2. Background
2.2 IEEE 802.11(e) QoS Enhancements
The MAC architecture can be described as shown in Figure 4 as providing the PCF and hybrid coordination function (HCF) through the services of the DCF. PCF is optional in all STAs.
Figure 4. IEEE 802.11e MAC architecture
In IEEE 802.11e, an additional coordination function called HCF is usable in QoS network (QBSS) configurations and shall be implemented in all QSTAs. It combines functions from the DCF and PCF with some enhanced, QoS-specific mechanisms and frame subtypes. The HCF uses both a contention-based channel access method, called the enhanced distributed channel access (EDCA) mechanism for contention-based transfer and a controlled channel access, referred to as the HCF controlled channel access (HCCA) mechanism, for contention-free transfer.
The EDCA mechanism provides differentiated, distributed access to the WM for QSTAs using eight different user priorities (UPs). The EDCA mechanism defines four access categories (ACs) that provide support for the delivery of traffic with UPs at the QSTAs. The AC is derived from the UPs as shown in Table 1.
Table 1. UP-to-AC mappings
For a transmit queue of each AC, an independent EDCA function contends for transmission opportunities (TXOPs) using a distinct set of EDCA parameters as following:
1. Arbitration IFS: the minimum specified idle duration time is not the constant value (DIFS) as defined for DCF, but is a distinct value assigned either by a management entity or by a QAP. The duration AIFS[AC] is a duration derived from the value AIFSN[AC] by the relation:
AIFS[AC] = AIFSN[AC] × aSlotTime + aSIFSTime ( 3 )
2. CWmin and CWman: the contention window limits, from which the random backoff is computed, are not fixed per PHY, as with DCF, but are variable and assigned by a management entity or by a QAP.
3. TXOP limit: during an EDCA TXOP, a QMS may initiate multiple frame exchange sequences within the same AC. The duration of this EDCA TXOP is called TXOP limit and bounded for an AC. A value of 0 for this duration means that the EDCA TXOP is limited to a single MSDU or MMPDU at any rate in the operational set of the QBSS.
4. Internal collisions: if collisions occur between contending EDCAFs within a QSTA, the data frames from the higher priority AC receives the TXOP and the data frames from the lower priority colliding AC(s) behave as if there were an external collision on the WM.
The default values used by QSTAs for the parameters in the EDCA Parameter Set element are defined in Table 2.
Table 2. default EDCA parameter values
The HCCA mechanism uses a QoS-aware centralized coordinator, called a hybrid coordinator (HC). The HC is collocated with the QAP of the QBSS and uses the HC’s higher priority of access to the WM to initiate frame exchange sequences and to allocate TXOPs to itself and other QSTAs in order to provide limited-duration controlled access
The HC differs from the PC used in PCF in several significant ways, although it may
ive different IFSs are defined to provide priority levels for access to the wireless med
.3 Admission Control at the HC
Admission control, in general, depends on vendors’ implementations of schedulers, ava
optionally implement the functionality of a PC. Most important is that HCF frame exchange sequences may be used among QSTAs associated in a QBSS during both the CP and any locally generated CFP (generated optionally by the HC) to o meet the QoS requirements of a particular TC or TS. Another significant difference is that the HC grants a non-AP QMS a polled TXOP with duration specified in a QoS(+)CF-Poll frame.
Non-AP QSTAs may transmit multiple frame exchange sequences within given polled TXOPs, subject to the limit on TXOP duration.
F
ia. Arbitration interframe space is added in IEEE 802.11e for QoS facility. The relationships of these IFSs are shown as Figure 5.
Figure 5. IEEE 802.11e IFS relationships
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ilable channel capacity, link conditions, retransmission limits, and the scheduling requirements of a given TSPEC. It is required when a QMS desires guarantee on the
amount of time that it can access the channel. The HC, which is in the QAP, is used to administer admission control in the network. As IEEE 802.11e supports two access mechanisms, there are two distinct admission control mechanisms: one for contention-based access and another for controlled access.
