《电子工程师手册》学习资料（英文版）Chapter 92 Computer Networks.pdf_大学文库

92 Computer Networks 92.2 Local Area Network Carrier Sense Buses. Token Ring. Token Bus.Wireless LANs. Asyncronous Transfer Mode(ATM) LANs Private Branch metropolitan Area Network Thomas g. robertazzi tate University of New York 92.4 Wide Area Network 92.5 The Future 92.1 Introduction Computer networks are geographically distributed collections of communication links and switching proces sors, the purpose of which is to transport data between computers, workstations, and terminals. In general the elements of a computer network must follow compatible rules of operation together to function effectively. These rules of operation are known as protocols. There are three broad categories of computer networks, distinguished by geographical extent. Local area networks(LANs)connect computer equipment in a single building or floor of a building. Metropolitan area networks(MANs) interconnect network users over a campus or metropolitan-sized region. Finally, wide area networks(WANs)interconnect users on a national or an international scale. In the following, key features of these types of networks will be outlined. 92.2 Local area networks There are six main types of local network architectures that have been commercially produced to date: carrier ense multiple-access buses with collision detection, token rings, token buses, wireless LANs, ATM LANS, and private branch exchanges. The first four have been standardized in the IEEE 802 series standards ner se Buses The IEEE 802.3 standard deals with a network architecture and protocol first constructed at Xerox in the 1970s is usually run through the ceiling near each user's computer equipment o% and termed Ethernet. All stations in an Ethernet can be connected, through interfaces, to a coaxial cable that The coaxial cable essentially acts as a private radio channel for the users. An interesting protocol called sense multiple-access with collision detection(CSMA/CD) is used in such rk. Each station cons monitors the cable and can detect when it is idle(no user transmitting), when one user is transm (successfully), or when more than one user is simultaneously transmitting (resulting in an unsuccessful collision on the channel). The cable basically acts as a broadcast bus. Any station can transmit on the cable if the station detects it to be idle. Once a station transmits, other stations will not interrupt the transmission. As there is no central control in the network, occasionally two or more stations may attempt to transmit at about the c 2000 by CRC Press LLC

© 2000 by CRC Press LLC 92 Computer Networks 92.1 Introduction 92.2 Local Area Networks Carrier Sense Buses • Token Ring • Token Bus • Wireless LANs • Asyncronous Transfer Mode (ATM) LANs Private Branch Exchange 92.3 Metropolitan Area Networks FDDI • DQDB 92.4 Wide Area Networks 92.5 The Future 92.1 Introduction Computer networks are geographically distributed collections of communication links and switching processors, the purpose of which is to transport data between computers, workstations, and terminals. In general the elements of a computer network must follow compatible rules of operation together to function effectively. These rules of operation are known as protocols. There are three broad categories of computer networks, distinguished by geographical extent. Local area networks (LANs) connect computer equipment in a single building or floor of a building. Metropolitan area networks (MANs) interconnect network users over a campus or metropolitan-sized region. Finally, wide area networks (WANs) interconnect users on a national or an international scale. In the following, key features of these types of networks will be outlined. 92.2 Local Area Networks There are six main types of local network architectures that have been commercially produced to date: carrier sense multiple-access buses with collision detection, token rings, token buses, wireless LANs, ATM LANs, and private branch exchanges. The first four have been standardized in the IEEE 802 series standards. Carrier Sense Buses The IEEE 802.3 standard deals with a network architecture and protocol first constructed at Xerox in the 1970s and termed Ethernet. All stations in an Ethernet can be connected, through interfaces, to a coaxial cable that is usually run through the ceiling near each user’s computer equipment. The coaxial cable essentially acts as a private radio channel for the users.An interesting protocol called carrier sense multiple-access with collision detection (CSMA/CD) is used in such a network. Each station constantly monitors the cable and can detect when it is idle (no user transmitting), when one user is transmitting (successfully), or when more than one user is simultaneously transmitting (resulting in an unsuccessful collision on the channel). The cable basically acts as a broadcast bus. Any station can transmit on the cable if the station detects it to be idle. Once a station transmits, other stations will not interrupt the transmission. As there is no central control in the network, occasionally two or more stations may attempt to transmit at about the same Thomas G. Robertazzi State University of New York, Stony Brook

