Understanding IP Telephone Systems: Components and Features

As we know, PBXs (Private Branch Exchanges) are local exchanges within an office’s premises. They act as a pool of telephone trunks or lines connecting to the local telephone company. A high-capacity path between the telephone company and the central office is referred to as a trunk. In this way, PBXs help hundreds of users share a common pool of just about 10 trunks.

This setup allows organizations to cut down the cost of calls between users within the office premises, as these calls will be free. Additionally, they only need to pay for a few trunk lines to the telephone company. PBXs eliminate the need for separate wirings to individual telephones, as all users are interfaced with PBXs located within the organization.

Telephone systems based on the Voice over IP (VoIP) protocol are referred to as IP PBXs.

IP telephone system

The figure above depicts the IP telephone system block diagram. As shown, the entire system is made up of the following components:

• Media servers
• Media Gateways
• Layer-2 switches
• Links between PBXs and IP systems
• Connectivity with PSTN (Public Switched Telephone Network)
• Routers and connectivity with the data network
• IP-enabled telephones and computers with VOIP softwares

Here, the PBX is connected with the PSTN.

The main function of the media server is call processing. In addition, the media server will have applications for email, speech recognition, web pages, contact centers, unified messaging, and voice mails. All IP-based telephone systems are equipped with redundant media servers so that in case of any fault, the standby server will take over for the faulty one.

Media gateways are composed of advanced DSPs (Digital Signal Processors) so that voice can be compressed to fit within LAN-compatible packet formats. They also handle analog-to-digital and digital-to-analog signal conversion.

As shown in the figure, layer-2 switches are connected on the LAN. Computers and telephones share the same cabling used by the layer-2 switches. The user’s computer is connected with the data outlet port of the telephone, and the telephone is connected with an 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 is also possible, as shown, to provide a greater degree of flexibility. Routers are also connected with this small network to provide connectivity with the data network of other branch offices, as well as the internet backbone.

A gateway is also used to provide functionalities such as IP to TDM and vice versa conversion, compression, and signaling.

IP PBXs support analog phones, softphones, and IP feature phones. Softphones are specific applications installed on user computers so that the user can use the computer itself as an IP telephone to make voice calls. Companies develop GUIs (Graphical User Interfaces) which provide various built-in functionalities on the user’s computer, such as caller ID, message alerts, and hold buttons.

Typical features of IP PBXs are as follows:

IP PBX Features

• 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 as Lease Line & Optical connectivity through suitable interface.
• It should be able to integrate with Third-party PBXs 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.
• The system should support codec-independent video calls for both IP Users and IP Trunks. It should support codecs such as H.263, H.263+, and H.264.
• Should support secure conversations through sRTP, SIPs along with TLS.
• The 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 a few popular vendors of IP telephone systems across the globe. Session Initiation Protocol (SIP) defines a way to set up calls and audio/video conferences so that telephones from different manufacturers can interoperate.

• 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.