Image Number 3 for United States Patent #6570915.
A DSL communication system including a DSL transmission unit at a central office (DTU-C) and a DSL transmission unit at a remote location (DTU-R) in communication over a communication link. DTU-C and DTU-R are capable of transmitting and receiving packets of data prior to synchronization or training. DTU-C transmits a discovery message to DTU-R and awaits receiving a discovery response message from DTU-R. After receiving the discovery response message, DTU-C transmits a probe message to DTU-R followed by a probe signal. DTU-R measures the line quality while receiving the probe signal. After receiving the probe signal, DTU-R transmits a probe signal to DTU-C, using which signal DTU-C measures the line quality. After transmitting the probe signal, DTU-R transmits a probe response message, including line quality measurements performed by DTU-R. DTU-C and DTU-R may negotiate a first data rate by transmitting data rate messages based on the line quality measurements. DTU-C and DTU-R synchronize or train at the first data rate. After synchronization, DTU-C and/or DTU-R measure the line quality at the first data rate. The line quality may be measured based on bit-error-rate, attenuation level and/or signal-to-noise ratio. A second data rate is then selected based on the line quality measurements. DTU-C may then initiate a rate change request, according to which DTU-C and DTU-R may re-synchronize at the second rate.