1、附录 A 外文翻译 -原文部分 Internetworking and Layered Models The Internet today is a widespread information infrastructure, but it is inherently an insecure channel for sending messages. When a message (or packet) is sent from one Website to another, the data contained in the message are routed through a numb
2、er of intermediate sites before reaching its destination. The Internet was designed to accommodate heterogeneous platforms so that people who are using different computers and operating systems can communicate. The history of the Internet is complex and involves many aspects technological, organisat
3、ional and community. Early arpanet researchers accomplished the initial demonstrations of packetswitching technology. In the late 1970s, the growth of the Internet was recognised and subsequently a growth in the size of the interested research community was accompanied by an increased need for a coo
4、rdination mechanism. The Defense Advanced Research Projects Agency (DARPA) then formed an International Cooperation Board (ICB) to coordinate activities with some European countries centered on packet satellite research, while the Internet Configuration Control Board (ICCB) assisted DARPA in managin
5、g Internet activity. In 1983, DARPA recognised that the continuing growth of the Internet community demanded a restructuring of coordination mechanisms. The ICCB was disbanded and in its place the Internet Activities Board (IAB) was formed from the chairs of the Task Forces. The IAB revitalised the
6、Internet Engineering Task Force (IETF) as a member of the IAB. By 1985, there was a tremendous growth in the more practical engineering side of the Internet. This growth resulted in the creation of a substructure to the IETF in the form of working groups. DARPA was no longer the major player in the
7、funding of the Internet. Since then, there has been a significant decrease in Internet activity at DARPA. The IAB recognised the increasing importance of IETF, and restructured to recognise the Internet Engineering Steering Group (IESG) as the major standards review body. Since the early 1980s, the
8、Internet has grown beyond its primarily research roots, to include both a broad user community and increased commercial activity. This growth in the commercial sector brought increasing concern regarding the standards process. Increased attention was paid to making progress, eventually leading to th
9、e formation of the Internet Society in 1991. In 1992, the Internet Activities Board was reorganised and renamed the Internet Architecture board (IAB) operating under the auspices of the Internet Society. The mutually supportive relationship between the new IAB, IESG and IETF led to them taking more
10、responsibility for the approval of standards, along with the provision of services and other measures which would facilitate the work of the IETF. 1.1 Networking Technology Data signals are transmitted from one device to another using one or more types of transmission media, including twisted-pair c
11、able, coaxial cable and fibre-opticable. A message to be transmitted is the basic unit of network communications. A message may consist of one or more cells, frames or packets which are the elemental units for network communications. Networking technology includes everything from local area networks
12、 (LANs) in a limited geographic area such as a single building, department or campus to wide area networks (WANs) over large geographical areas that may comprise a country, a continent or even the whole world. 1.1.1 Local Area Networks (LANs) A local area network (LAN) is a communication system that
13、 allows a number of independent devices to communicate directly with each other in a limited geographic area such as a single office building, a warehouse or a campus. LANs are standardised by three architectural structures: Ethernet, token ring and fibre distributed data interface (FDDI). (1) Ether
14、net Ethernet is a LAN standard originally developed by Xerox and later extended by a joint venture between Digital Equipment Corporation (DEC), Intel Corporation and Xerox. The access mechanism used in an Ethernet is called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). In CSMA/CD
15、, before a station transmits data, it must check the medium where any other station is currently using the medium. If no other station is transmitting, the station can send its data. If two or more stations send data at the same time, it may result in a collision. Therefore, all stations should cont
16、inuously check the medium to detect any collision. If a collision occurs, all stations ignore the data received. The sending stations wait for a period of time before resending the data. To reduce the possibility of a second collision, the sending stations individually generate a random number that
17、determinates how long the station should wait before resending data. (2) Token Ring Token ring, a LAN standard originally developed by IBM, uses a logical ring topology. The access method used by CSMA/CD may result in collisions. Therefore, stations may attempt to send data many times before a trans
