ADSL

ADSL

Recent technology has improved the digital performance of conventional telephone circuits. One example is Asymmetric Digital Subscriber Line (ADSL), a system that uses special electronics at the exchange to provide an astonishing data rate.

• ADSL data is conveyed using Internet protocols, making it ideal for domestic users or for organisations that already have an Internet-based network, sometimes known as an Intranet.

Speed and Cables

In theory, ADSL offers a rate of 9,216 kbit/s, or around 1.1 MB/s, for data received downstream and 640 kbit/s for what you send upstream. In practice, it usually runs at 384 kbit/s to 2.5 MB/s downstream and 128 to 256 kbit/s upstream.

• In the UK, ADSL usually gives 512 kbit/s downstream and 256 kbit/s upstream, although businesses may use downstream rates of 1,024 or 2,048 kbit/s. All are faster than a 56 kbit/s modem and adequate for multimedia data.
• Unlike a dial-up modem service, ADSL, when connected via an Ethernet router, gives a permanent connection to the Internet. However, some types of ADSL modem, as wired directly to a computer, may need a ‘click’ to get online.

The specified data rate is usually ‘shared’ with up to 50 other users, a situation known as a 50:1 contention ratio. Although not a problem at the moment, this could reduce your final speed to 10 kbit/s if ADSL became an over-popular method of connection. Fortunately, business users can use rates of 1,024 or 2,048 kbit/s with a superior 20:1 contention ratio.

• The issue of contention is a marketing ploy rather than a technical limitation. Each ADSL circuit is connected to DSLAM equipment at your local telephone exchange, which is linked to the company’s core IP routers, the latter connected via 34 Mbit/s ATM or 155 Mbit/s STM-1 links. With a50:1 contention ratio, each circuit can accommodate at least 3,400 users. In practice, it’s unlikely that so many would be connected via a single link.

Because the data travels so fast, a maximum cable length is imposed between each ADSL subscriber and the local telephone exchange. In the early days this was 2.5 kilometres (km), although this has now increased to 3.5 km. A new technology known as Rate-Adaptive ADSL (RADSL) is also available, extending the distance to 5.5 km, although not all providers support this system. In the United Kingdom, BT has announced that it can now support distances of over 6 km.

• If you’re too far away from a local telephone exchange you can’t use ADSL.
• The ADSL service provided by BT in the United Kingdom is known as BT Openworld. For domestic users the adaptor box connects to a USBport, but for a professional system Ethernet is used.

Connecting ADSL

ADSL data is usually conveyed via an existing phone line, although the line must be activated for this purpose by your telephone provider. The connection can be provided on an ADSL faceplate, which replaces the original faceplate on your ‘master’ telephone socket. Unfortunately, most providers charge for this kind of alteration, so it’s usually cheaper to employ the self-install method, involving the use of plug-in micro-filters and telephone splitters.

• Only a standard telephone circuit can be used to convey ADSL data. If your line is ‘divided’ into two circuits by means of the Digital Access Carrier Systems (DACS) you can’t use ADSL.

Self-installation is very easy: you insert a micro-filter into your main phone socket and then plug your non-ADSL devices, such as telephones andtelevision set-top boxes into the standard telephone socket on the filter, using standard telephone splitters as necessary. Finally, plug your ADSL equipment into the RJ11 ADSL socket on the front of filter.

• If your master socket is already wired to several outlets you may prefer not to disrupt the cabling. In this case, fit a micro-filter in every socket and plug each non-ADSL device into the standard socket on the front of each filter.
• A micro-filter is also known as an ADSL splitter. In the United Kingdom this has a standard 6-way modular plug for the incoming line, a matching socket for other telephone devices and an RJ11 socket for the ADSL data.
• If you don’t actually use a phone on your ADSL line you don’t have to use a micro-filter. Instead, you can connect your ADSL device via a standard cable that has a 6-way modular plug at one end and an RJ11 connector at the other.

