Although SDSL methodologies are not as widespread as those in the ADSL offerings, they are just as viable as broadband technologies. SDSL is available in the following forms:
■ SDSL (symmetric DSL)—Provides identical transfer rates, both downstream and upstream, ranging from as slow as 128 kbps to as fast as 2.32 Mbps. The most typical implementation is 768 kbps. SDSL is a rather general term that encompasses a number of varying vendor implementations providing variable rates of service over a single copper pair. SDSL has a distance limit of 21,000 feet.
■ G.SHDSL (symmetric high-data-rate DSL)—An industry-standard SDSL offering. SHDSL equipment conforms to the ITU-T Recommendation G.991.2. G.SHDSL outperforms older SDSL versions with a better loop reach (26,000 feet) and less crosstalk into other transmission systems in the same cable, and promises vendor interoperability. G.SHDSL systems operate in a range of transfer rates from 192 kbps to 2.3 Mbps. SHDSL is best suited to data-only applications that require higher upstream transfer rates than those typically available in DSL implementations.
■ HDSL (high-data-rate DSL)—Created in the late 1980s, this technology is meant to deliver symmetric service at upstream and downstream transfer rates up to 768 kbps in each direction (for a total of 1.544 Mbps). It is available in 1.544 Mbps (T1) as described or 2.048 Mbps (E1) depending on the country in which it is deployed. This symmetric fixed-rate service does not allow for standard telephone service over the same copper pair.
■ HDSL2 (second-generation HDSL)—Evolution of HDSL that allows 1.5 Mbps downstream and upstream transfer rates while still enabling the support of voice (Voice over IP), data, and video using either ATM or other technology over the same copper pair. HDSL2 does not provide standard POTS voice telephone service on the same wire pair. HDSL2 differs from HDSL in that HDSL2 uses one pair of wires to convey 1.5 Mbps, whereas ANSI HDSL uses two wire pairs.
■ IDSL (ISDN DSL)—Supports downstream and upstream transfer rates of up to 144 kbps (two 64-kbps channels plus one 16-kbps D channel for signaling) using existing phone lines. IDSL supports a local loop length of 18,000 feet but can be augmented to 45,000 feet using repeaters. It is unique in that it has the ability to deliver services through a digital loop carrier (DLC), a remote device that is typically located in remote terminals placed in newer housing developments to simplify the distribution of wiring from the telco. IDSL differs from traditional ISDN in that it is an always-available service rather than a dialup service. It is, however, capable of using the same terminal adapter (TA) used in traditional ISDN installations. IDSL is a data-only service and does not support traditional voice services.
The nature of DSL and its coexistence with POTS telephone service has been the subject of some discussion in both the industry at large and within xDSL circles. With terminology and limitations well in-hand, some additional depth regarding ADSL and how it is positioned in the teleworker architecture is in order.
As mentioned, DSL is a local loop or "last-mile" service. The CPE might be a DSL modem or a DSL-capable router. Both are very common depending on the type of access required or, more importantly, the number of hosts in the subscriber's home needing access to the Internet. The subscriber access device, be it a modem or a router with the modem functionality built in, is known as an ADSL Transmission Unit-remote (ATU-R). The ATU-R goes through a training process, similar in concept to that of a traditional modem, with the provider's modem—the ADSL Transmission Unit-central office (ATU-C). Multiple ATU-C units are bundled into a single chassis known as a DSLAM.
ADSL utilizes three distinct channels within the local loop. Each of these channels has a bit of buffer space between it and the adjacent channel. The manner in which the channels are utilized depends on the modulation type used for the particular ADSL deployment.
The modulation methods employed in bringing DSL to the CPE router simply exploit existing wires, making use of the untapped frequencies by setting aside channels or carriers for use by downstream and upstream data transfers.
Modulation, as defined earlier in the chapter, is the process of varying a periodic waveform to use that signal to convey a message. The ANSI standards for DSL define two modulation types:
■ Carrierless Amplitude Phase (CAP)
Line code represents a means of transmitting bits by increasing or decreasing (pulsing) voltage on the wire to represent a bit value of 1 in binary. A zero is represented with no alteration in voltage. The line-coding techniques associated with DSL technologies include a single-carrier method (CAP) as well as multicarrier methods (DMT and G.Lite). Until the advent and standardization of DMT, CAP was the more widely used modulation type. DMT's flexibility, coupled with the fact that it is an accepted industry standard, has allowed it to surpass CAP in today's market. The modulation method choice is, of course, made by the provider of the service.
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