How does ADSL compare to a dial-up or leased line connection?

Historically two methods of Internet access were available in the UK: dial-up and leased line.

Dial-up is suitable for the domestic or small business user who uses a MODEM or Terminal Adaptor to access his Internet Service. This limits the speed at which the user can connect to either 56 kbps for V.90 MODEM access or 128 kbps for dual-channel Basic Rate ISDN (BRI) access. The user normally connects on a periodic basis to send and receive email and News on a store and forward basis and can also connect on-demand for web browsing and other interactive purposes.

Leased-line access is normally for more intensive users of Internet services such as large corporates, and involves the user connecting to a Point of Presence (PoP) via a leased line at 64kbps, nx64kbps or 2Mbps. Normally, the ISP would deploy some form of Customer Premises Equipment (CPE) at the customer site, such as a router and firewall.

Leased line Internet access is generally more expensive than dial-up of, particularly at higher speeds. ADSL access offers a middle-ground between these two access types, offering the customer high speed access (up to 2 mbps), when compared with dial-in; and continuous connection for a low fixed monthly fee irrespective of how much the link is used.

Differences between ADSL and Leased Line Access

Conceptually, a leased line can be regarded as a sealed pipe of a particular size from your premises to your Internet service provider. ADSL access is a much more cost-effective method for Internet access, and allows users who historically have only been able to justify either dial-up or low-speed (64/128kbps) leased line access to take advantage of the broadband revolution.

There are fundamentally two differences between ADSL and leased-line Internet access:

Symmetry

A leased line offers symmetric access to the Internet - that is to say the downstream and upstream speed is the same. This could be important if you intend to host your own web or ftp servers on your own premises, rather than using the hosting facilities provided by the ISP as part of its standard service. If you do decide to host on your own premises, the upstream path to the ISP is 256 kbps - normally fast enough for most applications.

Overbooking
Capacity across BT's network is shared by other users. This 'overbooking' of capacity is 20:1. As a result, in the absolute worst case you could get a minimum of 1/20th of the maximum capacity of your circuit. For a 2 mbps downstream service, as a worst case you could get 100kbps of downstream capacity. This will not normally be the case, and you should usually expect much better performance. This is because access to the Internet tends to be 'bursty': typically a user downloads a page of information, but then spends considerably longer than the download time reading the information contained on the page. Statistically this 'averages out', meaning that although the capacity across the backbone is nominally overbooked, performance is usually very good, but can never be guaranteed.