location enhancement to ieee 802.11 dcfnadeem/papers/cstr4606.pdf · tamer nadeem1, lusheng ji2,...
TRANSCRIPT
Location Enhancement to IEEE 802.11 DCF
Tamer Nadeem1, Lusheng Ji2, Ashok Agrawala1, and Jonathan Agre2
1Department of Computer ScienceUniversity of Maryland
College Park, Maryland 20742{nadeem,agrawala}@cs.umd.edu
2Fujitsu Labs of America8400 Baltimore Ave., Suite 302College Park, Maryland 20740{lji,jagre}@fla.fujitsu.com
CS-TR-4606 and UMIACS-TR-2004-48
June 2004
Abstract
In this paper, we propose an enhancement to the existing IEEE 802.11 DCFMAC function to improve channel spatial reuse efficiency, and thus improve datathroughput. Our modification, named the Location Enhanced DCF (LED) forIEEE 802.11, incorporates location information in the RTS-CTS-Data-ACK hand-shakes of the IEEE 802.11 DCF so that other stations sharing the channel are ableto make better interference predictions and blocking assessments. Physical layerreceiver capture effect is also used in LED to encourage more concurrent trans-missions, after the result of the improved channel estimation. In this paper weanalytically study the potential performance enhancement of the LED over theoriginal IEEE 802.11 DCF. The results are also verified using thens-2simulator,which show that up to 35% of DCF blocking decisions are unnecessary and ourLED method can achieve up to 22% more throughput than the original DCF.
1 Introduction
The IEEE 802.11 [1] is the most popular standard for Wireless Local-Area Networks(WLANs). The IEEE 802.11 Medium Access Control (MAC) specifies two differ-ent mechanisms: the mandatory contention-based Distributed Coordination Function(DCF) and the optional polling-based Point Coordination Function (PCF). At present,DCF is the dominant MAC mechanism implemented by IEEE 802.11-compliant prod-ucts.
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Contention based protocols are the mainstream MAC protocols for distributed andself-organized wireless networks since in such networks the infrastructure is usually notpresent and there is no clear separation between the roles of access points and clientstations. The support of contention based DCF also made IEEE 802.11 equipmentpopular choices for various wireless ad hoc networks.
Most of the contention based MAC protocols, including the IEEE 802.11 DCF, arebased on Carrier Sense Multiple Access (CSMA). In CSMA, a station may transmit ifand only if the medium is sensed to be idle. The reason is to avoid this station causinginterference to the current ongoing transmission sensed on the medium. In additionto basic CSMA, the DCF also incorporates acknowledgement signals and a back-offmechanism. An optional channel reservation mechanism is also include in the DCF.
The IEEE 802.11 DCF is known to be not efficient in shared channel use due toits over-cautious approach towards assessing the possibility of causing interference. Inparticular, a station simply blocks its own transmission when it senses the medium isbusy, or it receives a channel reservation message sent by any other station. Howeverin many cases this channel assessing station’s own transmission may not introduceenough signal energy to disturb the on-going transmission at its receiver.
Finer channel assessment schemes which do consider the above possibility are dif-ficult to implement with information provided by the current IEEE 802.11 commu-nication protocol. If more information regarding an on-going transmission, such aslocations of the transmitters and receivers and transmission power levels can be pro-vided to the channel assessing stations, it is then possible for these stations to makebetter estimations to decide if indeed their transmissions will collide with the on-goingtransmission. In this way, more concurrent transmissions in wireless networks may beencouraged and the communication channel can be used more efficiently.
In this paper, we propose a novel contention-based distributed MAC scheme whichassesses the channel condition more accurately and exploits radio signal capture phe-nomena to increase the simultaneity of data transmissions to enhance wireless networkperformance. This scheme is designed as an enhancement to the DCF. In doing so,we will also develop a new MAC frame format in addition to the new MAC protocolto provide the additional information to help the stations in deciding whether to blocktheir transmissions or not, when there are on going communications occurring in theirvicinities.
2 Backgrounds and Related Works
2.1 IEEE 802.11 DCF Mode
Historically, the design of the IEEE 802.11 DCF is influenced by several other pro-tocols. MACAW protocol [4], extending the predecessor Multiple Access CollisionAvoidance (MACA) protocol [14], is based on the use of the Request-to-send andClear-to-Send (RTS/CTS) handshaking scheme. If a node has a packet to send, it firsttransmits a RTS packet to request the channel. If available, the receiver replies witha CTS packet. After the sender receives the CTS packet successfully, it proceeds totransmit the actual data packet. Nodes that hear the RTS packet will defer transmission
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for a sufficiently long period of time to allow the transmitter to receive the CTS packet.Nodes hear the CTS packet will back off for a period of time that is sufficiently long toallow the receiver to receive the entire data packet. Sender nodes in such mechanismdo not use the carrier sense mechanism to assess the channel availability. An extendedprotocol named Floor Acquisition Multiple Access (FAMA) is proposed in [8]. FAMAbears significant resemblance to IEEE 802.11, employing both local carrier sense, aswell as the RTS/CTS collision avoidance exchange for data transmission.
The basic MAC method of IEEE 802.11, the DCF, is a Carrier-Sense Multiple Ac-cess with Collision Avoidance (CSMA/CA) mechanism with a random back-off timewindow after sensing a busy medium. ACK messages are also used in the DCF foracknowledging the reception of unicast data frames.
The CSMA scheme of the DCF works as follows. Before a station transmits, itmust sense the wireless channel to determine if any other stations are transmitting.The channel is assessed as busy if there are detected carrier signals or the energy levelof the channel exceeds a threshold, or both, depending on each particular vendor’simplementation. If the carrier is assessed as busy, the station needs to wait until thecarrier becomes idle and then wait more for a period known as the Distributed Inter-Frame Space (DIFS). After the DIFS period is passed, the station again waits for arandom back-off interval and then transmits if the medium is still free.
After a directed transmission (unicast data frame) is correctly received, the receiv-ing station sends an ACK frame back after a Short Inter-Frame Space (SIFS). Thereception of an ACK frame following the transmission of a DATA frame notifies thetransmitter that its data has been received by the receiver without error. If no ACK isreceived after the transmission of a data frame, the transmitter schedules the data framefor retransmission.
In addition to the abovebasic transmission mechanism, the DCF employs an op-tional reservation based collision avoidance mechanism for unicast data packets. Thisoption requires the sender and the receiver to exchange short Request-To-Send (RTS)and Clear-To-Send (CTS) control frames, respectively, prior to the actual data frametransmission to reserve the channel. Any stations which hear either the RTS or theCTS block their own transmissions (if any) to yield to the communication between thissender and its receiver.
Each station maintains a timer called the Network Allocation Vector (NAV) whichtracks the remaining time of any on-going transmissions of other stations. After a sta-tion receives a RTS, CTS, DATA, or ACK frame not destined for itself, it sets its NAVaccording to the duration field of the frame. The duration field contains the framesender’s estimation for how long the whole data frame delivery message exchange se-quence (including all the SIFS waits and the acknowledgement) will take, or in otherwords, the reservation duration of this data frame delivery. After set, NAV may be ex-tended if a newly received frame contains a duration field pointing to a later completiontime. Figure 1 illustrates how nodes set their NAVs during a RTS-CTS-DATA-ACKhandshake. Checking one’s NAV before a station attempting to transmit is also knownas “virtual carrier sensing”. If the NAV is not zero, the node needs to block its owntransmissions to honor the channel reservations.
In summary, a node blocks its own transmissions if either physical carrier sensingor virtual carrier sensing returns channel busy.
