A High-Throughput MAC Strategy for Next-Generation WLANs
Seongkwan Kim Youngsoo Kim Sunghyun Choi
Multimedia & Wireless Networking Laboratory
Seoul National University
{skim,yskim}@mwnl.snu.ac.kr, [email protected]
Abstract
Today, IEEE 802.11 Wireless LAN (WLAN) has emerged
as a prevailing technology for the broadband wireless networking. Along with many emerging applications and services over WLANs, the demands for faster and highercapacity WLANs have been growing fast. In this paper, we
propose a new medium access control (MAC) scheme for the
next-generation high-speed WLANs such as IEEE 802.11n.
The proposed MAC, called Multi-user polling Controlled
Channel Access (MCCA), is composed of two sub-schemes.
The first one is multi-user polling in order to achieve higher
network utilization. We also introduce a frame aggregation scheme as the another proposed scheme, which performs aggregations at both MAC and physical (PHY) layers, and can achieve even higher throughput gain as a result. From simulations, we confirm that the proposed MCCA
scheme enhances the aggregate throughput of non-qualityof-service (non-QoS) traffic by an order of magnitude from
17.4 Mbps to 129.9 Mbps, while the aggregate throughput
and QoS requirements continue to be satisfied.
1. Introduction
In recent years, IEEE 802.11 Wireless LAN (WLAN)
has gained a prevailing position in the market for the (indoor) broadband wireless access networking. The IEEE
802.11 standard defines the medium access control (MAC)
layer and the physical (PHY) layer specifications [1]. Due
to the contention-based channel access nature of the mandatory channel access function (i.e., distributed coordination
function or DCF), it supports only the best-effort service
without guaranteeing any quality-of-service (QoS).
The new MAC protocol of the 802.11e is called the Hybrid Coordination Function (HCF), which will provide QoS
[3, 8, 9, 10]. The standardization of the IEEE 802.11e is still
on-going at the final stage. The HCF contains a contentionbased channel access mechanism (referred to as Enhanced
Distributed Channel Access or EDCA) and a centralized
Kyunghun Jang Jin-Bong Chang
Communication & Network Laboratory
Samsung Advanced Institute of Technology
{khjang,jinb.chang}@samsung.com
channel access mechanism, namely, HCF Controlled Channel Access (HCCA).
Recently, the needs for real-time services, such as Voice
over IP (VoIP) over WLANs have been increasing drastically. Due to such fast-growing demands, the market, where
IEEE 802.11b has gained its wide deployment base so far,
is switching to new IEEE 802.11g-based products supporting up to 54 Mbps [4]. This kind of trend, i.e., growing
demands for higher-speed WLANs, will be accelerated further along with the evolution of the wired counterparts, e.g.,
1 or 10 Gbit Ethernets.
IEEE 802.11 Task Group N (TGn) has been initiated in
September 2003 for a high-speed WLAN to provide a maximum throughput of at least 100 Mbps measured at the MAC
data service access point (SAP) [7]. This group will generate a high-speed WLAN standard called IEEE 802.11n
in the future. Currently, many different technical solutions covering both PHY and MAC are being proposed for
the new protocol architecture within TGn [6]. The 802.11
is known to have a high overhead for the MAC/PHY operations such as PHY preamble/header, MAC header, acknowledgment, and backoff procedures, thus yielding the
throughput performance much worse than the underlying
PHY transmission rate. In [11], Yang and Jon demonstrate
that by simply increasing the PHY rate without reducing the
MAC/PHY overhead, the enhanced throughput is bounded
under 100 Mbps, even if the PHY rate goes to infinity. It
means that we need to enhance the 802.11 MAC by reducing overheads in order to create the next-generation highspeed WLAN.
In this paper, we propose a new MAC scheme, called
Multi-user polling Controlled Channel Access (MCCA),
which is composed of two schemes including multi-user
polling and two-level frame aggregation. The proposed
scheme reduces the protocol overheads by eliminating the
backoff process via an efficient multi-user polling and reducing the MAC/PHY header overheads via frame aggregation at both MAC and PHY. From simulations, we confirm that the proposed MCCA achieves our design goal by
significantly improving throughput performance.
