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GEOPRIV                                                       M. Thomson
Internet-Draft                                           J. Winterbottom
Intended status: Standards Track                      Andrew Corporation
Expires: December 30, 2011                                 June 28, 2011

     Specifying Location Quality Requirements in Location Protocols


   Parameters that define the expected quality of location information
   are defined for use in location protocols.  These parameter can be
   used by a requester to indicate to a Location Server quality
   requirements for the location information that is requested.  The
   Location Server is able to use this information to control how
   location information is determined.  An optional indication of
   whether the quality requirements were met is defined to be provided
   by the Location Server alongside location information.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 30, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must

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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions used in this document  . . . . . . . . . . . .  4
   2.  Location Quality Operation . . . . . . . . . . . . . . . . . .  4
   3.  Location Quality Objects . . . . . . . . . . . . . . . . . . .  6
     3.1.  Location Quality Request . . . . . . . . . . . . . . . . .  6
       3.1.1.  Strict Quality Constraints . . . . . . . . . . . . . .  6
       3.1.2.  Maximum Uncertainty  . . . . . . . . . . . . . . . . .  6
       3.1.3.  Required Civic Elements  . . . . . . . . . . . . . . .  8
       3.1.4.  Maximum Age  . . . . . . . . . . . . . . . . . . . . .  9
     3.2.  Location Quality Indication  . . . . . . . . . . . . . . .  9
   4.  Location Quality Schema  . . . . . . . . . . . . . . . . . . . 10
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
     6.1.  URN Sub-Namespace Registration for
           urn:ietf:params:xml:ns:geopriv:lq  . . . . . . . . . . . . 13
     6.2.  XML Schema Registration for Location Quality Schema  . . . 13
     6.3.  Registration of HELD 'lowQuality' Error Code . . . . . . . 14
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 14
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15

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1.  Introduction

   Location determination methods produce results of varying accuracy.
   In general, the accuracy of location information increases as the
   effort expended in generating the information increases.  Accuracy is
   the primary aspect of the quality of location information that is
   relevant to a Location Recipient (LR).  Other aspects of quality can
   also be significant, such as the currency of the data.

   Means for expressing the quality of location information is outlined
   in [RFC5491] and [I-D.thomson-geopriv-uncertainty].  An entity
   requesting location information of a Location Server (LS) is unable
   to specify the quality of the location that it ultimately receives.
   This is inefficient because an LS either provides location
   information that is inadequate for the intended task; or the LS could
   waste resources generating location information that is of
   eccessively high quality.

   This document defines XML elements that can be added to any protocol
   that provides location information.  These elements provide the
   ability to communicate location quality requirements to a Location
   Server.  These requirements specify a desired uncertainty at a
   certain confidence, plus the maximum acceptable age where location
   information is stored.  Guidelines for deterministically evaluating
   location information against these rules are provided.

   Location quality requirements provide information that a LS is able
   to use in deciding how to generate location information, if the LS
   has that capacity, directly or otherwise.

   This document provides semantics, examples and security
   considerations for the HELD protocol [RFC5985] and the SIP presence
   event package [RFC3856].  The parameters and procedures described in
   this document are applicable to HELD when used either as a location
   configuration protocol (LCP) [RFC5687] or as a location dereference
   protocol [RFC5808].  Application of the parameters described in this
   document to other protocols is not described, but is relatively
   trivial for protocols that are able to convey XML.

   Specifying location quality requirements ensures that a Location
   Receipient (LR) receives information that is suited to their needs.
   It also provides information that any Location Generator (LG) can use
   to better decide how location information is generated.  This
   provides advantages to both requester and source of the information.
   In one example, a LS might be able to meet quality constraints more
   quickly than allowed for (for instance, using the HELD "responseTime"

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   This document also defines an object that can be used by the LS to
   indicate if the location quality meets the requirements.  These
   parameters can be used by a location recipient to ensure that the
   location is of adequate quality without requiring specific checking
   without having to examine the location object.  Response parameters
   are an optional optimisation that also indicates that the LS has
   understood the location quality requirements.

1.1.  Conventions used in this document

   Terms and procedures relating to uncertainty and confidence are taken
   from [I-D.thomson-geopriv-uncertainty].  Familiarity with terminology
   outlined in [RFC5687] and [I-D.ietf-geopriv-arch] is also assumed.

