Performance of TCp Several factors affect TCP performance inMANET-Wirelesstransmissionerrors- Multi-hop routes on shared wireless medium· For instance, adjacent hops typically cannot transmitsimultaneously- Route failures due to mobility
Performance of TCP • Several factors affect TCP performance in MANET − Wireless transmission errors − Multi-hop routes on shared wireless medium • For instance, adjacent hops typically cannot transmit simultaneously − Route failures due to mobility
Random Errors. If number of errors is small, they may becorrected by an error correcting code Excessive bit errors result in a packet beingdiscarded, possibly before it reaches thetransport layerProblems-Fastretransmissionoflostpacket- Reduction in congestion window→ reduces the throughput- Burst errors may cause timeouts
Random Errors • If number of errors is small, they may be corrected by an error correcting code • Excessive bit errors result in a packet being discarded, possibly before it reaches the transport layer • Problems − Fast retransmission of lost packet − Reduction in congestion window → reduces the throughput − Burst errors may cause timeouts
Throughput over Multi-HopWireless PathsConnections over multiple hops are at adisadvantage compared to shorter connections,because they have to contend for wirelessaccessat each hop1600140012001000TCP800TCP Throughput usingThroughtput(Kbps)6002Mbps 802.11MAC400200012345678910Numberofhops
Throughput over Multi-Hop Wireless Paths • Connections over multiple hops are at a disadvantage compared to shorter connections, because they have to contend for wireless access at each hop 0 200 400 600 800 1000 1200 1400 1600 1 2 3 4 5 6 7 8 9 10 Number of hops TCP Throughtput (Kbps) TCP Throughput using 2 Mbps 802.11 MAC
Throughput Degradations withIncreasing Number of HopsPacket transmission can occur on at most one hopamong three consecutive hops-Increasing the number of hops from 1 to 2, 3 results inincreased delay, and decreased throughputIncreasing numberofhopsbeyond3allowssimultaneous transmissions on more than one linkHowever,degradationcontinuesduetocontention betweenTCP Data and Acks traveling in opposite directionsWhen number of hops is large enough, the throughputstabilizes due to effective pipelining
Throughput Degradations with Increasing Number of Hops • Packet transmission can occur on at most one hop among three consecutive hops − Increasing the number of hops from 1 to 2, 3 results in increased delay, and decreased throughput • Increasing number of hops beyond 3 allows simultaneous transmissions on more than one link − However, degradation continues due to contention between TCP Data and Acks traveling in opposite directions • When number of hops is large enough, the throughput stabilizes due to effective pipelining