2. Related Work
Under the 802.11 PCF or the 802.11e HCCA, each station transmits frames upon being polled. Apparently, pollframe transmissions can be another type of overhead. The
number of poll-frames can be reduced by using multi-user
polling, and it can enhance wireless channel utilization.
Shou-Chih Lo et al. proposed a multi-user polling mechanism for WLAN [12]. Their approach is mainly focused on
supporting QoS traffic in order to satisfy the QoS requirement under the DCF environment. However, if one uses the
emerging 802.11e EDCA, the QoS traffic can be supported
more effectively.
In high-speed environment, the effect of overheads mentioned Section 1 increases since these overheads are fixed irrespective of the transmission rate. According to [13], high
throughput performance can be achieved via frame aggregation, though the aggregation is performed at the device
driver level. However, only the frames with the same destination and the same kind of traffic can be aggregated in [13].
It is surely desired to apply the aggregation of frames with
different destination addresses, i.e., downlink frames transmitted from access point (AP) to stations. As it will be
mentioned in Section 3, the AP’s downlink access is very
important since its performance could be a bottleneck of the
entire network performance. In order to achieve highly efficient aggregate throughput, the aggregation of frames with
different destinations should be supported.
3. Baseline MAC Protocols
In this section, we briefly describe how 802.11e EDCA
works since the proposed MCCA works on top of EDCA1 .
The IEEE 802.11e EDCA is designed to provide differentiated, distributed channel accesses for frames with 8 different user priorities by enhancing the DCF [3].
Each frame from the higher layer arrives at the MAC
along with a specific priority value (ranging from 0 to 7).
Then, each QoS data frame carries its priority value in the
QoS Control field of the MAC frame header. An 802.11e
STA shall implement four channel access functions, where
a channel access function is an enhanced variant of the
DCF. Each frame arriving at the MAC with a user priority
is mapped into an access category (AC).
Basically, a channel access function uses AIFS2 [AC],
CWmin[AC], and CWmax[AC] instead of DIFS, CWmin,
and CWmax, of the DCF, respectively, for the contention
process to transmit a frame belong to access category AC.
AIFS[AC] is determined by
AIF S[AC] = SIF S + AIF SN [AC] · SlotT ime
where AIFSN[AC] is an integer greater than 1 for STAs and
an integer greater than 0 for access points (APs). The backoff counter is selected from [0, CW[AC]].
The values of AIFSN[AC], CWmin[AC], and CWmax[AC], which are referred to as the EDCA parameter
set, are advertised by the AP via Beacons and Probe Response frames. The AP can adapt these parameter dynamically depending on the network condition. Basically, the
smaller AIFSN[AC] and CWmin[AC], the shorter the channel access delay for the corresponding priority, and hence
the more capacity share for a given traffic condition. However, the collision probability increases when operating with
smaller CWmin[AC]. These parameters can be used in order to differentiate the channel access among different priority traffic.
IEEE 802.11e EDCA also provides a new channel access
method called Transmission Opportunity (TXOP), which
is an interval of time when a particular STA has the right
to initiate frame exchange sequences onto the wireless
medium. A TXOP is defined by a starting time and a maximum duration. The TXOP is either obtained by the STA by
successfully contending or is assigned by the AP.
It should be also noted that the AP can use the EDCA
parameter values different from the announced ones for
the same AC. The 802.11 DCF originally is designed to
provide a fair channel access to every station including
the AP. However, since typically there is more downlink
(i.e., AP-to-stations) traffic than uplink (i.e., stations-toAP), AP’s downlink access has been known to be the bottleneck to the entire network performance. Accordingly,
EDCA, which allows the differentiation between uplink and
downlink channel accesses, can be very useful to control the
network performance.
The traffic identifier (TID) of a frame is a label, which
specifies the corresponding QoS requirements. There are 16
possible TID values, where the values from 0 to 7 specify
the user priority value of a frame, and the values from 8
to 15 specify the traffic stream which the frame belongs to.
The TID value is specified in the QoS Control field (from
bit 0 to bit 3) of the 802.11e QoS data frame’s MAC header.
