ch5handout交换与通信网理论

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

March30, 11/ARQStop-and-WaitARQGoBackNARQ(ARQN

March30, 12/Stop&Wait

Stop&发送端发出一个包后，等待收到ACK没有收到ACK

March30, 14/

Stop&

March30, 15/ Stop&ACK如果ACK不编号，因重复帧而回复的ACK

March30, 16/ACK

Stop&

March30, 17/

Stop&121个

March30, 18/

Stop&SN= 发送第SN给定时间内收到确认帧，如果RN>SN，设RN为SN，发送该SNSN帧

March30, 19/

Stop&RN=接受到一个无错的、SN序号等于RN的帧，向上层递交该帧，RN加1，在规定时间内，向发送端回复一个带有RN的帧

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

TD+2TP+TA

March30, 23/ Stop&XP{X=1}=1−P{X=2}=P(1−⋯1−P的几何分布（ X 1−

March30, 24/ Stop&超时重发次数：X−1=ToS

P效率E= TD+2TP+TA+

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

时延TP大于帧传输时延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收到一个包，就发一个ACK March30, 33/ GoBackN 双向数据传输时，在数据中携带确认序号 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及时到达，避免了重传帧0帧2RN=4未及时到达RN=4到达，避免了帧3的重传帧6，7应用层还没给，所以随机重传未收到ACK的帧4和5 March30, 41/ GoBackNanimations/gbn_sr/index.htm

March30, 42/ GoBackN反向长帧延缓RN重传会因确认到达而中途停止或改变(帧5，6应用层还没给，所以随机重传未收到

March30, 43/ GoBackN为连续发送，N1ACK返回:N≥无差错时效率：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帧以FCFS顺

March30, 49/ GoBackN发送端需N个缓存 重发N个帧MM>N（N

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

SelectiveRepeat发送一个明确的NAK信号请求特定帧

March30, 55/ SelectiveRepeatGO SelectiveRepeat接收端对所有正确接收到的帧都发送确认信号超NAK的帧(如果接收端采用NAK机制) 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只重发丢失的帧 SelectiveRepeatGoBackN好，SRP占优 March30, 64

SelectiveRepeatGoBackN+SelectiveTimeout值：RTT的 平均+4倍标准差（对 RTTave=a×RTTmeas+(1−a)RTTave;a≈Timeout=RTTave+

March30, 65/

SelectiveRepeatA