DIGITAL SUBSCRIBER LINE (DSL)

WHAT ARE BROAD BAND SERVICES ?

Services requiring transmission channels capable of supporting rates greater than the primary rates are called Broadband Services.

PROBLEMS IN EXISTING NETWORKS (PSTN,ISDN)1Bandwidth LimitationAnalog Modem Connections are limited to 56KBasic Rate ISDN limited to 144 KbpsSwitch CongestionVoice calls are usually short (say 5 min)Data calls are usually long(may be 120 mins.)Flat Charge /Low Charging by ISPs intends users for long internet usage.

DIGITAL SUBSCRIBER LINE- AN OVERVIEWWhat is X DSL?

DSL (Digital Subscriber Line) is a technology for bringing high-bandwidth information to homes and small businesses over ordinary copper telephone lines

A DSL line can carry both data and voice signals and the data part of the line is continuously connected

One may be able to receive data at rates up to 6.1 Mbps (millions of bits) per second (of a theoretical 8.448 megabits per second), enabling continuous transmission of motion video, audio, and even 3-D effects

More typically, individual connections provide from 1.544 Mbps to 512 Kbps downstream and about 128 Kbps upstream

DIGITAL SUBSCRIBER LINE4 kHz to 1.1 MHz - ADSL Data300 Hz to 3.3 kHz - Analog VoiceFrequency Range

56 kbps

AnalogPublic Telephone NetworkInternetVoice Switch(:

Customer Premises6MbpsDSLPublic Telephone Network(:Telephone LineInternetDSL Access MultiplexerCentral Office

DSL BASICSBell Labs, in the 30s, determined that most speech energy lay in the frequency range below 3500 HzThus, for faithful reproduction of speech, only a narrow range of frequencies (0-3500 Hz), needs to be transmitted

The telephone networks are designed to pass frequencies in the range 300 3400 HzDSL uses a much broader range of frequencies than the voice channel Using POTS transmission guidelines instead of the higher frequencies- WHY?

Crosstalk: The electrical energy transmitted across the copper wire line as a modulated signal also radiates energy onto adjacent copper wire loops which are located in the same cable bundle Near End Crosstalk(NEXT) is the most significant because the high-energy signal from an adjacent system can induce relatively significant crosstalk into the primary signal

Far End Crosstalk, FEXT, is typically less of an issue because the far end interfering signal is attenuated as it traverses the loop Attenuation: The use of higher frequencies to support higher-speed services results in shorter loop reach because high frequency signals attenuate energy faster than the lower frequency signals

One way to minimize attenuation is to use lower resistance wire Telephone companies have designed their cable plant using the thinnest gauge wire that could support the required servicesThe analog modems must force their data into the 4kHz channel used to carry voice trafficSince rates like 28kbps or 56kbps require much more bandwidth than that, modulation techniques and coding have to be employed to overcome the bandwidth limitations of the telephone channel

HOW TO MINIMIZE ATTENUATION

DSL enables telephone companies to use the worlds nearly 750 million existing copper wires to deliver affordable high-speed remote access to the Internet, corporate networks and on-line services over ordinary phone lines Unlike competing technologies, DSL eliminates the need for extensive and expensive infrastructure upgrades BENEFITS OF DSL

DSL enables new applications that require real-time, interactive multimedia and broadcast-quality video. Such applications include collaborative computing (Joint computing), video conferencing, distance learning and video-on-demand Because it is one customers dedicated line, transmission speeds are not affected by other users going on-line BENEFITS OF DSL

DSL is "always on" and connected - just like a standard telephone. There is no time wasted dialing up the serviceBoth residential and businesses properties around the world are already running out of spare lines on existing installed telephone cables. DSL provides service providers with the capability to leverage the existing infrastructure DSL-based networks are well suited for carrying IP and ATM traffic, thus future- proofing DSL technology for decades to come

VARIANTS OF DSL WHY ?There are a few key differences1Speed-How much data can be sent or received in a certain amount of time ?(bit rate)2Line Coding-How the information is encoded and sent on the line ?Number of Lines-What are the requirements in terms of twisted pairs ?Reach-How many feet/meters the signal can reliably travel ?

