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Glossary

ADSL/ADSL2+ (Asymmetric DSL)

DSL where uploads and downloads are not the same speed. ADSL1 allows a maximum 1.5Mbps download and 256kbps upload speed. The newer 2+ variant is rated at 24Mbps download and 1Mbps upload. However that speed can only be achieved within a few hundred metres of the exchange - and even then the ISP may limit service. A more realistic speed for the majority of the population is 5-10Mbps download and 200-600kbps upload.

Bandwidth

In computer networks, the maximum speed at which data can be transferred, usually measured in megabits per second (millions of bits per second).

Contention ratio

Contention is a major brake on ADSL performance. It is the ratio between actual bandwidth available and the theoretical bandwidth required to provide the promised bandwidth to all users sharing the service. On ADSL, for example, a contention ratio of 20:1 is typically used, so the actual bandwidth is only 1/20 of what would be required to supply the full bandwidth to all users. Contention is a major brake on ADSL performance.

DSL (Digital Subscriber Line)

A technology that allows digital signals to be transferred over standard telephone lines. There are different types of DSL, with the newer being faster.

Encryption

A system of securing data, making it intelligible only to the authorised party which is in possession of the 'key', a secret number almost impossible to guess.

Ethernet

A technology for interconnecting computing devices in a local area network (LAN) or wide area network (WAN).

Internet

The global network which connects hundreds of thousands of networks together using standardised protocols such as tCp/ip.

Jitter

A measure of the variability of latency in the network. The standards-based term for jitter is packet delay variation (PDV). PDV is an important quality of service factor in assessment of network performance.

Latency

The time it takes data to get from a source point in the network to its destination. Sometimes the return trip is also included in latency. Latency will be greater than the raw transmission time allowed by the bandwidth, because there are usually delays at various points in the network.

MAC address (Media Access Control address)

Nothing to do with Apple computers, this is a unique number assigned to a network interface device at the time of manufacture. One use of MAC addresses is MAC address filtering, which can enhance security. That is, a network can be configured to allow only devices with specified MAC addresses to connect.

Packet Loss

acket loss occurs when one or more packets of data traveling across a computer network fail to reach their destination. The rate of packet loss is measured as a percentage. Some protocols are designed to resend lost packets. Data integrity is ensured even if there is a high loss rate, however packet loss becomes problematic with applications involving voice or video streaming.

QoS (Quality of Service)

The ability to provide different priority to different types of data flows, or to guarantee a certain level of performance to a data flow. An important application of QoS is to ensure that video or audio data has priority through a network, since small delays in other data (such as websites or email) are not noticeable.

SHDSL (Symmetric High bit-rate DSL)

This variant of DSL offers symmetric upload speed and download speed. Circuits may be bonded to create a 4Mbps link. However, to achieve 2Mbps the distance of the circuit must be no greater than 3km. Once the circuit exceeds this distance, the circuit will run at slower speeds. Maximum distance from the exchange is 6km where the speed will fall to 192kbps.

TCP/IP (Transmission Control Protocol and Internet Protocol)

Communications protocols used for the Internet and other similar networks. TCP/IP comprises a set of layers, each solving a set of problems involving the transmission of data.

Telepresence

An enhanced form of video conferencing which uses high quality audio and video to give the participants the impression of being in the same room.

Video Conferencing

Communicating with one or more people at a different location using both sound and image. Traditionally, teleconferencing has provided only small, jumpy images of the other party, mainly due to limited bandwidth. See also telepresence (above).

VDSL / VDSL2 - Very High Bit-rate DSL

A high speed, short distance DSL technology which allows speed of up to 18Mbps download and 16Mbps upload within the high rise building. The second variants of VDSL will allow speeds of 50Mbps to 100Mbps within the same building. This technology is very popular in countries like Korea where the internet is used for mass distribution of video and TV.

VoIP – Voice over Internet Protocol

System for allowing voice communications over computer networks, including the Internet. From the user's point of view, VoIP can be identical to making traditional telephone calls. If the computer network does not perform adequately, call quality can suffer. However a properly designed VoIP system can offer better performance and features than regular telephony, and at much lower call cost.

VPN (Virtual Private Network)

A system to allow businesses to access their internal networks and computers over the Internet or other public network. Encryption and other techniques are used to ensure that data cannot be accessed without authorisation. VPNs are primarily used to connect branch offices and allow mobile staff to access the company's network.