08-11-2012, 06:02 PM
CLOCK SYNCHRONISATION
Clock Synchronization.pptx (Size: 67.51 KB / Downloads: 27)
Clock synchronization is a problem from computer science and engineering which deals with the idea that internal clocks of several computers may differ
Clock drift refers to several related phenomena where a clock does not run at the exact right speed compared to another clock.
Importance of clock Synchronization
Computer networks time synchronization is critical because every aspect of managing, securing, planning, and debugging a network involves determining when events happen.
Without synchronized time, accurately correlating log files between these devices is difficult, even impossible.
Tracking security breaches, network usage, or problems affecting a large number of components can be nearly impossible if timestamps in logs are inaccurate.
Problems and solutions
Many of these clocks are maintained by a battery-backed, clock-calendar device that may drift as much as a second per day.
there are problems associated with clock skew that take on more complexity in a distributed system in which several computers will need to realize the same global time.
In a centralized system the solution is trivial; the centralized server will dictate the system time
Solutions
In a distributed system the problem takes on more complexity because a global time is not easily known.
Cristian's algorithm relies on the existence of a time server
Berkeley algorithm- his algorithm is more suitable for systems where a radio clock is not present.
This algorithm highlights the fact that internal clocks may vary not only in the time they contain but also in the clock rate.
NETWORK TRANSPORT PROTOCOL (NTP)
The Network Time Protocol (NTP) is a protocol and software implementation for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks.
NTP uses a hierarchical, semi-layered system of levels of clock sources.
NTP provides Coordinated Universal Time (UTC) including scheduled leap second adjustments. No information about time zones or daylight saving time is transmitted.
Clock synchronization in Delay Tolerant Networks (DTN)
Timestamp Transformation protocol (TTP) solves the temporal ordering problem in sparse ad-hoc networks.
Clock inaccuracy increases linearly with time and number of hops.
Double-pair wise Time Protocol (DTP) is a reference node clock synchronization which assumes at least one reference node in the network.
It does not work in distributed environment where there is no reference node
Asynchronous Diffusion(AD) protocol
Asynchronously averaging clock values with contacted neighbors
Some effort has been made to provide clock synchronization in underwater acoustic networks.
Inefficient in DTNs where connection is dynamic and limited to just few nodes