E-MAC: Self-Organizing 802.11-Compatible MAC with Elastic Real-time
Scheduling
Imad Aad, Philipp Hofmann, Luis Loyola, Farhan Riaz, Jorg Widmer
DoCoMo Euro-Labs
lastnamegdocomolab-euro.com
Abstract
To overcome the lack of strict QoS guarantees in existing hotspots, in this paper we present a system for realtime traffic support in 802.11 networks that works in either infrastructure or ad-hoc mode. The proposed mechanism, called elastic MAC (E-MAC) protocol, helps stations
with real-time traffic to organize and establish a transmission schedule in a distributed manner, while coexisting with
standard 802.11 stations. This distributed scheduling guarantees very short delays and a minimum reserved data rate
for real-time stations, while protecting best-effort 802.11
traffic from starvation. Another feature of our mechanism
is time-slot reuse, which improves the network efficiency by
allowing other real-time stations to take over unused slots
(e.g., in case of using voice codecs with silence suppression). We evaluate the performance of our system using a
testbed implementation, ns-2 simulations and a mathematical model, and show how it outperforms other QoS schemes
(e.g., 802. le) in terms of throughput, delay, andjitter.
1. Introduction
network
m o
e sed,manyof
Thecatmore
peopll
used wirelesstem
themrequirinq
pliction
wil
be
rquirng qalit
of
service (QoS) guarantees (e.g., VoIP / 802.11 / dual mode
cellular phones). However, the more the wireless channel
gets occupied, the tougher it becomes to provide those guarantees. Throughout the literature several approaches to provide QoS support can be found. They can be split into two
statistical
guarantees and strict guarantees.
types:
Statistical guarantees are usually based on contention
among competing stations to access the channel, such as
in 802.1 le [7]. High priority traffic/stations are assigned
A TF .backhi,-riorit chne
(ep AIFS,
.k
high-priority
channel *cesaccess naaeeparameters (e.g.,
off to gra th chne m re ofe tha th lo-pioit
traffic/stations. On the one hand theses approaches are rela____________________
1-4244-1455-5/07/$25.OO ®) 2007 IEEE
tively simple, making them easy to implement, deploy, and
manage, therefore boosting their success in the market. On
the other hand the guarantees they provide are statistical,
still causing some problems for certain QoS requirements.
Strict guarantees are based on reservations (e.g., 802.11
infrastructure PCF mode [6], TDMA). The reservations can
be easily tailored to the needs of particular QoS applications. However they are very complex to implement and
manage, which has hindered their deployment.
Besides the drawback of implementation complexity,
strict guarantees rely on "medium reservations" that are often done on a periodic basis: a given station reserves the
channel for transmitting (P bytes) every T seconds. However this periodicity does not match the real-time traffic pattern from the data sources. Consider for instance voice traffic, where the communication channel is typically idle 1/3
of the time. Several voice codecs, e.g., the ITU-T G.711
,u-Law codec [8], tend to optimize bandwidth usage by applying "silence suppression", leaving several reserved timeslots empty. One may try to adapt the period of the slot
reservation to the voice codec pattern to optimize bandwidth
efficiency, or to reduce delays but never both at a time, i.e.
there is a compromise between reducing the delay, and increasing the efficiency. To reduce the packet delay, short
slot intervals must be used but for an over-provisioned reservation most of the reserved slots will be empty, therefore
drastically
reducing the efficiency, i.e.,ofthe ratio of the used
stsaover the er timeslts
Hvin the above problems imi wdiaes
ticM (b-MC protocol to p ind,es Qos gantees
to real-time traffic, with "flexible" or "elastic" reservations
to perform empty-slot reuse. An alternative option would
in 802.1 1 infrastructure
point
~ coor
~ ~ ~to~ ~ optimize
~be tion thetioscheduling
(cF) mode. T
t acce point
i(AP)
polls every staion before transmssion,
c
polls the next
t transmission, oportunnxty
station in the list if one skips itS transmission opportunity.
The rest of the "period", based on contention rather than on
polig is' ofee fo etefr rfic oee,n rd
~~~~~uct that uses the 802.11 infrastructure PCF mode can be
found in the market, due to its implementation complexity.
station
ieli
e
To fill this gap and offer good QoS guarantees for real-time
traffic we design our protocol that meets the QoS requirements for strict guarantees and empty-slot reuse, in both infrastructure and ad hoc mode. We implement it in a real
testbed, while keeping it compatible with existing 802.11
networks and already deployed hotspots.
The paper is organized as follows. In Section 2 we
present the design goals and the system model in detail.
In Section 3 we present the performance evaluation testbed
setup, the mathematical model and the simulation setup.
