Analysis of Protocol Capacity of IEEE Wireless LAN Medium Access Control (MAC) Protocol

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1 1 Analysis of Protocol Capacity of IEEE Wireless LAN Medium Access Control (MAC) Protocol Govind Vartikar, Kartikeya Tripathi and Mandeep Jawanda Abstract The efficiency of a WLAN depends upon its Medium Access Control Mechanism which dictates the sharing of the limited communication bandwidth of the wireless channel. In this paper, the generic IEEE Wireless LAN MAC protocol is studied and simulated as a sequential event process. The protocol capacity is defined as the fraction of channel bandwidth used by successfully transmitted messages. An analytical expression for this capacity is found as a ratio of time for successful transmission of a packet and the total time for that transmission inclusive of collision times and contention window times. An upper bound of the capacity is found and comparison between theoretical and simulation results is done. Index Terms Back-off Algorithm, Medium Access Protocol (MAC), Protocol Capacity. I. INTRODUCTION In this paper we focus on the IEEE WLAN MAC and analysis of the efficiency of this protocol. We then discuss the algorithms which can be used or modified to increase the overall throughput and performance. The most common standards include the initial IEEE standard which used the 2.4 GHz band and provided data rates up to 2 Mbps. The IEEE b provides data rates up to 11 Mbps while using the same spectrum as the initial standard. It also uses the same DSSS (Direct Sequence Spread Spectrum) techniques. However IEEE a (5 GHz band) and IEEE g (2.4 GHz band) promise a substantial increase in data rates by using OFDM techniques. The data rates are on the order of 54 Mbps. WLANs provide a flexible, mobile data communication system as an extension to or as an alternative for a wired LAN. But WLANs need to be much more concerned about bandwidth consumption than their wired counterparts due to the limited bandwidth available. Hence WLANs typically provide data rates of 1-2 Mbps vs Mbps using wired LANs. The simultaneous transmissions on the Wireless LAN medium are coordinated by the Medium Access Control (MAC) protocol. Control messages or messages that need to be retransmitted due to collisions on the channel reduce the effective data rate or the utilization of the channel. The maximum value of this utilization is the capacity of the protocol and we strive to maximize this channel capacity by using the different back off algorithms suggested in the research paper. II. IEEE MAC PROTOCOL The IEEE MAC protocol provides access to timebounded, asynchronous, and contention-free traffic on different kinds of physical layers. IEEE employs Carrier Sense Multiple Access, Collision Avoidance (CSMA/CA) protocol. We cannot use Collision detection for Wireless protocols as it requires the implementation of a full duplex radio capable of transmitting and receiving at the same time, an expensive option. Also, the detection of a collision is hindered by the multi-path and Doppler fading effects associated with a wireless channel. The access methods used by this protocol are the Distributed Coordination Function (DCF) and the Point Coordination Function (PCF). DCF is based on the CSMA/CA MAC protocol while PCF uses a polling system which determines the station having the right to transmit at a certain point of time to avoid any contention between the transmitting nodes. Although both the access methods can coexist we shall be using DCF (Basic access) scheme throughout our study. Before a node transmits it first senses the channel to check if any other node is using the channel. This check is done both physically and virtually. The physical carrier sensing is done at the air interface while the virtual carrier sensing happens at the MAC sub layer. To sense the channel physically each station detects the presence of other users by analyzing the relative signal strength from other sources. If no signal is detected then it is assumed that none of the other stations are transmitting and hence the node starts its transmission. However, there is a problem called hidden terminal problem associated with this scheme when two nodes fail to detect each others presence on the network due to their relative positions.

