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xDSL Technology Overview
What's behind DSL?
DSL is a digital broadband transmission technology that involves sending digital information over a subscriber's telephone line, also referred to as the local loop. The acronym DSL stands for digital subscriber line. The applications of DSL involve the transport of high-speed data, voice and recently video, to residential and business subscribers.
DSL in general describes the technology, while xDSL represents individual varieties of DSL technology. The x in xDSL is a variable that changes depending upon the speed and application of the xDSL technology (e.g., ADSL, VDSL, etc.).
ATU-C: Asymmetric digital subscriber line (ADSL) transceiver unit central
ATU-R: Asymmetric digital subscriber line (ADSL) transceiver unit remote
DSLAM: Digital subscriber line access multiplexer
POTS: Plain old telephone service
Figure 1: ADSL topology
For example, ADSL (where the A stands for asymmetrical) is best suited for the residential sector, as the bit rate from the network to the subscriber is higher than the bit rate from the subscriber to the network. SHDSL (where the SH stands for symmetrical high-speed) is a symmetrical service that is best suited for the business sector to replace a traditional T1 or E1 service.
ADSL2, ADSL Reach-Extended (RE), ADSL2+, VDSL, VDSL (1.5) and VDSL2 are all variations of asymmetrical services. Typically, they represent evolutions in the technology aimed at providing higher speeds, greater resiliency to noise and other impairments, and/or operation over longer local-loop distances.
DSL achieves higher data-transfer rates than dial-up modems by utilizing more of the available bandwidth in a local loop. Ordinary telephone service only makes use of a very limited amount of the lower frequencies—3400 Hz and below. Although a bandwidth of 3400 Hz is more than enough for transmitting reasonable-quality analog voice, it is not suitable for transmitting high-speed data.
DSL provides at least 256 kbit/s to as much as 100 Mbit/s. This allows for the simultaneous transmission of high-quality digital voice, data and even full motion video (a.k.a. triple-play services). Services based on ADSL and ADSL2+ have been widely deployed and deliver rich-content broadband experiences for residential and commercial consumers.
Still Analog, Still Copper
Most of the DSL technologies mentioned above send their digital signals over twisted-copper telephone lines (local loops) using amplitude-modulated analog tones. This technique is referred to as discrete multitone transmission (DMT). For example, ADSL uses up to 256 tones of different frequencies, each of which is modulated in amplitude, depending on the bit sequences to be carried.
From a test and measurement point of view, the local loop must carry as many tones as possible to achieve the highest possible bit rate with the least amount of disruptions due to noise interference. Short local loops do not attenuate the DSL signals as much as longer loops. In addition, local loops that have excellent electrical continuity, free from bad splices and galvanic corrosion, are best. Most telecommunications service providers have preventative maintenance practices to ensure the quality of their local loop plant. If not, they at least make every attempt to repair local loops that become defective over time.
Figure 2: ADSL carrier-load data
Data, Voice and Video
At the core of the DSL network are the DSL modem (installed at the customer's home) and the digital subscriber loop access multiplexer (DSLAM), which is located either at the telco's central office or in remote terminals in the neighborhoods. The DSLAM is a multiport device that serves hundreds to thousands of subscribers through a direct connection to the copper local-loop plant.
Most DSL networks are based on ATM and/or Ethernet/IP protocols. This makes it easier for service providers to offer Internet, VoIP and IPTV services to subscribers, while allowing the straightforward integration of DSL equipment into their overall networks. Customer data is carried, routed and managed using the various packet IP and ATM addresses in conjunction with encapsulation protocols.
Figure 3: Representation of a DSL network delivering triple-play services
In DSL networks, legacy PSTN phone service or ISDN coexists in the same local loop that carries the DSL signals. The two services simultaneously utilize different frequency bands to avoid interfering with each other. Once PSTN or ISDN signals arrive at the DSLAM, they are separated using splitters (low-pass and/or high-pass filters) and fed to the traditional public switched telephone network (PSTN) switch or integrated-services digital network (ISDN) equipment.
As an alternative to PSTN, service providers or third parties may offer VoIP services. In such cases, the voice communications of subscribers are converted into streams of digitized and packetized voice. From there, they are carried as IP packets through the network.
In the past few years, the evolution of DSL technology has been able to generate high enough bit rates that it can now deliver broadcast-quality video dubbed IPTV, which uses video content servers and broadcast channel management devices to serve up a few video channels at a time to each subscriber. Unlike coaxial-based CATV networks that simultaneously deliver hundreds of TV channels to each end point, irregardless of the channels actually being watched or recorded, DSL/IPTV delivers only those channels that are in use at any one time. This is how the lower inherent bandwidth of a twisted pair of telephone wires can offer a customer experience that is similar to the higher-bandwidth-capable CATV networks.
DSL service performance varies according to the following factors:
- The distance between a customer premises and the local telephone company's central office or remote terminal
- The capacity of the digital backbone that is available to support each DSLAM
- The number of users that are aggregated to the available digital backbone
- The behavior of the served community (i.e., how often they use the service, how much data they consume, the type of services offered)
- The quality of the local loop between the customer premises and the service provider's central office
- The proximity of noise sources to the DSL equipment and the local loop
- The state of the customer premises wiring
- The condition of the public power feed and grounding
Provisioning widespread deployments of DSL service is complex. Each customer can get a different connection rate and, in many cases, subscribers may choose from a wide array of services. When these factors are coupled with the complexity of setting up user names, passwords, DSLAM port settings, subscribed video channels (in the case of IPTV), Internet service provider settings, e-mail settings, as well as VoIP settings, it is quite typical to expect service-impairing errors for a certain percentage of subscribers. A test set that provides clear connection visibiliy to technicians has tremendous value (see example of test results above).
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