TSMP – Time Synchronized Mesh ProtocolIntroductionApproachFrequency DiversityTime synchronizationGraph routing – path redundancy, QoSMechanicsTime slotsCharge consumption of basic operationsLink layer acksTime SynchronizationClock skew and driftSynchronization traffic – keepalives and beaconsLinkChannel HoppingSuperframesGraphsSimilar to flow labelsPacket/graph priorityLink sharingAdvertising and joiningSecurityNetwork Keycrc doesn’t cut it for bit errorsPerformanceAnalysisCharge accountingExamplesLimitsw/ 10ms slotsultimate limits w/ 15.4 radiosComparisonVs. preamble samplingDiscussionGraph IDs and IPv6 Flow labelsDistributed routingBibliographyTSMP – Time Synchronized Mesh ProtocolIntroductionIn addition to the wide variety of research applications of wireless sensor networks(WSN), there is now a growing commercial application space. Commercial requirementsof the networking stack tend to be straightforward to state, although often difficult toachieve. Simply, the requirement is reliable, secure delivery of packets at low power in atimely manner. A survey of research applications shows that this is often the case in thatdomain as well [glaser].Diligent attention to these applications requirements led to the development of TSMP, theTime Synchronized Mesh Protocol. The success of this approach is evident in its recentadoption as the basis for the Wireless HART protocol, and for the ISA SP100 protocol,both intended for application in industrial automation. Industrial automation applicationsare extremely performance sensitive, and TSMP’s adoption in this regime came only afterextensive evaluation in hundreds of real-world deployments of periods of a several years.ApproachReliability in wireless systems is such a challenging problem that it is embedded in thepublic consciousness with advertising quotes such as “can you hear me now?”.Unfortunately, most wireless sensor motes cannot walk outdoors or move into a differentroom if they don’t have enough “bars”.Reliability in most WSN applications means the fraction of data transmitted (typicallymeasured in units of packets) that gets where it needs to go in the network within a givenlatency requirement. Requirements on latency vary widely, from milliseconds forFactory Automation, to days or months for some environmental monitoring applications.The longer end of this range generally collapses toward lower latency once it becomesclear that it is possible to have latencies of no more than tens of seconds for no highercost than longer latencies. Similarly, not all applications require 99.9% reliability, butmost would like it if they felt that it could be had at little or no cost above what otherprotocols provide.To maximize reliability, TSMP uses frequency diversity, time diversity, and spatialdiversity. To minimize power consumption, motes are time synchronized and mostbandwidth is dedicated, to minimize the power spent on idle listening, and the powerwasted on packet collisions. To provide different levels of quality of service (QoS),TSMP radio resources and packet flow are organized into independent graphs withsomething analogous to MPLS and IPv6 flow labels.Frequency Diversity802.15.4 radios provide 16 different channels in the 2.4GHz band. These channels usedirect sequence spread spectrum with a channel bandwidth of 2MHz. This provides somediversity against narrowband interference, but unfortunately 2MHz is just not enough of aspread to avoid deep fades due to multipath interference [Werb]. By adding channelhopping in addition to DSSS, TSMP realizes several advantages over single-channelprotocols:- Reliability- Longer effective range- More BW availableTime synchronizationAll motes in a TSMP network share the same sense of time, accurate to well under onemillisecond. This capability supports several application requirements:- Reliability Reliability requires channel hopping. Channel hopping requires timesynchronization.- Low power It is well known that having a shared sense of time between the twosides of a radio link allows them both to reduce their “on” time.- Sensor time-stamps There are several approaches to providing time stamps touser data even if the motes themselves are not synchronized. Having time-synchronized motes makes it trivial.Graph routing – path redundancy, QoSThere are often different classes of data flowing in WSNs. Examples include applicationdata, network performance, and configuration data. Users may have different prioritiesfor application data, which may be lower for regular reporting and higher for alarm data.These flows are typically going in more than one direction. Most data and alarms arelikely to flow to one or more gateways, whereas configuration information is likely toflow from gateways into the network. TSMP provides the flexibility to treat all of theseflows as equivalent, or to provide dedicated resources to some or all of them. Quality ofservice (QoS) on individual flows is directly related to power consumption.MechanicsThe basic operation of TSMP will be described here. Details of implementation andperformance will be given in the following sections. Unless otherwise indicated, thissection will assume 2.4GHz 802.15.4 radio PHY.Time slotsThe format of a timeslot is shown in figure xxx. Mote A is transmitting to mote B, withan optional initial clear channel assessment. Assuming that the motes are alreadysynchronized (discussed below), then B knows when to expect the first bit of thepreamble. With some model for worst-case clock skew between B’s clock and A’s clock,B turns on its receiver Tg seconds before the expected arrival of the first bit from A.Figure 1 Packet timing with a worst-case late transmitter. Block lengths are not to scale. Red indicates transmission, blue is reception, green is idle listening. For a perfectly synchronized transmitter and receiver, every block except “RX startup” and one “Tg” shift left by Tg. If B does not hear a preamble, then it turns off its receiver after the initial 2Tg idle listen. Assuming a valid preamble and start symbol are detected, the receiver verifies the 2 byteCRC and then 4 byte DLL MIC. If both of these are valid, then an ACK packet iscreated, MICed and