DSL FAQs

What is broadband?

Broadband has revolutionised business connectivity; it has replaced slower narrowband modems and ISDN and provided a cheaper alternative to leased lines. Broadband is a broad technology term that is defined by Ofcom as any data connectivity system that is "always on" and operates at data rates of 128kbps and above. This definition is broad by design because there are many different types of broadband connections including; ADSL (Asymmetric Digital Subscriber Line), SDSL (Symmetric Digital Subscriber Line), xDSL, cable modems, satellite and wireless.

What are the benefits of broadband?

There are so many different types of broadband service that it is hard to identify a common feature between them. Compared to narrowband services, broadband services generically offer faster connection times and higher potential data transfer rates. Other than that individual services differ widely in their other features and benefits.

How does DSL work?

Voice uses only a small fraction of the range of frequencies that a copper wire carry. Using frequency ranges outside of that used by voice (e.g. ADSL) or even using the whole frequency range of the wire (e.g. SDSL), can considerably increase the bandwidth of the circuit. By using special frequency filters at the local telephone exchange, and sometimes at the customer's end too, two or more additional data bands can be detected and separated to create an "upstream" data signal and a "downstream" data signal.

How does the data get from the telephone exchange to my ISP?

All local DSL connections are collected in a computer called a DSLAM (DSL Access Multiplexer). This device is connected to a large wide area network managed by companies such as BT. A virtual circuit is created from each telephone exchange to the ISP's network, usually terminating in London. Often a large number of local users will share the same virtual circuit, resulting in contention.

What is contention?

When two or more users share a circuit, there is a chance that they will both want to transfer data at the same time. Modern packet networks share bandwidth equally, meaning that all users get an equal share of the capacity. This is called contention. Broadband services have often been characterised by a contention ratio, which is the maximum number of users sharing the same circuit. The higher this ratio, the more unpredictable the connection performance.

Residential service providers usually offer contention ratios of around 50:1. Business service providers offer contention ratios from 20:1 down to 1:1 (uncontended).

What is a microfilter?

This is a small electronic device that separates out a voice signal and a data signal on a single copper circuit. It plugs into the telephone socket on the wall and, itself, has two sockets, one for a phone or fax, the other for a computer or router. Every extension connected to the same copper circuit enabled with ADSL requires a separate microfilter.

Care has to be taken that equipment plugged into the phone socket is compatible. ADSL is known to interfere with the operation of a number of telephone-based systems including, but not limited to; PBXs, credit card machines and alarm systems.

How does distance affect broadband?

At long range, noise on the copper wire makes it harder to get maximum data performance. For ADSL, the maximum range for 2Mbps downloads is 3.5km. For 1Mbps it is 6.0km. In theory all connected customers would be able to receive at least 512kbps downloads. In practice around 5% of customers will not be able to achieve even this level of performance. SDSL suffers from even more severe distance limitations.

What is ADSL?

Asymmetric DSL offers greater download capacity compared to its upload capacity. It usually co-exists with a telephone service on the same copper circuit and uses microfilters to separate the voice and data service at both "ends" of the circuit. The international standard for ADSL define a maximum performance of 8Mbps downstream and 800kbps upstream.

In 2003, an international protocol ADSL2+ has been developed that can offer a boosted maximum download data rate of, perhaps, 25Mbps. The first services using ADSL2+ are likely to be rolled out in the UK during 2007.

Does ADSL come with an SLA?

Most providers do not offer service level agreements with broadband services. This means that if there is a fault, then there are no guarantees of a fix time. Even where there is an SLA, the onus is usually on the customer to prove the breach of agreement and to claim the compensation, which is usually disproportionately small compared to the losses incurred in a network outage.

Opal recommends broadband customers employ a back up service using ISDN with automatic fail over between the two circuits, particularly in business-critical applications or for VPNs.

Is my broadband service secure?

Broadband is not inherently more or less secure than any other Internet connection although because broadband connections are always on, it could be argued that broadband-connected computers are more prone to attack compared to ones on narrowband connections.

In any event the bare minimum security required is some kind of firewall, to protect against hackers and worms and a reputable anti-virus defense particularly if using email. Computers should be routinely patched with security updates for all applications. People with broadband might also want to consider anti-spam measures, anti-spyware measures and for networks, intrusion prevention/protection.

Business customers are advised that the first step towards good security is in a written security policy, staff training and developing an understanding of the nature and scale of the online threats facing the connected company. Opal can assist in drawing up suitable plans for customers.

What is SDSL?

Symmetric DSL offers balanced upload and download data rates more suitable for businesses that send as much data as they receive. SDSL uses the portion of the frequency spectrum usually occupied by telephone voice traffic. This means that the two services cannot coexist on the same circuit - and thus there is no need for a microfilter.

As befits a more business-orientated connection method, SDSL services generally offer lower contention ratios than ADSL.

The roll out of SDSL in the UK has been very slow and it is still subject to limited availability. With ADSL2+ offering even higher performance, and leased line and ethernet prices coming down in the major cities, SDSL services may continue to be of limited utility.

What is xDSL?

Digital Subscriber Line refers to a collection of communication technologies that deliver high-speed data transfers over a normal copper telephone line. It is currently the most common form of broadband in the UK. xDSL is the generic term used to refer to all of the different DSL protocols. The two main types are symmetric (SDSL) and asymmetric (ADSL) in nature.


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