3
Contention Window
Nodes located
Destination
DIFS
SIFS SIFS SIFS
Data RTS
ACK CTS
NAV (RTS)
NAV (CTS)
Source
NAV (EIFS)
NAV (EIFS)
NAV (EIFS)
NAV (EIFS) between R and I
Nodes located
witihin R
Figure 1: IEEE 802.11 DCF Mechanism
2.2 Capture Effect
When a frequency modulation scheme, such as the Direct Sequence Spread Spectrum(DSSS) used by most IEEE 802.11 and 802.11b physical layer (PHY) implementations,is used in wireless communication, an effect known as the “capture effect” [3, 17, 18,15, 9] may occur. When two transmissions sent by two different transmitters at thesame frequency overlap in time space, and they are received at the same receiver, thesignals of the stronger transmission will capture the receiver modem, and signals of theweaker transmission will be rejected as noise.
Different works (e.g., [9, 6, 16, 21, 20]) studied the analytical and simulation mod-els for characterizing the capture effects. Among the results of these previous works,we adopt a simple yet widely accepted model to describe the capture effect. In ourmodel, a receiver captures the signals of a particular transmission if the received en-ergyPr of this transmission sufficiently exceeds all other received energyPi of n otherconcurrent interfering contenders combined by a minimum ratio. That is, the captureoccurs when:
Pr > α
n∑
i=1,i6=r
Pi (1)
This minimum ratioα is called the capture ratio. The received signals are assumed tohave phase terms varying quickly enough to allow incoherent addition of the receivedpower of each frame.
WLAN systems such as the IEEE 802.11 do not pay special attention to captureeffects mainly to keep the system simple. Also the contention based MAC protocollargely reduces the time and space overlapping of concurrent transmissions. Nonethe-less, the capture effect still exists in IEEE 802.11 DSSS networks and it has beenconfirmed by several published studies. Authors in [11, 22, 21, 20] have also studiedthe impact of capture effect on traffic fairness and throughput of UDP and TCP flowsfor both ad hoc and infrastructure modes of WLAN systems.
Another aspect, which has not been raised by many of the previous works, that
4
2 1
R
3 4
I
Figure 2: Example of network with 4 nodes, where R is transmission range and I is thecarrier sense range
needs some discussion before we proceed further is the capturing of asignal versuscapturing of aframe. We consider the case when the new (stronger) frame arrives afterthe receiver begins to receive the weaker frame. A receiver being able to capture astronger signal does not necessarily mean it can capture the stronger frame. Whether areceiver can capture a stronger frame also depends on several other factors such as: thearrival moment of the beginning of the stronger frame, the current receiving state of thereceiver, the capability of the receiver to realize that it is seeing the beginning of a new(stronger) frame, and capability of the receiver to jump to the appropriate receivingstate for beginning to process the new frame. If the receiver is not able to realize thatit has just seen the beginning of a new frame and reset its receiving state accordingly,the bits of the new frame may be interpreted as the bits of the weaker frame, whichtypically result in failure of the frame’s forward error checking and frame rejection.
We are interested in capture effects because we believe that it can be used to ouradvantage to improve channel sharing efficiency. Consider the following example asshown in 2. Two concurrent connections share the same wireless communication chan-nel. The first connection is from station 2 (source) to station 1 (destination) and the sec-ond is from station 3 to station 4. In current IEEE 802.11 DCF, whichever connectionacquires the channel first gets to complete its data frame delivery message exchangebecause stations of the other connection would have detected the carrier signals ofthis connection, or received reservation messages (RTS/CTS) of this connection, andremain blocked.
However, if the stations are positioned in such a way that the energy levels of station3 and 4’s transmissions as measured at station 1 and 2 are not strong enough that station1 and 2 can still capture each other’s transmissions, stations of the second connectionshould be permitted to communicate, even after stations of the first connection havebegun their frame exchange. Similarly station 1 and 2 can do the same if station 3 and4 have acquired the channel first.
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� �� �� �� �
x
v s
R
I
A(x)
r m
m '
x '
dm
Figure 3: Capture analysis wherex′ = x√α
andm′ =√
αm
Because IEEE 802.11 uses a combination of physical carrier sensing and channelreservation, and these mechanisms are generally effective in reducing packet collisions,the protocol is rather pessimistic and not very efficient in channel use because it doesnot encourage enough concurrent transmissions. Different researchers [10, 23, 13, 2]manipulated the functionality of RTS/CTS in order to enhance network performanceeither by reducing the collision rates or increase the channel spatial reuse. However,within the solutions proposed by [10, 2, 13], capture effect was not considered in theanalysis, and [23, 2, 13] built complex mechanism over assumptions that do not reflectthe complexities of WLAN system and wireless communication well.
It is the above stated observations and inspirations from various related workswhich lead us to our own modification to the IEEE 802.11 DCF protocol. We name themodification Location Enhanced DCF (LED).
3 Analysis of blocking probabilities with capture effect
In this section, we perform an analysis on the probabilities of successful transmissionin the presence of sensed signal(s). Using this probability, we illustrate the space forimprovement to the original DCF in terms of over-pessimistic blocking of transmission.
A free space omni-directional propagation channel model [19], in which manychannels, especially outdoor channels, have been found to fit in practice is assumed. In
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this propagation model, the received signal power,Pr, is calculated as the following:
Pr =
{Pt∗Gt∗Gr∗λ2
(4∗π)2∗D2∗L D ≤ Dcross
Pt∗Gt∗Gr∗h2t∗h2
r
D4∗L D > Dcross
(2)
wherePt is the transmission power,Gt is the transmitter antenna gain,Gr is the re-ceiver antenna gain,D is the separation between transmitter and receiver,ht is thetransmitter elevation,hr is the receiver elevation,L is the system loss factor not re-lated to propagation (≥ 1), λ is the wavelength in meters, andDcross is calculated asDcross = (4 ∗ π ∗ hr ∗ ht)/λ. The first sub-model of Equation 2 is called the Friisfree-space propagation model and only used when the distance between the transmitterand receiver is small. The second sub-model is called the two-ray ground reflectionmodel and used when the distance is large.
We assume that stations are uniformly distributed over an area with a density ofδ.Each station has a transmission rangeR and a carrier sense rangeI, with the formerbeing the range within which frames sent by the station can be received and decoded,and the latter being the range within which transmissions of the station can be detected(carrier busy). For the ease of analysis, we assume that all stations have the same trafficmodel. All data packets are of the same length. Each packet requires transmissiontime τ , and is randomly destined to a local neighborhood node. One data packet isgenerated at a randomly selected time within every time intervalT , whereT > τ . Wealso assume that all transmitters use the same transmission power and all antenna gainsare the same.
We are concerned about the scenarios where a nodev may cause interference toanother noder which is receiving a data packet delivery from nodes as shown inFigure 3. Nodev transmits only if its transmission doesn’t affect the reception of DATAand ACK frames at nodesr ands respectively. Using the Friis radio propagation modelas in equation 2 and receiver capture model as in equation 1, to allow nodess andr tocapture correctly each other’s packets in the presence of any transmission from nodev,the following should hold:
(v.s >√
α s.r) AND (v.r >√
α s.r) (3)
wherea.b is the distance between nodea and nodeb, andα is the capture ratio. weonly use the Friis propagation model for the sake of analysis simplification.
Figure 3 illustrates the situations for bothr to captures’ transmissions (DATA)and fors to capturer’s transmissions (ACK) in the presence ofv’s transmission. Forrto captures’ transmissions, givenm being the distance betweens andr, the distancebetweenv andr must be greater than
√αm. Fors to capturer’s transmissions, given
x being the distance betweenv ands, r must be within a circle of radiusmin(R, x√α).
