A SURVEY OF QOS TECHNIQUES IN 802.11 Drabu, Yasir Department of Computer Science, Kent State University Contents 1. Abstract 2. Introduction 3. Background 3.1. 802.11 Mac Sub layer 3.2. Distributed Coordination Function (DCF) 3.3. Virtual Carrier Sense 3.4. Point Coordination Function 3.5. Fragmentation 4. QoS Techniques in 802.11 4.1. Introduction 4.2. Classification of service differentiation techniques 4.2.1. Differentiated services based on DCF 4.2.1.1. Backoff Increase Function or Distributed Fair Scheduling 4.2.1.2. Varying DIFS 4.2.1.3. Maximum Frame Length 4.2.1.4. Extended DCF 4.2.1.5. Blackburst 4.2.2. Differentiated services based on PCF 4.2.2.1. Hybrid Coordination Function 4.2.2.2. Distributed TDM 5. Conclusion 6. Future Work 7. References
1. ABSTRACT Currently lot of work is being done to provide service differentiation in the Internet. However, in wireless environments where bandwidth is scarce and channel conditions are variable, IP differentiated services are sub-optimal without lower layers support. This paper discusses the 802.11 MAC layer and provides a survey of the different techniques used for differentiation of service. 2. INTRODUCTION The most important functions of the MAC layer for a wireless network include controlling channel access, maintaining Quality of Service (QoS),and providing security. Wireless links have characteristics that differ from those of fixed links, such as high packet loss rate, bursts of packet loss, packet re-ordering, and large packet delay and packet delay variation. Furthermore, the wireless link characteristics are not constant and may vary in time and place. The mobility of users poses additional requirements, as the end-to-end path may be changed when users change their point of attachment. Users expect to receive the same QoS after they have changed their point of attachment. This implies that the new end-to-end path should also support the existing QoS (i.e., a reservation on the new path may be required),and problems arise when the new path cannot support the required. In section 3 we discuss the 802.11 protocol and how it transports data under different scenarios. In section 4 we will survey the different service differentiation techniques that have been considered to work with 802.11. 3. 802.11 MAC SUB LAYER 3.1 Introduction The IEEE 802.11 MAC protocol provides two service types: asynchronous and synchronous (or contention free). These types of services can be provided on top of a variety of physical layers and for different data rates. The asynchronous type of service is always available whereas the contention free is optional. The asynchronous type of service is provided by the Distributed Coordination Function (DCF) which implements the basic access method of the IEEE 802.11 MAC protocol and is also known as the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol. The contention free service is provided by the Point Co-ordination Function (PCF) which basically implements a polling access method. The PCF uses a Point Coordinator, usually the Access Point, which cyclically polls stations giving them the opportunity to transmit. Unlike the DCF, the implementation of the PCF is not mandatory. Further more, the PCF itself relies on the asynchronous service provided by the DCF. MAC Sub layer Extent Point Coordination Function (PCF) Used for Contention free transmission Distributed Coordination Function (DCF) Used for Contention services and basis for PCF. Figure 1: MAC Architecture of 802.11 A survey of QoS techniques in 802.11 2
3.2 Distributed Coordination Function (DCF) According to the DCF (see figure 2) a station must sense the medium before initiating the transmission of a packet. If the medium is sensed as being idle for a time interval greater than a Distributed InterFrame Space (DIFS) then the station transmits the packets. Otherwise, Figure 2: Basic Access Scheme the transmission is deferred and the backoff process is started. Specifically, the station computes a random time interval, the backoff interval, uniformly distributed between zero and maximum called Contention Window (CW). This back-off interval is then used to initialize the backoff timer. This timer is decreased only when the medium is idle, whereas it is frozen when another station is transmitting. Specifically, each time the medium becomes idle, the station waits for a DIFS and then periodically decrements the backoff timer. The decrement period is referred to as the slot-time which corresponds to the maximum round-trip delay within the BSS and, hence, depends on the maximum BSS coverage. As soon as the backoff timer expires, the station is authorized to access the medium. Obviously, a collision occurs if two or more stations start transmission simultaneously. Unlike wired networks (e.g., with CSMA/CD), in a wireless environment collision detection is not possible. Hence, as shown in figure 2, a positive acknowledgement is used to notify the sending station that the transmitted frame has been successfully received. The transmission of the acknowledgement is initiated at a time interval equal to the Short InterFrame Space (SIFS) after the end of the reception of the previous frame. Since the SIFS is, by definition, less than the DIFS 1 the receiving station does not need to sense the medium before transmitting the acknowledgement. If the acknowledgement is not received the station assumes that the transmitted frame was not successfully received and, hence, schedules a retransmission and enters the backoff process again. However, to reduce the probability of collisions, after each unsuccessful transmission attempt, the Contention Window is doubled until a predefined maximum (CWmax) is reached. After a (successful or unsuccessful) frame transmission, if the station still has frames queued for transmission, it must execute a new backoff process. 