Performance evaluation of different wireless network
topologies using the IEEE 802.11 MAC Protocol
CHRIS BASIOS*, PANAGIOTIS KOSTARAKIS* and EVANGELOS PALLIS+
* Physics Department - Electronics, Telecommunications and Applications
Laboratory
+
Institute of Informatics and Telecommunications - Telecommunications Laboratory
*University of Ioannina
+National Center for Scientific Research ‘Demokritos’
*Panepistimioupolis, 45110, Ioannina
+Ag. Paraskevi Attikis, 15310, Attiki
GREECE
Absract: WLANs are an emerging part of the current wireless communications providing
high bandwidth to users in a limited geographical area. The IEEE 802.11 Standard is a widely
used protocol for this kind of communication. In this paper, we estimated the performance of
the above protocol for 3 different wireless networks; an ad-hoc network, a network that makes
use of the LMDS technology and an infrastructure network. The performance evaluation was
based on throughput values coming out of various simulations. Our main interest focused on
the transfer of fixed data packets under a varying network load (depending on the number of
users, the length of the packets etc.).
Key-Words: 802.11, DCF, throughput, ad-hoc, LMDS, infrastructure
1. Introduction
The main concept in our
simulations was the throughput
estimation for the case where every
user that resides in the WLAN requests
and downloads – using either TCP or
UDP transport protocol - a fixed size
message from a remote server. In the
ad-hoc network both users and the
server use the DCF access method –
specified by the IEEE 802.11 Standard
– in order to gain access to the wireless
medium. In the LMDS model, both
users and the AP (Access Point) of the
wireless cell, which is connected via a
point-to-point link to a wired server,
use the DCF access method. Finally, in
the infrastructure network the users use
the DCF in order to request and
download a message from the server
that resides in a wired LAN. We have
modeled various scenarios in order to
investigate the network efficiency
under different conditions. Thus, we
have varied the offered load (by
altering the number of the users, the
arrival rate of the users’ requests to the
server, the message length and the
length of the MSDUs the message
splits to) and we have also investigated
the utilization of the fragmentation
mode provided by IEEE 802.11
Standard.
The design of the simulation
models and the analysis of their
efficiency have been made by using
the COMNET III simulation program.
With this simulation tool we had the
possibility to built our own network
topology by using and altering many
parameters of the main building blocks
of the program - such as nodes, links,
network devices etc. - that constructed
our designed models.
2. The DCF access method and
simulation basic parameters
In all simulations the access
method that was used by the wireless
users was the DCF (Distributed
Coordination Function). DCF is the
primary access method provided by the
IEEE 802.11 Standard and is based on
CSMA/CA algorithm. In the following
lines we will give an overview of this
algorithm.
Before a station initiates
transmission of a data frame, it needs
to sense the channel in order to
determine whether another station is
currently transmitting. The station can
proceed with its transmission if the
medium is determined to be idle for a
time interval of DIFS (DCF InterFrame Space). After a data frame is
successfully
received
at
the
destination, the receiver must send an
acknowledgment
frame
(ACK),
because the transmitter cannot
determine whether a frame has been
faithfully delivered to its destination by
simply listening to the channel; the
sender may not observe frame
collisions the receiver detects with
other senders not observable by the
first (this is the so-called “hidden
terminal problem”). To transmit the
ACK, the receiver waits for the
channel to be idle for another time
interval of SIFS (Short Inter-Frame
Space). If the transmitter does not
receive an ACK packet within a certain
period (ACK timeout), it presumes that
the data frame is lost and schedules a
re-transmission.
If the medium is busy upon
transmitting a data frame or an ACK,
the transmission must be deferred until
the end of the ongoing transmission. In
this case, a random backoff interval is
selected, as follows. A backoff timer is
set with a random backoff integer (BV)
drawn from a uniform distribution over
the interval [0,CW - 1], where CW
(Collision Window) is an integer
within the range of CWmin and
CWmax. BV is the number of idle
“slots” the station must wait until it is
allowed to transmit. The value is
decremented by one for each idle slot
detected. The backoff timer suspends
when the medium becomes busy
before BV reaches zero. The timer
resumes only after the medium has
been idle longer than the designated
inter-frame space interval. The station
starts transmitting the frame when the
backoff timer reaches zero. For each
successive retransmissions, the value
of CW increases exponentially (i.e.
CWnew = CWold . 2 - 1), until it
reaches and then stays at CWmax . CW
will be reset to CWmin after a
successful transmission. The backoff
method is used to minimize collisions
and maximize throughput at both low
and high network utilizations.