Considering both conditions,r must be located within the shaded areaA(x) in thefigure. Hence, the probability thatv’s transmission doesn’t corrupt communicationbetweens andr is:
P (B|x) =A(x)πR2
(4)
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where the areaA(x) is calculated as follow:
A(x) =∫ min(R, x√
α)
0
2(π − arccos(x− x2−m2+(
√αm)2
2x
m))m dm
(5)
By unconditioningx, since we only worry about potential interferers within thecarrier sensing range, we obtain:
P (B) =∫ I
0
A(x)πR2
2x
I2dx (6)
Based on the traffic model, the probability that none of the nodes within the carriersensing range of a node will transmit is obtained by:
P1 = [1− τ
T]δπI2
(7)
and the probability thatv’s transmission will not interfere with other transmissions (ifany) in the interference range is:
P2 = [1− τ
T+
τ
TP (B)]δπI2
(8)
Therefore, the probability thatv can transmit with the presence of a nearby trans-mission without corrupting this transmission is given by:
Pb = P2 − P1 (9)
Note that the calculatedPb is still conservative because of the following two as-sumptions:
1. Only the Friis propagation model is used in analysis because we assumeI <Dcross. However, in practiceI may be greater thanDcross and thus the distancex could also be greater thanDcross. In this case the two-ray ground modelshould be used instead, which further reduces the probability of the interferenceand consequently increases thePb.
2. In the analysis, for simplicity, we assume that all nodes in the vicinity ofv havethe freedom of transmission. We do not take into accounts that some of thesenodes will have to block because of other ongoing transmissions in their vicini-ties. Accounting for these blocked nodes would reduceP1 and hence increasePb.
We verified our analytical results by generating random network topologies andtraffic patterns and studying the interference situation in each case. We also studyhow our simplified assumptions stated in the previous paragraph affect our blockingprobability estimation by relaxing them in simulation runs.
8
0
0.2
0.4
0.6
0.8
1
50 100 150 200 250 300 350 400 450 500
Pro
babi
lity
No. of Nodes
Anl. P_2Anl. P_1Anl. P_bSim. P_2Sim. P_1Sim. P_b
Figure 4: Blocking Probability
0
0.1
0.2
0.3
0.4
0.5
50 100 150 200 250 300 350 400 450 500
Pro
babi
lity
No. of Nodes
tau/T=0.01 (with assump.)tau/T=0.01tau/T=0.02tau/T=0.03tau/T=0.04tau/T=0.01tau/T=0.10
Figure 5:Pb with differentτ/T values
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For constructing each random network, we place thev node at the center of anarea of1000 × 1000. Transmitter nodes are distributed uniformly in this area. Then aposition is picked within the simulation area and uniformly within the circle with radiusR centered at each transmitter for its corresponding receiver to establish a connection.Then each transmitter starts transmitting following the traffic model described before:all packets require transmission timeτ and they are generated randomly at a constantrate: one packet every time intervalT , whereT > τ . Whenv has a frame to send, westudy if it will be blocked under the current IEEE 802.11 operations and when blockedif indeed v’s transmission will collide with other communications. The number ofsituations where unnecessary blocking is suggested by IEEE 802.11 is then dividedover the total number of simulated situations to derive the probability of unnecessaryblocking, which is compared to the analytical result.
Figure 4 plots both the analytical and the simulated values ofP1, P2, andPb forR=250, I=550,α=5, τ/T=0.01, and different numbers of nodes (thus varying the nodedensityδ). As we can see, the simulation results closely match the analytical resultswhich validates our analysis.
Figure 5 plots the simulation ofPb with the simplification assumptions relaxed.Also this figure plotsPb with different packet load values. ThePb plot for with theseassumptions, which is copied from Figure 4, is also include for easy comparison. Theplots show that our analytical results are conservative.
As expected the probability analyzed above only takes into account whether thechannel assessing node’s transmission may corrupt other ongoing data deliveries. Itdoes not address if this channel assessing nodes’ transmission will be received correctlyby its receiver. Such a transmission may still fail at its receiver if other ongoing datadeliveries produce enough interfering energy there.
The above analysis shows that the unnecessary blocking probability of DCF is largeenough (as high as 35%) to motivate us to consider modifying the MAC layer to exploitthe capture phenomena of the physical layer. In the following section we will describethe newly proposed IEEE 802.11 DCF modifications.
4 Location Enhanced DCF for IEEE 802.11
In this section, we describe our Location Enhanced DCF (LED) for IEEE 802.11. Be-fore we introduce our approach of using location information and capture effect toimprove channel efficiency, several terms which will be used during the descriptionneed to be clarified to avoid confusion.
In our description, we use the term “delivery” for the whole handshake procedurefor delivering a unicast packet. Depends on the packet size, a “delivery” may involvethe full RTS-CTS-DATA-ACK 4-way message exchange sequence or just DATA-ACK2-way exchange. A “source” is the station having data to send during a delivery. The“destination” of a delivery is the station to whom the source wishes to send data. The“sender” and “receiver” are the sender and receiver of individual RTS, CTS, DATA,or ACK frame. “Transmitter” is used interchangeably with “sender”. “Connection” isused to refer to both the source and destination stations collectively.
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4.1 Protocol Overview
Our approach is simple: to include more information about each transmission in thetransmission so that any other stations hearing the transmission are able to better assesswhether their own transmissions may collide with this transmissions. Among varioustransmission related parameters, the locations of the transmitters and receivers are themost important. We assume that each node is capable of acquiring its own location,e.g. GPS or other RF based localization methods. A station can retrieve other commu-nication parameters regarding its own transmitter/receiver easily as they are typicallyconfiguration parameters.
At a given station, for a particular on-going delivery that does not involve itself,if the locations and other communication parameters such as transmission power andantenna gains of the source and destination stations are known, using the propagationmodel1 in equation 2 this station can compute the received energy level of the framesof the data delivery at their receivers. Then if the capture ratio of the receiver is alsoknown, knowing its own location, antenna gain, and transmission power, this node canmake a prediction of whether its own transmission may interfere this on-going datadelivery.
This is a rather simplified estimation model as each channel assessing station onlyconsiders the effects from its own potential transmission. It may occur that several sta-tions simultaneously predict that their own transmissions will not cause collision to theon-going delivery. In this event, the aggregated energy from all these side transmissionsmay actually change the result of the capture effect and cause enough interference withthe ongoing delivery. We postpone this issue to future studies as we currently expectthat the probability of this happening is low.
Another issue is that in the current model, a station only is concerned about ifits own transmission will affect an on-going delivery. It does not consider if its owntransmission can be received correctly by its destination. This optimistic approach islargely for keeping the model simple at its current stage. Studies for the impact of thisadditional estimation refinement are also deferred.
4.2 PHY
As we point out earlier, the current 802.11 standards do not require a WLAN receiverto be able to capture a new (stronger) frame after the receiver has been tuned to receiveanother frame, even if the signals of the new frame are strong enough to be captured. Aswe explained before, unless the capture capability is specifically designed into WLANreceivers, they are usually not able to correctly capture the new frame. This may causeproblems in our approach. If a station decides to transmit after it estimates that its owntransmission will not interfere with an ongoing delivery, it will begin to send its ownframe. However, chances are that the intended receiver of this frame is already engagedin receiving another frame, one of the frames of the on-going delivery. As a result, thisreceiver will not receive and interpret the new frame correctly even if the signals arestrong enough.
