Swirling flow

SDSL Symmetric Digital Subscriber Information

>DSL Broadband is sold as the business critical broadband service aimed at businesses that require an enhanced service level over ADSL technology and increased up load speeds.

The main SDSL services available today are sold in 3 flavours of speed and also 3 flavours of contention ratio. Typically you will see 2Mbs SDSL, 1Mbs SDSL & 512Kbs SDSL services with contention ratios of 10-1, 5-1 and 1-1). One of the main issues with SDSL at the moment is the distance from local telephone exchange that you can receive service. Currently you can only expect to achieve 2Mbs if you are within 1Km straight-line distance.

The other major issue with SDSL is the availability. Currently there are only around 1400 exchanges covered in the UK, which equates to around 50% of national coverage. Some LLU providers (local Loop Unbundling) are still rolling out SDSL but with the possibility of 24Mbs ADSL coming soon, new SDSL exchange coverage may be slow to roll out.

The main reasons businesses should look at sdsl are as follows:

  • When using Voice Over IP (VOIP)
  • For a central site VPN connection where current ADSL connection is causing bottlenecks due to lack of up load bandwidth.
  • Leased Line Replacement

Symmetric Digital Subscriber Line (SDSL) borrows broadband technology from the local area network (LAN) world and uses it over ordinary copper phone lines. Unlike ISDN or domestic analogue modems no call is dialled specifically to send and receive data; the service is live continuously for data to be transmitted and received even when voice calls are in progress.

DSL's signal attenuation causes deterioration even over relatively short runs of 500m to 2000m, which could limit the distance customers can be from local exchanges unless some kind of signal regeneration is employed telecom providers' network cabinets. All DSL variants generate noise, which affects other copper pairs in the same bundle carrying DSL. ADSL is believed to be the worst offender.
A future answer may be VDSL - potentially offering 13Mbit/s outwards to 52Mbit/s inwards. It too, suffers from attenuation but will work better if telecom providers use an all-optical-fibre backbone network to join short run copper local loop in local neighbourhood cabinets.

DSL Variants

  • ADSL 256kbit/s upload, 2Mbit/s download
  • ADSL MAX Up to 800kbit/s upload, 8Mbit/s download
  • ADSL 2 Plus Up to 1Mbit/s upload 24Mbit/s download
  • Annex M Up to 2.5Mbit/s upload 16Mbit/s download
  • HDSL 1.5Mbit/s to 2Mbit/s both ways (full duplex)
  • SDSL 2Mbit/s both ways (full duplex)
  • VDSL 13Mbit/s upload, 52Mbit/s download