GSM (Global System for Mobile communication) is a digital mobile telephone system that is widely used in Europe and other parts of the world. GSM uses a variation of time division multiple access (Time Division Multiple Access) and is the most widely used of the three digital wireless telephone technologies (TDMA, GSM, and CDMA). GSM digitizes and compresses data, then sends it down a channel with two other streams of user data, each in its own time slot. It operates at either the 900 MHz or 1800 MHz frequency band.
GSM is the de facto wireless telephone standard in Europe. GSM has over 120 million users worldwide and is available in 120 countries, according to the GSM MoU Association. Since many GSM network operators have roaming agreements with foreign operators, users can often continue to use their mobile phones when they travel to other countries.
American Personal Communications (APC), a subsidiary of Sprint, is using GSM as the technology for a broadband personal communications service (personal communications services). The service will ultimately have more than 400 base stations for the palm-sized handsets that are being made by Ericsson, Motorola, and Nokia. The handsets include a phone, a text pager, and an answering machine.
GSM together with other technologies is part of an evolution of wireless mobile telemmunication that includes High-Speed Circuit-Switched Data (High-Speed Circuit-Switched Data), General Packet Radio System (General Packet Radio Services), Enhanced Data GSM Environment (Enhanced Data GSM Environment), and Universal Mobile Telecommunications Service (Universal Mobile Telecommunications System).
High-Speed Circuit-Switched Data (HSCSD) is circuit-switched wireless data transmission for mobile users at data rates up to 38.4 Kbps, four times faster than the standard data rates of the Global System for Mobile (GSM) communication standard in 1999. HSCSD is comparable to the speed of many computer modems that communicate with today’s fixed telephone networks. HSCSD is an evolutionary technology on the way to Universal Mobile Telecommunications Service (UMTS).
POTS is a term sometimes used in discussion of new telephone technologies in which the question of whether and how existing voice transmission for ordinary phone communication can be accommodated. For example, Asymmetric Digital Subscriber Line and Integrated Services Digital Network provide some part of their channels for “plain old telephone service” while providing most of their bandwidth for digital data transmission.
Frame relay is a telecommunication service designed for cost-efficient data transmission for intermittent traffic between local area networks () and between end-points in a wide area network (wide area network). Frame relay puts data in a variable-size unit called a frame and leaves any necessary error correction (retransmission of data) up to the end-points, which speeds up overall data transmission.
For most services, the network provides a permanent virtual circuit (Permanent Virtual Circuit), which means that the customer sees a continous, dedicated connection without having to pay for a full-time leased line, while the service provider figures out the route each frame travels to its destination and can charge based on usage. An enterprise can select a level of service quality – prioritizing some frames and making others less important. Frame relay is offered by a number of service providers, including AT&T.
Frame relay is provided on fractional T-1 or full T-carrier system carriers. Frame relay complements and provides a mid-range service between Integrated Services Digital Network, which offers bandwidth at 128 Kbps, and Asynchronous Transfer Mode (asynchronous transfer mode), which operates in somewhat similar fashion to frame relay but at speeds from 155.520 Mbps or 622.080 Mbps.
Frame relay is based on the older X.25 packet-switching technology which was designed for transmitting analog data such as voice conversations. Unlike X.25 which was designed for analog signals, frame relay is a fast packet technology technology, which means that the protocol does not attempt to correct errors. When an error is detected in a frame, it is simply “dropped.” (thrown away). The end points are responsible for detecting and retransmitting dropped frames. (However, the incidence of error in digital networks is extraordinarily small relative to analog networks.)
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GPRS (General Packet Radio Services) is a packet-based wireless communication service that, when available in 2000, promises data rates from 56 up to 114 Kbps and continuous connection to the Internet for mobile phone and computer users. The higher data rates will allow users to take part in video conferences and interact with multimedia Web sites and similar applications using mobile handheld devices as well as notebook computer computers. GPRS is based on Global System for Mobile (GSM) communication and will complement existing services such circuit-switched cellular phone connections and the Short Message Service (SMS).
Integrated Services Digital Network (ISDN) is a set of CCITT/ITU standards for digital transmission over ordinary telephone copper wire as well as over other media. Home and business users who install an ISDN adapter (in place of a modem) can see highly-graphic Web pages arriving very quickly (up to 128 Kbps). ISDN requires adapters at both ends of the transmission so your access provider also needs an ISDN adapter. ISDN is generally available from your phone company in most urban areas in the United States and Europe.