18、mission captures a perfect link. This redundancy can create delays of indeterminable length if traffic is heavy. There is no way to predict either the occurrence of collisions or the delays produced by multiple stations attempting to capture the link at the same time. Token ring resolves this uncert
19、ainty by making stations take turns in sending data. As an access method, the token is passed from station to station in sequence until it encounters a station with data to send. The station to be sent data waits for the token. The station then captures the token and sends its data frame. This data
20、frame proceeds around.the ring and each station regenerates the frame. Each intermediate station examines the destination address, finds that the frame is addressed to another station, and relays it to its neighbouring station. The intended recipient recognises its own address, copies the message, c
21、hecks for errors and changes four bits in the last byte of the frame to indicate that the address has been recognised and the frame copied. The full packet then continues around the ring until it returns to the station that sent it. (3) Fiber Distributed Data Interface (FDDI) FDDI is a LAN protocol
22、standardised by ANSI and ITU-T. It supports data rates of 100 Mbps and provides a high-speed alternative to Ethernet and token ring. When FDDI was designed, the data rate of 100 Mbps required fibre-optic cable. The access method in FDDI is also called token passing. In a token ring network, a statio
23、n can send only one frame each time it captures the token. In FDDI, the token passing mechanism is slightly different in that access is limited by time. Each station keeps a timer which shows when the token should leave the station. If a station receives the token earlier than the designated time, i
24、t can keep the token and send data until the scheduled leaving time. On the other hand, if a station receives the token at the designated time or later than this time, it should let the token pass to the next station and wait for its next turn. FDDI is implemented as a dual ring. In most cases, data
25、 transmission is confined to the primary ring. The secondary ring is provided in case of the primary rings failure. When a problem occurs on the primary ring, the secondary ring can be activated to complete data circuits and maintain service. 1.1.2 Wide Area Networks (WANs) A WAN provides long-dista
26、nce transmission of data, voice, image and video informationover large geographical areas that may comprise a country, a continent or even the world.In contrast to LANs (which depend on their own hardware for transmission), WANs can utilise public, leased or private communication devices, usually in
27、 combination. (1) PPP The Point-to-Point Protocol (PPP) is designed to handle the transfer of data using either asynchronous modem links or high-speed synchronous leased lines. The PPP frame uses the following format: Flag field: Each frame starts with a one-byte flag whose value is 7E(0111 1110). T
28、he flag is used for synchronisation at the bit level between the sender and receiver. Address field: This field has the value of FF(1111 1111). Control field: This field has the value of 03(0000 0011). Protocol field: This is a two-byte field whose value is 0021(0000 0000 0010 0001) for TCP/IP. Data
29、 field: The data field ranges up to 1500 bytes. CRC: This is a two-byte cyclic redundancy check. Cyclic redundancy check (CRC) is implemented in the physical layer for use in the data link layer. A sequence of redundant bits (CRC) is appended to the end of a data unit so that the resulting data unit
30、 becomes exactly divisible by a predetermined binary number. At its destination, the incoming data unit is divided by the same number. If there is no remainder, the data unit is accepted. If a remainder exists, the data unit has been damaged in transit and therefore must be rejected. (2) X.25 X.25 i
31、s widely used, as the packet switching protocol provided for use in a WAN. It was developed by the ITU-T in 1976. X.25 is an interface between data terminal equipment and data circuit terminating equipment for terminal operations at the packet mode on a public data network. X.25 defines how a packet
32、 mode terminal can be connected to a packet network for the exchange of data. It describes the procedures necessary for establishing connection, data exchange, acknowledgement, flow control and data control. (3)Asynchronous Transfer Mode (ATM) ATM is a revolutionary idea for restructuring the infras
33、tructure of data communication. It is designed to support the transmission of data, voice and video through a high data-rate transmission medium such as fibre-optic cable. ATM is a protocol for transferring cells. A cell is a small data unit of 53 bytes long, made of a 5-byte header and a 48-byte pa
34、yload. The header contains a virtual path identifier (VPI) and a virtual channel identifier (VCI). These two identifiers are used to route the cell through the network to the final destination. An ATM network is a connection-oriented cell switching network. This means that the unit of data is not a packet as in a packet switching network, or a frame as in a frame relay, but a cell.
