Physical Layers MAC - Layer. Contention Free Service used for. Contention Service. used for. time bounded traffic and for contention free

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1 Analyzing and Improving the IEEE MAC Protocol for Wireless LANs Jost Weinmiller, Hagen Woesner, Adam Wolisz Electrical Engineering Department Technical University Berlin, Sekr. FT5 Einsteinufer 25, Berlin, Germany Abstract 1 The MAC-part of the upcoming IEEE standard for wireless LANs - named 2 is based on a CSMA/CA-type access protocol with a rotating backo window and an optional RTS/CTS message exchange. In this paper, following a short presentation of we will present the results of our simulative analysis of focussing on analysis and improvements of the basic key mechanisms - the RTS/CTS message exchange and the backo algorithm. 1 Introduction Wireless LANs (WLANs) are expected to be a major growth factor for the network industry in the upcoming years. They will be used as an extension of the wired network with a wireless last link to attach the large number of mobile terminals. In order to avoid limitations of mobility due to technical boundaries, standardization is currently under work for the IEEE standard specifying physical layer (PHY) and media access control (MAC) for wireless LANs [1] as well as for the ETSI HIPERLAN [2] standard with the same task. In this paper we concentrate on the analysis of the MAC part of the IEEE protocol. In order to gain experience on the characteristics of this newly designed protocol we simulated a network cell with several stations using the Distributed Coordination Function (DCF) as the access scheme. We focussed our simulations on two key mechanisms - the backo mechanism that incorporates the collision avoidance functionality and the RTS/CTS message exchange intended to solve the hidden terminal problem in wireless LANs. 1 This work was funded by the German Research Agency DFG with a grant in the Priority Program 'Mobile Communication' 2 Distributed Foundation Wireless Medium Access Protocol In the second section we briey describe the draft standard for, in the third section we present our simulations concerning the backo mechanism followed by section 4 where we present the results of our simulations on the RTS/CTS message exchange. The paper is concluded by pointing out open questions and directions for future work. 2 Contention Free Service used for time bounded traffic and for contention free asyncronous traffic Power Saving PCF Superframe Structure BB-IR Inter Spaces RTS/CTS Contention Service used for asyncronous traffic DCF CSMA/CA with rotating backoff FH-SS DS-SS Figure 1: Structure of the Concept The IEEE standard will oer two operation modes - a distributed coordination and a centralized point-coordination mode. The MAC-part of the IEEE working group's proposal - - de- nes an access mechanism based on a CSMA scheme with collision avoidance that applies a rotating backo window mechanism. Optionally this mechanism is extended by an RTS/CTS message exchange to increase robustness towards hidden terminal scenarios. Further elements of the proposal are dierent minimal interframe spaces that enable prioritized access for urgent Physical Layers MAC - Layer

2 control messages on the one side, for the point coordination function on the other side. In point coordinated mode both coordination modes share the bandwidth usage in a superframe structure. This enables contention and contention-free service for all kinds of time-bounded, synchronous or asynchronous trac. A power saving-mode is additionally specied. The elements of the standard are illustrated in Figure 1 Generally, a station that intends to transmit and senses the channel busy will wait for the end of the ongoing transmission, then waits for a time period of DIFS 3 length, and then randomly selects a time slot within the backo window. If no other station started transmitting before this slot is reached (i.e another station that selected an earlier slot) it starts its own transmission. Collisions can now only occur in the case that two stations selected the same slot. If another station selected an earlier slot, the station freezes its backo counter, waits for the end of this transmission and now only waits for the slots remaining from the previous competition. (Figure 2) This access mechanism can optionally be extended Station A Defer Station B Defer Station C Station D DIFS Defer Backoff Backoff Remaining Backoff = Contention Window Figure 2: Access Scheme by the RTS/CTS 4 message exchange: After gaining access to the medium and before starting transmitting the data packet itself a short control packet is sent to the receiving station announcing the upcoming transmission. This packet is answered by a packet to indicate the readiness to receive the data. Both packets contain the projected length of the transmission and thus inform all stations within radio-range of both stations how long the channel will be used. The RTS/CTS message exchange has been designed to solve the hidden terminal problem: The successful exchange of the two small messages reserves the space 3 Distributed coordination Inter Space 4 Ready To Send / Clear To Send within range of the receiver and the sender for the intended transmission guaranteeing an undisturbed media for the longer data packet. 