-============================
-Link Quality Exchange Protocol (LQEP)
-============================
+============================================================================
+The Link Estimation Exchange Protocol (LEEP)
+============================================================================
-:TEP: 114
+: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
Abstract
============================================================================
-The memo documents the Link Quality Exchange Protocol (LQEP). Nodes
-use LQEP to exchange information about the quality of links to the
-neighbors.
+The memo documents the Link Estimation Exchange Protocol (LEEP). Nodes
+use LEEP to estimate and exchange information about the quality of
+links to the neighbors.
1. Introduction
============================================================================
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 LQEP to determine
-the out-bound link qualities. Bi-directional link quality is computed
-using thus learned in-bound and out-bound link qualities.
+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. 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
---------------------------------------------------------------------
-
-Quality of the link between any two nodes describes the probability
-with which data link layer packets can be received without any
-error. The link between the nodes A and B may have different quality
-in A to B and B to A directions.
-
-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 defined as a value ranging from 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 (LQI,RSSI) provided by the radio on
-the node B, or some other methods.
-
-2.3 Out-bound Link Quality
+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 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
quality is defined as the quality of the link from B to A. B can
determine the out-bound link quality if A advertises its in-bound link
-qualities. LQEP is the protocol that is used to exchange the in-bound
+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
--------------------------------------------------------------------
-Link Information Entry is a tuple (n,q) where q is the in-bound link
-quality from the node n.
+Link Information Entry created by node k is a tuple (n,q) where q is
+the in-bound link quality from node n to k.
-2. Link Quality Exchange Protocol (LQEP)
+3. The Link Estimation Exchange Protocol (LEEP)
============================================================================
-2.1 Assumptions
+3.1 Assumptions
--------------------------------------------------------------------
-Following are the assumptions made by LQEP:
+Following are the assumptions made by LEEP:
-2.1.1. The data link frame has a single-hop source field.
-2.1.2. The data link layer provides a broadcast address.
-2.1.3. The data link layer provides the length of the LQEP frame.
+3.1.1. The data link frame has a single-hop source field.
+3.1.2. The data link layer provides a broadcast address.
+3.1.3. The data link layer provides the length of the LEEP frame.
-2.2 The Protocol
+3.2 The Protocol
--------------------------------------------------------------------
To compute the bi-directional link quality, in-bound link quality must
-be exchanged among the neighbors. LQEP maintains a sequence number
-that is incremented by one for each outgoing LQEP frame. LQEP frame
-transmits this sequence number and Link Information entries describing
-the in-bound link qualities for a subset of its neighbors. When a node
-receives the LQEP frame and finds its node id in one of the Link
-Information entries, the node MUST use the second element of that
-tuple as its out-bound link quality to the neighbor identified by the
-data link source address. The LQEP sequence number SHOULD be used to
-estimate the in-bound link quality.
-
-2.3 LQEP Frame
+be exchanged among the neighbors. LEEP maintains a sequence number
+that is incremented by one for each outgoing LEEP frame. The sequence
+number in the LEEP frame MUST be incremented by one even if the data
+link layer retransmits the LEEP frame. The LEEP sequence number MAY be
+used to count the number of missing packets to estimate the in-bound
+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. 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
--------------------------------------------------------------------
-A LQEP frame has a header, the payload, and a footer with the Link
-Information (LI) entries as shown in this diagram:
+A LEEP frame has a header, the payload, and a footer with the Link
+Information (LI) entries as shown in this diagram::
-------------------------------------------------------------
- | LQEP | Payload | LI Entry | LI Entry | ... | LI Entry |
+ | LEEP | Payload | LI Entry | LI Entry | ... | LI Entry |
| Header | | 1 | 2 | | n |
-------------------------------------------------------------
+The number of Link Information entries can be different in each
+outgoing LEEP frame. The number of Link Information entries MUST not
+increase the size of the LEEP frame beyond the maximum payload length
+allowed by the data link layer. A LEEP frame can have 0 Link
+Information entry.
-2.3.1 LQEP header
+3.3.1 LEEP header
+--------------------------------------------------------------------
+
+The following diagram shows the LEEP header format::
-The following diagram shows the LQEP 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:
-nentry - Number of Link Information entries in the footer
-seqno - LQEP sequence number.
-rsrvd - Reserved
+ * nentry - Number of Link Information entries in the footer
+ * seqno - LEEP sequence number.
+ * rsrvd - Reserved and must be set to 0.
-2.3.2 Link Information Entry
+3.3.2 Link Information Entry
+--------------------------------------------------------------------
+
+The following diagram shows the Link Information Entry format::
-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 |
+-+-+-+-+-+-+-+-+
Field definitions:
-node id: the link layer address of the neighbor
-link quality: The in-bound link quality from the node identified by node id
+ * node id: the link layer address of the neighbor
+ * link quality: The in-bound link quality from the node identified by node id
+ to the node that transmits this Link Information entry
-7. Implementation
+4. Implementation
============================================================================
-The following files in ``tinyos-2.x/tos/lib/net/le`` provide a reference
-implementation of LQEP described in this TEP.
+The following files in ``tinyos-2.x/tos/lib/net/le`` provide a
+reference implementation of LEEP described in this TEP.
+
+ * ``LinkEstimator.h`` and ``LinkEstimatorP.nc``
- * ``LinkEstimator.h`` and ``LinkEstimatorP``
+The reference implementation uses the LEEP sequence number to count
+the number of missing packets to estimate the in-bound link
+quality. The implementation tries to append Link Information entry for
+all the neighbors in its neighbor table by sending the largest
+possible data link frame. If there is still not enough room to fit all
+the Link Information entries, it uses a round-robin policy to select
+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.
-The reference implementation uses the LQEP sequence number to estimate
-the in-bound link quality. The implementation tries to append Link
-Information entry for all the neighbors in its neighbor table by
-sending the largest possible data link frame. If there is still not
-enough room to fit all the Link Information entries, it uses a
-round-robin policy to select the entries to be exchanged that could
-not fit in the previous LQEP frame.
+Another reference implementation resides in
+``tinyos-2.x/tos/lib/net/4bitle``. This implementation is described in
+detail in [2]_.
-8. Author's Address
+5. Author's Address
============================================================================
| Omprakash Gnawali
| email - gnawali@usc.edu
|
+6. Citations
+============================================================================
+
+.. [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.
projects / tinyos-2.x.git / blobdiff