Digital Subscriber Line

C H A P T E R Chapter Goals Identify and discuss different types of digital lector stock certificate (digital proofreader line) technologies. Discuss the benefits of using xdigital subscriber line technologies. Explain how ASDL works. Explain the canonic concepts of communicateing and modulation. Discuss additional DSL technologies (SDSL, HDSL, HDSL-2, G. SHDSL, IDSL, and VDSL). Digital suppressorser crimp Introduction Digital proofreader crease (DSL) applied science is a modem technology that uses existent reprobate-pair call off lines to transport high-bandwidth info, much(prenominal) as multimedia and video, to receipts subscribers.The term xDSL coers a fleck of homogeneous yet competing forms of DSL technologies, including ADSL, SDSL, HDSL, HDSL-2, G. SHDL, IDSL, and VDSL. xDSL is drawing signifi kindlet attention from implementers and advantage providers be form it promises to deliver high-bandwidth info evaluate to dispersed locations with relatively small changes to the existing telco infrastructure. xDSL servings are dedicated, point-to-point, public cyberspace rec everyplace over twisted-pair copper conducting wire on the local loop (last mile) between a ne twork help providers (NSP) teleph cardinal exchange region and the node site, or on local loops compeld either intrabuilding or intracampus.Currently, most DSL deployments are ADSL, mainly delivered to residential customers. This chapter focus mainly on defining ADSL. irregular Digital lecturer livestock Asymmetric Digital lector production line (ADSL) technology is asymmetric. It al broken ins to a greater extent bandwidth downriverfrom an NSPs central office to the customer sitethan upstream from the subscriber to the central office. This asymmetry, combined with always-on nettle (which eliminates call setup), makes ADSL ideal for electronic network/intranet surfing, video-on-demand, and remote LAN access. Users of these applications typically download much to a greater extent than selective cultivation than they send.Internetworking Technologies Handbook 1-58705-001-3 21-1 Chapter 21 Asymmetric Digital lector Line Digital Subscriber Line ADSL transmits more than 6 Mbps to a subscriber and as much as 640 kbps more in both directions (shown in depend 21-1). Such order expand existing access capacity by a compute of 50 or more without new cabling. ADSL piece of tail literally transform the existing public information network from genius limited to vocalise, text, and low-resolution graphics to a powerful, present system capable of obstetrical delivery multimedia, including full-motion video, to every home this century.Figure 21-1 The Components of an ADSL Network Include a Telco and a CPE Core network Existing copper Server ADSL ADSL 1. 5 to 9 Mbps 16 to 640 kbps Internet ADSL connection ADSL will swordplay a crucial role over the next decade or more as predict companies enter new markets for delivering information in video and multimedia formats. New broadband cabling will tug decades to r severally all prospective subscribers. Success of these new services depends on reaching as some(prenominal) subscribers as possible during the first few years.By bringing movies, television, video catalogs, remote CD-ROMs, corpo deem LANs, and the Internet into homes and small telephone circuites, ADSL will make these markets viable and profitable for telephone companies and application suppliers a exchangeable. ADSL Capabilities An ADSL circuit connects an ADSL modem on each end of a twisted-pair telephone line, creating three information driveways a high-velocity downriver channel, a medium-speed duplex channel, and a sanctioned telephone service channel. The basic telephone service channel is ruin off from the digital modem by filters, thus guaranteeing uninterrupted basic telephone service, steady if ADSL fails.The high-speed channel ranges from 1. 5 to 9 Mbps, and duplex evaluate range from 16 to 640 kbps. to each one channel squeeze out be submultiplexed to form multiple lower-rate channel. ADSL modems provide data rates consistent with North American T1 1. 544 Mbps and European E1 2. 048 Mbps digital hierarchies (see Figure 21-2), and can be purchased with various speed ranges and capabilities. The minimum configuration provides 1. 5 or 2. 0 Mbps downriver and a 16-kbps duplex channel others provide rates of 6. 1 Mbps and 64 kbps for duplex.Products with downstream rates up to 8 Mbps and duplex rates up to 640 kbps are available today. ADSL modems accommodate Asynchronous maneuver Mode (ATM) transport with variable rates and compensation for ATM overhead, as well as IP protocols. Internetworking Technologies Handbook 21-2 1-58705-001-3 Chapter 21 Digital Subscriber Line Asymmetric Digital Subscriber Line Figure 21-2 This Chart Shows the Speeds for downriver Bearer and Duplex Bearer Channels downstream bearer channels n x 1. 