Asymmetrical Digital Subscriber Line (ADSL) - Online Article

Asymmetrical Digital Subscriber Line (ADSL) is a method to use the existing analog local loop lines for digital data transfer to and from the home. It is asymmetrical in that the upstream transfer rate is slower than the downstream data rate. This means that the data transfer from the premise (home) to the CO is a different rate than the data transfer from the CO to the home.

The data transfer is rate adaptive. This means that depending on the condition of the local loop lines, ADSL will automatically compensate and find the fastest transfer rate possible. The range for upstream data transfer is 64 kbps to 768 kbps. The range for downstream data transfers is 1.5 Mbps to 8 Mbps. The reasoning for the asymmetrical transfer rate is that most users will be surfing the Internet, upstream requests tend to be small webpage addresses. The downstream data consists of downloads of large graphic intensive webpages. Small upstream requests, larger downstream response.

The data transfer rate depends on the distance from the central office, the quality of the line and the wire gauge. If the distance from the central office is 15,000 to 18,000 ft, then the maximum transfer rate is 1.5 Mbps. If the distance is 9,000 ft or less, the maximum transfer rate is 8 Mbps.

How ADSL  Works ?

On the Wire

Currently, most ADSL communication is full-duplex. Full-duplex ADSL communication is usually achieved on a wire pair by either frequency-division duplex (FDD), echo-cancelling duplex (ECD), or time-division duplexing (TDD). FDD uses two separate frequency bands, referred to as the upstream and downstream bands. The upstream band is used for communication from the end user to the telephone central office. The downstream band is used for communicating from the central office to the end user.

With standard ADSL (annex A), the band from 25.875 kHz to 138 kHz is used for upstream communication, while 138 kHz – 1104 kHz is used for downstream communication. Each of these is further divided into smaller frequency channels of 4.3125 kHz. During initial training, the ADSL modem tests which of the available channels have an acceptable signal-to-noise ratio. The distance from the telephone exchange, noise on the copper wire, or interference from AM radio stations may introduce errors on some frequencies. By keeping the channels small, a high error rate on one frequency thus need not render the line unusable: the channel will not be used, merely resulting in reduced throughput on an otherwise functional ADSL connection.

Vendors may support usage of higher frequencies as a proprietary extension to the standard. However, this requires matching vendor-supplied equipment on both ends of the line, and will likely result in crosstalk problems that affect other lines in the same bundle.

There is a direct relationship between the number of channels available and the throughput capacity of the ADSL connection. The exact data capacity per channel depends on the modulation method used.

ADSL Standards

At the time of this writing, there are 3 competing standards for ADSL:

  • Carrierless Phase Modulation ADSL,
  • Splitterless ADSL
  • Discrete Multitone ADSL.

Carrierless Phase Modulation (CAP) ADSL is a modulation technique similar to Quadrature Amplitude Modulation. It provides Echo Cancellation and overlaps upstream and downstream signals.

Splitterless ADSL (also called ADSL Lite, G.Lite, PnP ADSL, Universal ADSL) has a lower transmitting rate and is easier to implement.

DMT - Discrete Multitone is an ANSI T1.413 standard which uses a broadband modem that covers the 4 kHz to 2.2 MHz range. It has 256 channels of 4 kHz, each channel is assigned 15 bits of data to transfer. In addition each channel is checked for signal quality and bits assigned accordingly. A poor responding channel may less bits assigned or none at all. DMT adjusts for the local loop line conditions and attempts to make the fastest transfer rate possible.

ADSL OSI Model

ADSL is a Physical layer protocol which covers the transmission of data, and cabling requirements.

ADSL Premise Equipment

ADSL shares the bandwidth of the local loop with the existing phone system. It does not require modification to the central office switch. Instead a splitter combines the ADSL information with the POTS switch's analog information. At the central office end, the ADSL signal is sent to the Digital Subscriber Line Access Module (DSLAM) and then to a communication server.

At the premise end, another splitter separates the ADSL information from the analog information. An ADSL modem called an ATU-R device decodes the ADSL information and sends it to the Service Module (SM). The Service Module translates it to Ethernet. In plain network terms, in comes ADSL and out comes an Ethernet signal for connection to a network interface card (NIC).

ADSL Advantages

  • No expensive modification is required to CO switch.
  • Simple splitter splits ADSL signal from the existing analog line.
  • High bandwidth is available.
  • The POTS works regardless of ADSL.
  • ADSL has competitive pricing versus other technologies

ADSL Disadvantages

  • The transfer rate depends on distance from the central office.
  • The presence of bridged taps and load coils on the local loop affect the transfer rate.
  • ADSL must be installed to test if it will work.
  • 25% of existing local loops will not work with ADSL
  • There is an 18,000 ft distance limit from the central office.
  • There can be a bottleneck at the communication server at central office.

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