There are two levels of service: the Basic Rate Interface (BRI), intended for the home and small enterprise, and the Primary Rate Interface (PRI), for larger users. Both rates include a number of B-channels and a D-channels. Each B-channel carries data, voice, and other services. Each D-channel carries control and signaling information.
The Basic Rate Interface consists of two 64 Kbps B-channels and one 16 Kbps D- channel. Thus, a Basic Rate user can have up to 128 Kbps service. The Primary Rate consists of 23 B-channels and one 64 Kpbs D-channel in the United States or 30 B-channels and 1 D-channel in Europe.
Integrated Services Digital Network in concept is the integration of both analog or voice data together with digital data over the same network. Although the ISDN you can install is integrating these on a medium designed for analog transmission, broadband ISDN (BISDN) will extend the integration of both services throughout the rest of the end-to-end path using fiber optic and radio media. Broadband ISDN will encompass frame relay service for high-speed data that can be sent in large bursts, the Fiber Distributed-Data Interface (FDDI), and the Synchronous Opical Network (SONET). BISDN will support transmission from 2 Mbps up to much higher, but as yet unspecified, rates.
IDSL is a system in which digital data is transmitted at 128 Kbps on a regular copper telephone line (twisted pair) from a user to a destination using digital (rather than analog or voice) transmission, bypassing the telephone company’s central office equipment that handles analog signals. IDSL uses the Integrated Services Digital Network (Integrated Services Digital Network) Basic Rate Interface in ISDN transmission code.
IDSL is a technology developed by Ascend Communications (now part of Lucent Technologies). IDSL is only one possible technology in the Digital Subscriber Line approach (of which Asymmetric Digital Subscriber Line or Asymmetric Digital Subscriber Line is best known) and an expedient approach that allows use of existing ISDN card technology for data-only use.
The differences between IDSL and ISDN are:
| · ISDN passes through the phone company’s central office voice network; IDSL bypasses it by plugging into a special router at the phone company end
· ISDN requires call setup; IDSL is a dedicated service
· ISDN may involve per-call fees; IDSL may be billed at a flat rate with no usage charges |
AppleTalk is a set of local area network communication protocol originally created for Apple computers. An AppleTalk network can support up to 32 devices and data can be exchanged at a speed of 230.4 kilobits per second (Kbps). Devices can be as much as 1,000 feet apart. AppleTalk’s Datagram Delivery Protocol corresponds closely to the Network layer of the Open Systems Interconnection (OSI) communication model.
EDGE (Enhanced Data GSM Environment), a faster version of the Global System for Mobile (GSM) wireless service, is designed to deliver data at rates up to 384 Kbps and enable the delivery of multimedia and other broadband applications to mobile phone and computer users. The EDGE standard is built on the existing GSM standard, using the same time-division multiple access (TDMS) frame structure and existing cell arrangements. Ericsson notes that, when available, its base stations can be updated with software. EDGE is expected to be commercially available in 2001. It is regarded as an evolutionary standard on the way to Universal Mobile Telecommunications Service (UMTS).
A satellite is a specialized wireless receiver/transmitter that is launched by a rocket and placed in orbit around the earth. There are hundreds of satellites currently in operation. They are used for such diverse purposes as weather forecasting, television broadcast, amateur radio communications, Internet communications, and the Global Positioning System. The first artificial satellite, launched by Russia (then known as the Soviet Union) in the late 1950s, was about the size of a basketball. It did nothing but transmit a simple Morse code signal over and over. In contrast, modern satellites can receive and re-transmit thousands of signals simultaneously, from simple digital data to the most complex television programming.
There are three types of communications satellite systems. They are categorized according to the type of orbit they follow.
A geostationary satellite orbits the earth directly over the equator, approximately 22,000 miles up. At this altitude, one complete trip around the earth (relative to the sun) takes 24 hours. Thus, the satellite remains over the same spot on the earth’s surface at all times, and stays fixed in the sky from any point on the surface from which it can be “seen.” So-called weather satellites are usually of this type. You can view images from some of these satellites on the Internet via the Purdue Weather Processor. A single geostationary satellite can “see” approximately 40 percent of the earth’s surface. Three such satellites, spaced at equal intervals (120 angular degrees apart), can provide coverage of the entire civilized world. A geostationary satellite can be accessed using a dish antenna aimed at the spot in the sky where the satellite hovers.
