题目内容
Networks can be interconnected by different devices. In the physical layer, networks can be connected by. (31) or hubs, which just move the bits from one network to an identical network. One layer up we find bridges and switches, which operate at data link layer. They can accept (32) , examine the MAC address, and forward the frames to a different network while doing minor protocol translation in the process. In the network layer, we have routers that can connect two networks. If two networks have (33) network layer, the router may be able to translate between the packet formats. In the transport layer we find transport gateway, which can interface between two transport connections. Finally, in the application layer, application gateways translate message (34) . As an example, gateways between Internet E-mail and X. 400 e-mail must (35) the e-mail message and change various header fields. （34）是（）
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Networks can be interconnected by different devices. In the physical layer, networks can be connected by. (31) or hubs, which just move the bits from one network to an identical network. One layer up we find bridges and switches, which operate at data link layer. They can accept (32) , examine the MAC address, and forward the frames to a different network while doing minor protocol translation in the process. In the network layer, we have routers that can connect two networks. If two networks have (33) network layer, the router may be able to translate between the packet formats. In the transport layer we find transport gateway, which can interface between two transport connections. Finally, in the application layer, application gateways translate message (34) . As an example, gateways between Internet E-mail and X. 400 e-mail must (35) the e-mail message and change various header fields. （32）是（）
The usual way to ensure reliable delivery is to provide the (16) with some feedback about what is happening at the other end of the line. Typically, the protocol calls for the receiver to send back special (17) frame bearing positive or negative (18) about the incoming frames. If the sender receives a positive acknowledgement about a frame, it knows the frame has arrived safely.On the other hand, a negative acknowledgement means that something has gone wrong, and the frame must be transmitted again. An additional complication comes from the possibility that hardware troubles may cause a frame to (19) completely. In this case, the receiver will not react at all, since it has no any reason to react. It should be clear that a protocol in which the sender transmits a frame and then waits for an acknowledgement, positive or negative, will hang forever if a frame is ever lost due to, for example, (20) hardware. （17）是（）
MIDI enables people to use (51) computers and electronic musical instruments. There are actually three components to MIDI, the communications " (52) ", the Hardware Interface and a distribution (53) called "Standard MIDI Files". In the context of the WWW, the most interesting component is the (54) Format. In principle, MIDI files contain sequences of MIDI Protocol messages. However, when MIDI Protocol (55) are stored in MIDI files, the events are also time-stamped for playback in the proper sequence. Music delivered by MIDI files is the most common use of MIDI today. （53）是（）
The purpose of the requirements definition phase is to produce a clear, complete, consistent, and testable (36) of the technical requirements for the software product.During the requirements definition phase, the requirements definition team uses an iterative process to expand a broad statement of the system requirements into a complete and detailed specification of each function that the software must perform and each (37) that it must meet. The starting point is usually a set of high-level requirements from the (38) that describe the project or problem.In either case, the requirements definition team formulates an overall concept for the system and then defines (39) showing how the system will be operated, publishes the system and operations concept document, and conducts a system concept review (SCR).Following the SCR, the team derives (40) requirements for the system from the high level requirements and the system and operations concept. Using structured or object-oriented analysis the team specifies the software functions and algorithms needed to satisfy each detailed requirement. （36）是（）
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