A communications system employs frequency division duplexing to accommodate symmetric and asymmetric services, while substantially eliminating near-end cross talk. Upstream and downstream channels are separated by a guard band in order to insure that near-end cross talk is kept to a minimum. An asymmetric...http://www.google.de/patents/US6333920?utm_source=gb-gplus-sharePatent US6333920 - Frequency division duplexing system which accommodates symmetric and asymmetric channels