A low-earth-orbit (LEO) satellite system employs a large fleet of “birds,” each in a circular orbit at a constant altitude of a few hundred miles. The orbits take the satellites over, or nearly over, the geographic poles. Each revolution takes approximately 90 minutes to a few hours. The fleet is arranged in such a way that, from any point on the surface at any time, at least one satellite is on a line of sight. The entire system operates in a manner similar to the way a cellular telephone functions. The main difference is that the transponders, or wireless receiver/transmitters, are moving rather than fixed, and are in space rather than on the earth. A well-designed LEO system makes it possible for anyone to access the Internet via wireless from any point on the planet, using an antenna no more sophisticated than old-fashioned television “rabbit ears.”
Some satellites revolve around the earth in elliptical orbits. These satellites move rapidly when they are near perigee, or their lowest altitude; they move slowly when they are near apogee, or their highest altitude. Such “birds” are used by amateur radio operators, and by some commercial and government services. They require directional antennas whose orientation must be constantly adjusted to follow the satellite’s path across the sky.
Frame relay is a telecommunication service designed for cost-efficient data transmission for intermittent traffic between local area networks () and between end-points in a wide area network (wide area network). Frame relay puts data in a variable-size unit called a frame and leaves any necessary error correction (retransmission of data) up to the end-points, which speeds up overall data transmission.
For most services, the network provides a permanent virtual circuit (Permanent Virtual Circuit), which means that the customer sees a continous, dedicated connection without having to pay for a full-time leased line, while the service provider figures out the route each frame travels to its destination and can charge based on usage. An enterprise can select a level of service quality – prioritizing some frames and making others less important. Frame relay is offered by a number of service providers, including AT&T.
Frame relay is provided on fractional T-1 or full T-carrier system carriers. Frame relay complements and provides a mid-range service between Integrated Services Digital Network, which offers bandwidth at 128 Kbps, and Asynchronous Transfer Mode (asynchronous transfer mode), which operates in somewhat similar fashion to frame relay but at speeds from 155.520 Mbps or 622.080 Mbps.
Frame relay is based on the older X.25 packet-switching technology which was designed for transmitting analog data such as voice conversations. Unlike X.25 which was designed for analog signals, frame relay is a fast packet technology technology, which means that the protocol does not attempt to correct errors. When an error is detected in a frame, it is simply “dropped.” (thrown away). The end points are responsible for detecting and retransmitting dropped frames. (However, the incidence of error in digital networks is extraordinarily small relative to analog networks.)
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T1 – The T1 (or T-1) carrier is the most commonly used digital line in the United States, Canada, and Japan. In these countries, it carries 24 pulse code modulation (PCM) signals using time-division multiplexing (TDM) at an overall rate of 1.544 million bits per second (Mbps). T1 lines use copper wire and span distances within and between major metropolitan areas. A T1 Outstate System has been developed for longer distances between cities.
A fractional T-1 or T-3 line is a T-1 or T-3 digital phone line in the North American T-carrier system that is leased to a customer at a fraction of its data-carrying capacity and at a correspondingly lower cost. A T-1 line contains 24 channels, each with a data transfer capacity of 64 Kbps. The customer can rent some number of the 24 channels. The transmission method and speed of transfer remain the same. Overhead bits and framing are still used, but the unrented channels simply contain no data. UMTS
E1 (or E-1) is a European digital transmission format devised by the ITU-TS and given the name by the Conference of European Postal and Telecommunication Administration (CEPT). It’s the equivalent of the North American T-carrier system format. E2 through E5 are carriers in increasing multiples of the E1 format.
The E1 signal format carries data at a rate of 2.048 million bits per second and can carry 32 channels of 64 Kbps* each. E1 carries at a somewhat higher data rate than T-1 (which carries 1.544 million bits per second) because, unlike T-1, it does not do bit-robbing and all eight bits per channel are used to code the signal. E1 and T-1 can be interconnected for international use.
E2 (E-2) is a line that carries four multiplexed E1 signals with a data rate of 8.448 million bits per second.
E3 (E-3) carries 16 E1 signals with a data rate of 34.368 million bits per second.
E4 (E-4) carries four E3 channels with a data rate of 139.264 million bits per second.
E5 (E-5) carries four E4 channels with a data rate of 565.148 million bits per second. * In international English outside the U.S., the equivalent usage is “kbps” or “kbits s-1.”