All the wireless users access
the wireless channel of each one of the
3 network topologies by using the
basic DCF method provided by the
802.11 Standard, while at the physical
layer they use the FHSS technique.
The bit rate of the wireless medium is
fixed to 2 Mbps.
The transport protocols we
used are TCP and UDP. The
congestion
control
algorithm
implemented in TCP was selected to
be the ‘Fast recovery’ algorithm. We
also modeled the establishment and
disconnection of a TCP connection
using 2 fixed size (40 bytes) packets
for every message (‘Open’, ‘Close’
packets), while the ACK packet
acknowledging every data packet is 40
bytes long.
The main 802.11 parameters
and their values used in the simulations
are summarized in the next table.
Table 1: Parameters used for the
simulation of the IEEE 802.11 MAC
protocol
802.11 Parameters
Values
Bandwidht
Frame max
ACK timeout
Frame TxLife Time
Slot time
SIFS
FH dwell time
CWmin
CWmax
2 Mbps
2304 bytes
0.2 ms
512 ms
0.05 ms
0.028 ms
128 ms
7
255
The main assumption that have
been made for the wireless networks is
that the ‘hidden-node’ problem is not
addressed in the simulation models,
while the wireless users are all in fixed
locations (no mobility).
3. Simulation
results
models
and
In this section, we present the 3
different simulation models and the
individual scenarios being used. We,
briefly, analyze the network topology
and we demonstrate the throughput
graphics coming out for each scenario.
Figure 1: Simulation model of the ‘adhoc’ network.
1st scenario
Every BSS STA (user) requests
(1000 bytes packet) for a single time a
2 MB file from the server located in
the same BSS. The requests take place
in the first 120 sec of the overall
simulation run time (3600 sec)
according to a uniform distribution.
Both users and server make use of TCP
and UDP protocols (TCP have a fixed
sized packet consisted of 1460 data
bytes and 40 overhead bytes, while
UDP packet is consisted of 1480 data
bytes and 20 overhead bytes). The
offered load in the network varies
according to the number of users that
join the ad-hoc network (the min.
number of users is 1 and the maximum
is 30).
3.1 Ad-hoc model





Packetize delay: 0.01 ms
Buffer port delay: 0.05 ms
Propagation delay: 1 μs
BER: 10-3
ACK BER: 0.112
throughput (Mbps)
In this model we simulated the
transfer of a fixed-size message from a
wireless server to a variable number of
wireless users with both users and the
server residing in the same IBSS.
Some simulations took place for 2
different environments; the first was an
error-free environment and in the
second we simulated the following
basic delays and error factors:
2
TCP (error free
env.)
1,5
UDP (error free
env.)
1
TCP (error env.)
0,5
UDP (error env.)
0
0
10
20
30
40
users
Figure 2: Effect of number of users for
different simulation environments.
2nd scenario
The requests’ arrival for every
user in the BSS follow a Poisson
distribution, making the interarrival
throughput (Mbps)
2
1,8
1,6
1,4
1,2
1
0,8
0,6
0,4
0,2
0
TCP, f-1000
UDP, f-1000
TCP, f-2000
UDP, f-2000
TCP, f-1460
UDP, f-1460
0
10
20
30
40
users
Figure 3: Effect of number of users for
different MSDU lengths.
3rd scenario
In this scenario the number of
users is fixed (equal to 10) and the
varying parameter of the system is the
length of the message being
downloaded by all 10 users. Each
request to the server is generated once
(by every user) according to a uniform
distribution. The request arrivals take
place in the first 120 sec of the overall
simulation run time (3600 sec). The
message size varies from 1MB to
5MB, while the size of each transport
protocol packet (TCP or UDP) remains
the same equal to 1500 bytes (plus the
overhead).
throughput (Mbps)
time between the request messages
follow an exponential distribution with
a mean value of 600 sec. In this
scenario - that was only modeled for
error-free
environment
we
demonstrate the effect of the MSDU
packet length in the user throughput.
We used 3 different TCP-UDP packets
by changing the data bytes of every
packet (1000 bytes, 1460 bytes and
2000 bytes) that contract the whole
2MB message. Thus, the MSDUs
coming out from the server will take 3
different values, with none of them
exceeding the maximum allowable
MSDU length (2304 bytes).
1,6
1,4
1,2
1
0,8
0,6
0,4
0,2
0
TCP (error free
env.)
UDP (error free
env.)
TCP (error env.)
UDP (error env.)