In Section 5.1, we use TID value in the proposed MCCA in
order to aggregate frames with different TIDs.
In order to improve the MAC efficiency, a new acknowledgment scheme, called Block Ack (BA), is defined in IEEE
802.11e. The scheme basically works as follows: during a
TXOP, a STA (or AP) can transmit a number of frames without receiving any Ack. After all the frame transmissions
within the TXOP are completed, the STA (or AP) sends a
control frame, called Block Ack Request (BAR). The receiving STA (or AP) of the BAR should respond with BA 3 .
1 We
assume the 802.11a PHY in this paper because the target of our
scheme is the next-generation high-speed WLAN such as TGn.
2 AIFS : Arbitration Interframe Space [3]
3 There are two types of Block Ack procedures in 802.11e, which are
Immediate and Delayed Block Ack. The former is explained here.
MAC Protocol Data Unit (MPDU)
F
MAC
Header MSDU C
S
DA1:TID1
F
MAC
Header MSDU C
S
DA1:TID1
F
MAC
Header MSDU C
S
DA1:TID2
DATA MPDU
M
D
M CHDATA - 1 M CHDATA - 2
D
D
MPDU
MPDU
PSDU
P
D
rate 1
basic rate
2
6
0 ~ 2304
4
Sequence
Control
Address
4
Frame
Body
FCS
DATA MPDU
Aggregate PHY Protocol Data Unit (PPDU)
P
Preamble/
PLCP Header D
6
Address
3
Figure 2. Type-1 compressed header data
Aggregate PHY Service Data Unit (PSDU)
M
D
2
Frame
Control
Octets:
F
MAC
Header MSDU C
S
DA2:TID1
PSDU
Octets:
2
6
2
6
2
0 ~ 2304
4
Frame
Control
Address
3
Sequence
Control
Address
4
QoS
Control
Frame
Body
FCS
rate 2
Figure 3. Type-2 compressed header data
Figure 1. Two-level frame aggregation
4. Underlying PHY Model
The purpose of this paper is to enhance aggregate
throughput and network channel utilization. As mentioned
in Section 1, the overheads such as PHY preamble/header,
MAC header, ACK, and backoff procedure should be reduced in order to achieve the goal. Moreover, we also consider faster PHY technology to achieve more than 100 Mbps
throughput gain at the MAC SAP which is one of the goals
of the 802.11 TGn. Accordingly, we assume Multiple Input Multiple Output (MIMO) technology as the underlying
PHY model. MIMO is one of the underlying PHY schemes
being considered for 802.11 TGn. However, MIMO is considered as the dominant technology of the next-generation
WLAN in the nearest time4 . In this paper, we assume that
the PHY model uses 2 × 2 MIMO, which can provide 2
times faster transmission rate than 802.11a PHY.
In addition, we assume that our PHY in consideration uses multiple channel bonding scheme. That is, two
802.11a channels (of 20 MHz) are combined together, and
the bonded channel of 40 MHz is used for the communications. Therefore, we can achieve 2 times faster rate due to
this bonding scheme. Accordingly, if we employ both 2 × 2
MIMO and two channel bonding schemes, the PHY transmission rate, which is 4 times faster than that of 802.11a
PHY (i.e., up to 216Mbps), can be achieved.
5. Multi-user polling Controlled Channel Access (MCCA)
In this paper, we propose two MAC schemes to enhance network utilization and maximize aggregate throughput, namely, two-level frame aggregation and multi-user
polling. The name of MCCA, i.e., Multi-user polling Con4 At the last 802.11 TGn meeting on Sep. 2004, while there are 36
proposals offered as how 802.11n will look eventually, all the proposals
use MIMO.
trolled Channel Access, is coined from the access manner
to the wireless channel of our scheme.
5.1. Two-Level Aggregation
As part of MCCA, we employ a two-level frame aggregation scheme, which is composed of two types of hierarchical aggregations, namely, aggregate PHY Service
Data Unit (PSDU) and aggregate PHY Protocol Data Unit
(PPDU) schemes. Proposed aggregation schemes are performed at both MAC and PHY layers, respectively, according to the characteristics of the aggregated frames, such as
the destination address and TID.