3 The Backo Mechanism The following section does not attempt to present a complete analysis of the problem but describes it in a qualitative way: As described before, the protocol attempts to give longer waiting stations an increasing possibility to gain access to the medium with the help of reduced wait-intervals after each lost competition. On the other hand, collisions only occur if two or more stations select the same slot. (These stations have to reenter the competition with an exponentially increasing value i.e. twice, 4-times if they collide again etc.) We will show, that these two facts in the design of the access mechanism has significant inuence on the system`s performance. 3.1 Probability distributions of slot selection We will take a look at the contention window and the probability that a certain slot within the contention window is selected. We assume several stations are continuously competing for access to the medium. In initial state this scheme results in an equally distributed probability for each slot to be selected. (Figure 3a) In the following cycle all stations p n (x) 0 p n (x) p n (x) Fig a slot selection probability caused by stations remaining from the previous cycle slots slot selection probability caused by new or returning stations 0 Fig b slots - selected 0 Fig c slots Figure 3: Slot selection probability distribution that have already been competing for access in the cycle before all have reduced their backo time by Selected (if Selected represents the winning slot). Still within this reduced contention window (from 0 to (- Selected)) all slots are selected with the same probability by the remaining stations.

3 If a station newly enters the competition or returns from a collision they will choose their slot with equally distributed probability however within the whole range of the contention window. This results in a slot selection probability as shown in Figure3b, the slots later than - Selected have a signicant lower probability to be chosen compared to the slots before this mark. In equilibrium state (Figure3c) after several cycles, we see that slots positioned early in the contention window have a much higher probability to be chosen. The result is a decreasing staircase function for the slot selection probability. This however causes a very much unwanted eect: slots that are more likely to be chosen also are more likely to be chosen twice or more times, in which case a collision would result. This fateful connection of slot selection probability, collision rates and resulting performance parameters can be seen in our simulations in Figure 4-6. An equally distributed probability for every slot to be chosen is the favored situation in terms of collision avoidance. 3.2 Modications of the Backo Mechanism The trivial method to solve this problem would be to have remaining stations select a new random backo time within the whole contention window in every cycle again. This however results in a certain possibility that a station waits forever to gain access - there is no mechanism to limit the maximal wait time which would make this design unsuitable for support of QoS guarantees. We investigated two other approaches to solve this problem, both of them attempt to keep the newly entering stations out of the way of the stations that have lost the previous competition i.e. away from the early slots Weighted Selection Probabilities We started o with the idea to have stations, that arrive new in the competition, select later slots with higher probabilities. We heuristically used dierent probability functions for the slot selection probabilities starting from a linear increasing function to functions of higher order. Since the distribution function for all slot selections has to be 1 for x=, the slot selection probability function changes into: f(x) = n + 1 xn n n+1 0; 1 x For the linearly increasing probability function (n=1) we don't get signicantly improving behavior, however we achieve higher throughput with increasing exponents n up to a certain maximum. We got best results with n in the area of 10. The obvious drawback of this scheme is that even if load is low in the WLAN and thus there would be no need to choose late slots since no stations are waiting for access from previous cycles the late slots are chosen anyway. This causes a slight increase in average access delay but since delay caused by collision adds up much more to the overall delay the reduced collision probability of the modied scheme can be considered advantageous (as shown in section 3.3) Load Adaptive Selection The second approach limits newly entering stations to slots that undoubtedly are not selected by stations from the previous cycle. If the winning slot in the previous competition cycle was at position Selected, all the other stations that lost this competition cycle will have a slot position below (- Selected). Therefore we restrict new entering stations in their slot selection process to the slots between (- Selected) and i.e. if slot 4 was the winning slot in the previous competition and there are 32 slots overall, new stations only get to chose between slot 29 and 32. We can assume that, if the network is running under high load, that the winning slot in the previous competition is likely to have been positioned early in the contention window since this small backo time is likely to be the remains of a larger backo time from the competition cycles before. Early position of the winning slot is a sign of `rough' competition and can eventually be interpreted as a sign of high