Chapter 11: Time Synchronization

Chapter 11: Time Synchronization

Time Synchronization What is time synchronization in sensor networks?
HH:MM:SS 12:01:10 12:01:10 12:01:10 Objective: Allow all sensor nodes to maintain the same time frame

Time Synchronization Temporal relations play an important role in sensor fusion  Which event happened first? Physical time is itself part of information Estimation of target position: Direction, Speed Providing fire breaking time Due to the clock drift, the local clock needs to be periodically synchronized to maintain an accurate global time

Time Synchronization Challenges
Why is it so difficult to synchronize the sensor nodes? Low-end clock crystals are used Clock drifting may be significant and clock jitters may occur often Communication links are noisy Some sensor nodes may become unsynchronized Node failures occur often Cannot depend on a single sensor node to be the master clock

Factors Influencing Time Synchronization
Temperature Temperature variations during day may cause the clock speed up or down (a few sec/day). Phase Noise Access fluctuation at the hardware interface, response variation of the operating system to interrupts, jitter in delay, etc. Frequency Noise The frequency spectrum of a crystal has large sidebands on adjacent frequencies.

Factors Influencing Time Synchronization
Asymmetric Delay The delay of a path may be different for each direction Clock Glitches Hardware or software anomalies may cause sudden jumps in time

Sources of Time Synchronization Error
Sending Time (Time spent at the sender to construct the message) Kernel protocol processing Variable delays caused by OS Transfer within the host to NIC Access Time (Delay caused to wait for access to the channel) Specific to MAC protocol

Sources of Time Synchronization Error
Propagation Time Can be neglected for air Important for underground, underwater Receiving Time Processing required for the antenna to receive the message from the channel and notify the processor of its arrival (A/D conversion) Common Denominator: non-deterministic!!!!

Further Difficulties Periodic message exchange is not guaranteed to occur among nodes Transmission delay between two nodes is hard to estimate

TERMINOLOGY FREQUENCY: The rate at which a clock progresses.
CLOCK OFFSET: Difference between the time reported by a clock and the real time. SKEW OF A CLOCK: Difference in the frequencies of the clock and the perfect clock. DRIFT (RATE) of a CLOCK: Second derivative of the clock value with respect to time.

Traditional Synchronization
Problem for SYNCH: Many sources of unknown, nondeterministic delays (send time, access time, propagation time and receive time) between sender and receiver Sender Receiver Send Time Receive Time At: t=1 Radio Radio Access Time Physical Media Propagation Time

Reference Broadcast Synchronization (RBS) J. Elson, et. al
Reference Broadcast Synchronization (RBS) J. Elson, et. al., “Fine-Grained Network Time Synchronization using Reference Broadcasts,” Proc. of the Fifth Symp. on Operating Systems Design and Implementation (OSDI 2002), Boston, December 2002. Does not try to sync Sender & Receiver Tries to sync receivers Sender Receiver Receiver Receive Time Radio Radio Radio I saw it at t=4 I saw it at t=5 Propagation Time Physical Media

TRADITIONAL SYNCH vs RBS
Radio Radio Sender Sender Receiver Receiver 1 Time Receiver 2 Critical Path Critical Path Traditional Critical Path: From the time the sender reads its clock, to when the receiver reads its clock Nondeterministic delay: Send time and Access time. Receiver time small. RBS: Only sensitive to the differences in receive time and propagation delay Critical path length is shortened to include only the time from the injection of the packet into the channel to the last clock read. (Send time and Access time are eliminated!)

Reference-Broadcast Synchronization (RBS)
Algorithm to Estimate the Phase Offset between the Clocks of Two Receivers A transmitter broadcasts a reference packet to two (or more) receivers Each receiver records the time at which the packet was received according to its local clock

Reference-Broadcast Synchronization (RBS)
3. The receivers exchange the observed times at which they received the packet. 4. The clock offset between two receivers is computed as the difference of the local times at which the receivers received the same message.

Observations about RBS
RBS removes SEND and ACCESS TIME errors Broadcast packet is used as a relative time reference

Observations about RBS
Each receiver synchronizing to a reference packet Ref. packet is injected into the channel at the same instant for all receivers Broadcast packet does not contain timestamp Almost any broadcast packet can be used, e.g ARP, RTS/CTS, route discovery packets, etc

Phase Offset Estimation
RBS can produce highly accurate results if Message reception by each receiver is high Each receiver can record its local clock reading as soon as the message is received.