1This model can be easily changed to any other propagation model depending on the operation environ-ment of the system without affecting how the protocol operates.
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Figure 6: Packet Capture of 802.11
Below is a more detailed analysis of what may happen at a receiver which do notprovide support for capturing of a stronger frame if it arrives after the receiver hasbegun receiving a weaker frame. Depending on at what exact moment the beginningof the stronger frame arrives during the reception of a weaker frame, there are threedifferent situations as shown in Figure 6. Each IEEE 802.11 DSSS frame has threesections based on their affects on receiver’s physical and MAC layer operations: thePhysical Layer Convergence Protocol (PLCP) Preamble section is used to train the re-ceiver modem for synchronization; the PLCP Header section contains medium (DSSS)dependent information such as modulation choice and frame length; and the PSDUsection contains the actual MAC layer data.2 Accordingly, the reception of a frame isalso broken into three stages.
1. If the stronger frame arrives during the training period of the modem towardsreceiving the weaker frame (stage 1), the modem is able to get retrained andswitch to reception of the new stronger frame.
2. If the stronger frame arrives during the reception stage of the weaker framesPLCP header, or stage 2 in the illustration, the new signal would likely destroy
2In the original IEEE 802.11 standard, this section is called MAC Protocol Data Unit (MPDU). The laterIEEE 802.11b standard changes it to PSDU (PLCP Service Data Unit).
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First Transmission
Second Transmission
Recepition
t
Receiver energy increase
Figure 7: Message In Message
the data contained in the weaker frames PLCP header and result in PLCP re-ception error or CRC failure, in which case the receiver goes back to idle state.Then, if this happens soon enough, the receiver may still be able to detect thenew carrier for the stronger frame and get trained for receiving it, if the strongertransmission is still in its SYNC portion of the frame.
3. If the stronger frame arrives during the reception stage of the weaker framesPSDU (stage 3), it would most likely destroy the reception due to two majorreasons. First the demodulation algorithm the receiver is currently engaged infor the weaker frame may be different from the modulation used by the strongerframe. Second the bits of the stronger frame, even correctly demodulated, areinterpreted as part of the PSDU of the weaker frame and passed up for MACprocessing. After the whole message is received, the MAC forward error detec-tion mechanism will fail and the frame is dropped. Unless the stronger transmis-sion is still in its SYNC section and the receiver can catch it quick enough, thestronger frame will not be received correctly either.
For nodes that have received a frame in error, obviously they can not determine theduration of the reservation, they need to wait for an Extended Inter-Frame Space (EIFS)after the carrier becomes idle. The EIFS will leave enough time for the on-going frameexchange to finish.
Fortunately, WLAN receiver designs which do support the capture of a new frameafter the receiver has already begun to receive another frame do exist. One exampleof such a receiver PHY design is Lucent’s PHY design with “Message-In-A-Message”(MIM) support [5]. In this design, the newly arrived frame is referred to as the “(new)message in the (current) message”.
A MIM receiver is very similar to normal WLAN PHY designs. Except that it con-tinues to monitor the received signal strength after the PHY transits to data receptionstate. If the received signal strength increases significantly during the reception of aframe, as shown in Figure 7, the receiver considers that it may have detected the be-ginning of a MIM frame and hence switches to a special MIM state to handle the newframe.
While under the MIM state, the receiver tries to detect a carrier for a new frame. Itthe carrier signal is detected, the receiver begins to receive the initial portion (namely
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PLCP Header Preamble
SYNC SFD Signal,Service,Length CRC MAC Frame � �� �
ENH
MPDU/PSDU
LOCT PWRT GAINT LOCR PWRR GAINR Header Body FCS DUR
Figure 8: Frame Structure
the preamble) of the new frame and gets retrained to be synchronized with the newtransmission. If no carrier is detected, which means the energy increase is caused bynoise, the PHY will remain in this MIM state until either a carrier is detected or thescheduled reception termination time for the first frame is reached.
With a MIM capable design, a WLAN receiver is always able to correctly detectand capture a strong frame regardless the current state of the receiver, unlike in regularPHY designs the strong frame can only be correctly detected and captured while thePHY is under certain states during its reception of a weak frame.
4.3 MAC
Our enhanced design for a DCF WLAN MAC is atop of a MIM capable PHY. Figure8 shows the frame format to support the enhanced functionalities of the new MAC.
We propose to insert a block of information called ENH (“Enhanced”) to providethe additional information needed for LED. Since the earlier the ENH block is received,the sooner the receiver can decide if it needs to block its own transmission, the ENHblock should be inserted before the true MAC data section, or the PLCP (Physical LayerConvergence Protocol) Service Data Unit (PSDU). However, inserting ENH before theend of the DSSS PLCP header or at the beginning of the PSDU needs a little morethinking. In current design, we have the ENH as part of the PLCP header mainly dueto the fact that firstly the PLCP header has its own CRC field so the contents of theENH block can immediately be verified and utilized, and secondly all stations withinthe service set can understand the ENH block since the PLCP header is transmitted atthe base rate.
The ENH block is further divided into seven fields. The LOCT field contains the lo-cation of the frame transmitter, the PWRT field describes the transmission power of thetransmitter, and the GAINT field specifies the transmission antenna gain. The LOCR,PWRR, and GAINR fields contain the same pieces of information for the receiver. TheDUR field is a copy of the Duration field of a RTS, CTS, DATA, or ACK messageMAC format, except that it is in PLCP header instead of PSDU.
When a source has a unicast data packet to send, it starts by sending out an RTSmessage to reserve the channel. In this message, the source fills the LOCT, PWRT, andGAINT fields with its own parameters, and the LOCR, PWRR, and GAINR with thedestination’s parameters. If these parameters are not known at that time, they are setto NIL. Upon receiving the RTS, the destination of the data packet copies the LOCT,PWRT, and GAINT fields into the corresponding fields of its CTS message. It also fills
14
PLCP Header Preamble
SYNC SFD Signal,Service,Length CRC Data
MAC
PHY PLCP
DSSS PHY PMD
PMD_ED/ PMD_CS
PHY_CCA. ind(BUSY)
not PMD_ED/ PMD_CS
PHY_CCA. ind(IDLE)
PHY_RXSTART. ind(RXVECTOR)
PHY_RXEND. ind(RXERROR)
ENH
MPDU/PSDU
PMD_DATA. ind
PMD_DATAi nd
PMD_DATAi nd
PDM_RATE.req PDM_RATE.req PMD_MODULATION.req
PHY_DATA. ind(DATA)
PMD_DATAi nd
PHY_DATA. ind(DATA) LED
Figure 9: PHY-MAC Interactions
the LOCR, PWRR, and GAINR fields of the CTS message with its own parameters. Insubsequent DATA and ACK messages, full descriptions of both the source and the des-tination are included. In case of the frame size being less than the RTS/CTS thresholdand no RTS/CTS handshake being conducted, the DATA message will have its fieldsset in the same fashion as the RTS message, and the ACK message is filled the sameway as the CTS message.
In the standard IEEE 802.11, the PHY (PLCP in particular) does not deliver anydata bits to the MAC layer until the PSDU reception has begun. Then the receiver willproceed all the way till the end of the message (unless the carrier is lost in the middle ofthe reception). Received bits are passed to the MAC layer as they are decoded for beingassembled into the MAC frame. At the end of the PSDU is a forward error detectionCRC block called FCS. If the MAC frame passes the CRC check, it is accepted andpassed up for further 802.11 MAC processing. If the CRC fails, the frame is dropped.In LED, because of the ENH block’s location, additional data namely the ENH blockneeds to be passed from the PLCP layer to the LED part of the MAC layer for pro-cessing. The signal and data interactions among the layers are illustrated in Figure9.
