Asymmetric Digital Subscriber Line (ADSL)
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Asymmetric Digital Subscriber Line (ADSL)
Asymmetric Digital Subscriber Line (ADSL) is a new modem technology that converts existing twisted-pair telephone lines into access paths for multimedia and high speed data communications. ADSL will play a crucial role over the next ten or more years as telephone companies enter new markets for delivering information at high speeds in various formats including video and multimedia formats. It has become the most familiar method of internet communication for home and small business users. ADSL works on over 60-80% of the existing phone lines without any modification, thus initial cost of installation is very less and this allows a large number of users to access internet through ADSL technology. Asymmetrical digital subscriber line (ADSL) is emerging as the optimal solution to high-speed Internet access technology. ADSL matches the asymmetric pattern of Internet traffic with speeds of upto 8 Mb/s downstream from the network to the end user, and upto 640 kb/s upstream from the end user to the network.This seminar and presentation provides an overview of ADSL technology and a detailed discussion on how ADSL works.
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Traditional phone service (sometimes called "Plain Old Telephone Service" or POTS) connects your home or small business to a telephone company office over copper wires that are wound around each other and called twisted pair. Traditional phone service was created to let voice information to be exchanged with other phone users and the type of signal used for this kind of transmission is called an analog signal. An input device such as a phone set takes an acoustic signal (which is a natural analog signal) and converts it into an electrical equivalent in terms of volume (signal amplitude) and pitch (frequency of wave change). Since the telephone company's signaling is already set up for this analog wave transmission, it's easier for it to use that as the way to get information back and forth between your telephone and the telephone company. That's why computer has to have a modem - so that it can demodulate the analog signal and turn its values into the string of 0 and 1 values that is called digital information.
Because analog transmission only uses a small portion of the available amount of information that could be transmitted over copper wires, the maximum amount of data that you can receive using ordinary modems is about 56 Kbps (thousands of bits per second). (With ISDN, which one might think of as a limited precursor to DSL, receiving data rate can be up to 128 Kbps.) The ability of your computer to receive information is constrained by the fact that the telephone company filters information that arrives as digital data, puts it into analog form for your telephone line, and requires modem to change it back into digital. In other words, the analog transmission between home or business and the phone company is a bandwidth bottleneck.
Digital Subscriber Line is a technology that assumes digital data does not require change into analog form and back. Digital data is transmitted to your computer directly as digital data and this allows the phone company to use a much wider bandwidth for transmitting it to you. Meanwhile, if you choose, the signal can be separated so that some of the bandwidth is used to transmit an analog signal so that you can use your telephone and computer on the same line and at the same time.
Digital Subscriber Line (DSL) is a broadband transmission line developed for use with existing physical (copper) phone line runs. DSL was developed since the early 90's but was never brought to market because of the many constraints it involved then for both users and providers. However, technology today is winning the battle against these limitations. DSL is being successfully deployed more and more everywhere in North America and the world.
DSL requires excellent structured phone wiring in the residential environment if part of it is to be used for your premise DSL equipments. It surely requires the physical copper lines from the Central Office to the home to be in optimum conditions. DSL requires a maximum of 3.5 miles of copper wire runs between the home and the providing Telephone Central Office (CO). It might not work well for you even if you are at about 2.5 miles to the CO. At that distance the bit rate drop might be significant and performance fairly inconsistent. Therefore, in a word, you might not be eligible for DSL for the time being. DSL provides simultaneous voice and data transmission over the twisted pair. However the voice network provider and the data network provider are not necessarily the same.
No one can guarantee at which exact bit rate your DSL line will perform. While it will certainly be much higher than the 56 Kb/s of a dial-up modem by multiple folds, it might not reach the nominal rate of DSL line type. ADSL, which is the DSL type for the residential environment, runs at 148 Kb/s from your computer to the public network and 768 Kb/s from the network to your computer. Higher bit rates and symmetric DSL (SDSL: same high rate in both directions) are also available.
