P O Z N A N U N I V E R S I T Y O F T E C H N O L O G Y A C A D E M I C J O U R N A L S
No 52 Electrical Engineering 2007
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Dariusz KOŚCIELNIK*
Jacek STĘPIEŃ*
ANALYSIS OF THE QUALITY OF SERVICE OF
ISOCHRONOUS DATA STREAMS IN IEEE 802.11e WLAN
The new IEEE 802.11 standard version - 802.11e, was developed in order to introduce mechanisms enabling handling of isochronous streams in the network. The first part of this paper contains short descriptions of the original and modified method of controlling access to the transmission channel. Using the further presented results of simulation tests it is pos-sible to determine system operation intervals in which the network guarantees the required quality of service to traffic streams of a given class. The summary contains most important conclusions from the comparison of both mechanisms as well as the executed experiments. Keywords: wireless network, transmission protocol, EDCF mechanism.
1. INTRODUCTION
The basic IEEE 802.11 standard version was presented in 1999. The defined Wireless Local Area Network may operate in two basic configurations. The first one assumes use of a temporary, volatile system (ad-hoc). This system is based on a required number of stations gathered within a small area, which create an inde-pendent computer network. In such a system, there are no units of special privilege.
The second configuration requires auxiliary fixed infrastructure. This will be normally a wired network, using any transmission protocol. Wires, cooperating sta-tions and necessary software create the distribution system (DS). Mobile stasta-tions (STA) connect to the system using access points (AP). These elements, apart from data exchange between the wired and wireless parts of the system, perform a num-ber of very important tasks, such as: synchronization of station clocks, demarcation of superframe boundaries and power saving process control..
1.1. Basic version of transmission protocol
Transmission in a IEEE 802.11 network proceeds with transactions. In the sim-plest case, a transaction consists of two packets. The first one carries data and is denoted as Data. If transfer has been successful, then, after the time of short inter-frame space (SIFS), the data recipient should confirm it with acknowledge inter-frame (ACK). In general, a transaction may contain a few pairs of packets: Data and
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ACK. At its beginning there may be an additional exchange of packets reserving the channel: ready to send frame (RTS) and clear to send frame (CTS).
Before the next transaction may begin, the station must obtain access to the channel. In the basic IEEE 802.11 standard version, two mechanisms are defined to manage this process. The first one is known as the distributed coordination func-tion (DCF) and may be used in systems with both fixed and temporary infrastruc-ture. Its operation is based on the CSMA/CA protocol, ensuring multiple access to the channel with content detection and collision avoidance. Before a new transac-tion begins, the statransac-tion must ensure that the previous transactransac-tion has already fin-ished (Fig. 1). The respective test checks silence in the radio channel which cannot be shorter than the time identified as DIFS (DCF inter-frame space). After this time, the station measures additional, random delay time (but only when the chan-nel was already busy at the time of receiving the transmission request). If no other station begins sending first, the node will get the right to open its own transaction.
Fig. 1. Operation of DCF mechanism
The random time duration is a multiple of the time-slot – TSL. The determined
transmission back-off period value – TBO is within the range [0, CW]. CW
parame-ter deparame-termines the maximum duration of the contention window (Fig. 1).
Two additional mechanisms were also used in the protocol. The first one is called the continued countdown principle. When a station is taken over by another system user during the contention period, then in the next channel takeover attempt it will use the clock value remaining after the previous unsuccessful countdown.
The second mechanism is to double the CW parameter increment in the event of collision occurring during the last contention period. A similar principle is utilized in IEEE 802.3 networks and referred to as binary exponential back-off. However, this time, after a packet has been successfully sent, the CW window size does not immediately come back to its minimum value (CWmin), but is only reduced by half. The second method of channel access may be used only in systems equipped with fixed infrastructure. It makes use of the polling method and is referred to as point coordination function (PCF). Coordination station (CS) is responsible for ac-tivating selected stations, according to the prepared list. This function is normally delegated to an access point (AP). Querying is repeated at specified intervals, ac-cording to the duration of the superframe, which begins when the Beacon frame appears in the channel. This signal starts the first part of the superframe, known as contention free period (CFP). In this phase, the point coordination principle (PCF) is followed. End of the collision-free interval is announced by appearance of
CF-End frame. The remaining part of the superframe creates the contention period (CP), in which channel access is controlled by DCF mechanism. As PCF procedure was not covered by the tests, it will not be discussed here in more detail.