0
2
4
6
message size (MB)
Figure 4: Effect of message size for
different simulation environments.
3.2 LMDS model
The idea of the LMDS model
was the investigation of the efficiency
for a network topology where the users
of a single wireless cell request and
download a fixed size message from a
service provider that resides in a
remote wired LAN. The connection
between the AP of the cell and the
router that routes the packets up and
down the server is implemented via an
RF point-to-point link (2Mbps
bandwidth). The service provider
resides in a wired LAN, which uses the
802.3 10BASET technology, while the
access method in the cell is carried out
using the CSMA/CA algorithm for
both uplink and downlink; that means
that both the users and the AP make
use of the 802.11 protocol. Some
simulations took place for 2 different
environments (error-free and errormodeling).
The parameters and their values
used for the simulation of the IEEE
802.3 MAC protocol are according to
the 802.3 Standard, while these of the
802.11 MAC protocol are the same
with what was referred in the ad-hoc
model.
In
the
error
simulation
environment we have modeled the
same errors and delays with the
previous model plus the BER of the
wired medium - which was set to 10-5 and the processing packet delay in both
router and AP (Processing/packet:
0.0067 ms).
Figure 5: Simulation model of the
LMDS network.
fragmentation mode. Firstly, we must
mention that the arrivals of the users
requests are modeled with exponential
interarrival times with a mean value of
600 sec. The transport layer that was
used in order to simulate the behavior
of the fragmentation mode was UDP
with a fixed size packet of 1500 bytes
(1480 data bytes + 20 OH bytes). The
fragmentation threshold was set in 712
bytes. In addition, we simulated the
use of TCP in order to compare the
efficiency of the 2 protocols when the
load of the network increases (due to
the increment of the wireless users).
1st scenario
throughput (Mbps)
2
TCP (non error
env.)
1,5
UDP (non error
env.)
1
TCP (error env.)
0,5
UDP (error env.)
0
0
10
20
30
40
users
Figure 6: Effect of number of users for
different simulation environments.
2nd scenario
have
In this second scenario, we
modeled
the
802.11
throughput (Mbps)
2
Every BSS STA (user) requests
(using a 1000 bytes packet) and
downloads a 2MB from the wired
server. Every user that joins the
wireless cell generates only one
packet-request. The requests take place
in the first 120 sec of the overall
simulation run time (3600 sec)
according to a uniform distribution.
Both users and server make use of TCP
and UDP protocols (TCP have a fixed
sized packet consisted of 1460 data
bytes and 40 overhead bytes, while
UDP packet is consisted of 1480 data
bytes and 20 overhead bytes).
1,5
TCP (without
frg)
1
UDP (without
frg)
UDP (with frg)
0,5
0
0
10
20
30
40
users
Figure 7: The effect of fragmentation
technique.
3rd scenario
In this scenario we investigate
the effect of the message size in the
users throughout under the 2 different
simulation environments. The number
of the users is fixed (equal to 10) and
each request to the server is generated
once (by every user) according to a
uniform distribution in the first 120 sec
of the overall simulation run time. The
message size varies from 1 MB to 5
MB, while the size of each transport
protocol packet (TCP or UDP) remains
the same equal to 1500 bytes (plus the
overhead).
2
1,8
throughput ( Mbps)
1,6
1,4
TCP (non error env.)
1,2
UDP (non error env.)
1
TCP (error env.)
0,8
UDP (error env.)
0,6
0,4
0,2
0
0
1
2
3
4
5
6
message size (MB)
Figure 8: Effect of message size for
different simulation environments.
wired and wireless users make a single
request for the message and the
number of both LANs’ users is
incremented with the same rate (e.g.
when in the WLAN reside 10 users,
the same number of users shall reside
in the wired LAN). The arrival of each
request-packet will take place in the
first 120 sec and will follow a uniform
distribution.
10
In this simulation model a
802.11 wireless BSS is interconnected
via a wireless portal with a 802.3 wired
LAN (10BASET technology). Both
wireless and wired LAN consist of a
varying number of users (1 up to 30).
Every one of the above users will
request and try to download a fixed
data message from a server that is
connected to the 802.3 LAN, using the
TCP and UDP transport protocols. The
main aim of this simulation model was
to investigate the way the wireless
users’ throughput is affected by the
presence of the wired users. All the
simulations took place for error-free
environments.
8
throughput (Mbps)
3.3 Infrastructure model
802.11, TCP
6
802.11, UDP
4
802.3, TCP
802.3, UDP
2
0
0
10
20
30
40
users
Figure 10: Effect of number of users
for different transport protocols.
