题目内容
SDLC was invented by IBM to replace the older Bisynchronous protocol for wide area connections between IBM equipment. A varietion of the (71) protocol called NRM is essentially the same as SDLC. SDLC is (72) protocol like HDLC, Frame Relay or X.25.An SDLC network is made up of (73) that controls all communications, and (74) . It is capable of (75) operation, but almost all practical applications are strictly half duplex.
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Frame Relay is simplified form of (51) , similar in principle to (52) , in which synchronous, frames of data are routed to different destinations depending on header information. Packets are routed throught one or more Virtual Circuits known as (53) . Most Virtual Circuits are (54) , which means that the network provider sets up all DLCI connections at subscription time. (55) are also part of the Frame Relay specification. They privide a link that only lasts only as long as the session.
Packet-switching wireless networks are preferable (41) when transmissions are (42) because of the way charges are (43) per packet. Circuit-switched networks are preferable for transferring large files or for other lengthy transmissions because customers are (44) for the (45) of time they use the net-work.
Networks can be interconnected by different devices in the physical layer networks can be connected by (1) 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 (2) examine the MAC address and forward the frames to a different network while doing minor protocol translation in the process in me network layer, we have routers that can connect two networks, If two networks have (3) 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 (4) . As an example, gateways between Internet e-mail and X.400 e-mail must (5) the e-mail message and change various header fields.
In the following essay, each blank has four choices. Choose the most suitable one from the four choices and write down in the answer sheet. The key to SNMP (76) is its simplicity. It has a small command set that does a good job of collecting information from (77) any network device. In a SNMP environment, (78) of the work is handled by the network management system. Devices that are being managed are not (79) with processing (80) that might affect their performance.
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