536 Mbps 1. 536 Mbps 3. 072 Mbps 4. 608 Mbps 6. 144 Mbps 2. 048 Mbps 4. 096 Mbps x 2. 048 Mbps Duplex bearer channels C channel Optional channels 16 Kbps 64 Kbps 160 Kbps 384 Kbps 544 Kbps 576 Kbps Downstream data rates depend on a number of factors, including the length of the copper line, its wire gauge, the presence of bridged taps, and cross-coupled throw innce. Line attenuation increases with line length and frequency, and decreases as wire diameter increases. Ignoring bridged taps, ADSL performs as shown in Table 21-1. Table 21-1 Claimed ADSL Physical-Media Performance Data Rate (Mbps) 1. 5 or 2 1. 5 or 2 6. 1 6. 1 Wire Gauge (AWG) 24 26 24 26 surmount (feet) 18,000 15,000 12,000 9,000 Wire Size (mm) 0. 5 0. 4 0. 5 0. 4 Distance (km) 5. 5 4. 6 3. 7 2. 7 Although the measure varies from telco to telco, these capabilities can cover up to 95 percent of a loop plant, depending on the desired data rate. Customers beyond these distances can be reached with fiber-based digital loop carrier (DLC) systems. As these DLC syst ems become commercially available, telephone companies can oblation virtually ubiquitous access in a relatively unforesightful time. Many applications envisioned for ADSL involve digital besotted video.As a real-time signal, digital video cannot use link- or network-level error control procedures usually found in data communication theory systems. Therefore, ADSL modems incorporate forward error correction that dramatically withers errors cause by impulse affray. Error correction on a symbol-by-symbol basis in like manner reduces errors caused by continuous hindrance coupled into a line. ADSL Technology ADSL depends on advanced digital signal processing and creative algorithms to abridge so much information through twisted-pair telephone lines. In addition, many advances have been required in transformers, analog filters, and analog/digital (A/D) converters.Long telephone lines may attenuate signals at 1 megahertz (the outer edge of the band used by ADSL) by as much as 90 dB, forcing analog sections of ADSL modems to work very hard to realize large dynamic ranges, separate channels, and Internetworking Technologies Handbook 1-58705-001-3 21-3 Chapter 21 Asymmetric Digital Subscriber Line Digital Subscriber Line maintain low noise figures. On the outside, ADSL looks simpletransparent synchronous data pipes at various data rates over ordinary telephone lines. The inside, where all the transistors work, is a miracle of modern technology. Figure 21-3 displays the ADSL transceiver-network end.Figure 21-3 This Diagram Provides an Overview of the Devices That Make Up the ADSL Transceiver-Network End of the Topology Downstream channel(s) Duplex channel(s) Mux Error control XMTR D/A and A/D Line coupler Channel separation (FDM or ECH) canonic telephone service splitter Line Demux Duplex channel(s) Error control RCVR Basic telephone service ADSL transceivernetwork end (Premises end is reverberate image) To create multiple channels, ADSL modems divide the ava ilable bandwidth of a telephone line in one of two ways frequency-division multiplexing (FDM) or echo cancellation, as shown in Figure 21-4.FDM assigns one band for upstream data and another band for downstream data. The downstream path is because divided by time-division multiplexing into one or more high-speed channels and one or more low-speed channels. The upstream path is also multiplexed into alike low-speed channels. Echo cancellation assigns the upstream band to overlap the downstream, and separates the two by means of local echo cancellation, a technique well known in V. 32 and V. 34 modems. With either technique, ADSL splits off a 4-kHz part for basic telephone service at the DC end of the band.Internetworking Technologies Handbook 21-4 1-58705-001-3 Chapter 21 Digital Subscriber Line augury and Modulation Figure 21-4 ADSL Uses FDM and Echo Cancellation to Divide the Available Bandwidth for Services FDM Upstream Basic telephone service Downstream Frequency Echo cancella tion Upstream Basic telephone service Downstream 1 Mhz Frequency 1 Mhz An ADSL modem organizes the aggregate data stream created by multiplexing downstream channels, duplex channels, and maintenance channels together into blocks, and it attaches an error correction code to each block.The receiver then corrects errors that occur during transmission, up to the limits implied by the code