ADSL2 addresses this problem by including the ability to seamlessly adapt the data rate on-line. This new innovation, called seamless rate adaptation (SRA), enables the ADSL2 system to change the data rate of the connection while in operation without any service interruption or bit errors. ADSL2 simply detects changes in the channel conditions — for example, a local AM radio station turning off its transmitter for the evening — and adapts the data rate to the new channel condition transparently to the user.
SRA is based on the decoupling of the modulation layer and the framing layer in ADSL2 systems. This decoupling enables the modulation layer to change the transmission data rate parameters without modifying parameters in the framing layer which would cause the modems to loose frame synchronization resulting in uncorrectable bit errors or system restart. SRA uses the sophisticated online reconfiguration (OLR) procedures of ADSL2 systems to seamlessly change the data rate of the connection. The protocol used for SRA works as follows:
1. The receiver monitors the SNR of the channel and determines that a data rate change is necessary to compensate for changes in channel conditions.
2. The receiver sends a message to the transmitter to initiate a change in data rate. This message contains all necessary transmission parameters for transmitting at the new data rate. These parameters include the number of bits modulated and transmit power on each subchannel in ADSL multicarrier system.
3. The transmitter sends a "Sync Flag" signal which is used as a marker to designate the exact time at which the new data rate and transmission parameters are to be used.
4. The Sync Flag signal is detected by the receiver and both transmitter and receiver seamlessly and transparently transition to the data rate.