File time. The transmissions will overlap and be unintelligible(collision). The transmitting stations will detect suck a situation and each will retransmit at a randomly chosen later time. Ethernet and 802.3 networks have raw speeds of up to 10 million bits per second(Mbps). Idle time and llisions, however, can reduce the useful information throughput significantly. The maximum length of these networks is limited by signal propagation delay. An 802.3 coaxial bus differs from an internal computer bus in size and in the lack of a bus controller. Ethernet connections can also be made over unshielded twisted pair or for long runs, over fiber optics Since the introduction of ethernet more than a dozen years ago, desktop computer capabilities and network ing requirements have increased significantly. To meet these demands a 100-Mb/s(fast) ethernet has been developed by a consortium of companies. This will be standardized within IEEE 802.3. Media options for the 100-Mb/s ethernet include shielded or unshielded twisted pair as well as fiber optics. wiring distances of up to 100 m between an end system and wiring closet can be supported. Adapters as well as repeaters that can operate at either 10 or 100 Mb/s will be available. In 1994 there were 50 million ethernet nodes with 15 million new nodes being added each year. Token ring Token ring LANs were developed by IBM in the early 1980s. Topologically, stations are arranged in a circle with point-to-point links between neighbors. Transmissions flow in only one direction(clockwise or counter clockwise). A message transmitted is relayed over the point-to-point links to the receiving station and then forwarded around the rest of the ring and back to the sender to serve as an acknowledgment Only a station possessing a special digital code word known as a token may transmit. when a station is finished transmitting, it passes the token to its downstream neighbor. Thus, there are no collisions in a token ring, and utilization can approach 100% under heavy loads. Because of the use of point-to-point links, token rings can use various transmission media such as twis pair wire or fiber-optic cables. The transmission speed of a token ring can range from 1 to 16 Mbps, depend on the type of point-to-point links used. Token rings are often wired in star configurations for ease of nodes with 3-4 million new nodes being added each yeals ndard. In 1994 there were 10 million token ring installation. Token rings are covered by the IEEE 802.5 sta Token bus a token bus uses a coaxial cable along with the token concept to produce a Lan with improved throughput ompared to the 802.3 protocol. That is, stations pass a token from one to another to determine which station currently has permission to transmit. Also, in a token bus(and in a token ring), response times can be e 2000 by CRC Press LLC

© 2000 by CRC Press LLC time. The transmissions will overlap and be unintelligible (collision). The transmitting stations will detect such a situation and each will retransmit at a randomly chosen later time. Ethernet and 802.3 networks have raw speeds of up to 10 million bits per second (Mbps). Idle time and collisions, however, can reduce the useful information throughput significantly. The maximum length of these networks is limited by signal propagation delay. An 802.3 coaxial bus differs from an internal computer bus in size and in the lack of a bus controller. Ethernet connections can also be made over unshielded twisted pair or, for long runs, over fiber optics. Since the introduction of ethernet more than a dozen years ago, desktop computer capabilities and networking requirements have increased significantly. To meet these demands a 100-Mb/s (fast) ethernet has been developed by a consortium of companies. This will be standardized within IEEE 802.3. Media options for the 100-Mb/s ethernet include shielded or unshielded twisted pair as well as fiber optics. Wiring distances of up to 100 m between an end system and wiring closet can be supported. Adapters as well as repeaters that can operate at either 10 or 100 Mb/s will be available. In 1994 there were 50 million ethernet nodes with 15 million new nodes being added each year. Token Ring Token ring LANs were developed by IBM in the early 1980s. Topologically, stations are arranged in a circle with point-to-point links between neighbors. Transmissions flow in only one direction (clockwise or counterclockwise). A message transmitted is relayed over the point-to-point links to the receiving station and then forwarded around the rest of the ring and back to the sender to serve as an acknowledgment. Only a station possessing a special digital code word known as a token may transmit. When a station is finished transmitting, it passes the token to its downstream neighbor. Thus, there are no collisions in a token ring, and utilization can approach 100% under heavy loads. Because of the use of point-to-point links, token rings can use various transmission media such as twistedpair wire or fiber-optic cables. The transmission speed of a token ring can range from 1 to 16 Mbps, depending on the type of point-to-point links used. Token rings are often wired in star configurations for ease of installation. Token rings are covered by the IEEE 802.5 standard. In 1994 there were 10 million token ring nodes with 3–4 million new nodes being added each year. Token Bus A token bus uses a coaxial cable along with the token concept to produce a LAN with improved throughput compared to the 802.3 protocol. That is, stations pass a token from one to another to determine which station currently has permission to transmit. Also, in a token bus (and in a token ring), response times can be FIGURE 92.1 Bus-type local area network

