol is referred as IP PBX.

IP telephone system

Figure depicts the IP telephone system block diagram. As shown the entire system is made up of following components:
•  Media servers
•  Media Gateways
•  Layer-2 switches
•  Links between PBXs and IP systems
•  Connectivity with PSTN
•  Routers and connectivity with data network
•  Ip enabled telephones and computers with VOIP softwares

Here PBX is connected with PSTN(Public Switched Telephone Network). The main functio of media server is call processing. In addition media server will have applications for email, speech recognition, web pages, contact centers, unified messaging and voice mails. All the IP based telephone systems are equipped with redundant media servers so that in case of any fault other standby server will take over the faulty one. Media gateway composed of advanced DSPs (Digital Signal Processors) so that voice can be compressed to accomodate within LAN compatible packet formats. They also take care of analog to digital and digital to analog signal conversion.

As shown in the figure, layer-2 switches are connected on LAN. computers and telephones share the same cabling used by layer-2 switches. The user's computer is connected with data outlet port of the telephone and telephone is connected with RJ45 ethernet port over a LAN or WAN.Cisco Unified IP phone 7900 series are very popular IP enabled phones. Separate dedicated wiring to layer-2 switches are also possible as shown to provide greater degree of flexibility.

Routers are also connected with this small network to provide connectivity with data network of other branch offices as well as internet backbone. Gateway is also used to provide functionalities viz. IP to TDM and vice versa conversion, compression and signaling etc.

IP PBXs support analog phones, softphones and IP features phones. Softphones are specific applications installed on user computers so that user can use the computer itself as IP telephone to make voice calls. Companies developed GUI which provides various built in functionalities on the user computer such as caller ID, message alerts and hold buttons etc. Typical features of IP PBX are as follows.


•  Need to provide connectivity with Analog Public Network (P&T lines), ISDN Public Network (BRI /PRI/E1R2/T1), Private Network of E&M, LD Trunk as well Lease Line & Optical connectivity through suitable interface.
•  It should be able to integrate with Third party PBX using Analog, digital or SIP trunks.
•  It should support Computer Telephony Integration.
•  shall support H.323/SIP trunking & SIP Devices supporting RFC 3261, RFC 3264, RFC 3310, RFC 2327, RFC 4028.
•  System should support codec independent video calls for both IP Users and IP Trunks. It should support codecs viz. H.263, H.263 and H.264.
•  Should support secure conversations through sRTP, SIPs along with TLS.
•  System should support a wide range of directory integration options that include integration with the PBX telephone directory, Microsoft Active Directory® or LDAP (lightweight directory access protocol).

Vendors or manufacturers

Following are few of popular vendors of IP telephone system across the globe. Session Initiation Protocol(SIP) defines way to set up calls, audio video conferences so that telephones from difference manufacturers can inter-operate.
•  3Com
•  Cisco Systems
•  Avaya communications
•  Nortel Networks
•  Mitel Networks
•  Compagnie Financiere Alcatel
•  Siemens ICN
•  Nexspan system
•  Pingtel corporation
•  Vertical networks Inc.
•  Altigen
•  ShoreTel Inc.
•  EADS Telecom
•  Interactive Intelligence Inc.

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