When the HC provides controlled channel access to non-AP QSTAs, it is resp
n ADDTS (add traffic stream) Request frame shall be transmitted by a non-AP QM
. Accept the request, or
hen the HC aggregates the admitted TS, a QAP shall schedule the transmissions in onsible for granting or denying polling service to a TS based on the parameters in the associated TSPEC. If the TS is admitted, the HC is responsible for scheduling channel access to this TS based on the negotiated TSPEC parameters. The HC should not tear down a TS unless explicitly requested by the MS or at the expiry of the inactivity timer. The polling service based on admitted TS provides a “guaranteed channel access” from the scheduler in order to have its QoS requirements met. The nature of wireless communications may preclude absolute guarantees to satisfy QoS requirements. However, in a controlled environment (e.g., no interference), the behavior of the scheduler can be observed and verified to be compliant to meet the service schedule.
A
S to the HC in order to request admission of traffic in any direction (i.e., uplink, downlink, direct, or bidirectional) employing an AC that requires admission control.
The QAP shall respond to an ADDTS Request frame with an ADDTS Response frame that may be to accept or deny the request. On receipt of an ADDTS Request frame from a non-AP QSTA, the QAP shall make a determination about whether to
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2. Deny the request.
W
HC
.4 The Sample Scheduler and Admission Control Unit
In this section, the design of a simple scheduler and admission control unit (the unit tha
The sample scheduler uses the mandatory set of TSPEC parameters to generate a CA TXOPs and communicate the service schedule to the non-AP QSTA. The service schedule is communicated to the non-AP QMS in a Schedule element contained in an ADDTS Response frame. In the ADDTS Response frame, the modified service start time shall not exceed the requested service start time, if specified in ADDTS Request frame, by more than one maximum service interval (SI). The HC uses the maximum SI for the initial scheduling only as there may be situations that HC may not be able to service the TS at the scheduled timing, due to an EDCA or DCF transmission or other interferences interrupting the schedule. The Service Interval field value in the Schedule element shall be greater than the minimum SI. The service schedule could be subsequently updated by a QAP as long as it meets TSPEC requirements. A non-AP QMS may affect the service schedule by modifying or deleting its existing TS. Section 2.4 provides guidelines for deriving an aggregate service schedule for a single non-AP QMS from the non-AP QSTA’s admitted TS. The schedule shall meet the QoS requirements specified in the TSPEC.
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t administers admission policy in the HC SME) meets the minimum performance requirements of controlled-access admission control and use the minimum set of mandatory TSPEC parameters.
schedule: Mean Data Rate, Nominal MSDU Size, and Maximum Service Interval or Delay Bound. If both Maximum Service Interval and Delay Bound parameters are specified by the non-AP QMS in the TSPEC, the scheduler uses the Maximum Service Interval parameter for the calculation of the schedule. The schedule for an admitted stream is calculated in two steps:
1. Calculate the scheduled SI.
of the stream for a given SI.
In first step, the calculation of the scheduled service interval is done as follows: First, the
For the calculation of the TXOP duration for an admitted stream, the scheduler uses 2. Calculate the TXOP duration
scheduler calculates the minimum of all maximum SIs for all admitted streams. Let this minimum be m. Second, the scheduler chooses a number lower than m that is a submultiple of the beacon interval. This value is the scheduled SI for all non-AP QSTAs with admitted streams. See Figure 6.
Figure 6. Schedule for stream from QMS i
the following parameters: Mean Data Rate (ρ) and Nominal MSDU Size (L) from the negotiated TSPEC, the Scheduled Service Interval (SI) calculated above, Physical Transmission Rate (R), Maximum Allowable Size of MSDU, i.e., 2304 bytes (M), and Overheads in time units (O). The physical transmission rate is the minimum PHY rate negotiated in the TSPEC. If the minimum PHY rate is not committed in the ADDTS Response frame, the scheduler can use the observed PHY rate as R. The overheads in time include IFSs, ACK frames and CF-Poll frames. For simplicity, details for the overhead calculations are omitted in this description. The TXOP duration is calculated as follows: First, the scheduler calculates the number of MSDUs that arrived at the mean data rate during the SI:
(4)
hen the scheduler calculates the TXOP duration as the maximum of
SDU at Ri (plus overheads):
(5)
An example is shown in Figure 6. Stream from QMS i is admitted. The beacon inte
a new stream is admitted with a maximum SI smaller than the current SI, the
sch of
n admission control unit (ACU) administers admission of TS. The ACU uses the sam
T
— Time to transmit Ni frames at Ri and
— Time to transmit one maximum size M
rval is 100 ms and the maximum SI for the stream is 60 ms. The scheduler calculates a scheduled SI (SI) equal to 50 ms using the steps above. The same process is repeated continuously while the maximum SI for the admitted stream is larger than the current SI. An example is shown in Figure 7.
