phils comments

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

diff --git a/doc/txt/tep124.txt b/doc/txt/tep124.txt
index ab5c8c595370548d1b357248212ac56717ee54d3..24024230397c4ff8ec6e9cb949cbca9a4503d11a 100644 (file)
--- a/doc/txt/tep124.txt
+++ b/doc/txt/tep124.txt
@@ -34,10 +34,16 @@ 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. These in-bound link
-qualities are exchanged among the neighbors using LEEP to determine
-the out-bound link qualities. Bi-directional link quality is computed
-using thus learned in-bound and out-bound link qualities.
+messages and 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. 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
 ============================================================================
@@ -45,11 +51,13 @@ using thus learned in-bound and out-bound link qualities.
 2.1 Link Quality
 --------------------------------------------------------------------
 
-Quality of the link between any two nodes describes the probability
-with which data link layer packets can be received without any
-error. This definition assumes uncorrelated link losses. The link
-between the nodes A and B may have different quality in A to B and B
-to A directions.
+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
 --------------------------------------------------------------------
@@ -101,7 +109,17 @@ 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
 --------------------------------------------------------------------
@@ -126,10 +144,10 @@ Information entry.
 The following diagram shows the LEEP header format::
 
                            1
-       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
-      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-      |nentry | rsrvd |      seqno      |
-      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+      |nentry | rsrvd |     seqno     |
+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
 
 Field definitions:
@@ -145,10 +163,10 @@ Field definitions:
 The following diagram shows the Link Information Entry format::
 
                            1
-       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
-      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-      |             node id             |
-      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+      |            node id            |
+      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | link quality  |
       +-+-+-+-+-+-+-+-+
 
@@ -177,6 +195,10 @@ 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/le``. This implementation is described in
+detail in [2]_.
+
 5. Author's Address
 ============================================================================
 
@@ -193,3 +215,6 @@ 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.