{"id":862,"date":"2019-10-13T02:55:23","date_gmt":"2019-10-12T18:55:23","guid":{"rendered":"http:\/\/149.129.74.185\/?p=862"},"modified":"2019-10-13T02:55:23","modified_gmt":"2019-10-12T18:55:23","slug":"%e8%ae%a1%e7%ae%97%e6%9c%ba%e7%bd%91%e7%bb%9c%e7%ac%ac%e4%b8%89%e7%ab%a0-%e4%bd%9c%e4%b8%9a","status":"publish","type":"post","link":"http:\/\/www.magicmipamipa.com\/?p=862","title":{"rendered":"\u8ba1\u7b97\u673a\u7f51\u7edc\u7b2c\u4e09\u7ae0 \u4f5c\u4e1a"},"content":{"rendered":"

Q. <\/b> A bit string, 0111101111101111110, needs to be transmitted at the data link layer. What is the string actually transmitted after bit stuffing? <\/span>
\n <\/span>

\u4e94\u4e2a1\u63d2\u4e2a0<\/p>\n

011110111110011111010<\/span><\/p>\n

\n

Q. <\/b>What is the remainder obtained by dividing x^7 + x^5 + 1 by the generator polynomial x^3 + 1? (give your answer as bit string)<\/span>
\n <\/span>

10100001 \/ 1001 =\u00a0 101110 … 111<\/p>\n

111\u662f\u4f59\u6570<\/p>\n

\n

Q.<\/b>A channel has a bit rate of 4 kbps and a propagation delay of 20 msec. For what range of frame sizes does stop-and-wait give an efficiency of at least 50 percent?<\/span>
\n <\/span>A.<\/b> bits<\/span><\/p>\n

\u53d1\u9001\u4e00\u5e27\u7684\u65f6\u95f4\u7b49\u4e8e\u4fe1\u9053\u4f20\u64ad\u5ef6\u8fdf\u76842\u500d\uff0c\u4fe1\u9053\u5229\u7528\u7387\u4e3a50.<\/p>\n

\u73b0\u5728\u53d1\u9001\u901f\u7387\u4e3a4kbp\/s\uff0c\u53d1\u4e00\u4f4d\u9700\u89810.25\u5fae\u79d2<\/p>\n

\uff0820*10-3<\/sup>*2\uff09\/\uff080.25*10-6<\/sup>\uff09 = 160000 bits<\/p>\n

 <\/p>\n

\n

Q.<\/b>Consider an error-free 64-kbps satellite channel used to send 512-byte data frames in one direction, with very short acknowledgements coming back the other way. What is the maximum throughput for window sizes of 1, 7, 15? The earth-satellite propagation time is 270 msec. (give your answer as an integer)<\/span>
\n <\/span>A.<\/b>A. for window size=1: <\/span> bps <\/span>
\n <\/span>A.<\/b>A. for window size=7: <\/span> bps <\/span>
\n <\/span>A.<\/b>for window size=15: <\/span> bps<\/span><\/p>\n

\u6ce8\u610f512-byte = 512 * 8 bits<\/p>\n

Tframe<\/sub> = 512 * 8bits\/64kbps = 64ms<\/p>\n

Transmission cycle: T = 270ms * 2 + 64ms = 604ms<\/p>\n

Transmission window: 604ms\/64ms = 9.4, \u5927\u4e8e9\u7684\u65f6\u5019channel\u5c31\u6ee1\u4e86<\/p>\n

W = 1 ; throughout = 512 * 8b \/ 604ms = 6781bps = 6.78kbps<\/p>\n

W = 2 ; throughout = 7 * 512 * 8b \/ 604ms<\/p>\n

W = 15 > 9 ; throughout = 65kbps<\/p>\n

\n

Q.<\/b>A 100-km-long cable runs at the T1 data rate. The propagation speed in the cable is 2\/3 the speed of light in vacuum. How many bits fit in the cable?<\/span>
\n <\/span>A.<\/b> <\/span> bits<\/span><\/p>\n