If
eduler needs to change the current SI to a smaller number than the maximum SI the newly admitted stream. Therefore, the TXOP duration for the current admitted streams needs also to be recalculated with the new SI.
Figure 7. Schedule for streams from QMS i to k
A
e set of parameters that the sample scheduler uses. When a new stream requests admission, the admission control process is done in three steps:
1. The ACU calculates the number of MSDUs that arrive at the mean data rate
. The ACU calculates the TXOP duration that needs to be allocated for the
3. The ACU determines that the stream can be admitted when the following
(6)
here
is the number of existing streams
1 is used as index for the newly arriving stream
ffic
The ACU ensures that all admitted streams have guaranteed access to the channel.
An
during the scheduled SI. The scheduled SI (SI) is the one that the scheduler calculates for the stream as specified in K.3.3.1. For the calculation of the number of MSDUs, the ACU uses the equation for Ni shown in K.3.3.1.
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stream. The ACU uses the equation for TXOPi shown in K.3.3.1.
inequality is satisfied:
w
k k +
T
indicates the beacon intervalTcp is the time used for EDCA tra
y modification can be implemented for the design of the ACU. For example, UP-based ACU is possible by examining the UP field in TSPEC to decide whether to admit, retain, or drop a stream. If the UP is not specified, a default value of 0 is used. If a higher UP stream needs to be serviced, an ACU might drop lower UP streams.
3. System Design and Implementation
This section is divided into two parts. The first one describes the architecture of the overall system, including the design of the MAC layer, which is necessary for the dynamic admission control unit and the priority-based scheduler. The second one describes the detailed implementation of the proposed scheduler and how the dynamic assignment works.
3.1 System Architecture
Figure 8 illustrates a model of a MAC implementation that adopts the regulate channel access procedures specified by the 802.11e standard. A MAC service data unit (MSDU) with its user priority (UP) is sent to the MAC layer from the upper application. At first, it is categorized as either EDCA or the HCCA procedure. The EDCA packets will be inserted to one of the four transmission queues depending on the UP field. For each transmission queue, the channel access is based on its EDCA parameters and the CSMA/CA algorithm. The HCCA traffic, however, is needed to be polled by the hybrid coordinator (HC). If the MAC layer receives an HCCA MSDU, it first recognizes the traffic stream to which the MSDU belongs and then dispatches the MSDU to a corresponding transmission queue. In the case that the determined traffic stream has not been registered in a HC yet, the MAC layer needs to send an ADDTS Request and waits for the decision of the HC.
A traffic controller in Figure 8 manages the register of TSs and other corresponding behavior. It also handles the polling service from a HC. Besides, the transmission coordinator in Figure 8 is responsible for the cooperation of the HCCA and EDCA channel access mechanisms. Suppose that both two channel access schemes inside a node want to access WM, which of them could get the access right first? If one of them
wants to access the medium while the other is transmitting, how this conflict is solved?
The transmission coordinator is used to deal with these problems and important to the efficiency of 802.11e MAC.
Figure 8. The MAC architecture of 802.11e
In the preceding discussion, a model of MAC layer is depicted. In addition, the network architecture is described in the following. In such a network, there are several access points operating in different channels and a number of mobile stations in a limited region. Because of the overlapping transmission ranges of theses access points, a mobile station may have more than one access point to select to do association. To enhance the channel utilization of each access point, the messages describing guaranteed throughputs in each access point are exchanged in wired links.
Based on the obtained information about the statuses of other access points, the current access point will try to decide which access point the traffic stream can be registered in after receiving an ADDTS Request which can not be served. If an access point is chosen by the distributed, priority-based algorithm, the current access point will send an modified ADDTS Request to the chosen access point over a wired network.
