Various DSL Technologies And How They Differ (Part 1)

This article explores the information carrying capability of different types of DSL technologies along with the limitations of various existing high-speed data services like coaxial cable, analogue modem, cable modem, ISDN and satellite data services.

The rapid development in the field of information technolo­gies has led to the appearance of new services that require high-speed data transmission technologies. For example, services like voice-over-IP, video streaming, teleworking, tele­medicine, telecommuting, broadcast­ing of TV programs, high-speed file sharing, user-generated video, online video gaming, online education, shop­ping etc, require high-speed Internet access for effective operation.

Until now, existing high-speed ser­vices (coaxial cable, analogue modem, etc) were not well-suited to the real needs of these services due to several different reasons. The idea of using twisted-pair cabling seemed the best since throughout the world millions of connections of this type were already in place and it just needed equipment to be added to the telephone exchange along with a small installation at the user location to be able to access digi­tal subscriber line (DSL) technology. Unshielded copper pairs used in the telephone network carry voice signals in the frequency range of 300 to 3400 Hz but are capable of transporting in­formation at much higher rates. These cables have been used to transport data in LANs up to 10 MHz or more.

Various DSL technologies and how they differ

The performance of analogue mo­dems is very poor in comparison to the DSL. Another solution lies in modem bonding, where two modems are close- coupled to one computer, which may theoretically double the performance of a single modem. Theoretically, this arrangement increases network performance by a factor of two, but many Internet users need much more improvement in their connectivity speeds to support features like video streaming, online gaming and large file transfer at the home or workplace. For these purposes, DSL is a better option.

Cable modems support high-speed, 'always-on' Internet access using the cable television lines, which is compa­rable to that of DSL. But, the difference between DSL and cable line technology lies in the distribution of bandwidth. In DSL the bandwidth is dedicated locally to all the subscribers, i.e., speed won't drop when others use it at the same time. On the other hand, cable modem service involves locally shared bandwidth. This means, the realized performance of a customer's cable will depend on how many other customers in that local area subscribe to the same service.

Integrated services digital network (ISDN) technology provides data rates just twice that of the ordinary dial-up connection. But this much speed is not comparable to data rates of either cable modem technology or DSL technology. ISDN has been more widely available for several years from the telecom com­panies, but the very fast expansion of DSL networks superseded the ISDN.

For remote areas, which are out of reach of DSL service, satellite data ser­vice is a good option. But, here again, data rates are nearly one-third the data rates of DSL.

The advances in electronics have largely made DSL a successful, faster and cheaper technology, though dig­ging trenches in the ground for new copper or fibẻ-optic cables remains expensive. All types of DSL utilize very complex digital signal processing to overcome the inherent limitations of the existing copper pair. Until the late 1990s, digital signal processors for DSL were very expensive. But the rapid de­velopment in very large-scale integra­tion (VLSI) technology has significantly lowered the cost of signal processors that supported DSL as a commercially successful technology.

DSL technology

Digital subscriber line (DSL) is referred to as a broadband technology because it is an 'always-on' data connection that is able to support interactive ser­vices including Internet access. It supports the minimum download speed of 256 kbps to an individual subscriber from the point of presence (POP) of the service provider where multiple such individual broadband connections are aggregated. The subscriber is able to access these interactive services includ­ing the Internet through this POP.

DSL technology is a modem tech­nology that uses existing twisted-pair telephone lines to transport high-bandwidth data, such as multimedia and video, to service subscribers. xDSL services, a family of technologies, are dedicated, point-to-point, public net­work access over twisted-pair copper wire on the local loop between an In­ternet service provider's (ISP's) central office and the customer site, or on local loops created either intra-building or intra-campus.

In order to provide DSL connec­tions, the existing telephone network is utilized and a network element called digital subscriber line access multiplex­er (DSLAM) is installed at the central office/telephone exchange along with a modem placed in subscribers' home or workplace. The DSLAM works like a concentrator. It provides multiple DSL connections for access to the bandwidth available to the DSLAM from the Internet backbone. The con­nection to the subscriber is then given from DSLAM via copper lines. The DSL modem performs the task of con­verting the DSL physical layer signal into a format that can be understood by a computer or any other equipment connected to it.

The DSL modem performs the task of con­verting the DSL physical layer signal into a format that can be understood by a computer or any other equipment connected to it.

The pair from the DSLAM is termi­nated into the main distribution frame (MDF), from where connections are provided to subscribers via twisted copper pair lines. These twisted pair lines terminate at DP box placed near a subscriber's home/workplace. The connection from DP box is first run to the splitter that sits in the customer's premises. The splitter, which is basi­cally a low-pass filter, removes the plain old telephone system (POTS) signal (voice signal, 300-3400 Hz) from the incoming DSL signal. The high- frequency filtered DSL signal is then given to a DSL modem. Now the con­nection is taken out from Ethernet port of DSL modem and terminated to the subscriber's computer.

Now the con­nection is taken out from Ethernet port of DSL modem and terminated to the subscriber's computer.

The underlying technology of transport across DSL facilities is a high- frequency sinusoidal carrier modula­tion, which is an analogue-based signal transmission. Each end of a DSL circuit has a modem that modulates patterns of bits into certain high-frequency tones, representing that bit pattern, for transmission across the length of the facility. Tones received from the far-end modem are demodulated back into a corresponding bit pattern that the near-end modem retransmits, in true digital form as pulses of voltage, to its interfaced equipment (such as a computer, router and switch).

Unlike traditional dial-up modems, which modulate bits into carrier that can fall only in the 300-3400Hz base­band (voice service), DSL modems modulate frequencies from 4 kHz to as high as 4 MHz. This frequency band separation enables DSL service and POTS to coexist on the same copper pair facility.

DSL technology divides the fre­quencies used in a single phone-line into two primary bands. The high-frequency band is used for data services and lower-frequency band (below 4 kHz) is utilized for voice. Ideally, DSL service provides 'always-on' connection to the customers to access the Internet. DSL service implemented with point- to-point over Ethernet (PPoE) does not support 'always-on' connection, but even in this case a DSL router/modem can automate the connection process.