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

vides the interface used to publish events.
No restriction is placed on how generators create
events. Some generators may read precomputed
state information from input files and publish
events on time boundaries. Others may create
event data algorithmically in real-time and publish
the events based on update threshold logic. When
new EMANE event types are introduced, no modifications
are necessary to existing EMANE components,
provided those components are not required
to process the new events. Only the Event Generator
and the destination EMANE components need
to know the more specialized form of the generically
transmitted event. Events are addressable using a
three field tuple: NEM Platform Server identifier,
NEM identifier, and component identifier.
An Event Agent is an EMANE component that
translates events from their emulation domain representation
to a format usable by application domain
entities. They facilitate the reuse of any experiment
scenario information propagated via an
event that is of interest outside of the emulation domain.
For example, position information contained
in the EMANE Location Event which is used by
some PHY layers to compute path loss may also be
of interest to application domain entities that require
GPS location.
2.4 NEM Advanced Topics
A Shim Layer plugin provides a mechanism to interact
with the OTA messages and cross-layer messages
exchanged between contiguous layers. Shims
may generate their own OTA messages for communication
with their respective Shim layer contained
in a different NEM, and can append and strip layer
specific headers to OTA messages.
It is possible to insert one or more Shims between
the layers of a NEM stack or at the top and bottom
of the layer stack. A Shim plugin implements the
same generic interface as a MAC and PHY plugin.
This generic interface allows Shims to be inserted
between components without requiring those components
to have knowledge of their presence. Figure
3 shows what a NEM layer stack looks like after
inserting Shim layers and the resulting physical and
logical connections.
A NEM containing a MAC and PHY layer is formally
known as a structured NEM. A NEM may
also be unstructured. An unstructured NEM relaxes
NEM layer stack constraints to allow for the
creation of NEMs that may or may not contain
MAC, PHY or Shim layers. Unstructured NEMs
are useful when porting legacy emulation models
that do not support a separation of MAC and PHY
4
Figure 3: NEM Layer stack as it appears without
Shims and with three Shims inserted. Two Shims
reside between the MAC and PHY layers and one
Shim resides between the PHY Layer and the OTA
message interface.
layer functionally.
3 Network Emulation Modules
Within the EMANE architecture, NEMs address
the “hard problem” in wireless network emulation
associated with representing the behavior of the underlying
wireless technology at both the Media Access
and Physical Layers. Physical and MAC layer
phenomena such as frequency interference, multipath/fading,
collisions, channel access protocols,
adaptive power and data rate controls, quality of
service, and others vary significantly based on the
underlying wireless commercial or tactical technology.
Depending on the research/test objective, failure
to account for these or any corresponding cross
layer interactions can provide misleading and outright
false results. Conversely, the ability to account
for some of these complex phenomena within
a real-time emulation environment can impose requirements
(hardware, timing, scale, etc.) which
may not be critical to the research/test objective.
As such, the modular EMANE architecture is designed
specifically to support independent development
of NEMs to emulate current and evolving wireless
technology of varying fidelity under a single emulation
framework. These independently developed
modules can be contributed to the EMANE project
as add-ons for use by the community at large or
can be maintained as proprietary/sensitive modules