   The term Location Server (LS) is used as a generic label, since these
   paramters apply in all cases where location information is served to
   a requesting entity.  From the perspective of this document, the LS
   could be a Location Information Server (LIS).  Similarly, the term
   Location Recipient (LR) is used to refer to the requester of location
   information, which could be a Device or Target for HELD.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Location Quality Operation

   Location quality parameters are provided by a location recipient when
   it requests location information.  These parameters identify a
   minimum quality for parameters.

   Figure 1 shows an example HELD message where a Device requests
   location information of a specified quality.

     <locationRequest xmlns="urn:ietf:params:xml:ns:geopriv:held">
       <locationType exact="true">geodetic</locationType>
       <quality xmlns="urn:ietf:params:xml:ns:geopriv:lq"
         <maxUncertainty confidence="95">

                  Figure 1: Example HELD Location Request

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   A presence application that uses event notification filtering
   [RFC4660] and the XML format for expressing event notification
   filters [RFC4661] can include this element in the "what" element of
   the filter document.  A SIP presence application might include this
   in a filter document as shown in Figure 2.

       <filter id="123" uri="sip:presentity@example.com">
           <lq:quality strict="false">
             <lq:requiredCivic>ca:RD ca:HNO</lq:requiredCivic>

                     Figure 2: Example Filter Document

   An LS that supports the location quality element uses the information
   contained in the request to choose how it serves the query.  If the
   LS sources the information from an LG, this information might be
   passed to the LG to determine how it generates the information.

   The response to a request for location of a particular quality MAY
   contain a quality indicator element that includes a list of the
   quality requirements that were understood and met.  Figure 3 shows a
   HELD location response that includes a quality indicator.

     <locationResponse xmlns="urn:ietf:params:xml:ns:geopriv:held">
       <presence xmlns="urn:ietf:params:xml:ns:pidf"
         <!-- Actual location information omitted -->
       <qualityInd xmlns="urn:ietf:params:xml:ns:geopriv:lq">
         maxUncertainty/vertical maxAge

                 Figure 3: Example HELD Location Response

   An LS provides an indication of the requirements that have been met.
   The actual quality of the location estimate SHOULD be included in the
   actual PIDF-LO document, expressed in the uncertainty inherent in the
   location information and tuple timestamp.

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3.  Location Quality Objects

   This section defines the format and semantics of the location quality
   parameters for requests and the indication that is included with

3.1.  Location Quality Request

   The "quality" element is included in a HELD request to indicate the
   requirements set by the Location Recipient (LR) on the quality of
   returned location information.  This document defines three elements
   that are included.

   Extensions to this specification can specify XML elements that are
   included as children of the "quality" element.  Elements that are not
   understood MUST be ignored.

3.1.1.  Strict Quality Constraints

   The "strict" attribute of the "quality" element contains a Boolean
   value that indicates if the constraints are to be followed strictly.
   If the "strict" attribute is present and set to "true", the LS is
   requested to respond with an error code when it is unable to provide
   location information of the requested quality.

   A HELD error code, "lowQuality", is registered in Section 6.3.  A
   code of "lowQuality" indicates that the requested quality couldn't be
   provided.  The example in Figure 4 shows how the "lowQuality" error
   code might be used.

     <error xmlns="urn:ietf:params:xml:ns:geopriv:held"
       <message xml:lang="en">
         Could not provide location of requested quality.</message>

                           Figure 4: HELD Error

   The "strict" attribute defaults to "false".  This indicates that the
   LS provides location information even when the quality constraints
   aren't met.

3.1.2.  Maximum Uncertainty

   The "maxUncertainty" element describes an upper limit on uncertainty
   at a given confidence.  Uncertainty is divided in to horizontal and

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   vertical components.  Horizontal uncertainty is the maximum distance
   from the centroid of the area to the point in the shape furthest from
   the centroid on the plane of the horizontal at the centroid.
   Vertical uncertainty is the absolute difference in altitude from the
   centroid to the point in the shape with the greatest or least

   The "horizontal" and "vertical" elements are numerical values that
   contain a decimal value in metres.  Maximum uncertainty values MUST
   be greater than zero.

   A location estimate that does not contain uncertainty (i.e. a Point
   shape), never meets location quality requirements.  Where uncertainty
   is unknown, it is assumed to be arbitrarily large for any non-zero
   confidence.  In particular, this applies to vertical uncertainty
   where the location estimate is two-dimensional only; location
   estimates without a vertical component of uncertainty never meet
   vertical uncertainty requirements.

