Long-distance connection technologies refer to the continually evolving suite of technologies that allow users to connect to the Internet or their corporate WANs (Wide-Area Networks) or LANs (Local-Area Networks) remotely and across large distances. Technically, access describes the transport connection between service providers (whether telephone company, cable, satellite, or wireless) and customer premises. Long-distance access options today include T1 and T3 connections, ISDN, SONET and other fibre-optic connection technologies, DSL and cable modem connections, Frame Relay, ATM, satellite, and wireless access. The speed at which connection technologies have expanded from analog modems introduced in the 1960s, to fiber-optic connections and DSL in the 1990s, to satellite and wireless in the 2000s, is linked to the exponential growth of the Internet and the explosion of multimedia applications of diverse scope on the World Wide Web. Increasingly connection technologies offer broadband capabilities and high-speed access, which means that bit-heavy multimedia, voice, video, and data can be transmitted simultaneously, and at record velocities to users.
ISDN, which stands for Integrated Services Digital Network, is a system of digital phone connections that was introduced in 1983 and offered a speed of 128 Kilobits per second; analog modems, though then offering only 2400 bits per second, were still evolving and provided slightly higher connection speeds every few years. Analog modems were not destined, however, to approach the very high speeds of ISDN. Today, broadband ISDN can support data rates of 1.5 million bits per second (bps), while analog modems offer only 56 kilobits per second.
Using the same twisted pair of copper phone lines that telephones and analog modems use, but using them to send digital data, ISDN can work with other digital technologies such as Frame Relay and ATM, which makes higher speeds possible in the future. Since telephone companies converted all long distance transmission early in the 1960s to cheaper digital signaling, ISDN's advantage over analog modems is clear. ISDN can combine voice and data services, as well as fax, sophisticated switching, and speedy data transmission over the same wires, since computers can connect directly via the digital ISDN to the telephone network without first converting their signals to an analog audio signal, as modems do. ISDN currently provides a raw data rate of 144 Kbps on a single copper twisted-pair. The service is divided into three channels: two 64Kb B (Bearer) channels and a 16Kb D (Data) channel for signaling, assuming that the D-channel is operating at the basic rate. If the D channel operates at the primary rate, then it too is operating at 64Kbps. (Primary and basic rates define broadband or narrowband ISDN, related to the number of available channels for data transmission.)
ISDN's alternatives, most of which offer methods for going digital directly to the subscriber, include existing copper-wire digital services, such as T1. Dedicated T1 lines, close in concept to ISDN, were deployed in 1984 and offer speeds of 1.544 Mega bits per second (Mbps). Other technologies include Frame Relay (FR) at 56 Kbps to 1.544 Mbps, Asynchronous Transfer Mode (ATM) at 25 Mbps to 100 Mbps, and Switched Multimegabit Data Service (SMDS) at 35 Mbps. Most of these services are very expensive and generally used to fuel the backbone of the high-speed networks forming the core of the Internet rather than as access technologies. Another alternative is to replace the base copper wiring with fiber-optic cabling, which would reach every subscriber and offer essentially unlimited bandwidth.
Fiber-optic technology for networks has expanded in recent times through the implementation of SONET, or Synchronous Optical Network, a standard for connecting fiber-optic transmission systems, established in the middle 1980s and now an ANSI (American National Standards Institute) standard. SONET defines network interface standards at the physical layer of the OSI seven-layer model, establishing a hierarchy of speeds and formats for its Optical Carrier (OC) Levels defining rates of data transfer. These levels can be multiplexed (combined through switching) to form higher-rate signals. Because it is a standard, SONET ensures seamless connectivity across compliant equipment and facilitates efficient switching and transport, eliminating the need for the bulky hard-wired infrastructure used in T1s or T3s. SONET's high-speed Optical Carrier (OC) levels range from 51.8 Mbps on OC-1 (about the same as a T3 line) to 13.271 Gbps on OC-256. SONET is used mainly in enterprise backbone networks and fault-tolerant rings around major metropolitan areas. Communications carriers across the world can also use SONET to very smoothly interconnect their existing digital carrier and fiber-optic systems.
Current and developing industry high-speed access options include: xDSL technologies, hybrid fiber-coax (HFC) utilized by cable modems, Ethernet technology, satellite, and wireless communications. Cable modems offer a scalable solution for home and small-business users over cable TV networks. Downstream speeds range from 1.5 Mbps to 35 Mbps, depending on the cable provider; 1.5 Mbps is 26 times what dial-up analog modems allow.
xDSL refers to a variety of Digital Subscriber Line technologies that allow digital modems to send data over existing telephone company installations of copper wire, making it a convenient and inexpensive transport medium. xDSL is designed to enable Internet/Intranet access at speeds many times higher than today's 56 Kbps modems or 128 Kbps ISDN links. Certain DSL technologies can enable high-speed data services, which can operate concurrently with plain old telephone service (POTS). HDSL or High-data Rate DSL provides up to 1.5 Mbps of two-way traffic over two copper pairs, and has been used as an economical way to deploy T1 services. ADSL or Asymmetric Digital Subscriber Line technology provides up to 8 Mbps downstream and up to 1 Mbps upstream in conjunction with basic telephony service on the same line. Of all the DSL variants, ADSL has received the most attention from the industry primarily because it is ideally suited to Internet traffic. In addition to Internet and Intranet access, key applications for ADSL likely will be video streaming and multicast entertainment, which are predicated on higher rate downstream speeds. RADSL or Rate Adaptive DSL automatically adjusts to copper quality and can run at different rates adjustable up to the maximum rates for ADSL. SDSL or Single-line/Single-Pair DSL is a symmetric technology that delivers up to 768 Kbps in both directions, and is a data-only service also suited to T1 replacement services, and is appropriate for remote LAN access, videoconferencing, and Web hosting.
VDSL or Very High Rate DSL, a high-speed asymmetric DSL technology is most attractive in close proximity to the switching equipment--typically residing in the same building. VDSL is best suited to services such as videoconferencing, digital video, and high speed Internet access for in-building applications, and is currently a steadily expanding technology. IDSL or ISDN DSL is a data-only service fully interoperable with ISDN terminal adapters and routers, and is applicable in the ISDN replacement market for remote LAN access, Internet access, and videoconferencing.
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