X-Lisa: Cross-Layer Information
Sharing Architecture
Cross-layer interactions are a well researched area. Many gains can be obtained from
knowing the local nodes' remaining energy, capabilities, location, etc. Many protocols
make use of such information, thus keeping redundant data structures at various levels
in the protocol stack or compromising flexibility and ease of use for specialization.
X-Lisa proposes to formally standardize cross-layer interactions, which have been
done in an ad-hoc manner. We wish to create a rational open source development
of services and cross-layer protocols that other researchers can use.
X-Lisa is a new architecture that retains a layered protocol stack while
providing enough flexibility for today's sensor network protocol needs. It contains
three information repositories that give a local view of the network and of the
node's working conditions:
- A neighbor table that carries information about the node and its neighbors. The neighbor
table keeps track of valuable information such as node ID, location, remaining energy,
sensing capabilities, status, etc.
- A message pool that indicates which packets are about to be transmitted.
- A sink table that retains information about data sinks, their location and QoS needs.
Each protocol has access to these information repositories through an interface named
CLOI (Cross-Layer Optimization Interface). The information contained in the
tables and message pool may not be modified directly to ensure stability.
X-Lisa also encompasses various services that maintain the information repositories.
These services include, but are not limited to:
- Node ID assignment,
- Location assessment,
- Remaining energy measurement, and
- Sensing abilities, which indicates what physical variables a node may monitor.
Services may be selectively loaded at compilation, and they may be turned on or off
during runtime. Figure 1 illustrates the X-Lisa architecture.
Figure 1: X-Lisa, a Cross-Layer Information Sharing Architecture.
X-Lisa retains a layered structure while remaining flexible enough for protocol
replacement and maintenance.
CLOI automatically piggy-backs an information vector that updates all
or parts of the neighbor table. The neighbor table population and clean-up can
be done without supervision from protocols in the stack.
The neighbor table, message pool, and sink table as well as the
information they contain are illustrated in Figures 2, 3, and 4.
Figure 2: Fields of the neighbor table.
Figure 3: Fields of the message pool.
Figure 4: Fields of the sink table.
X-Lisa retains several advantages over existing protocol architectures:
- It helps protocol designers focus only on their protocol's key functionalities.
- It is flexible: replacing or modifying a particular protocol in the stack is no
longer an arduous task since CLOI> standardizes information exchange.
- It gives the most up-to-date local view of the network to all protocols
at the same time. For instance, when a protocol realizes that a neighbor can
no longer be reached, all other protocols in the stack are notified at the same time.
- It helps save memory space by avoiding redundant tables at different levels in the stack.
- In many cases, it helps reduce overhead by concurrently transmitting
information needed by many protocols.
We have implemented X-Lisa (with a few services) in TinyOS. For more information,
please refer to the
UR Technical Report or email Chris Merlin.