One possible way around the problem of the limited spectrum available for wireless LANs is to use infrared light as the transmission medium. There are both direct infrared systems(range: 1-3 mi)and nondirect systems bounced off walls or ceilings). For small areas data rates are consistent with those of existing ethernet and token ring networks with 100-Mb/s systems on the horizon Asyncronous Transfer Mode (ATM) LANs2 ATM LANs are relatively new to the Lan marketplace. This is a packet switching technology utilizing relatively short, fixed length packets to provide networking services. ATM was originally seen as a way to develop the next generation wide area telephone network using packet switching, rather than the more conventional circuit itching technology. It was envisioned as a way to transport video, voice, and data in an integrated network a short packet size was chosen to meet several requirements including minimizing real-time queueing delay While progress on the original goal of using aTm technology in wide area telephone networks proceeded slowly because of the complexity of the challenge and the large investments involved, a number of smaller companies introduced ATM local area network products using much the same technology An ATM lAn consists of a switch into which are wired end users in a star-type topology. There are several possibilities for the internal architecture of the switch. A low-cost switch may essentially be a computer bus in a box. More sophisticated switches may use switching fabrics. These are very large-scale integrated (VLSI implementations of patterned networks of simple switching elements, sometimes referred to as space division switching. A great deal of effort has gone into producing cost-effective ATM switches over the past 12 years. It should be pointed out that many of the issues that are not yet resolved for wide area network ATM (i.e, traffic policing, billing)are more tractable in the private ATM LAN environment. ATM LANs can support a relatively small number of users at high data access rates(low megabit per second) Although ATM is good at handling mixed media traffic at high speeds, it remains to be seen if enougH applications are developed requiring its high-bandwidth capability to make it a success. The cost effectiveness of ATM technology is another issue awaiting resolution Private branch exchange Historically, private branch exchanges(PBXs) were privately owned telephone switching computers that would be placed in the basement of a building and serve to interconnect phones in the building and provide access to outside lines provided by common carriers. However, PBXs are now available that offer both telephone and data service. In a typical system a phone may have a data socket for terminals or workstations. PBXs are wired in a star topology with the PBX at the center of the star and each user wired directly to it. 92.3 Metropolitan Area Networks While several network architectures have been proposed for use as MANs, the two that are closest to widespread commercial implementation are fiber-distributed data interface(FDDI) and distributed queue dual bus (DQDB)interface. A key feature of a MAN is the ability to interconnect LANs. This is a problem because of the high data rates at which LANs operate FDDI The FDDI is similar to a token ring lan except that two rings, instead of one, may be used. Stations needing high-reliability communication are connected to both rings. In the case of a break in the rings the network can e automatically reconfigured. FDDI rings operate at 100 Mbps with a maximum of 500 nodes and a maximum fiber length of 200 km. In fact, most actual FDDI installations have only a small number of nodes(such as routers). There is an American National Standards Institute(ANSI)standard for FDDI. This material was previously published in The Mobile Communications Handbook, J D. Gibson, Ed, Boca Raton, Fla. CRC Press. 1996 e 2000 by CRC Press LLC

