题目内容
Both bus and tree topologies are characterized by the use of multipoint （1）. For the bus, all stations attach, through appropriate hardware （2）known as a tap, directly to alinear transmission medium, or bus. Full-duplex operation between the station and the tap allowsdata to be transmitted onto the bus and received from the （3）. A transmission from anystation propagates the length of the medium in both directions and can be received by all other（4）. At each end of the bus is a （5）, which absorbs any signal, removing it from thebus. （2）是（）
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An internet is a combination of networks connected by （1）. When a datagram goes from a source to a （2）, it will probably pass many （3） until it reaches the routerattached to the destination network. A router receives a （4）from a network and passes it toanother network. A router is usually attached to several networks. When it receives a packet, towhich network should it pass the packet The decision is based on optimization: Which of theavailable（5） is the optimum pathway （2）是（）
The presentation layer is concerned with the（1） and semantics of the information exchanged between two systems. The processes in two systems are usually exchanginginformation in the form of character（2）numbers, and so on. The information should bechanged to bit （3） before being transmitted. Because different encoding systems, thepresentation layer is responsible for （4）between these different encoding methods. Thepresentation layer at the sender changes the information from its sender-dependent format into acommon format. The presentation layer at the receiving machine changes the common format intoits （5） dependent format. （2）是（）
A transport layer protocol has several responsibilities, One is to create a process-to-process (program-to-program) communication; TCP uses port（1） to accomplish this. Anotherresponsibility of a transport layer protocol is to create a（2）and error-control mechanism atthe transport level. TCP uses a sliding（3）protocol to achieve flow control. It uses theacknowledgment packet, time-out, and retransmission to achieve（4） control. The transportlayer is also responsible for providing a connection mechanism for the application program. Theapplication program sends （5）of data to the transport layer. It is the responsibility of thetransport layer at the sending station to make a connection with the receiver. （1）是（）
For（1） service, we need a virtual-circuit subnet. Let us see how that works. The idea behind virtual circuits is to avoid having to choose a new （2） for every packet sent. Instead,when a connection is established, a route from the （3）machine to the destination machine ischosen as part of the connection setup and stored in tables inside the（4）That route is used forall traffic flowing over the connection, exactly the same way that the telephone system works.When the connection is released, the virtual circuit is also terminated. With connection-orientedservice, each packet carries an （5）telling which virtual circuit it belongs to. （2）是（）
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