A token ring network is a local area network (LAN) in which all computers are connected in a ring or star topology and a binary digit- or token-passing scheme is used in order to prevent the collision of data between two computers that want to send messages at the same time. The token ring protocol is the second most widely-used protocol on local area networks after Ethernet. The IBM Token Ring protocol led to a standard version, specified as IEEE 802.5. Both protocols are used and are very similar. The IEEE 802.5 token ring technology provides for data transfer rates of either 4 or 16 megabits per second.
Very briefly, here is how it works:
| 1. Empty information frames are continuously circulated on the ring.
2. When a computer has a message to send, it inserts a token in an empty frame (this may consist of simply changing a 0 to a 1 in the token bit part of the frame) and inserts a message and a destination identifier in the frame.
3. The frame is then examined by each successive workstation. If the workstation sees that it is the destination for the message, it copies the message from the frame and changes the token back to 0.
4. When the frame gets back to the originator, it sees that the token has been changed to 0 and that the message has been copied and received. It removes the message from the frame.
5. The frame continues to circulate as an “empty” frame, ready to be taken by a workstation when it has a message to send. |
The token scheme can also be used with bus topology LANs. The standard for the token ring protocol is Institute of Electrical and Electronics Engineers (IEEE) 802.5. The Fiber Distributed-Data Interface (FDDI) also uses a token ring protocol.
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DSL (Digital Subscriber Line) is a technology for bringing high-bandwidth information to homes and small businesses over ordinary copper telephone lines. xDSL refers to different variations of DSL, such as ADSL, HDSL, and RADSL. Assuming your home or small business is close enough to a telephone company central office that offers DSL service, you may be able to receive data at rates up to 6.1 megabits (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 will provide from 1.544 Mbps to 512 Kbps downstream and about 128 Kbps upstream. A DSL line can carry both data and voice signals and the data part of the line is continuously connected. DSL installations began in 1998 and will continue at a greatly increased pace through the next decade in a number of communities in the U.S. and elsewhere. Compaq, Intel, and Microsoft working with telephone companies have developed a standard and easier-to-install form of ADSL called G.Lite that is accelerating deployment. DSL is expected to replace ISDN in many areas and to compete with the cable modem in bringing multimedia and 3-D to homes and small businesses.
How It Works:
Traditional phone service (sometimes called POTS for “plain old telephone service”) 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 you exchange voice information 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 signalling 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 your 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, you can receive 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 your modem to change it back into digital. In other words, the analog transmission between your 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.
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A cable modem is a device that enables you to hook up your PC to a local cable TV line and receive data at about 1.5 Mbps. This data rate far exceeds that of the prevalent 28.8 and 56 Kbps telephone modems and the up to 128 Kbps of Integrated Services Digital Network (ISDN) and is about the data rate available to subscribers of Digital Subscriber Line (DSL) telephone service. A cable modem can be added to or integrated with a set-top box that provides your TV set with channels for Internet access. In most cases, cable modems are furnished as part of the cable access service and are not purchased directly and installed by the subscriber.
A cable modem has two connections: one to the cable wall outlet and the other to a PC or to a set-top box for a TV set. Although a cable modem does modulation between analog and digital signals, it is a much more complex device than a telephone modem. It can be an external device or it can be integrated within a computer or set-top box. Typically, the cable modem attaches to a standard 10BASE-T Ethernet card in the computer. All of the cable modems attached to a cable TV company coaxial cable line communicate with a Cable Modem Termination System (CMTS) at the local cable TV company office. All cable modems can receive from and send signals only to the CMTS, but not to other cable modems on the line. Some services have the upstream signals returned by telephone rather than cable, in which case the cable modem is known as a telco-return cable modem.
The actual bandwidth for Internet service over a cable TV line is up to 27 Mbps on the download path to the subscriber with about 2.5 Mbps of bandwidth for interactive responses in the other direction. However, since the local provider may not be connected to the Internet on a line faster than a T-carrier system at 1.5 Mpbs, a more likely data rate will be close to 1.5 Mpbs.
Ethernet is the most widely-installed local area network (LAN) technology. Specified in a standard, IEEE 802.3, Ethernet was originally developed by Xerox and then developed further by Xerox, DEC, and Intel. An Ethernet LAN typically uses coaxial cable or special grades of twisted pair wires. The most commonly installed Ethernet systems are called 10BASE-T and provide transmission speeds up to 10 Mbps. Devices are connected to the cable and compete for access using a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) protocol.
