Modeling TCP throughput on Ad hocWireless NetworksJaiwant Virk{[email protected]}Dec 2002Abstract: Many different models of TCP have been proposed over wired links but notmuch effort has been put into mathematical modeling of flow control behavior of TCP inthe mobile Ad hoc networks. TCP has serious drawbacks in a wireless link due to its nondifferentiating nature of packets lost due to congestion or transmission loss. I have triedto model this behavior of regular TCP throughput with respect to loss rate and windowsize.1. IntroductionAd hoc networks are rapidly deployable multi hop networks that route through each otherto reach a destination. Communication is relayed in chains of router nodes when thenodes are not in direct connection with each other. All nodes are assumed to route thepackets for the peers. The routes are in constant flux due to change of topology inducinga change in route reconstruction.Most Ad hoc nodes may face a number of constraints ranging from the common sharedbandwidth to power, high BER and security.WLAN networks may operate in Access point mode and Independent Infrastructuremode. The Latter will be of interest to us for the simulation part..Independent Mode is a peer-to-peer setup for communication, using other nodes to find apath to the destination using some specific routing protocols and then maintaining theconnection.TCP Congestion control behavior uses AIMD [9], linear additive increase andexponential decrease in the window size or the number of packets sent based on the lossof packets lost due to congestion on the link.However wireless scenario has a lot of medium specific problems. The packets may belost due to a variety of reasons like congestion at the router node, transmission loss due topath loss, mobility of nodes. These may lead to high BER, change in topology, reroutingand delay associated with it. Another problem may be Power Consumption due toretransmission. As we would see later that power control is an important factor in generalin the Ad Hoc networks which has led to development of power aware protocols likePAMAS[10], [11].The method of congestion detection is when the sender receives Triple duplicate ACKand Time out notification. Depending on these the Congestion window is increased ordecreased. Moreover on a time out the window size goes into an exponential back-off.This is a serious drawback in the wireless networks. Unless there is a mechanism [13]such as ECN [12] or RED to distinguish between congestion loss and transmission lossthe exponential back-off creates extra delay.2. Previous WorkFrom Previous research its been proven that TCP performs quite badly in Wirelessnetworks for a variety of reasons. Most of the TCP wireless models and protocols forimprovements have been suggested for Cellular networks or wireless networks withaccess point mode. These studies have led to the development of Split Connection, End-to-End, Link Level Notification and Explicit Congestion Notification.Some of the Protocols that may be relevant in the Pure Ad Hoc scenario worth notingAre:Split connection – Split Connection is relevant only in Overlay or Cellular Networksinvolving two different versions running on the wired and the wireless part of thenetwork. Some of them are MTCP, ITCP, and M-TCP [12].End-to-End – In this approach the sender and receiver adapt to the packet loss. Some ofthe solutions are Selective ACK [3], Fast Retransmit, and TCP-New Reno [6] andATCP[16].Link Level Retransmission – These Protocols implement their own link levelretransmission techniques at the wireless links in addition to the TCP retransmission.Snoop Protocol [7], TCP –Aware, Delayed Duplicate ACK [8] fall under this category.Explicit Notification –ECN, TCP-F explicitly notify the cause of the loss to the sender.As far as Modeling is concerned the most notable one are [1] and [2]. There have beennumerous ways to model the TCP behavior based on different distribution method oranalysis notably the Markov analysis and Queuing theory.3. ModelFor the modeling the assumption is of a single sender to destination over an Ad hocnetwork. The Path taken by the packets may include multiple hops over similar nodeswith a fixed processing speed. It’s also assumed that during the relay, the SIR remains thesame for all the nodes and all paths. It’s not a simple assumption to make but to formulatethe model these variables have to be constant to reduce the complexity.TFigure 1: Window Size adjustment over timeAs stated earlier the packet loss may occur due to the congestion or a link loss. The lossprocess is modeled as an occurrence between two timeout outs. The loss notificationevents arrive as a Poisson distribution. The time out is not included in the throughputcalculation as there is no transmission during that period and therefore would notcontribute towards it, although the average throughput over a period of time must includetime out.In general, in Ad hoc wireless networks it is observed thatThroughput= f (Loss rate, No. of hops, Window size, Packet Size, Power, mobility)Loss rate = ƒ (Loss on congestion, transmission loss)Window Size =ƒ (RTT)RTT =ƒ (No. of hops)A Simple Model: This approach is based on the throughput formula developed by [4]This model bases that throughput is dependent on the of probability of packet lossR= (1/RTT)* (1.22/√P)Where P is the loss probability of the packet and RTT is the return Trip Time.Since Probability of loosing a packetP (L) = P (Ak) + P (Bk)Where Ak and Bk are losses due to Congestion and Transmission respectivelyTherefore substitutingR= (1/RTT)* (1.22/√(P(Ak)+P(Bk))Link LossCongestion LossTimeOutLoss EventSince there in no way for the sender to know or judge the exact probability of any ofthese events we can perhaps assign them some weights given by ữ based on the conditionof the network.R= (1/RTT)* (1.22/√ (ữP (Ak) + (1-ữ) P (Bk)) …..(1)Fixed point model: In this Model the packet loss is seen in terms of network side view ,i.e. packet loss notification arrivals are studied instead of giving a probability of loss toeach packet sent out[1]. Packets arrive as a Poisson stream to the sender, andaccordingly the window size is adjusted.These loss are bundled together as a loss cycle Lk separated by
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