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Transport Issues in Ad Hoc Transport Issues in Ad Hoc and Sensor Networksand Sensor Networks

ΠαναγιώτηςΠαναγιώτης ΠαναγιώτουΠαναγιώτουΤµήµα Ηλεκτρολόγων Μηχανικών και Μηχανικών ΥπολογιστώνΤµήµα Ηλεκτρολόγων Μηχανικών και Μηχανικών Υπολογιστών

Πολυτεχνική Σχολή Πολυτεχνική Σχολή Πανεπιστήµιο ΚύπρουΠανεπιστήµιο Κύπρου

Computer Networks (ECE654) Project PresentationComputer Networks (ECE654) Project Presentation19 November 200419 November 2004

Ad Hoc NetworksAd Hoc Networks

Wireless networks that operate without anycentralized structure used for directingcommunication.Similar to users of cellular phonescommunicating without the aid of a phonetower or laptop users communicating withone another without a wireless access pointavailable.

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Ad Hoc Networks Ad Hoc Networks Nodes send and receive messages via end-toendcommunication and also act as forwarding agentsfor other mobile stations.

The multi-hop network consists ofautonomous,mobile nodes that act as relaystations for paths of communication.

Because there are no centralized routers or basestations, the nodes must also be able to handleconstant route changes and terminations

Ad Hoc NetworksAd Hoc Networks

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Transport LayerTransport LayerIntended to provide a more reliable method ofcommunication that is not offered by the network layer. Transport layer protocols were initially intended to offerservices regardless of the underlying network. Receives data from the user and transmits it to the receiverin order and without error. Makes it possible for the users to send and receive datawithout concern of network congestion or loss of packets.Ability to dynamically adjust window size to provide betterflow control.

Common Transport ProtocolsCommon Transport Protocols

UDP:UDP:

User Datagram Protocol

Connectionless protocol that does not offerguaranteed packet delivery.

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Common Transport ProtocolsCommon Transport Protocols

Traditional TCP:Traditional TCP:

Transmission Control ProtocolMost commonly used transport protocol.End to end protocol that provides reliable, in-order transmission over the unreliable IP. Operates independently of the routing mechanisms implemented in the network layer.Traditional TCP was designed to be used on wired networks. As it is, does not work well in wireless environments.

Traditional TCP ProblemTraditional TCP Problem

It assumes every packet loss to be due tocongestion even though the loss couldbe due to a number of possibilities.

Mechanisms:Slow StartCongestion Avoidance

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Congestion Control in TCPCongestion Control in TCP

–– Threshold := CongestionWindow/2Threshold := CongestionWindow/2–– CongestionWindowCongestionWindow ::= 1= 1

TCP vs. Ad Hoc NetworksTCP vs. Ad Hoc Networks

Bit ErrorsBit Errors– TCP immediately reduces its transmission

speed to avoid further congestion. – A reduction in transmission speed is the exact

opposite of what needs to be done to remedythe problem.

– The optimal way would be to immediatelyresend the packet to the receiver.

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TCP vs. Ad Hoc NetworksTCP vs. Ad Hoc Networks

Path ChangePath Change– In ad hoc networks, the mobility of the nodes

causes the probability of path change toincrease.

– TCP will timeout and incorrectly assumenetwork congestion

–– The optimal way would be to The optimal way would be to stop transmittinguntil the network has found a new path and thenresume sending at the rate determined by thenew node(s) and the network layer.

Important Issues in Important Issues in Ad Hoc NetworksAd Hoc Networks

Variable link qualityVariable link qualityMulti-user interferencePower consumptionTopological changesPropagation path lossPropagation path loss

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Link QualityLink Quality

Quality of transmitters and receiversDistance from the Mobile Station to thereceiverThe degradation of each link can have anenormous effect on throughput.

Multi user interferenceMulti user interferenceWired networks: there is not really any threat of multiplenodes receiving packets even if they were not meant forthem.Ad hoc wireless networks: transmitters do not have apinpoint location to send data to.

they must broadcast data / request to send data toevery node in range.

Need to decipher whether or not the broadcasted messageis for the node. – YES accept the message and read it. – NO ignore it and go on.

These problems can be seen in both the hidden andexposed station problems.

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Power ConsumptionPower Consumption

An important issue that all mobile stations have.Not consistent or stable source of power. Wired networks: transmitting power does notreally need to change.An ad hoc wireless networks: power consumptionchanges per mobile stationTransmitting power needs to be adjusteddepending on that distance which cancontinuously change with the inconsistent topologyof ad hocwireless networks.