CHAPTER I: DSL
1.1: INTRODUCTION TO DSL
DSL is an abbreviation for "Digital Subscriber Line". Digital Subscriber Line (DSL ) technology is a modem technology that uses existing twisted-pair telephone lines to transport high-bandwidth data, such as multimedia and video, to service subscribers. This is also known as “ 2Wire Digital Lifestyle”. DSL runs on regular copper phone wires which is a direct dedicated connection between your location and the phone company central office. A DSL modem is attached to the DSL phone line, and run another cable from the DSL modem to the computer. This aplication requires 10BaseT Ethernet card.Looking into internet, waiting for pages to be downloaded, modem to be connected to internet, busy signals, giving up phone line while surfing are the few very common scenarios that are faced by conventional 56.6Kbps internet user. DSL is here to soothe all those problems away!
Digital Subscriber Line (DSL) is a high-speed Internet connection over regular twisted copper pair phone lines that makes use of previously unused bandwidth on the phone line and uses it to transport large amounts of data to and from computers while allowing voice calls over the same line.
As data's share of network traffic increases, Digital Subscribe Line (DSL) can offer numerous benefits for carriers which may need multiple DSL flavors to suit business and residential markets. As telephone technology enters its second full century, the business model for local exchange carriers is changing quickly from one based on providing circuit-switched voice services to one based on providing differentiated packet-switched voice and data services.
DSL has had incredible market acceptance spurred on by very aggressive pricing from companies like Pacific Bell (SBC) here in Northern California. Unfortunately, the pricing has been so low to attract customers (and keep them from trying cable), that many DSL service providers have been losing money on each account.
Most basic home DSL lines include the ability to connect regular analog phones to RJ-11 jacks, just like normal phone service. In some cases a POTS (Plain Old Telephone Service) splitter, which is basically a box that connects the analog phone wires is used.More expensive DSL service, sometimes called "enhanced" may provide higher data rates but more often than not is enhanced by the addition of features like static IP addresses (more than one).
With the industry change from focus on voice service to focus on data services, Local Exchange Carriers (LECs) and other network access providers are being forced to reassess their service offerings and the technology used to deliver them. They are seeking cost-effective high-bandwidth solutions that can transmit data for high-speed Internet as well as for corporate Intranet applications. They increasingly are looking at Digital Subscriber Line (DSL) technology to fill this need.Access providers are already seeing their voice-dominated business models shift. In the past several years, telephone companies in North America have seen the majority of their growth comes not from voice, but from data traffic, which has become the main engine of business expansion - a trend that undoubtedly will continue and accelerate. There are two main reasons for data traffic growth:
• the Internet and on-line communications are becoming important vehicles to access and distribute information for both business and residential consumers.
• telecommuting or because they own a home-based business. These non-traditional workers rely upon dependable data and multimedia transmission to operate their businesses.
DSL is a solution that can meet critical requirements for providing high-speed data services. With DSL, access providers can carry numerous high-bandwidth services across copper wire. It ensures the unfettered flow of both voice and data traffic by providing high quality, differentiated services at a reasonable cost to both the provider and consumer.
1.2 ADVANTAGES AND DISADVANTAGES OF DSL
Network Access Provider Benefits
For the network access provider, the principal benefit of DSL is its ability to permit the rapid flow of information while using the existing copper wire foundation. Therefore, access providers do not need to install more copper or lay down miles of expensive fiber, as they would for other potential solutions.
DSL also eliminates providers' need to constantly upgrade their pricey Class 5 switch installations. If providers were to continue to provide data and voice services over the same network, they would have to buy more switches to ensure a congestion-free network.
As widely reported, a typical voice call lasts about five minutes, compared to an average 30 minutes for an Internet call. This means that for every new Internet customer, the carrier would need to provision six additional lines to guarantee the previous level of service availability to Plain Old Telephone Service (POTS) users.
DSL bypasses the Class 5 switch for data-only calls, so access providers can accept more traffic without investing in a huge switch upgrade. With DSL, voice and data paths are separated, which means that providers can meet the burgeoning need for high-speed data services without breaking their budgets. Furthermore, by keeping costs low, service providers can make money off the traditionally low-margin business of providing Internet access.
