ADHOCTCP: Improving TCP Performance in Ad Hoc Networks
Seyed Mohsen, Fatemeh Torgheh · InTech eBooks · 2011
Protocol Design 122 works.The design and implementation of ADHOCTCP are presented in Section 4. Simulations results are given in Sections 5.we conclude the chapter in Section 6. Challenges for TCP in MANETsTCP assumes that network congestion has happened whenever a packet is lost.It then invokes appropriate congestion control actions including window size reduction.Although this assumption is reasonable for wired networks, it is questionable for wireless networks especially MANETs.Other than congestion, possible causes of packet losses in MANETs include, wireless link errors, MAC layer losses due to channel contention, and link breakages due to node mobility.All those causes that are not related to congestion can result in unnecessary congestion control, which will degrade the TCP performance.Unlike wired networks, some unique characteristics of mobile ad hoc networks seriously deteriorate TCP performance.These characteristics include the unpredictable wireless channels due to fading and interference, the vulnerable shared media access due to random access collision, the hidden terminal problem and the exposed terminal problem, and the frequent route breakages due to node mobility.Undoubtedly, all of these pose great challenges on TCP to provide reliable end-to-end communications in mobile ad hoc networks.From the point of view of network layered architecture, these challenges can be broken down into six categories: lossy channels, hidden and exposed stations, network partitions, path asymmetry, route failures, and Energy Efficiency. Lossy channelsWireless links posses high bit error rates that cannot be ignored.But TCP interprets packet losses caused by bit errors as congestion.As a result, its performance suffers in wireless networks when TCP unnecessarily invokes congestion control, causing reduction in throughput and link utilization.The main causes of errors in wireless channels are the following: •Signal attenuation: This is due to a decrease in the intensity of the electromagnetic energy at the receiver (e.g.due to long distance), which leads to low signal-to-noise ratio (SNR).• Doppler shift: This is due to the relative velocities of the transmitter and the receiver.Doppler shift causes frequency shifts in the arriving signal, thereby complicating the successful reception of the signal.• Multipath fading: Electromagnetic waves reflecting off objects or diffracting around objects can result in the signal traveling over multiple paths from the transmitter to the receiver.Multipath propagation can lead to fluctuations in the amplitude, phase, and geographical angle of the signal received at a receiver.In order to increase the success of transmissions, link layer protocols implement Automatic Repeat reQuest (ARQ) or Forward Error Correction (FEC), or both.For example, IEEE 802.11 implements ARQ, so when a transmitter detects an error, it will retransmit the frame; error detection is timer based.Bluetooth implements both ARQ and FEC on some synchronous and asynchronous connections.Note that packets transmitted over a fading channel may cause the routing protocol to incorrectly conclude that there is a new one-hop neighbor.This one-hop neighbor could