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The accelerated growth of content rich applications and online gaming, which demand high bandwidth, has changed the nature of information networks. High-speed communication is now an ordinary requirement throughout business, government, academic, and home office environments. Internet access, telecommuting, and remote LAN access are three of the clearly defined services that network access providers are offering now. These rapidly growing applications are placing a new level of demand on the telephone infrastructure.
In particular, the local loop portion of the network (i. e. , the local connection from the subscriber to the local central office) has become a challenge for telephone companies. Historically, this local loop facility has been provisioned with copper cabling which cannot easily support high bandwidth transmission. This environment is now being stressed by the demand for increasingly higher bandwidth capacities. Although this infrastructure could be replaced by a massive rollout of fiber technologies, the cost to do so would be insupportable in today's business models and, more importantly, the time to accomplish such a transition is unacceptable because the market demand exists today! Telephone companies are already faced with growing competition and unprecedented customer demands A new category of companies, Internet Service Providers (ISPs), has emerged in this market as providers of data services.
Traditionally ISPs have used the telephone company infrastructure. However, thanks to deregulation, they now have direct access to the physical cable plant. ISPs will be formidable competitors in this quest for the customers. Network service providers around the world fill this moment with great potential for remarkable success. A new technology called high-speed Digital Subscriber Line (xDSL) has emerged onto this scene. xDSL, which encompasses several different technologies, essentially allows the extension of megabit bandwidth capacities from the service provider central office to the customer premises on a customer by customer basis over the existing copper cabling, without the need for massive infrastructure replacement and at very reasonable costs.
These new xDSL solutions satisfy the business need to provision the network in a fast, cost effective manner, while preserving the infrastructure and allowing a planned migration into newer technologies. xDSL has the ability to meet the customer demand for high bandwidth right now, at costs that make sense. xDSL is a group of emerging Digital Subscriber Line (DSL) modem technologies for supporting high-rate traffic transmission over POTS lines. X stands for asymmetric in ADSL, rate adaptive in RADSL, high-speed in HDSL, and very high speed in VDSL.
xDSL Delivers Broadband over Copper The best thing about xDSL technologies is their ability to transport large amounts of information across existing copper telephone lines. This is possible because xDSL modems leverage signal processing techniques that insert and extract more digital data onto analog lines. The key is modulation, a process in which one signal modifies the property of another. ADSL Development and Deployment Progress Of all the emerging xDSL technologies, ADSL is receiving the most attention because there is a standard (DMT) for it, and its capabilities provide NSPs with a competitive offering to cable modems.
But there is increasing interest in symmetrical xDSL offerings such as HDSL and SDSL. As a local access service, Adsl's implementation has no critical drawbacks. It can be deployed as an overlay network where there is subscriber demand, eliminating the need for NSPs to risk building out their infrastructure unnecessarily in the hope that the technology will catch on. ADSL development and deployment is focused primarily in North America, followed by northern Europe and the Pacific Rim. In North America, US West, GTE, Ameritech, SBC, BellSouth, and Edmonton Tel (Canada) are the service providers leading the current wave of ADSL/xDSL deployment. Covad, Northpoint, and a handful of other CLECs are entering high-density metropolitan areas typically offering a portfolio of xDSL offerings at different classes of service and price points, and competing with incumbent local exchange carriers.
Chicago-based Inter Access was the first ISP to offer ADSL. Telia (Sweden), Telenor (Norway), British Telecom (UK), and Telfonica (Spain) are leading xDSL proponents in Europe. In the Pacific Rim, Telstra (Australia), Hong Kong Telecom, and Singtel (Singapore) are deploying xDSL for data and video applications. ADSL modems have been tested successfully by more than 40 telephone companies, and close to 50, 000 lines have been installed in various technology trials and commercial deployments. Increasingly, alternative service providers such as enterprises, multi-tenant building owners, hospitality businesses (hotels and resorts), and office park developers are offering or considering offering ADSL to their users as private network operators. In early 90 's xDSL technologies were tested by some of the regional Bell operating companies in the United States, as well as several European telephone companies.
At that time, the driving applications behind xDSL were video on demand (VOD) and interactive TV (ITV). Those applications were seen as potentially explosive sources of revenue growth for the residential market. In 1995, interest shifted toward the online world and more specifically the World Wide Web. The increasing demand for bandwidth with which to access the Web is one of the primary applications at which xDSL technologies are now targeted. However, xDSL technologies are also being looked at in conjunction with several other applications. These applications may produce a far greater revenue stream in near future compared to broadband Web access for residential market.