Token Ring/802.5
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T3 – a line providing 44.736 Mbps, is also commonly used by Internet service provider. Another commonly installed service is a fractional T-1, which is the rental of some portion of the 24 channels in a T-1 line, with the other channels going unused.
OC-1 The Synchronous Optical Network (SONET) includes a set of signal rate multiples for transmitting digital signals on optical fiber. The base rate (OC-1) is 51.84 Mbps. OC-2 runs at twice the base rate, OC-3 at three times the base rate, and so forth. Planned rates include OC-1, OC-3 (155.52 Mbps), OC-12 (622.08 Mpbs), and OC-48 (2.488 Gbps). Asynchronous transfer mode (ATM) makes use of some of the Optical Carrier levels.
High-Speed Serial Interface (HSSI) is a short-distance communications interface that is commonly used to interconnect routing and switching devices on local area networks (LANs) with the higher-speed lines of a wide area network (WAN). HSSI is used between devices that are within fifty feet of each other and achieves data rates up to 52 Mbps. Typically, HSSI is used to connect a LAN router to a T-3 line. HSSI can be used to interconnect devices on token ring and Ethernet LANs with devices that operate at Synchronous Optical Network (SONET) OC-1 speeds or on T-3 lines. HSSI is also used for host-to-host linking, image processing, and disaster recovery applications.
Like ISDN and DSL, HSSI operates at the physical layer of a network, using the standard Open Systems Interconnection (OSI) model. The electrical connection uses a 50-pin connector. The HSSI transmission technology uses differential emitter-coupled logic (ECL). (ECL is a circuit design in which two transistor emitters are connected to a resistor that is switched between the emitters, producing high bit rates.) HSSI uses gapped timing. Gapped timing allows a Data Communications Equipment (DCE) device to control the flow of data being transmitted from a Data Terminating Equipment (DTE) device such as a terminal or computer by adjusting the clock speed or deleting clock impulses.
For diagnosing problems, HSSI offers four loopback tests. The first loopback tests the cable by looping the signal back after it reaches the DTE port. The second and third loopbacks test the line ports of the local DCE and the remote DTE. The fourth tests the DTE’s DCE port. HSSI requires two control signals (“DTE available” and “DCE available”) before the data circuit is valid.
The HSSI cable uses the same number of pins and wires as a SCSI-2 cable, but uses the HSSI electrical interface. It is not recommended to use a SCSI-2 cable with an HSSI interface.
Fast Ethernet is a local area network () transmission standard that provides a data rate of 100 megabits per second (referred to as “100BASE-T”). Workstations with existing 10 megabit per second (10BASE-T) Ethernet card can be connected to a Fast Ethernet network. (The 100 megabits per second is a shared data rate; input to each workstation is constrained by the 10 Mbps card.)
FDDI (Fiber-Distributed Data Interface) is a standard for data transmission on fiber optic lines in a that can extend in range up to 200 km (124 miles). The FDDI protocol is based on the token ring protocol. In addition to being large geographically, an FDDI local area network can support thousands of users.
An FDDI network contains two token rings, one for possible backup in case the primary ring fails. The primary ring offers up to 100 Mbps capacity. If the secondary ring is not needed for backup, it can also carry data, extending capacity to 200 Mbps. The single ring can extend the maximum distance; a dual ring can extend 100 km (62 miles).
FDDI is a product of American National Standards Committee X3-T9 and conforms to the open system interconnect (OSI) model of functional layering. It can be used to interconnect LANs using other protocols. FDDI-II is a version of FDDI that adds the capability to add circuit-switched service to the network so that voice signals can also be handled. Work is underway to connect FDDI networks to the developing Synchronous Optical Network Synchronous Optical Network.
OC-3
SDH (Synchronous Digital Hierarchy) is a standard technology for synchronous data transmission on optical media. It is the international equivalent of Synchronous Optical Network. Both technologies provide faster and less expensive network interconnection than traditional PDH (Plesiochronous Digital Hierarchy) equipment.
In digital telephone transmission, “synchronous” means the bits from one call are carried within one transmission frame. “Plesiochronous” means “almost (but not) synchronous,” or a call that must be extracted from more than one transmission frame.
SDH uses the following Synchronous Transport Modules (STM) and rates: STM-1 (155 megabits per second), STM-4 (622 Mbps), STM-16 (2.5 gigabits per second), and STM-64 (10 Gbps).
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OC-12
Gigabit Ethernet provides an even higher level of backbone support at 1000 megabits per second (1 gigabit or 1 billion bits per second).
