[ {"bn": "urn:dev:ow:10e2073a01080063",
      "bt": 1320067464,
      “bto”: 10,
      "bu": "%RH",
      "v": 21.2},
      { "v": 21.3},
      { "v": 21.4},
      { "v": 21.4},
      { "v": 21.5},
      { "v": 21.5},
      { "v": 21.5},
      { "v": 21.6},
      { "v": 21.7},
      { "v": 21.5},
   ... 

Figure 2: SenML Package with multiple records using base time offset

It can be seen that it results in a record and overall pack size reduction. The receiver of the record would use the base time “bt” for the initial data point, e.g. “v”: 21.2 would have a timestamp of 1320067464. Each subsequent record would have a time stamp equal to the previous record plus the base time offset “bto”, e.g. “v”: 21.3 would have a timestamp of 1320067474, “v”: 21.4 would have a timestamp of 1320067484, and so on.

2. Terminology

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119].

See [I-D.ietf-core-senml] for further definitions.

3. SenML Structure and Semantics

3.1. Base Attributes

This document adds an additional “base time offset” attribute.

Base Time Offset: The base time offset represents a fixed time offset between each record in a SenML pack. The first record represents time zero (or the base time if provided) and the offset is then applied to each subsequent record. Either a positive or negative base time offset is allowed.

3.2. Regular Attributes

This document modifies the behaviour of the time attribute.

Time: If the base time offset attribute is used in a SenML pack then the time attribute shall not be used in the individual records.

Editor’s note: One theoretical use case may be to include the time attribute if the entry’s time deviates from the regular offset. It is not seen that such a corner case warrants the extra complexity.

3.3. Considerations

To simplify implementation, the use of base time offset is limited to:

• Fixed time increasing or decreasing records from the time stamp associated with the first record in the SenML pack. This means that usages such as described by the 2nd example in 5.1.2/[I-D.ietf-core-senml] where negative time offset from time zero being provided by the last record in a SenML pack are not possible.

Editor’s note: It could be possible to facilitate this use case by allowing a t=0 to be set on the last record with the use of a negative base time offset value. However again it’s not seen that such a corner case warrants the extra complexity in having to store values and calculate offsets at the end of transmission/reception.

• It is not possible to for two records in the SenML pack to have the same time. For example the inclusion of a record for a voltage measurement followed by a current measurement would result in the records having times with a difference of the time offset.

4. JSON Representation (application/senml+json)

This document defines an additional SenML label (JSON object member name) as shown in Table 1 below.

  senml = element senml {
     attribute bn { xsd:string }?,
     attribute bt { xsd:double }?,
     attribute bv { xsd:double }?,
     attribute bs { xsd:double }?,
     attribute bu { xsd:string }?,
     attribute bver { xsd:int }?,
     attribute bto { xsd:double }?,

     attribute l { xsd:string }?,

     attribute n { xsd:string }?,
     attribute s { xsd:double }?,
     attribute t { xsd:double }?,
     attribute u { xsd:string }?,
     attribute ut { xsd:double }?,

     attribute v { xsd:double }?,
     attribute vb { xsd:boolean }?,
     attribute vs { xsd:string }?,
     attribute vd { xsd:string }?
   }

   sensml =
     element sensml {
       senml+
   }

   start = sensml

<?xml version="1.0" encoding="utf-8"?>
   <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
   elementFormDefault="qualified"
   targetNamespace="urn:ietf:params:xml:ns:senml"
   xmlns:ns1="urn:ietf:params:xml:ns:senml">
     <xs:element name="senml">
       <xs:complexType>
         <xs:attribute name="bn" type="xs:string" />
         <xs:attribute name="bt" type="xs:double" />
         <xs:attribute name="bv" type="xs:double" />
         <xs:attribute name="bu" type="xs:string" />
         <xs:attribute name="bver" type="xs:int" />
         <xs:attribute name="bto" type="xs:double" />
         <xs:attribute name="l" type="xs:string" />
         <xs:attribute name="n" type="xs:string" />
         <xs:attribute name="s" type="xs:double" />
         <xs:attribute name="t" type="xs:double" />
         <xs:attribute name="u" type="xs:string" />
         <xs:attribute name="ut" type="xs:double" />
         <xs:attribute name="v" type="xs:double" />
         <xs:attribute name="vb" type="xs:boolean" />
         <xs:attribute name="vs" type="xs:string" />
         <xs:attribute name="vd" type="xs:string" />
       </xs:complexType>
     </xs:element>
     <xs:element name="sensml">
       <xs:complexType>
         <xs:sequence>
           <xs:element maxOccurs="unbounded" ref="ns1:senml" />
         </xs:sequence>
       </xs:complexType>
     </xs:element>
   </xs:schema>