A parameter cache may be maintained by stations to store the location, power, andantenna information of already known stations. This way when sending data to a stationin cache, the cached parameters may be used in the corresponding fields of the ENHblock instead of NIL values. Cache entries are updated if newer information is receivedfrom their corresponding stations. Cache entries are removed after its expiration time.
Upon receiving a frame with ENH block, a station other than the intended receiverof the frame begins interference estimation. That is, if this station’s own transmissionwill cause interference to the delivery which the just received frame belongs to. Thestation needs to calculate the power level of its own transmission at both the source
15
P si and destinationP d
i of the ongoing data delivery using Equation 2. The station alsoneeds to calculate the received power level of the destination at the sourceP s
d and thatof the source measured at the destinationP d
s . If (P sd > αP s
i ) and(P ds > αP d
i ), thestation does not block its own transmissions. Otherwise, it blocks its transmissions. Inthe case that the communication parameters of either the source or the destination areunknown, the assessing node assumes the worst and blocks its own transmission.
Normally, after a station other than the intended receiver of the frame receives aRTS, CTS , DATA, or ACK message, it needs to set its NAV value according to theDuration field of the message, which is set to the time required for the full RTS-CTS-DATA-ACK message exchange to finish. In LED, a non-receiver station will only setthe NAV according to the standard when it determines that its own transmission willinterfere with this on-going delivery. Otherwise, the NAV is not set.
In addition, if the non-receiver station determines that it does not need to block itown transmission to yield to the on-going delivery, it also needs to set a special vec-tor called CCA-Suppression Vector (CSV). The CSV is a suppression timer. It is setaccording to the Duration field of the received RTS, CTS , DATA, or ACK message,which means the timer will run till the whole on-going delivery is completed. Togetherwith an active CSV each station needs to remember the source and destination stationsof the delivery the CSV is referring to so that this station will block its own transmis-sions addressed for either. Just like NAV, if a later message prolongs the duration ofthe delivery, the CSV can also be extended. Once the CSV counts down to zero, aCCAReset signal is issued to the PHY layer so the CCA mechanism will retest the car-rier and report the result. Upon receiving a busy CCA indicator, the station will blockuntil either the carrier is idle again, or another frame is captured.
On the source or destination station of the on-going delivery, according to the stan-dard, the NAV is not set for the duration of the delivery. In LED, this specification isstill followed. However, a LED receiver does set its CSV to the end of the delivery.The reason is that since LED permits concurrent transmissions by other stations as longas they do not produce enough energy to disturb the on-going delivery. Thus, if anyother station is indeed transmitting, their carrier may cause the source and destinationof the on-going delivery to abort their RTS-CTS-DATA-ACK handshake. Thus, theCCA should be suppressed on the source and destination stations till the end of theirdelivery.
In total, a LED station has three indicators related to the channel estimation. TheCCA is the physical carrier indicator. It is “TRUE” when the PHY layer detects carrier(or energy exceeding threshold, or both, depending on equipment vendor implemen-tation). The NAV indicator is the virtual carrier indicator. It is “TRUE” when thereis a reservation which needs to be honored. That is, if this station transmits, then thetransmission will interfere with the on-going delivery. Finally, the CSV indicator tellsthe station if it should ignore the CCA. It is “TRUE” when the suppression timer is run-ning. The decision of whether this station should block its own transmission is madeas follows.
if ((CCA AND (NOT CSV)) OR NAV) then BLOCK (10)
Since the CSV is implemented in MAC layer, even when Equation 10 indicatesnot to block, the PHY layer may not have the PHYTXSTART signal enabled if it hasengaged in receiving. Thus, a PHY reset signal is needed to force the PHY to leave
16
the receiving state and enable PHYTXSTART signal when the MAC has a frame tosend. Following the reset, the PHY layer enters transmission state to send out theframe. However, on the source or destination station of an ongoing delivery, if duringthe SIFS gaps within the message exchange sequence a station detects a carrier andbegin to receive a new frame, it needs to block till its PHY to finish the PLCP headerpart of the packet. Then the test of Equation 10 is carried to see if continuing with thedata delivery message sequence will cause frame collision at the receiver of this newframe. Only if the test result is no blocking, the delivery sequence proceeds.
Another issue occurs if a channel assessing node only detects carrier but can notdecode the frame. In this case this node is not able to estimate whether its transmissionwill affect this on-going transmission. Either an aggressive approach or a conservativeapproach can be taken. In the aggressive approach this node will not block its owntransmission in the event of “detecting a carrier but not being able to decode the frame”,while in the conservative approach this node will block its own transmission.
A tricky issue requiring more discussion is for a station to receive messages frommore than one delivery. We refer to this situation as the “stacked delivery”. Simplestacking situations can be handled by using CSV and NAV together. If the first deliverydoes not require this station to block its own transmission (non-blocking delivery), theNAV is not set and the CSV is set till the end of this delivery. Now the second deliveryis started and it is a blocking delivery. In this case, the NAV is set immediately tothe end of the second delivery, and the CSV is not changed. The overall result is thestation will block from the moment the first frame of the second delivery is receivedtill the completion of the second delivery. The opposite case is that the first delivery isa blocking delivery and the second is non-blocking. Now the NAV is set for the firstdelivery and the CSV is set for the second. Overall, the station only blocks till the endof the first delivery when the NAV is cleared. More complex stacking situations are leftfor future works.
5 Performance Evaluation
In this section, we present extensive simulation-based studies on the performance of theLED mechanism. The performance comparisons were done using thens-2simulator,enhanced with the CMU-wireless extensions (the underlying link layer is IEEE 802.11with 11 Mbps data rate). In doing this, we extended ns-2 as follows:
• We modified the capture model to allow receivers to capture the stronger packetout of the weaker packet(s), as in Equation 1, if the stronger packet comes afterthe weaker to reflect the PHY design as discussed in the previous section.
• Current implementation of ns-2 allows the node to compare the newly comingpacket only with the one it is receiving. In order to implement the capture Equa-tion 1, we extended the PHY layer in ns-2 to allow each node to keep track ofall its incoming packets and the aggregated background signals. Also in order tocreate a more realistic environment, we allow each node to aggregate the signalsthat have lower values than the CSThreshused by ns-2.
17
• We enhanced the IEEE 802.11 MAC layer by extending it with the implementa-tion of our LED mechanism.
Each of our simulated networks consists of a set of connections which are con-structed as pairs of stationary sender and receiver nodes. The senders and receiversare placed in a1000m× 1000m area in the same fashion as the simulations describedbefore in Section 3. We assume that each sender has already cached the location ofits corresponding receiver. Other parameters such as transmission power levels andantenna gains are also assumed to be fixed and known to all stations therefore not in-cluded in simulation. In simulation, the ENH header only contains LOCT and LOCRfields of 32 bits each.
The transmission radiusR of a node is selected to be 250m while the interferenceradius is 550m. Each connection is a UDP flow with packets of 1000 bytes which aretransmitted at 11Mbps. To simplify the simulation implementation, all RTS and CTSmessages, as well as the PLCP headers, are also sent as 11Mbps. Such a simplificationshould not affect the correctness of the evaluation method since we are more interestedin relative performance improvement. Each simulation is run for a fixed duration of 50seconds. Each point on the curves to be presented is an average of 5 simulation runs.