Fig. 1 illustrates how the proposed two-level aggregation
works. Multiple MPDUs addressed to the same STA are aggregated into an aggregate PSDU. Each aggregated MPDU
follows a MPDU Delimiter (MD), which plays a role to robustly delimit the MPDUs within the aggregate PSDU. An
MD is composed of several fields including Unique pattern,
CRC, and MPDU length.
The aggregate PSDU is for a single destination. The first
case of the aggregate PSDU scheme is as follows. If there
are buffered data MPDUs with the same destined address
and the same TID, they can be aggregated into a PSDU. The
first data MPDU has the legacy MAC header, and the subsequent MPDUs use the format of CHDATA-1, i.e., Type1 Compressed Header DATA, and the format of this new
frame is illustrated in Fig. 2. Because all the MPDUs are
separated by MD, the receiving high throughput STA (HT
STA)5 can recognize the existence of another MPDU after
the MD. The CHDATA-1 header should be rebuilt based on
the first MPDU’s MAC header by the receiver.
If there are buffered data MPDUs with the same destination address, but different TIDs, they can also be aggregated into a PSDU, and this type of aggregation is the
second case of the aggregate PSDU. The rule to make an
aggregate PSDU is the same as the first case mentioned
5 We refer a STA, which supports the MCCA proposed in this paper, to
as an HT STA.
above, except for the compressed header format, i.e., Type2 Compressed Header DATA (CHDATA-2) is used instead
of legacy MAC header for the subsequent MPDUs. The
format of CHDATA-2 header is shown in Fig. 3. In this
CHDATA-2 header, QoS field is needed since MSDUs contained in this MPDU have different TIDs. Similar to the
operation for receiver to rebuild the CHDATA-1 header, the
CHDATA-2 header should be rebuilt on the equivalent way
by receiver.
For the composite case of the above two cases, i.e.,
there are buffered data MPDUs with the same and different
TIDs, MPDUs which have either CHDATA-1 or CHDATA2 headers can be aggregated into a PSDU. In this case,
the first MPDU should have the legacy MAC header and
it should be referenced for rebuilding the MAC header of
CHDATA-1 and CHDATA-2 MPDUs. Fig. 1 is an example
of this case of aggregate PSDU. Apparently, with the header
compression scheme, we can use channel bandwidth more
efficiently and transmit more data during a fixed channel
access opportunity.
The second-level aggregation, referred to an aggregate
PPDU, can be applied if there are buffered PSDUs with
different destination addresses. With the aggregate PPDU
scheme, the buffered PSDUs can be aggregated into a
PPDU without additional preambles as shown in Fig. 1. If a
number of PSDUs are aggregated into a PPDU, PSDU Delimiters (PDs) with own unique patterns precede each aggregate PSDU, and this allows to transmit each PSDU at a
different rate according to the optimal rate of the destined
HT STA. A PD should be transmitted at the basic rate, i.e.,
6 Mbps of the 802.11a PHY, in order for all the HT STAs to
be able to decode it.
There is a limit of aggregate PPDU. According to the
802.11a, the PLCP in a PPDU has Length (of 12 bits) and
Rate (of 4 bits) fields in the legacy SIGNAL field. The
Length and Rate fields are virtually set in order to cover the
duration of the PPDU. The length of an aggregate PPDU
is limited so that its duration may not exceed 5.46 milliseconds, because the maximum duration of the PPDU in
802.11a is 5.46 millisecond, which can be derived from the
maximum length (4095 bytes) divided by the lowest rate (6
Mbps). In other words, an aggregate PPDU can spoof only
the duration up to 5.46 milliseconds.
Basically, we assume the infrastructure mode of the
IEEE 802.11 because a kind of controlled channel access
scheme is proposed in our scheme. Accordingly, there are
two different directions of data transmission, i.e., uplink and
downlink and the policies, which the proposed two-level
aggregation scheme is applied, are different depending on
the transmission direction. In an uplink phase (U/L), in
which uplink data is transmitted, an HT STA gaining access to the channel can transmit a PPDU containing an aggregate PSDU destined to HT AP. In this phase, HT STAs
octets : 2
Frame
Control
2
6
Dur/ID
BSSID
2
2
D/L
U/L
Count(n) Count(m)
5*n
4*m
4
D/L MAP
U/L MAP
FCS
Figure 4. Format of MP-frame
may transmit more than one aggregate PSDUs after SIFS
if the transmission is finished within a given TXOP limit.
