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Rate-adaptive services are intended to offer the best possible speed attainable, which may vary over time. The maximum speed permitted is determined both by current line conditions and the level of noise, and also by recent history based on factors including the rate of communications errors and the best and worst DSL modem sync speeds achieved during some recent period of time. The highest speed ADSL Max services enable customers to receive up to 7.15 Mbit/s (termed 'downstream') over a standard BT telephone line on lines of sufficient quality. Various kinds of lower speed rate-limited adaptive services are also available as a reduced cost option. Customers with long lines or poor quality lines or who experience high levels of noise or interference will be limited to much slower transfer rates, and some customers whose lines are very poor or who are affected by high levels of noise may be unable to obtain service at all.
ADSL Max services are available to both residential and business customers. Aimed at business users, the IPStream Max Premium service offers prioritised delivery of traffic over the BT network compared to traffic from non-'Premium' customers and allows a maximum transmit speed of 832 kbit/s (notional, DSL 'upstream' sync rate). The non-'Premium' services, aimed at residential users, offer a lower maximum transmit speed of a notional 448 kbit/s (notional, DSL 'upstream' sync rate).
Rate adaptation and BT Wholesale's 'Dynamic Line Management' (DLM)
During the first ten days after the service has been provided, the line's performance is monitored and the lowest connection speed is noted. This figure is used from then on to define minimum service standards below which the service can be considered to have a fault. Both during this period and afterwards a system known as dynamic line management (DLM) constantly assesses the performance of the line in order to provide information to the end user's DSL modem to allow it to choose a suitable sync rate with which to connect to the DSLAM, balancing speed against the risk of errors due to changing noise conditions. DLM can make adjustments to the DSLAM output power, suggested target signal-to-noise-ratio (SNR) margin and sync rate and can choose to apply a technique known as interleaving which aids error correction. Several factors dictate the sync rate, which limits the maximum attainable speed, such as the presence of noise within the frequency spectrum used by ADSL, but chiefly it is the distance from the Central Office (telephone exchange) which has the largest influence on the line's performance.
Software within the BT Wholesale network limits the rate at which inbound IP packets are delivered to the user, this rate being based on a parameter termed the IP Profile. The IP profile mechanism imposes an upper limit on the rate at which data destined for the user is transmitted. This rate restriction is imposed at the point where inbound data enters the BT Wholesale IP network from the Internet. The rate is chosen so that throughput can not exceed the throughput of the ADSL link. The IP Profile set by DLM may unfortunately be set to a figure that is somewhat below that which the DSL modem could support, which will limit the maximum inbound data delivery rate over the ADSL link to be below the maximum achievable rate.
The IP profile changes over time, and is derived from a consideration of the worst downstream sync rate used by the DSL modem during some recent period of time. If circumstances cause a drop in sync rate, the IP profile decreases immediately. If conditions later improve, the IP profile will only increase after a certain delay time. The time taken for this increase to occur depends on a number of factors and can be between 75 minutes and several days. As frequent disconnections may be misinterpreted as connection problems, it is recommended that modems are left connected while not in use, in order to avoid unnecessary decreases in the IP profile.
IP Profile values are chosen from a certain fixed list of rate values taken from a table and are determined by the downstream sync rate of the ADSL connection. Many sync rate values will correspond to the same IP Profile figure. It is often the case that a user's downstream sync rate, when overheads are accounted for, equates to a true maximum inbound throughput figure that falls between two permitted IP Profile levels and, in such a case, the lower level is used. As an example, if the downstream sync rate of a DSL modem is set to 1984 kbit/s, the associated IP Profile is 1500 kbit/s so the maximum permitted inbound data rate is 1500 kbit/s, but if the sync rate were to rise to 2016 kbit/s the IP Profile would be 1750 kbit/s.
Congestion and performance guidelines
At busy times, congestion in the network may limit real throughput still further. BT Wholesale suggests as a guideline that inbound throughput at very busy times may perhaps be not much more than 2 Mbit/s even if the DSL modem's downstream sync rate is currently much higher. Presumably, users who use the ADSL Max 'Premium' services will be less affected at busy times, since their traffic is prioritised.
Notes and references
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