IEEE 802.11 equipment does not come with capture ratio specifications. The cap-ture ratio used in simulation is derived by the following method. Given a specific BitError Rate (BER) the theoretical required Signal to Noise Ratio (SNR) for a particu-lar modulation technique can be calculated. In the case of 11mbps CCK modulation,according to calculations described by [19], it can be determined that 18dB of SNR isneeded to achieve10−8 BER, as specified by Orinoco’s WLAN cards. The 11 mbpsCCK uses 8 chip/symbol, which is 9dB spreading gain. In addition, CCK coding pro-vides about 2dB additional coding gain. All together the processing gain is 11dB.When only considering signals before receiver processing, the SNR requirement is7dB. Roughly, this maps to 5 times signal power over interference. We adopt the samenumber as the capture ratio. In our model, when a station is receiving frame A andframe B arrives, if the received power of frame A,PA, is more than 5 timesPB , thereceiver captures A and continuously receives frame A; ifPB is more than 5 times ofPA, the receiver captures B and drops A; and in all other situations, packets collide andno frame is received.
We modeled various scenarios of different node densities, work loads, transmissionand interference ranges (transmission power levels), and errors in location estimationand their effects on performance. To study the performance of our suggested schemes,we compare our LED with both theOriginal IEEE 802.11 andMACAW protocols3.As described in Section 4, we experiment with two different flavors of LED:LED CSandLED RX. LED CS mechanism is an aggressive (optimistic) version of LED mech-anism in which a node receiving a frame with signal level lower than RXThresh, froman on going transmission, assumes its transmission will not interfere with that ongoingtransmission and therefore should not block. On the other hand, LEDRX is a con-servative (pessimistic) version of LED in which a node assumes its transmission willinterfere with the ongoing transmission of a frame with a signal lower than RXThresh.
For evaluation, we take the following measurements:
3Both Original and MACAW protocols use the extended ns-2 capture model as described earlier.
18
SIFS
Node 1
SIFS
RTS
�
CTS
�RTS
� �� �
RTS � �� � SIFS
CTS
�
Timeout (SIFS+Data)
SIFS
Node 2
Node 3
Node 4 �Data
Blocking period
ENH header
Figure 10: Medium under utilization scenario of LED mechanism
1. Effective Throughput: This counts the total number of packets received by allthe receiver nodes over the simulation period.
2. Collision Packets: This counts the total number of collisions that involve datapackets by all the node over the simulation period. Note that this metric considersthe collision of the data packets at all nodes, not only destination nodes.
3. Fairness Index: To measure the bandwidth sharing of the connections underdifferent mechanisms, we use Jain’s fairness index [7, 12] which is defined asthe following:
F =(∑N
i=1 γi)2
N∑N
i=1 γ2i
(11)
whereN is the number of connections andγi is the number of received packetsfor connectioni.
We start with experimenting using RTS/CTS access mode. Although the LEDmechanism forces each node to be blocked during the ENH header of each receivedframe, we found that forcing the node to be blocked during the RTS/CTS period of theother connections will increase the network throughput. The reason for this is morerelated to the particular ns2 implementation of the physical layer. To explain this, con-sider Figure 2 in which each node of node 2 and node 3 has packet to send to node 1 andnode 4 respectively. Assume node 2 would start the transmission cycle by transmittingRTS packet to node 1 ashown in Figure 10. When node 1 receives the RTS packet, itwaits for SIFS period and, if the medium is still idle, transmit the CTS back to node2. After transmitting the CS packet, the ns2 implementation of node 2 sets a timer forperiod equal to the SIFS period plus the period for transmitting the data packet. In ns2implementations, node 2 is expecting to receive the data packet within this period andif it timeouts with no received data packet, it indicates the failure of the current trans-mission cycle. Meanwhile, as shown in Figure 10, node 3 detects the CTS packet andfigure out that its transmission will not affect the on going transmission and hence de-cided to start its own transmission cycle by transmitting the RTS packet that happenedto be within the SIFS period after the CTS packet of node 1. Since LED mechanism
19
60000
80000
100000
120000
140000
160000
180000
200000
50 100 150 200 250 300 350
Effe
ctiv
e T
hrou
ghpu
t
# of Connections
OriginalLED_CSLED_RXMACAW
Figure 11: Effective throughput ver-sus node density using RTS/CTS accessmode.
0
5
10
15
20
25
50 100 150 200 250 300 350
Enh
ance
men
t Per
cent
age
# of Connections
LED_CSLED_RXMACAW
Figure 12: Throughput enhancementover Original protocol versus node den-sity using RTS/CTS access mode.
forces each node to be blocked during the ENH header of each received frame, node 2will be blocked during the ENH period of the RTS of node 3 which happen to last morethan the SIFS period. Therefore, node 2 detects the failure of its current transmissioncycle and tries to start another transmission cycle by sending new RTS packet to node1 as shown in the Figure after waiting for DIFS and a doubled contention window.However, during the ns2 implementation, node 1 which is expecting data packet willdrop such RTS packet. Node 2 will timeouts and detects an unsuccessful transmissionand then tries again to send RTS packet after the DIFS and new doubled contentionwindow. Node 1 will drop any non data packet (e.g. RTS packets) while it is waitingfor its timeout to expire. This mechanism/implementation under utilizing the mediumand hence reduce the network throughput. IEEE 802.11 standards do not specify howa node should react when it receives non data packet while it is expecting to receivea data packet during a certain time period. To enhance the network performance byeliminating such problem, we forced each node to be blocked during the transmissionperiod of RTS/CTS cycle. This could be done by including the blocking duration infor-mation in the ENH header or set one of the locations in ENH to null to force the nodesto be blocked for the whole packets and then set NAV to the end of RTS/CTS cycle in-stead of the while transmission cycle. Back to Figure 10, with such mechanism, node3 will be blocked from transmission until the transmitting the ENH of the data packetby node 2 and therefore the on going transmission cycle will not be disturbed.
Figure 11 shows the effective throughput of the networks with different numbersof connections. The UDP flows are constant bit rate (CBR) at a rate of 20 packets persecond. LEDCS, LED RX, MACAW have higher throughput than the original IEEE802.11 DCF. Figure 12 further illustrates the improvements by showing the percentagethroughput gain of LEDCS, LED RX, and MACAW over Original. At their peaks,LED CS could reach about 20% more than the Original amd LEDRX could reach to22% higher throughput than Original while MACAW could reach to 8%. LEDRXexperiences higher throughput than LEDCS for scenarios especially with the numberof connections is large because of the aggressive nature of the LEDCS mechanism,which also leads to high number of collisions and retransmissions. The aggressiveness
20
0
10000
20000
30000
40000
50000
60000
70000
80000
50 100 150 200 250 300 350
Col
lisio
n P
acke
ts
# of Connections
OriginalLED_CSLED_RXMACAW
Figure 13: Packet collisions versus nodedensity using RTS/CTS access mode.
0.25
0.3
0.35
0.4
0.45
0.5
50 100 150 200 250 300 350
Fai
rnes
s In
dex
# of Connections
OriginalLED_CSLED_RXMACAW
Figure 14: Fairness index versus nodedensity using RTS/CTS access mode.
20000
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200000
0 50 100 150 200 250 300 350 400
Effe
ctiv
e T
hrou
ghpu
t
Packets/Second/Node
OriginalLED_CSLED_RXMACAW
Figure 15: Effective throughput ver-sus network load using RTS/CTS accessmode.
