faq
What is adsl & sdsl?
ADSL or 'asynchronous digital subscriber line' is a connection which runs over a plain old telephone system - POTS - line; i.e. the normal phone system. It is a shared service & the connection speed is faster incoming (downlink) to your network than it is outgoing (uplink) from your network. The generally accepted maximum ADSL speed is 2Mb/s downlink, 256kb/s uplink; i.e. 8 times faster in than out. This is fast changing to ADSL Max & ADSL2 whereby the accepted maximum bandwidth will be 8Mb/s downlink, 800kb/s uplink. It is dependent on the local telephone exchange being correctly enabled, the quality of the telephone line & how far it is from the exchange as to what bandwidth is available at a specific location. The next generation is ADSL2+ which can reach downlink speeds up to 24 Mb/s & doubling the uplink speed. This is not yet widely available.
SDSL or 'synchronous digital subscriber line' is very similar to ADSL except in one important area. Uplink & downlink speeds are exactly the same.
ADSL & its variants are normally shared services, that is to say the bandwidth within your Internet Service Provider's (ISP) network is shared to a certain degree with other subscribers to the ISPs network services. This is called contention ratio & can be important when selecting your ISP; be warned though not all ISPs are equal & contention ratio is not everything. As in all things you pay for what you get. If a non-contended connection is required then a leased-line is an option.
One thing to note. The contention ratio quoted by an ISP is often only for the traffic which is passed over that ISP's network. Once the connection passes from the ISP's network the contention ratio can be & often is completely different.
What is a leased-line?
In simplistic terms a leased-line is a dedicated office-office or office-ISP connection. It is not shared with any other subscriber at any point until it arrives in the Internet "cloud". In essence there is no contention ratio within the service provider's network.
What is contention ratio?
Contention ratios are related to planning rules, & planning rules are used by network engineers when designing networks. Contention ratios are becoming less important as bandwidth increases & is sold more on actual usage than on contention ratio. If done properly contention ratios should have no impact on the service & the end-user experience.
Contention ratio describes the maximum number of users sharing the bandwidth on the connection between your local exchange & 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 rarely happens, as not all users will be using the service at the same time. Typical contention ratios are 50:1 for home users & 20:1 down to 5:1 for business use.
NB. Often the contention ratio quoted by an ISP is only for the traffic which is passed over its own network. Once the connection passes from the ISP's network the contention ratio is often completely different & something the ISP has no control over.