2 2 Figure 2 : Backoff Procedure Figure 1: Hidden Terminal Problem e.g. As shown in the figure, wireless stations A and C can see B, but A cannot see C and hence node A is hidden from C. So if both A and C try to transmit at the same time a collision takes place. However this problem can be tackled by sensing the channel virtually in addition to the physical check. In DCF the node wanting to transmit first senses the channel. If the medium is found to be idle then the station waits for a DIFS amount of time and then starts the transmission. However if the medium is busy, then the station waits for the ongoing transmission to get over. DCF uses a Binary Exponential Back off technique whereby a random interval called a back off interval is selected which initializes a backoff timer. The station then has to keep sensing the channel for this additional random time after detecting the channel as being idle for a minimum duration of DIFS. The backoff timer is then decremented as long as the channel is idle and stopped when a transmission is detected on the channel. When the backoff timer of a particular station reaches zero it gets the right to transmit. The time following a DIFS is slotted and a station can start transmitting only at the beginning of a slot frame. The backoff time is chosen from a Uniform distribution over the interval [0, CW], where CW (Contention Window) is an integer within the range of values of CWmin and CWmax which are defined by the physical characteristics. After each unsuccessful transmission attempt, another Back-off timer is allocated to the colliding stations with the length of the CW doubled. This reduces the collision probability in case there are multiple stations attempting to access the channel. The receiving station then checks the (Cyclic Redundancy Check) CRC of the received packet and sends an acknowledgement (ACK) packet. This receipt indicates to the transmitting station that there were no collisions detected. If the sender does not receive ACK, then it re-transmits the last fragment. Consider the instant when station A is transmitting a frame. At this time all the other stations B, C, D and E defer their transmission as they sense the medium to be busy. After A is done with its transmission it picks a random number again and waits for its turn. In the meanwhile all other stations sense the channel to be idle for a DIFS time and start counting down their back off counter. Since C has the next smallest backoff counter value, it gets the chance to transmit and the stations B, D, E freeze their backoff counter at the instant C starts transmitting. When C is done it chooses a random number again and the other stations once again sense the channel to be idle and hence start the back off counter from the value they stopped at the last time. This procedure is then repeated for the other nodes.

3 3 DIFS 128 µsecs Back off slot time 50 µsecs Bit rate 2 Mbps Propagation delay 1 µsecs ACK 50 µsecs CWmin 32 CWmax 256 IV. IEEE CAPACITY ANALYSIS Figure 3 : Data Transmission using RTS/CTS Virtual carrier sensing is accomplished using the (Clear to send) CTS and (Request to Send) RTS. After gaining access to the channel (after a successful physical check) and waiting for a DIFS duration of time a short control packet called RTS (Request to send) is sent to the destination node which includes the source, destination and the duration of the following transaction. The receiving station then responds with another control packet called CTS (Clear to send), after having waited for SIFS time, if the medium is free to transmit which includes the same duration information. All stations receiving either the CTS or RTS set their Virtual Carrier Sense indicator (called NAV, Network Allocation Vector) for the given duration and use this information along with Physical Carrier Sense to determine the length of time the medium shall be busy. The sender then sends the data and waits for an ACK from the receiver. In case there is a timeout before the ACK can be received the data is considered lost and is retransmitted. This protocol reduces the probability of collision. Also the overhead of collision is reduced since the control packets are small when compared to the data packets and the collision between them is recognized faster. III. MATHEMATICAL ANALYSIS In Wireless LAN the medium access control (MAC) protocol is the main element for determining the efficiency in sharing the limited communication bandwidth of the wireless channel. This paper focuses on the efficiency of the IEEE standard for wireless LANs. Specifically an analytical formula for the protocol capacity has been derived. From the analysis the theoretical upper bound of the IEEE protocol capacity was found and was also concluded that the standard can vary depending on the network configuration and also depending on the appropriate tuning of the backoff algorithm. A distributed algorithm which enables each station to tune its backoff algorithm is discussed. The performances of the IEEE protocol, enhanced with our algorithm, are investigated via simulation. The results indicate that the enhanced protocol is very close to the maximum theoretical efficiency. The typical values in the simulation are as follows: SIFS 28 µsecs Protocol Capacity varies across the various MAC protocols, but it is also influenced by several other parameters, such as the number of active stations and the way active stations contribute to the offered load. In this paper, ρmax denotes the capacity when there are M active stations in asymptotic conditions (i.e. all the network stations, M, always have a packet ready for transmission); ρsingle denotes the capacity in the extreme case of a single active node. In a MAC protocol which is ideal from a utilization standpoint, both ρmax and ρsingle must be equal to one. In this paper the IEEE MAC protocol capacity is analytically estimated by evaluating, in asymptotic conditions, the ratio between the average message length and the average time t, the channel occupied in transmitting a message; t v is also referred to as the average virtual transmission time. To perform this analysis let S indicate the time required for a successful transmission, i.e. the time interval between the start of a transmission which does not experience a collision and the reception of its ACK plus a DIFS. Packet transmission time is given by: S m + 2τ + SIFS + ACK + DIFS Where m= average message length τ = maximum propagation delay between two WLAN stations SIFS= Short Inter-frame Space ACK= Acknowledgement length DIFS= Distributed Inter-frame Space Let us assume that the successful transmission is performed by station A which at time t0, transmits a packet to station B, τ AB is the propagation delay between these two stations, without any loss of generality we assume τ AB < m. Figure 4 : Events in a successful transmission

