ch5handout交換與通信網(wǎng)理論

March30, 1/ March30, 2/112233 March30, 5/ March30, 8/ARQAutomaticRepeat

March30, 11/ARQStop-and-WaitARQGoBackNARQ(ARQN

March30, 12/Stop&Wait

Stop&發(fā)送端發(fā)出一個包后，等待收到ACK沒有收到ACK

March30, 14/

Stop&

March30, 15/ Stop&ACK如果ACK不編號，因重復(fù)幀而回復(fù)的ACK

March30, 16/ACK

Stop&

March30, 17/

Stop&121個

March30, 18/

Stop&SN= 發(fā)送第SN給定時間內(nèi)收到確認(rèn)幀，如果RN>SN，設(shè)RN為SN，發(fā)送該SNSN幀

March30, 19/

Stop&RN=接受到一個無錯的、SN序號等于RN的幀，向上層遞交該幀，RN加1，在規(guī)定時間內(nèi)，向發(fā)送端回復(fù)一個帶有RN的幀

March30, 20/ Stop& March30,21/ Stop& March30, 22/ Stop&Stop-and-WaitARQ:TA:TP: E:鏈路利用率SE=TDS

TD+2TP+TA

March30, 23/ Stop&XP{X=1}=1?P{X=2}=P(1??1?P的幾何分布（ X 1?

March30, 24/ Stop&超時重發(fā)次數(shù)：X?1=ToS

P效率E= TD+2TP+TA+

March30, 25/ Stop&Stop&waitE TD+2TP+TA每個RTT

時延TP大于幀傳輸時延TD，效率

March30, 26/

GoBackN March30, 28/ GoBack 到的RNSNmin+N? March30, 29/

GoBackNSNmin=0;SNmax=SendpacketSNmaxSNmax=SNmax+SNmin=

March30, 30/ GoBackNIfSNmin<SNmax(therearestillsomeanynewpacketsrecentlysent

March30, 31/ GoBackNACKN

March30, 32/ GoBackN Stop&wait收到一個包，就發(fā)一個ACK March30, 33/ GoBackN 雙向數(shù)據(jù)傳輸時，在數(shù)據(jù)中攜帶確認(rèn)序號 March30, 34/

GoBackNRN=Acceptpacketfromtheotherdirection

March30, 35/

GoBackN

March30, 36/

GoBackNanimations/gbn_sr/index.htm

March30, 37/ GoBackN

March30, 38/ GoBackNanimations/gbn_sr/index.htm

March30, 39/ GoBackNRN=2 March30, 40/ GoBackN GoBackNRN=2及時到達(dá)，避免了重傳幀0幀2RN=4未及時到達(dá)RN=4到達(dá)，避免了幀3的重傳幀6，7應(yīng)用層還沒給，所以隨機(jī)重傳未收到ACK的幀4和5 March30, 41/ GoBackNanimations/gbn_sr/index.htm

March30, 42/ GoBackN反向長幀延緩RN重傳會因確認(rèn)到達(dá)而中途停止或改變(幀5，6應(yīng)用層還沒給，所以隨機(jī)重傳未收到

March30, 43/ GoBackN為連續(xù)發(fā)送，N1ACK返回:N≥無差錯時效率：E=min{1NTD/S} March30, 44 GoBackNWX>E=R=

March30, 45/ GoBackNACKWX<E=WXR=W/D

March30, 46/

GoBackNS=10s,TD=1,S/TD=

March30, 47/

GoBackNN=?S/TD?P:NX:1

(1+iN)Pi(1?E=

1 NP1?

March30, 48/

GoBackN幀以FCFS順

March30, 49/ GoBackN發(fā)送端需N個緩存 重發(fā)N個幀MM>N（N

March30, 50/ GoBack GoBackN例GoBackNARQwithN=7or March30, 51 GoBackNN March30, 52 SelectiveRepeat March30, 54

SelectiveRepeat發(fā)送一個明確的NAK信號請求特定幀

March30, 55/ SelectiveRepeatGO SelectiveRepeat接收端對所有正確接收到的幀都發(fā)送確認(rèn)信號超NAK的幀(如果接收端采用NAK機(jī)制) March30, 56

SelectiveRepeat

March30, 57/

SelectiveRepeatanimations/gbn_sr/index.htm

March30, 58/ SelectiveRepeat

March30, 59/ SelectiveRepeat

March30, 60/ SelectiveRepeatanimations/gbn_sr/index.htm

March30, 61/ SelectiveRepeatanimations/gbn_sr/index.htm

March30, 62/ SelectiveRepeatMM≥

March30, 63/ SelectiveRepeatSRP只重發(fā)丟失的幀 SelectiveRepeatGoBackN好，SRP占優(yōu) March30, 64

SelectiveRepeatGoBackN+SelectiveTimeout值：RTT的 平均+4倍標(biāo)準(zhǔn)差（對 RTTave=a×RTTmeas+(1?a)RTTave;a≈Timeout=RTTave+

March30, 65/

SelectiveRepeatA