However, the aggregate PPDU scheme cannot be used for
this phase because the destination of a HT STA is always
the AP. On the other hand, HT AP can utilize the aggregate
PPDU scheme as well as the aggregate PSDU for the downlink phase (D/L) when the controlled access scheme is used,
which will be explained in detail in the next subsection.
5.2. Multi-user Polling
As the second part of MCCA, we propose a controlled
channel access scheme, based on multi-user polling. Our
multi-user polling scheme, which we referred to as multipolling, has the advantages of high channel utilization.
One of the main features of our multi-user polling is that,
unlike PCF and HCCA, it can deal with downlink traffic
streams during the controlled access period. Moreover, a
modified hybrid coordinator (HC) 6 in our scheme can send
multiple polls at the same time. To support these new and
intelligent features, the format of the multiple poll frame,
namely, MP-frame, should be designed well. Fig. 4 shows
the format of MP-frame.
The MP-frame provides the control of flows for D/L and
U/L within a service period. It conveys a number of D/L
MAPs and the number of U/L MAPs, where the MAPs handle the aggregation exchanges during a service period.
The Dur/ID field is used to set a long NAV value to protect the service period as soon as the receipt of the MPframe at the legacy STAs associated to the HT AP. The D/L
Count field and U/L Count field, respectively, indicate the
number of PSDUs in D/L to be used by the HT AP and the
number of HT STAs to have a transmission opportunity in
U/L. They shall be referenced by D/L MAP and U/L MAP
fields which are successively repeated within the MP-frame.
Fig. 5 illustrates how the overall access mechanism of
multipolling works. Whenever an HT AP transmits an MPframe, it initiates a service period. The HT AP gains the access to the channel by transmitting an MP-frame after waiting for a PCF Interframe Space (PIFS) idle time interval,
which is shorter than DIFS and any AIFS. When the STA
receives the MP-frame, it sets the backoff counter to the appropriate value, that is implicitly assigned by AP in the MP6 HC
is the new notion in the IEEE 802.11e specification [3].
HT STA1
TXOP Limit
0
Preamble
PLCP
Preamble
PLCP
MBA MPDU
Preamble
PLCP
MBA MPDU
Aggregate
PSDU
Preamble
PLCP
MBA MPDU
Data MPDU
Data MPDU
MBAR MPDU
HT AP
HT STA2
2
1 0
Aggregate
PSDU
1
0
Preamble
PLCP
Data MPDU
MBAR MPDU
HT STA1
1
Preamble
PLCP
MBA MPDU
Data MPDU
Data MPDU
MBAR MPDU
0
HT STA2
HT STA3
TXOP Limit
4 3
PIFS
DIFS
SIFS
SIFS
DIFS
SIFS
SIFS
SIFS
PIFS
Figure 6. Example of MCCA Block Ack
Service Period
PIFS
DIFS
2 1 0 DATA
NAV
Non-polled
(HT) STA4
PIFS
CF-END Frame
Within TXOP limit
Preamble
PLCP
MBA MPDU
Data MPDU
MBAR MPDU
Aggregate
PPDU
Preamble
PLCP
Data MPDU
Data MPDU
MBAR MPDU
Data MPDU
Data MPDU
MBAR MPDU
HT AP
Preamble
PLCP
MP-Frame
MP
Within TXOP limit
CFEND
Aggregate Aggregate
PSDU
PSDU
Preamble
PLCP
MBA MPDU
collision-free CSMA/CA
dynamic TDM
DIFS
PIFS
AIFS[AC]
Figure 5. Multi-user polling operation
frame according to the polling order. It is also possible that
CF-End frame may be used when there is no more uplink
traffic from the polled HT STAs, which this functionality
is the same as that of PCF. During a service period, AIFS
is set to DIFS irrespective of the AC of the frames being
transmitted.