OC-48
OC-192
OC-256
TDM (time-division multiplexing) is a scheme in which numerous signals are combined for transmission on a single communications line or channel. Each signal is broken up into many segments, each having very short duration.
PCM (pulse code modulation) is a digital scheme for transmitting analog data.
The signals in PCM are binary; that is, there are only two possible states, represented by logic 1 (high) and logic 0 (low). This is true no matter how complex the analog waveform happens to be. Using PCM, it is possible to digitize all forms of analog data, including full-motion video, voices, music, telemetry, and virtual reality (VR).
Twisted pair is the ordinary copper wire that connects home and many business computers to the telephone company. To reduce crosstalk or electromagnetic induction between pairs of wires, two insulated copper wires are twisted around each other. Each connection on twisted pair requires both wires. Since some telephone sets or desktop locations require multiple connections, twisted pair is sometimes installed in two or more pairs, all within a single cable. For some business locations, twisted pair is enclosed in a shield that functions as a ground. This is known as shielded twisted pair (STP). Ordinary wire to the home is unshielded twisted pair (UTP).
Coaxial cable is the kind of copper cable used by cable TV companies between the community antenna and user homes and businesses. Coaxial cable is sometimes used by telephone companies from their central office to the telephone poles near users. It is also widely installed for use in business and corporation Ethernet and other types of local area network.
Coaxial cable is called “coaxial” because it includes one physical channel that carries the signal surrounded (after a layer of insulation) by another concentric physical channel, both running along the same axis. The outer channel serves as a ground. Many of these cables or pairs of coaxial tubes can be placed in a single outer sheathing and, with repeaters, can carry information for a great distance.
Optical fiber (or “fiber optic”) refers to the medium and the technology associated with the transmission of information as light pulses along a glass or plastic wire or fiber. Optical fiber carries much more information than conventional copper wire and is in general not subject to electromagnetic interference and the need to retransmit signals. Most telephone company long-distance lines are now of optical fiber.
Transmission on optical fiber wire requires repeater at distance intervals. The glass fiber requires more protection within an outer cable than copper. For these reasons and because the installation of any new wiring is labor-intensive, few communities yet have optical fiber wires or cables from the phone company’s branch office to local customers (known as local loop).
Single mode fiber is used for longer distances; multimode fiber fiber is used for shorter distances.
The term wireless refers to telecommunication in which electromagnetic waves (rather than some form of wire) carry the signal over part or all of the communication path. Some monitoring devices, such as intrusion alarms, employ acoustic waves at frequencies above the range of human hearing; these are also sometimes classified as wireless. The first wireless transmitters went on the air in the early 20th century using radiotelegraphy (Morse code). Later, as modulation made it possible to transmit voices and music via wireless, the medium came to be called “radio.” With the advent of television, fax, data communication, and the effective use of a larger portion of the spectrum, the term “wireless” has been resurrected.
Common examples of wireless equipment in use today include:
| · Cellular phones and pagers — provide connectivity for portable and mobile applications, both personal and business
· Global Positioning System (GPS) — allows drivers of cars and trucks, captains of boats and ships, and pilots of aircraft to ascertain their location anywhere on earth
· Cordless computer peripherals — the cordless mouse is a common example; keyboards and printers can also be linked to a computer via wireless
· Cordless telephone sets — these are limited-range devices, not to be confused with cell phones
· Home-entertainment-system control boxes — the VCR control and the TV channel control are the most common examples; some hi-fi sound systems and FM broadcast receivers also use this technology
· Remote garage-door openers — one of the oldest wireless devices in common use by consumers; usually operates at radio frequencies
· Two-way radios — this includes Amateur and Citizens Radio Service, as well as business, marine, and military communications
· Baby monitors — these devices are simplified radio transmitter/receiver units with limited range · Satellite television — allows viewers in almost any location to select from hundreds of channels
· Wireless LANs or local area networks — provide flexibility and reliability for business computer users |
Wireless technology is rapidly evolving, and is playing an increasing role in the lives of people throughout the world. In addition, ever-larger numbers of people are relying on the technology directly or indirectly. (It has been suggested that wireless is overused in some situations, creating a social nuisance.)
