In conclusion, depending on the question which viewpoint is more important, minimizing the waiting time or maximizing utilization, a trade-off choice has to be taken into account. We point out that it doesn’t exist a batch collection rule which is generally valid so each individual case has to be considered separately. Except of designing and implementing an algorithm which would be able to control the server’s initiation mechanism besides the server starting threshold according to some pre-set parameters. This method will dynamically determine and adapt the starting based upon the traffic’s characteristics, the packet maximum delay requirements and the server’s maximum capacity.
IEEE 802.11n and QoS in
Conjunction
The IEEE 802.11n latest amendment attains rates of 100+ M b/s by introducing innovative enhancements at the PHY and MAC, e.g. MIMO and Frame Ag- gregation, respectively. However, the performance improvement potentials may be limited by the interaction between prioritized and parameterized channel ac- cess scheduling mechanisms defined for the QoS support and the enhanced asyn- chronous data service for the increased MAC efficiency. So, if STAs of multiple priorities share the wireless medium at the same time, the IEEE 802.11e amend- ment defines a prioritization method where the higher priorities STAs should maintain shorter waiting channel access periods. Consequently, as we will show here, the higher priority STAs will tend to have small aggregate sizes and lower priority STAs a small channel access frequency, both effects result in poor channel utilization and overall poor network performance. In the following sections, we will briefly describe 802.11e’s EDCA mechanism and how this interferes with the new HT enhancements.
4.1
IEEE 802.11e and EDCA
Several related aspects of traffic grade of service standards for the most recent HT amendment builds upon IEEE 802.11e’s probabilistic priority mechanisms. This
QoS standard is considered of critical importance for delay-sensitive applications, such as VoIP over WLAN and streaming multimedia. It enhances the legacy DCF and PCF, through a new coordination function, known as HCF. Within the HCF, there are two methods of channel access, similar to those defined in the legacy 802.11 MAC, the HCCA and the EDCA. Both EDCA and HCCA have been thoroughly studied and discussed about the QoS improvements over the legacy standard by the research and academic community [99, 100, 101, 102, 103]. Since the distributed coordination mechanism that is based on the CSMA/CA function is of interest for this study, HCCA will not be discussed any further. A simple illustration of EDCA’s operation and mechanism can be seen in Figure 4.1.
(a) The four ACs (b) IEEE 802.11e interframe space relationship
Figure 4.1: An example of EDCA operation
The QoS support in EDCA is provided by the introduction of prioritization via distinguishing the traffic flows into ACs including a set of backoff entities for each AC, such as minimum and maximum CW and AIFS duration, seen in Figure 4.1a. Note that, the AIFS timers assigned by IEEE 802.11e are all defined as one SIFS value plus a variable number of slots times, known as AIFS-number (AIFSN), times Tslot, the duration of a time slot set by the physical layer encoding method in-use. The formula to calculate the AIFS in time slots for each AC is given by the equation AIF S[AC] = SIF S + AIF SN ∗ Tslot. There are four ACs defined as FIFO queues, according to their target application, i.e., Best Effort (BE), Background (BK), Video (VI) and VO, also known as [AC 0, AC 1, AC 3, AC 4] with the enumeration denoting the order of importance from low to high prior- ity and following the same order of the applications given above. So, BE traffic
maintain the lowest priority while VO the highest. Consequently, there are four distinct sets of contending entities with separate values between them that define relative priority in medium access per AC. The main idea is to use four coupled CSMA/CA queue mechanisms one for each AC that behaves as a single enhanced DCF contending entity, and all to contend for access to the same medium. How- ever, each AC is parametrised with different set of values, so higher priority traffic has certain parameters to allow it to gain access to the channel earlier than the lower priority traffic which have longer backoff timers and Inter-Frame Space (IFS) periods. An example of the default set values for the parametrization of each AC as defined in the IEEE QoS standard are given in Table 4.1.
Parameter AC BE AC BK AC VI AC VO
AIFSN 7 3 2 2
CWmin 15 15 7 3
CWmax 1,023 1,023 15 7
TXOP Limit 802.11a,g,n 0 µs 802.11a,g,n 0 µs 802.11a,g,n 3,008 µs 802.11a,g,n 1,504 µs 802.11,b 0 µs 802.11,b 0 µs 802.11,b 6,016 µs 802.11,b 3,264 µs
Table 4.1: EDCA parameters for each AC
An 802.11e STA that obtains medium access must not utilize radio resources for duration longer than a specified limit. This important new attribute of the 802.11e MAC is referred to as a TXOP. A TXOP is an interval of time during which a backoff entity has the right to deliver MSDUs and therefore is an impor- tant means to control delivery delay. A TXOP is defined by its starting time and duration which is limited by a parameter that takes a default set value from the standard. When TXOP is equal to 0, the standard defines that a single MSDU, PPDU, A-MSDU or A-MPDU is allowed to be transmitted. But in a nutshell, an HT STA that is a TXOP holder may transmit multiple MPDUs of the same AC within an A-MPDU as long as the duration of transmission of the A-MPDU plus any expected BlockAck response is less than the remaining Transmitter Network Allocation Vector (TXNAV) timer value that was initialized with the duration from the Duration/ID field in the frame most recently transmitted successfully.
As high priority flows have poor channel utilization because of their traffic characteristics, the low priority flows throughput can be amerced even further. Apart from the traffic load, where a high offered load from the application will sig- nify a big pile in the MAC stack, we need to investigate analytically the operation of EDCA on each prioritized flow.