Bandwidth pipedreamsGetting more bandwidth into the home via xDSL is more than just attaching a modem at one end and a splitter box at the other. Steve Bush reports. It is said that we will all be screaming for more and more bandwidth into the home. This may or may not actually be the case, but supposing that it is. ADSL and its relatives (jointly known as xDSL) are some of the technologies which will compete to bring it to you. Digital subscriber line variants Acronym x means? Number of pairs Bit rate Maxloop length VDSL Very-high-bit-rate 1 52.8Mbit/s # 1.4km * ADSL Asymmetric 1 9Mbit/s # 5.5km * HDSL High-bit-rate 2 1.544Mbit/s 3.6km * SDSL Single-pair 1 784kbit/s 3.6km * IDSL Integrated 1 160kbit/s 5.5km *
# Variable bit rate
* Not at maximum rate Source: Tektronix Much of the hype around xDSL suggests that you just ask your friendly phone company to put an appropriate modem at its end of the phone line, and perhaps a splitter box at your end, and off you go. This may actually be true for subscribers that live near their local exchange, but there are a lot of hidden complexities for those who live further away. The type of xDSL installed makes a difference. In good conditions ADSLwill reach 2.7km at 8.4Mbit/s, and 5.5km at 1.5mbit/s. VHDL on the other hand can only provide its maximum data rate at 0.3km. The type of wire in the ground also makes a difference. US experience shows that if the installed wire is 26 rather than the thicker 24AWG size, maximum working length at 1.5Mbit/s drops from 5.5 to 4.6km (See table). Allowable cable lengths in plain cables
24AWG 26AWG ADSL 1.5Mbit/s 5.5km 4.6km 6.1Mbit/s 3.6km 2.7km 8.4Mbit/s 2.7km –
VDSL 13Mbit/s 1.4km – 27Mbit/s 0.9km – 55Mbit/s 0.3km – Source: Tektronix Another issue is the way that the line was originally installed. Not all connections run point to point with no modifications. Side branches are a feature of some loops. These can be an old connection left after a loop has been diverted. Another way these occur is the bridged tap. This is where two routes that pass through a common junction box are joined to make one long connection. This leaves two still-connected tails out to where the unused ends originally went. An even more obscure problem is called split pairs, where one wire from each of two twisted pairs is accidentally used to form a loop. Sometimes the problem is not rectified, but bodged by re-joining the pairs in the correct arrangement at another junction box. Strong cross talk is a feature of split pairs. Water ingress also plays havoc with signals. These features will be left in a line from when it was originally set up. There are no reasons to make any changes all the time that the voice service is acceptable. xDSL systems are far less tolerant, each is affected differently, but lines with branches, splits and moisture will frequently have to be ‘cleaned up’ before digital services will operate. Time domain reflectometry (TDR) is a technique commonly used to spot these problems. It can be deployed to give the all-clear from the exchange end, but will have to be repeatedly deployed if disruptive features are found until all are dealt with. Load coils are another problem. These are sometimes put in series with longer subscriber loops to maintain the high frequency content of voice signals. No xDSL system is compatible with them and they have to be removed. TDRcan find them, but only one at a time as TDR signals do not pass through coils either. Once coils have been removed, the subscriber has lost the high-frequency content of their voice signals. This is a problem when voice and data are expected to run together on a single pair. Not all of these problems occur in all countries, standards vary, and they can often be cured by removing unwanted loop features or restricting bandwidth, but not everyone will get 55Mbit/s delivered down their phone line.