The performance results are shown in Section 4. We discuss
in Section
reld
various issues and future work in Section 55 and the related
work in Section 6. We conclude the paper in Section 7.
vaeperiousrisuande fsultureworkn
the same frame get BE priority (to contend fairly with
BE traffic). Such packets are denoted as best-effort
real-time (BE-RT).
* To deal with frame and schedule synchronization
among nodes, without requiring any changes at the AP,
a"maestro"stationtransmitsperiodicreservedaccess
markers (RAM).
Toke -A akadcmail,i sdsge
such that there are absolutely no requirements to change the
802. 11 stations that coexist with the real-time stations.
andWthe
2.1. Self-organizing Setup Phase
2. System Description
Before a real-time station starts transmitting real-time
packets, it overhears the channel for a given duration of
time. If it does not receive a RAM during this time, it is
the first station starting a real-time session. We refer to this
station as the maestro station for the remainder of this paper.
We consider a network with n stations among which nhrt
have real-time (RT) traffic to transmit, using our protocol,
and nhbe= n - nrt are legacy 802.11 stations with besteffort (BE) traffic. All n stations are in the same collision
domain, contending for access to the channel. Without loss
of generality, we consider that all transmit to a common destination, e.g., the AP of a hotspot. However, one can also
use the protocol without changes in an ad-hoc, single collision domain network.
The basic components of E-MAC (Fig. 1) are the following, required from real-time stations only (e.g., A, B, C and
D in Fig. 1):
A
B
C
0D
BE/BE-RT
A
B
2.1.1. Maestro Station
The maestro has to broadcast a RAM packet every T to indicate the start of a new round for all RT stations in a synchronous way. The RAM is sent with high priority, i.e.,
after SIFS Short Inter Frame Space and a backoff of one
IEEE 802.1 l time slot tslot (SIFS = I Os and tslot = 20,s
for 802.1 lb). The maestro continously overhears the chan-
Time
1 Round (RAM interval)
Figure 1. Conceptual model
* To get guaranteed access priority, real-time stations
must establish a transmission sequence among themselves in a distributed manner.
.
To avoid colliding with BE (802.11) stations, real-time
tn
stations transmit
stastationsbibob
the channel by having higher access priority.
beforeranBe(2
* To reuse empty reserved slots (e.g., when using voice
codecswith
silence
suppression), when a real-time sta-~
codecs with silence
stasuppression),
tion skips a reserved transmission slot (B in Fig. 1), its
successor(s) take over the slot, making more space for
BE stations.
* To preserve fairness among RT stations and to protect BE traffic from starvation, a RT station uses only
one high priority slot in a given interval (that we call
"Round"). Additional RT packets from that station in
nel to notice when new real-time stations join. Packets of
real-time stations are identified by the IP ToS (type of service) field. Hence, the maestro can maintain a table of all
active real-time stations, including their MAC address, sequence number i (explained in Subsection 2.1.2) and transmission time required for their real-time packets. Additionally, the time of the last real-time packet transmission of the
respective station is stored. Note that other stations should
also maintain such a table in case they become the maestro (see Section 2.4). The maestro broadcasts information
about the total number of real-time stations nrt and the total time ttot required for transmitting all real-time packets
If all transmission
per
roundpackets,
thetotal
RAM.
real-time stations
throughthe
time transis (cf.
mit their
required
Fig. 2):
~~~~~~~~~~~~~
\ DATA
RAM
ACK ||SIFS
|DATA, | DATA, |
g
tS,o (deterministic backoff) W t5,o (random backoff)
DATADIES
BE
Figure 2. Basic system model
___________
1This can be added to the AP's beacons if the AP is made E-MAC
compliant.
2.2. Slot Reuse, Backward Compatibility
ttot
(2nrt + 1) SIFS + (nrt + 1) tSlot +
nrt tack + tram +
tdata,i
tk t
Once a real-time station has (sent or) received a RAM,
(1)
The transmission duration of RAM (tram), data packets
(tdata,i), and ACK (tack) depends on the current channel
rate and thus may vary. How to cope with variable data
transmission times, different data rate requirements and mobility is discussed in Section 5.
To maintain compatibility with conventional 802.11 devices, the RAM is a normal 802.11 data frame with specific
information in the payload. It is not acknowledged by other
stations as it is transmitted using MAC layer broadcast.
Conventional stations are not aware of the notion of the
rounds and RAMs and may not finish their packet transmission before the end of a round, thus overlapping the next one
by time period of AT. In this case, the maestro would send
the RAM with a delay AT and schedule the next RAM after
duration T - AT (shortening the BE traffic period) to compensate for this delay, keeping the average round duration
T constant. This however means that the complete round
T can not be used exclusively for real-time transmissions.
