Chapter 16: xDSL

Overview             

                     One of the major problems facing the incumbent local exchange carriers (ILEC) is the ability to maintain and preserve their installed base. Ever since the Telecommunications Act of 1996, there has been mounting pressure on the ILECs to provide faster and more correct Internet access. In order to provide the higher−speed communications abilities, these carriers have continually looked for new means of providing the service.

                  However, the ILECs have an installed base of unshielded twisted pair in the local loop that cannot be ignored or abandoned. Therefore, a new form of communications was needed to work over the existing copper cable plant. One of the technologies selected was the use of xDSL. The DSL family includes several variations of what is known as digital subscriber line.

a.  ADSL

                   ADSL is a modem technology used to transmit speeds of between 1.5 Mbps and 6 Mbps under current technology.ADSL is the new modem technology to converge the existing twisted pair telephone lines into the high−speed communications access capability for various services. Most people consider ADSL as a transmission system instead of a modification to the existing transmission facilities.

Data rates for ADSL, based on installed wiring at varying gauges

The Analog Modem History

                   In the early days of modem communications, the Bell telephone companies provided all services across North America.Leased lines were used when specific speeds or volumes were anticipated, but not guaranteed by the dial−up services. Regardless of the modem and lines used, the main provider was the key ingredient.

               In 1968, things began to change. With court decisions allowing the introduction of competitive devices and the connection of these devices on the regulated carrier's network, demands began to escalate. Restrictions on power output and energy levels were in place to prevent any interference from the modems on the voice network. Also, the customer−provided modems were interconnected through a data coupler provided by the local regulated carriers. This, of course, involved a fee for the connection through the telephone company that provided protection equipment.

           Later, the Federal Communications Commission (FCC) in the United States and the Communications Radio and Television Commission (CRTC) in Canada allowed changes in the way the interconnection was handled. Modem manufacturers were allowed to produce their products according to a set of specifications and registrations, eliminating the need for the telephone company protection equipment and the fee associated with the monthly rentals.

2. IDSL

                    IDSL is a system in which digital data is transmitted at 128Kbps on a regular copper telephone line from a user to a destination using digital transmission, bypassing the telephone company's central office equipment that handles analog signals.IDSL uses ISDN-based technology to provide a data communication channel across existing copper telephone  lines at a rate of 144 kbit/s, slightly higher than a bonded dual channel ISDN connection a 128 kbit/s.

The IDSL line connection enables 128 Kbps in total simultaneously.

3. HDSL

          

                     HDSL delivers 1.544 Mbps of bandwidth each way over two copper twisted pairs. Because HDSL provides T1 speed, telephone companies have been using HDSL to provision local access to T1 services whenever possible. The operating range of HDSL is limited to 12,000 feet (3658.5 meters), so signal repeaters are installed to extend the service. HDSL requires two twisted pairs, so it is deployed primarily for PBX network connections, digital loop carrier systems, interexchange POPs, Internet servers, and private data networks.

4. SDSL

                 SDSL was developed to provide high-speed communications on that single cable pair but at distances no greater than 10k. Despite the distance limitation, SDSL was designed to deliver 1.544Mbps on the single cable pair. Typically, the providers offer SDSL at 768 Kbps. This creates a dilemma for the carriers because HDSL dan do the same things as SDSL.

5. RADSL

                

                      This is a popular variation of ADSL that allows the modem to adjust the speed of the connection depending on the length and quality of the line. This gives the flexibility to adapt to the changing conditions and adjust the speeds in each direction to potentially maximize the throughput on each line. Additionally, as line conditions change, you can see the speeds changing in each direction during the transmission. Many of the ILEC's have installed RADSL as their choice, given the local loop conditions. Speeds of up to 768 Kbps are the preferred rates offered by the incumbent providers.

6. CDSL

                     CDSL does not use, nor need, a splitter on the line. Moreover, speeds of up to 1 Mbps in the download direction and 160 Kbps in the upward direction are provided. It is expected that the speeds and DSL will meet the needs of the average consumer for some time to come.

7. SHDSL

                SHDSL supports repeaters, which further increase the reach capability of this technology. Another critical advantage of SHDSL is its increase in symmetric bandwidth. SHDSL is also rate adaptive, enabling flexible revenue−generation models and enabling service providers to offer service−level agreements that ensure businesses get the service they want, when they want it.

8. VDSL

                      VDSL provides an incredible amount of bandwidth, with speeds up to about 52 megabits per second (Mbps). Compare that with a maximum speed of 8 to 10 Mbps for ADSL or cable modem and it's clear that the move from current broadband technology to VDSL could be as significant as the migration from a 56K modem to broadband. As VDSL becomes more common, you can expect that integrated packages will be cheaper than the total amount for current separate services.

Summary of DSL speeds and operations using current methods

Source: http://en.wikipedia.org/wiki/ISDN_digital_subscriber_line
                    http://searchnetworking.techtarget.com/definition/IDSL
             http://www.cisco.com/en/US/tech/tk175/tk318/tsd_technology_support_protocol_home.html
                   http://computer.howstuffworks.com/vdsl3.htm
                   Broadband Telecommunications Handbook (VPN 3GW GPRS MPLS VoIP SIP).pdf