Digital Subscriber Line Technology Lecture 7: Digital Subscriber Line Technology DSL

Outline DSL Technology What is DSL? DSL and Dial-up Modems DSL Configuration DSL equipment the variants of DSL Modulation DSL Benefits

Introduction To send computer data over the telephone network, dial- up modem was used. They do this by modulating data onto voice frequencies, those below 4000 Hz. This is a rather inefficient approach: Voice frequencies are not well suited to carrying data. The telephone network has evolved from an analog medium to a mostly digital medium. These processes are slow (not more than a 56 Kbps) and error prone modems require significant set-up time Since the late 1950s, modems have been used to transmit data over the public telephone network. They do this by modulating data onto voice frequencies, those below 4000 Hz, and carrying it over twisted pair copper cable to a central office. The central office forwards it into the telephone network and ultimately to its destination. This is a rather inefficient approach. Voice frequencies are not well suited to carrying data. Moreover, the telephone network has evolved from an analog medium to a mostly digital medium, forcing a modem transmission to undergo multiple analogto- digital and digital-to-analog conversions in transit. These processes are slow and error prone and generally unable to manage more than a nominal 56 Kbps, and usually much lower speeds than that. In addition, modems require significant set-up time. The traditional phone service (also known as the Plain Old Telephone Service or POTS) was created to exchange voice information using an analogue signal. Computers, however, use digital signals to communicate, so in order for this signal to travel over the telephone network, a modem is needed to convert the digital data to analogue and back again. The telephone analogue frequency uses only a small proportion of the bandwidth on a line (under 4kHz).The maximum amount of data that conventional dial-up modems can transmit through a POTS system is about 56Kbps. Using this method to send data, the transmission through the telephone company is a bandwidth bottleneck. Typical telephone cabling is capable of supporting a greater range of frequencies (around 1MHz). With DSL modems, the digital signal is not limited to 4kHz of voice frequencies, as it does not need to travel through the telephone switching system. DSL modems enable up to 1MHz of bandwidth to be used for transmitting digital (data) alongside analogue (voice) signals on the same wire by separating the signals, thereby preventing the signals from interfering with each other. Figure 2 shows how the analogue and digital frequencies are split.

DSL Technology The DSL technology exploits the fact that typical telephone cabling (local loop) is capable of supporting a greater range of frequencies (around 1MHz) and all telephone signals are below 4kHz in frequency. With DSL modems: The digital signal is not limited to 4kHz of voice frequencies. It enable up to 1MHz of bandwidth to be used for transmitting digital (data) alongside analogue (voice) signals on the same wire by separating the signals, thereby preventing the signals from interfering with each other  DSL makes use of the rest of the 1MHz.

What is DSL? DSL (Digital Subscriber Line) is a modem technology that uses existing telephone lines to provides high-speed data transmissions. DSL provides dedicated, point-to-point, public network access. This DSL connection is typically between a network service provider central office (CO) and the customer site. There are a number of different types of DSL and they are referred to collectively as xDSL.

DSL and Dial-up Modems DSL offers several basic improvements on the dial-up modem: It dispenses with the need to modulate digital data into a voice (analog) signal. Uses higher frequencies, with correspondingly higher information carrying potential. Unlike a modem, a DSL line can be kept permanently active, requiring no call setup delays. DSL permits the twisted pair to support data traffic and voice traffic simultaneously. It dispenses with the need to modulate digital data into a voice signal, preserving it as digital throughout its transmission

DSL Configuration

A DSL modem is required at each end of the local loop. In DSL network, each user is having separate P2P dedicated access to the service provider’s CO. A DSL modem is required at each end of the local loop. DSL actually signifies the pair of modems at each end of a line and not the line at all. Internal splitter is used at users end for separating POTS from computer signal that is transmitting data. Multiplexers (called DSLAMs ) are installed at the CO to receive signals from customers. The CO separates the DSL service from the POTS service, the voice service being sent onward by means of the ordinary trunk network, whereas the data carried by the DSL may be sent to a data network that is separate from the switched voice network.

DSL equipment DSL equipment resides on the customer premise, on the local loop, and in the central office DSL Modems: typically is associated with a user PC and may be internal or external. Splitters: Some DSL technologies require installation of a splitter at the user-end to separate voice and data frequencies. Splitting can also be provided at the CO, but at the cost of lower data rates than user-end splitting. Splitters. Some DSL technologies require installation of a splitter at the user-end to separate voice and data frequencies. From a service standpoint, this approach has its pros and cons. It requires a customer site visit, but provides enhanced performance.

DSL equipment Digital Subscriber Line Access Multiplexers (DSLAMs). These devices are CO equipment. They accept input from remote DSL users and forward it to a DSL access concentrator . Copper Cable. The local loop that connects the DSL modem with the DSLAM is twisted pair copper cable. The length of cable is critical in determining allowable DSL bandwidths.

DSL equipment Digital Loop Carriers (DLCs): These are multiplexers that exist outside the central office, closer to the user than a DSLAM. They extend access to areas that are too far from the central office to receive service otherwise. DSL Access Concentrators. These devices are edge routers that accept traffic from DSLAMs and forward it onto a service network..

What are the variants of DSL? DSL technologies can be subdivided into three broad classes: Symmetric: the data rate transmitted in both directions (downstream and upstream) is the same. This is a typical requirement of business customers. It includes the following DSL types:- HDSL, SDSL and SHDSL Asymmetric. In this case, there is asymmetry between the data rates in the downstream and upstream directions, with the downstream data rate typically higher than the upstream (usually appropriate for applications such as Web browsing). . It includes the following DSL types: ADSL, ADSL lite, ADSL 2, ADSL 2+ and ADSL 2++ or ADSL4 There are numerous different DSL technologies. The range of DSL types reflects the numerous different applications requiring different data rates. Generally speaking, the variations of DSL technology have been implemented to meet the needs of different users, such as home users, small to medium sized businesses, schools and colleges. See table 1 on page 9 for a comparison of the data rates and their applications. Symmetric DSL transmits data at the same rate upstream and downstream. It includes the following DSL types:- • High data rate Digital Subscriber Line (HDSL) • Symmetric Digital Subscriber Line (SDSL) • Symmetric High bit rate Digital Subscriber Line (SHDSL)

What are the variants of DSL? Symmetric and Asymmetric DSL: it can transmit data both symmetrically and asymmetrically. It includes the following DSL type: VDSL.

Modulation ADSL modems use specialized modulation technology to divide the available bandwidth on a copper pair line and create multiple channels for sending and receiving signals. DSL uses several modulation techniques. Two of the most common are: Discrete Multitone Technology (DMT) which uses 256 narrow-band carriers, all transmitting simultaneously. Carrierless Amplitude Modulation (CAP), modulates the data onto two wide frequency bands in a manner similar to quadrature amplitude modulation (QAM) used by modems.

What are the benefits of DSL? DSL is a more cost effective option than many other broadband connections, such as leased lines, terrestrial broadcast, cable and cable modem, satellite and fiber optic connections, because it is able to take advantage of the existing telephone infrastructure for both voice and data traffic. Only the user’s modem and the telecommunications equipment needs to be upgraded when moving to a DSL connection because it utilizes the existing cable infrastructure. DSL is always on, always fast and always reliable. DSL connections are point-to-point dedicated circuits that are always connected, so there is no time lost dialling up.

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