Therefore, a fraction tguard must be kept unreserved to allow for those overlaps and the reduction of time available
for RT traffic in the round. In the worst case, a best-effort
station starts sending a data packet shortly before the end
of the round. Hence, tguard should be set appropriately depending on the channel rate, maximum packet size, and T.
2.1.2. Sequence Establishment and Admission Control
If a real-time station wants to join and is not the maestro,
it first has to check whether sufficient transmission time
in T is available to accommodate its real-time packets. If
ttot +tguard +2 SIFS+tslot +tack+tdata < T, the real-time
station may join. Otherwise, it has to refrain from transmit-
ting real-time packets, contending instead using BE priority.
Thmetr
assqune ube
Time saetiojin obtainsi
1,me2,...)nbyosimplyningobtain
n
ne
se.
quene
ownetotheq
nwrel
nubewre
number t'
t
= I1,2, .... .) by simply adding one to the total
number
(i
of real-time stations nrt previously advertised by the maestro in the RAM. A real-time station with sequence number
selects a non-random backoff tim tback,i ( - 1)X
(i
f
electsrealnon-timerackt bachootin
scribedtwayresultsmeinpac
thbackoff In theot
d
=
transmsin aonfseenche,aode-
scribed way results in a given
given transmission sequence, avoid' is. a
ing collisions among real-time stations. However, there
small probability that two stations join at the same time and
hence select the same backoff. This would though result in
a collision (detected by absence of an ACK). To resolve this
conflict, the two colliding stations wait for a duration r T
(r being a random integer number, e.g., between 1 and 10)
before trying to join again.
it is allowed to transmit one real-time packet. If a real-time
packet is already waiting in the buffer upon receiving the
RAM, the real-time station starts decrementing its backoff
after the channel gets idle for AIFS (= SIFS + tslot) time.
If another real-time station is transmitting, the backoff is
frozen until the channel becomes idle again. Hence, if all
real-time stations have a packet in their buffer upon receiving the RAM, any two consecutive real-time packets are being transmitted with an idle time of AIFS between them.
This case is illustrated in Fig. 2.
In case a real-time station has no packet ready upon receiving the RAM, it skips its turn. The subsequent station in the sequence will then transmit next with an idle
time of AIFS+tslot after the previous transmission. Generally, if k consecutive stations refrain from transmitting a
RT packet after the RAM, the idle time between two packets becomes AIFS+k tslot. This idle time might be longer
than the DIFS of legacy 802.11 stations for large k (or nrt)
resulting in possible collisions between RT and BE stations.
To avoid this, all real-time stations except the one with the
highest sequence number set the duration field in their packets to 2 SIFS + tack + (nrt + 1) tsiot, which in turn sets
the Network Allocation Vector, NAV, of legacy 802.11 stations. The last RT station in the sequence announces a duration (for the NAV of legacy 802.11 stations) of SIFS+tack.
Hence, best-effort stations will refrain from transmitting until all real-time stations with buffered packets have transmitted. Also, the duration field needs to be set in the RAM to
SIFS+(nrt + 1) tslot. The duration field is ignored by realtime stations.
among RT
Stations
Stations may receive real-time packets from the application layer at any time during a "round". The first packet of a
given station is assigned high priority, as described before.
However
if a second packet of the same station arrives dur.
i~~~~~~~~~~~~~~~ng
i the same round, it iS either queued until the next round
or contends as BE (as illustrated in Figure 3). We refer to
such "degraded" packets as BE-RT packets in the rest of the
paper. Upon hearing a new RAM, a station with a waiting BE-RT packet "promotes" it back to high priority to be
trnmteinheRprodPooigBETpakshs
in the RT period. Promotingg BE-RT packets has
~transmitted
the advantage of:
* Avoiding that packets of the same flow get reordered
at the MAC layer.
* Not delaying RT packets in the queue waiting for a
BE-RT packet to get transmitted.
RAM
U
DATA,
|
DATA,
DIFS[A|g::
|
BE
T
DATA,
DIFS
BE
CWmax
AIESCWmin(DIES)
/IZ
B
Figure 3. Real-time transmission during besteffort phase
|Traf. type
Station ID 1 2 3 xx 1 2 3 xx 1 2 3 xx
"E-MAC"
802.1 1 e
For simplicity we assume that RT stations are allowed
to transmit only one RT packet per round. Extra RT packets are "degraded" to BE-RT priority to compete equally
with 802.11 stations. The general case of different stations
with different RT traffic requirements is discussed in Section 5. Stations can have (unrestricted) BE traffic sources in
addition to RT traffic sources, in which case the two traffic types should be managed separately by two different
queues. However, this scenario is out of scope of the paper.