load. Our scheme can thus be seen as a load adaptive scheme applying dierent degrees of access control depending on the load. 3.3 Simulations and Results For our simulations we used PTOLEMY [3], an object oriented simulation tool, developed at the University of California Berkeley which proved suitable for our intentions due to its ability for concurrent process oriented simulations. We simulated a WLAN that uses 's distributed coordination function as the access scheme with a network throughput of 2Mbit/s. Our setup involves 8 stations with and without hidden terminals. The channel and packet source model is basically the same as it is described in [4], i.e. poisson distributed packet sources in a wireless channel model. In addition to that we simulated the signal run time, which can be left out if one only considers a range of a microcell, but which becomes considerable at distances of m and more.the packet sources are simulated as Poisson processes with an attached

4 innite queue, the packet sizes are read from a trace le of an Ethernet [5]. We simulated with the following xed values: DIFS=32ms, Slottime=4ms, PHY preamble= 30 byte, min=32 Slots, max=256 Slots, RTS/CTS o. The values for DIFS and slottime for have been dened longer later on - over 30ms for DIFS and s for the (DSSS-)slottime slot # slot # slot # Fig. a Fig b Fig. c Figure 4: Distribution of slot selection for a) DFW- MAC, b) weighted, c) load adaptive scheme Our results show a signicantly better distributed slot selection probability for our modied schemes. (Slots above are only selected from colliding stations that choose their backo slot in the enlarged contention window. Therefore the more slots are selected above the more collisions have occurred.) Figure 4a shows the expected steady decrease in slot selection towards later slots, Figure 4b obviously has more equally distributed slot selections, however still shows a good number of selections beyond, i.e. has more collisions than the third scheme in Figure 4c. Figure 5 presents the corresponding collision rates where the advantages of the two modied schemes become obvious.. The remarkable result of the above gure is, that under high load, we get almost no collisions for the load adaptive scheme. The lower rate of collisions of course results in a signicant increase in throughput and decrease in mean access delay (Figure6). The peak for the collision rate of the load adaptive Throughput in % Load adaptive scheme Weighted Backoff Delay in ms Weighted Backoff Load adaptive scheme Figure 6: Throughput and mean access delay over load, all three schemes % of collided packets scheme in the lower load range (Figure5) results from the fact that at this point there may be many stations entering the contention phase at one time, that, if limited to few slots, will collide. This obviously is inuenced by the simulation approach, especially the source modelling (innite queue). In order to `atten` the collision rate curve, our rst intention was to work not with the latest number of free slots, but with a mean of the last n cycles. This didn`t lead to any signicant improvement - the collision rate even increased with n. Nevertheless, this mean seems to be a good measure for the overall net load. In another aporiginal scheme % of collided packets simplified scheme Weighted Figure 7: Load adaptive schemes (original and simpli- ed version) compared to Load adaptive Figure 5: Collision rates over load, all three schemes proach we conducted one promising simulation where every station selects from those slots, that were later than its own last winning access, no matter if this happened in the previous cycle or any time before that. I.e. if that station last gained access with slot p, it

5 will select its next slot in the range between -p and. This mechanism also does not demand continuous monitoring of the network and its main advantage is that dierent stations have dierent ranges to select from at the same time. We performed simulations of this simpler strategy (Figure 7). The result shows slightly lower collision rates in the low load range but also slightly higher collision rates for higher load. 4 RTS/CTS Message Exchange In radio networks the 'hidden terminal scenario' poses a special problem on MAC design. This scenario is explained in Figure 8: If station B is send- RTS A B C D CTS =Area of reception from Station B =Area of reception from Station C =Area of reception from StationB and C + + =reserved space after RTS/CTS Figure 8: Hidden Terminal Situation ing to station C the medium appears busy only for the stations located within the range of the sending station B - other stations cannot sense a signal and consider the medium idle. Therefore station D might start a transmission since it does not notice B's ongoing transmission. However since the receiving station C is within range of B and D and thus receives two signals at once it will not receive any undisturbed signal (whether destined for it or not). This collision however cannot be detected at the sending station B unless it notices the lacking acknowledgment from station C after a certain time-out. 4.1 RTS/CTS in To solve this situation the RTS/CTS mechanism, developed in [7] and analyzed in [8], has been integrated into : Each station competes for access as described in section 2. When the RTS/CTS mechanism is applied, the winning station does not send data packets right away but sends a RTS packet