4 4 As shown in the figure above, the sequence of events in a successful transmission is: 1. A begins transmission at time t 0 2. B begins reception at time t + τ AB 3. A completes its transmission at time t 0 + m 4. B completes reception at time t 0 +τ AB + m 5. B begins the ACK transmission at time t 0 +τ AB + m + SIFS 6. A begins the ACK reception at time t 0 +τ AB + m + SIFS + ACK 7. B completes the ACK transmission at time t 0 +τ AB + m + SIFS +τ AB 8. A completes the ACK reception at time t 0 +τ AB + m + SIFS +τ AB + ACK 9. A can start the next transmission at time t0 +τ AB + m + SIFS +τ AB + ACK + DIFS Hence S = t 0 + 2τ AB + m + SIFS + ACK + DIFS ρ single can be computed by noting that when only one station is active its average backoff time is E[CW], and hence tv = E[S] + E[CW] ρ single = ( 2τ + m E [ CW 1]) Where m = average transmission time E[CW1] = half of the minimum CW value The paper assumes that the packet lengths are an integer multiple of the slot length and also that the packet lengths are independent identically and geometrically distributed with parameter q. Hence When more than one station is active, the virtual transmission time includes a successful transmission and collision intervals m + SIFS + ACK + DIFS + m = tslot /( 1 q) medium is idle due to back off algorithm(idle periods). Note that an additional overhead is associated with a collision: due to the carrier sensing mechanism colliding messages prevent the network stations from observing that the channel is idle for a further time interval less or equal to the maximum propagation time. Furthermore according to the MAC protocol, after each collision the medium must remain idle for an interval equal to a DIFS. From these observations it follows that Ni tv = E[ ( Idle _ pi + Colli + τ + DIFS)] i= 1 + E[ Idle _ p Ni + 1 ] + E[ S] where Idle_p i and Coll i are the lengths of the i-th period and collision respectively: N c is the number of collisions in a virtual time. In IEEE protocol the length of a collision is equal to the maximum length of the colliding packets and hence it depends on the packet size distribution and on the backoff algorithm which determines the number of colliding stations. The length of the idle periods and the number of colliding stations depends on the backoff algorithm. To compute the unknown quantities in the above equation by exactly taking into consideration the backoff algorithm used in the standard is very difficult, due to the temporal dependencies which it introduces. In this paper to simplify the protocol analysis it is assumed that the backoff times do not have uniform distribution. Specifically it is assumed that the tagged station for each transmission attempt uses a backoff interval sampled from a geometric distribution with parameter p, where p=1/ (E[B]+1) and E[B] is the average value of the backoff times. E[B] =(E[CW]-1)/2 where E[CW] is the average contention window. [2] The assumption of the backoff algorithm implies that the future behavior of the station ins not dependent on the past and hence in virtual transmission the idle periods are i.i.d. sampled from a geometric distribution with average E[Idle_p] and the collision lengths are i.i.d with average E[Coll]. For large values of M the number of stations ready for transmission is less dependent on the virtual time evolution hence assumptions made above become more realistic as M increases. The results presented in this paper also indicate that for M=10 the above assumptions do not introduce significant errors in capacity analysis. The average contention window is estimated by focusing on a tagged station and computing the average contention window used by this station. Figure 5 : Structure of a virtual transmission i From the figure we see that before successful transmission may occur along with periods in which the transmission V. RESULTS Simulative experiments have been used to validate the iterative algorithm which estimates the average window size.

5 5 Specifically for three different network configurations with M=10, 50, 100 the simulative estimates were compared to analytical estimates. The parameter q of the geometric distribution was taken to be 0.99 Figure 7: t v (p) function for several M values (q=0.99) Improving IEEE Capacity Figure 6: MAC protocol capacity For each network configuration the figure reports both the analytical and simulative estimates. The results obtained indicate that the analytical model provides close approximation of the real behavior and in all experiments the analytical results are slightly higher than the simulative results. Also the capacity decreases when M increases. This is due to the increase I the collision probability as the backoff mechanism does not take into consideration the number of active stations. Analytical bounds on the MAC protocol capacity As the capacity is the ratio between the average packet length and the average virtual transmission time, for a given packet length distribution its maximum value corresponds to the minimum value of the average virtual transmission time. Transmission Time is function of: M: no of Active nodes p: parameter of geometrically distributed backoff interval q : parameter of geometrically distributed packet length For low p values the high t v value is mainly due to the high number of empty slots before a transmission. At the other extreme we have a significant number of collisions before a successful transmission. The minimum of tv corresponds to a p value for which these two effects are balanced. The results presented earlier indicate that the protocol is very far from its theoretical limits. Specifically the critical point is the average backoff time. Hence if the number of active stations is known at runtime then the contention window size can be selected from amongst the interval [0, M-1] and thus the number of empty slots can be reduced. Now we look at how we went about doing the simulation. VI. THE APPROACH The generic MAC protocol outlines the algorithms for efficient access of channels in order to minimize the total time required (inclusive of any time spent in collisions etc) for successful transmission of packets. The algorithms are inherently event driven in that all nodes work independently, generating their own packets, sensing the channel, deciding their own back-off times, and colliding in the event of two or more simultaneous transmissions. However, we approached the problem from a sequential perspective. We broke down the situation into discrete events happening one after the other, and programmed the scenario as a discrete-event simulation with different functional blocks. This allowed us to explore software platforms other than those affording a multi-threaded environment (e.g. Java). Following is the outline of the sequence of events that we decided would constitute a close approximation of the real scenario, along with their implementation in the functional blocks of our program. Get the number of nodes. This is implemented in the function getnodes. The user is prompted to enter the number of nodes and also their locations as X and Y co-ordinates. She may also choose to let a random location generator assign positions to the nodes (the location generator confines all nodes to within a 50x50 square region). Binary addresses are then assigned to each node and distances are calculated