2.4. Releasing Reservation
If a real-time station has finished its session, the previously reserved resources must be released. If a station has
not transmitted a real-time packet for a duration I T (I being
a predefined integer number valid for all stations, e.g., 100),
the maestro supposes that it has finished its real-time session. The maestro then informs the other real-time stations
about this fact in the next RAM together with the sequence
number of the station that has left. Then, all real-time stations with a higher sequence number can decrement their
non-random backoff by one.
If a station has not transmitted a real-time packet for
more than I T although it has not finished its real-time session yet, it has to re-join as described in Section 2.1 before transmitting the next real-time packet. Alternatively,
it might send keep-alive messages (real-time packets without payload) with an interval of (I - 1) T during its inactive
period.
In case the maestro wants to finish its real-time session,
it adds the number of remaining RAMs it will still broadcast to the last j (e.g., 10) RAMs. Thus, other real-time
stations know when they have to decrement their sequence
number. Furthermore, the real-time station with sequence
number two then knows when it has to take over the role
of the maestro. In the event of sudden maestro disconnection (e.g. mobility), one can easily adapt the scheme using
timeouts. Due to space constraints, the analysis of such scenarios was kept out of scope of this paper.
3. Performance evaluation
We evaluate the performance of our protocol by means
of testbed implementation, mathematical analysis and ns-2
Real-time BE Real-time BE Real-time BE
802.1 1 g
1 3 5
0 0 0
-3
2
2
0 0 0
15
7
7
128
|
1023
Figure 4. AIFS/DIFS, CWmin and CWmax values used for the performance comparison
simulations.
Since the number of stations in our testbed is limited (6
stations + 1 AP), we set the channel capacity to 2 Mbit/s
in order to saturate it with a reasonably low number of
packets per round. The various inter-frame parameters used
for comparing 802.11, 802.1 le, and our protocol (denoted
"elastic") are shown in Fig. 4. No RTS/CTS is used in any
of the settings.
In regard to the packet delay computation, the buffer size
B used is 50 packets throughout the evaluations.
3.1. Mathematical Analysis
As the real-time stations are also allowed to contend during the best-effort period, the analysis focuses on two periods: a) a real-time period where nrt real-time stations transmit in a TDMA-like way, and b) a best-effort period where
all the nrt + nbe stations contend for the channel access (including nrt BE-RT stations). The analysis focuses on the
behavior of the network under saturated conditions, i.e., at
any time both real-time and non-real-time stations have at
least one packet in their transmission buffer.
3.1.1. Throughput Analysis
During the reserved real-time period all real-time stations
transmit their packets without any contention in a TDMAlike way. Under the assumption of saturated conditions
the real-time stations have always a packet to transmit so
they also participate in the best-effort period. Hence, all
(nrt + nbe) stations participate in the contention during the
best-effort period. After the end of the current time frame,
any BE-RT packet which could not be sent during the besteffort period is promoted to real-time priority again and
transmitted during its corresponding time slot in the real-
time period.
During the real-time period, as all real-time stations are
always transmitting there are no time-slot takeovers and
thus the separation between the end point and the start point
of any two consecutive data packets is AIFS + t,i0t. Assuming that acknowledgments are transmitted after realtime packets, and that the packet size Pt and the data rate
Rrt have the same value for all real-time stations, ttot can
be calculated as in Eq. (1). As the time frame has a fixed
length T the length of the best-effort period is T - ttot.
During the best-effort period, like in previous work
[2],[4] it has been assumed that the probability of a packet
collision p is constant and does not depend on the number
of previous transmissions. Our analysis follows the work
based on mean values carried out by Lin et al. [9]. During
the best-effort period, the number of transmissions experienced by each packet follows a geometric distribution with
probability of success (1 - p). As the contention window
doubles in size after every retransmission, the average contention window size W for the nrt + nb, stations is given
by:
W
=
(1
p) CWmtin + p (l
+p2 (1 -p) 22 CWmin +
= CWmIlin(1-P)
(1
(2)
(1
(I~
2p)
-p) (1 (2p)m±l) CWmin
=1I
-
(I
.