Power Consumption: DistancePower Consumption: Distance

At t1 mobile station A needs power P1 totransmit to mobile station B successfully.

At t2 Y mobile station A needs power P2 totransmit successfully.

Mobile station B has either moved closer orfarther away from mobile station A.

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Path LossPath LossAttenuation undergone by an electromagnetic wave intransit between a transmitter and a receiver.Many of the transport protocols treat path loss ascongestion andimplement traditional congestion controlmechanisms to compensate.Ad hoc wireless networks: path loss does not necessarilymean congestion. The destination node has moved out of range of the senderor is now on a different path. Congestion Control in this case is not useful and wastesbandwidth and power (this is not needed).

TCP based modificationsTCP based modifications

Wired TCP:Wired TCP:TCP-RenoTCP-VegasTCP-TahoeTCP-Sack

Ad Hoc TCP:TCP-BuSTCP-F

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TCPTCP--BuSBuS

Proposed by Dongkyun Kim, C.K. Toh, andYanghee Choi.Main differentiating property is theintroduction of buffering capabilities in themobile nodes.Also does feedback information for detectingroute disconnection.

ModificationsModifications

1. Explicit notifications for route failures and route reestablishment

ERDN (explicit route disconnection notification) Stop TransmissionERSN (explicit route successful notification) Start Transmission

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ModificationsModifications

Use of extended timeout valuesPackets are still sent by the sender and the nodethat sent the ERDN message buffers thesepackets. Problem: the time it takes for a new path to bediscovered might be smaller than the time thesender waits before thinking that the packet hasbeen lost.

The timeout value for the buffered packets isdoubled.

ModificationsModifications

3. Selective retransmission of lost packets.4. Avoidance of unnecessary requests for

fast transmission.5. Reliable transmission of control messages.

2 timers: ERDN_RET_TIMER andERSN_RET_TIMER

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TCPTCP--FF

Created by Chandran, Raghunathan, Venkatesan, and Prakash

Uses feedback to overcome the limitations of TCP

Goal: "source informed of route failure so that isdoes not unnecessarily invoke congestion controland can refrain from sending any further packetsuntil the route is restored."

TCPTCP--FFWhen an intermediate node in the path between thesender and the receiver encounters a route disruption itpropagates a route failure notification (RFN) packet tothe source and records the occurrence. Each node that receives the RFN invalidates the particularroute and prevents incoming packets intended for thedestination from passing though that route. If one of the intermediate nodes knows an alternate route, the RFN can be discarded and communication can beginon the new route. If not,the RFN gets propagated all the way back to thesender.

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Snooze StateSnooze StateWhen sender receives a RFM, it goes into a "snooze" state:

1. Ceases sending all packets. 2. Marks all timers as invalid.3. Freezes the packet window, the retransmit timers, and

window size. 4. Starts a route failure timer.

It remains in a snooze state until it receives notification of areestablished route by a RRN (route reestablishmentnotification packet).

If a node discovers a working route,it propagates themessage back towards the sender.

All further RRNs for the same source-destination combinationare then discarded.

ATCPATCP

Ad hoc Transmission Control Protocol

Acts as an intermediary between thenetwork layer and standard TCPIt is important for applications to be able touse standard TCP without anymodifications. ATCP is a thin layer between the networklayer and TCP.

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Packet LossPacket LossWhen ATCP detects packet loss due to a routefailure or path change, it does not notify TCP ofthe packet loss.Instead, it places TCP into persist modeby changing the receiver’s advertised window sizeto 0. This causes TCP to temporarily stop sendingwithout attempting congestion control. Once ATCP has detected that the network layerhas reestablished a path, it will notify TCP to beginsending again by setting the receiver’s window tothe previous size.

High Bit Error RateHigh Bit Error Rate

Same technique for high bit error rate. When ATCP detects that a packet has beencorrupted, it places TCP in persist mode andretransmits the packet itself without notifyingTCP of the packet loss.

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AdvantagesAdvantages

Current method of congestion control (TCP) isavailable without any modifications. When ATCP detects packet loss due tocongestion, it simply steps aside and lets TCP handle it as it would in a wired network.It supports the use of current TCP implementations, which will allow betterinteroperability with pre-existing applications.Programmers will not need to learn a new protocolto use for ad hoc network applications.

Data FlowData Flow

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ATP (1)ATP (1)Application controlled Transport Protocol

The application should be able to decide the QoS offered by thetransport layer.

ATP also allows for the priority levels for different applications to bedetermined dynamically, allowing for more flexibility in multipleapplication environments.

ATP is based on a good idea that applications should be abletodynamically set their priority.

Example: Example: parameticparametic needs to look up patient data and communicatewith another doctor at the same time.