3.3 Virtual Carrier Sense In radio systems based on medium sensing, a phenomenon known as the hidden station problem may occur. This problem arises when a station is able to successfully receive frames from two different transmitters but the two transmitters cannot receive signals from each other. In this case a transmitter may sense the medium as being idle even if the other one is transmitting. This results in a collision at the receiving station. A survey of QoS techniques in 802.11 3
To solve the hidden terminal problem, an optional RTS/CTS (Request To Send and Clear To Send respectively) scheme is used in addition to the previous basic scheme, as shown in Figure. 3: a station sends an RTS before each frame transmission to reserve the channel. Figure 3: RTS-CTS-DATA-ACK Access Scheme Note that a collision of RTS frames (20 octets) is less severe and less probable than a collision of data frames (up to 2346 octets). The destination replies with a CTS if it is ready to receive and the channel is reserved for the packet duration. When the source receives the CTS, it starts transmit-ting its frame, being sure that the channel is reserved for itself during all the frame duration. All other STAs in the BSS update their Network Allocation Vector (NAV) whenever they hear an RTS, a CTS or a data frame. NAV is used for virtual carrier sensing, as detailed in the next paragraph. The overhead of sending RTS/CTS frames becomes considerable when data frames sizes are small, and the channel is suboptimally used. Very large frames may reduce trans-mission reliability too. e.g. an uncorrectable error in a large frame wastes more bandwidth and transmission time than an error in a shorter frame. So another optimization parameter is used, which is fragmentation threshold, above which packets are fragmented. Not all packet types have the same priority. For example, ACK packets should have priority over RTS or data frames. This is done by assigning to each packet type a different Inter Frame Spacing (IFS), after the channel turns idle, during which a packet cannot be transmitted. In DCF two IFSs are used: Short IFS (SIFS) and DCF IFS (DIFS), where SIFS is shorter than DIFS (See Fig. 1 and 2). As a result, if an ACK (assigned with SIFS) and a new data packet (assigned with DIFS) are waiting simultaneously for the channel to become idle, the ACK will be transmitted before the new data packet (the first has to wait SIFS whereas the data has to wait DIFS). Carrier sensing can be performed on both physical and MAC layers. On the physical layer, physical carrier sensing is done by sensing any channel activity caused by other sources. On the MAC sub-layer, virtual carrier sensing can be done by updating a local NAV with the value of other terminals transmission duration. This duration is declared in data, RTS and CTS frames. Using the NAV, a STA s MAC knows when the current transmission ends. NAV is updated upon hearing an RTS from the sender and/or a CTS from the receiver, so the hidden node problem is avoided. The collision avoidance part of CSMA/CA consists of avoiding packet transmission right after the channel is sensed idle for DIFS time, so it does not collide with other waiting packets. Instead, a WT with a packet ready to be transmitted waits the channel to become idle for DIFS time, then it waits for an additional random time, backoff time, after which the packet is transmitted, as shown in Figure. 2 and 3. Collision avoidance is applied on data packets in the basic scheme, and on RTS packets in the RTS/CTS scheme. The backoff time of each STA is A survey of QoS techniques in 802.11 4
decreased as long as the channel is idle (during the so called contention window). When the channel is busy, backoff time is frozen. When backoff time reaches zero, the WT transmits its frame. If the packet collides with another frame (or RTS), the WT times out waiting for the ACK (or the CTS) and computes a new random backoff time with a higher range to retransmit the packet with lower collision probability. This range increases exponentially as where (initially equal to 1) is the transmission attempt number. Therefore, the backoff time equation is: where Slot_time is a function of the physical layer and rand() is a random function with a uniform distribution in [0,1]. 3.3 Point Coordination Function (PCF) Time-bounded data such as voice and video is supported in the 802.11 MAC specification through the Point Coordination B PCF Super frame Contention Free Period Contention Period DCF Figure 4: PCF and DCF working together Busy Function (PCF). As opposed to the DCF, where control is distributed to all stations, in PCF mode a single access point controls access to the media. If a BSS is set up with PCF enabled, time is spliced between the system being in PCF mode and in DCF (CSMA/CA) mode. During the periods when the system is in PCF mode, the access point will poll each station for data, and after a given time move on to the next station. No station is allowed to transmit unless it is polled, and stations receive data from the access point only when they are polled. Since PCF gives every station a turn to transmit in a