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DSL: Digital Subscriber Line

Last Edit: 19/05/17

bullet Introduction

DSL stands for 'Digital Subscriber Line', and is a fixed wire Internet access technology that provides a broadband service across the 'local loop' (telephone line that loops from the customers premises to the telephone exchange) of the UK's copper line telephone network. DSL access technologies, such as ADSL, replaced dialup (narrowband access technology) as the most popular way to access the Internet (from 2000-2005) in the UK. Before DSL was launched, dialup was the only access technology available to most domestic premises (UK). DSL and dialup are similar in one respect: they are both provided on the 'local loop' of the copper line telephone network. The difference between the two technologies is that DSL is a broadband technology and dialup is a narrowband technology. DSL is categorised as a broadband technology because it can support multiple signals / channels: which enables the 'plain old telephone service' (POTS) to be used simultaneously with Internet access (dialup uses the frequency band as POTS). DSL separates the analogue phone signal from the DSL data signal by placing them on a different frequency; the analogue voice signal is placed on a lower frequency (below 4KHz).

DSL's advantage is accessibility: it uses an established telecommunications infrastructure that can connect to virtually every building in the UK. When DSL was launched in the UK, it was at least 10 times faster than dialup (dialup is maxed at 60 kbit/s, dsl should provide 512 kbit/s), and has continued to increase in speed with the launch of new DSL technologies like ADSL2+ -- BT has been upgrading its equipment at telephone exchanges to evolve from a 20CN to a 21CN network. Improvements to BT's core telephone network infrastructure -- to improve DSL speeds -- has not effected end-users: as the changes are at the telephone exchanges and not on the local loop.

DSL's disadvantage is that its signal strength/quality degrades over distance, and many premises struggle to achieve a download speed of 8 mbit/s; even though ADSL2+ is capable of providing 24 mbit/s. Therefore, while DSL is cost effective, its data capacity is not suitable for modern Internet services: high definition video streaming etc. Fiber optic networks are slowly being implemented in the UK, and should eventually replace DSL as the most common fixed wire access technology (within 10-20 years). Fiber optic is better at retaining signal strength over distance, provides a superior speed, and has superior protection against electrical/atmospheric interference.

bullet Equipment

DSL (UK) technologies are provided upon BT's 'local loop' copper pair (tip and ring) telephone infrastructure -- technical problems on the local loop are fixed by BT Wholesale. The local loop can be over-ground, via telegraph poles, or undergound, and connects a customers premises to the local telephone exchange. The local telephone exchange has DSL equipment (DSLAM) from BT Wholesale and from independent LLU network providers. The DSL equipment at the exchange will then connect to the Internet via switches, gateways, edge routers, colocation centres and central data pipes. The end-user is typically unaware of what occurs beyond their local telephone exchange.

To connect to the local telephone exchange via the local loop, end-users will need a device that is capable of supporting TCP/IP (Internet software); such as a desktop/laptop computer with a modern operating system. Alongside a TCP/IP capable device, end-users will also need: DSL router/modem, ethernet cable, modem cable and a DSL filter (microfilter). DSL routers/modems take digital data from the TCP/IP capable device and communicate with the DSLAM at the telephone exchange; DSL routers are capable of providing a range of stats on the status of the local loop: such as the Signal to Noise Ratio (SNR). Some routers are noted for their superior ability at handling poor line conditions on the local loop: such as the Netgear DG834G.

Some popular DSL routers are the following: ASUS DSL-AC88, Belkin F5D7630, Belkin F5D7630, Belkin F5D7632, Belkin F5D7632, Belkin F5D7633 rouBelkin F5D763, Billion BiPAC 5200, Billion BiPAC 7300, Binatone 500, Binatone 1000, Binatone 2000, BT Home Hub 1, BT Home Hub 2, BT Home Hub 3, BT Home Hub 4, BT Home Hub 5, BT Voyager 100, BT Voyager 105, BT Voyager 205, BT Voyager 210, DLink 300, DLink 604, D-Link DSL-3580, D-Link DSL-G604T, Draytech Vigor 2600, Draytech Vigor 2800, Draytek Vigor 2860, Edimax AR-7287, Fujitsu FDX310, Huawei HG612, Huawei MT882, Linksys AG-241, Linksys WAG 160, Linksys WAG 354, Netgear DG814, Netgear DG834, Netgear D6000, Netgear D6400, Plusnet Hub, Sagem 800, Sky Hub, Speedtouch 330, TP-LINK Archer VR400, Zoom X7N and Zyxel VMG 1312.

bullet Technology

There are two categories of DSL: asymmetric digital subscriber line (ADSL) and symmetric digital subscriber line (SDSL). ADSL refers to a DSL technology that has a higher bit rate is one direction (higher download bit rate), and SDSL refers to a DSL technology that has an equal bit rate in both directions. ADSL is usually marketed to home users - who download data more than they upload data - and SDSL tends to be marketed to business users - who upload more data to provide business services. John Cioffi, an American engineer, is credited as being the 'father' of DSL, or essential in it's development, due to his research of discrete multitone modulation (DMT) in the mid 1980's. DSL technologies have been developed by a range of research organisations and telecommunication companies, such as: AT&T Paradyne, Texas Instruments, and the American National Standards Institute (ANSI).

ADSL is by far the most popular DSL technology in the UK, because it supports the highest download bit rate (up to 24 mbit/s), and the phone can be used at the same time as the Internet. The drawback with ADSL is that it's speed drops considerable the further away a user is to the telephone exchange; making it a bad technology for people who are over 3 miles from the telephone exchange: as they will probably have a download speed of under 2 mbit/s. Openreach (BT) has deployed the DSL technology Broadband Enabling Technology (BET) - which is a version of Symmetrical high speed digital subscriber line (SHDSL) - to improve download speeds for rural areas who are over 8 miles from the telephone exchange. Standard ADSL only supports a download speed of 8 mbit/s (1 mbit/s upload), but has been updated to Asymmetric digital subscriber line 2 (ADSL2) download 12 mbit/s and upload 3.5 mbit/s, and Asymmetric digital subscriber line 2 plus (ADSL2+) download 24 mbit/s and upload 3.5 mbit/s.

SDSL is nowhere near as popular as ADSL, due to it being more suitable for medium sized businesses: who upload far more data than home users or small business users. Some versions of SDSL can support an upload bit rate of up to 22 mbit/s when paired over multiple copper lines. Some popular versions of SDSL are: High bit rate digital subscriber line (HDSL), High bit rate digital subscriber 2 (HDSL2), High bit rate digital subscriber line 4 (HDSL4), and Single pair high speed digital subscriber line (G.SHDSL). HDSL2 is provided over two copper wires and HDSL4 is provided over four copper wires; HDSL4 is more expensive to implement, but is more reliable for essential business services.

Some niche DSL technologies are used to improve performance over distance, tolerance of copper line noise distortion, or to increase bandwidth up to 400 mbit/s; some examples are: Rate adaptive digital subscriber line (RADSL), DSL Rings (DSLR), Uni-DSL (UDSL), Multi rate symmetric DSL (MSDSL / MDSL), Etherloop, Very high bitrate digital subscriber line (VDSL), Very high bitrate digital subscriber line 2 (VDSL2) and DSL bonding.