More specialized and exotic examples of wireless communications and control include:
| · Global System for Mobile Communication (GSM) — a digital mobile telephone system used in Europe and other parts of the world; the de facto wireless telephone standard in Europe
· General Packet Radio Service (GPRS) — a packet-based wireless communication service that provides continuous connection to the Internet for mobile phone and computer users
· Enhanced Data GSM Environment (EDGE — a faster version of the Global System for Mobile (GSM) wireless service
· Universal Mobile Telecommunications System (UTMS) — a broadband, packet-based system offering a consistent set of services to mobile computer and phone users no matter where they are located in the world
· Wireless Application Protocol (WAP) — a set of communication protocols to standardize the way that wireless devices, such as cellular telephones and radio transceivers, can be used for Internet access
· i-Mode — the world’s first “smart phone” for Web browsing, first introduced in Japan; provides color and video over telephone sets |
Wireless can be divided into:
| · Fixed wireless — the operation of wireless devices or systems in homes and offices, and in particular, equipment connected to the Internet via specialized modems
· Mobile wireless — the use of wireless devices or systems aboard motorized, moving vehicles; examples include the automotive cell phone and PCS (personal communications services)
· Portable wireless — the operation of autonomous, battery-powered wireless devices or systems outside the office, home, or vehicle; examples include handheld cell phones and PCS units
· IR wireless — the use of devices that convey data via IR (infrared) radiation; employed in certain limited-range communications and control systems |
In the U.S., Kbps stands for kilobits per second (thousands of bits per second) and is a measure of bandwidth (the amount of data that can flow in a given time) on a data transmission medium. Higher bandwidths are more conveniently expressed in megabits per second (Mbps, or millions of bits per second) and in gigabits per second (Gbps, or billions of bits per second).
The term RF radio frequency (abbreviated RF, rf, or r.f.) refers to alternating current (AC) having characteristics such that, if the current is input to an antenna, an electromagnetic (EM) field is generated suitable for wireless broadcasting and/or communications. These frequencies cover a significant portion of the electromagnetic radiation spectrum, extending from nine kilohertz (9 kHz), the lowest allocated wireless communications frequency (it’s within the range of human hearing), to thousands of gigahertz (GHz).
BRI & PRI: In the Integrated Services Digital Network (ISDN), there are two levels of service:
The Basic Rate Interface (BRI), intended for the home and small enterprise.
The Primary Rate Interface (PRI), for larger users. Both rates include a number of B-channels and a D-channel. Each B-channel carries data, voice, and other services. The D-channel carries control and signaling information.
The Basic Rate Interface consists of two 64 Kbps B-channels and one 16 Kbps D-channel. Thus, a Basic Rate Interface user can have up to 128 Kbps service.
The Primary Rate Interface consists of 23 B-channels and one 64 Kpbs D-channel in the United States or 30 B-channels and 1 D-channel in Europe.
Mbps stands for millions of bits per second or megabits per second and is a measure of bandwidth (the total information flow over a given time) on a telecommunications medium. Depending on the medium and the transmission method, bandwidth is sometimes measured in the Kbps (thousands of bits or kilobits per second) range or the Gbps (billions of bits or gigabits per second) range.
A megabit is a million binary pulses, or 1,000,000 (that is, 106) pulses (or “bits”). For example, a U.S. phone company T-carrier system line is said to sustain a data rate of 1.544 megabits per second. Megabits per second is usually shortened to Mbps.
Some sources define a megabit to mean 1,048,576 (that is, 220) bits. Although the bit is a unit of the binary number system, bits in data communications are discrete signal pulses and have historically been counted using the decimal number system. For example, 28.8 kilobits per second (Kbps) is 28,800 bits per second. Because of computer architecture and memory address boundaries, bytes are always some multiple or exponent of two. See kilobyte, etc.
An ISP (Internet service provider) is a company that provides individuals and other companies access to the Internet and other related services such as Web site building and virtual hosting. An ISP has the equipment and the telecommunication line access required to have POP on the Internet for the geographic area served. The larger ISPs have their own high-speed leased lines so that they are less dependent on the telecommunication providers and can provide better service to their customers. Among the largest national and regional ISPs are AT&T WorldNet, IBM Global Network, MCI, Netcom, UUNet, and PSINet.
ISPs also include regional providers such as New England’s NEARNet and the San Francisco Bay area BARNet. They also include thousands of local providers. In addition, Internet users can also get access through online service providers (OSP) such as America Online and Compuserve.
The larger ISPs interconnect with each other through MAE (ISP switching centers run by MCI WorldCom) or similar centers. The arrangements they make to exchange traffic are known as peering agreements. There are several very comprehensive lists of ISPs world-wide available on the Web.