DSL offers customers access to a range of information services they previously could not receive through Internet communication. Users can experience high-speed Web browsing and can send and receive large documents.
Another benefit is to use DSL's always-on capability to receive information, such as news, video clips and stock quotes, that is pushed toward the user at the time it is generated, according to a personalized profile, without the need for the user to dial-up.
Always-on capability will help turn the PC into an appliance, making it easier for the mass consumer market to use. People will be able to walk to a PC and send e-mail, check for the latest airfare promotion or download a promotional clip for a new movie - and they will be charged on a usage basis, not a connect-time basis.
Consumers also may benefit by receiving all of their telephone-related services through a single provider. Customers are buried under a monthly pile of bills from utility service companies. By being able to receive voice and data services from one source-and paying for them through a single bill-consumers will gain significant added value from their service provider. This one-stop shopping will give providers an important means of differentiating their services from those of competitors such as cable companies.
For the business customer, DSL represents a very compelling investment for several key reasons:
• First, DSL provides a cost-effective way for telecommuters and branch offices to access the corporate network, provisioned via secure virtual private network services.
• Additionally, a DSL-based approach can improve the quality and reduce the cost of communications among employees via videoconferencing. It also can improve productivity by making intranet applications available to the distributed work force and by creating a cost-effective communications channel with the corporation's partners via extranets.
• Finally, DSL can displace the costs associated with separate voice, data and video networks.
To the business, though, a faster pipe does not represent a significant advantage if the service provider cannot guarantee bandwidth on that pipe. The availability of different classes of services and quality of service, provisioned at different prices, are essential requirements for the business. The granularity of these service classes also will help the carrier market its services to multiple tiers of customers, helping to accelerate the return on its overall investment.
• Fast - Modems are much faster than analog modems. Different varieties of DSL provide different maximum speeds, from twice as fast to approximately 125 times faster than a 56.6K analog modem. The only speed limit with DSL is the speed of the Internet and all the different computers attached to it. The speed can go up to 1.5Mbps.
• Doesn’t tie up to phone line - DSL doesn't interfere with phone calls, even though it uses regular phone line. This means that one can leave the Internet connection open and still use the phone line for voice calls.
• Always on - DSL connection is always available. No more traditional dialing-up procedure is required and user doesn’t have to be worried being line-dropped while browsing or downloading. One only need to set up the computer to check for new e-mail or to browse through the internet.
• Reliable - DSL is reliable said to be reliable since it runs on phone line and phone company networks are among the most reliable in the world, experiencing only minutes of downtime each year
• DSL doesn't necessarily require new wiring; it can use the phone line you already have.
• The company that offers DSL will usually provide the modem as part of the installation.
• A router, along with a DSL modem, allows up to eight computers to access the Internet over a single DSL line.
• DSL provides the bandwidth you need for high-bandwidth applications. Streaming audio and video will come across in real-time, rather than interrupted segments.
• DSL offers reliable high-speed Internet connections that are far less expensive than other options such as T1 lines.
• DSL is flexible and scalable, ideal for growing business computer networks.
• Enable users to work from home, listen to streaming audio or watch streaming video while working on your computer.
• A DSL connection works better when user is closer to the provider's central office.
• The connection is faster for receiving data than it is for sending data over the Internet.
• The service is not available everywhere
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ADSL - Asymmetric Digital Subscriber Line
• ADSL is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines
• ADSL is capable of providing up to 50 Mbps, and supports voice, video and data.
• ADSL is the #1 Broadband Choice in the World with over 60% market share
• ADSL is now available in every region of the world
What does ADSL mean
• Asymmetric - The data can flow faster in one direction than the other. Data transmission has faster downstream to the subscriber than upstream
• Digital - No type of communication is transferred in an analog method. All data is purely digital, and only at the end, modulated to be carried over the line.