VARIANTS OF DSLHDSL (High Bit Rate Digital Subscriber Line)HDSL transceivers can transmit a 2.048 Mbps data signal over two, 24 gauge (0.5mm), twisted wire pair loops at a distance of up to 13 kft (4.2 km) without the need for repeatersMost Mature type and cheaperUsing a technique called echo cancellation,HDSL allows data to be sent in both directions at the same frequency on the same lineHDSL requires a smaller frequency band to send the same amount of data

VARIANTS OF DSLHDSL (High-bit-rate Digital Subscriber Line)The smaller frequency band means only the lower frequencies must be needed,which helps in reducing some losses on the line and reduces Near End Cross Talk (NEXT).HDSL is designed to run at the same bit rates as T1/E1 (1.55/2.048 Mbps),but it can achieve this same bit rate on less expensive lines over a greater distanceThe Line Code for HDSL is called 2B1Q (2-binary 1 quaternary).It encodes two bits at one time.Each level represents two bits of data.But it doesn't make efficient use of available bandwidth.

VARIANTS OF DSLHDSL ADVANTAGES The primary advantage of HDSL is that it is a mature and proven technology It is easy and economical to install. It has a decent transfer rate in both directions at 1.544/2.048 Mbps (784/1040 kbps *2)HDSL DISADVANTAGES The primary disadvantage of HDSL is that it requires two twisted pairs of wires to operate HDSL is slower than some other forms of DSL

VARIANTS OF DSLHDSL 2 HDSL2 or second-generation HDSL can be thought of as offering everything traditional HDSL offers, but it can be done on a single pair of copper.This helps to reduce the cost of the system.The Line Code used is Overlapped Pulse Amplitude Modulated (PAM) .This provides excellent use of the available bandwidth and doesn't suffer severely from cross talk.HDSL2 has excellent reach.1.544Mbps can be achieved 13200 ft from the central office,and 384 Kbps can be maintained up to 22500 ft.

3ADSL (Asymmetric DSL)ADSL is called "asymmetric" because most of its two-way bandwidth is devoted to the downstream direction, sending data to the user. Only a small portion of bandwidth is available for upstream or user-interaction messages. Using ADSL, up to 6.1 Mbps of data can be sent downstream and up to 640 Kbps upstreamThe data rate of ADSL is highly dependent upon the distance from Central Office (CO) to the Consumer Premises(CPE).ADSL is designed to co exist with a regular phone line (phone operates at frequencies up to 4 KHz).In order not to interfere with POTS and to avoid interference from POTS ,ADSL operates at frequencies above 4 KHz,(upstream is in the range from 30 to 138 KHz, down stream ranges from 138 KHz To 1.1 MHzSeparate frequency band for voice and data allow the signals are obtained by using Filters.For ADSL ,these filters are combined into one central piece of equipment called Pots Splitter.This splitter takes the signal received from the Service Provider and separates them in to two home networks (regular telephone network and home computer network)

ADVANTAGES AND DISADVANTAGESADSL is well suited to residential application.It uses lines that are already installed virtually everywhere and peacefully co-exists with current phone service. It has large downstream data capacity (up to 8 Mbps)ADSL (like most flavors of xDSL) is always connected For Large business upstream data rates are often just as important as downstream ,which would make ADSL a POOR CHOICEADSL data rates also suffer dramatically as line length increases.ADSL will extend out to 18,000 ft,but its ideal rates only operate out to 9,000 ft.ADSL

ADSL (Asymmetric DSL)

To create multiple channels ,ADSL Modem divide the available bandwidth of a telephone line in one of the two waysFrequency Division Multiplexing (FDM)Echo Cancellation (technique is used in V.32 and V.34 modems) In FDM ,one band for upstream and one band for downstream is assigned Modulation in ADSL is Discrete Multi tone Modulation (DMT).DMT System divide the frequency band into 256 sub channels that are 4.3125 KHz wide.32 of these sub channels are reserved for upstream data.Each channel independently sends a data using QAM encoding