   Note:  An LS MAY provide location information using the Point shape
      and indicate that the requested uncertainty is met, if the LS has
      access to uncertainty information and is prevented from sharing
      this information due to policy constraints.  An LS SHOULD NOT omit
      uncertainty in this fashion, because the LR has no way of
      independently verifying that the uncertainty meets their

   The "confidence" attribute of this element includes the confidence
   level (expressed as a percentage) that the uncertainty is evaluated
   at.  Desired confidence has a default value of 95.  The definition of
   this attribute is taken from [I-D.thomson-geopriv-confidence].

   To evaluate uncertainty, the location estimate is first scaled so
   that the confidence of the estimate matches or exceeds the requested
   confidence.  The LS SHOULD convert the shape of the uncertainty to a
   circle or a sphere prior to scaling to simply the scaling process.
   For consistency - and contrary to the rules in
   [I-D.thomson-geopriv-uncertainty] - it is RECOMMENDED that a normal
   PDF be assumed for all location information except where confidence
   is reduced for a rectangular PDF.  Other scaling methods MAY be
   applied where better information about the distribution is known.

   Horizontal uncertainty is evalulated by removing the altitude and
   altitude uncertainty components from the location estimate.  While
   removing altitude components from a location estimate might normally
   increase confidence, confidence MUST NOT be increased at this step;
   the confidence value has already been considered.  The shape is then
   converted to a circle, if it is not already in that shape.  The

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   radius of the resulting circle is compared to the maximum horizontal

   Vertical uncertainty is evaluated for shapes that contain altitude
   uncertainty.  The value used for evaluating vertical uncertainty
   depends on the shape type: the vertical axis value for the Ellipsoid
   shape; the radius of the Sphere shape; half the height of the Prism
   shape.  A constraint on vertical uncertainty cannot be met if
   vertical uncertainty is not known.

   The LS MAY use location quality parameters to alter the way that it
   generates location information and to provide location information
   that more closely matches what is requested.  If maximum value is
   provided for vertical uncertainty, the LS SHOULD provide a location
   estimate that includes altitude and altitude uncertainty if possible.
   The LS SHOULD provide location information with the confidence
   included in the request, if scaling is possible.  Scaling MAY be
   avoided if the location information is significantly degraded by the
   scaling process.

3.1.3.  Required Civic Elements

   The "requiredCivic" element represents the requirements of an LR for
   civic address information.  An LR can specify the address elements
   that need to be present in the civic address in order for the
   location information to meet their quality requirements.

   The "requiredCivic" element contains a whitespace-separated list of
   element names.  These can be interpreted as XPath
   [W3C.REC-xpath20-20070123] expressions that are evaluated in the
   context of the "civicAddress" element [RFC5139].  These XPath
   statements are restricted to use of qualified names only (using the
   response document namespace context) and the "/" separator; that is,
   the only permitted axis is the "child::" axis.  All child nodes of
   elements (including attributes and textual content) are treated as
   belonging to an element.

   Figure 5 shows an example request where an LR requires country, state
   (or equivalent) and post code civic address elements in the location
   information provided by the LS.

     <quality xmlns="urn:ietf:params:xml:ns:geopriv:lq">
         ca:country ca:A1 ca:PC

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        Figure 5: Example Specifying Required Civic Address Fields

   This does not force the LS to limit the civic address fields provided
   to just those requested.  Any additional address fields that are
   known can be provided as long as policy permits their inclusion.

3.1.4.  Maximum Age

   Where location information is stored or cached, an LR can specify a
   limit on the age of this information.  This is particularly important
   if location information is generated in advance.  The "age" of
   location information is indicated by the the "timestamp" element in
   the PIDF tuple.  The age parameter specifies the minimum value for
   this field; that is, the oldest location information that is

   Location information that has greater age than requested SHOULD be
   determined anew.  A value of "now" can be used to indicate that
   stored location information of any age is not acceptable to the LR.

   Age is calculated from the time that the LS receives a request.
   Location information generated after this time has an effective age
   that is less than zero.  The age of location information generated
   after the request is received is always acceptable.

3.2.  Location Quality Indication

   The "qualityInd" element is used in responses to indicate which of
   the location quality requirements from a request were met.  The
   presence of this element indicates that a request for a given
   location quality was understood and lists the quality requirements
   that the accompanying location information meets.