and the block length. At the users option, the whole also can create superblocks by interleaving data inside subblocks this allows the receiver to correct any combination of errors within a specific span of bits. This, in turn, allows for effective transmission of both data and video signals. Signaling and Modulation This section includes the following cowl and DMT Modulated ADSL ADSL Standards and Associations CAP and DMT Modulated ADSLDMT and CAP are line- cryptology methods for modulating the galvanic signals sent over the copper wire in the local loop. Carrierless(prenominal) Amplitude and Pha se (CAP) is a familiar line-coding method. CAP is a well-understood technology because of its similarity with QAM. Although CAP is well-understood and relatively inexpensive, some argue that it is difficult to ordered series because it is a maven-carrier modulation technique and is susceptible to narrowband interference. DMT uses multiple carriers. At this point, DMT is capable of more speed than CAP. This is one reason that the ANSI committee T1E1. accorded it standards status in document T1. 413. This standard calls for 256 subbands of 4 KHz each, thereby occupying 1. 024 GHz. Each subband can be modulated with QAM 64 for clean subbands, down to QPSK. If each of the subbands can support QAM-64 modulation, then the forward channel supports 6. 1 Mbps. On the return path are 32 subbands, with a probable for 1. 5 Mbps. Internetworking Technologies Handbook 1-58705-001-3 21-5 Chapter 21 Signaling and Modulation Digital Subscriber Line CAP and DMT Compared CAP is a single-carrier t echnique that uses a wide passband.DMT is a multiple-carrier technique that uses many narrowband channels. The two have a number of engineering differences, even though, ultimately, they can strait similar service to the network layers discussed previously. adaptative leveling Adaptive rests are amplifiers that shape frequency response to compensate for attenuation and phase error. Adaptive leveling requires that the modems learn line characteristics and do so by sending probes and looking at the return signals. The equalizer then knows how it must amplify signals to get a nice, flat frequency response.The greater the dynamic range, the more complex the tearing down. ADSL requires 50 dB of dynamic range, complicating adjustive equalisation. Only with recent advances in digital signal processing (number crunching) has it become possible to have such equalization in relatively small packaging. Adaptive equalization is required for CAP because noise characteristics vary signific antly across the frequency passband. Adaptive equalization is not needed for DMT because noise characteristics do not vary across any given 4-KHz subband.A major have it away in comparing DMT with CAP is determining the point at which the complexity of adaptive equalization surpasses the complexity of DMTs multiple Fourier transform calculations. This is determined by further implementation experience. function Consumption Although DMT cl archaean scales and does not need adaptive equalization, other factors must be considered. First, with 256 channels, DMT has a disfavor regarding power consumption (and, therefore, cost) when compared with CAP. DMT has a high peak-to-average power ratio because the multiple carriers can constructively interfere to yield a strong signal.DMT has higher computational requirements, resulting in more transistors than the transceiver chips. Numbers are in the main proprietary at this point, but it is estimated that a single transceiver will consume 5 W of power, even with further advances. Power consumption is important because hundreds or thousands (as carriers dearly hope) of transceivers might be at the central office, or CEV. This would require much more heat dissipation than CAP requires. Latency Another issue for DMT is that latencies are somewhat higher than with CAP (15). Because each subband uses only 4 KHz, no bit can travel faster than permitted by a QAM-64.The trade-off between throughput and latency is a historical one in data communications and has normally been settled in the marketplace. Speed DMT appears to have the speed improvement over CAP. Because narrow carriers have relatively few equalization problems, more aggressive modulation techniques can be used on each channel. For CAP to hit comparable bit rates, it might be necessary to use more bandwidth, far beyond 1 MHz. This creates new problems associated with high frequencies on wires and would reduce CAPs current advantage in power consumption. Internet working Technologies Handbook 21-6 1-58705-001-3Chapter 21 Digital Subscriber Line Additional DSL Technologies ADSL Standards and Associations The American National Standards make up (ANSI) Working Group T1E1. 