2nd scenario
In this scenario we have
modeled and simulated 2 different
cases. In the first one we vary the
number of wired users for a fixed
number of wireless users, while on the
second one we vary the number of
wireless users for a fixed number of
wired users. The arrival of requests
from each station is modeled with
exponential interarrival times with a
mean value of 600 sec.
10
9
Figure 9: Simulation model of the
infrastructure network.
throughput ( Mbps)
8
7
6
wireless users
5
wired users
4
3
2
1st scenario
1
0
0
5
10
15
20
wired users
The message size that each user
will try to download from the wired
server is fixed and equal to 2 MB. Both
(a)
25
30
35
4. Conclusions
10
9
throughput (Mbps )
8
7
6
wireless users
5
wired users
4
3
2
1
0
0
5
10
15
20
25
30
35
wireless users
(b)
Figure 11: The effect of (a) increment
of the wired users-the number of
wireless users remains fixed and (b)
increment of the wireless users-the
number of wired users remains fixed.
3rd scenario
In this scenario we simulated
the transfer of a fixed data packet from
the wired server to 10 wireless users
without the presence of any wired user.
Both the users and the server make use
of UDP with the number of the packet
data bytes remaining fixed to 1480
bytes. The varying magnitude in this
scenario is the message size, which
varies from 0.01 MB to 5 MB. In order
to estimate the throughput variation in
association with the offered load, we
used 2 different rates for the Poisson
arrivals of every user request. Thus,
the interrarival time is following an
exponential distribution with 2
different values of 300 and 600 sec,
respectively.
throughput (Mbps)
2
1,5
exp(600)
1
exp(300)
0,5
0
0.01 0.1
0.5
1
2
3
4
5
message size (MB)
Figure 12: Effect of message size for
different interarrival times.
The main conclusion coming
out is the clearly better performance of
UDP over TCP, whether the simulation
environment used is error-free or not.
This result has to do with the ARQ
error mechanism provided by the
802.11 that makes the use of TCP
congestion
control
and
acknowledgement technique bouncy,
especially for the networks that do not
experience high load and collisions. As
it was expected, TCP enters greater
overhead than UDP because of the
extra use of the ‘Open’, ‘Close’ and
ACK packets. Another result for both
ad-hoc and infrastructure networks is
that the increase of the users turns to a
throughput decrease for any kind of
traffic; this means that downloading
such large messages (1-5 MB) has a
serious affection on the users’
throughput.
In particular, the use of larger
fixed data packets (that construct the
overall message) in the ad-hoc model
is, generally, more efficient than the
use of smaller ones. This is due to the
‘error-free’ simulation environment
and the fact that when the 2 MB
message splits in smaller MSDUs,
more packets will have to contend for
the wireless medium, thus increasing
the probability of collisions. This
explains and the decreasing of the
throughput when the message size
increases (from 1 to 5 MB).
In the LMDS model, the most
remarkable conclusion is the limit
estimation of the number of wireless
users that makes the use of
fragmentation either useful or not for
the entire network. When the users of
the wireless cell exceed the number of
15, the fragmentation mode leads to
higher values of throughput. Despite
the extra overhead bytes for every
fragment, fragmentation increases the
probability of successful transmission
of the MSDU (because of the use of
smaller frames) when the offered
network load comes to higher values.
The results of figure 2.3 show that for
the error-free simulation model the
offered load we applied in the network
(10 users, one message request per
user) leads in a slight throughput
decrement despite the message size
increment.
For
the
infrastructure
simulation model, the existence of 10
fixed wired users does not affect in any
way the throughput of the wireless
users
which
remains
fixed,
independently of their number residing
in the WLAN. The same wireless user
throughout is slightly affected when
the number of the wired users varies
from 1 to 30. By varying the request
generation rate and the length of the
message size (figure 3.3), we turned
out with 2 main goals. The first one
has to do with the existence of 3
throughout
levels;
increment,
saturation and decrement that lead to a
clear limit of the best achievable
throughput. The second one is the
estimation of the maximum allowable
message size value (1MB) over which
the arrival rate affects the user
throughput.
In order to overcome the
significant
TCP
overhead
in
infrastructure
networks,
many
optimization techniques have been
introduced. The main feature of most
of these techniques is splitting the TCP
end-to-end connection in individual
TCP connections for the wireless and
wired parts of the network. The
efficiency of such techniques is
included in our current interest
research fields.
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ISWC’99
IEEE
International Symposium on Wireless