predetermined fashion, a maximum latency is guaranteed. A downside to PCF is that it is not particularly scalable, in that a single point needs to have control of media access and must poll all stations, which can be ineffective in large networks. The PCF is an optional connection-oriented capability. The PCF needs a Point Coordinator (PC) that initiates and controls the contention-free period(cf) where PCF is used. The PC first senses the channel for PIFS seconds (priority over regular DFS traffic) and then starts a CF period by broadcasting a beacon signal. All regular terminals add CFP maxduration (the maximum possible duration of the contention free period) to their NAV. Later, active users with time-bounded packet streams are continuously polled. The PC can end the contention free period at any time by transmitting a CF-end packet; this occurs frequently when the network is lightly loaded. When a terminal's turn in the poll comes, the PC sends a data packet to it (if any such data is buffered) piggybacked by a poll token or simply a poll token. The receiver sends back an ACK after SIFS seconds or any buffered data piggibacked with an ACK. Note that all packets are separated by SIFS seconds, this is why piggibacking is very useful in this transmission scenario. Priority polling mechanisms can be used if different QOS levels are requested by different polled users. Delay B PCF DCF A survey of QoS techniques in 802.11 5
Users who are idle repeatedly are removed from the poll cycle after k idle periods and polled again at the beginning of the next CF period. k=1 was found to be optimal by Crow et. al. [8] when all time-bounded data are voice data streams; this is explained by the fact that relative to the duration of the CF period, voice streams are sent in slow onoff bursts. 3.5 Fragmentation The MAC also supports a concept called fragmentation that provides for flexibility in transmitter/receiver design, and can be useful in environments with RF interference. An 802.11 transmitter can optionally break messages into smaller fragments for sequential Figure 5: Frame Fragmentation in 802.11 transmission. A receiver can more reliably receive the shorter date bursts because the shorter duration of each fragment transmission reduces the chance for errors due to signal fading or noise. Moreover, the smaller fragments have a better chance of escaping burst interference such as that from a microwave source. The 802.11 standard mandates that all receivers support fragmentation but leaves such support optional on transmitters. Fig. 5 shows how a STA would send a fragmented packet We can see there are no RTSs between packet fragments, so a given WT keeps sending its packet fragments as long as it is receiving the corresponding ACKs. Meanwhile, all other STAs are quiet. This leads us to almost the same data rate shares as if there were no fragmentation, unless there is fragment loss (thus a new RTS), due to a noisy channel for example. In the case of no fragment loss, both above cases can then be described by the former one, i.e. limiting packet lengths to a given value. This technique limits the risk to have to retransmit a package and thus improves overall the performances of the network without wire. The MAC layer is responsible for the reconstitution of the received fragments, the processing being thus transparent for the protocols of higher level. 4. QOS TECHNIQUES IN 802.11 4.1 Introduction There is more than one way to characterize Quality of Service (QoS). Generally, QoS is the ability of a network element (e.g. an application, a host or a router) to provide some level of assurance for consistent network data delivery. Some applications are more stringent about their QoS requirements than others, and for this reason (among others) we have two basic types of QoS available: A survey of QoS techniques in 802.11 6
Resource reservation (integrated services): network resources are apportioned according to an application s QoS request, and subject to bandwidth management policy. Prioritization (differentiated services): network traffic is classified and apportioned network resources according to bandwidth management policy criteria. To enable QoS, network elements give preferential treatment to classifications identified as having more demanding requirements. These types of QoS can be applied to individual application flows or to flow aggregates, hence there are two other ways to characterize types of QoS: Per Flow: A flow is defined as an individual, uni-directional, data stream between two applications (sender and receiver), uniquely identified by a 5-tuple (transport protocol, source address, source port number, destination address, and destination port number). Per Aggregate: An aggregate is simply two or more flows. Typically the flows will have something in common (e.g. any one or more of the 5-tuple parameters, a label or a priority number, or perhaps some authentication information). In the next section we classify and discuss some of the techniques that can be applied to obtain differentiated services in a 802.11 base network. 4.2 Classification Of Service Differentiation Techniques Differentiation in services can be achieved by modifying the parameters that define how a STA would access the wireless medium. These can be broadly classified in to DCF and PCF based techniques. Further classification is done based on the parameter that is used to achieve service differentiation. QoS techniques in 802.11 DCF Based Techniques PCF Based Techniques Backoff Increase Function Varying DIFS Hybrid Coordination Function Distributed TDM Extended DCF Maximum Frame Length Blackburst Fig 5: Classification of QoS techniques in 802.11 4.2.1 Differentiated Services Using DCF To introduce priorities in the IEEE 802.11 using the DCF (Distributed Coordination Function), the following techniques were surveyed. A survey of QoS techniques in 802.11 7