Messages can be corrupted. Receiving node may not be able to record the time of the message arrival promptly (maybe busy with other computations) Solution: Use a sequence of reference messages from the same sender rather than a single message

Receiver i will compute its offset relative to any other receiver j as the average of clock differences for each packet received by nodes i and j: Result: For all i,j  n: m Offset[i,j] = 1/m  (Tj,k - Ti,k) k=1 n: Number of receivers m: Number of reference broadcasts Ti,k: Node i’s clock when it received the broadcast k

RBS: Phase Offset Estimation Performance Evaluation
In each trial, n nodes were given random clock offsets, m message transmission times were selected at random. Each synchronization message was delivered to every receiver and time-stamped using the receiver’s clock.

RBS: Phase Offset Estimation Performance Evaluation
Since every receiver computes its offset with every other receiver, O(n2) offsets were computed and then compared with the “real/actual” offsets. The maximum difference between computed and actual offsets was considered to be the Group Dispersion.

RBS: Phase Offset Estimation Numerical Analysis Results
2-receiver case 30 ref broadcasts improve precision from 11 sec to 1.6 sec 20-receiver case Dispersion reduced down to 5.6 sec

RBS - Multi-hop Communication
Features: Use nodes that receive two or more reference broadcasts from different transmitters as translation nodes Receivers Transmitter Translation Nodes

RBS - Multi-hop Communication
Multiple hops could introduce a high degree of variability in message transmission time between multiple receivers of the same message  RBS would lose its accuracy To avoid loss of precision Two nodes located in different neighborhoods are synchronized using a third node lying in the intersection of the two neighborhoods

Time Routing Compute a “time route” -> dynamically determine a series of nodes through which time can be converted to reach a desired final time-base The physical topology can be easily converted to a logical topology; links represent possible clock conversions

Time Routing B A C D Use shortest path search to find a “time route”;
1 2 5 1 2 5 A B 6 6 3 4 7 3 4 7 C 8 9 8 9 D 10 11 10 11 Use shortest path search to find a “time route”; Edges can be weighted by error estimates

TIME ROUTING The nodes A,B,C,D send synchronization packets.
Other nodes are receivers. The transmission radii are shown by circles

TIME ROUTING Logical Topology: e.g., E1(R1)->E1(R4)->E1(R8)->E1(R10) Edges are drawn between nodes that have received a common broadcast and therefore have enough information to relate their clocks to each other. Receivers 8 and 9 share two logical links because they have two receptions in common from C and D.

ADVANTAGES OF THE RBS SCHEME
Largest sources of error (send and access times) are removed from the critical path by decoupling the sender from the receivers. Clock offset and skew are estimated independently of each other In addition, clock correction does not interfere with either estimation because local clocks are never modified

ADVANTAGES OF THE RBS SCHEME
Post-facto synchronization prevents energy from being wasted on expensive clock updates Multi-hop support is provided by using nodes belonging to multiple neighborhoods (i.e., broadcasting domains) as gateways

DISADVANTAGES of the RBS SCHEME
For a single-hop network of n nodes, it requires O(n2) message exchanges (computationally expensive for dense networks) Convergence time can be high due to large number of message exchanges.

Reference sender is left unsynchronized in this method. If reference sender needs to be synchronized, it will lead to a significant waste of energy

Limited to only one broadcast domain Translation errors and forwarding delays: make RBS impractical and time difference may occur Different time-of-occurrences may occur at different sinks

Timing-Sync Protocol for Sensor Networks (TPSN) S. Ganeriwal, et. al
Timing-Sync Protocol for Sensor Networks (TPSN) S. Ganeriwal, et.al., “Timing-Sync Protocol for Sensor Networks,” ACM SenSys, November 2003. Features: Have many root nodes Organize into multiple level hierarchy Synchronize level-by-level Root Nodes A B synchronizes to A C synchronizes to B B C

TPSN: Basic Concept First create a hierarchical topology in the level discovery phase As a result, each node is assigned a unique level: 0 (root node)12…N

TPSN: Basic Concept A time synchronization phase is initialized by the root node Each node synchs to the node which is one level lower than itself

TPSN: Algorithms Level Discovery Time Synchronization
New Node: Level Request Root Node Dies: Local Leader Election

TPSN: Level Discovery Root node assigned level 0
Root node initiates the phase by broadcasting a “level-discovery packet” Upon receiving the packet, each node assigns itself a +1 level and broadcasts its own new “level-discovery packet”

TPSN: Level Discovery Eventually each node in the network is assigned a unique level Once a node’s level has been decided, it ignores all such packets in the future.

