Glossary

What is ADSL?

ADSL (Asymmetric Digital Subscriber Line) is a technology that allows very high-speed data transmission over existing, ordinary telephone lines. It is normally used for very high-speed "always on" Internet access technology.

How does DSL work?

Conventional "voice band" modems use the same frequency band (0-4 kHz) as the telephone service. This means that if your modem is in use, your voice telephone line is unavailable.

Since ADSL operates at frequencies (100 kHz to 1.1 MHz) above the voice channel, ADSL "splits" your telephone line into a voice channel and an ADSL high-speed data channel. Therefore you have two channels available on the one line, one for voice and one for data..

Why is ADSL called "Asymmetric"?

Asymmetric means that the speed of download on the line is different to the speed of upload. For example, a 512kbps/128kbps connection means you are receiving data at up to 512kbps and outputting data at up to 128kbps. ADSL makes more bandwidth (or speed) available downstream than upstream to allow greater downloading capability.

Firewall

A system designed to prevent unauthorized access to or from a private network. Firewalls can be implemented in both hardware and software, or a combination of both. Firewalls are frequently used to prevent unauthorized Internet users from accessing private networks connected to the Internet, especially intranets. All messages entering or leaving the intranet pass through the firewall, which examines each message and blocks those that do not meet the specified security criteria.

Types of firewall

Packet filter: Looks at each packet entering or leaving the network and accepts or rejects it based on user-defined rules. Packet filtering is fairly effective and transparent to users, but it is difficult to configure.

Application gateway: Applies security mechanisms to specific applications, such as FTP and Telnet servers. This is very effective, but can impose performance degradation.

Circuit-level gateway: Applies security mechanisms when a TCP or UDP connection is established. Once the connection has been made, packets can flow between the hosts without further checking.

Proxy server: Intercepts all messages entering and leaving the network. The proxy server effectively hides the true network addresses.

In practice, most firewall vendors use two or more of these techniques together. What is stateful inspection

Also referred to as dynamic packet filtering. Stateful inspection is a firewall architecture that works at the network layer. Unlike static packet filtering, which examines a packet based on the information in its header, stateful inspection tracks each connection traversing all interfaces of the firewall and makes sure they are valid.

A stateful firewall may examine not just the header information but also the contents of the packet up through the application layer in order to determine more about the packet than just information about its source and destination. A stateful inspection firewall also monitors the state of the connection and compiles the information in a state table. Because of this, filtering decisions are based not only on administrator-defined rules (as in static packet filtering) but also on context that has been established by prior packets that have passed through the firewall. Stateful firewalls also close ports that are not currently in use.

VPN

Short for virtual private network, a network that is constructed by using public wires to connect nodes. For example, there are a number of systems that enable you to create networks using the Internet as the medium for transporting data. These systems use encryption and other security mechanisms to ensure that only authorized users can access the network and that the data cannot be intercepted. VPN supports a variety of protocols to allow for interconnection between different vendor hardware.

ISDN

Abbreviation of integrated services digital network, an international communications standard for sending voice, video, and data over digital telephone lines or normal telephone wires. ISDN supports data transfer rates of 64 Kbps (64,000 bits per second).

There are two types of ISDN:

In Ireland, the service known as eircom Hi-Speed is based on ISDN and provides for 2 standard analogue telephone connections and 2 ISDN digital connections (only 2 connections can be active concurrently). It is often used in small office or home office environments to provide additional telephone services and faster more reliable internet access.





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