For the D/L, which starts immediately after the MPframe transmission with a PIFS deference, the aggregate
PPDU scheme is applied, and hence we referred to this
D/L as dynamic Time Division Multiplexing (TDM), because each HT STA receives a PSDU destined to itself at
its receiving time. After the end of the D/L, uplink transmission opportunity is handed over to the HT STA, whose
backoff counter becomes zero. In this U/L, each HT STA
follows EDCA rule except that its backoff number is assigned from HC in advance with AIFS value set to DIFS in
all occasions. Within a TXOP of each HT STA scheduled
via the U/L MAP, an HT STA can transmit multiple frames
including one or more aggregate PSDUs consecutively. If
an HT STA scheduled via the U/L MAP does not transmit
any frame, the next scheduled HT STA can transmit after
a backoff slot time according to the EDCA channel access
rule.
An HC shall know the decrease of the backoff counters
while the STAs transmit their data according to the U/L
Count field in the MP-frame. The backoff counter of the
last STA decreases from U/L Count value to zero eventually
at the end of the service period. If the uplink transmission
finishes and the service period remains enough to transmit
the CF-End frame, then the HC sends the CF-End frame to
inform both HT STAs and legacy STAs of the end of the
multipoll operation before the originally-scheduled end of
the service period.
In a controlled channel access scheme such as PCF and
MCCA, the whole network operation can be collapsed by
hidden stations. The hidden station problem has been a
inveterate obstacle in order for these centralized access
schemes to operate smoothly until now. The emerging IEEE
802.11k [5], however, defines hidden node report method
using a hidden station detection scheme, which basically
works as follows. First of all, if STA i detects a frame,
which the AP transmits to STA j via downlink, but STA i
cannot detect the corresponding Ack from STA j. If this
happens persistently,7 and STA i never receives any frame
transmitted by STA j, STA i can conclude that STA j is
hidden from itself. By using this mechanism, AP can manage the STA list, in which the STAs are not hidden each
other. Accordingly, if we use 802.11k hidden node report
scheme described above in our MCCA scheme, our centralized channel scheme is getting more robust against hidden
STAs. From now on, we assume that the proposed MCCA
use the hidden node report scheme basically.
5.3. Error Recovery via Block Ack
There can be transmission errors due to the unreliable
channel behavior caused by background noise, fading, and
so on. As we take into account an aggregation scheme as
a major part of the proposed scheme, an Ack mechanism
such as Block Ack, which is defined in IEEE 802.11e, is
needed. We propose a Block Ack mechanism which is appropriate in our MCCA scheme. We assume that the frame
formats of proposed BA and BAR are identical to those defined in 802.11e. However, the usage is different from the
802.11e’s. For example, the proposed BA is a delayed Ack
for a number of aggregated transmitted frames, not one for
a chain of frames, 802.11e Block Ack. Our BA and BAR
themselves can also be aggregated into an aggregate PPDU
frame or an aggregate PSDU frame. Therefore, we use the
new term of MCCA Block Ack (MBA) and MCCA Block Ack
Request (MBAR) to denote them in this paper.
Fig. 6 shows an example of error recovery by the proposed MBA. After an MP-frame transmission and PIFS
time interval, the HT AP transmits aggregated PSDUs destined to multiple HT STAs including an MBAR to each HT
STA. By uplink transmission rule mentioned above, the first
7 Note
that this can happen occasionally due to the channel error.
EDCA
Internet streaming
(2Mbps)
Video conferencing
(1Mbps)
Throughput (Mbps)
Table 1. Application Characteristics
Delay Bound Offered Load
Application
(msec)
(Mbps)
VoIP
30 msec
0.096
Video conferencing
100 msec
1
A/V streaming
200 msec
2
Internet file transfer
N/A
N/A
Local file transfer
N/A
N/A
1
VoIP (0.096Mbps)
0.1
HT STA transmits its own aggregated MPDUs to the HT
AP including an MBA MPDUs for the aggregated MPDUs
from HT AP and an MBAR MPDU. As shown an example
of Fig. 6, if some of MPDUs are not received successfully,
the HT STA informs the HT AP of these errors using the
Block Ack Bitmap. If there are any error frames as shown
in Fig. 6, the HT AP shall retransmit the frame with the corresponding MBAR. There is another example of the transmission error recovery procedure at the right side of Fig. 6.
