DSL Technology

DSL (Digital Subscriber Line) is a family of technologies that facilitate Internet access by transmitting digital data over the twisted pair of conventional copper wires of the basic or switched telephone network.

DSL service is provided simultaneously with telephone service on the same line, since the former uses higher frequency bands to transmit data over the analog voice signal carried between the telephone exchange and subscribers with conventional twisted-pair wiring.

The bit rate of DSL services varies, depending on the type of technology installed, the condition of the telephone line, and the application of quality parameters.

DSL technologies use two types of data transmission modes: symmetric and asymmetric. In telecommunications, the term ADSL (Asymmetric DSL) or Asymmetric Digital Subscriber Line represents the most commonly installed DSL technology, where the downstream data rate (in the direction of the customer) is higher than the upstream one, hence the term asymmetric. In SDSL (Symmetric DSL) or Symmetric Digital Subscriber Line services, the downstream and upstream data rates are equal.

Equipment

DSL service providers' equipment is located in the same central facility that provides the telephone voice communication service. In that respect, the customer circuit is then rewired to a hardware interface, which combines frequencies for DSL and telephone voice communication frequencies in the same twisted pair installation.

ADSL technology uses two important pieces of equipment, one on the user's side and the other on the service provider's side, whether it is the telephone company or another provider. On the client or user side a transceiver or DSL modem is placed, which can provide other services as well. On the provider side, a DSL access multiplexer, called DSLAM, is installed to receive the customer connections.

The transport technology for DSL service uses the analog technique of high frequency sine wave modulation. A DSL circuit modulates bit patterns in certain high frequency pulses to transmit to the other modem.

The signals received by the customer's modem are demodulated to produce the corresponding bit pattern, which in turn will be retransmitted, now in digital form, to interconnected equipment such as a computer, router, or any other device.

DSL modems generally emit signals in the 25 kHz to 1.1 MHz band, much higher than the telephone voice baseband, which operates between 300 Hz and 3400 Hz; this band separation allows DSL and traditional telephone services to coexist.

At the customer end of the circuit (twisted pair), “low pass” DSL filters are installed on each phone to filter out the high frequency noise that would otherwise be heard. The “high pass” filters are correspondingly incorporated in DSL modems to filter the voice frequencies.

The DSLAM access multiplexer, which is usually located in the telephone exchange that provides the DSL service, is what actually allows the final connection to be established.

DSLAM collects connections from many users, and aggregates them into a single high-capacity Internet connection. These devices are generally flexible and capable of supporting multiple types of DSL; in fact, additional features such as routing or dynamic IP address assignment (DHCP) for clients are often included.

It should be noted that DSLAMs provide a big difference between a DSL service and a connection over a cable modem. Cable modem networks usually share a common network connection that is valid for everyone who wants to use it, which means that the more users are using it, the lower the throughput. DSL technology provides a dedicated connection from the user to the DSLAM, so you won't notice too many problems as more devices join.

On the other hand, if there will be any slowness in accessing the network using DLS technology, it is mainly due to the fact that the number of users connected simultaneously on the DSLAM starts to exceed the bandwidth of the available Internet connection. At this point, depending on the provider, additional service can be provided by means of an alternative DSLAM.

DSL Types (xDSL)

Various types of technologies used in DSL to transmit data are referred to as “x”, and for this reason are often referred to as xDSL, where “x” stands for different types of DSL technologies, and there are two types of modes for data transmission: symmetric and asymmetric, with their particular implementations implemented for each of them.

Symmetric DSL

Symmetric DSL (Symmetric Digital Subscriber Line) technology implements identical bandwidth, both upstream (from the customer to the network provider) and downstream, and can be considered as the counterpart of asymmetric DSL, but with slight variations. Some of the variations implemented for this mode are:

• HDSL: HDSL (High Bit Rate Data Subscriber Line) is implemented under the symmetric bidirectional data transmission mode and is considered the first DSL technology. It uses two pairs of twisted-pair copper cables to transmit data at up to 2 Mbps speed and 3.6 km range.
• SDSL: SDSL (Single Line DSL) is a version of HDSL, also known as SHDSL, that uses a single copper pair cable. It provides bidirectional, high-speed data transfer with a maximum range of 3 km and up to 1.5 Mbps. Compared to HDSL, it is easier to deploy, as you can reserve a cable in the twisted pair.
• IDSL: also known as ISDN Digital Subscriber Line, provides a basic ISDN service using DSL technology. ISDN (Integrated Services Digital Network) connections Red Digital de Servicios Integrados) are established over regular telephone lines to transmit digital rather than analog signals (transmitting the data, not voice), allowing information to travel slightly faster than it would with a traditional modem, and keeping the ISDN connection always active. It is rarely used, due to the relatively low speeds it supports.

Asymmetric DSL

In asymmetric DSL technology, the upstream bandwidth (from the customer to the network provider) is lower than the downstream bandwidth. It provides bidirectional bandwidth for different requirements such as web browsing, multimedia on demand and information distribution. Some of its most widespread variations are:

• ADSL: Asymmetric DSL also uses a twisted pair of copper telephone lines with a downstream speed capacity of up to 8Mbps and upstream speed of 1Mbps. It can transmit up to 3 or 5 kilometers. Typically in a household, ADSL technology only requires equipment to be installed at both ends of the line to provide high-speed broadband services without the need for new cables.
• RADSL: Rate-Adaptive DSL is a new generation access technology developed on the basis of ADSL. It allows service providers to adjust the bandwidth of xDSL connections to meet actual needs and resolve line length and quality issues. It provides a reliable access means to the data network for remote users. It uses a twisted-pair transmission pair; supports synchronous and asynchronous transfer; speed adaptation: downstream from 1.5 Mbps to 8 Mbps, and upstream from 16 Kbps to 640 Kbps; supports simultaneous data and voice transmission, especially suitable for extreme climatic environments.
• VDSL: Very High Speed Digital Subscriber Line is considered to be the high-end broadband data transfer technology over the twisted pair. The transmission speed is high on both the uplink and downlink, thus overcoming the insufficient bandwidth in the uplink direction (very useful for residential buildings where LAN cabling is very expensive). Its use should be focused on those areas with high speed requirements, good telephone line quality, and short distance, so that its advantages can be fully used.
• G.Lite: The G.Lite standard is a low-speed ADSL standard (also known as G.992.2). Its key feature is that it eliminates the customer-side filter (splitter) to separate the audio and data frequency bands, providing a downstream speed of 1.5 Mbps and an upstream speed of 512 Kbps over a telephone line.

Advantages and Disadvantages

DSL is a way to connect to the Internet using the regular telephone line, without interrupting the dial tone signal from the telephone, unlike dial-up connections, which use the same part of the line, and require dial tone to access an Internet phone number.

Some of the advantages of DSL technology are:

• The ability to connect to the Internet during the use of the telephone line concurrently used for voice calls.
• Higher speed than a dial-up modem.
• Use of each user's existing telephone line, without the need for new wiring.
• Generally, the company providing Internet access provides a DSL modem needed to obtain the service.

The main disadvantage for DSL Internet access is related to distance limitations: the closer the customer is to the service provider, the better the signal quality and connection speed.

On the other hand, the download speed varies depending on the type of DSL technology used, the line conditions, and the level of service/traffic applied to the telephone pair, all of which limit its use.

However, DSL services have proven to be extremely flexible and have become one of the most rapidly evolving and widely used telecommunications technologies to meet the demand of mainly residential customers with good quality and performance.