求达人帮忙解答！！[求助]

alextxc

最好能给出推导过程，谢谢！！


l         用户A与用户B通过卫星链路通信时，传播延迟为270ms，假设数据速率是64Kb/s，帧长4000bit，若采用停等流控协议通信，则最大链路利用率为（30）；

若采用后退N帧ARQ协议通信，发送窗口为8，则最大链路利用率可以达到（31）。

（30）A、0.104         B、0.116         C、0.188               D、0.231
（31）A、0.416         B、0.464           C、0.752               D、0.832

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 swiches which operate at data link layer.They can accept (67) examine the MAC address and forward the frames to a differdnt 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 (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 Intemet e-mail and X.400 e-mail must (70) the e-mail message and change various header fields.

（66）A.reapers             B.relays           C.packages       D.modems

（67）A.frimes              B.packets          C.packages       D.cells

（68）A.special             B.dependent         C.similar        D.dissimilar

（69）A.syntax             B.semantics          C.language      D.format

（70）A.analyze            B.parse              C.delete        D.create