DisadvantagesDisadvantages

It omits much of the functionality required by anideal transport layer protocol. Reliable delivery is left to the user application, which can query ATP to see if an ACK has arrivedbut must determine itself when to retransmit. It completely overlooks congestion control, packeterrors, and path loss. These issues areleft to the user application for implementation.

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ENICENICENhanced Inter-layer Communication and control

The OSI model layers should have bettercommunication with the other layers. In order to avoid the hidden station and exposedstation problem in wireless networks, the ENIC protocol utilizes Request-To-Send (RTS) andClear-To-Send (CTS) packets.

Hidden and Exposed StationHidden and Exposed Station

A ΓB

∆ ΗΖΕ

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ENICENICAbility to freeze the TCP state when a route has failed. The network layers in ENIC will monitor the connectionsbetween the sender and receiver and will notify thetransport layer of a failure with an Explicit Route StateNotification (ERSN).At this point, the protocol will save the current state of itsvariables (retransmission time, congestion window, etc.) until the network layer has determined a new route.The transport layer will periodically probe the network layerfor a new route by using a Route Recovery Timer (RRT).

ENIC handles route change and path loss without a serious reduction in throughput.

In the case ofnetwork congestion, ENIC will use standardTCP slow-start algorithm.

ATP (2)ATP (2)

Ad hoc Transport Protocol

Communication between layers needed for startup rateestimation, congestion control, and path loss/failure.

This is a common theme among the ad hoc protocols, somost lower layers should support this communication.

The intermediate nodes are required to piggyback datarates onto packets to the nodes. This allows the sender todetermine the amount of congestion in the network.

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Sensor Networks Sensor Networks Transport ProtocolsTransport Protocols

PSFQ: Pump Slowly and Fetch QuicklyPSFQ: Pump Slowly and Fetch Quickly

ESRT EventESRT Event--toto--Sink Reliable Transport for Sink Reliable Transport for Wireless Sensor NetworksWireless Sensor Networks

CODA: Congestion Detection and CODA: Congestion Detection and Avoidance in Sensor NetworksAvoidance in Sensor Networks

Sensor NetworksSensor Networks

Expectations for transport protocol for Expectations for transport protocol for sensor networks sensor networks

Reliability, congestion controlReliability, congestion control

ConstraintsConstraintsResource constraints Resource constraints –– power, storage, power, storage,

computation complexity, data ratescomputation complexity, data ratesThese constraints are application These constraints are application specific.specific.

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Sensor NetworksSensor Networks

General notion for sensor networksGeneral notion for sensor networks

Low data Rate High data RatePower limited Not power limitedStorage limited Not storage limited

Sink

User

PSFQPSFQ

Pump Slowly and Fetch QuicklyPump Slowly and Fetch Quickly

A Reliable Transport Protocol for Wireless Sensor Networks

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MultiMulti--Hop Packet Forwarding Hop Packet Forwarding

1 2 3 4

1

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22 2

33 3

When No Link Loss – Multi-Hop Forwarding takes place

Recovering from Errors Recovering from Errors

2 431

2 lost

1 1 1

33

3

Recover 2

Recover 2

Recover 2

Error Recovery Control Messages are wasted

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How PSFQ Recovers from ErrorsHow PSFQ Recovers from Errors“Store and Forward”“Store and Forward”

2 31 4

2 lost

Recover 2

1

22

3

11

33 2

2

No wastage of the Error Recovery control messages

PSFQ Pump SchedulePSFQ Pump Schedule

If not duplicate and in-order and TTL not 0Cache and Schedule for Forwarding at time t (Tmin<t<Tmax)

Tmin

TmaxTminTmax

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1

1

1

t

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““Fetch Quickly” OperationFetch Quickly” Operation

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11

2 lost2

3

Tmin

Tmax

TrRecover 2Tr 2

2

““Proactive Fetch”Proactive Fetch”

Tproc

1 2last-1

last

last

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CODACODACongestion Detection and Avoidance in

Sensor Networks

Energy efficient congestion control schemeEnergy efficient congestion control schemeThree mechanisms are involvedThree mechanisms are involved–– Congestion DetectionCongestion Detection–– OpenOpen--loop hoploop hop--byby--hop backpressurehop backpressure–– ClosedClosed--loop multiloop multi--source regulationsource regulation

