● The TTL field was originally designed to hold a time stamp, which was decremented by each visited router. The datagram was (   ) when the value became zero. However, for this scheme, all the machines must have synchronized clocks and must know how long it takes for a datagram to go from one machine to another. Today, this field is used mostly to control the (   ) number of hops (routers) visited by the datagram. When a source host sends the datagram, it (   ) a number in this field. Each router that processes the datagram decrements this number by 1. If this value, after being decremented, is zero, the router discards the datagram.This field is needed because routing tables in the Internet can become corrupted.A datagram may travel between two or more routers for a long time without ever getting delivered to the (   ).This field limits the (   ) of a datagram.

（1）A．received

B.discarded

C.rejected

D.tansferred

（2）A．maximum

B.minimum

C.exact

D.certain

（3）A．controls

B.transmits

C.stores

D.receives

（4）A．switch

B.router

C.source host

D.destination host

（5）A．lifetime

B.moving time

C.receiving time

D.transmitting time

● With circuit switching a __(1)__path is established between two stations for communication . Switching and transmission resources within the network are __(2)__for the exclusive use of  the circuit for the duration of the connection . The connection is__(3)__：Once it is established ，It  appears to attached devises as if there were a direct connection. Packet switching was  designed to  provide a more efficient faciltiy than circult switching for (   ) data tratffic.  Each packet contains some portion of the user data plus control information needed for proper functioning of the network. A  key distinguishing element of packet_switching networks is  whether the internal operation is datagram or virtual circuit . With internal virtual circuits,a route is defined between two endpoints and all packets for  that virtual circuit follow the__(4)__ route, With internal datagrams,each packet is treated independently,and packets intended for the same destination may follow different routes.

（1）A．unique

B. dedicated

C. nondedicated

D.independent

（2）A．discarded

B. abandoned

C. reserved

D. broken

（3）A． indistinct

B.direct

C.indirect

D. transparent

（4）A．casual

B.bursty

C.limited

D.abundant

（5）A． same

B. different

C. single

D.multiple

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