計算機英語課件Unit 8 Computer Network Section C Network Connecting Devices

1、Section C Network Connecting Devices We can divide computer networks into three broad categories: local area networks (LANs), metropolitan area networks (MANs), and wide area networks (WANs). The three network types can be connected using connecting devices. The interconnection of networks makes glo

2、bal communication from one side of the world to the other possible. Connecting devices can be divided into four types based on their functionality as related to the layers in the OSI model: repeaters, bridges, routers, and gateways. Repeaters and bridges typically connect devices in a network. Route

3、rs and gateways typically connect networks into internetworks (Figure 8C-1). A repeater is an electronic device that regenerates data. It extends the physical length of a network. As a signal is transmitted, it may lose strength, and a weak signal may be interpreted erroneously by a receiver. A repe

4、ater can regenerate the signal and send it to the rest of the network. Figure 8C-2 shows a network with and without a repeater. Repeaters operate only in the physical layer of the OSI model. They do not recognize physical or logical addresses. They simply regenerate every signal they receive. Repeat

5、ers, popular when the dominant topology was the bus topology, often connected two buses to increase the length of the network. When a network uses a bus topology, the medium is shared between all stations. In other words, when a station sends a frame, the common bus is occupied by this one station,

6、and no other station is allowed to send a frame (if it does, the two frames collide). This implies a degradation of performance. Stations need to wait until the bus is free. This is similar to an airport that has only one runway. When the runway is used by one aircraft, the other aircraft ready for

7、takeoff must wait. A bridge is a traffic controller. It can divide a long bus into smaller segments so that each segment is independent trafficwise. A bridge installed between two segments can pass or block frames based on the destination address in the frame. If a frame originates in one segment an

8、d the destination is in the same segment, there is no reason for the frame to pass the bridge and go to the other segments. The bridge uses a table to decide if the frame needs to be forwarded to another segment. With a bridge, two or more pairs of stations can communicate at the same time (Figure 8

9、C-3). In addition to its traffic controlling duties, a bridge also functions as a repeater by regenerating the frame. As discussed previously, this means that a bridge operates at the physical layer. But because a bridge needs to interpret the address embedded in the frame to make filtering decision

10、s, it also operates at the data-link layer of the OSI model. In recent years, the need for better performance has led to the design of a new device referred to as a second-layer switch, which is simply a sophisticated bridge with multiple interfaces. For example, a network with 20 stations can be di

11、vided into four segments using a four-interface bridge. Or the same network can be divided into 20 segments (with one station per segment) using a 20-interface switch. A switch in this case increases performance; a station that needs to send a frame sends it directly to the switch. The media are not

12、 shared; each station is directly connected to the switch (Figure8C-4). Routers are devices that connect LANs,MANs, and WANs. A router operates at the third layer of the OSI model. Whereas a bridge filters a frame based on the physical (data-link layer) address of the frame, a router routes a packet

13、 based on the logical (network layer) address of the packet. Whereas a bridge may connect two segments of a LAN or two LANs belonging to the same organization, a router can connect two independent networks: a LAN to a WAN, a LAN to a MAN, a WAN to another WAN, and so on. The result is an internetwor

14、k (or a internet). The Internet (the unique global internet) that connects the whole world together is an example of an internetwork where many networks are connected together through routers. Figure 8C-5 shows an example of an internetwork. Traditionally, a gateway is a connecting device that act a

15、s a protocol converter. It allows two networks, each with a different set of protocols for all seven OSI layers, to be connected to each other and communicate. A gateway is usually a computer installed with the necessary software. The gateway understands the protocols used by each connected network and is therefore able to translate from one to another. For example, a gateway can connect a network using the AppleTalk protocol to a network using the Novell Netware protocol. Today, however, the term gateway is used interchangeably w