(3)
(3)
T)nrt+nbe-1
(4)
From (4), T can be also expressed as:
T
±SIFS±tack±DIFS±tslot (nrt+nbe+1)
The average number of packets from best-effort stations
thatfitintothecontention-basedperiodis:
Fbe
T );
R( 72rt±(2nrt±1) SIFS±()rt±1) tslotthrt
tack±tRAM )(7
W
pb(,t+nbe+1)
±SIFS±tac,k±DIFS±tslot
Rrb
Hence, the average throughput for real-time stations
reaches:
1
1-(1-p)1/(nrt+nbe-1)
Prt
Srt
(5)
Using numerical methods, the probability of collision p
can be calculated from (3) and (5), and hence the average
contention window W can be obtained from 2. In average, the separation between the end point and the start point
of two consecutive packets during the best-effort period is
given by SIFS + tack + DIFS + tslot W/ (nrt + nbe + 1)
where tack is the time length of an ACK frame. Thus, if the
packet size and channel data rate of all best-effort stations
are denoted as Pbe and Rbe, respectively, the average number of packets transmitted by real-time stations Frt that can
FrtPrt
T +
b
T(nrt+Tnbe)
The average throughput for non-real-time stations is:
T
Sbe
The probability that a packet transmitted by a station during the contention-based period collides is equivalent to the
probability that at least one of the other stations transmits in
the same idle slot and hence it is given by:
P
( R rt ±(2mrt±1) SIFS±(nrt±1) tslotthrt tack±tRAMl()
+ pm (1 -p) 2m CWmin
In (2) the parameter m is the maximum number of allowed retransmissions. Consequently, the probability of
transmission during the idle time slots of the best-effort period can be calculated as:
W
Frt
p) 2CWm,,in
...
(1 - (2p)m±)
12
=
1 I=
fit into a single best-effort period is:
T(f +be
(8)
(9)
)
3.1.2. Delay Analysis
Since in saturated conditions buffers of both real-time and
best-effort stations are full, the average delay for besteffort packets can be calculated based on the average intertransmission period between two consecutive packets from
the same best-effort station and the buffer size as:
Delaybe
=
B X
1/(Sbe/Pbe)
B
T (rt + be)
Fbe
(10)
Similarly, the average time elapsed between two consecutive BE-RT packets transmitted by the same station can be
approximated as:
Delayrt
=
B x 1/(Srt/Prt)
=
B T ((rt + nbe)
nrt + nbe + Frt
As can be seen in (10) and (11), two consecutive
best-effort packets from a given station have an intertransmission period generally greater than the round length
T while in case of real-time packets the inter-transmission
period is guaranteed to be less than T. The latter feature
represents a great advantage for real-time applications since
other statistical QoS-guarantee mechanisms as 802.1 le cannot guarantee delay for real-time packets, especially under
heavy traffic load.
3.2. Testbed Implementation
For the testbed implementation we use 7 Linux laptops
with Proxim wireless network cards that use the madwifi
driver. One of the laptops is used in AP mode as a sink
to the CBR data flows from the other 6 laptops. We use
Multi-generator (MGEN) to generate the traffic. For the
delay analysis we synchronize all the machines using the
Network Time Protocol (NTP). Since the wireless channel
is usually overloaded (beyond its capacity), we put the NTP
control traffic on cable connections connecting all the stations. Synchronization is made every 1 s, and the resulting
time drift is less than 1 ms. Values are averaged over 180second-long tests.
sim-Elastic-BE
800
700
sim-Elastic-RT
sidm-802.11e-RT---
600
testbed-Elastic-RT
testbed-802.1 le-BE
400
testbed-802. 1 e-RT
3
200
C
100
00
3.3. Simulation Setup
----------
5
15
10
20
Number of BE stations
To evaluate the scalability of our protocol we perform ns2 simulations with the default network configuration cited
above. Particular ns-2 and network parameters are as follows:
Figure 5. Throughput (per station) with variable number of BE stations
* Stations have a transmission range of 250 m
* The network area size is 200 m x 200 m, therefore all
nodes are within receive range of each other
* The channel model is two-ray-ground
* The number of RT and BE stations varies according to
the scenario in consideration
of BE stations increases. Using E-MAC however, RT stations always get 200 Kbit/s throughput independent of the
number of BE stations. Testbed and simulations yield qualitatively similar results showing how E-MAC outperforms
802.1 le in providing strict guarantees.
600
* All values are averaged over 400-second simulation
runs
* We consider a warmup period for the first 70 s, with
low traffic load, to make sure all nodes have the ARP
entry of the AP. The final measurements do not include
this warmup phase.
* No routing protocol is used (single hop to the AP)
Elastic BE
stme80211dERTiE
~~~~~~~~~~~~~~~~~~testbed-Elastic-RT
Sim
:802.11-BE
e-BE
\0~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Ssim-802.11
s~~~~~~~~~~~~~~~im-802.1
1e-RT
\
te
\;,
testbed-802.11-RT
400
400
testbed-802.11e BE
300
200
E
V X
-
E
---------
4. Performance Results
The performance comparison of 802. 11, 802. 1 1e, and
elastic MIAC is performed with three RT stations and a variable number of BE stations or vice versa. RT stations send a
packet with 250-byte payload every 10 ms (i.e., 200 Kbit/s)
and BE stations send a 1400 bytes packet every 5.5 ms (i.e.,
2 Mbit/s). The real-time packet generation interval is set to
10 ms. The evaluation of the slot reuse is performed with
different parameters (see Subsection 4.2).
