Tech Primer

Voice over Internet Protocol

What is VoIP?

Voice over Internet Protocol technology leverages the connectionless nature of TCP/IP in which packets can take different paths between endpoints and all paths are shared by packets from different transmissions. Special headers are used to give voice packets priority over data.

How does VoIP work?

It sends voice information in digital form as discrete packets rather than in the traditional circuit-committed protocols of the public switched telephone network (PSTN). Voice is digitised with an ADC (analogue to digital converter), transmitted, and at the other end it gets transformed back to analogue with a DAC (digital to analogue converter).

What are the advantages using VoIP over PSTN?

Generally VoIP is cheaper than PSTN and companies can save money on long distance calls. It lets telcos and IT heads offers value-added services such as sending images, graphs and videos. In the past year, the quality and reliability of VoIP technology has improved to the point that many Indian companies have started using their existing data networks to carry voice as well.

How many voice calls are possible on a 64 Kbps leased line?

Usually 8 Kbps of bandwidth is required for a normal VoIP call and it provides reasonable clarity. Eight calls can be made simultaneously on a 64 Kbps leased line.

How does VoIP to phone communication takes place?

A VoIP set-up needs a gateway device that receives packetised voice transmission and routes it to its destination using a carrier system interface and sends the data over a WAN or the public Internet.

Can VoIP be used in a public network?

Technically, there’s nothing stopping VoIP from working on a public network. The law however restricts VoIP usage to a closed user group (CUG) within an enterprise, say over a leased line or a Frame Relay network.

What is the role of IP-PBXs within an enterprise CUG?

An IP-PBX is the PBX attached to an IP phone network. It acts as a call manager and server that handles call routing, transferring extension numbers and streamlines new upgrades to the IP system. In conventional PBXs a lot of physical upgrades are required, whereas with an IP-PBX the software handles new upgrades. Companies do not need to keep a separate person to manage an IP-PBX and it can help cut manageability costs. In India there is a mixed trend with some enterprises having IP phones upgrading their conventional PBXs to IP while others use station gateways eschewing the use of an IP-PBX. Green field set-ups in the BPO space tend to go in for IP-PBXs while deploying IP telephony.

What is the role of voice compression in VoIP?

Voice compression is vital in Voice over IP because voice packets usually travel over low-speed links. Some small and medium-sized enterprises, for instance, may connect to a virtual private network (VPN) at only 28.8 kbps. Microsoft Netmeeting, for example, a popular voice application for PCs and laptops, supports ITU G.723.1 voice compression for transmission over dial-up modems. The ITU G.723.1 standard for voice compression over IP ensures toll quality voice.

Who are the vendors offering VoIP equipment in India?

Cisco, Nortel, Avaya, D-Link, Net4India, Alcatel and ITXC.