Internet DRAFT - draft-perkins-manet-rsw
draft-perkins-manet-rsw
Mobile Ad Hoc Networks [manet] C. Perkins
Internet-Draft Futurewei
Intended status: Standards Track April 18, 2016
Expires: October 20, 2016
Received Signal Weakness (RSW) Metric
draft-perkins-manet-rsw-00.txt
Abstract
The Received Signal Weakness (RSW) metric is a simple cost metric
that enables selection of a route with the high end-to-end signal
strength.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Received Signal Weakness Metric . . . . . . . . . . . . . . . 2
3. Units for RSW metric . . . . . . . . . . . . . . . . . . . . 3
4. Cost() and Loop_Free() functions for the RSW metric . . . . . 3
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
7. Informative References . . . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4
1. Introduction
It is often desirable to identify which of several available routes
offers the best signal strength for data transmission, de-emphasizing
other considerations such as number of hops. However, signal
strength is in certain ways less suitable for use as a routing
metric; in particular, the signal strength of a path with several
hops is not as easy to calculate as cost metrics such as hop count.
Instead of signal strength, we calculate a metric proportional to the
weakness of the signal, in order to obtain a cost metric. The route
having the links with the best signal strength is then chosen in
preference to other routes, by choosing the route presenting the
lowest cost as measured by the Received Signal Weakness (RSW) metric.
The total signal weakness cost for a route is the sum of the signal
weakness measurements at each hop, so that the RSW cost metric is
additive, monotonic, and easy to calculate.
2. Received Signal Weakness Metric
The received signal strength for packets received from a neighbor is
an important factor relevant to the reliability of the link between
the receiving node and its neighbor. Notice that the received signal
strength can vary over time even if the neighboring devices are not
moving.
For a route R as follows composed of links between nodes N_1 ... N_k:
N_1 <--> N_2 <--> N_3 <--> .... <--> N_k
denote the link between N_{i} and N_{i+1} by L_{i,i+1} and the
received signal weakness over link L_{i,i+1} by RSW_{i,i+1}. The RSW
cost for route R is the sum of the RSW costs for each link, or in
other words M_rsw(R) = SUM M_rsw(L_{i,i+1}) [i == 1..k-1], where
M_rsw is the metric value for the RSW metric.
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3. Units for RSW metric
The received power as measured (say, in mW) for incoming packets may
have quite a large dynamic range, but the measurements are also quite
variable and so great precision is unlikely to be required. In order
to fit in eight bits, the received power measurement is normalized to
be within the range from 0 to 1, where the minimum measurable power
P_min maps to 1 (the highest cost value) and the maximum measurable
power P_max maps to 0 (the lowest cost value). In other words, the
measured received power P_meas maps to a normalized value P_norm =
(P_max - P_meas) / (P_max - P_min).
It is desirable to increase the cost of low signal strength so that
weak signals are strongly disfavored. For this purpose, P_norm,
which is a positive number no greater than 1, can be exponentiated.
Using RSW_exponent = (1/8) is proposed for this purpose, and
effectively reduces the cost associated with using links that have
good measured values for the received signal strength.
For the purposes of this initial draft, it is proposed to use a
precision that can be carried in an 8 bit metric. That would allow
Max_RSW to attain the value 255, but that value should be reserved to
indicate a route cost of "infinity"; i.e., the route cost is too
large to be represented. For that reason, Max_RSW is defined to be
254. In addition, we define Min_RSW to be 1, so that there is some
nonzero RSW cost for every link even if the measurement of the
received signal strength is the same as P_min. These definitions of
Max_RSW and Min_RSW determine the scaling factor for P_norm, namely
(Max_RSW - Min_RSW).
Given the scaling factor and shaping function P_norm^RSW_exponent as
above, the RSW metric is defined as M_rsw = floor((Max_RSW-Min_RSW) *
(P_norm^RSW_exponent)) + MinRSW
4. Cost() and Loop_Free() functions for the RSW metric
To be useful with AODVv2 [I-D.ietf-manet-aodvv2], it is helpful to
define functions Cost() and Loop_Free() for the RSW metric. The
purpose of the Loop_Free() function is to provide assurance that a
selected route is loop-free.
The definition of the Cost() function for RSW is exactly the same as
the RSW metric, M_rsw. In other words, using RSW, Cost(L) = M_rsw(L)
and Cost(R) = M_rsw(R) for a link L and a route R.
For routes R1 and R2, Loop_Free(R1, R2) for RSW is defined as
follows:
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LoopFree(R1,R2) := M_rsw(R1) < M_rsw(R2)
or, in other words, LoopFree(R1,R2) returns TRUE if the cost of R1 is
less than the cost of R2 (cost as measured by the RSW metric).
5. Security Considerations
This document does not introduce any security mechanisms, and does
not have any impact on existing security mechanisms.
6. IANA Considerations
The routing metric defined in the document should be assigned a value
from the "AODVv2 Metric Types" registry [I-D.ietf-manet-aodvv2].
7. Informative References
[I-D.ietf-manet-aodvv2]
Perkins, C., Ratliff, S., Dowdell, J., Steenbrink, L., and
V. Mercieca, "Ad Hoc On-demand Distance Vector Version 2
(AODVv2) Routing", draft-ietf-manet-aodvv2-14 (work in
progress), April 2016.
[IEEE_L2R_RSW]
Perkins, C., "RSW for IEEE 802.15.10 Layer-2 Routing
(https://mentor.ieee.org/802.15/dcn/15/15-15-0925-03-0010-
received-signal-weakness-rsw-metric-specification.docx)",
2015.
Author's Address
Charles E. Perkins
Futurewei Inc.
2330 Central Expressway
Santa Clara, CA 95050
USA
Phone: +1-408-330-4586
Email: charliep@computer.org
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