   The list of requirements is represented as a whitespace-separated
   list of element names.  These can be interpreted as XPath
   [W3C.REC-xpath20-20070123] expressions that are evaluated in the
   context of the original location quality request.  These statements
   follow the same constraints as the list of elements in Section 3.1.3.

   Where elements are nested, such as the "maxUncertainty" element, the
   outer element can be included to indicate an entire constraint is
   met; or, each individual child element can be identified.  Two
   quality indications that are roughly equivalent are shown in
   Figure 6.

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       <qualityInd xmlns="urn:ietf:params:xml:ns:geopriv:lq">

       <qualityInd xmlns="urn:ietf:params:xml:ns:geopriv:lq">
         maxUncertainty/horizontal maxUncertainty/vertical

                   Figure 6: Similar Quality Indications

   A LS that is unable to determine if a constraint is met for any
   reason MUST NOT list that constraint in this element.  This includes
   the case where the constraint is not supported by the LS.  This list
   MAY be empty if none of the requested quality requirements could be

   The special value "##all" indicates that all quality requirements
   were met.  A value of "##all" cannot be used if there are unknown or
   unsupported elements in the quality request.

4.  Location Quality Schema

   Note that the pattern rules in the following schema wrap due to
   length constraints in RFC documents.  None of the patterns contain

   <?xml version="1.0"?>

         HELD Location Quality
       <xs:documentation source="http://www.ietf.org/rfc/rfcXXXX.txt">
         <!-- [[NOTE TO RFC-EDITOR: Please replace above URL with URL of
              published RFC and remove this note.]] -->
         This schema defines a framework for location quality requests
         and indications of whether they are met.

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     <xs:import namespace="urn:ietf:params:xml:ns:geopriv:conf"/>

     <xs:element name="quality" type="lq:qualityType"/>
     <xs:complexType name="qualityType">
         <xs:restriction base="xs:anyType">
             <xs:element ref="lq:maxUncertainty" minOccurs="0"/>
             <xs:element ref="lq:requiredCivic" minOccurs="0"/>
             <xs:element ref="lq:maxAge" minOccurs="0"/>
             <xs:any namespace="##other" processContents="lax"
                     minOccurs="0" maxOccurs="unbounded"/>
           <xs:attribute name="strict" type="xs:boolean"
           <xs:anyAttribute namespace="##any" processContents="lax"/>

     <xs:element name="maxUncertainty" type="lq:maxUncertaintyType"/>
     <xs:complexType name="maxUncertaintyType">
         <xs:restriction base="xs:anyType">
             <xs:element name="horizontal" type="lq:limitType"/>
             <xs:element name="vertical" type="lq:limitType"/>
           <xs:attribute name="confidence" type="conf:confidenceBase"

     <xs:simpleType name="limitType">
       <xs:restriction base="xs:decimal">
         <xs:minExclusive value="0.0"/>

     <xs:element name="maxAge" type="lq:maxAgeType"/>
     <xs:simpleType name="maxAgeType">
       <xs:union memberTypes="xs:dateTime">
           <xs:restriction base="xs:token">
             <xs:enumeration value="now"/>

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     <xs:element name="requiredCivic" type="lq:childOnlyXPathList"/>

     <xs:element name="qualityInd" type="lq:qualityIndType"/>
     <xs:simpleType name="qualityIndType">
       <xs:union memberTypes="lq:childOnlyXPathList">
           <xs:restriction base="xs:token">
             <xs:enumeration value="##all"/>
             <xs:enumeration value="##none"/>

     <xs:simpleType name="childOnlyXPathList">
       <xs:list itemType="lq:childOnlyXPath"/>
     <xs:simpleType name="childOnlyXPath">
       <xs:union memberTypes="xs:QName lq:childOnlyXPath2"/>
     <xs:simpleType name="childOnlyXPath2">
       <xs:restriction base="xs:token">
         <xs:pattern value="(([\i-[:]][\c-[:]]*:)?[\i-[:]][\c-[:]]*/)+


5.  Security Considerations

   Location information might be cached by a location server to
   alleviate load on location generation functions.  A location server
   might provide a means for an attacker to increase the location
   generation load by forcibly circumventing the cache using the
   "maxAge" quality constraint.  Where caching is used to reduce
   location generation load, a location server needs mechanisms to
   control how caching is bypassed.

   An entity that is concerned about the privacy implications of making
   its preferences known can choose not to specify location quality

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6.  IANA Considerations

   This section registers a schema for the location quality objects and
   a HELD error code.