6 6 between them in a square matrix. Propagation delays (in the units of microseconds) between pairs of nodes are also found. Decide the number of microseconds the simulation must run for. The user is prompted to choose that value. The reason for using all time values in microseconds is to save memory from usage in floating point numbers. Pass the value of simulation time to the function timetable. This function is the heart of the program and uses many other functions to construct a timetable matrix with rows representing nodes and columns representing their respective contentionwindow sizes after each iteration. During each iteration, the contention window sizes of all nodes (assigned to them by the function contentionwindowtime: this draws the CW from a uniform distribution) are checked and the one with the smallest CW is allowed to transmit. That node chooses a random destination and makes a packet of length chosen from a geometric distribution (done by the function makeframe). Assuming that the frame lengths are multiples of single slots, the time for transmission of this packet is calculated according to the formula T=2*(prop_delay) + SIFS + ACK + FrameTime + DIFS Here is assumed that the length of all acknowledgement packets is the same (50 microseconds). The next column of the time-table is updated with respect to this transmission time. In case of a collision, the column corresponding to that iteration is filled with negative numbers representing the time of the collision (so that a glance at the timetable can indicate collisions), and the nodes are assigned new values of contention windows. This continues till the simulation time entered by the user is reached. Display the transmissions. Before the function timetable is called, a plot of all nodes and their locations is displayed. During the running of timetable, each iteration generates a plot showing the transmission between two nodes, represented as a line drawn between those nodes. The title of the plot tells which node is the source and which is the destination. Test all these functions. There is a lot of randomness incorporated at different levels of the simulation, and that makes it difficult to verify whether or not the results are legitimate. So testing of each of the functions is done with the random components removed from them and matching their test results with expected values. It is logical to deduce the correctitude of the program from their performance in the test bed, and extrapolate it to when the random elements are brought back in. When the simulation is run, it gives us primarily the timetable, apart from location and propagation delay matrices and a display of transmissions and collisions. The time-table shows the contention windows assigned to different nodes after each transmission or collision, the number of collisions that took place in the simulation time, the times at which the collisions took place, and the colliding nodes. The few approximations and assumptions in this simulation are as follows: There are no hidden nodes in the system. All nodes can hear everybody else and so the only cause of collision is when by chance two or more nodes get assigned the same contention window length. Once the node locations are entered by the user, the nodes are assumed fixed for the time of the simulation. This is a close approximation of the real system in that the time (in microseconds) the simulation runs for is so small that the locations of the nodes would not change in the real situation either. All times (CW times, slot time, SIFS, DIFS, ACK, etc) are in microseconds. If units of seconds were chosen, the decimal numbers would be very small (e.g sec for 20 microseconds) and would occupy more memory. Every time a packet is made for a pair of communicating nodes, its time length is calculated by multiplying the length with slot time, and this assumes that packet lengths are multiples of single slots. The Acknowledgement frames are assumed to be all of equal length (50 microseconds). The paper we have referred to does not mention any variation in the size of the ACK frame either. VII. EXAMPLE An illustration of the simulation is presented here. As described in the Approach section, the program asks the user to give the number of nodes, their locations, and the time for which the simulation has to run. In this run of the simulation, the number of nodes is 10, the locations have been selected randomly by the location generator, and the simulation is run for 50,000 microseconds. The locations generated by the program are:

7 7 The first set of CW lengths assigned are: Node # CW Size The sequence of transmission starts with node 3 transmitting first as the value of its CW is the least. Here, the Back-off times of 2 nodes have finished at the same time (since their CWs were the same), there is a collision. The subsequent transmissions and collisions are shown below, in the order in which they were generated by the simulation -