4.1. Throughput, Delay, and Jitter
The average throughput with respect to the number of
BE stations is illustrated in Fig. 5. IEEE 802.1 le can provide a throughput of 200 Kbit/s for each RT station only if
the number of BE stations is one. However, real-time stations do not get their requested data rate when the number
able n um ber of RT stations
The average throughput with respect to the number ofRT
stations is illustrated in Fig. 6. It shows how E-MAC yields
a higher throughput than 802.1 le as the number of RT stations increases. This throughput is guaranteed to remain
fixed unless some RT stations do not adhere to the admission control, therefore going beyond the system capacity of
5 RT stations in the example (we show this to the sake of
understanding).
The average packet delay with respect to the number of
BE stations is illustrated in Fig. 7. Using B-MAC, the simulated average packet delay of BE stations increases to 10
s when increasing the number of BE stations to 10. In contrast, the average packet delay of RT stations is always be-
10
0.35
-0.3
0
sim-Elastic-BE
sim-Elastic-RT
.3testbed-Elastic-BE
testbed-Elastic-RT
J
n.
0.1
a)
0.25
testbed-802.1 1e-BE
-------- --- - -------- -testbed-802.1 le-RT
X 0.2
testbed-802.11-BE
testbed-802.1 1-R
0.15
sim-Elastic-BE
sim-Elastic-RT
simn82-11 e-BE
0.01
0.001
sim-802.1 1 e-RT
testbed-Elastic-BE
testbed-Elastic-RT
testbed-802.1 1 e-BE
l testbed-802.1
lllllll°--I1 e-RT
3
4
5
6
7
8
9
Number of BE stations
0.1
I-
I
I
1
2
10
Figure 7. Packet delay with variable number
of BE stations
low 30 ms. Furthermore, the experimental results show that
802.1le is not able to guarantee low transmission delays for
RT stations as opposed to our approach. With three BE stahe vergeackt
dlayof
T stations
satins iss about
aout600
thetios,
of RT
600
tions,
average
packet delay
ms with 802.1 le compared to 5 ms with B-MAC. The guarantee of such low delays for real-time packets is a key feaBMACthat
tothe
estof
or knowledge,
kowlege,no
ture oftureof
E-MAC
the best
of our
no oher
other
that, to
sceecmail'ihIEE821 rvds h
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smlcrease cofpathbledelay forErEal-t
packidetsin g. 7e
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collasnons
BE
oftefis
stationsaicreases.
Unfortunapelodu the limited
du
t
number of stations available it was not possible to obtain
more testbed results for this parameter.
According to [12], we calculate the jitter after reception
of packet i as
J
J(i - 1) + ( D(i -1, i) -J(i-1))
16
(
where D(i - 1, i) is the difference of the transmission
delay of two successively received packets. The average jitter depending on the number of BE stations is illustrated in
Fig. 8. The simulatedjitter of BE stations goes up to 350 ms
when increasing their number to ten. RT stations using EMAC on the other hand only experience a very low jitter of
less than 10 ms independent of the number of BE stations in
both, simulations and experiments. The very low jitter provided by B-MAC represents another great advantage when
compared to 802.1 le, which performs poorly in terms of
jitter as the number of BE stations increases, as shown in
Fig.8.
-
0.05
0
-
0
2
4
6
Number of BE stations
8
10
Figure 8. Packet jitter with variable number of
BE stations
4.2. Slot Reuse and Throughput Gain
As mentione in ot the Introducto an EMSys
Model description, one of the major features of B-MAC is
the time slot reuse that allows real-time stations taking over
unused time slots reserved for other stations. By offering
a flexible -"elastic"- scheduling for real-time packets this
fetrpovdsamcmrefiintuefbnwdhrfeature provides a much more efficient use of bandwidth re-
sources, which results in an increased total throughput of
the network. In order to measure the throughput gain obtained by the time slot reuse feature, we compare E-MAC
with apseudo-TDA approach. Pseudo-TDMA means that
RT stations can reserve time slots at the beginning of each
round for the transmission of their real-time packets similar to E-MAC. The main difference of pseudo-TDMA to
E-MAC, however, is that real-time slots can not be reused
by another station, i.e., if a RT station does not transmit a
packet, the respective time slot remains empty. In contrast,
E-MAC does not waste resources as BE stations can reuse
resources not used by RT stations.
We assume that up to five RT stations have to transmit real-time packets generated by a ITU-T G.711 voice
codec [8] with silence suppression. This codec generates a
packet with 240-byte payload every 30 ms (i.e., 64 Kbit/s).
