Bandwith Issues Essay, Research Paper
xDSL, present and future. By IcePick firstname.lastname@example.org email me if you use this. 26-April-1998 Since the birth of the World Wide Web, bandwidth has been a concern. Computer users surfing the Web are always in need of more bandwidth. In an effort to attract more surfers, content providers are jazzing up their pages using more multimedia than ever. The new generation of multimedia-laced Web pages contain bigger pictures and have embedded sound files; all these items take more bandwidth to download. The growth of the World Wide Web and its large content pages has led many computer users to seek faster ways of connecting than through analog modems. ADSL and the other products in the xDSL family are leading the way to faster Internet connections. The main advantage of xDSL is that the service is provided using digital signals as opposed to the analog signals used with regular voice modems. Digital circuits use signal levels or changes in signal levels to represent binary 1s and 0s. Digital signals are less susceptible to line noise then analog signals. Circuits based on digital signals can be manipulated using Digital Signal Processors, which have the ability to filter out line noise and other undesirable properties of a circuit. Analog signal transmissions manipulate properties of a carrier signal using either one or a combination of the following to convey data: amplitude, frequency, and phase (Cisco NA). xDSL is a family of products and standards that allows telephone companies to use existing copper-based POTS (plain old telephone service) lines to bring high speed digital services to consumers (Strauch 1997). The x in xDSL stands for one of many implementations of the family of products known as Digital Subscriber Line technology (UUNET 1997). Except for the relatively slow analog connection from the user’s computer and the ISP’s (Internet Service Provider) modems, the Internet is based on high speed digital circuits. The influx of slow connections from users’ computers and ISP modems creates a bottle-neck. xDSL is designed to eliminate the existing bottle- neck (TeleChoice DSL White Paper 1997). "DSL can literally transform the existing public information network from one limited to voice, text, and low resolution graphics to a powerful, ubiquitous system capable of bringing multimedia, including full motion video, to everyone’s home this century" (ADSL Tutorial 1998). The different DSL products include High bit rate Digital Subscriber Line (HDSL); Asymmetric Digital Subscriber Line (ADSL); Universal Asymmetric Digital Subscriber Line, (UADSL); Single-line High bit rate Digital Subscriber Line (S-HDSL); ISDN-based Digital Subscriber Line (IDSL); Rate Adaptive Digital Subscriber Line, (RADSL); and Very high bit rate Digital Subscriber Line (VDSL). S-HDSL is also known as SDSL, Symmetric Digital Subscriber Line (UUNET 1996). Many differences exist between the members of the xDSL family and some differences exist among companies on how to implement individual types of xDSL. Advanced digital signal processing is used with xDSL to increase the throughput and signal quality (ADSL Tutorial 1997). While not all members of the xDSL family exist in any form save on paper, HDSL is a proven product with years of service. It is estimated that there are over 200,000 HDSL installations (Strauch 1997). Bellcore developed HDSL as a way of establishing T-1/E-1 links between the PSTN (Public Switched Telephone Network) and the customer’s premises (PairGain 1996). HDSL is a symmetric system using two pairs of copper cables. HDSL can deliver between 512Kbps and 2.048Mbs (Scheuble NA). HDSL links can be established over spans 3.6 km using 24-gauge wire. Longer distances can be reached using heavier gauge copper (PairGain 1996). Due to the successful track history and ease of installation HDSL is well suited for use in PBX systems, campus-based networks, and digital loop services (Aber 1997). SDSL uses only one pair of copper wires and can be used to transmit data 384Kbps at 18,000 feet and can approach T-1 speeds of 1.544Mbs with loops of less than 10,000 feet (Cisco 1998). ADSL uses one pair of copper lines and operates in the frequency range of 4Khz to 2.2Mhz (ADSL Forum FAQ 1997). "ADSL provides asymmetric transfer rates of 1.5Mbs to 9Mbps downstream, 15Kbps to 640Kbps upstream" (Scheuble NA). The asymmetric nature of ADSL is provided by the fact that the upstream and downstream transfer rates are different. This asymmetric setup complements the fact that when surfing the Web most of the bandwidth used is for the downloading of Web pages and very little information must be transmitted upstream. The upstream of a connection is the data transfer path from the customer to the telephone company, downstream is from the phone company to the customer. Two forms of modulation systems are implemented, CAP and DMT. CAP, or Carrier-less Amplitude/Phase, modulation is a system in which a single carrier is modulated, then suppressed, before the signal is transmitted. The carrier is then recreated at the receiver’s end of the connection (ADSL Forum FAQ 1997). DMT, or Discrete Multi-Tone, is a system in which data is transmitted over several carrier signals. There are advantages to both of these modulation methods. CAP based systems can interpolate with CAP systems built by other manufactures, while DMT is said to be better at longer span distances (TeleChoice 1996). ADSL is ideally suited for use Internet access, remote LAN access and video on demand services (Aber 1997). UADSL, (Universal Asymmetric Digital Subscriber Line) also known as splitterless ADSL or ADSL lite, is a low-speed form of ADSL technology (Lawton 1998). It has an upstream rate of 1Mbps (Surkin 1998). UADSL