Net 435: Wireless sensor network (WSN) Networks and Communication Department Lecture 8: time synchronization in WSN

Synchronization Synchronization is the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time. Systems operating with all their parts in synchrony are said to be synchronous or in sync. 18-Sep-18 Networks and Communication Department

Synchronization in Communication systems In electrical engineering for digital logic and data transfer, a synchronous circuit requires a clock signal. However, the use of the word "clock" in this sense is different from the typical sense of a clock as a device that keeps track of time-of-day; the clock signal simply signals the start and/or end of some events. 18-Sep-18 Networks and Communication Department

18-Sep-18 Networks and Communication Department

Motivation for Time Synchronization Most applications require some synchronization accuracy Fire and flood tracking Animal movement Vehicle movement Gunshot detection

18-Sep-18 Networks and Communication Department

Existing Synchronization Solutions Global Positioning System (GPS) Power-hungry Network Time Protocol (NTP) Computationally infeasible for wireless sensors Timing-sync Protocol for Sensor Networks (TPSN) Transmitter-receiver synchronization

GPS The Global Positioning System (GPS) is a space- based navigation system that provides location and time information in all weather conditions, anywhere on or near the Earth where there is a line of sight to four or more GPS satellites. 18-Sep-18 Networks and Communication Department

Definitions Local clocks are maintained at designated time servers, which are timekeeping systems belonging to a synchronization subnet, in which each server measures the offsets between its local clock and the clocks of its neighbor servers or peers in the subnet. stability of a clock is how well it can maintain a constant frequency, the accuracy is how well its time compares with national standards and the precision is how precisely time can be resolved in a particular timekeeping system. 18-Sep-18 Networks and Communication Department

UTC Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. A time standard is a specification for measuring time: the rate at which time passes It is one of several closely related successors to Greenwich Mean Time (GMT). For most purposes, UTC is considered interchangeable with GMT, but GMT is no longer precisely defined by the scientific community. 18-Sep-18 Networks and Communication Department

to synchronize frequency means to adjust the clocks in the subnet to run at the same frequency, to synchronize time means to set them to agree at a particular epoch with respect to Coordinated Universal Time (UTC), as provided by national standards. to synchronize clocks means to synchronize them in both frequency and time. 18-Sep-18 Networks and Communication Department

NTP Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable-latency data networks. 18-Sep-18 Networks and Communication Department

Historical facts Developed in 1985 by David Mills at the University of Delaware, USA. David is still maintaining NTP. NTP is one of the oldest Internet protocols still in use. Latest version is NTP version 4 (version 5 is under development) 18-Sep-18 Networks and Communication Department

Differences to Cristian‘s method and the Berkley algorithm Cristian‘s method(CM) and the Berkley algorithm(BA) are both designed for primarily use in intranets. An intranet is a network based on TCP/IP protocols (an internet) belonging to an organization, usually a corporation, accessible only by the organization's members, employees, or others with authorization. The Network Time Protocol was designed for use in the Internet (or other unreliable networks) right up from the start. CM and BA both synchronize against one time server. NTP synchronizes against many time servers. 18-Sep-18 Networks and Communication Department

Design aims of NTP Adjust system clock as close to UTC as possible over the Internet. (statistic techniques for filtering timing data) Handle bad connectivity. (redundant paths, reconfigurable servers) Enable sufficiently frequently resynchronizations. (scaling well on large numbers of clients and servers) Security. (distrust server’s time, authentication) 18-Sep-18 Networks and Communication Department

Hierarchical structure The NTP service is provided by a network of servers located across the Internet. Primary servers are connected directly to a time source. (e.g. a radio clock receiving UTC, GPS). Secondary servers are synchronized with primary servers. The servers are connected in a logical hierarchy called a synchronization subnet. 18-Sep-18 Networks and Communication Department

Synchronization subnet Each level of the synchronization subnet is called stratum. (e.g. primary servers are stratum 1, secondary stratum 2 and so on) Lowest-level (leaf) servers execute in users‘ workstations. (e.g. ntp under Linux) Servers with high stratum numbers are liable to have less accurate clocks than those with lower stratum numbers. The synchronization subnet can reconfigure as servers become unreachable or failures occur. 18-Sep-18 Networks and Communication Department

18-Sep-18 Networks and Communication Department

Server synchronization There are three possible synchronization modes: multicast (sometimes called broadcast) procedure-call (sometimes called client) symmetric (sometimes called peer) 18-Sep-18 Networks and Communication Department

Multicast mode One or more servers periodically multicasts the time to the servers in the network. Receivers set their clock assuming a small delay. Receivers don‘t reply. Problem: Relative low accuracy. Due to hardware limitations this mode only works in IP multicast enabled networks like LAN. 18-Sep-18 Networks and Communication Department

procedure-call mode One server accepts requests from other computers. The server replies with its timestamp. Higher accuracy. 18-Sep-18 Networks and Communication Department

symmetric mode Intended to be used on higher levels of the synchronization subnet. A pair of servers exchange messages to improve the accuracy of their synchronization over time. (reduction of the synchronization dispersionتشتت) Highest accuracy. 18-Sep-18 Networks and Communication Department

18-Sep-18 Networks and Communication Department

18-Sep-18 Networks and Communication Department

18-Sep-18 Networks and Communication Department

18-Sep-18 Networks and Communication Department

18-Sep-18 Networks and Communication Department

18-Sep-18 Networks and Communication Department

Timing-sync Protocol for Sensor Networks Timing-sync Protocol for Sensor Networks (TPSN) that works on the conventional approach of sender receiver synchronization. We argue that for sensor networks, the classical approach of doing a handshake between a pair of nodes is a better approach than synchronizing a set of receivers. Traditional handshake approach Timestamp at the MAC layer Two stages Level Discovery Phase (Flooding) Synchronization Phase

TPSN Model – Level Discovery Phase Assign root (level 0) node Broadcast level_discovery packet Nodes 1 hop away assigned to level 1 Ignore all subsequent level_discovery packets Broadcast level_discovery packet …

18-Sep-18 Networks and Communication Department

18-Sep-18 Networks and Communication Department

18-Sep-18 Networks and Communication Department

TPSN Model – Synchronization Phase Each node (A) broadcasts synchronization_pulse Timestamped at T1 Node B receives pulse at T2, broadcasts ack at T3 Node A receives ack at T4 Δ is clock drift d is propagation delay

COMPUTER CLOCK AS AN APPROXIMATION OF REAL TIME 18-Sep-18 Networks and Communication Department

TPSN Energy Usage Transmissions and receptions grow as O(n) Large energy savings over RBS for large n Less efficient for small n

Thank You 18-Sep-18 Networks and Communication Department