• Subscriber Line - The data is carried over a single twisted pair copper loop to the subscriber premises
ADSL standards :
• ADSL Speed Comparison
• In general, the maximum range for DSL without a repeater is 5.5 km
• As distance decreases toward the telephone company office, the data rate increases
• For larger distances, you may be able to have DSL if your phone company has extended the local loop with optical fiber cable
ADSL Speed Factors
• The distance from the local exchange
• The type and thickness of wires used
• The number and type of joins in the wire
• The proximity of the wire to other wires carrying ADSL, ISDN and other non-voice signals
• The proximity of the wires to radio transmitters.
• ADSL network components
• The ADSL modem at the customer premises(ATU-R)
• The modem of the central office (ATU-C)
• DSL access multiplexer (DSLAM)
• Broadband Access Server (BAS)
• Splitter - an electronic low pass filter that separates the analogue voice or ISDN signal from ADSL data frequencies DSLAM.
ADSL Loop Architecture
• Phone-line, activated by your phone company for ADSL
• Filter to separate the phone signal from the Internet signal
• Subscription with an ISP supporting ADSL
How does ADSL work
• ADSL exploits the unused analogue bandwidth available in the wires
• ADSL works by using a frequency splitter device to split a traditional voice telephone line into two frequencies
• Modulation is the overlaying of information (or the signal) onto an electronic or optical carrier waveform
• There are two competing and incompatible standards for modulating the ADSL signal:
– Carrierless Amplitude Phase (CAP)
– Discrete Multi-Tone (DMT)
Carrierless Amplitude Phase
• Carrierless Amplitude Phase (CAP) is an encoding method that divides the signals into two distinct bands:
– The upstream data channel (to the service provider), which is carried in the band between 25 and 160kHz
– The downstream data channel (to the user), which is carried in the band from 200kHz to 1.1MHz .
• These channels are widely separated in order to minimize the possibility of interference between the channels.
Discrete Multi-tone (DMT)
• Discrete Multi-Tone (DMT) separates the DSL signal so that the usable frequency range is separated into 256 channels of 4.3125kHz each.
• DMT has 224 downstream frequency bins (or carriers) and 32 upstream frequency bins.
• DMT constantly shifts signals between different channels to ensure that the best channels are used for transmission and reception.
• The DMT frequency bands
• Frequency Division Multiplexing (FDM)
• Echo Cancellation
Asynchronous Transfer Mode
• ATM is a connection-orientated technique
• ATM provides cell sequence integrity
• Cells are much smaller than standard packet-switched networks (53 bytes)
• The quality of transmission links has lead to the omission of overheads
• There is no space between cells
Types of ATM services
• Constant Bit Rate (CBR)
• Variable Bit Rate (VBR)
• Unspecified Bit Rate (UBR)
• Available Bit Rate (ABR)
• The ATM layer transport information across the network
• ATM uses virtual connections for the information transport
• The connections are divided into two levels:
– The Virtual Channels
– The Virtual Path
– This mechanism is used to provide quality of service (QoS)
• The connection between two endpoints is called a Virtual Channel (VC).
• A Virtual Path (VP) is a term for a bundle of virtual channel links that all have the same endpoints.
• Each VC and VP has a unique identifier
• Virtual paths are used to simplify the ATM addressing structure.
ATM Adaptation Layer (AAL)
• The ATM Adaptation Layer (AAL) converts information from the upper layers into ATM cells
• The standard used for ATM over ADSL services is AAL5
• AAL5 Encapsulation Methods
– Virtual Channel Multiplexing (VCMux)
– For detailed information please refer to the RFC 1483
ADSL Protocol stacks
• Point-to-Point Protocol over Ethernet over ATM (PPPoEoA)
• Conclusion: Pros & Cons
• Simultaneous Internet and voice/fax capabilities over a single telephone line
• Uninterrupted, high-speed Internet access that's always on-line
• Cost-effective solution for society
• Data Security that exceeds other technologies
• Fast download speeds
• Slower upload speeds
• Phone line required