参考答案：
1．a．           2．b．      3．c．          4．d．     5．a． 
6．b．           7．c．      8．d．         9．a．     10．b．
11．c．          12．d．     13．a．        14．d．     15．a
16．d            17．a．     18．b．        19．c．     20．d．
21．a．          22．b．     23．c．        24．d．     25．a．
26．b．          27．c．     28．d．        29．a．     30．b．
31．c．          32．d．     33．a．        34．b．     35．c．
36．d．          37．a．     38．b．        39．c．     40．d．
41．a．          42．b．     43．c．        44．d．     45．a．
flow control is a function that prevents network congestion by ensuring that _(1)_ do not overwhelm_(2)_ with data.
there are three commonly used methods for handling network congestion . _(3)_is used by network devices to temporarily store bursts of excess data in memory until they can be processed. _(4)_ are used by receiving devices to help prevent their buffers from overflowing . _(5)_ is a flow-control scheme in which the source device requires an acknowledgment from the destination after a certain number of packets have been transmitted.
⑴⑵a.transmitting devices   b.transmitted devices
c.receiving devices    d.received devices
⑶⑷⑸a.buffer      b.source-quench messages 
   c.window   d.windowing      e.buffering
answer：acebd
the integrated services digital network(isdn) is a set of international standards for access to advanced,all-digital public _(1)_network.
isdn standards have been defined by the _(1)_,a branch of the united nations’ internaational telecommunications union (itu), in the series i and q recommendations.
there are five isdn standard devices.a_(1)_ is any piece of communicating equipment that complies with the isdn standard. _(1)_performs the logical interface functions of switching and local-device control. _(1)_performs the physical interface conversion between the dissimilar customer and network sides of the interface.
⑴a.information b.telecommunications c.global  d.local
⑵a.itu-t  b.itu-u   c. itu-a  d. itu-r
⑶⑷⑸a.ta  b.te  c.nt1  d.nt2  e.et
answer：babcd
sdlc was invented by ibm to replace the older bisynchronous protocol for wide area connections between ibm equipment . a varietion of the _(1)_ protocol called nrm is essentially the same as sdlc.
sdlc is _(2)_ protocol like hdlc , frame relay or x.25.an sdlc network is made up of _(3)_ that controls all communications , and _(4)_ .it is capable of _(5)_ operation, but almost all practical applications aare strictly half duplex.
⑴a.hdlc   b.frame relay c.x.25   d.x.21
⑵a.a peer to peer     b.a point-to-point  
c.not a peer to peer    d.a peer to peers
⑶a.a primary station    b.one or more primary stations   
c.a secondary station       d.one or more secondary stations
⑷a.a primary station    b.one or more primary stations   
c.a secondary station       d.one or more secondary stations
⑸a.half duplex      b.full duplex   
c.half or full duplex    d. half and full duplex