MaxDSL survey - background
This is background information to a survey of user experiences with the MaxDSL broadband service provided by BT in the UK and used by many ISPs to provide ADSL services to their customers.
MaxDSL was introduced at the beginning of April 2006 as an "up to 8M" service, the line speed depending on the quality and length of the phone line and the level of interference.
In the "fine print" of the trial product description it was made clear that the end user of a >2M MaxDSL service may see the same data throughput as a 2M fixed speed user in times of congestion, in other words it is a "best efforts" service to give faster than 2M speeds where possible. AOL describe it as a "2M up to 8M service" which covers this. The maximum IP rate in the spec is 7320 kbits/s which is 3.6 times that of the fixed speed 2M service.
Originally the MaxDSL trial was limited to lines running at 2M fixed speed. On release of the final product this eligibility constraint was removed however the product retains 0.5M steps in the "data rate" profile applied to the line to control the ATM capacity of the circuit. This rather clunky arrangement means a user originally on 512k fixed speed may migrate to MaxDSL and get a line speed of 1120 kbits/s on their modem but their data rate profile will remain at 0.5M. The only benefit they get is the increased upload speed of 400 kbit/s or 750 kbits/s on the "Max Premium" business service.
MaxDSL is the first BT product to use rate adaption rather than fixed speed. This means the lines run at the fastest speed they can, rather than the previously conservative regime which gave high reliability but limited speed to 2M on lines capable of far more. The trade-off for higher speed is reduced SNR/noise margin, more errors and more retrains / disconnections. BT has a process of "Dynamic Line Management" designed to find a compromise between stability and speed by upping the target SNR margin (which reduces the speed) on unstable lines. DLM can also apply interleaving, unless the end user has opted out of this via their ISP. Interleaving reduces errors and increases stability on lines with "the right sort of interference", it adds 10-20 ms to ping times and reduces the maximum speed possible from 8128 to 7616 kbits/s, details in the BT SIN.
The online survey was conducted to assess the user experiences of actual data throughputs, stability, line sync rates etc. It was promoted on Usenet and on the ADSLguide forums and attracted over 600 responses.
MaxDSL was introduced at the beginning of April 2006 as an "up to 8M" service, the line speed depending on the quality and length of the phone line and the level of interference.
In the "fine print" of the trial product description it was made clear that the end user of a >2M MaxDSL service may see the same data throughput as a 2M fixed speed user in times of congestion, in other words it is a "best efforts" service to give faster than 2M speeds where possible. AOL describe it as a "2M up to 8M service" which covers this. The maximum IP rate in the spec is 7320 kbits/s which is 3.6 times that of the fixed speed 2M service.
Originally the MaxDSL trial was limited to lines running at 2M fixed speed. On release of the final product this eligibility constraint was removed however the product retains 0.5M steps in the "data rate" profile applied to the line to control the ATM capacity of the circuit. This rather clunky arrangement means a user originally on 512k fixed speed may migrate to MaxDSL and get a line speed of 1120 kbits/s on their modem but their data rate profile will remain at 0.5M. The only benefit they get is the increased upload speed of 400 kbit/s or 750 kbits/s on the "Max Premium" business service.
MaxDSL is the first BT product to use rate adaption rather than fixed speed. This means the lines run at the fastest speed they can, rather than the previously conservative regime which gave high reliability but limited speed to 2M on lines capable of far more. The trade-off for higher speed is reduced SNR/noise margin, more errors and more retrains / disconnections. BT has a process of "Dynamic Line Management" designed to find a compromise between stability and speed by upping the target SNR margin (which reduces the speed) on unstable lines. DLM can also apply interleaving, unless the end user has opted out of this via their ISP. Interleaving reduces errors and increases stability on lines with "the right sort of interference", it adds 10-20 ms to ping times and reduces the maximum speed possible from 8128 to 7616 kbits/s, details in the BT SIN.
The online survey was conducted to assess the user experiences of actual data throughputs, stability, line sync rates etc. It was promoted on Usenet and on the ADSLguide forums and attracted over 600 responses.
posted by PhilT at 3:49 PM
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