takes for the node to send a unicast packet whose acknowledgment is
successfully received.
-CTP has several mechanisms in order to improve delivery reliability,
-but it does not promise 100\% reliable delivery. It is best effort, but
-a best effort that *tries very hard.*
+CTP has several mechanisms in order to achieve high delivery
+reliability, but it does not promise 100\% reliable delivery. It is a
+best effort protocol.
-CTP is designed for relatively low traffic rates. Bandwidth-limited systems
-might benefit from a different protocol, which can, for example, pack
-multiple small frames into a single data-link packet.
+CTP is designed for relatively low traffic rates such that there is
+enough space in the channel to transmit and receive routing frames
+even when the network is forwarding collection data
+frames. Bandwidth-limited systems or high data rate applications might
+benefit from a different protocol, which can, for example, pack
+multiple small frames into a single data-link packet or employ rate
+control mechanisms.
3. Collection and CTP
of routing loops. If a node suppresses origin packets, then if
asked to forward the same packet twice due to a routing loop, it will
drop the packet. However, if it suppresses packet instances, then it
-will route succesfully in the presence of transient loops unless the
+will route successfully in the presence of transient loops unless the
THL happens to wrap around to a forwarded packet instance.
A node MUST send CTP data frames as unicast messages with link-layer
small value when one or more of the following conditions are met:
1) The routing table is empty (this also sets the P bit)
- 2) The node's routing ETX increases by >= 1 trasmission
+ 2) The node's routing ETX increases by >= 1 transmission
3) The node hears a packet with the P bit set
The implementation augments the LEEP link estimates with data
transmissions. This is a direct measure of ETX. Whenever the data path
-transmits a packet, it tells the link estimator the destimation and
+transmits a packet, it tells the link estimator the destination and
whether it was successfully acknowledged. The estimator produces an
ETX estimate every 5 such transmissions, where 0 successes has an ETX
of 6.
the transmission rate, then it can quickly detect a broken link and
switch to another candidate neighbor.
-The component ``tos/lib/net/le/LinkEstimatorP`` implements the
-link estimator. It couples LEEP-based and data-based estimates.
+The component ``tos/lib/net/4bitle/LinkEstimatorP`` implements the
+link estimator. It couples LEEP-based and data-based estimates as
+described in [4]_.
6.2 Routing Engine
------------------------------------------------------------------------------
------------------------------------------------------------------------------
The component ``tos/lib/net/ctp/CtpForwardingEngineP`` implements the
-forwarding engine. It has five repsonsibilities:
+forwarding engine. It has five responsibilities:
1) Transmitting packets to the next hop, retransmitting when necessary, and
passing acknowledgment based information to the link estimator
in Wireless Sensor Networks." In Proceedings of the First USENIX
Conference on Networked Systems Design and Implementation (NSDI), 2004.
.. [3] TEP 119: Collection.
+.. [4] Rodrigo Fonseca, Omprakash Gnawali, Kyle Jamieson, and Philip Levis.
+ "Four Bit Wireless Link Estimation." In Proceedings of the Sixth Workshop
+ on Hot Topics in Networks (HotNets VI), November 2007.
compute the routes. Nodes can estimate the quality of the in-bound
link from a neighbor by estimating the ratio of successfully received
messages and the total transmitted messages. LEEP appends in-bound
-packet reception rate (PRR) estimates to packets. Other nodes hearing
+packet reception rate (PRR) estimates to packets. Other nodes hearing
these packets can combine the in-bound PRR values with their own
-in-bound values to compute bi-directional link quality.
+in-bound values to compute bi-directional link quality. Thus, LEEP is
+a discovery and link table bootstrapping mechanism. The link quality
+is often fine-tuned using different mechanisms.
+
+Link quality estimates obtained using LEEP are often used as a
+bootstrapping values in the link quality table; data transmission
+statistics are later used to make these estimates more accurate.
2. Definitions
============================================================================
entries describing the in-bound link qualities for a subset of its
neighbors. The Link Information entry on the LEEP frame allows the
receiver node to find the out-bound link quality to the transmitter
-node identified by the data link source address.
+node identified by the data link source address. Thus, LEEP is also a
+way for nodes to discover new nodes and links in the network.
+
+Link quality estimation is inherently imperfect - data transmission
+and link quality estimation might be done at different timescales. The
+PRR for LEEP frames (broadcast) and data frames (unicast) might be
+different. So LEEP is better used as a link quality bootstrapping
+mechanism. The link quality estimate can be made more accurate later
+using data transmission statistics.
+
+
3.3 LEEP Frame
--------------------------------------------------------------------
frame. The LEEP frames are transmitted whenever the CTP [1]_ beacons,
sent as a LEEP payload, are sent.
+Another reference implementation resides in
+``tinyos-2.x/tos/lib/net/le``. This implementation is described in
+detail in [2]_.
+
5. Author's Address
============================================================================
============================================================================
.. [1] TEP 123: The Collection Tree Protocol.
+.. [2] Rodrigo Fonseca, Omprakash Gnawali, Kyle Jamieson, and Philip Levis.
+ "Four Bit Wireless Link Estimation." In Proceedings of the Sixth Workshop
+ on Hot Topics in Networks (HotNets VI), November 2007.
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