If an HT STA obtaining a channel access opportunity transmits aggregated MPDUs and some errors occur, the HT AP
shall inform the HT STA of the erroneous MPDUs by using
Block Ack bitmap like the first case error recovery operation as mentioned above.
0
10
20
30
Flow Number
40
50
60
Figure 7. Throughput in Large Enterprise
model
In this section, we comparatively evaluate the performance of the IEEE 802.11e EDCA and our MCCA using
the ns-2 simulator [14].
which satisfy their corresponding delay bound, are counted
to the aggregate throughput calculation.
Basically, we assume that all the STAs are fully controlled by the HC in service periods, and hence that there
is not any collision which can cause the transmission error.
As mentioned in Section 5.3, however, other error sources
such as RF interference, channel fading, and so on can exist.
To take into account these erroneous conditions, we apply
strict error model in which a system can suffer from errors
per MPDU-basis at a randomized and uniformly distributed
rate.
The EDCA parameter set used for each traffic type are
the default values defined in [3].
6.1. Simulation Environments
6.2. Simulation Results and Discussion
For the simulation scenario, we use one of the models,
called an Large Enterprise scenario, defined by TGn [7].
Three different types of traffic are considered for our simulation, namely, voice, video, and data. The voice traffic
is modeled by a two-way constant bit rate (CBR) session
according to G.711 codec. The video traffic is modeled by
three kinds of applications, including video conferencing
and the A/V Internet streaming. The data traffic is modeled
by a unidirectional FTP/TCP flow to represent a file transfer. These traffic models are described in Table 1 in detail.
In the simulation, we use the 802.11a parameters except
the transmission rate. We assume that all the stations use the
transmission rate of 216 Mbps, which can be achieved by
MIMO-aware PHY techniques described in Section 4. With
the Large Enterprise scenario, we compare the throughput performances of (1) EDCA, (2) EDCA with aggregate
PSDU, and (3) MCCA. When we consider different types
of QoS traffic including voice and video, only the packets
Fig. 7 presents the throughput performance of the EDCA
scheme. The first 30 flows are downlink flows and the other
30 flows are uplink flows. Each station has one downlink
and one uplink flow, respectively, e.g., flow i and flow 30+i
are for station i. A flow with zero throughput represents that
there is no active flow. There are three kinds of QoS flows,
which are indicated in Fig. 7. The remaining flows are nonQoS flows. In order to show the low throughput values more
clearly, e.g., those of VoIP traffic, we use the logarithmic
scale for the y-axis. From the simulation results, all of the
QoS traffic flows are found to satisfy their QoS requirement
with default EDCA parameters.
Fig. 8 shows the aggregate throughput performance of all
three schemes. The proposed MCCA can achieve very high
throughput performance. Approximately, 7.5-time larger
aggregate throughput gain can be achieved by MCCA compared to EDCA. This result is even higher than that of aggregate PSDU with 2.2-time gain, approximately.
6. Performance Evaluations
140
EDCA
Aggregate PSDU
MCCA
EDCA
Aggregate PSDU
MCCA
129.9
delay bound : 30
100
End-to-end Delay (msec)
Aggregate Throughput (Mbps)
delay bound : 200
delay bound : 100
100
120
80
59.3
60
10
1
40
17.4
20
9.2
0
0.1
QoS traffics
non-QoS traffics
VoIP
Figure 8. Aggregate throughput performance
Downlink AC_BE queue size (packet)
120
100
80
60
40
20
0
AC_BE
17
17.1
17.2
17.3
17.4
17.5
Time (sec)
Figure 9. Downlink EDCA queue status with
aggregate PSDU scheme
To evaluate the performance of our new MAC scheme,
first of all, we compare aggregate PSDU scheme to EDCA,
with the same simulation model as shown in Fig. 7. We
apply the aggregate PSDU scheme to all stations including AP, and hence it improves the aggregate throughput of
non-QoS traffic by 3.4 times, approximately, while throughput of all QoS traffic still remain the same as those of the
EDCA. However, if the characteristic of non-QoS traffic,
i.e., TCP flows, is considered, the more improvement can be
achievable. It is because the TCP flow has the burst characteristics. It means that more TCP Ack packets are received
by the TCP sender, and hence more packets will be sent
up to the TCP receive window size. Therefore, we check
the status of EDCA queues at the AP to determine whether
these EDCA downlink queues are bottleneck or not, in the
simulation of the aggregate PSDU scheme.