© 2000 by CRC Press LLC One possible way around the problem of the limited spectrum available for wireless LANs is to use infrared light as the transmission medium. There are both direct infrared systems (range: 1–3 mi) and nondirect systems (bounced off walls or ceilings). For small areas data rates are consistent with those of existing ethernet and token ring networks with 100-Mb/s systems on the horizon. Asyncronous Transfer Mode (ATM) LANs2 ATM LANs are relatively new to the LAN marketplace. This is a packet switching technology utilizing relatively short, fixed length packets to provide networking services. ATM was originally seen as a way to develop the next generation wide area telephone network using packet switching, rather than the more conventional circuit switching technology. It was envisioned as a way to transport video, voice, and data in an integrated network. A short packet size was chosen to meet several requirements including minimizing real-time queueing delay. While progress on the original goal of using ATM technology in wide area telephone networks proceeded slowly because of the complexity of the challenge and the large investments involved, a number of smaller companies introduced ATM local area network products using much the same technology. An ATM LAN consists of a switch into which are wired end users in a star-type topology. There are several possibilities for the internal architecture of the switch. A low-cost switch may essentially be a computer bus in a box. More sophisticated switches may use switching fabrics. These are very large-scale integrated (VLSI) implementations of patterned networks of simple switching elements, sometimes referred to as space division switching. A great deal of effort has gone into producing cost-effective ATM switches over the past 12 years. It should be pointed out that many of the issues that are not yet resolved for wide area network ATM (i.e., traffic policing, billing) are more tractable in the private ATM LAN environment. ATM LANs can support a relatively small number of users at high data access rates (low megabit per second). Although ATM is good at handling mixed media traffic at high speeds, it remains to be seen if enough applications are developed requiring its high-bandwidth capability to make it a success. The cost effectiveness of ATM technology is another issue awaiting resolution. Private Branch Exchange Historically, private branch exchanges (PBXs) were privately owned telephone switching computers that would be placed in the basement of a building and serve to interconnect phones in the building and provide access to outside lines provided by common carriers. However, PBXs are now available that offer both telephone and data service. In a typical system a phone may have a data socket for terminals or workstations. PBXs are wired in a star topology with the PBX at the center of the star and each user wired directly to it. 92.3 Metropolitan Area Networks While several network architectures have been proposed for use as MANs, the two that are closest to widespread commercial implementation are fiber-distributed data interface (FDDI) and distributed queue dual bus (DQDB) interface. A key feature of a MAN is the ability to interconnect LANs. This is a problem because of the high data rates at which LANs operate. FDDI The FDDI is similar to a token ring LAN except that two rings, instead of one, may be used. Stations needing high-reliability communication are connected to both rings. In the case of a break in the rings the network can be automatically reconfigured. FDDI rings operate at 100 Mbps with a maximum of 500 nodes and a maximum fiber length of 200 km. In fact, most actual FDDI installations have only a small number of nodes (such as routers). There is an American National Standards Institute (ANSI) standard for FDDI. 2 This material was previously published in The Mobile Communications Handbook, J. D. Gibson, Ed., Boca Raton, Fla.: CRC Press, 1996

The DqDb forms the basis of the IEEE 802. 6 standard for MANs DQDB is descended from the earlier QPSX, which was developed at the University of Western Australia and Telecom Australia. DQDB uses two unidirec tional linear fiber-optic buses. Stations are connected to both buses. Through the clever use of counters the DQDB protocol provides approximate first in, first out(FIFO) service to arriving packets. There are no collisions in DQDB, so utilization can approach 100%. Bus speeds of 150 Mbps are possible. 92.4 Wide Area Networks Data are generally transmitted over long distances by wide area packet networks. These networks generally lease telephone lines from telecommunications carriers that are used to carry data exclusively Packet switching technology was first used on a large scale in the arpanet beginning in the 1960s. The Internet(which replaced the earlier ARPANET)serves to connect universities, industrial and government research centers, and private users One problem area unique to wide area packet networks is that of routing. Unlike the previously mentioned networks, there are usually multiple routes available between sources and destinations. Distributed routing algorithms have been developed that route based on current traffic conditio 92.5 The future The future is likely to see an increase in data rates as fiber-optic cables are widely deployed. This will spur the development of faster switching nodes through the use of parallel processing and VLSI implementation. Protocols will have to be simplified to increase processor throughput. New forms of traffic such as video and graphics will become