uses the same frequency bandwidth as ADSL but carries less data. By using the same frequency bandwidth as ADSL but utilizing a lower transfer rate, UADSL is more tolerant of line interference. Packing less data into the same bandwidth also allows the components needed to complete the system to be simpler. This allows UADSL to be more reliable and easier to implement. Making UADSL easier to implement comes at a cost; it is considerably slower than ADSL. UADSL also forgoes the usage of a splitter. With regular ADSL a splitter is used at the customer’s premises to divide the incoming signal into two parts, the POTS line and the ADSL signal. The lack of a splitter saves money but it can also cause problems. UADSL and telephone signals can interfere with each other and cause signal degradation in both (Lawton1998). RADSL (Rate Adaptive Digital Subscriber Line) combines both SDSL and ADSL technologies (Web Techniques 1997). RADSL changes the signaling speed and data rate in order to get the most out of a particular phone line. RADSL offers downstream rates of 600Kbps to 12Mbps and upstream rates of 128Kbps to 1Mbps (Web Techniques 1997). IDSL is an on-all-the-time ISDN-based system. It provides data rates of 128Kbps (Scheuble NA) and is one of the most inexpensive of the xDSL line of products. It uses the same customer premises equipment that has always been used by ISDN (Probe Net 1997). ISDN (Integrated Services Digital Network) has the ability to bring voice and data services to the customer. In the United States, it was observed by companies providing ISDN service that the majority of customers only used the service for data communications and did not use the voice options provided. Rather than providing services that are only half utilized, IDSL, also known as dedicated ISDN, was developed to replace ISDN installations where only data was being transmitted. Very high bit rate digital subscriber line (VDSL) connections can be used to transfer data from 13Mbs to 55Mbps downstream and 1.6Mbps to 2.3Mbs upstream (ADSL Forum VDSL Tutorial). The rate that can be achieved with a VDSL system varies greatly depending upon the distance of the circuit. 1,000 to 4,500 feet are the distances that VDSL can successfully span (Cisco 1998). In the future the large bandwidth provided by VDSL could be used to transfer High-Definition TV signals (Aber 1997). DSL services have a wide range of applications including telecommuting, high speed access to corporate networks; Internet access, and high speed access directly to the Internet. Other potential functions include desktop video conferencing, based on H.323 video applications; computer-telephony integration, DSL services with POTS and IP capabilities and distance learning, based on H.323. Also, continuous-on applications, such as stock tickers, continuously updated sports reports, and other applications benefiting from non-switched network connections (US West NA). Below is a diagram from Pacific Bell about how ADSL connects customers to the Internet or a corporate LAN. The DSLAM in the PacBell diagram is a Digital Subscriber Line Access Multiplexer. The DSLAM provides the access between several ADSL links. It divides the incoming ADSL data into voice traffic directed to the PSTN (Public Switched Telephone Network) and digital data directed to high-speed links (Aber 1997). Above is a diagram from 3com showing how non-splitterless ADSL will be implemented in both home and business environments. RDSLAM is a Remote Digital Subscriber Line Access Mulitplexer, this unit is not located in the telephone company’s central office but instead resides in the remote digital carrier system (Aber 1997). Although the future seems bright for ADSL and the xDSL family, many obstacles must be overcome before most end-users can enjoy the bliss of high speed Internet connections at home. Even though a few companies have brought splitterless ADSL products to market there is no standard (Nobel 1998). G.lite, the name of the as yet incomplete ITU standard, does offer some hope for compatibility between products. Some industry experts believe that a standard for UADSL is two years from becoming a realization (Lawton 1998). All the RBOCs (Regional Bell Operating Companies) have xDSL trials in place (Greene 1998). It may seem reckless to invest in any technology with no standards, but telephone companies are in a race with other types of Internet service providers and must have some way of keeping customers from flocking to non-telephone company based solutions. Although telephone companies could start implementing technology from UADSL-pioneering firms throughout there networks, many telephone companies will not invest the huge amounts of capital needed to make the service widely available until products based on industry accepted standards are available. In spite of the fact that few standards exist some sort of xDSL technology seems to be the heir apparent to modern digital connections to the Internet. Market researchers project the number of xDSL products deployed by 2003 to be 3.7 to 20 million (TeleChoice 1997). Many firms are conservatively placing the number of installed circuits some where in the middle of that range. Microsoft has incorporated ADSL drivers into Windows 98. Compaq is working with Ameritech to ensure compatibility with Compaq’s Presario PC and Ameritech’s ADSL deployment (Nobel 1998). DSL modems are now being developed for USB (Universal Serial Bus) by Intel (Nobel 1998). Although not all xDSL solutions will be needed or work for all customers, among the many forms of DSL technologies that exist there should be at least one member of the family that can provide benefits over existing technologies for everyone.
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