Fig. 9 shows the status of the downlink EDCA queues
Video Conferencing
Streaming A/V
Figure 10. Comparison of end-to-end delay
corresponding to AC BE. Approximately, 120 packets are
buffered at the AC BE queue almost always. We use TCP
new Reno with Delayed Ack option with TCP receive window size of 6. Since there are 16 downlink and 10 uplink
TCP flows, the maximum number of packets, which can be
buffered at the AC BE queue, is 126 packets. This means
that the downlink AC BE queue is a bottleneck in our simulation scenario when the aggregate PSDU MAC scheme is
used. Accordingly, if those downlink packets (AC BE) can
be served faster, the aggregate throughput will be increased.
This is the reason why we could achieve a very high
throughput performance with MCCA as shown in Fig. 8.
Even though we get higher throughput gain with MCCA
than EDCA scheme or EDCA with aggregate PSDU
scheme, we lose nothing. In Fig. 8, this can be confirmed by
observing the aggregate throughput of QoS traffic. Fig. 10
also shows that the MCCA does not sacrifice the delay performance of QoS traffic either. As shown in Fig. 10, which
has a log-scale y-axis, there are still enough margins to each
delay bound for all three schemes.
Until now, we have simulated in the environments, where
the channel errors do not occur, thus not utilizing the proposed error recovery mechanism described in Section 5.3.
Fig. 11 shows the effect of the error recovery mechanism
when the frame (MPDU) error rate varies. In this figure,
with a log-scale y-axis, the performance of MCCA nonQoS traffic is observed lower than that in Fig. 8 due to additional frame overheads such as MBA and MBAR frames.
For this simulation, we assume that the sender retransmits
a frame at most once. In other words, even if a frame suffers consecutive errors, it is retransmitted just once. We first
observe that both MCCA and EDCA sustain the throughput
for QoS traffic irrespective of the frame error rate. On the
other hand, the throughput for non-QoS traffic decreases as
the frame error rate increases for both systems. However,
References
Throughput (Mbps)
100
10
MCCA : non-QoS
MCCA : QoS
EDCA : non-QoS
EDCA : QoS
1
0
2
4
6
Frame error rate (%)
8
10
Figure 11. Error recovered performance
non-QoS traffic of MCCA still preserves 72% of the overall
throughput even if frame errors occur with the rate of 10%,
As a result, we can conclude that MCCA with the error recovery mechanism still has a great throughput gain compared to EDCA even in unreliable channel environments.
7. Conclusion
In this paper, we propose a new high-efficient MAC
scheme, called MCCA, for the next-generation high-speed
WLANs. The MCCA is based on the EDCA-based multiuser polling (multipolling) and a two-level frame aggregation scheme performing aggregation at both MAC and
PHY layers. Therefore, the proposed scheme can aggregate
frames with different QoS requirements and different destinations. The aggregate PPDU scheme, however, has the
nature that this aggregated frame should be well scheduled
in order for the receiving HT STAs to recognize easily and
correctly. Therefore, we have introduced our multipolling
for the purpose of scheduling the aggregate PPDU frames.
Moreover, by using the multipolling, we also achieve very
high efficiency of the channel utilization thanks to using a
minimum size of backoff counter value, which is uniquely
assigned by U/L MAPs of HT AP.
With simulation runs, we show that the MCCA provides
very high throughput performance while the delay performance remains acceptable. The achieved performance enhancement is approximately 646 % in terms of the aggregate throughput of non-QoS traffic when the MCCA is employed while the aggregate throughput of QoS traffic is not
sacrificed. In the future, we plan to evaluate the proposed
scheme further. Those include the consideration of more
realistic channel environments such as multipath fading and
multiple transmission rates depending on the channel condition of each STA.
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