0
5
10
15
20
25
0 50 100 150 200 250 300 350 400
Enh
ance
men
t Per
cent
age
Packets/Second/Node
LED_CSLED_RXMACAW
Figure 16: Throughput enhancementover Original protocol versus networkload using RTS/CTS access mode.
of the LED compared to the Original is indicated in Figure 13 which shows the totalnumber of collisions that occur in the networks. On the other hand, MACAW doesnot utilize the spatial reuse as LED mechanisms. For example, a node using MACAWblocks it transmission once it hears CTS packet regardless if its own transmission willinterfere with others or not. Therefore as the node density increases, MACAW per-formance approach Original since CTS packets will cover most of the network area.Another reason for low performance of MACAW is due to its high aggressiveness. Anode hearing RTS packet but not CTS packet will assume its transmission will not in-terfere with others and start to transmit its packet. Since such node decision may beincorrect, large number of collisions happen as shown in Figure 13. Figure 14 showsthe fairness index for the different mechanisms. Using the extended capture model in-creases the unfairness in the network, however the newly proposed mechanisms havebetter fairness levels than the Original. An explanation for this is the LED mechanismsreduce the well-known “exposed node” problem in IEEE 802.11 DCF which is one ofthe sources for unfairness.
21
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0.4
0.5
0.6
0.7
0.8
0.9
1
0 50 100 150 200 250 300 350 400
Fai
rnes
s In
dex
Packets/Second/Node
OriginalLED_CSLED_RXMACAW
Figure 17: Fairness index versus net-work load using RTS/CTS access mode.
20000
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180000
200000
0 50 100 150 200 250 300 350 400
Effe
ctiv
e T
hrou
ghpu
t
Packets/Second/Node
OriginalLED_CSLED_RX
Figure 18: Effective throughput versusnetwork load using basic access mode.
0
2
4
6
8
10
12
14
16
18
0 50 100 150 200 250 300 350 400
Enh
ance
men
t Per
cent
age
Packets/Second/Node
LED_CSLED_RX
Figure 19: Throughput enhancementover Original protocol versus networkload using basic access mode.
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 50 100 150 200 250 300 350 400
Fai
rnes
s In
dex
Packets/Second/Node
OriginalLED_CSLED_RX
Figure 20: Fairness index versus net-work load using basic access mode.
Next, we experiment with different network packet loads to see their effect on theprotocol performance. We fix the number of connections in the network to be 50 andchange the packet rate at each source node from 10 packets per second to 400 packetsper second. Figures 15 and 16 show the effective throughput and the relative en-hancements of each mechanism over the Original respectively. As shown, LEDCShas the highest throughput over LEDRX and MACAW. LED RX performs not as wellas LEDCS and MACAW since it is a conservative mechanism and with small numberof 50 connections as in our experiment, a node will block long period of times while itcan transmit within such period with no interference with other transmissions. This isopposite to LEDCS which takes an advantage of its aggressive mechanism and avoidsuch blocking periods. With network with larger number of connections as shown pre-viously in Figure 11, LEDRX will yield a better results than LEDCS since it reducesthe number of collisions in the networks. Figure 17 shows the fairness index of all themechanisms. LEDCS, LED RX, and MACAW protocols have similar fairness indexmeasurements which are higher than the Original since they try to resolve the exposed
22
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160000
165000
0.5 1 1.5 2 2.5 3
Effe
ctiv
e T
hrou
ghpu
t
Capture Factor
LED_CSLED_RX
Figure 21: Effective throughput versuscapture factor (β) using RTS/CTS accessmode.
145000
150000
155000
160000
165000
0 20 40 60 80 100
Effe
ctiv
e T
hrou
ghpu
t
Error Range (meters)
LED_CSLED_RX
Figure 22: Effective throughput versuserror range using RTS/CTS access mode.
node problem.Basic access mode shows similar performance to the RTS/CTS mode when we
experimented it using different network packet loads. Figures 18 and 19 show the ef-fective throughput and the relative enhancements of each mechanism over the Originalrespectively. Similarly, Figure 20 shows the fairness index of all the mechanisms. Notethat MACAW protocol does not implement the basic access mode.
As pointed out earlier, it may occur that several nodes simultaneously predict thattheir own transmissions will not cause interference to the on-going delivery and hencestarts their own transmissions. In this event, the aggregated energy from all these sidetransmissions may actually change the result of capture effect and cause interferencewith the ongoing delivery. To eliminate this problem, we multiply the capture ratioαused in the blocking Equation 1 by capture factorβ. By increasingβ value over 1, wedecreases the chance that the aggregated energy from all these side transmissions wouldinterfere with the ongoing transmission. At the same time, increasingβ has the sameeffect of increasing the capture ratio in reducing the network throughput. Figure 21shows the LEDCS and LEDRX performance, using RTS/CTS mode, over differentvalues ofβ for 50 connections with CBR of 100 packets per second.
Settingβ to values less than 1 degrades the performance of both mechanisms sincethere are more chances competing nodes decide to transmit and their aggregated trans-mission will cause packet collision. Asβ increases over 1, the throughput increasessince we reduce the number of interferences caused by the aggregated signals. How-ever, increasingβ to large values has a negative effect on the throughput since it underutilizes the capture mechanism. What is more interesting is that for our experimentconfigurations, usingβ = 1.2 results in the optimal performance.
Next, we study the effect of errors in estimating node locations due to the inaccu-racy of the location estimation systems. We experimented with network configurationof 50 connections with CBR of 100 packets per second. Each node adds a distance er-ror selected randomly from the range[−Err,Err] to each accurate X and Y positionof the nodes. We tested, using RTS/CTS mode, with different values ofErr as shownin Figure 22. Surprisingly, the effective throughput increases with the lower values
23
Transmission Transmission Carrier Sense
Power Range (R) Range (I)
0.282W 250m 550m
1.427W 375m 825m
4.510W 500m 1100m
22.829W 750m 1650m
72.151W 1000m 2200m
Figure 23: Different transmission powers and their corresponding ranges.
0
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200 300 400 500 600 700 800 900 1000
Effe
ctiv
e T
hrou
ghpu
t
Transmission Range
OriginalLED_CSLED_RXMACAW
Figure 24: Effective throughput versustransmission range using RTS/CTS ac-cess mode.
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
200 300 400 500 600 700 800 900 1000
Fai
rnes
s In
dex
Transmission Range
OriginalLED_CSLED_RXMACAW
Figure 25: Fairness index versus trans-mission range using RTS/CTS accessmode.
of Err. This could be explained as using small random errors to emulate the effectof the capture factorβ as described earlier in reducing the interference possibility be-cause of the aggregated energy. However with high errors the performance of the LEDprotocols degrades where the degradation in LEDRX is higher than LEDCS sinceLED RX depends on the location information only in deciding of the blocking statuswhile LED CS depends on the signal energy in addition to the location information.
All the mechanisms under consideration are based on the transmission and the in-terference ranges in the network. To examine the performance of those mechanismsunder different ranges, we fix the maximum distance for a connection to be within250m while changing the node transmission power. Table 23 shows the used transmis-sion powers and their corresponding transmission and interference ranges used in ourexperiments using the propagations models defined by Equation 2. Figure 24 showsthe effective throughput of the network, using RTS/CTS mode, versus the transmis-sion ranges for network configuration of 50 connections with CBR of 100 packets persecond. Although in addition to transmission range, performance of the LED mech-anism depends on the network topology and node locations, the effective throughputof LED CS decreases as the transmission range increases because of the following:1) with larger ranges, more nodes hear the transmission and have to block during theRTS/CTS exchange, 2) as the transmission range increases, many of the unblocked
24
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200 300 400 500 600 700 800 900 1000
Effe
ctiv
e T
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t
Transmission Range
OriginalLED_CSLED_RX
Figure 26: Effective throughput versustransmission range using basic accessmode.