4.2.1.1 Backoff Increase Function or Distributed Fair Scheduling In [7] an access scheme called Distributed Fair Scheduling (DFS) which utilizes the ideas behind fair 1 queuing [2] in the wireless domain is presented. It uses the backoff mechanism of IEEE 802.11 to determine which station should send first. Before transmitting a frame, the backoff process is always initiated. The backoff interval calculated is proportional to the size of the packet to send and inversely proportional to the weight of the flow. This causes stations with low weights to generate longer backoff intervals than those with high weights, thus get-ting lower priority. Fairness is achieved by including the packet size in the calculation of the backoff interval, causing flows with smaller packets to get to send more often. This gives flows with equal weights the same bandwidth regardless of the packet sizes used. If a collision occurs, a new backoff interval is calculated using the backoff algorithm of the IEEE 802.11 standard. 4.2.1.2 Varying DIFS Apart from changing the backoff algorithm, we can vary the DIFS for differentiation. In IEEE 802.11 ACK packets get higher priority than RTS packets, simply by waiting SIFS which is shorter than DIFS (for RTS). Figure 6: Introducing Priority using DIFS Using the same idea to introduce priorities for data frames (in the basic scheme) and for RTS frame (in the RTS/CTS scheme). In this approach each priority level is given a different DIFS, like DIFSl where DIFSj+1 < DIFSj. So the STAs having priority j will wait DIFS j idle period before transmitting the packet. To avoid same priority frames collision, the backoff mechanishm is maintained in a way that the maximum contention window size added to DIFSj is DIFS j-1 - DIFS j as shown in the Fig 6. This ensures that no STA of priority j+1 has queued frames when STA of priority j starts transmission. Low priority traffic will suffer as long as these high priority frames are queued. It could also be the case that the maximum random range (RR j ) after DIFS j can be made greater than DIFS j-1 - DIFS j, so the previous rule becomes less severe. In this case, a packet which failed to access the channel at the first attempt will "probably" have its priority reduced after consecutive attempts, depending on the DIFSs and RRs values. This technique maybe useful for real-time application, where we have more constraints on delays than on packet drops. 4.2.1.3 Maximum Frame Length The third mechanism that can be used to introduce service differentiation into IEEE 802.11 is to limit the maximum frame length used by each STAs. Here, we should distinguish between two possibilities: Either to drop packets that exceed the maximum frame length assigned to a given STA (or simply configure it to limit its packet lengths), or A survey of QoS techniques in 802.11 8
To fragment packets that exceed the maximum frame length. As mentioned in section II, this mechanism is actually used to increase transmission reliability, we ll also use it for differentiation. Figure. 5 shows how a STA would send a fragmented packet. We can see there are no RTSs between packet fragments, so a given WT keeps sending its packet fragments as long as it is receiving the corresponding ACKs. Meanwhile, all other STAs are quiet. This leads us to almost the same data rate shares as if there were no fragmentation, unless there is fragment loss (thus a new RTS), due to a noisy channel for example. In the case of no fragment loss, both above cases can then be described by the former one, i.e. limiting packet lengths to a given value. 4.2.1.4 Extended DCF The contention window is a time window following the transmission of a frame. During this time window, the various stations on the network contend for access to the network. But every station can' t just attempt to seize the wire after the completion of the previous packet transmission. To avoid collisions, the MAC protocol requires that each station first wait for a randomly-chosen time period. Since this period is chosen at random by each station, there is less likelihood of collisions between stations. Extended DCF involves using the contention window as a way to give higher priority to some stations than to others. Assigning a short contention window to those stations that should have higher priority ensures that in most (though not all) cases, the higher-priority stations will be able to transmit ahead of the lower-priority ones. 4.2.1.5 Blackburst The main goal of Blackburst [2] is to minimize the delay for real-time traffic. Unlike the other schemes it imposes certain requirements on the high priority stations. Blackburst requires: 1) all high priority stations try to access the medium with equal, constant intervals, t sch ; and 2) the ability to jam the medium for a period of time. When a high priority station wants to send a frame, it senses the medium to see if it has been idle for a PIFS and then sends its frame. If the medium is busy, the station waits for the medium to be idle for a PIFS and then enters a black burst contention period. The station now sends a so called