Data Communications Networking Devices - 4th Ed.pdf

500 _________________________________ WIDE AREA NETWORK TRANSMISSION EQUIPMENTTable 4.3664 and 256 QAM element codingInputs Previous inputs Phase change Outputsb5 n b4 ndegrees)A n A n 1 0 00A n B n 1 270 10A n C n 1 180 11A n D n 1 90 01B n A n 1 90 01B n B n 1 0 00B n C n 1 270 10B n D n 1 180 11C n A n 1 180 11C n B n 1 90 01C n C n 1 0 00C n D n 1 270 10D n A n 1 270 10D n B n 1 180 11D n C n 1 90 01D n D n 1 0 00of DSL modems being developed, each having features and operating characteristicsdeveloped to support different transmission rates over copper twisted-pairwire for different transmission distances. Two of the more popular types of DSLmodems are HDSL and ADSL, both of which we will examine in this section.In our previous examination of modems we noted that their operating rate islimited to approximately 33.6 kbps due to the voice channel bandwidth ofapproximately 3300 Hz limiting modem transmission capacity. One methodrecognized many years ago to transmit at a higher operating rate was to removeloading coils on a subscriber line, in effect increasing the bandwidth on a twistedpairwire to approximately 1 MHz. Doing so enables a modem that packs two bitsper baud to obtain a data transmission rate of 2 Mbps, while one that packs four bitsper baud theoretically has the potential to operate at 4 Mbps when a 1 MHzbandwidth is available for use. Unfortunately, we do not live in a perfect world andthe use of Digital Loop Carrier DLC) multiplexers, a bridge tap on a subscriberline as well as the attenuation of high frequencies served as constraints whichlimited development of DSL modems. Improvements in line coding techniquesresulted in the development of High-bit-rate Digital Subscriber Line HDSL)technology during the early 1990s as a substitute for T1 and E1 transmissionfacilities, while Asymmetrical Digital Subscriber Line ADSL) technology wasdeveloped to provide a high-speed transmission capability into homes and of®ces.HDSLHDSL represents a transmission technique which uses a dibit coding schemereferred to as 2 Binary, 1 Quaternary 2B1Q) that was originally developed for usewith ISDN. Instead of transmitting on a single wire pair, HDSL uses two pairs