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
200 300 400 500 600 700 800 900 1000
Fai
rnes
s In
dex
Transmission Range
OriginalLED_CSLED_RX
Figure 27: Fairness index versus trans-mission range using basic access mode.
nodes which were not able to decode the transmission frames before would be able todecode those frames now and may find out that they have to block during those trans-missions. On the other hand, increasing the decoded frames because of using highertransmission powers results in many unblocked nodes that formerly would block un-necessarily because of their inability of decoding frames. Increasing the transmissionpower also reduces the LEDRX throughput as shown in the figures because of: 1) withlarger ranges, more nodes hear the transmission and have to block during the RTS/CTSexchange, 2) As the transmission power increases, the interference range increase andadditional nodes become able to hear the transmission but unable to decrypt it andhence force the nodes to block. As the transmission range increases, the area where thenodes are unable to decode the frames becomes smaller since we conduct experimentswithin a fixed square region and hence the performance of LEDRX becomes similarto the LEDCS performance. On the other hand, the performance of original 802.11DCF and MACAW keep degrading as the transmission range increases because now asingle RTS/CTS frame exchange will block more nodes. For 802.11 DCF, more nodeswill also be blocked because they sense the carrier as busy. As shown in the figure,when the transmission range is large, the performance of LED mechanisms is superiorto the original 802.11 DCF and MACAW protocols.
Figure 25 shows the effect of transmission ranges on fairness index for the RTS/CTSmode. LED mechanisms experience fixed fairness index over the different transmis-sion ranges while both original 802.11 and MACAW protocols has in increase in theirfairness index as the transmission range increases since the hidden and exposed nodeproblems are reduced. Similar results has been shown by the basic access mode withdifferent transmission ranges. Figure 26 shows the effective throughput of the networkwhile Figure 27 shows the fairness index for the basic access mode with different trans-mission ranges for network configuration of 50 connections with CBR of 100 packetsper second.
25
6 Conclusion and Future Works
In this paper we have introduced an enhancement of the IEEE 802.11 DCF (LED).This enhancement, known as the Location Enhanced DCF, includes more communica-tion parameters especially the locations of transmitters and receivers than the original802.11 DCF frames. These parameters may assist stations to better assess the channelcondition. We have shown that the 802.11 DCF is conservative in terms of channelassessment, causing as much as 35% of unnecessary blocking. On the other hand, ourLED may improve throughput as much as 22% over DCF with better fairness at thesame time.
It should be noted that although the LED achieves better throughput, it is at thecost of trying harder (or blocking less). This is indicated by the higher collision countscompared to the original DCF. Although many of these collisions occur at other nearbynodes rather than the packet destinations, they do increase overall network energy ex-penditure, which may become an issue when applying LED to energy constrained net-work applications such as sensor networks.
Among the collisions we suspect those do occur at packet destinations are mostlythe result of collision estimation errors made by channel competing nodes, whichlargely due to the fact that each node only considers how its own transmission mayaffect the ongoing delivery and not leaving space for other channel assessing nodes tomake the same decision. The studies of the effect of a static and network wideβ factorincluded in this paper are only a beginning. In future works we would also like to studythe use of variableβ factor and transmission power, as a mechanism to further improvespacial reuse, to enhance performance.
References
[1] IEEE 802.11, Part II: Wireless LAN Medium Access Control (MAC) and PhysicalLayer (PHY) Specifications.
[2] A. Acharya, A. Misra, and S. Bansal. MACA-P : A MAC for Concurrent Trans-missions in Multi-hop Wireless Networks. InFirst IEEE International Confer-ence on Pervasive Computing and Communications (PerCom’03), Fort Worth,Texas, 2003.
[3] H. Ahmadi, A. Krishna, and R.O. LaMaire. Design issues in wireless LANs.Journal of High Speed Networks, 5:87.
[4] V. Bharghavan, A. J. Demers, S. Shenker, and L. Zhang. MACAW: A mediaaccess protocol for wireless LAN’s. InProceedings of SIGCOMM, 1994.
[5] J. Boer and et. al. Wireless LAN With Enhanced Capture Provision, U.S. Patent5987033. Technical report, 1999.
[6] K. Cheun and S. Kim. Joint delay-power capture in spread-spectrum packet radionetworks.IEEE Transaction on Communications, 1998.
26
[7] D.-M. Chiu and R. Jain. Analysis of the Increase and Decrease Algorithms forCongestion Avoidance in Computer Networks.Computer Networks and ISDNSystems, 17:1–14, 1989.
[8] C. Fullmer and J.J. Garcia-Luna-Aceves. Floor Acquisition Multiple Access(FAMA) for Packet-Radio Networks. InProceedings of SIGCOMM, Cambridge,MA, 1995.
[9] E. Geraniotis and M. Soroushnejad. Probability of Capture and Rejection of Pri-mary Multiple Access Interference in Spread Spectrum Networks.IEEE Trans.on Communications, 39(6), 1991.
[10] X. Guo, S. Roy, and W. S. Conner. Spatial reuse in wireless ad-hoc networks. InVehicular Technology Conference, 2003.
[11] Z. Hadzi-Velkov and B. Spasenovski. Capture Effect in IEEE 802.11 WirelessLANs. In Proc. of IEEE ICWLHN 2001, 2001.
[12] R Jain. The Art of Computer Systems Performance Analysis. John Wiley andSons, 1991.
[13] H. Ju, I. Rubin, and Y. Kuan. An adaptive RTS/CTS control mechanism for IEEE802.11 MAC protocol. InProc. of IEEE Vehicular Technology Conference- VTC2003, Jeju, Korea, 2003.
[14] P. Karn. MACA - A New Channel Access Method for Packet Radio. InARRL/CRRL Amateur Radio 9th Coputer Networking Conference, 1990.
[15] S. Kubota, K. Mutsuura, O. Akizuki, and S. Ooshita. A random access micro-cellular sustem.IEICE Trans. Fundamentals, (7):1241.
[16] C.T. Lau and C. Leung. Capture Models for Mobile Packet Radio Networks.IEEE Transactions on Communications, page 917.
[17] J.J. Metzner. On improving utilization in ALOHA networks.IEEE Trans. Com-mun., page 447.
[18] K. Mutsuura, H. Okada, K. Ohtsuki, and Y. Tezuka. A New control schemewith capture effect for random access packet communications. InProc. IEEEGLOBECOM, page 938.
[19] J.G. Proakis. Digital Communications,4th edition. Page 270. New York,McGraw-Hill, 2000.
[20] A. Vasan, A. Kochut, and A. U. Shankar. Sniffing out the correct Physical LayerCapture model in 802.11b. Technical Report UMIACS-TR-2004-26 and CS-TR-4583, Department of Computer Science, University of Maryland College Park,April 2004.
27
[21] C. Ware, J.F. Chicharo, and T. Wysocki. Modelling of capture behaviour in ieee802.11 radio modems. InIEEE International Conference on Telecommunications,2001.
[22] C. Ware, J. Judge, J. Chicharo, and E. Dutkiewicz. Unfairness and capture be-haviour in 802.11 adhoc networks. InIEEE International Conference on Com-munications, 2000.
[23] F. Ye, S. Yi, and B. Sikdar. Improving Spatial Reuse of IEEE 802.11 Based AdHoc Networks. InProceedings of IEEE GLOBECOM, San Francisco, CA, 2003.
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