black burst to jam the channel. The length of the black burst is determined by the time the station has waited to access the medium, and is calculated as a number of black slots. After transmitting the black burst, the station listens to the medium for a short period of time (less than a black slot) to see if some other station is sending a longer black burst which would imply that the other station has waited longer and thus should access the medium first. If the medium is idle, the station will send its frame, otherwise it will wait until the medium becomes idle again and enter another black burst contention period. By using slotted time, and imposing a minimum frame size on real time frames, it can be guaranteed that each black burst contention period will yield a unique winner [4]. After the successful transmission of a frame, the station schedules the next transmission attempt t sch seconds in the future. This has the nice effect that realtime flows will synchronize, and share the medium in a TDM fashion [4]. This means that unless some low priority traffic comes and disturbs the order, very little blackbursting will have to be done once the stations have synchronized. Low priority stations use the ordinary CSMA/CA access method of IEEE 802.11. A survey of QoS techniques in 802.11 9
4.2.2 Differentiated Services Using PCF Some of the techniques surveyed using the PCF to achieve service differentiation are discussed below. 4.2.2.1 Hybrid Coordination Function Hybrid Coordination Function (HCF) uses the wireless access point as a traffic director. The access point uses a polling technique as the traffic control mechanism. (The AP sends polling packets to a succession of stations on the network. The individual stations can reply to the poll with a packet that contains not only the response, but also any data that needs to be transmitted. But it must first wait to be polled.) Instead of polling in a round-robin, or any other purely-fair, unweighted basis, the AP would establish a polling priority based on what the QoS priority should be. 4.2.2.2 Distributed TDM This mechanism uses polling like regular PCF, but using this technique we can also set up time division multiplexing (TDM)-like timeslot periods, and specify which station gets which timeslot. Once the timeslots have been assigned, each station will "know" when it can transmit, and the packet transmissions can take place with very little intervention from the AP (in contrast with HCF, where the AP must use its polling capability to direct the transfer of every frame to be sent). 5. CONCLUSION This survey presents some of the techniques that can be used to attain differentiated services in wireless LANs based on the 802.11 protocol. Some of these techniques can be achieved will little modification to the actual protocol, other are more elaborate to implement. How well these techniques work with real data is yet to be evaluate. These techniques are just part of the complete Diffserv architecture and a mapping has to be established with the different classes of services used in Diffserv. Other issues like admission control too need to be addressed. The 802.11e standard is planning to introduce QoS into the 802.11 standard. 6. FUTURE WORK Firstly these techniques need to be simulated and evaluated for different metrics like throughput, bandwidth utilization and transmission delays under different load conditions. Also simulation to evaluate TCP and UDP traffic can be done. One thing that would be interesting to do in the future is an authentic evaluation of the mechanisms in a real wireless LAN. Since evaluations based on simulations do not consider node mobility and roaming between several base stations, these aspects need to be investigated further. Another aspect that needs to be studied is admission control. To be able to provide service differentiation that gives certain guarantees to high priority traffic, it does not suffice to just have a QoS aware access mechanism. If no admission control is used, it is very likely that too many users will use the higher priority class. This results in an overload that can't be handled, thus reducing the performance of high priority stations. A survey of QoS techniques in 802.11 10
7. REFERENCES [1] IEEE 802.11: Wireless LAN medium access control (MAC) and physical layer (PHY) specifications. IEEE. [2] D-J. Deng and R-S. Chang. A priority scheme for IEEE 802.11 DCF access method. IEICE Transactions on Communications, E82-B(1), January 1999. [3] A. Wolisz F. H.P. Fitzek, "QoS support in wireless networks using Simultaneous MAC packet transmission (SMPT)", in ATS, April 1999 [4]Imad Aad and Claude Castelluccia, "Differentiation mechanisms for IEEE 802.11", IEEE Infocom 2001, April 22-26, 2001 [5] G. Anastasi and L. Lenzini, "QoS provided by the IEEE 802.11 wireless LAN to advanced data applications: a simulation analysis" Page 99-108, ACM, Wireless Networks, Volume 6, Issue 2, 2000. [6] H.S. Chhaya and S. Gupta, Performance of asynchronous data transfer methods of IEEE 802.11 MAC protocol", IEEE Personal Communications 3(5) (October 1996). [7] D-J. Deng and R-S. Chang. A priority scheme for IEEE 802.11 DCF access method. IEICE Transactions on Communications, E82-B-(1), January 1999. [8] Brian P. Crow, Indra Widjaja, Jeong Geun Kim and Prescott Sakai. IEEE 802.11 wireless local area networks. IEEE communication Magazine pages 1160126, Sept. 1997. [9] HomeRF Wireless Lan, web page. URL: http://homerf.org A survey of QoS techniques in 802.11 11