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In the CAP modulation process, once a group of bits are mapped to a predefined point in the signal constellation, the in-phase and quadrature filters implement the positioning in the signal constellation. Because this technique simply adjusts the amplitude and phase without requiring a constant carrier, the technique is referred to as carrierless.
The ADSL modem developed by Paradyne uses a 256-point signal constellation for downstream operations. This constellation pattern is in the 120 KHz to 1,224 KHz frequency range and produces a composite signaling rate of 960K baud, with 7 bits packed into each signal change. Although the resulting downstream operating rate is 6.72M bps, the use of the Reed-Solomon forward error correction code reduces the actual payload to 6.312M bps, plus a 64K-bps control channel. In the upstream direction, the Paradyne ADSL modem uses a CAP-16 line code in the 35-KHz to 72-KHz frequency band to obtain a composite signaling rate of 24K baud across 16 subchannels. By packing 3 bits per signal change, an upstream line rate of 72K bps is obtained, of which 64K bps is available for data. Similar to an ADSL DMT modem, an ADSL CAP modem can use time-division multiplexing to derive multiple channels on the upstream and downstream channels.
Although the ANSI DMT standard defines an operating rate of 6M bps on local loops up to 12,000 feet in length, some vendors now offer rates approaching 8M bps downstream and up to 800K bps upstream. This additional data rate is accomplished through an automatic rate adaptation capability that correlates the connected data rate to distance and line noise, enabling additional bandwidth to be gained which translates into a higher operating rate.
In the US, GTE Telephone Operations (Stamford CT) began testing the use of ADSL technology as part of a public data trial in the Dallas-Fort Worth area during 1996. This trial tests downstream data transmission at operating rates of up to 4M bps and upstream transmission at data rates up to 500K bps. The testing involves the Irving Public Library system, several bookstores, and area GTE employees. ADSL modems used during this trial are provided by Western Technologies of Oswego IL and Aware, Inc. of Bedford MA.
On the West Coast, U.S. WEST (Olympia WA) is testing ADSL-based services through its ENTERPRISE business unit and will use equipment from several vendors before deciding on which equipment to deploy commercially. Other communications carriers are expected to announce field trials as well as commercial deployment of ADSL technology soon.
In addition to ADSL, three related technologies warrant a degree of discussion:
HDSL modems are used on either end of one or more pair of twisted-pair wires to obtain a T1 or E1 operating rate. Currently, a T1 operating rate requires two lines and E1 requires three. HDSL is primarily used on college campuses to provide a T1 or E1 link to interconnect LANs.
SDSL represents HDSL over a single telephone line and is used where multiple lines do not exist or would be expensive to install.
VDSL modems for twisted-pair access operate at data rates from 12.9 to 52.8M bps on 24-gauge cable. Because of the higher operating rate, the maximum transmission distance supported by VDSL is limited to under 4,000 feet. Higher rates over shorter distances may limit VDSLs ability to support communications carrier customers.
VDSL standards may not be finalized for several years. There are four VDSL line coding methods discrete multitone, discrete wavelet multitone, carrierless amplitude/phase modulation, and simple line coding. DMT for VDSL is similar to the technology used in the ADSL standard, with more carriers (supporting ADSL) being the primary difference between the two. Discrete wavelet multitone coding represents a multicarrier system that uses wavelet transformers for individual carriers. The carrierless amplitude/phase modulation method under consideration is similar to Paradynes CAP, representing a version of suppressed carrier quadrature amplitude modulation. The fourth coding technique, simple line coding, represents a baseband signaling and filtering system.
Based on the higher transmission capacity of VDSL, it will probably be deployed by communications carriers to businesses as a mechanism for asynchronous transfer mode (ATM) connectivity. The ATM Forum, which produced a 51.84M-bps standard for a private user-to-network interface, is currently examining the potential delivery of ATM via VDSL transmission.
From the viewpoint of a corporate network manager, ADSL is probably the only digital subscriber line technology that will be offered by enough carriers in the next few years to deserve consideration for interconnecting geographically separated locations. Although two different types of ADSL technology, based on either DMT or CAP modulation, are being considered by communications carriers, from the perspective of the end user the most important factors are:
The availability of ADSL will govern whether or not the technology can be used to interconnect LANs, provide remote access to mainframes, support videoconferencing between locations, or perform other communications functions. Although it may be possible, for example, to use an ADSL connection in one location and a conventional T1 connection in another location for communications, carriers still have not addressed the conversion of data between different transmission facilities.
The second area of concern is the operating rate of ADSL service. Because the transmission rate varies depending on line quality, the distance of the users location from a carriers central office, and the wire gauge of the wire pair users cannot fully appreciate its transmission capability until an ADSL line becomes operational. Vendor specifications should be used as a guide and ADSL should be considered for applications that require a high data transmission capability but that can also be effectively performed if only a fraction of that capability is actually achieved.
Thus, videoconferencing or LAN access to the Internet are two applications for which ADSL may prove effective, even if the user achieves half or even a quarter of the theoretical ADSL rate due to a bad subscriber loop, long subscriber wire distance from the carriers central office, or a similar factor.
A third area that should be examined before considering ADSL service is the scope of the carriers internal broadband network. The user should determine whether the network interconnects only the ADSL offerings of that carrier, if it provides connectivity to other carriers, and if it provides connectivity to other data services offered by that carrier. By doing so, the user can determine if it will be possible to use ADSL services as a transport mechanism for such data services as X.25 and frame relay.
By contacting their communications carrier, users can ascertain when ADSL may be deployed in their areas and its projected cost. By asking the carrier key questions concerning the availability of the service in different areas where an organization may have offices, the operating rate of the service, and the capability of the carriers internal broadband network, users can determine if ADSL service will be suitable for trial. If so, a test plan should be developed to compare ADSL with traditional T1 service to determine if it can be used as a cost-effective transport replacement.
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