1.2. Modified DCF – EDCF protocol version
The new protocol combines features of both mechanisms: DCF and PCF. This is why it has been named hybrid coordination function (HCF). During the super-frame, both periods (CFP and CP) are still distinguished. This time however, the hybrid coordinator (HC) can take over system control during both parts of the su-perframe. Examining system operation in terms of the possibility to transmission of isochronous data, more significant is the modification in the DCF protocol, whose new version has been called extended distribution coordination function (EDCF).
Requests sent to the system can be assigned different priority levels. It is pro-posed to use eight privilege classes, numbered [0 ÷ 7]. Class 7 corresponds to the highest service priority. QSTA (QoS station) stores packets of each of the classes in independent transmitter buffer queues. Data having the highest priority at the given time are served first. Each class is assigned different contention mechanism control parameters (Fig. 2). First of all, the lower the sent data priority, the longer the minimum delay time between transactions – arbitration inter-frame space (AIFS). Therefore, it is more probable that contention will be won by the station that has more important data to send.
Fig. 2. Operation of EDCF mechanism
The second change relates to the size of contention windows (CW). The lower the class of transmitted data, the higher the minimum (CWMIN) and maximum
(CWMAX) values of this parameter. With this solution, it is more likely that high
class packets will be preceded by shorter back-off periods – TBO.
2. EDCF MECHANISM PERFORMANCE SIMULATION TESTS
The new IEEE 802.11 standard version was subjected to simulation tests. Their main purpose was to determine the way how the network handles isochronous traf-fic streams in a situation when the system is loaded with requests of threshold in-tensity value, as well as in a condition of contention. The most important of theevaluated parameters was respective quality of service guaranteed by the network for data of a given priority.
2.1. Model of examined system
Four types of request streams were used in the tests, The first one contained asynchronous requests, corresponding to typical computer data transfer. Frames transporting this type of information were assigned the lowest priority (class 0). Request intensity, depending on the character of tests, could have infinitely large value (restricted by available system throughput) or predefined maximum value.
Streams of the other three types were generated by isochronous data. Each of them was assigned a different level of priority: 1, 2, or 3, respectively. Data por-tions, generated in equal intervals, were placed in packets of the same length. All the results presented further were obtained given the assumption that each of Data frames transports a 1024-octet data field.
For streams of different priorities, respective parameter sets were defined to be used by the transmission channel access procedure. The most important of them, determining the contention process and effects, are presented in Tab. 1.
Table 1. Contention window parameters for different priority levels Type of data AIFS CWmin CWmax
asynchronous (class 0) DIFS+6TSL 63 1023
isochronous (class 1) DIFS+4TSL 31 127
isochronous (class 2) DIFS+2TSL 15 63
isochronous (class 3) DIFS 7 15
The other network operation parameters are consistent with the existing stan-dards (Tab. 2). During the simulations, the number of utilized priority levels was reduced to four. This operation was supposed to facilitate data analysis, and did not affect by any means the accuracy or generality of the results.
Table 2. Transmission system parameters
Parameter Value
Transmission rate 11 Mb/s
Time-slot – TSL 20 µs
Short inter-frame space – SIFS 10 µs
Number of priority levels 4
Data field length – DFL 1024 octets
2.2. Single isochronous data stream
The first results set presents how the system behaves when loaded with a single isochronous data stream. The background for the observed requests is asynchro-nous traffic (class 0) with unlimited intensity. This means that after sending each data portion, subsequent blocks are already waiting for transmission in transmitter buffers. In this way the effective throughput of asynchronous requests is automati-cally adjusted to available network throughput.
Isochronous traffic packets are assigned priority 2. This stream intensity was gradually increased in subsequent experiments by 0.5 Mb/s. The obtained curves presenting effective throughhput offered by the system are shown on Fig. 3.
Isochronous data gradually displace asynchronous stream packets from the net-work. With the priority assigned to them isochronous requests are handled across full offered throughput. This takes place until total traffic intensity completely ex-hausts system's transmission resources. As isochronous requests intensity grows further, it no longer causes major changes in the distribution of available system throughput. In the remaining part of the chart, both curves gradually come closer to their horizontal asymptotes.
