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xDSL FAQ

  knowledgebase  :: network status

Q: What is xDSL?
A: xDSL is an umbrella term that covers a range of Digital Subscriber Line (DSL) technologies, including ADSL, RADSL, SDSL and VDSL.

Q: What is DSL?
A: Digital Subscriber Line (DSL) technology allows data to be delivered at high speed down a standard copper phone line. DSL is actually a signalling specification which defines which part of the phone line is used for voice, which part is used for data, and the details of each region.

Q: What is ADSL?
A: Asymmetric DSL means that download speeds are different (usually faster) than upload speeds. The maximum download speed available is 24Mbit/sec with ADSL2+ (8Mbit/sec with regular ADSL), however the maximum rates are often not available as they depend on line quality, which drops off rapidly as distance to the exchange increases. Upload speeds are generally an 8th or a quarter of the download speed. To obtain faster upload speeds, SDSL must be used.

Q: What is broadband?
A: Broadband is an umbrella term which, in the context of data communications, refers to a digital connection to the internet, rather than one made over an analogue modem (such as a 56.6k dialup connection). A digital connection can be made using a number of technologies, including ISDN, cable, xDSL, Ethernet and FTTP, and at a variety of speeds, from 64Kbit/sec upwards. Care should be used when reading advertising promoting only "broadband" rather than more specific details, as the meaning of the term varies from provider to provider, depending on what it is that provider has for sale.

Q: Can I make a voice call on a line with ADSL enabled?
A: Yes, however you may need to install splitters, or microfilters, one per wall socket, to eliminate the sound of the ADSL signal. Microfilters are around £2.50 each, and often come free with ADSL routers.

Q: Can I use a fax machine or a modem on a line with ADSL enabled?
A: Yes. These devices work in the "voice" region of the phone line, and consequently you can use them just as you always did. As with telephones, faxes and modems also need microfilters installed.

Q: Must I log on and off with ADSL?
A: No. An ADSL connection is "always on", and you can stay logged in for as long as you like.

Q: Can I plug ADSL into my LAN, and give all my computers internet access?
A: You sure can, however you will need to configure a few things. We can help ..

Q: What is the difference between USB and Ethernet ADSL?
A: Ethernet ADSL is delivered in the form of a router which plugs into a 10-BaseT (twisted pair) network. This allows you to integrate it easily with a LAN. In contrast, USB ADSL is delivered to a single computer through the USB port. This configuration can still be shared across a LAN, but it requires the computer connected to the USB ADSL device to run routing and DNS forwarding services (such as Microsoft's Internet Connection Sharing). Should this computer be switched off or crash, internet connectivity to the entire LAN is lost. For this reason, Ethernet-based ADSL is recommended for business or professional use.

Q: Can I upgrade to Ethernet ADSL from USB?
A: Yes, however you'll need to purchase an ADSL router. Also, your USB ADSL modem will become redundant.

Q: What is NAT?
A: Network Address Translation is the mechanism by which multiple computers on a LAN can share an internet connection which is allocated a single IP address. A router (or a computer acting as a router) on the LAN hosts the connection, and as clients on the LAN send packets over the internet, the router transparently replaces their internal IP addresses with its own IP address. This works well in most setups, however it can confuse certain applications (such as VPNs and videoconferencing).

Q: What are static IP addresses?
A: They are IP addresses allocated to a specific computer - if that computer loses its connection to the internet, when it reconnects, it retains the same IP address it had previously. This is in contrast to dynamically allocated IP addresses, which change each time a computer reconnects.

Q: What are contention ratios?
A: They are the rate at which your data gets bandwidth, relative to everyone else on your DSLAM. For example, a 20:1 contention ratio means that for every 20 bits of data on the wire, 1 bit of it is yours. This is faster than a 50:1 ratio, because users on 50:1 ratios may have to wait up to 49 bits before it is their turn. The need for contention ratios arises because the network is of a finite width, while demand for bandwidth is infinite.

