Fine-Grained Network Time Synchronization using Reference Broadcasts Jeremy Elson, Lew Girod, and Deborah Estrin OSDI Boston, MA Speaker : hsiwei-Chen

Outline  Introduction  Reference-Broadcast Synchronization(RBS)  Implementation on Berkeley Motes  Conclusions and FutureWork

Sources of Time Synchronization Error(1/2) Send Time Send Time  Time spent at the sender to construct the message, e.g. context switches and system call overhead  The time required to transfer the message from the host to its network interface. Access Time Access Time  Wait for the channel to be clear before transmitting

Sources of Time Synchronization Error(2/2) Propagation Time Propagation Time  The time needed for the message to transit from sender to receivers Receive Time Receive Time  Time to receive the message and notify the host of its arrival Send time & access time are most non-deterministic

Reference-Broadcast Synchronization(RBS) Traditional protocols Traditional protocols Synchronize the sender of a message with its receiver Synchronize the sender of a message with its receiver RBS RBS A set of nodes synchronize with each other (not with the sender) A set of nodes synchronize with each other (not with the sender)  reduces non-determinism

Critical Path Analysis Traditional RBS Send Time Access Time Propagation Delay Receive Time

RBS Critical Path Remove the send time & access time from the critical path Remove the send time & access time from the critical path  Receive time is independent of the sender or network load  Propagation time can be ignored in a short distance

RBS Critical Path (Cont.) Use broadcast as a relative time reference Use broadcast as a relative time reference  Broadcast packet does NOT include timestamp generated by sender  Any broadcast, e.g., RTS/CTS, route discovery packets

Simple RBS One transmitter broadcasts a reference packet to two receivers One transmitter broadcasts a reference packet to two receivers Each receiver records the time the reference packet arrived according to the local clock Each receiver records the time the reference packet arrived according to the local clock Receivers exchange the timestamps Receivers exchange the timestamps We consider the Propagation Time to be effectively 0 We consider the Propagation Time to be effectively 0 Sensitiving to differences in the receive time of messages within a set of receivers. Sensitiving to differences in the receive time of messages within a set of receivers.

Simple RBS Sender Receiver 1 Receiver 2 Broadcast exchange the timestamps

Distribution of Inter-Receiver Phase Offset Pairwise Difference in Packet Reception Time (usec) Number of Trials Phase offset recorded by a logic analyzer for 1,478 broadcast packets(total) (sender emitted 160 broadcast packets) Gaussian distribution  = 0,  = 11.1us Confidence = 99.8% 5 motes: receivers 1 mote: transmitter

Estimation of Phase Offset A sender broadcasts m reference packets A sender broadcasts m reference packets Each of the n receiver records the time at which the reference was received Each of the n receiver records the time at which the reference was received Receivers exchange timestamps Receivers exchange timestamps Receiver i computes its phase offset to any other receiver j as the average of m differences Receiver i computes its phase offset to any other receiver j as the average of m differences

Estimation of Clock Skew Clock skew due to crystal oscillators errors Clock skew due to crystal oscillators errors  Frequency error Difference between the expected and actual frequencies Difference between the expected and actual frequencies  Instability Short-term: Environmental factors, e.g., variations in temperature, supply voltage, and shock Short-term: Environmental factors, e.g., variations in temperature, supply voltage, and shock Long-term: Oscillator aging Long-term: Oscillator aging  Least square method (linear regression) Assume phase error is changing at constant rate Assume phase error is changing at constant rate Continuously adjust frequency based on recent observations relating the local oscillator to a reference Continuously adjust frequency based on recent observations relating the local oscillator to a reference

Implementation on Berkeley Motes One sender, five receivers One sender, five receivers 1000 trials 1000 trials

Receiver Group Dispersion #Reference packet broadcasts Receiver Group Dispersion (us) 25 broadcasts can reduce the dispersion below 10us 20 Receivers 2 Reeivers

Effect of Clock Skew on RBS Time (sec) Phase Offset (us) Fit Error (us)

Performance Comparisons Compare to NTP Compare to NTP UNIX implementation of RBS for fair comparisons UNIX implementation of RBS for fair comparisons  User space UNIX daemon  UDP datagram  StrongArm-based Compaq IPAQ PDA’s “Familiar” Linux (2.4 kernel) “Familiar” Linux (2.4 kernel)  Lucent Technology 11Mbit wireless Ethernet adapters 300 trials, 每個 trial 間格 8 秒鐘,Total time is 40 minuters 300 trials, 每個 trial 間格 8 秒鐘,Total time is 40 minuters

Synchronization Error with Light Network Load Error (us) Cumulative Error Prob. RBS NTP NTP-Offset Sync. Error between two IPAQ’s

Synchronization Error with Light Network Load RBS: / us RBS: / us NTP: / us NTP: / us NTP-Offset: 204 +/ us NTP-Offset: 204 +/ us RBS works better due to the reduced non- determinism RBS works better due to the reduced non- determinism

Synchronization Error with Heavy Network Load Error (us) Cumulative Error Prob. RBS NTP NTP-Offset

Synchronization Error with Heavy Network Load RBS: / us RBS: / us  With kernel support which timestamps at interrupt time, / us NTP: / us NTP: / us NTP-Offset: / us NTP-Offset: / us RBS is not affected by Medium Access Delay RBS is not affected by Medium Access Delay

Multi-Hop Time Synchronization 在 node A 面積 local 中,node 1 與 node 4 同步 2. 再以 node 4 當 getway, 與 node B 面積 local 中,node 4 與 node 7 同步 3. 即可達到 Global 同步. A 5 67 B

Multi-Hop RBS Performance Hop Mean Error( )

Conclusions and FutureWork RBS provides more precise, flexible, and resource-efficient network time synchronization than traditional algorithms RBS provides more precise, flexible, and resource-efficient network time synchronization than traditional algorithms But such as automatic, dynamic election of the set of nodes to act as beacon senders ? But such as automatic, dynamic election of the set of nodes to act as beacon senders ?

Summary 本篇作者去除掉了 Send Time and Acces 本篇作者去除掉了 Send Time and Acces Time 同步 error 的兩個來源 Time 同步 error 的兩個來源 以相對的時間代替絕對的時間 以相對的時間代替絕對的時間

Thank you