ADSL Max

ADSL Max is a cover term for the UK telco BT's range of commercial ADSL services that were rate-adaptive and deployed over phone lines. BT's ADSL Max services were launched in March 2006, however ADSL Max, and the back-end IP Stream platform were formally retired in June 2014, although the technology is still the only product available in many rural exchanges where BT has no plans whatsover to upgrade these to 21CN.

ADSL Max has been replaced in most exchanges by a new service Wholesale Broadband Connect.

Rate-adaptive services were intended to offer the best possible speed attainable, which could vary over time. The maximum speed permitted was determined by current line conditions, 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 enabled customers to receive up to 7.15 Mbit/s (termed 'downstream') over a standard BT telephone line of sufficient quality. Various kinds of lower-speed, rate-limited adaptive services were also available as a reduced cost option. Customers using ADSL Max with long lines, poor quality lines or who experienced high levels of noise or interference are limited to much slower transfer rates, and some customers whose lines are very poor or who are affected by high levels of noise are unable to obtain service at all.

ADSL Max services were available to both residential and business customers. Aimed at business users, the IPStream Max Premium service offered a prioritised delivery of traffic over the BT network compared to traffic from non-'Premium' customers and allowed a maximum transmit speed of 832 kbit/s (notional, DSL 'upstream' sync rate). The non-'Premium' services, aimed at residential users, offered a lower maximum transmit speed of a notional 448 kbit/s (notional, DSL 'upstream' sync rate).

During the first ten days after the service was provided, the line's performance was monitored and the lowest connection speed is noted. This figure was 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 assessed 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 could make adjustments to the DSLAM output power, suggested target signal-to-noise-ratio (SNR) margin and sync rate and could 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 telephone exchange which had the largest influence on the line's performance.

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