Enhanced audio transmission over ADSL using prioritised DMT modulation and retransmissions

Abstract

Asymmetric Digital Subscriber Line (ADSL) is a broadband communications technology which is widely used for transmitting high bandwidth data such as audio and video over the internet. However, ensuring a good Quality of Service (QoS) in the transmission of such data over ADSL is challenging due to channel impairments such as noise and crosstalk. Moreover, given that the total monthly fixed data traffic is increasing at a phenomenal rate, (QoS) management is becoming a crucial issue. This paper proposes an enhanced transmission scheme for audio over ADSL. The proposed scheme exploits the unequal importance of the subbands generated by an MP1 codec to offer varying protection to them during ADSL transmission. This is achieved by multiplexing the bit streams from the subbands before Discrete Multi-tone Modulation (DMT) so that bit streams corresponding to more important subbands are offered higher protection than the least important ones. Additionally, a sophisticated retransmission protocol is proposed which allows frames from more important subbands to benefit from a greater number of retransmissions than least important ones. The retransmission protocol uses eight different Negative Acknowledgement (NACK) packet types which allow adaptive multiplexing of frames at the transmitter. The proposed Unequal Error Protection (UEP) scheme provides an average gain of over 28 dB in Segmented Signal to Noise Ratio (SSNR) over a conventional Equal Error Protection (EEP) one but incurs a negligible loss in throughput.