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. Received Signal Weakness Metric
• 3. Units for RSW metric
• 4. Cost() and Loop_Free() functions for the RSW metric
• 5. Security Considerations
• 6. IANA Considerations
• 7. Informative References
• Author's Address

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.

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:

• 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

Author's Address