A guide to VoIP telephony
Published: 26 Jul 2006
How VoIP works
VoIP works by converting speech into a digital signal, compressing it and encoding it using one of a range of speech CODECs (COder/DECoder) and breaking it into data packets that are sent over the Internet. All VoIP systems support a range of CODECs that can juggle call quality and bandwidth. Call quality, or QoS (Quality of Service), is affected by Internet traffic density and, on any individual call, the call may switch to a lower bandwidth (and therefore lower quality) CODEC if the available Internet bandwidth is limited.
At the destination, the packets are reassembled, decoded using the appropriate CODEC, and converted to analogue audio that can be heard by the call recipient. The compression introduced by the CODEC and the packet switching makes VoIP much more efficient, and therefore cheaper, than a point-to-point analogue phone call. The standard Internet protocols are used to route the packets between addresses just like any other internet traffic -- hence the term Voice over Internet Protocol. Additional protocols are required to handle call control.
Unlike the traditional telephone service, the destination ID (or telephone number) is not assigned to a specific run of cable, but by the user to the VoIP device itself. This means that VoIP IDs/phone numbers are portable. A VoIP phone can be moved by the user to various locations and it will still receive calls made to its ID.
All VoIP devices (apart from software VoIP systems like Skype) have a built-in Web server. Each VoIP device must be assigned an IP address. If the device is a phone, this can be done either manually from the phone's keypad or via a DHCP server. Users can then use a browser to gain protected access to this Web server by entering a user name and password. Once access is gained, the user can set the VoIP ID, password, domain and an outbound proxy. These values are usually stored in NVRAM inside the VoIP device.
Setting up a VoIP device can be tricky because, as with so much network equipment, terms and syntax vary widely from one manufacturer to another. The VoIP service providers are the best source of setup information, and each provider supports a limited range of phones, so your choice of hardware and provider are quite closely linked.
VoIP protocols -- SIP & H.323
Handling telephone calls over the Internet requires an extra protocol, in addition to the IP, to manage call setup, voice data transfer and call termination. There are several standard protocols designed to do this; ITU-T H.323 (known as H.323 for short), Media Gateway Control Protocol (MGCP) and the IETF Session Initiation Protocol otherwise known as SIP. The latter is one of the simplest and by far the most popular: SIP is used by Vonage and many other VoIP service providers.
There are also proprietary VoIP protocols -- the Minet protocol that Mitel Networks uses on its range of VoIP equipment and the peer-to-peer Skype protocol, for example.
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