However, if the user is silent, no packets are generated.
One BE station is saturating the channel with a data rate
of 2 Mbit/s. We use Ethereal to obtain voice traces of a
VoIP real-time communication with GnomeMeeting (now
called Ekiga). We then generate dummy packets according to those voice traces on the RT stations in our testbed
to compare the efficiency of E-MAC and pseudo-TDMA.
The resulting throughput is shown in Fig. 9. As illustrated,
B-MAC yields a considerably higher value than the pseudoTDMA scheme for both total network throughput and total
best-effort throughput, and the gap becomes larger as the
number of RT stations increases.
1750
1700
1650R \
-? 1600
1550
- 1500
o-1 1450
-~1400
1350
13 0
1300
1250
1
400
. _
350
---
n 300
|
.sim-Elastic-BE
|
sim-Elastic-RT
math-Elastic-BE
'
math-Elastic-RT
250
-----
150
ioo
100
testbed-Elastic-BE
testbed-Elastic-Total
testbed-TDMA-BE
testbed-TDMA-Total ------2
3
4
Number of RT stations
50
0
5
Figure 9. Throughput, using variable number
of voice sources with silence suppression
4.3. Protecting best-effort traffic from starvation
Another useful feature of E-MAC is that, even under
heavy-load traffic conditions, it does not starve best-effort
stations. Figure 10 shows the throughput for both realtime and best-effort stations with respect to the real-time
packet generation interval. As it can be observed in Fig. 10
for small intervals (i.e., high packet generation rates), EMIAC yields a higher throughput for real-time stations than
802.11 e while keeping a minimum guaranteed data rate and
without decreasing the throughput of best-effort stations,
which is almost the same as in 802.1 1e. As the interval
increases (i.e., the generation rate decreases) real-time stations obtain exactly what they require in both E-MAC and
IEEE 802.1 le, but best-effort stations get a considerably
higher throughput with E-MAC than with IEEE 802.1 le.
The reason for the latter behavior is that E-MAC yields a
0
5
10
15
Number of BE stations
T
a
5
sim Elastic-BEr
simEastic-RT
si m-802 .1 1 e
-RT
350
300
7
sim-802.1-g-O si-802.
1
n
150
r
_
20
Figure 11. Throughput, with increasing number of BE stations. (3 RT stations, each can
saturate the channel alone)
total throughput that is higher than that of 802.1 le due to
the scheduled real-time transmissions in E-MAC. Therefore
there are less collisions and retransmissions.
Figure 11 shows the throughput obtained by each of the
three real-time stations under saturated conditions when the
number ofbest-effort stations increases for both simulations
and mathematical analysis. It can be observed in Fig. 11
how E-MAC keeps the minimum guaranteed data rate (200
Kbit/s) for any number of best-effort stations. At the same
time, best-effort stations do not starve but always get their
fair share of the best-effort period in the round. Moreover, it can be seen that, as predicted in Eqs. (8) and (9),
the throughput obtained by real-time stations in E-MAC is
equivalent to the share of the channel obtained by the besteffort stations (normalized by the packet's time length) in
addition to the minimum guaranteed data rate. The latter
result has been verified by the simulation results in Fig. 11.
100
400
----
200
sim-Elastic-BE
sim-Elastic-RT
-
math-Elastic-BE
math-Elastic-RT
10
O -S-g-BE
^
\
2
7
100-----
50
0 2 4 6 8 10 12 14 16 18 20
Packet generation interval of (each of the) RT stations (ms)
Figure 10. Throughput, as RT stations get
more greedy
0.1
0
2
4
6
8
10
12
Numberof BE stations
Figure 12. Packet delays, with increasing
number of BE stations. (3 RT stations, each
can saturate the channel alone)
single round. In contrast, real-time stations in 802.1 le
can consume the whole channel capacity depending on
the data rates at the sources.
Results from both simulations and mathematical analysis show that E-MAC guarantees a minimum data-rate for
RT stations while protecting BE stations from starvation,
even in presence of a high number of BE stations. Figure
12 depicts the delay obtained by E-MAC for both real-time
and best-effort packets. It shows how the delay for realtiepcesi gurnte readls of th nubrofB
scomplement
the
statiospacs
resus
the
obtained through mathematical analysis regarding
regarding the very
very
short delay guarantees for real-time traffic provided by BM\AC. Last, we ran video streaming under heavy BE con-
Te mecan ismintsre afet by any channel
asa cnowledg t frame mut olloweany scessful transmission of a data frame. Not overhearing the RAM
5. Discussion and Future Work
lision domain, i.e., with no hidden nodes. For a multi-hop
predicguatedein(11).dT
oframe broadcast by the maestro is not a problem either since
any station can wait for the next RAM in order to synchronieistasiso.ihte
otation trouhmthemaical analysis
tes
h
ytmcnwr
with variable
packet lengths
rates as the
wt
aibepce
egh and
n transmission
rnmsinrtsa
h
time-slots of the real-time period do not have a fixed time
length associated but only define a sequential order that the
real-time stations must follow.