4 tardily approved an ADSL standard at rates up to 6. 1 Mbps (DMT/ANSI Standard T1. 413). The European Technical Standards Institute (ETSI) contributed an annex to T1. 413 to reflect European requirements. T1. 413 currently embodies a single terminal interface at the expound end. Issue II expands the standard to include a multiplexed interface at the premises end, protocols for configuration and network management, and other improvements.The ATM meeting place and the Digital Audio-Visual Council (DAVIC) have both recognized ADSL as a physical layer transmission protocol for UTP media. Additional DSL Technologies This section discusses the following DSL technologies SDSL HDSL HDSL-2 G. SHDSL ISDN Digital Subscriber Line (DSL) VDSL SDSL Symmetric Digital Subscriber Li ne (SDSL) is a rate-adaptive version of HDSL and, like HDSL, is symmetric. It allows equal bandwidth downstream from an NSPs central office to the customer site as upstream from the subscriber to the central office. SDSL supports data only on a single line and does not support analog calls.SDSL uses 2B1Q line coding and can transmit up to 1. 54 Mbps to and from a subscriber, or can be configured to tin a variable range of bandwidth up to 1. 45 Mbps. The symmetry that SDSL offers, combined with always-on access (which eliminates call setup), makes it a favorable WAN technology for small to medium businesses and branch offices, and can be an affordable choice to dedicated leased lines and Frame Relay services. Because traffic is cruciate, file transfer, web hosting, and distance-learning applications can effectively be implemented with SDSL. HDSLOriginally positive by Bellcore, high bit-rate DSL (HDSL)/T1/E1 technologies have been standardized by ANSI in the United States and by ET SI in Europe. The ANSI standard covers two-pair T1 transmission, with a data rate of 784 kbps on each twisted pair. ETSI standards exist both for a two-pair E1 system, with each pair carrying 1168 kbps, and a three-pair E1 system, with 784 kbps on each twisted pair. HDSL became democratic because it is a better way of provisioning T1 or E1 over twisted-pair copper lines than the long-used technique known as alternate Mark Inversion (AMI).HDSL uses less bandwidth and requires no repeaters up to the CSA range. By using adaptive line equalization and 2B1Q modulation, HDSL transmits 1. 544 Mbps or 2. 048 Mbps in bandwidth ranging from 80ntrast to the 1. 5 MHz required by AMI. (AMI is still the encoding protocol used for the majority of T1. ) Internetworking Technologies Handbook 1-58705-001-3 21-7 Chapter 21 Additional DSL Technologies Digital Subscriber Line T1 service can be installed in a day for less than $1,000 by installing HDSL modems at each end of the line.Installation via AMI costs much more and takes more time because of the requirement to add repeaters between the subscriber and the CO. Depending on the length of the line, the cost to add repeaters for AMI could be up to $5,000 and could take more than a week. HDSL is heavily used in cellular telephone buildouts. Traffic from the base station is backhauled to the CO using HDSL in more than 50 percent of installations. Currently, the vast majority of new T1 lines are provisioned with HDSL. However, because of the embedded base of AMI, less than 30 percent of existing T1 lines are provisioned with HDSL.HDSL does have drawbacks. First, no provision exists for analog role because it uses the voice band. Second, ADSL achieves better speeds than HDSL because ADSLs asymmetry deliberately keeps the crosstalk at one end of the line. Symmetric systems such as HDSL have crosstalk at both ends. HDSL-2 HDSL-2 is an emerging standard and a promising alternative to HDSL. The intention is to offer a symmetric serv ice at T1 speeds using a single-wire pair kind of than two pairs. This will enable it to operate for a larger potential audience.It will require more aggressive modulation, shorter distances (about 10,000 feet), and better phone lines. Much of the SDSL equipment in the market today uses the 2B1Q line code developed for Integrated Services Digital Network. The Bell companies have insisted that using this SDSL at speeds higher than 768 kbps can cause interference with voice and other services that are offered on copper wire within the very(prenominal) wire bundle. The biggest advantage of HDSL-2, which was developed to serve as a standard by which different vendors equipment could interoperate, is that it is knowing not to interfere with other services.However, HDSL-2 is full rate only, offering services only at 1. 