| 摘要 |
A DSL communication system includes a DSL transmission unit at a central office (DTU-C) (310) and a DSL transmission unit at a remote location (DTU-R) (350) in communication over a communication link. DTU-C (310) and DTU-R (350) are capable of transmitting and receiving packets of data prior to synchronization or training. DTU-C (310) transmits a discovery message (315) to DTU-R (350) and awaits receiving a discovery response message (355) from DTU-R (350). After receiving the discovery response message (355), DTU-C (310) transmits a probe message (320) to DTU-R (350) followed by a probe signal. DTU-R (350) measures the line quality while receiving the probe signal. After receiving the probe signal, DTU-R (350) transmits a probe signal to DTU-C (310), using which signal DTU-C (310) measures the line quality. After transmitting the probe signal, DTU-R (350) transmits a probe response message (365), including line quality measurements performed by DTU-R (350). DTU-C (310) and DTU-R (350) may negotiate a first or startup data rate by transmitting data rate messages (330) based on the line quality measurements and/or based on other information that may be exchanged between DTU-C (310) and DTU-R (350). DTU-C (310) and DTU-R (350) synchronize or train at the first data rate. After synchronization, DTU-C (310) and/or DTU-R (350) 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 (310) may then initiate a rate change request, according to which DTU-C (310) and DTU-R (350) may re-synchronize at the second rate. The second rate may be several data rates higher or lower than the first data rate.
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