to the receiving station, that responds with a CTS packet. If a station captures a RTS packet from another station and it is not the destination of the RTS packet it reads the intended transmission duration from the RTS packet and stays silent for that time. The same happens if only a CTS packet is received i.e. by a station outside of the transmission range of the sender but within the range of the receiver. This guarantees that all stations within range of either sender or receiver have knowledge of the transmission as well as of the duration of it. The eects of the RTS/CTS mechanism are as follows It increases bandwidth eciency by its reduced collision probability since the ongoing transmission has been made known everywhere within the range of it It increases bandwidth eciency since if collisions occur they do not occur with the long data packets but with the relative small control packets It decreases bandwidth eciency since it transmits two additional packets without any payload It decreases bandwidth eciency since it reserves geographical space for its transmission where or when it might actually not need it. Due to the above listed eects of the RTS/CTS mechanism, the draft standard allows its usage but does not demand it. Usage policy is set on a per-station-basis with the help of a manageable object RTS Threshold, determining the payload-size, above which RTS/CTS should be used. This parameter is not xed in the draft standard and has to be set separately by each station. The packet size is the only parameter that is used to decide whether the mechanism is applied. 4.2 RTS/CTS in the hidden terminal scenario As explained above the RTS/CTS mechanism was introduced to solve the hidden terminal problem. Therefore our rst simulations intended to evaluate the eects of this mechanism in this case. We continued with simulations on RTS/CTS in the fully meshed case to determine dependencies and usage parameters. We simulated the following scenario: 8 stations are present in a cell, where station 1 is "hidden" to stations 2 and 3, and vice-versa. All of the \outer" stations (1,2 and 3) try to send data to the \inner" stations. The positions and the destinations of data trac are shown in Figure9. The result of these simulations (Figure10 without RTS/CTS and Figure11 with RTS/CTS) was that using the RTS/CTS message exchange does not completely solve the hidden terminal problem, even though signicant improvements can be achieved.

6 Meter Meter Figure 9: Map of the stations in the hidden terminal simulation Throughput per station in % stations 5,7 stations 4,6 station 8 stations 1,2,3 Offered Load per station in % Figure 10: Throughput per Station, Hidden Terminals, RTS/CTS o Throughput per station in % all other stations stations 2,3 station 1 Offered Load per station in % Figure 11: Throughput per station, Hidden Terminals, RTS/CTS on The breakdown of inbound data trac in the case of higher load stems from the fact that the mutually hidden stations become synchronized by an earlier data exchange in the area between them. In result, they start their backo counters at the same time but they are unable to detect the begin of transmission of the other station. Figure10 shows, that the stations that are hidden to other stations hardly get any packets through due to the above mentioned synchronization eect. The stations that attempt to send towards the `hidden stations' have signicantly lower throughput than the two stations 5 and 7 that only send to non-hidden stations. The rst group successfully gets packets through, however many acknowledgment packets are destroyed by trac from the hidden stations. Figure11 shows that still the hidden terminal scenario is not solved: Station 1 still hardly gets any packets through, but its throughput is improved compared to the gure without RTS/CTS. The same goes for stations 2 and 3 - all of the hidden stations benet from the captured CTS packets. The non-hidden terminals all achieve the same (high) throughput due to the fact that outbound trac is protected by the RTS packets. 4.3 Determining RTS Threshold In order to determine the RTS Threshold parameter, we simulated a scenario with 8 stations without hidden terminal scenario. We determined the gain or loss of throughput and delay versus load, if RTS/CTS is used, depending on the packet size. Since the packet size on the wireless channel consists of the MAC-layer packet size and the physical layers preamble the length of this latter part has to be taken into account when determining the RTS Threshold parameter. Our simulations gave larger "best" values for RTS Treshold for larger physical preambles (Figure 12). Since this physical preamble has dierent lengths depending on the channel used - for the dened physical layers in the specication those values are 128 bits plus a variable number of stu bits for the 2.4GHz FHSS channel, 192 bits for the 2.4GHz DSSS channel and between 96 and 112 slots of 250ns length plus 32 bits for the baseband infrared channel - this value has to be taken into account when conguring the RTS Threshold parameter. This gets even more dicult in the case of the 2.4GHz FHSS channel as well as for the infrared channel, where no xed length is added but a variable number of bits or timeslots, respectively. We can say that for the proposed PHY preamble sizes of the draft standard we recommend, based on our simulations, a value of around byte MPDU size for the RTS threshold.

