DSL Technology

The subscriber line PSTN (public switched telephone network) was initially designed to transmit analog voice transmissions (telephone conversions) and signaling. Although since the concept information transmission as we know of it today didn't exist. The phone systems regular frequency of transmitting voice transmissions is between 300 and 3,400 Hz, which is known as the range that human speech is required for it to be clearly intelligible. This is more commonly referred to as voiceband.

Once the analog voice transmissions reach the central office (telephone exchange) speech is digitized into a 64 kbit/s data stream in the form of an 8 bit signal using a  sampling rate of 8,000 Hz. Through the Nyquist–Shannon sampling theorem all signals above 4,000 Hz will not be passed by the phone network.

Through the laws of the Shannon–Hartley theorem (the Shannon limit) there are limits to the speed of data transmission. In the earlier days of telephony it was thought that a standard phone line couldn't be pushed beyond the low speed limits of data transmission (generally under 9,500 bps). During the 1950s 4 MHz television signals were commonly transmitted through standard twisted pair telephone wire, suggesting that the Shannon limit would allow for the transmission of many Megabits per second. Although there was a fault with said suggestion, the current twisted pair telephone wire had many impairments, thus limiting these rates and making them far to impractical. During the 1980s new techniques where developed for that limit to be greatly exceeded.

The local loop that connections the telephone switch to subscribers is able to route frequencies far beyond the plain old telephone service limit of 3.4 kHz. Depending on the distance and strength of the local loop, limits can reach up to tens of megahertz. DSL uses this unused bandwidth of the loop by forming 4312.5 Hz wide channels, beginning at 10 and 100 kHz, based on how the system was configured. Channels are constantly checked for usability in a similar way an analog modem would with a plain old telephone service connection. Very similar to an analog modem a DSL transceiver will frequently monitor the durability and quality of each channel, than add or remove them from service based on their quality.

ADSL service supports two modes for transmission, interleaved channel and fast channel. Fast channel is primarily for streaming video and audio, where Latency is everything. Interleaved channel is primarily for file transfers, where transfer errors are unacceptable.

The success of DSL largely reflects on how elections in recent decades have improved and become less expensive, while laying fiber optic and copper cable still remains very costly. During the early 1990s the cost of Digital signal microprocessors for DSL technologies was far to expensive for DSL to reach the popularity that it has today, with advancements in VLSI technology in the late 1990s the cost has been lowered to the point where phone companies could profit off this technology. The DSL service could be deployed over an existing cable line rather than the much more expesive task of installing a new, fiber-optic cable. With the competition in Internet access the costs of ADSL have dropped drastically, thus making ADSL much more economical compared to a dial-up service. These are the major factors of popularization of DSL technology today.

Every DSL type employs a very complex DSP (Digital signal processing) algorithm procedure to eliminate the inherent limits of exisiting twisted pair cabling. During the 1990s the price of such a signal processing system would be far to expensive but thanks to the VLSI technology, the price of installing DSL on a standard existing local loop from DSLAM to DSL modem is far less than it would cost to install a brand new fiber-optic cable over the same distance.

Many residential and small-office DSL services reserve low frequencies for voiceband services(telephone conversions), so that splitters are able to split the DSL and voiceband frequencies from one an another allowing you to use existing voice service and operate a DSL connection. Thus allowing standard telephone services, fax machines and analog modems to share the same wires as DSL. Only one DSL connection is aloud to use the subscriber line at a time, the most common way to connect multiple computers to a DSL service is through a router or hub that will establish a connection from the DSL modem to a Ethernet, Wi-Fi or Powerline network on the customer's residence. The amount of landlines in the US has dropped from 189,000,000 in 2000 to 172,000,000 in 2005, while the amount of cellphone subscribers has grown. It is expected with VoIP becoming largely popular that this number will dramatically decrease again.

Edited by Administrator - 05 Apr 2007 at 6:31pm