TPSN: Time Synchronization
Root node starts this phase by broadcasting a time_synch packet Nodes in level one wait for some random time and start a two way message exchange to synch their local clocks This is carried out throughout the whole network

At time T1, A sends a sync pulse packet to B, which contains the level number of A and T1. Node B receives this packet at T2 T2 = T1 + D + d D= clock drift between the two nodes d= propagation delay Node B Node A T1 T4 At time T3, ‘B’ sends back an acknowledgement packet to ‘A’ with values of T1, T2, T3 and level # of B.

A can calculate the phase (clock) offset D (clock drift) and delay d as (T2-T1) + (T4-T3) (T2-T1) - (T4-T3) d= D= 2 2 Node A corrects its clock to synchronize with node B, based on the computed offset.

TPSN When a new node joins a network or when it does not get a level discovery packet The node waits for some time to be assigned a level (listens for a level discovery packet). If it is not assigned a level within that period, it times out and broadcasts a level-request packet. The neighbors reply to this request by sending their own level and the new node defines its level to be one greater than the level it received.

TPSN: When a node i loses all its neighbors with level i+1
Protocol is vulnerable to node failures When this happens, it is possible for a node at level i not to have a neighbor (i.e., a synchronization server) at level i-1 In such cases, the node at level i would not receive an ACK to its synchronization message.

TPSN Node retransmits a message for a fixed number of times before assuming that it has lost all its neighbors If the node does not receive any responses to its synchronization messages, it broadcasts a level-request packet with a new level

TPSN Maximum number of retransmissions of a synchronization message is subject to two constraints. If too large, it will increase the time taken for synchronization (i.e., the convergence time) If too small, a node may erroneously conclude that its server has died Unnecessary message flooding in the network (optimal number of retransmissions is four!)

TPSN: When the root node dies
Level 1 nodes will time-out on ACK packets to root Instead of “Level Request” query, they run an election algorithm and select a new root node A brand new level, “Discovery Phase” is started until all nodes get reassigned their new level.

Percentage of time error is less than or equal to average error
TPSN vs RBS TPSN RBS Average error (µs) 16.9 29.13 Worst case error (µs) 44 93 Best case error (µs) Percentage of time error is less than or equal to average error 64 53

ADVANTAGES: TPSN Scalable
Synchronization accuracy does not degrade significantly as the size of the network is increased Network-wide synchronization is effectively achieved Computationally less expensive

DRAWBACKS:TPSN Energy conservation is not very effective
Requires a physical clock correction to be performed on local clocks of sensors while achieving synchronization Requires a hierarchical infrastructure Unsuitable for applications with highly mobile nodes Support for multi-hop communication is not provided

DRAWBACKS: TPSN Forms islands of time
Whole network does not have the same time frame Nodes become unsynchronized when mobile All nodes are predefined in a hierarchy. No end-to-end common time frame Assumed the end users will be able to interpret the time gathered from the network Requires a pre-defined reliable hierarchy of nodes.

Time-Diffusion Synchronization Protocol
W. Su and I.F. Akyildiz, “Time-Diffusion Synchronization Protocol for Sensor Networks,” IEEE/ACM Transactions on Networking, April 2005 Principle problem with other protocols: Rely on particular nodes to be time servers or masters Need for a robust solution that automatically self-configures sensitive to energy requirements

CASE 1: Precise time servers (sinks act) are present: Sinks broadcast a reference time to all master nodes which are randomly elected to synchronize their neighbors Master nodes in turn use the received reference time to synchronize their neighbor nodes. Goal: Equilibrium time that the sensor network reaches is the reference time broadcast by the sinks

CASE 2: No precise time servers are present Sinks cannot be used as time servers Line-of-sight or connection to all master nodes from sinks may not be possible The sensor network should still synchronize on a consistent time.

Translates the TDP time to the time used by the users. Focus on TDP

All sensors have a local time that is within a small bounded time deviation from the network-wide “equilibrium” time. Protocol operates in alternating active and inactive phases. Within each active phase there are multiple cycles (iterations), each cycle lasting a duration t.

TDP SCHEDULING Protocol operates in alternating active and inactive phases Active period depends on the convergence time Inactive period depends on the tolerance range due to local clock drifting Performs several iterations (cycles t) frequently re-electing master nodes in active periods. Does nothing in inactive period.