Congestion DetectionCongestion Detection

Accurate and efficient congestion detection Accurate and efficient congestion detection is importantis important–– Buffer queue length or Buffer occupancy Buffer queue length or Buffer occupancy –– not a not a

good measure of the congestion.good measure of the congestion.–– Channel loading Channel loading –– sample channel at sample channel at

appropriate time to detect congestion.appropriate time to detect congestion.–– Report rate/Fidelity measurement Report rate/Fidelity measurement –– slow, slow,

observed over a longer periodobserved over a longer period

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OpenOpen--Loop HopLoop Hop--byby--Hop Hop BackpressureBackpressure

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1 2

4

5

3

Congestion detected

Closed Loop MultiClosed Loop Multi--Source Source RegulationRegulation

1 2

1,2,3ACK

4,5,6 Congestion detected

7,8

Regulate bit is set

ACK

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ESRTESRT

EventEvent--toto--Sink Reliable Transport for Wireless Sink Reliable Transport for Wireless Sensor NetworksSensor Networks

EventEvent--toto--sink reliabilitysink reliabilitySelfSelf--configurationconfigurationEnergy awareness [low power Energy awareness [low power consumption requirement!]consumption requirement!]Congestion ControlCongestion ControlVariation in complexity at source and Variation in complexity at source and sink. [computation complexity]sink. [computation complexity]

S

ESRTESRT

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ESRT’s Definition of ReliabilityESRT’s Definition of Reliability

Reliability is measured in terms of the Reliability is measured in terms of the number of packets received. Or reporting number of packets received. Or reporting frequency (number of packets / decision frequency (number of packets / decision interval).interval).Observed reliability:Observed reliability: number of received number of received data packets in decision interval at the sink.data packets in decision interval at the sink.Desired reliability:Desired reliability: number of packets number of packets required for reliable event detection.required for reliable event detection.Normalized reliabilityNormalized reliability is observed reliability is observed reliability / desired reliability./ desired reliability.

Algorithm for ESRTAlgorithm for ESRT

If congestion and low reliability: decrease reporting If congestion and low reliability: decrease reporting frequency aggressively. (exponential decrease)frequency aggressively. (exponential decrease)If congestion and high reliability: decrease If congestion and high reliability: decrease reporting to relieve congestion. No compromise on reporting to relieve congestion. No compromise on reliability (multiplicative increase)reliability (multiplicative increase)If no congestion and low reliability: increase If no congestion and low reliability: increase reporting frequency aggressively (multiplicative reporting frequency aggressively (multiplicative increase)increase)If no congestion and high reliability: decrease If no congestion and high reliability: decrease reporting slowing (half the slope)reporting slowing (half the slope)

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ReferencesReferences1. Tanenbaum, A. Computer Networks, Third Edition.

Prentice Hall2. John Calagaz, Wade Chatam, Brian Eoff and John A.

Hamilton, Jr. On the Current State of Transport Layer Protocols in Mobile Ad hoc Networks.

3. Liu, J., Singh, S. ATCP: TCP for mobile ad hoc networks. Selected Areas in Communications, IEEE Journal on, Volume: 19 Issue: 7, (July 2001).

4. Dongkyun K., Toh, C.K., and Choi, Y. TCP-BuS: improving TCP performance in wireless ad hoc networks. In Communications, 2000 IEEE International Conference on, Volume: 3, (18-22 June 2000), 1707 -1713.

ReferencesReferences

5. Sun, D., and Man, H. Performance comparison oftransport control protocols over mobile ad hoc networks. In Personal, Indoor and Mobile Radio Communications12th IEEE International Symposium on, Volume: 2, (30 Sept.-3 Oct. 2001), G-83 -G-87.

6. Chandran, K., Sudarshan, R., Venkatesan, S., Prakash, R. A Feedback-Based Scheme for Improving TCP Performance in Ad Hoc Wireless Networks. In IEEE Personal Communications, (February 2001).

7. Holland, G., Vaidya, N. Analysis of TCP performanceover mobile ad hoc networks, Wireless Networks, Volume 8 Issue 2/3 March 2002.

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ReferencesReferences8. Liu, J. and Singh, S., ATP: Application controlled

Transport Protocol for Mobile Ad Hoc Networks. Wireless Communications and Networking Conference, 1999. IEEE, (21-24 Sept. 1999) 1318 -1322 vol.3.

9. Sundaresan, K., Anantharaman, V., Hsieh, H., andSivakumar, R. ATP: a reliable transport protocol for ad-hoc networks. Proceedings of the 4th ACM internationalsymposium on Mobile ad hoc networking & computing. (June 2003).

10. Sun, D., and Man, H. ENIC - an improved reliabletransport scheme for mobile ad hoc networks. GlobalTelecommunications Conference, 2001. GLOBECOM '01. IEEE, Volume: 5, (25-29 Nov. 2001).

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