An ISP is also sometimes referred to as an IAP (Internet access provider). ISP is sometimes used as an abbreviation for independent service provider to distinguish a service provider that is an independent, separate company from a telephone company.
10BASE-T, one of several physical media specified in the IEEE 802.3 standard for Ethernet local area networks (LANs), is ordinary telephone twisted pair wire. 10BASE-T supports Ethernet’s 10 Mbps transmission speed. In addition to 10BASE-T, 10 megabit Ethernet can be implemented with these media types:
| · 10BASE-2 (Thinwire coaxial cable with a maximum segment length of 185 meters)
· 10BASE-5 (Thickwire coaxial cable with a maximum segment length of 500 meters)
· 10BASE-F (optical fiber cable)
· 10BASE-36 (broadband coaxial cable carrying multiple baseband channels for a maximum length of 3,600 meters) |
This designation is an Institute of Electrical and Electronics Engineers (IEEE) shorthand identifier. The “10” in the media type designation refers to the transmission speed of 10 Mbps. The “BASE” refers to baseband signalling, which means that only Ethernet signals are carried on the medium. The “T” represents twisted-pair; the “F” represents fiber optic cable; and the “2”, “5”, and “36” refer to the coaxial cable segment length (the 185 meter length has been rounded up to “2” for 200).
A local area network (LAN) is a group of computers and associated devices that share a common communications line and typically share the resources of a single processor or server within a small geographic area (for example, within an office building). Usually, the server has applications and data storage that are shared in common by multiple computer users. A local area network may serve as few as two or three users (for example, in a home network) or many as thousands of users (for example, in an FDDI network).
In 100 Mbps (megabits per second) Ethernet (known as Fast Ethernet), there are three types of physical wiring that can carry signals:
| · 100BASE-T4 (four pairs of telephone twisted pair wire)
· 100BASE-TX (two pairs of data grade twisted-pair wire)
· 100BASE-FX (a two-strand optical fiber cable) |
This designation is an Institute of Electrical and Electronics Engineers shorthand identifier. The “100” in the media type designation refers to the transmission speed of 100 Mbps. The “BASE” refers to baseband signalling, which means that only Ethernet signals are carried on the medium. The “T4,” “TX,” and “FX” refer to the physical medium that carries the signal. (Through repeaters, media segments of different physical types can be used in the same system.)
The TX and FX types together are sometimes referred to as “100BASE-X.” (The designation for “100BASE-T” is also sometimes seen as “100BaseT.”)
Gbps stands for billions of bits per second and is a measure of bandwidth on a digital data transmission medium such as optical fiber. With slower media and protocols, bandwidth may be in the Mbps (millions of bits or megabits per second) or the Kbps (thousands of bits or kilobits per second) range.
In general, synchronous (pronounced SIHN-kro-nuhs, from Greek syn-, meaning “with,” and chronos, meaning “time“) is an adjective describing objects or events that are coordinated in time. In information technology, the term has several different usages.
1) In telecommunication signaling within a network or between networks, synchronous signals are those that occur at the same clock rate and all clocks are based on a single reference clock. (plesiochronous signals are almost but not quite in synchronization (and a method is used to adjust them) and asynchronous signals are those that run from different clocks or at a different transition rate.)
2) In program-to-program communication, synchronous communication requires that each end of an exchange of communication respond in turn without initiating a new communication. A typical activity that might use a synchronous protocol would be a transmission of files from one point to another. As each transmission is received, a response is returned indicating success or the need to resend. Each successive transmission of data requires a response to the previous transmission before a new one can be initiated.
In general, asynchronous (pronounced ay-SIHN-kro-nuhs, from Greek asyn-, meaning “not with,” and chronos, meaning “time“) is an adjective describing objects or events that are not coordinated in time. In information technology, the term has several different usages.
1) In telecommunication signaling within a network or between networks, an asynchronous signal is one that is transmitted at a different clock rate than another signal. (Plesiochronous signals are almost but not quite in synchronization – and a method is used to adjust them – and synchronous signals are those that run at the same clock rate.
2) In computer programs, asynchronous operation means that a process operates independently of other processes, whereas synchronous operation means that the process runs only as a result of some other process being completed or handing off operation. A typical activity that might use a synchronous protocol would be a transmission of files from one point to another. As each transmission is received, a response is returned indicating success or the need to resend. Each successive transmission of data requires a response to the previous transmission before a new one can be initiated.
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