Q: What is my DSLAM?
A: Your Digital Subscriber Line Access Multiplexer is a big box in your local telephone exchange. It routes all your incoming and outgoing ADSL traffic, plus that same traffic for every other ADSL subscriber in your geographical vicinity, onto BT's ATM network.

Q: What is ATM?
A: Asynchronous Transfer Mode is the core transmission technology used to move ADSL data around the UK. It can transport data over very long distances in 3ms or less. Your outbound internet traffic flows through your ADSL router and onto BT's ATM network, which routes it to your ISP - who in turn route it onto the internet. Your inbound traffic goes from the internet to your ISP - who then route it onto BT's ATM network, which in turn, sends it to your ADSL router.

Q: What are the differences between ADSL-based broadband and cable-based broadband?
A: Cable modems use the cable TV network rather than the telephone network to provide internet connectivity. These networks suffer from "head-end bottlenecks" - essentially congestion, which gets worse as more users join the network. The cable network itself is less resilient than the copper network, suffering from "distortion" if modems are disconnected. And, by misconfiguring his modem, your neighbour can disrupt your service (cause unknown; source: NTL installation guide). Also, cable users share subnets, and consquently cable networks are less secure than ADSL networks - on a cable network, it is easy for neighbours to eavesdrop upon one another. Finally the cable modem itself often requires extra drivers and "bridging" software to make it work. In short, the difference between the two is that ADSL is scalable and robust, while cable is not.

Q: What is LLU?
A: Local Loop Unbundling (LLU) refers to removing control of the backbone network from the incumbent telephone company (such as BT). National telephone systems are usually controlled by a single company, which is bad for the consumer, the market, and the environment. To redress this, governments and communications regulatory agencies (such as Ofcom) are pressing the incumbents to separate their backbone network business from their customer wiring business. This process is known as LLU. Once the separation (unbundling) is done, the customer may select from alternative providers. Companies participating in LLU must build their own backbone network in order to compete with the incumbent, and as such, can provide alternative pricing models and featuresets.

Q: What is RADSL?
A: Rate-Adaptive DSL (RADSL) dynamically adjusts its transmission speed to allow for noisy or degraded lines. It's most useful in areas with older infrastructure at some distance from the exchange.

Q: What is SDSL?
A: Symmetric DSL (SDSL) fills both the upstream and downstream portions of the phoneline with DSL data. This means SDSL connections can upload at the same rate they download. SDSL lines do not support voice traffic, as the upstream region of the line is consumed by DSL signal, however this is not usually as problem, as SDSL lines are usually purchased to provide high-speed internet uplinks. Compared to ADSL, SDSL is considerably more expensive (as of this writing).

Q: What is VDSL?
A: Very-High-Data-Rate DSL (VDSL) can run at up to 54Mbit/sec, however it can only run over short cable lengths (around 300m). It's suited for use in areas where there is Fibre-To-The-Neighbourhood (FTTN) - the fibre does not actually run to customer premises. In this configuration, the fibre terminates in the street and standard telephone lines are used to provide a VDSL connection to the fibre termination point. However, the infrastructure in most countries does not yet provide FTTN, and thus VDSL is currently more theoretical than a product you can buy.

Q: What is FTTP?
A: Fibre-To-The-Premises (FTTP) describes a network where fibre-optic cables run all the way from the communications provider to the customer premises. It represents the complete replacement of the copper-wire telephone network. It will provide massive bandwidth, however it requires re-wiring every customer premises. For this reason it will not happen quickly. In turn, this means transitionary technologies are needed; which is where xDSL fits in, as it provides increased bandwidth over standard copper wiring.

Q: What is ISDN?
A: Integrated Services Digital Network (ISDN) is an older broadband technology, still widely deployed by businesses. However, it is relatively slow and expensive, and cumbersome to use, when compared to xDSL.

Q: What is the difference between Mb and Mbps?
A: The notation Mb refers to a unit of storage a megabyte in size, while the notation Mbps refers to the bandwidth (carrying capacity) of a cable or wire in megabits per second.

Want to know more about ADSL? Check out Wikipedia, thinkbroadband or whirlpool