5 Mbps. G. SHDSL G. SHDSL is a standards-based, multirate version of HDSL-2 and offers symmetrical service. The advantage of HDSL-2, which was developed to serve as a s tandard by which different vendors equipment could interoperate, is that it is designed not to interfere with other services. However, the HDSL-2 standard addresses only services at 1. 5 Mbps. Multirate HDSL-2 is part of Issue 2 of the standard known as G. SHDSL, and is ratified by the ITU. G.SHDSL builds upon the benefits of HDSL-2 by offering symmetrical rates of 2. 3 Mbps. ISDN Digital Subscriber Line ISDN digital subscriber line (IDSL) is a cross between ISDN and xDSL. It is like ISDN in that it uses a single-wire pair to transmit full-duplex data at 128 kbps and at distances of up to RRD range. Like ISDN, IDSL uses a 2B1Q line code to enable transparent operation through the ISDN U interface. Finally, the user continues to use existing CPE (ISDN BRI terminal adapters, bridges, and routers) to make the CO connections.The big difference is from the carriers point-of-view. Unlike ISDN, ISDL does not connect through the voice switch. A new piece of data communications equipment ter minates the ISDL connection and shuts it off to a router or data switch. This is a key feature because the overloading of central office voice switches by data users is a growing problem for telcos. Internetworking Technologies Handbook 21-8 1-58705-001-3 Chapter 21 Digital Subscriber Line Summary The limitation of ISDL is that the customer no longer has access to ISDN signaling or voice services.But for Internet service providers, who do not provide a public voice service, ISDL is an arouse way of using POTS dial service to offer higher-speed Internet access, targeting the embedded base of more than quint million ISDN users as an initial market. VDSL Very-High-Data-Rate Digital Subscriber Line (VDSL) transmits high-speed data over short reaches of twisted-pair copper telephone lines, with a range of speeds depending on actual line length. The maximum downstream rate under amity is between 51 and 55 Mbps over lines up to 1000 feet (300 m) in length.Downstream speeds as low as 13 Mbps over lengths beyond 4000 feet (1500 m) are also common. Upstream rates in early models will be asymmetric, just like ADSL, at speeds from 1. 6 to 2. 3 Mbps. both data channels will be separated in frequency from bands used for basic telephone service and Integrated Services Digital Network (ISDN), enabling service providers to overlay VDSL on existing services. Currently, the two high-speed channels are also separated in frequency. As needs maturate for higher-speed upstream channels or symmetric rates, VDSL systems may need to use echo cancellation. SummaryASDL technology is asymmetric, allowing more bandwidth for downstream than upstream data flow. This asymmetric technology combined with always-on access makes ASDL ideal for users who typically download much more data than they send. An ASDL modem is connected to both ends of a twisted-pair telephone line to create three information channels a high-speed downstream channel, a medium-speed duplex channel, and a basic telepho ne service channel. ADSL modems create multiple channels by dividing the available bandwidth of a telephone line using either frequency-division multiplexing (FDM) or echo cancellation.Both techniques split off a 4-kHz region for basic telephone service at the DC end of the band Synchronous Digital Subscriber Line (SDSL) provides variable, symmetric, high-speed data communication up to 1. 54 Mbps. But SDSL doesnt allow analog on the same line, as ADSL does. SDSL uses 2B1Q line coding, a technology employed in ISDN and T1 services. SDSL is a viable business option because of its capability to transmit high-speed data over longer distances from the CO and because of its ease of deployment make possible by its spectral compatibility.High Bit-Rate DSL (HDSL) is a symmetric version of DSL that uses 2B1Q like SDSL, but over two-wire pairs. HDSL is targeted at business deployment because it offers full-rate symmetrical 1. 5 Mbps service. HDSL-2 is a standards-based version of HDSL offering symmetrical 1. 5 Mbps service like HDSL, but with a single twisted pair of wires. HDSL is full-rate and does not offer variable rates. G. SHDSL does offer multirate service with symmetrical speeds of up to 2. 