\u7535\u7f06\u4e2d\u4f20\u64ad\u901f\u5ea6\u662f\u6bcf\u79d2200000km\uff0c\u6bcf\u6beb\u79d2200km\uff0c100km\u5c06\u4f1a\u57280.5ms\u5185\u586b\u6ee1\u3002<\/p>\n

T1\u901f\u7387125\u5fae\u79d2\u4f20\u9001193\u4f4d\u5e27\uff0c0.5ms\u53ef\u4ee5\u4f20\u90014\u4e2aT1\u5e27\uff0c\u5373193*4 = 772bits<\/p>\n

\n
Assume the sequence number has 4 bits. What is the maximum number of outstanding sending frames for a go back N protocol?<\/div>\n
<\/div>\n
\n

Box 1: Enter your answer as a number. Examples: 3, -4, 5.5
\nEnter DNE for Does Not Exist, oo for Infinity<\/p>\n

\u5e8f\u5217\u53f7\u67094\u4f4d\uff0c\u56de\u9000N\u534f\u8bae\uff0c\u6700\u591a\u6ca1\u6709\u5b8c\u6210\u53d1\u9001\u5e27\u6709\u591a\u5c11\uff1f<\/p>\n

\u56db\u4f4d\u53ef\u4ee5\u8868\u793a0~15\u517116\u4e2a\u6570\u5b57 \u6700\u591a\u6ca1\u6709\u5b8c\u621015\u4e2a\u3002<\/p>\n

\n
Assume the sequence number has 5 bits. What is the maximum number of outstanding sending frames for a selective repeat protocol?<\/div>\n
<\/div>\n
\n

Box 1: Enter your answer as a number. Examples: 3, -4, 5.5
\nEnter DNE for Does Not Exist, oo for Infinity<\/p>\n

\u5e8f\u5217\u53f7\u67094\u4f4d\uff0c\u9009\u62e9\u91cd\u4f20\u534f\u8bae\uff0c\u6700\u591a\u6ca1\u6709\u5b8c\u6210\u53d1\u9001\u5e27\u6709\u591a\u5c11\uff1f<\/p>\n

\u4e94\u4f4d\u53ef\u4ee5\u8868\u793a0~31\u517132\u4e2a\u6570\u5b57 \u6700\u591a\u6ca1\u6709\u5b8c\u6210\uff08max+1\uff09\/2 = 33\/2 = 32\/2 = 16<\/p>\n

\n
Which is not the CSMA\uff0fCA rule of 802.11?<\/div>\n
\n
    \n
  • If station X received RTS of station A, X must remain silent for a short time<\/li>\n
  • If station X received RTS, but did not receive CTS, then X may not transmit its data.<\/li>\n
  • If station X has not received RTS, but received CTS, then X may not transmit its data<\/li>\n
  • If station X has received both RTS and CTS, then X may not transmit its data<\/li>\n<\/ul>\n

    Box 1: Select the best answer<\/p>\n

    B<\/p>\n

    \n
    After the sender first sends frames from 0 to 6 and at the end of timeout receives the acknowledgements for frame 1, 3, and 5, the next frame it will re-transmit is frame ________. (assume the protocol is go-back-n)<\/div>\n
    \n
      \n
    • 1<\/li>\n
    • <\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"

      Q. A bit string, 0111101111101111110, needs to be […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[19,16],"tags":[],"_links":{"self":[{"href":"http:\/\/www.magicmipamipa.com\/index.php?rest_route=\/wp\/v2\/posts\/862"}],"collection":[{"href":"http:\/\/www.magicmipamipa.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/www.magicmipamipa.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/www.magicmipamipa.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.magicmipamipa.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=862"}],"version-history":[{"count":1,"href":"http:\/\/www.magicmipamipa.com\/index.php?rest_route=\/wp\/v2\/posts\/862\/revisions"}],"predecessor-version":[{"id":863,"href":"http:\/\/www.magicmipamipa.com\/index.php?rest_route=\/wp\/v2\/posts\/862\/revisions\/863"}],"wp:attachment":[{"href":"http:\/\/www.magicmipamipa.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=862"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/www.magicmipamipa.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=862"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/www.magicmipamipa.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=862"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}