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Perkins 3 Internet-Draft Futurewei 4 Intended status: Standards Track April 18, 2016 5 Expires: October 20, 2016 7 Received Signal Weakness (RSW) Metric 8 draft-perkins-manet-rsw-00.txt 10 Abstract 12 The Received Signal Weakness (RSW) metric is a simple cost metric 13 that enables selection of a route with the high end-to-end signal 14 strength. 16 Status of This Memo 18 This Internet-Draft is submitted in full conformance with the 19 provisions of BCP 78 and BCP 79. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF). Note that other groups may also distribute 23 working documents as Internet-Drafts. The list of current Internet- 24 Drafts is at http://datatracker.ietf.org/drafts/current/. 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 31 This Internet-Draft will expire on October 20, 2016. 33 Copyright Notice 35 Copyright (c) 2016 IETF Trust and the persons identified as the 36 document authors. All rights reserved. 38 This document is subject to BCP 78 and the IETF Trust's Legal 39 Provisions Relating to IETF Documents 40 (http://trustee.ietf.org/license-info) in effect on the date of 41 publication of this document. Please review these documents 42 carefully, as they describe your rights and restrictions with respect 43 to this document. Code Components extracted from this document must 44 include Simplified BSD License text as described in Section 4.e of 45 the Trust Legal Provisions and are provided without warranty as 46 described in the Simplified BSD License. 48 Table of Contents 50 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 51 2. Received Signal Weakness Metric . . . . . . . . . . . . . . . 2 52 3. Units for RSW metric . . . . . . . . . . . . . . . . . . . . 3 53 4. Cost() and Loop_Free() functions for the RSW metric . . . . . 3 54 5. Security Considerations . . . . . . . . . . . . . . . . . . . 4 55 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 56 7. Informative References . . . . . . . . . . . . . . . . . . . 4 57 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4 59 1. Introduction 61 It is often desirable to identify which of several available routes 62 offers the best signal strength for data transmission, de-emphasizing 63 other considerations such as number of hops. However, signal 64 strength is in certain ways less suitable for use as a routing 65 metric; in particular, the signal strength of a path with several 66 hops is not as easy to calculate as cost metrics such as hop count. 68 Instead of signal strength, we calculate a metric proportional to the 69 weakness of the signal, in order to obtain a cost metric. The route 70 having the links with the best signal strength is then chosen in 71 preference to other routes, by choosing the route presenting the 72 lowest cost as measured by the Received Signal Weakness (RSW) metric. 73 The total signal weakness cost for a route is the sum of the signal 74 weakness measurements at each hop, so that the RSW cost metric is 75 additive, monotonic, and easy to calculate. 77 2. Received Signal Weakness Metric 79 The received signal strength for packets received from a neighbor is 80 an important factor relevant to the reliability of the link between 81 the receiving node and its neighbor. Notice that the received signal 82 strength can vary over time even if the neighboring devices are not 83 moving. 85 For a route R as follows composed of links between nodes N_1 ... N_k: 87 N_1 <--> N_2 <--> N_3 <--> .... <--> N_k 89 denote the link between N_{i} and N_{i+1} by L_{i,i+1} and the 90 received signal weakness over link L_{i,i+1} by RSW_{i,i+1}. The RSW 91 cost for route R is the sum of the RSW costs for each link, or in 92 other words M_rsw(R) = SUM M_rsw(L_{i,i+1}) [i == 1..k-1], where 93 M_rsw is the metric value for the RSW metric. 95 3. Units for RSW metric 97 The received power as measured (say, in mW) for incoming packets may 98 have quite a large dynamic range, but the measurements are also quite 99 variable and so great precision is unlikely to be required. In order 100 to fit in eight bits, the received power measurement is normalized to 101 be within the range from 0 to 1, where the minimum measurable power 102 P_min maps to 1 (the highest cost value) and the maximum measurable 103 power P_max maps to 0 (the lowest cost value). In other words, the 104 measured received power P_meas maps to a normalized value P_norm = 105 (P_max - P_meas) / (P_max - P_min). 107 It is desirable to increase the cost of low signal strength so that 108 weak signals are strongly disfavored. For this purpose, P_norm, 109 which is a positive number no greater than 1, can be exponentiated. 110 Using RSW_exponent = (1/8) is proposed for this purpose, and 111 effectively reduces the cost associated with using links that have 112 good measured values for the received signal strength. 114 For the purposes of this initial draft, it is proposed to use a 115 precision that can be carried in an 8 bit metric. That would allow 116 Max_RSW to attain the value 255, but that value should be reserved to 117 indicate a route cost of "infinity"; i.e., the route cost is too 118 large to be represented. For that reason, Max_RSW is defined to be 119 254. In addition, we define Min_RSW to be 1, so that there is some 120 nonzero RSW cost for every link even if the measurement of the 121 received signal strength is the same as P_min. These definitions of 122 Max_RSW and Min_RSW determine the scaling factor for P_norm, namely 123 (Max_RSW - Min_RSW). 125 Given the scaling factor and shaping function P_norm^RSW_exponent as 126 above, the RSW metric is defined as M_rsw = floor((Max_RSW-Min_RSW) * 127 (P_norm^RSW_exponent)) + MinRSW 129 4. Cost() and Loop_Free() functions for the RSW metric 131 To be useful with AODVv2 [I-D.ietf-manet-aodvv2], it is helpful to 132 define functions Cost() and Loop_Free() for the RSW metric. The 133 purpose of the Loop_Free() function is to provide assurance that a 134 selected route is loop-free. 136 The definition of the Cost() function for RSW is exactly the same as 137 the RSW metric, M_rsw. In other words, using RSW, Cost(L) = M_rsw(L) 138 and Cost(R) = M_rsw(R) for a link L and a route R. 140 For routes R1 and R2, Loop_Free(R1, R2) for RSW is defined as 141 follows: 143 LoopFree(R1,R2) := M_rsw(R1) < M_rsw(R2) 145 or, in other words, LoopFree(R1,R2) returns TRUE if the cost of R1 is 146 less than the cost of R2 (cost as measured by the RSW metric). 148 5. Security Considerations 150 This document does not introduce any security mechanisms, and does 151 not have any impact on existing security mechanisms. 153 6. IANA Considerations 155 The routing metric defined in the document should be assigned a value 156 from the "AODVv2 Metric Types" registry [I-D.ietf-manet-aodvv2]. 158 7. Informative References 160 [I-D.ietf-manet-aodvv2] 161 Perkins, C., Ratliff, S., Dowdell, J., Steenbrink, L., and 162 V. Mercieca, "Ad Hoc On-demand Distance Vector Version 2 163 (AODVv2) Routing", draft-ietf-manet-aodvv2-14 (work in 164 progress), April 2016. 166 [IEEE_L2R_RSW] 167 Perkins, C., "RSW for IEEE 802.15.10 Layer-2 Routing 168 (https://mentor.ieee.org/802.15/dcn/15/15-15-0925-03-0010- 169 received-signal-weakness-rsw-metric-specification.docx)", 170 2015. 172 Author's Address 174 Charles E. Perkins 175 Futurewei Inc. 176 2330 Central Expressway 177 Santa Clara, CA 95050 178 USA 180 Phone: +1-408-330-4586 181 Email: charliep@computer.org