7 Total throughput in % RTS/CTS on Total throughput in % RTS_threshold =256 RTS/CTS off RTS/CTS on Fig. a Total throughput in % Fig. b RTS/CTS off RTS_threshold= RTS/CTS on RTS/CTS off Fig. c Figure 12: Throughput vs. Load, a) PHY preamble=0 bytes b) PHY preamble=30 bytes a) PHY preamble= bytes 4.4 Asymmetric Usage Policy The use of RTS/CTS in the draft standard is managed on a per-station basis. This can result in asymmetric congurations in the network, e.g. one station does never use the mechanism, all the others do. We simulated such an asymmetric conguration to determine whether the stations behaving dierent than the rest might win performance at the cost of the others, whether they will loose or whether they will cause degradation of the overall network performance. The results can be seen in Figure 13. It showed that Total throughput per group in % using RTS/CTS not using RTS/CTS % Offered Load per group Figure 13: Throughput vs. Load 4 Stations use RTS/CTS, 4 Stations don't there is no individual gain for a station which does (for whatever reason) not behave like the others, but that there is a small decrease in the overall performance. 5 Conclusions In the previous sections we presented the results of our simulative analysis of two key mechanisms within the distributed coordination function MAC protocol - the backo strategy and the RTS/CTS message exchange. Without adding much to the protocols complexity we can gain up to 25% in throughput and decrease the average access delay at about % by applying a modied backo scheme. We also showed that the RTS/CTS mechanism does not completely solve the hidden terminal case, however some improvements can be achieved. In our future work we will experiment with dierent approaches how to apply the RTS/CTS mechanism e.g. based on load condition. One (or several) experienced collision may be interpreted as a sign for high load making the use of RTS/CTS advisable - the next attempt to access the media would have to be preceded by a RTS/CTS exchange. Another topic for further work could be the exploitation of the hidden terminal scenario: Since the collision can only occur at the receiving side of the communication it is not necessary to reserve space around the sending side. A station only hearing the RTS packet and the data packets but not the CTS packet can assume, that it is out of range of the receiving station and thus will not disturb the ongoing upstream communication (downstream ACKs will be disturbed!). References [1] Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specications, Draft Standard IEEE , P802.11/D1; The editors of IEEE [2] HIPERLAN Functional Specication Section 6 MAC Layer, Draft Version 0.6, ETSI Secretariat, December 1994 [3] PTOLEMY, anonymous ftp site: ftp.ptolemy.eecs.berkeley.edu, www: Copyright The Regents of the University of California. [4] A Wireless MAC Protocol comparison, W. Diepstraten; IEEE working paper P /51, May 1992 [5] High Time-Resolution Measurement and Analysis of LAN Trac: Implications for LAN Interconnection; W Leland, D Wilson; INFOCOM '91, ftp://ftp.bellcore.com/pub/wel /Lel- and+wilson INFOCOM 91.ps.Z [6] On the Self-similar nature of Ethernet trac (extended version), W. Leland et al.ieee Transactions on Networking, Vol 2, No 1, Feb 1994 [7] MACA - A new Channel Access Method for Packet Radio, P. Karn, ARRL/CRRL Amateur Radio 9th Computer Networking Conference, Sept [8] MACAW: A Media Access Protocol for Wireless LAN's; V. Bharghavan, A. Demers, S. Shenker, L Zhang, SIGCOM 94