Every iteration is made of several subparts: Election/Re-election of Leaders False Ticker Isolation Load Distribution Peer Evaluation Time Diffusion Time Adjustment

Flow Chart of TDP

Entire Protocol Operation During One Cycle
Peer evaluation procedure (PEP) in each cycle is performed To determine the master node eligibility for the next cycle Diffused leader responsibility for the remainder of the current cycle

Step 1: A master (level 1) and its neighbors Master sends a large number timestamped scan messages to its neighbors Neighbors send back ACKs containing the 2-sample Allen variance of the local clock from the master’s clock

Based on the received samples, master calculates i) an outlier ratio gyz for itself y and each neighbor z. ii) average of Allen variances iii) average of Allen deviations. All of these are sent to each neighbor z in a RESULT message. STEP j (j=2,3,…,n): Repeat the above between each level j diffused leader node and its neighbors

RESULT: All sensors will get their outlier ratios and average Allen deviation which are used to evaluate the quality of their clocks with respect to their neighbors. If outlier ratio is > 1, its local clock deviates from the clocks of its neighbors by more than twice the Allen variance Node does not become a diffused leader during the Time Diffusion Procedure of the current cycle or a master in the next cycle.

Nodes eligible for becoming masters in the next cycle, will be decided based on their energy availability (threshold) Same with diffused leaders in the current cycle depends on their energy level

STEP 3: TDP performs the main function of diffusing the time from each master in a tree-like manner for n hops where n is some pre-determined parameter

Message M (tM,I,n,bM,k) tM,I is the diffused time of the master M to which the nodes synchronize in round I n is the number of levels (i.e., depth) to which the timing information is to be diffused. bM,k is the deviation of the corresponding tM,I at a node k hops from the master M (used by time adjustment algorithm (TAA) the weight for the diffused time tM,I at level k.)

Do broadcast for each round i: 1. Master (level 1) and its neighbors i) Send a timing message M (tM,I,n,bM,k) to neighboring nodes ii) Elected diffused leaders at the next level respond with a time-stamped ACK message

iii) Master computes D = average (Dj) where Dj is the round trip time between master and diffused leader j. Diffused time from master node: tM,i = tM,i + D/2 + d d is the amount of time that the nodes wait (relative to tM,i) before adjusting their clocks at the end of the round.

Standard deviation a of the rtt Dj gives an estimate of the quality of diffused time tM,i, and is accumulated in bM,k at each hop from the master. The accumulated deviation is bM,k = bM,k-1 + a where k<=n is the number of hops from the master

b) On receiving a timing message M (tM,I,n,bM,k) for j=2,3,..,n Repeat the above between the elected diffused leaders at level j and their neighbors. In each round, each node builds a “TABLE” with rows <master_ID M, bM,k, tM,i> and populates it with the information received within that round.

After each round i within a cycle, each node resets its time to ti, the weighted sum of the times tM,i in its table, ti is based on the values of all the messages received in this round from different master initiators ti is recorded in the local TABLE The table is cleared at the end of each round i

∑ ti = ( βM,k , tM,i ) in Table [ ∑ βM’,k’ ] – βM,k βM’,k’ in Table tM,i * [[ ∑ βM’,k’ ] – βM’’,k’’ ] ∑ ( βM’’,k’’ , tM’’,i ) in Table βM’,k’ in Table

For any tM,I i) The numerator of the weight is the sum of all the deviations of all the diffusion messages, less the deviation for this particular message ii) The denominator of the weight is the sum of all such numerators for all the timing entries in the table.

Thus, the value of each clock is set to the weighted average of the clock values of the different master nodes of that round After averaging the data collected from multiple messages received within that round. Due to the weighted averaging all the nodes tend towards a common equilibrium time

ADVANTAGES: Time-Diffusion Synchronization Protocol
The protocol achieves a system-wide “equilibrium” time across all nodes, Computed using an iterative weighted averaging technique Involves all the nodes in the synchronization process

Although there is a hierarchical structure, that is neutralized by having multiple master nodes distributed across the network Designed to be robust to node failures

Network-wide time converges for both static and mobile environment Small variation in network-wide time Small energy variation throughout the network Better performance than TPSN

DRAWBACKS: Time-Diffusion Synchronization Protocol
High complexity Each active period has multiple cycles, and each cycle has multiple rounds, in each of which a diffusion broadcast is initiated by multiple masters

DRAWBACKS: Time-Diffusion Synchronization Protocol
The convergence time tends to be high when no external precise time servers are used However, if the servers are used, the convergence time is comparable to a server-based technique

Chapter 11: Time Synchronization

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