Merge TinyOS 2.1.1 into master.

[tinyos-2.x.git] / doc / txt / tep124.txt

diff --git a/doc/txt/tep124.txt b/doc/txt/tep124.txt
index 3a16cfe9128ce91d86f7327015a9af3ee91ba97e..b57e503668d1beaa8f08bba427c1695645ef9396 100644 (file)
--- a/doc/txt/tep124.txt
+++ b/doc/txt/tep124.txt
@@ -5,8 +5,8 @@ The Link Estimation Exchange Protocol (LEEP)
 :TEP: 124 
 :Group: Network Protocol Working Group 
 :Type: Documentary
-:Status: Draft
-:TinyOS-Version: 2.x
+:Status: Final
+:TinyOS-Version: > 2.1
 :Author: Omprakash Gnawali
 
 :Draft-Created: 05-Feb-2006
@@ -34,36 +34,33 @@ links to the neighbors.
 Routing protocols often require bi-directional link qualities to
 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 
+messages to the total transmitted messages. LEEP appends in-bound
+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
+bootstrapping values in the link quality table; data transmission
+statistics can later be used to make these estimates more accurate.
 
 2. Definitions
 ============================================================================
 
-2.1 Link Quality
---------------------------------------------------------------------
-
-The link quality between a directed node pair (A,B) is the probability
-that a packet transmitted by A will be successfully received by B. The
-bidirectional link quality of an undirected node pair (A,B) is the 
-product of the link quality of (A,B) and (B,A). This definition
-assumes independent link losses. It also includes the case when
-the link quality of (A,B) and (B,A) are different; this can occur
-due to local interference or noise.
-
-2.2 In-bound Link Quality
+2.1 In-bound Link Quality
 --------------------------------------------------------------------
 
 In a node pair (A,B), with B as the node of reference, in-bound link
 quality is a value in the range of 0 to 255 that describes the quality
-of the link from A to B estimated by B by counting the successfully
-received packets from A among all the transmitted packets or using
-link quality indicators such as LQI and RSSI provided by the radio on
-the node B, or some other methods.
-
-2.3 Out-bound Link Quality
+of the link from A to B estimated by B as the ratio of successfully
+received to all packets transmitted by A. Thus, in-bound link quality
+is the empirical probability that a packet will be successfully
+received on a given link. A value of 255 represents a probability of 1
+and a value of 0 represents a probability of 0 of successfully
+receiving a packet on a given link.
+
+2.2 Out-bound Link Quality
 --------------------------------------------------------------------
 
 In a node pair (A,B), with B as the node of reference, out-bound link
@@ -72,6 +69,17 @@ determine the out-bound link quality if A advertises its in-bound link
 qualities. LEEP is the protocol that is used to exchange the in-bound
 link qualities.
 
+2.3 Bi-directional Link Quality
+--------------------------------------------------------------------
+
+LEEP does not define or compute bi-directional link quality. LEEP
+provides a way to exchange sufficient information to compute in-bound
+and out-bound link qualities. These two link qualities can be used to
+compute the bi-directional link quality. Routing protocols often
+compute the bi-directional link quality of a node pair (A,B) as a
+function (product, min, etc.) of the link quality of (A,B) and (B,A).
+
+
 2.4 Link Information Entry
 --------------------------------------------------------------------
 
@@ -103,7 +111,17 @@ link quality from the transmitter. LEEP MUST transmit Link Information
 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
 --------------------------------------------------------------------
@@ -179,6 +197,10 @@ the entries to be exchanged that could not fit in the previous LEEP
 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/4bitle``. This implementation is described in
+detail in [2]_.
+
 5. Author's Address
 ============================================================================
 
@@ -195,3 +217,6 @@ sent as a LEEP payload, are sent.
 ============================================================================
 
 .. [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.