When a correct response is answered, the current access point then notifies the mobile station which sends the original ADDTS Request of the chosen access point. In such case, the mobile station can re-associate with the chosen AP.
3.2 System Implementation
In the proposed scheme, some problems need to be taken into account, such as identifying that whether the network is fully overlapped or partially overlapped in terms of the coverage of access points, the mobility of mobile stations, etc. The solutions of these problems will be discussed in the following sections.
3.2.1 Basic Mechanism
According to the IEEE 802.11e standard, the traffic stream of HCCA will send an ADDTS Request to ask its associated AP for guaranteeing the bandwidth it requires.
Nevertheless, it is possible that a number of mobile stations connect to the same access point, and thus this phenomenon will cause the starving problem of traffic streams in QoS provision. Using several dynamic assignment schemes, traffic streams can be moved from the current access point to a chosen one. The movements of traffic streams among access points will increase the QoS ability and total performances of the network.
In next section, the schemes of the dynamic assignment are briefly introduced.
First of all, the access point information (AP_Info) packets are exchanged over
wired links every one second or each time a scheduling is changed. Every access point has to maintain a list of the AP_info structures based on those exchanged packets and keep track of the latest statuses of other access points. Table 3 shows how an AP_info structure describes AP1 and also explains the meanings of those fields in an AP_info structure. Based on the information in Table 3, our dynamic admission schemes try to decide which AP can be chosen for re-register the unsatisfied traffic stream. The following schemes are used while the current access point does not have the ability to satisfy an incoming ADDTS Request.
Parameters Meanings
BSSID The Basic Service Set ID of AP1.
AP_addr The MAC address of AP1.
Channel The using channel of AP1.
Max_SI The maximum service interval of AP1 scheduler.
TXOP_sum The sum of transmission opportunities guaranteed by AP1.
Timestamp The generation time of the latest AP_Info packet from AP1.
Bandwidth The bandwidth of AP1.
Select A value indicates AP1 has been selected and its status shall be refreshed before assigning another stream to AP1.
Table 3. The information of AP1 stored in the struct of AP_Info.
1. Scheme A: to find the target access point which can directly support the QoS requirement of a traffic stream, the scheduler checks every unselected access point in the list. It tries to find an access point which uses the smallest bandwidth and shall be freer than the current one. If such an access point exists, the scheduler will calculate the traffic stream’s TXOP using the information of this access point to make sure that it has the ability of serving this traffic
stream. If so, the ADDTS Request is redirected to the chosen access point over a wired link. The chosen access point still has the right to admit or deny this ADDTS Request by responding an ADDTS Response to the current access point over the wired link. However, if the chosen access point accepts the ADDTS Request, the current access point will send a response to the mobile station owning the traffic stream and inform it of the chosen AP for re-association. If Scheme A fails, the scheduler will use the next schemes to handle the request.
2. Scheme B: first of all, the registered traffic streams which ask for smaller bandwidth than the incoming traffic stream does are picked out. The scheme detects if there is any of these traffic stream which can be moved out to satisfy the QoS requirement of the incoming traffic stream. In such a case, the scheduler needs to find another access point which is able to accept the moved-out traffic stream. After putting the selected traffic stream to another access point successfully, the current access point can accept the incoming traffic stream and add it into the polling schedule.
3. Scheme C: if the incoming traffic stream can be satisfied by neither Scheme A nor Scheme B, Scheme C just takes the utilization of each AP into account and moves the incoming traffic stream to an access point which has less utilization.
Scheme C leaves the TS-support decision to the target AP, which may result in vibration behavior that a mobile station repeatedly changes its associated access point. To solve this unsteadiness problem, the ADDTS Request needs to carry a timestamp, which indicates the time when the first ADDTS Request of this traffic stream generates. If an access point receives an ADDTS Request, the timestamp of
Scheme C leaves the TS-support decision to the target AP, which may result in vibration behavior that a mobile station repeatedly changes its associated access point. To solve this unsteadiness problem, the ADDTS Request needs to carry a timestamp, which indicates the time when the first ADDTS Request of this traffic stream generates. If an access point receives an ADDTS Request, the timestamp of