4 G.Lite (or DSL Lite) G.Lite (also known as DSL Lite, splitter less ADSL, and Universal ADSL) is a slower version of ADSL that doesn't require splitting of the line at the user end but manages to split it for the user remotely at the telephone company. G.Lite, officially ITU-T standard G-992.2, provides a data rate of 1.544 Mbps downstream and from 128 Kbps to 384 Kbps upstream. Because of the lower bit rate ,hardware design for G.Lite is much easier than ADSL.

Where wiring in the home is old or done poorly,the voice and data signals can interfere with each other .In those cases ,small Micro Filters are placed on each phone line in the house.The three divided segments are :-0 to 4KHz for POTS26KHz to 138 KHz range is used to transmit data upstream138KHz to 550 KHz range is used to transmit data downstream

RADSL (Rate-Adaptive DSL) is an ADSL technology in which software is able to determine the rate at which signals can be transmitted on a given customer phone line and adjust the delivery rate accordingly.It can deliver from 640 Kbps to 2.2 Mbps downstream and from 272 Kbps to 1.088 Mbps upstream over an existing line. It is generally used to extend reach of systems to area that are far from the central office.It operates over single twisted pair copper loop.Down stream is 1 to 12 Mbps and upstream is 0.128 to 1 Mbps.RADSL (RATE-ADAPTIVE DSL)

SDSL (Symmetric DSL) is similar to HDSL with a single twisted-pair line, carrying 1.544 Mbps (U.S. and Canada) or 2.048 Mbps (Europe) symmetrically on a duplex line. Its a concept rather than a specific technology. It can be used to provide variety of rates between 272 and 2,320 Kbps and can extend 11 km @ 256 Kbps.

SDSL (SYMMETRIC DSL)

IDSL provides DSL technology over existing ISDN Lines.It is closer to ISDN data rates and service at 128 Kbps than to the much higher rates of ADSL. Beyond 18000 feet, typical DSL applications break down but IDSL can still function. IDSL circuits can only carry data (not voice)It is always in Connected position ,as apposed to dial up this eliminating call set up delay.The transmission of data is over the data network rather than PSTN.

IDSL (ISDN DSL)

VDSL provides very high bandwidth asymmetrically (up to 52 Mbps in one direction and 2 Mbps in the other) to businesses and residences with broadband access requirements over a Fiber-To-The-Curb (FTTC) network.

VDSL (VERY-HIGH-BIT-RATE DSL)

COMPETING TECHNOLOGIESISDNWhat It Is: A digital line for both voice and data transmission Two varients ISDN BRIISDN PRIPros: Widely availablePermits fax, data, and voice on the same line

Cons: Top speed of only 128KbpsNot available in all areasOften difficult to set upExpensive phone-company and ISP charges

Conclusion: A reasonable upgrade for many analog modem users, but not smart if you have access to satellite or cable

SatelliteWhat It Is: Uses a mini-dish antenna to receive Internet data via satellite at up to 400KbpsPros: Available to anyone with an unobstructed view of the sky

Cons: Top download speed of 400Kbps is only a fraction of cable or ADSLRequires an analog modem for uploads; tricky installationPricing includes per-megabyte charges and can be expensive

Conclusion: The immediate choice where cable is not offeredThe only choice in rural areas

Cable ModemWhat It Is: Uses the existing cable TV infrastructure and a special modem to let you surf the Net at speeds of up to 30MbpsThere are two types: the most prevalent is the HFC type which offers data communications in both directions while the other is the older type which runs over the standard coaxial cable networks but which provides data communications only downstream

Pros: Potentially available wherever cable TV isProvides high-speed downloads and uploadsInexpensive