Fig. 3. Effective throughput offered to different data streams as function of class 2 request intensity
It is necessary to explain why the asynchronous traffic stream was not com-pletely suppressed by the growing isochronous data stream. The reason are the standard rules of contention for the right to begin transmission for different class packets. The changes do not guarantee absolute priority in channel access to higher class packets, but only increase their chances of winning. In addition, with the con-tinued countdown method, even after losing the contention process, maximum in-crease of the CW contention window and very unfavourable sampling result, the station sending asynchronous data will have a possibility to perform another at-tempt and at the same time its output stream will not be completely suppressed.
One should also note that the total effective system throughput is higher when the network has been loaded with large isochronous traffic, at the expense of re-stricted service for asynchronous data. This happens because packets whose trans-mission is preceded by shorter contention period are becoming more predominant in the process of contention for channel access (Tab. 1). In consequence, smaller are throughput losses resulting from delay times between subsequent transactions.
In order to be able to measure the suitability of the EDCF protocol for transmis-sion of isochronous streams, the size and character of delays of subsequent data blocks must be examined. The results of the simulations are presented on Fig. 4.
Fig. 4. Packet waiting time in the transmitter buffer as a function of class 2 isochronous traffic request intensity; a) average transmission delay time, b) standard deviation of delay times
A valuable feature of the tested system are permanent and small average delay times for isochronous stream packets (Fig. 4.a). This situation changes only in a condition of congestion. In this condition, regularly generated class 2 data start to be gathered in the transmitter queue whose length is quickly growing. As a result, the average transmission delay time value grows exponentially.
For isochronous data, the permanent character of the delay time is more impor-tant than the delay time itself. Fig. 4.b presents standard deviation of this parame-ter. In a non-overloaded system, this value is relatively small. The delay time vari-ability must be regarded as absolutely acceptable for all levels of quality and ser-vice defined in this network, including real time audio streaming.
2.3. Contention between different class isochronous streams
Another interesting aspect of the new protocol operation are certainly mutual re-lations between multiple isochronous streams contending for common transmission resources. Each of them is assigned a different priority, respectively: 1, 2 or 3. In order to determine the further presented characteristics, streams with priorities 1 and 3 were assigned constant load of 2.4 Mb/s. Class 2 data were generated with gradually increased intensity, changing in the range 0 ÷ 5.5 Mb/s.
Fig. 5 presents effective throughput of every stream in three network operation stages. When the total offered traffic value is within the system's transmission ca-pacities, loads of either of the three request types are not suppressed. The situation changes when the entire available throughput is used. This corresponds to class 2 data stream growth to ca. 1 Mb/s. In this situation, its packets gradually begin to force class 1 traffic out from the channel. The mechanism of this process is still the same as described above, but this time the basis are differences in minimum and maximum contention window sizes and delay time between transactions (AIFS).
It is worth noting that the top priority stream (class 3) does not experience any restrictions in the period concerned. Its effective throughput remains constant and
equal to offered traffic intensity, even when too large class 2 traffic starts to be lim-ited in the overloaded system. This situation occurs when priority 2 requests ex-ceed the intensity of 3.5 Mb/s (Fig. 5).
Fig. 5. Effective throughput offered to different data streams as function of class 2 isochronous traffic request intensity
In order to be able to correctly assess the quality of service guaranteed to differ-ent class data, delay times in transmitter queues must be examined. Respective characteristics are presented on Fig. 6.a.
Fig. 6. Average packet waiting time in the transmitter buffer as a function of class 2 isochronous traf-fic request intensity; a) average transmission delay value, b) delay time standard deviation
Average delay time calculated for different streams is very similar to the one presented on Fig. 4. Also this time, when requests of a given priority are sup-pressed, queues in transmitter buffers begin to grow quickly. As a result, the time when successful transmission of the packet may begin increases exponentially.
A very interesting effect was observed during examination of the transmission delay time standard deviation (Fig. 6.b). Its value shows characteristic local maxi-mums in both cases. Their location corresponds to the beginnings of suppression
zones for a given type of offered traffic. In order to understand the reasons for this effect, one must note that contending isochronous data streams consist of fixed length packets, generated in fixed time intervals. Service requests sent to the sys-tem from different class sources may therefore appear in various mutual relations, not interfering with each other, or overlapping to cause a collision. Different packet generation frequencies for different classes cause periodical interference between data streams. Their presence can make temporary collision window width (CW) grow to the maximum value, and later fall down to the assumed minimum level.