Your computer can be connected to the incoming line using one of the following methods:-

PCI Card

An ADSL modem card, when fitted into a spare PCI slot in your computer, offers a marginally higher speed than an Ethernet connection (see below), is easily plugged to your phone line via a standard RJ11 cable and gives an ‘always on’ connection.

Ethernet

ADSL via Ethernet is slightly faster than USB (see below). For a single user, you’ll need an Ethernet ADSL modem, which often gives you ‘dial-up’ access via Point-to-Point Protocol over Ethernet (PPPoE). However, more recent products operate similarly to an ADSL router (see below), giving you an ‘always on’ connection. These devices also use a fixed numerical IP address, which avoids the need for Network Address Translation (NAT), a particular advantage if you’re using the computer as a Web server.

If you have your own local network, an Ethernet connection gives all the computers on the network access to a common ADSL circuit. And if a hardware router is used you don’t even need to install any extra driver software on the computers.

In any event, the RJ11 cable from your incoming line plugs into the Ethernet ADSL modem or ADSL router, which in turn connects to your computer or existing network hardware via standard Ethernet wiring. Note that a crossover Ethernet cable may be required if you only have one computer or where an Ethernet ADSL modem is connected to a separate Ethernet router or Ethernet hub. Here’s a fairly complex arrangement for a networked connection, involving three separate devices:-

although a combined hub and router, as shown below, is more convenient:-

In this case, the ADSL modem is connected via a standard Ethernet cable to the wide area network (WAN) jack or uplink port on the hub. However, if you want to connect the modem directly to a standard Ethernet port on the hub (the same as other computers), you must use a crossover cable.

• A combined hub and router doesn’t usually support AppleTalk. However, Mac OS X and Mac OS 9.x accommodate AppleTalk over IP or TCP/IP file sharing, which can be used with some printers. You can set up a suitable LPR desktop printer for TCP/IP in the Mac OS from the Printer Setup Utility, Print Center orDesktop Printer Utility.
• Some kind of gateway may be necessary to enable computers on the network to reach the modem. This can provided as software on one machine, known as the gateway computer. In the Mac OS, the Network Preferences pane or TCP/IP panel on this machine must be configured as instructed, whilst other machines should be set to Using DCHP Server.

Finally, the simplest option is an ADSL router, which has the added benefit of providing an ‘always on’ connection:-

Most routers use Network Address Translation (NAT) to protect your computers from the Internet. With NAT, the IP for your local area network (LAN) is in one of the following groups:-

Class A: 10.x.x.x

Class B: 172.x.x.x

Class C: 192.168.x.x

These are private addresses, of which Class C is the most common, although 10.0.0.2, 10.0.0.138, 192.168.0.1, 192.168.1.1 and 192.168.7.1 are used by several types of router.

• The IP address for a Wi-Fi base station connected to the router should be on same subnet as the router itself, or on the same subnet as a firewall.

To use a router in the Mac OS, choose Using DCHP Server in Network Preferences or in the TCP/IP panel. Then plug in the router, open your Web browser and enterhttp://xx.xx.xx.xx, where xx.xx.xx.xx is the router’s IP address. Finally, fill in the configuration screens: for a BT connection in the United Kingdom you’ll normally enter 0 (zero) for VPI and 38 for VCI.

The computers on your LAN can be automatically configured by selecting Using DHCP Server or entering a unique IP address and a subnet mask. You should also enter the firewall or router LAN IP address in the Router Address field.

• Your ISP may require you to use Point-to-Point Protocol over Ethernet (PPPoE) to accommodate ASDL or other wideband connections via a modem or hub.

USB

Although slightly slower than the above, this low-cost form of connection is ideal for single-computer users, even though it may be a ‘dial-up’ service. The phone line is plugged via an RJ11 cable to an USB ADSL modem, which is connected with a USB cable to a spare USB port on your computer. Note that USB ADSL modems normally require driver software.