6.1.  URN Sub-Namespace Registration for

   This section registers a new XML namespace,
   "urn:ietf:params:xml:ns:geopriv:lq", as per the guidelines in

      URI: urn:ietf:params:xml:ns:geopriv:lq

      Registrant Contact: IETF, GEOPRIV working group,
      (geopriv@ietf.org), Martin Thomson (martin.thomson@andrew.com).


           <?xml version="1.0"?>
           <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
           <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
               <title>Location Quality</title>
               <h1>Namespace for Location Quality</h1>
   [[NOTE TO IANA/RFC-EDITOR: Please update RFC URL and replace XXXX
       with the RFC number for this specification.]]
               <p>See <a href="[[RFC URL]]">RFCXXXX</a>.</p>

6.2.  XML Schema Registration for Location Quality Schema

   This section registers an XML schema as per the guidelines in

   URI:  urn:ietf:params:xml:schema:geopriv:lq

   Registrant Contact:  IETF, GEOPRIV working group, (geopriv@ietf.org),
      Martin Thomson (martin.thomson@andrew.com).

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   Schema:  The XML for this schema can be found in Section 4 of this

6.3.  Registration of HELD 'lowQuality' Error Code

   This section registers the "lowQuality" error code in the "Geopriv
   HELD Registries, Error codes for HELD" IANA registry.

   lowQuality:  This error code indicates that the location information
      that was available did not meet the strict quality constraints
      specified in the request.

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.

   [RFC3856]  Rosenberg, J., "A Presence Event Package for the Session
              Initiation Protocol (SIP)", RFC 3856, August 2004.

   [RFC4661]  Khartabil, H., Leppanen, E., Lonnfors, M., and J. Costa-
              Requena, "An Extensible Markup Language (XML)-Based Format
              for Event Notification Filtering", RFC 4661,
              September 2006.

   [RFC5139]  Thomson, M. and J. Winterbottom, "Revised Civic Location
              Format for Presence Information Data Format Location
              Object (PIDF-LO)", RFC 5139, February 2008.

   [RFC5491]  Winterbottom, J., Thomson, M., and H. Tschofenig, "GEOPRIV
              Presence Information Data Format Location Object (PIDF-LO)
              Usage Clarification, Considerations, and Recommendations",
              RFC 5491, March 2009.

   [RFC5985]  Barnes, M., "HTTP-Enabled Location Delivery (HELD)",
              RFC 5985, September 2010.

              Thomson, M. and J. Winterbottom, "Representation of
              Uncertainty and Confidence in PIDF-LO",
              draft-thomson-geopriv-uncertainty-06 (work in progress),
              March 2011.

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              Thomson, M., "Expressing Confidence in a Location Object",
              draft-thomson-geopriv-confidence-03 (work in progress),
              October 2010.

7.2.  Informative References

   [RFC4660]  Khartabil, H., Leppanen, E., Lonnfors, M., and J. Costa-
              Requena, "Functional Description of Event Notification
              Filtering", RFC 4660, September 2006.

   [RFC5687]  Tschofenig, H. and H. Schulzrinne, "GEOPRIV Layer 7
              Location Configuration Protocol: Problem Statement and
              Requirements", RFC 5687, March 2010.

   [RFC5808]  Marshall, R., "Requirements for a Location-by-Reference
              Mechanism", RFC 5808, May 2010.

              Barnes, R., Lepinski, M., Cooper, A., Morris, J.,
              Tschofenig, H., and H. Schulzrinne, "An Architecture for
              Location and Location Privacy in Internet Applications",
              draft-ietf-geopriv-arch-03 (work in progress),
              October 2010.

              Fernandez, M., Berglund, A., Simeon, J., Chamberlin, D.,
              Boag, S., Robie, J., and M. Kay, "XML Path Language
              (XPath) 2.0", World Wide Web Consortium
              Recommendation REC-xpath20-20070123, January 2007,

Authors' Addresses

   Martin Thomson
   Andrew Corporation
   Andrew Building (39)
   Wollongong University Campus
   Northfields Avenue
   Wollongong, NSW  2522

   Email: martin.thomson@andrew.com

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   James Winterbottom
   Andrew Corporation
   Andrew Building (39)
   Wollongong University Campus
   Northfields Avenue
   Wollongong, NSW  2522

   Email: james.winterbottom@andrew.com

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