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Exploring
Wireless LAN Throughput and Interoperability
with other Protocols
Click
here to view and execute the VisualSim
model
This
shows the topology of a WiFi network
in VisualSim. Nodes and Access Points
can be arbitrarily added or removed.
This model contains two nodes, an Access
Point, Arbiter and statistics block.
The traffic enters the system at each
node and leaves the network at the Access
Point.
Click
here to view the interactive VisualSim
Block Diagram and model
The
VisualSim model of an 802.11 node contains
a State Machine, with state being the
state of the network. The transmitter
and receiver are contained in a single
window. The parameters of the model
can be modified to create multiple operating
conditions. The MAC layer is model signaling
and timing-accurate.
This
model has been created based on the
specification laid out by the IEEE 802.11
Working Group. For more details on the
Wireless LAN MAC layer details, please
refer to the Wireless LAN specifications.
Modeling can be a very effective tool
for analyzing a variety of architecture
and performance issues. Some of the
issues to be considered by System Architects
include:
- How
to account for frame errors that
cause retries, or contention from
other LAN users, when the application
is based on a Constant Bit Rate
(CBR) algorithm?
- How
big should the application buffer
be in order to handle interruptions?
- What
are the resource trade-offs in implementing
new applications and features in
custom hardware or software?
- What
is the buffer tradeoff versus end-to-end
frame latency?
- How
many users can the local coffee
shop WiFi network support before
information loss renders the Internet
access unusable?
The
complexity of these technical decisions
and the non-deterministic nature of
the traffic profile require accurate
modeling efforts to predict the resource
requirements, and the effective maximum
throughput for a WiFi network.
The
purpose of this model is to evaluate
the throughput of a Wireless LAN network
for a number of constraints including:
1. Impact of packet fragmentation on
throughput
2. Frame latency due to preemption by
voice traffic
3. Effective utilization based on Protocol
signal overhead, and variable node and
Access points Wire-rates.
Model
Overview
The Discrete-Event simulation model
in VisualSim consists of traffic generators,
nodes, Access Points, arbiters, and
statistics generation. The VisualSim
model has been constructed as a hierarchical
design to enable easy of understanding
and modification. The nodes have been
modeled to contain both the transmitter
and receiver. The arbiter was separated
from the nodes to improve simulation
performance and to allow independent
evaluation of the arbitration algorithm.
VisualSim model of 802.11 transmits
frames from multiple client nodes to
an access point. Nodes are contending
for service based on the standard 801.11
backoff procedure. The overall frame
latency is measured for each packet
exiting the central access point.
Each
wireless node is defined as a state
machine with five states: IDLE, RDY,
CTS, RTS and NEXT. The signaling and
arbitration can accommodate single frame,
burst mode and fragmentation modes of
operation. The separate arbiter block
provides arbitration between the node
devices, performs backoff, and slot
retention/improvement for the losing
node in the arbitration process.
Data
Structure
Station_Name String Null
Station_X_Pos double 0.0
Station_Y_Pos double 0.0
Station_Direction double 0.0
Station_Velocity double 0.0
Station_Power double 0.0
State_Name String IDLE
Frame_Receiver String Null
Frame_Access String Null
Frame_Transmitter String Null
Frame_Type String Null
Frame_Number int 0
Frame_Priority int 0
Frame_Slot int 0
Frame_Size int 0
Frame_Duration int 0
Frame_Retry int 0
Frame_Fragment int 0
Frame_Sequence int 0
Frame_Select boolean false
Frame_Time double 0.0
Frame_Delta_Time double 0.0
The Wireless LAN Data Structure carries
the information associated with each
frame for analysis and signalling. The
key fields used in the model are source,
destination, frame size and timestamp.
The model generates frames at periodic
rates but can be easily modified to
accommodate a network trace for more
accurate modeling. Provision is provided
for mobile position, direction and velocity
in the input definition. The Data Structure
can be enhanced to add other details
as required for the modeling including
adding the actual data that needs to
be transmitted.
Model
Details
The protocol data (FRAME) and control
frames (RTS, CTS, and ACK) are based
on the 802.11 specification. Network
Access Vectors (NAV_RTS and NAV_CTS)
are signals to discourage other stations
from accessing.
The
model has been constructed with expandability
in mind. The model can be enhanced to
add the Physical Layer. In addition,
application such MPEG-4 and H.264 can
be developed on top of this model. A
model analyzing hardware design on top
of this Wireless LAN system is also
available under the Wireless Demo heading.
Modifying
Parameters to create new scenarios
The frame size and fragmentation limit
are maintained as parameters that can
be modified to test protocol functionality
and system throughput. The network frequency
and individual node timing can be varied
from 1 Mhz to 11.0 or 54.0 Mhz. The
DCF Inter Frame Spacing (DIFS) and Short
Inter Frame Spacing (SIFS) delays are
also parameters of the model. In addition,
each node and the Access points have
a parameters that can be modified. The
traffic rate can also be generated by
modifying the Mean time and using a
different distribution.
- Vary
the simulation time by double-clicking
on the Gear (top-right) and modifying
the Stop Time parameter.
- Vary
the Node timing by double-clicking
on any node and changing the Frequency-MHz
to 1.0 or 54.0. First modify only
one and then modify all of them.
Notice the change in the Frame Lantency.
- Vary
the Frame_Size_Bytes parameter by
double-clicking on it and changing
the value. The fragmentation has
been set at 500 bytes.
- Double-click
on the DS_Gen blocks and change
the Mean time. Also pull-down and
select a different distribution
such as Exponential. If selecting
Uniform make sure that Spread is
greater than the Mean as these corresponds
to Minimum and Maximum value of
the distribution. For the Normal,
the spread is the standard deviation.
Analysis
Three analysis graphs are displayed
on this page below.
1. The timeline plot titled Wireless
Protocol contains all the 802.11 signals:
RTS, CTS, ACK, NAV_RTS, NAV_CTS and
FRAME. The plot shows fragmentation
frames being transmitted. Contention
is shown in the first sequence where
there are two RTS frames. This plot
represents the functional accuracy of
the implemented algorithm and evaluates
the protocol correctness for scenarios
such as fragmentation, arbitration and
contention.
2. Frame Latency shows the variation
of the latency against various parameters.
Vary the size of the input frame or
the node timing to see the latency get
modified.
3. Access Out text display shows the
values of the DS fields at the Access
Point.
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