41● Networks can be interconnected by different devices.In the physical layer, networks can be connected by （66） 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 （67） , 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 （68） network layer, the router may be able to translate between the packer 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 （69） . As an example, gateways between Internet e-mail and X.400 e-mail must （70） the e-mail message and change various header fields.
A.repeaters
B.relays
C.connectors
D.modems
开始考试练习点击查看答案42● The purpose of the requirements definition phase is to produce a clear, complete, consistent, and testable （71） 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 （72） that it must meet.The starting point is usually a set of high-level requirements from the （73） that describe the project or problem.In either case, the requirements definition team formulates an overall concept for the system and then defines （74） 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 （75） 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.
A.function
B.definition
C.specification
D.statement
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