8 8 times against varying parameter of distribution of back-off time for three different numbers of active nodes Active Nodes= Virtual Transmission Time Parameter of Geometrically Distributed Backoff Interval 6450 Active Nodes= Virtual Transmission Time Here, the Back-Off times of 3 nodes finish at the same time. Below is the Time-Table obtained at the end of the simulation. The various columns have the times when each node will transmit. Two negative columns indicate 2 collisions (as depicted in the above plots) and the times at which the collisions took place Parameter of Geometrically Distributed Backoff Interval x 10-3 Active Nodes= Virtual Transmission Time We observe that the virtual transmission times for packets vary with the parameter of the Geometric Distribution from which Back-Off times are drawn. The trend is for the virtual transmission time to drop, reach a minimum value, and increase again. Below are the plots of virtual transmission Parameter of Geometrically Distributed Backoff Interval x 10-3 So choosing a good parameter of the Geometric Distribution of Back-Off times (the minima) will theoretically decrease the virtual time and increase the capacity.

9 9 VIII. SIMULATION TOOL MATLAB When we decided to work on a network simulation, we first wanted to explore the standard platforms available as means. NS-II is a popular and widely available network simulation tool that came to our minds. But after a little digging on the subject, we realized that even though NS-II would make the task of comprehensively simulating the protocol very easy, it would take a great deal of experience with it as an initial investment. Our opinion was that the format of programming in NS-II, although learnable, would take considerable time to get accustomed to. So, in recognition of the inherent eventdriven nature of the situation at hand, we looked at Java. The multi-threaded environment of Java would be suitable for representing the independent activities of all nodes with respect to time, and applets would afford a fine display of results. Some initial level programming was done till we realized that none of us was well-versed with dynamic memory allocation techniques in Java. This proved to be a big impediment, as linked lists and other array handling methods were to be an integral component of the program. Finally we turned to MATLAB. MATLAB doesn t support multi-threaded programming. But it offers incredible leverage in dynamic memory handling and array manipulation. Even though the choice of MATALB required us to formulate a discrete event model of the process, and also necessitated some sacrifice in the display department of the simulation (it is not as flashy as that of Java would have been), our opinion is that its disadvantages were outweighed by its limberness. Also, MATLAB was much more easily accessible than Java, and experts tell us that a similar simulation in Java would take longer to run. On a whole, our experience with MATAB was quite satisfactory. [2] Cali, F., Conti, M. and Gregori, E., IEEE Wireless LAN: capacity analysis and protocol enhancement in CNUCE, Network Group Report, November 1997 [3] Cali, F., Conti, M. and Gregori, E., Dynamic tuning of the IEEE protocol to achieve a theoretical throughput limit in Networking, IEEE/ACM Transactions on, Volume: 8 Issue: 6, Dec Page(s): [4] Cali, F., Conti, M. and Gregori, E., IEEE protocol: design and performance evaluation of an adaptive backoff mechanism in Selected Areas in Communications, IEEE Journal on, Volume: 18 Issue: 9, Sept Page(s): [5] Akyildiz, I.F., McNair, J., Martorell, L.C., Puigjaner, R. and Yesha, Y., Medium access control protocols for multimedia traffic in wireless networks in IEEE Network, Volume: 13 Issue: 4, July-Aug Page(s): [6] IEEE Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, IEEE, 1997 [7] K.Pahlavan and P.Krishnamurthy, Principles of Wireless Networks: A Unified Approach, from Prentice Hall; ISBN: ; 1st edition (December 11, 2001) ACKNOWLEDGMENTS Dr. H. A. Latchman is the professor of Electrical and Computer Engineering at the University of Florida. He has been the guiding light for us in all our endeavors leading to this presentation. He encouraged us to explore the path of simulations of networks as a part of the curriculum for the course Queuing Theory in Data Networks that he taught in the Fall of 2002, and was both insightful and resourceful at all times when our own capabilities failed. Gratitude is due also to Mr. David Pinto, a graduate level student at ECE in UF, in no small amounts. He is a person of extraordinary talents in computer programming, and has provided us with invaluable help in some fundamental aspects of the simulation. REFERENCES [1] Cali, F., Conti, M. and Gregori, E., IEEE Wireless LAN: capacity analysis and protocol enhancement in INFOCOM '98, Seventeenth Annual Joint Conference of the IEEE Computer and Communications Societies. Proceedings. IEEE, Volume: 1, 1998W.-K. Chen, Linear Networks and Systems (Book style). Belmont, CA: Wadsworth, 1993, pp