more important. Computer networks will proliferate throughout the world, making possible the ubiquitous transport of data between any two points. These networks are likely to consist of both private networks and new service offerings from telecommunications companies. Defining Terms Area networks: LAN, within single building: MAN, metropolitan-sized region; WAN, national/international Coaxial cable: A shielded cable that conducts electrical signals and is used in bus-type local area networks. Fiber-optic cable: a glass fiber cable that conducts light signals and can be used in token ring local area networks and metropolitan area networks. Fiber optics can provide higher data rates than coaxial cable. They are also immune to electrical interference. IEEE standards: 802.3, CSMA/CD bus; 802.4, token bus; 802.5, token ring; 802.6, DQDB MAN. Related Topics 72. 3 Local-Area Networks .75.3 Stochastic Processes References U. Black, Data Networks: Concepts, Theory and Practice, Englewood Cliffs, N.J. Prentice-Hall, 1989 M. De Prycker, Asynchronous Transfer Mode: Solution for Broadband ISDN, New York: Simon and Schuster, 1991 A. De Simone and S Nanda,Wireless data: Systems, standards, services, Wireless Networks, 1(3): 241-253, 1995 N. J. Muller, Wireless Data Networking, Boston, Mass: Artech House, 1995 L. Peterson and B Davie, Computer Networks: A System Approach, San Francisco, Calif Morgan Kaufman, 1995 T G. Robertazzi, Performance Evaluation of High Speed Switching Fabrics and Networks: ATM, Broadband ISDN and MAN Technolgoy, Piscataway N.J. IEEE Press, 1993 P Scherer, The 100 Mbps Ethernet Standard In Distinguished Lecture Series(IX)(videotape), Stanford, Calif Univ Video Communications, 1994 e 2000 by CRC Press LLC

© 2000 by CRC Press LLC DQDB The DQDB forms the basis of the IEEE 802.6 standard for MANs. DQDB is descended from the earlier QPSX, which was developed at the University of Western Australia and Telecom Australia. DQDB uses two unidirectional linear fiber-optic buses. Stations are connected to both buses. Through the clever use of counters the DQDB protocol provides approximate first in, first out (FIFO) service to arriving packets. There are no collisions in DQDB, so utilization can approach 100%. Bus speeds of 150 Mbps are possible. 92.4 Wide Area Networks Data are generally transmitted over long distances by wide area packet networks. These networks generally lease telephone lines from telecommunications carriers that are used to carry data exclusively.Packet switching technology was first used on a large scale in the ARPANET beginning in the 1960s. The Internet (which replaced the earlier ARPANET) serves to connect universities, industrial and government research centers, and private users. One problem area unique to wide area packet networks is that of routing. Unlike the previously mentioned networks, there are usually multiple routes available between sources and destinations. Distributed routing algorithms have been developed that route based on current traffic conditions. 92.5 The Future The future is likely to see an increase in data rates as fiber-optic cables are widely deployed. This will spur the development of faster switching nodes through the use of parallel processing and VLSI implementation. Protocols will have to be simplified to increase processor throughput. New forms of traffic such as video and graphics will become more important. Computer networks will proliferate throughout the world, making possible the ubiquitous transport of data between any two points. These networks are likely to consist of both private networks and new service offerings from telecommunications companies. Defining Terms Area networks: LAN, within single building; MAN, metropolitan-sized region; WAN, national/international region. Coaxial cable: A shielded cable that conducts electrical signals and is used in bus-type local area networks. Fiber-optic cable: A glass fiber cable that conducts light signals and can be used in token ring local area networks and metropolitan area networks. Fiber optics can provide higher data rates than coaxial cable. They are also immune to electrical interference. IEEE standards: 802.3, CSMA/CD bus; 802.4, token bus; 802.5, token ring; 802.6, DQDB MAN. Related Topics 72.3 Local-Area Networks • 75.3 Stochastic Processes References U. Black, Data Networks: Concepts, Theory and Practice, Englewood Cliffs, N.J.: Prentice-Hall, 1989. M. De Prycker, Asynchronous Transfer Mode: Solution for Broadband ISDN, New York: Simon and Schuster, 1991. A. De Simone and S. Nanda, “Wireless data: Systems, standards, services,” Wireless Networks, 1(3):241–253, 1995. N. J. Muller, Wireless Data Networking, Boston, Mass: Artech House, 1995. L. Peterson and B. Davie, Computer Networks: A System Approach, San Francisco, Calif.: Morgan Kaufman, 1995. T. G. Robertazzi, Performance Evaluation of High Speed Switching Fabrics and Networks: ATM, Broadband ISDN and MAN Technolgoy, Piscataway N.J.: IEEE Press, 1993. P. Scherer, The 100 Mbps Ethernet Standard. In Distinguished Lecture Series (IX) (videotape), Stanford, Calif.: Univ. Video Communications, 1994