3 Mbps. ISDN digital subscriber line (IDSL) is similar in many ways to ISDN. The primary difference is that IDSL is always on and can reach speeds up to 512 kbps with compression.IDSL uses 2B1Q line coding and does not support analog. On the other hand, IDSL does allow data communications over longer distances than other DSL options (up to 26,000 feet) and is considerably less expensive than ISDN service, in most cases. Because IDSL supports existing ISDN CPE, it makes it slack to convert from ISDN to IDSL. Internetworking Technologies Handbook 1-58705-001-3 21-9 Chapter 21 check out Questions Digital Subscriber Line Very-High-Data-Rate Digital Subscriber Line (VDSL) transmits high-speed data over short distances through twisted-pair copper telephone lines.VDSL technology is still in the definition stage, but additional research is required before it can be standardized. VSDL and ADSL are similar technologies. However, although VSDL transmits data at nearly 10 times the rate of ADSL, ADSL is the more complex transmission technology. Review Questions Q Name the current versions of DSL technology. A ADSL, SDSL, HDSL, HDSL-2, G. SHDL, IDSL, and VDSL. Q What are the two-line coding methods used for ADSL? A DMT and CAP. Q Which versions of DSL offer symmetrical service? A SDSL, HDSL, and HDSL-2.Q What symmetrical version of DSL offers multirate service over a single pair of wire? A G. SHDSL Q How far of a reach can IDSL achieve from the CO? A 26,000 feet. Q What downstream and upstream rates are proposed for VDSL? A The maximum downstream rate under consideration is between 51 and 55 Mbps over lines up to 1000 feet (300 m) in length. Downstream speeds as low as 13 Mbps over lengths beyond 4000 feet (1500 m) are also common. Upstream rates in ear ly models will be asymmetric, just like ADSL, at speeds from 1. 6 to 2. 3 Mbps. For More Information ADSL Forum (http//www. adsl. com/) Cisco DSL Depot (http//www. isco. com/warp/public/779/servpro/promotions/dsldepot/) Glossary Terms G. SHDSL Asymmetrical Digital Subscriber Line. The upstream data rate is different from the downstream (typically the downstream is greater than the upstream). It is applicable to many DSL technologies offered today however, this term typically assumes DMT as defined in the ANSI T1. 413 specification. CPE Customer premises equipment, including devices such as CSU/DSUs, modems, and ISDN terminal adapters, required to provide an electromagnetic termination for wide-area network circuits before connecting to the router or access server.This equipment was historically provided by the telephone company, but it is now typically provided by the customer in North American markets. Internetworking Technologies Handbook 21-10 1-58705-001-3 Chapter 21 Digital Subscriber Line Glossary Terms CSU/DSU Channel service unit/data service unit. Provides electromagnetic termination of the digital (WAN) signal at the customer premises. Performs line teach and equalization functions, and responds to loopback commands sent from the central office.In North America, the customer supplies the device providing CSU/DSU functionality outside North America, the telecommunications service provider usually provides this device. DMT Discrete Multitone is the ANSI specified modulation technique for G. SHDSL (ANSI-T1. 413). DMT is theoretically capable of more speed than CAP. The key providers of DMT are Alcatel, Amati, Aware/ADI, and Orckit. Downstream Refers to the transmission of data from the central office (CO or COE) to the customer premise equipment (CPE). HDSL High-speed Digital Subscriber Line. This is a symmetrical modulation technique that uses two or three pairs of wires.HDSL2 High-speed Digital Subscriber Line. This is a symmetrical modulat ion technique that can achieve speeds in the T1 (1. 5 Mbps) range using one copper pair. POTS Plain old telephone service. QAM Quadrature premium phase modulation. RG. SHDSL Rate Adaptive Digital Subscriber Line. This refers to the CAP2 and QAM technologies that use variable data rates to maximize the utilization of various loop lengths. SDSL Symmetric Digital Subscriber Line. This indicates a subscriber line service that utilizes the same data rate for upstream and downstream. This term is applicable to MDSL and HDSL technologies.Upstream Refers to the transmission of data from the customer premises equipment (CPE) to the central office equipment (CO or COE). VDSL Very-High-Data-Rate Digital Subscriber Line. This is a high-speed asymmetrical service in the 10 to 25 Mbps range, typically limited to less than 5,000 feet. The targeted application for this technology is a crossbred fiber copper system (fiber to the neighborhood). Internetworking Technologies Handbook 1-58705-001-3 21-11 Chapter 21 Glossary Terms Digital Subscriber Line Internetworking Technologies Handbook 21-12 1-58705-001-3