Microwave frequency band in the range of 40 - 75 GHz that corresponds to wavelength from 0.75 cm to 0.4 cm.

An ITU-T standard for dial-up modem that provides data rate of 28.8  - 33.6 kbps. Also known as V.Fast.

An ITU-T standard for dial-up modem that provides data rates of 56 kbps (downstream) and 33.6 kbps (upstream). Also known as V.Last.

A revision of V.90 protocol with increased upstream data rate (from 33.6 to 48 kbps), faster connection time, and Modem On Hold feature.
Modem On Hold feature means when there is a telephone call you can answer the call while your Internet connection is suspended and when the call ends you can get back to the Internet without having to dial up again.

Variable Bit Rate. A service class in ATM which specifies an average cell rate. There are 2 variations of VBR, i.e. rt-VBR (real-time VBR) that supports interactive multimedia applications and nrt-VBR (non real-time VBR) for bursty traffic.
Also see ATM.

Virtual Container in SDH/SONET or Virtual Circuit in ATM/Frame Relay.

Very Fast IrDA. An IrDA standard for short-range, point-to-point, and half-duplex communication using infrared that provides maximum data rate of 16 Mbps.
Also see IrDA and infrared.

Very High-bit-rate Digital Subscriber Line. A variant of DSL with maximum downstream data rate of 52 Mbps and upstream of 6 Mbps over a telephone cable. VDSL comes at a price, that is shorter distance from the Central Office (or the Access Node) to the subscribers, i.e. 300 - 1500 meters. As with other DSL technologies, the farther a user from the Central Office, the lower the data rate. VDSL is defined in ITU-T G.993.1 (G.vdsl).
Also see DSL.

Very High-bit-rate Digital Subscriber Line 2. The latest addition to xDSL family that provides broadband access over a standard telephone cable, of up to 100 Mbps both downstream and upstream. VDSL2 reaches subscribers' homes through fiber optic access network that extends from the service provider's Central Office down to a neighborhood VDSL2 access node.
VDSL2 is defined in ITU-T G.993.2. It enables telcos and ISPs to deliver triple play services (e.g. Internet, VoIP, VoD, IPTV, HDTV) to their subscribers at their best quality.

Very High Frequency. A frequency band in the range of 30 - 300 MHz, corresponding to wavelength of 10 m - 1 m. Commonly used for television broadcast, terrestrial navigation systems, and aircraft communications. FM radio station also broadcasts at VHF band, from 88 MHz to 108 MHz.

Virtual LAN. A logical LAN that groups nodes based on their functions rather than their locations. The nodes may be located at different locations and different physical LANs, but they are connected as if they were in the same LAN.

Very Low Frequency. A frequency band in the range of 3 - 30 kHz, corresponding to wavelength of 100 km - 10 km. VLF is used for communication with submarines and radio navigation.

Voice over Internet Protocol. Also known as IP Telephony. VoIP is the technology that enables voice (telephone) calls to be carried over IP network (over LAN or Internet) instead of telephone network (PSTN).

A VoIP user doesn't need special hardware, because she can use her computer sound card, microphone, speaker, and softphone (special software for VoIP) as a telephone set. A VoIP user also can use a VoIP telephone or a regular telephone equipped with a VoIP adapter.

The advantages of using VoIP are portability, rich features, and Web integration. VoIP call can be integrated with other Internet traffic like e-mail, file transfer, and web browsing. VoIP drawback is its voice quality which is less stable compared to regular telephone (POTS). However, voice encoding and compression technology added with IP traffic prioritization technology has improved over time to make the voice quality better.

VoIP is more popular among broadband Internet users because its quality is more acceptable with broadband connection rather than dial-up. VoIP service is offered by many operators now with lower subscription fee than that of traditional telephone service. If you use softphone offered by several VoIP operators, you can enjoy VoIP for free (you still have to pay your Internet subscription anyway!).

Also see VoIP protocols: H.323, H.248, MEGACO, MGCP, SIP, RTP, RTCP.

Virtual Private Network. Private network that runs over public network infrastructure. VPN uses tunneling mechanism to maintain privacy and security of the data. VPN is often used by companies to provide access to their internal network resources to their home or mobile workers.
See picture. Also see tunnel and VPN protocols: L2F, L2TP, PPTP, IPSec.

Voltage Standing Wave Ratio. VSWR describes how much energy radiated by an antenna is reflected back to the antenna.
Also see antenna.