Listed below are just a few examples of how xDSL technology can be utilized: Intranet access for organizations that are standardizing on a Web based, client server model is one of primary xDSL applications. An organization that has implemented an Intranet will require higher bandwidth afforded by xDSL in order to link their remote offices and telecommuters to the more demanding business oriented applications running on their private Web servers. xDSL technologies have the potential to prove far more effective in low cost, high throughput, LAN to LAN connectivity than ISDN or traditional leased lines. Since xDSL operates at the physical layer, it could emerge as the most cost effective method of carrying frame relay traffic from the service subscriber to the frame relay network. As with frame relay the xDSL technologies can also be used to carry ATM cells to an ATM access device, where they are statistically multiplexed over an ATM backbone.
Digital Subscriber Line, or DSL, is fundamentally another name for an ISDN-BRI channel operating at the Basic Rate Interface with two 64 kbps switched channels and one 16 kbps packet switching and signaling channel. This circuit can carry both voice and data in both directions at the same time. xDSL refers to those various arrangements in which advanced modulating techniques are imposed onto the local channel in order to derive higher throughput in one or both directions. The various types of xDSL are described in the following paragraphs.
High-bit-rate Digital Subscriber Line (HDSL) derives its name from the high bandwidth that is transmitted in both directions over two copper loops. HDSL has proven to be a reliable and cost effective means for providing repeater-less T 1 and E 1 services over two twisted pair loops. This proven technology has already resulted in the deployment of over 300, 000 HDSL equipped circuits throughout the local access infrastructure. HDSL transceivers can reliably transmit a 2. 048 Mbps data signal over two non-loaded, 24 gauge (0. 5 mm), unconditioned twisted wire pair loops at a distance of up to 13 kft (4. 2 km) without the need for repeaters.
Eliminating the need for repeater equipment and removal of bridged taps significantly simplifies the labor and engineering effort to provision the service. This attribute eliminates the need to identify, modify, and verify a controlled environment, with power, secured access, and other factors needed to support repeater equipment. It also reduces the time, cost, and effort of isolating faults and taking corrective action when a failure does occur. Studies by some service providers have indicated that trouble shooting and replacing defective repeater equipment often costs significantly more than the cost of the equipment itself. These attributes translate into increased network up time and reduced engineering time; making possible T 1 provisioning in a matter of days, as opposed to weeks. Faster service provisioning and greater up time leads to increased customer satisfaction and increased service revenues.
To provision a 12 kft (3. 6 km) local loop with traditional T 1 transmission equipment requires two transceivers and two repeaters. To provision the same loop with HDSL, requires only two HDSL transceivers, one at each end of a line. Single-pair or Symmetric High-bit-rate Digital Subscriber Line (S-HDSL/SDSL) operate on a single copper pair as opposed to the traditional two pair HDSL described above. S-HDSL/SDSL allows easy implementation of applications that require symmetric data rates on a single local loop while maintaining the existing POTS on the same loop. Because only one pair is needed in this arrangement, the capacity of the entire local loop infrastructure is greatly magnified. With this capability, local providers can extract the maximum value from their existing plant, or deploy new capacities both more quickly and at a lower capital expenditure.
This allows for rapid and cost effective deployment of intermediate data rate services. Potential uses for this technology include fractional T 1 with a particular advantage in 768 kbps systems, Home Office, LAN Access, Distance Learning, Internet Access, and Campus or Large Facility LAN to LAN connectivity. Since S-HDSL/SDSL can be implemented with and without POTS and at multiple data rates, it can have different capacity and reach limitations. This allows for easy, cost effective implementation of such services as remote cell site support of PCs, remote LAN access, distance education and training, digital imaging, or any other service which requires a larger amount of bandwidth.
Probably the most common xDSL type is Asymmetric Digital Subscriber, which takes its name from the comparatively high bandwidth in one direction, with low bandwidth in the opposite direction. ADSL uses a single phone line for transmission. Many service providers have also come to recognize its potential to support a range of data applications. Additionally, ADSL's ability to operate at speeds of up to 6 Mbps positions it to support real time broadcast services and pre-recorded interactive video services; and to have multiple video and data activities underway simultaneously. ADSL supports applications with asymmetric traffic demands such as: Rate Adaptive Digital Subscriber Line (RADSL) is a simple extension of ADSL used to encompass and support a wide variety of data rates depending on the line's transmission characteristics. This is advantageous in situations in which there is a lower bandwidth demand and in situations in which the line quality is less than needed for full bandwidth implementations.
Very High-bit-rate Digital Subscriber Line (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. Within the FTTC architecture, VDSL will address the last section of copper cabling to the subscriber premises. Typical distance and implementation of VDSL is 1 km @ 26 Mbps. Unfortunately, this type of xDSL is not very common because of lack of FTTC networks available today. XDSL signals are designed to maximize the rate of transmission of digital signals through non loaded twisted pairs, making use of ba...
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