Of course, having two signals on a line where most equipment is only expecting one, could be problematic. This created the need to apply micro-filters (“splitters”) to all telephone outlets at the premises, in order to filter the analogue signal, from the data signal. Without using micro-filters, the data signal interferes with the telephone line and causes unacceptable noise, and the analogue line interferes with the data signal, causing data corruption and/or intermittent link loss.
Original trialists and home users were equipped with ADSL (USB) modems in order to connect to the data service. BT identified the need for specific ADSL routers, which could be added to already-modular 10/100BaseT networks, providing Internet connectivity to entire networks, making it convenient, professional and efficient, but also as an additional sweetener to the smaller businesses looking to have multiple users, using the same link.
BT’s Network
The true marvel of the ADSL network in the UK, is probably the sophistication and complexity of BT’s systems in place, in order to provide ADSL services to customers. Note, that BT itself (or at least, BT Ignite, formerly BT Wholesale) do not actually provide Internet connections, they merely provide the end-to-end connectivity, and essentially provide a path between the end user, and the ISP. In terms of billing, BT charges the ISP for the use of the ADSL link, a charge which is passed on to the end user, on top of bandwidth charges and other profit margins.
Buildings and sites are connected directly to their nearest ADSL-enabled exchange, where the signal is separated: analogue signals continue their journey along the PSTN network, whereas the data signal is re-routed to the exchange’s DSLAM – the Digital Subscriber Line Access Multiplexer. The DSLAM multiplexes a number of ADSL signals together from other customers, and sends them along a large-bandwidth infrastructure, known as BT’s ATM Network [4]. ATM stands for Asynchronous Transfer Mode, which is a high-speed, “on demand” transfer protocol, common to larger data communication links.
Successfully routed, the signal is then taken to the Home Gateway (server) of the ISP associated with the ADSL user’s telephone number.
Now at the ISP’s premises, the data can be stripped of its ADSL overheads, treated as ordinary TCP/IP (Transmission Control Protocol/Internet Protocol) data, and can continue its ordinary journey along conventional IP networks.
It should be noted that with one exception, both the DSLAM and relevant de-multiplexer (to unravel the different combined signals) are at BT’s premises.
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