Cons: Actual speeds reach only around 1.5Mbps downstream and 300Kbps upstreamShared bandwidth

Difficult to get in many older, urban buildings as well as business parks Inadequate security In order to offer the two-way service, the cable company must have a hybrid fiber/coax cable network but as of 1999, only about 30% of the total cable lines in the USA had been converted to HFC

Conclusion: A long-term choice for homes but not for businesses

FTTH / FTTCWhat It Is: For FTTC , optical fiber is terminated within a few hundred feet of the residence and the d/wire is coaxial cable or phone wireWhile for FTTH, the fiber is carried all the way to the home giving upstream speeds of 1.6~19.4 Mbps along with 12.9~51.8 Mbps downstreamPros: Fiber is immune to noiseEnough bandwidth to support customers web sites

Cons: Huge infrastructure costsProblems regarding how to transition current customers rewiringLagging behind more established technologies

Conclusion: Bandwidth offerings are quite good but the infrastructure costs render it prohibitive

DSL-BASED SERVICES OFFERED WORLDWIDEInternet accessDSL Internet access is competitive with ISDN pricing and lower than corresponding fractional T1/E1 access.

Remote LAN connectivityRemote LAN connectivity provides network connections between a remote user and a corporate LAN, using DSL as the last mile at the remote end. DSL access, and the end-to-end connection are included in the service

Frame relay or ATMFrame Relay and ATM are public data networking services offered by telecommunication companies for LAN-to-LAN connections. Customers connect to Frame Relay networks 56K leased lines, fractional T1/E1, or full T1/E1 lines. Pricing for local and long haul Frame Relay network connections varies Virtual Private NetworksVPNs support private communications over a public network, such as connecting two users or network locations over the Internet. The DSL loop enables tele-workers to link up to a company network through the Internet and work securely, as if they were accessing the LAN at the office

Voice Over DSLVoDSL supports up to 24 voice channels today, with greater capacity in the future, plus a channel for data communication on a single copper pair Another voice application is PBX extension, which is targeted at tele-workers. PBX extension supports a voice channel over the DSL line, effectively connecting a remote worker to a PBX located at the corporate headquarters VideoElectronic whiteboards combined with video conferencing over a DSL connection can create a powerful set of real-time, interactive tools that dramatically improve the communication of geographically dispersed teams

DSL COMPONENTSCentral Office :DSLAM (Digital Subscriber Line Access Multiplexer)It is a mechanism at a phone Companys central location that links many DSL customers (Connections) to a single high speed ATM LineDSLAMs are generally flexible and able to support multiple types of DSL in a central office and different varieties of protocol and modulationThe DSLAM may provide additional functions,including routing or dynamic IP address assignment for the customerDSL customer Terminates individual subscriber line and integrates it into the telephone and data networks.

DSL ComponentsCustomer Premises:CPE (Customer Premise Equipment)Splitter - Separates voice and data feeds.Bridge - Couples DSL to PC or Ethernet Equipment (typically a PC DSL Modem).Router - Couples and routes DSL or Ethernet feed to customer PCs or other network devices (Including network capable alarm controls!).Filters - Blocks high frequencies so that voice devices can work properly.

DSL COMPONENTSCentral Office :DSLAMCisco 6160Courtesy: SBC -TRI

DSL COMPONENTSCustomer Premises:CPE (Customer Premise Equipment)Splitter - Separates voice and data feeds.

DSL COMPONENTSCustomer Premises:CPE (Customer Premise Equipment)Splitter - Separates voice and data feeds.

DSL COMPONENTSCustomer Premises:CPE (Customer Premise Equipment)Bridge - Couples DSL to PC or Ethernet Equipment (typically a PC DSL Modem).Courtesy: 3Com

DSL COMPONENTSCustomer Premises:CPE (Customer Premise Equipment)Router - Couples and routes DSL or Ethernet feed to customer PCs or other network devices (Including network capable alarm controls!).DSL RouterEthernet HubAnalog Modem

DSL INFRASTRUCTURE