Emulation Experimentation Using the Extendable Mobile Ad-hoc ...

lect a random slot within a given contention window
once the channel becomes idle. The size of a
slot is on the order of tens of microseconds, sufficiently
long enough in real systems to ensure multiple
nodes selecting different slots will not step on
one another. Within an emulated system, however,
the time it takes for a packet to get from one emulated
node to another can be significantly larger
than the contention delay implying that actual implementation
of the protocol will not provide an
accurate representation of the collision and backoff
phenomena associated with CSMA/CA. Advanced
statistical models or simplistic approximations can
be utilized to account for such a phenomena, but the
determination of which model is appropriate should
be based on the research/test objective.
3.2 Models
The EMANE project currently contains three
models each having varying features and complexity
supporting research and evaluation of wireless
technologies within DoD, Academia, and Industry:
RF-Pipe, Comm Effects, and IEEE 80211abg. In
addition, there are two high-fidelity tactical models
(SRW and HNW) currently under development
with controlled/restricted access.
3.2.1 RF-Pipe
The RF-Pipe model is a simplistic yet feature rich
model which can be configured to provide capabilities
similar to existing emulators, such as NRL’s
MANE[3] and Boeing’s Core[4], to emulate throughput
and loss of various wireless technologies such
as satellite comms, 802.11, and tactical waveforms.
In addition, the model also serves as a template to
guide independent NEM developers of more complex
models. The majority of the functionality of
the RF-Pipe model is contained within the PHY
layer and can be summarized as follows:
• Filters out-of-band packets.
• Adds transmission delay based on data rate,
propagation delay, and jitter.
• Computes propagation effects based on runtime
Path Loss or Location events. Path Loss
Event values are computed off-line with events
dispatched in real-time during the emulation
experiment. Location Event based path loss is
computed in real-time using one of two configured
propagation models (Free Space or 2-Ray
Flat Earth).
• Performs packet treatment based on Receive
Power (Rxpower) and configured min/max
6
thresholds as depicted in Figure 4.
Rxpower = T xpower+T xgain+Rxgain−P athloss
(2)
The model utilizes the common EMANE PHY
header to provide the Txpower and Txgain in
each over-the-air packet transmission providing
the ability to emulate different types of
platforms such as base stations, vehicles, dismounts,
and sensors.
Figure 4: RF-Pipe Packet Treatment.
3.2.2 Comm Effects
The Comm Effect model provides the ability to
control network impairments commonly provided
via traditional COTS network emulators on a per
link basis asymmetrically. The Comm Effect model
is a Shim-only implementation utilizing unstructured
EMANE NEMs as described in Section 2.4.
The Comm Effects model provides the ability to
define the following network impairments:
• Loss: The percentage of packets that are
dropped based on a uniform loss distribution
model.
• Delay: Defines the delay for a packet to traverse
a specific link. The delay is composed of
a fixed and variable (jitter) component. Delay
is computed at the receiver and the packet is
delivered up the stack once the delay expires
providing the ability to emulate out of order
packet reception.
• Duplicates: Defines the percentage of packets
to be duplicated at the receiver.
The network impairments defined above are controlled
based on time and/or predefined filters.
Time based impairments are controlled via the
use of Comm Effect events dispatched in real-time
during the emulation experiment. The events provide
each node with the effects/impairments to every
other node within the scenario.