Glossary

Access Latency

Maximum time that a port will take to either successfully transmit a packet or discard it as measured from the time the packet is presented for transmission.

Access Line

The physical telecommunications circuit connecting an end user location with the serving central office in a local network environment. Also called the local loop or "last mile." See also Local Loop.

Always on

Current dial up services require the user to 'make a call' to the ISP. The connection is only active during the duration of the call. Most xDSL implementations (including ADSL and SDSL) enable the connection to be always on.

Attenuation

The decrease in the magnitude of the power of the signal transmitted over a wire (as a function of distance) measured in decibels. As distance increases, attenuation also increases and hence signal power decreases.

ADSL

High speed Internet access that works over existing telephone cables. ADSL works by splitting the phone line into two frequency ranges. The frequencies below 4 kHz are reserved for voice, and the range above that is used for data. This is what makes it possible to use the line for phone calls and data network access at the same time. ADSL provides download speeds of up to 8Mbit/s and upload speeds of up to 1Mbits per second. The different upstream and downstream rates define the Asymmetry. Home and small business users would predominantly use this form of DSL.

Bandwidth

The amount of data that can be sent through a given communications circuit per second

Bandwidth on demand

Designed for organisations that require different amounts of bandwidth at different times of the day.

Broadband

Broadband is any technology that provides users with a bandwidth of more than 100kbit/s. That technology can use either copper twisted pairs, cable from television service providers, satellites or wireless technologies.

Contention Ratio

This describes the maximum number of users sharing the bandwidth on the connection between your local exchange and the Internet Service Provider. A customer with a contention ratio of 20:1 may have to share this bandwidth with up to 20 other users at peak loading. However this is unlikely as not all users will be using the service at the same time.

CPE

Customer Premise Equipment. This refers to any equipment located at the customer's premises. DSL modems, bridges and routers are examples of CPE

Dial up

A term used to refer to slower Internet Access using a phone modem. Usually, the user dials a telephone number and using the computer in order to connect to the Internet. The maximum bandwidth of a dial up connection is 56kbit/s.

DNS

(Domain Name System) Name resolution software that lets users locate computers on a UNIX network or the internet (TCP/IP network) by domain name. The DNS server maintains a database of domain names (host names) and their corresponding IP addresses.

Downstream

Data flowing from the Internet to your computer (surfing the net, getting email, downloading a file)

DSL

DSL (Digital Subscriber Line) provides high-speed Internet Access using regular telephone lines. It has the ability to move data at speeds up to 8Mb/s or eight million bits per second, or 140 times quicker than the fastest analogue modems available today.

DSLAM

A DSLAM (Digital Subscriber Line access Multiplexer) is a device located in the central office switch that enables it for xDSL capabilities.

Dynamic IP Address

An IP address that is automatically assigned to a client station in a TCP/IP network, typically by a DHCP server. Network devices that serve multiple users, such as servers and printers, are usually assigned static IP addresses. (See static IP address)

Firewall

A firewall is designed to keep a network safe from intruders. It can be a single router that filters out unwanted packets or may comprise a combination of routers and servers each performing some type of firewall processing.

IP address

(Internet Protocol address) The address of a computer attached to a TCP/IP network. Every client and server station must have a unique IP address. Client workstations have either a permanent address or one that is dynamically set assigned to them each dial up session (see Static & Dynamic IP address for more detail) IP addresses are written as four sets of numbers separated by periods; for example, 204.171.64.2

Loop Qualification Check

A test that the telephone company can perform to see how far a customer is from the Central Office Switch.

NAT

Network Address Translation. This allows an organisation to present itself to the Internet with far fewer IP addresses than there are nodes on its internal network. The NAT technology, which is typically implemented in a router, converts the private IP addresses (such as in the 10.0.0.0 range) of the node on the internal private network to one IP address or of several IP addresses for the public Internet. It changes the packet headers to the new address and keeps track of each session, so that when the packets come back from the Internet, it performs the reverse conversion to the IP address of the client machine. NAT also serves as a firewall by keeping internal addresses hidden from the outside world.

SDSL

SDSL (Symmetric DSL) is a single twisted pair line, carrying 2.3Mbit/s each direction on a duplex line. The term symmetric is used in this context, as the data rate is the same in both directions.

Static IP Address

A permanent IP address that is assigned to a node in a TCP/IP network. Servers and routers are usually assigned static IP addresses, while client stations are often assigned dynamic IP addresses from a DHCP server each time they come online. (See Dynamic IP address)

Upstream

Data flowing from your computer to the Internet (sending emails, uploading a file)

VOD (Video On Demand)

The ability to activate a stored or live motion picture stream; in xDSL the application that allows subscribers to view movies or other video programming on request, similar to cable television's Pay-Per-View. See Pay-Per-View.

VDSL

VDSL (Very high bandwidth DSL) is in the early development phase but promises much higher data rates over relatively short distances (between 51 and 55 Mbit/s over lines up to 1,000 feet or 300 metres in length)

xDSL

This is a 'catch-all' phrase encompassing the family of DSL technologies (ADSL, SDSL...)