To conclude the analysis of this case, it is worth noting that the stream of top priority requests (class 3) did not experience any effect of system overload. Both its effective throughput (Fig. 5) as well as transmission delay time (Fig. 6.a) and delay time variability (Fig. 6.b) did not significantly change in the entire examined range. Class 2 stream was serviced in a similar way until it began to contend for system resources with higher priority data. The results confirm that the examined
IEEE 802.11 standard version maintains the required quality of service for top
prior-ity streams, provided that their total intensprior-ity does not exceed transmission channel throughput. To reverse the situation, one may say that if a top priority data stream does not cause system overload in the network itself, any growth in lower class re-quest intensity is not able to reduce the guaranteed quality of service.
2.4. Contention of same priority streams
The last discussed aspect is contention of multiple isochronous streams of the same priority. In subsequent simulations, attached to the system were subsequent class 1 sources and one class 2 source. Each stream had the same intensity of 1 Mb/s and the same packet length (1024 octets). The results are shown on Fig. 7.
Class 1 offered traffic intensity was suppressed when the number of class 1 streams exceeded 5. After then, each of the sources could use throughput reduced to the same extent, and its value gradually decreased as subsequent users were at-tached to the system (Fig. 7). Higher priority request intensity (class 2) was not limited in this time at all. The results confirm that top quality data have guaranteed quality of service, even when the network is overloaded with lower class traffic.
The growing number of request streams unfortunately resulted in a slight growth in average transmission delay time for class 2 packets (Fig. 8.a). The earlier presented results did not show such an effect when intensity of one of the service request sources was growing. However, this time it is not only the total intensity of generated packets that grows, but also the number of independent streams. There-fore, the number of collisions is growing as is the likelihood that some of them will be lost by top priority packets. As a result of differences in parameter values: CWmin, CWmax and AIFS, the higher the discrepancy in numbers of contending classes, the weaker the effect of this phenomenon. In order for the discussed mechanism to be visible, the presented characteristics were determined for data streams with adjacent priority values.
Fig. 7. Operation of the system loaded with a variable number of class 1 isochronous data streams – effective throughput calculated for different data streams
Fig. 8.b presents standard deviation of the average transmission delay time. In a non-overloaded system, this value is relatively small for class 1 isochronous data streams. The standard deviation is very small for all class 2 packet.
Fig. 8. Operation of the system loaded with a variable number of class 1 isochronous data streams; a) effective throughput calculated for different data streams, b) average transmission delay time value
3. SUMMARY
A very important advantage of the new IEEE 802.11 standard version is com-patibility with previous solutions. Thanks to that, stations utilizing both DCF pro-tocol and its modernized version – EDCF – may work together without problems within one network. Nonetheless, compatibility had to imply a number of limita-tions. The most significant is the fact that higher class data no longer have absolute priority in access to the transmission medium over lower priority packets. Instead, different streams have different values assigned of probability of winning subse-quent contentions. The scope of operation of the binary exponential back-off mechanism was also differentiated.
In spite of significant limitations, EDCF protocol shows very good performance in ensuring the guaranteed quality of service for different request streams. Input traffic is never suppressed in a non-congested system. When network overload ap-pears, lowest priority data service intensity is reduced in the first place. All high class streams whose total throughput does not exceed system's transmission capaci-ties are then still serviced with assumed quality. In particular, the network exam-ined will never restrict top priority traffic, unless its intensity (plus protocol and transmission overhead) is close to or higher than the applied channel throughput.
The band not used by isochronous data can be fully used by asynchronous re-quests that don't need guaranteed small or constant delay time values (Fig. 3). This effect can be certainly regarded as a valuable advantage.
The tests also disclosed a mechanism that prevents lower priority data from be-ing completely forced out from the data channel by higher class packets (Fig. 3 and Fig. 5). As a result, the maximum throughput available for the stream of most de-manding (or most important) information is limited when other, less dede-manding or less important types of requests are also present in the network. One should note though that this effect can be in many cases considered as a valuable advantage. It prevents other streams from being blocked by too intensive top priority transmis-sion, and thus eliminates a specific case of system throughput monopolization.
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