The system has been designed to operate in a single col-
tntIonovertB-MAC,anvdomparedmit
to 8
BvaluCan
aention
ation rresultvie
videos can bemfound
be found inin[1].
[I].reltmsaiosutflow
ea
network this assumption does not hold anymore. In the latter case, some real-time stations could not hear the transmission of the RAM or even the preceding station during
the real-time period, causing the whole system to lose synchronization. Even a best-effort station that is not able to
overhear a given transmission during a real-time slot may
cause synchronization problems for real-time stations. The
design of E-MAC for a proper operation in a multi-hop network is part of our future work.
The scenario in which RT stations have different packet
lengths due to different data rates and/or packet sizes does
not cause any problem to our system as the transmission sequence number, which is the most important parameter, is
not changed because of this. In the worst situation in which
several RT stations are transmitting large packets at very
low data rates, the real-time period length value conveyed
by the maestro through the beacon should reflect the stringent conditions and make new stations that are trying to join
the system defer until the traffic conditions become better.
Another important issue is how to deal with different
No collisions for real-time stations during the real-time
data rate requirements from RT stations. This problem is
period, due to the order imposed by the sequence of
easily solved by allowing RT stations to transmit a burst of
backoff values. In 802.1 le, high-priority stations still
several packets during the RT period instead of only one as
suffer from increasing collisions when the number of
previously described. In this solution RT stations have an
real-time stations increases.
internal counter hi set to the maximum number of packets
to be transmitted during the RT period, which reflects their
Strict throughput and delay guarantees for real-time
own bandwidth requirement. Thus, for instance if station
due
t
I
A
has this counter hA set to 3 it is allowed to transmit 3
te "reervation"cofperictes.
contrafi,
contrast,
802.1 leoffersonlysatisticalguarantees.packets during the round. Any other packet in its buffer
exceeding this limit should contend during the best-effort
A very low guaranteed delay even under heavy-load
period and promoted to the RT class after next RAM as detraffic conditions. In IEEE 802.1 le the delay perforscribed in the paper. In order to keep the AIFS distance
mance deteriorates as the number of high-priority stabetween consecutive packets, the AIFS value associated to
tions increases.
each RT packet from station A should follow the rule: the
Better protection for best-effort traffic, due to the limfirst packet in the transmission queue pick AIFSJA, the secitation of one RT transmission! round! station, and the
ond one AIFSJA± 1, the third one AIFSJA± 2 an so forth.
assignment of best-effort backoffs to additional realIn this case, the maestro announces in the RAM the total
time packets transmitted by a given station within a
number of RT packets (Z,_1 hi) and the expected length
In comparison with TDMA, our mechanism has the advantage of slot reuse, making it more efficient since no time
slots are wasted due to silence suppression by the corresponding (e.g., voice) application codecs.
The differentiation based on traffic categories defined by
IEEE 802.1 le does not give any guarantees for real-time
traffic since at high load there is a high number of collisions even for real-time flows. Hence, under high load
traffic the delay performance of IEEE 802.1 le deteriorates.
Moreover, previous work [7] showed that in heavy-loaded
networks, low priority traffic has extremely low transmission probability when using EDCA, an effect called starvation of low-priority applications. Conversely, the proposed
E-MAC guarantees a minimum data rate and a very low delay for all real-time stations almost irrespective of the network load while avoiding the starvation of best-effort stations. In summary, E-MAC has the following advantages
with respect to IEEE 802.1 le:
*
*
*
*
of the real-time period in the current frame.
6. Related Work
(MadWifi). The system is self-organizing, completely distributed, and requires no changes to existing legacy 802.11
stations.
We show how E-MAC provides strict QoS guarantees:
a minimum-reserved throughput, and extremely low packet
delays. All testbed measurements, mathematical model and
the simulations showed consistently good results with EMAC outperforming 802.1 le, pseudo-TDMA and 802.11.
Our system is operational and ready to use in existing
802.11 networks.
In the past decades there has been considerable research
in wireless networks with particular focus on QoS due to the
increasing demand of new multimedia applications [3, 5,
14]. Several proposed approaches occupy a wide spectrum
of applications, requirements, and assumptions. They differ
in complexity as well as in performance results. However,
in contrast to our system, the lack of feasibility is a common
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