Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Reexamination Certificate
1998-09-15
2001-12-04
Olms, Douglas (Department: 2661)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C375S371000
Reexamination Certificate
active
06327274
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to communication systems, and more particularly, to a method and system for measuring/estimating the relative skew between clocks in a packet based communication networks.
2. Description of Related Art
Estimating the relative skew between clocks on different host machines (or referred to network nodes) in a packet network environment, such as the Internet, is relatively new since real-time applications and usage based pricing are relatively new concepts. The issue of measuring one way transit times of packets is becoming increasingly important with the emergence of real-time applications such as voice and video over the Internet. This concept of one-way transit time measurements involves comparing (specifically, taking differences of) times measured with respect to different clocks (e.g. the sender and receiver clocks). Two of the main sources of clock timing discrepancy are offset and skew. Offsets can be readily corrected. The most widely used remedy for clock offsets is a form of synchronization (e.g. using Network Time Protocol (NTP), or Global Positioning System (GPS), etc.). It should be noted, though, this synchronization zeros the clock difference between two clocks at one instant, but the clock skew will inherently cause the clocks to drift apart as time progresses. Accordingly, a clock skew can impose a serious clock timing discrepancy problem for a real-time application.
Accordingly, there is a need to develop a method to detect/correct clock skews. Specifically, there is a need to develop a method of measuring/estimating the relative skew between clocks on different host machines (or referred to network nodes) in a packet network environment such as the Internet. The present invention provides a solution to the above and other problems and offers advantages over the existing network systems.
SUMMARY OF THE INVENTION
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and system for estimating the relative skew between clocks in a packet communication network.
The present invention provides a method of detecting the existence of a clock skew between two different clocks. The clock offsets are predefined by a separate synchronization mechanism based on Network Time Protocol (NTP) and/or Global Positioning System (GPS) based systems. It is appreciated that other clock synchronization mechanisms can be used within the scope of the present invention.
In one embodiment of the present invention, a method for determining a clock skew between at least a first and second clocks in a network includes: transmitting a plurality of packets from a first network node to a second network node, each of the packets having a timestamp which records time measured by the first clock at the first network node; receiving one of the packets at the second network node and recording time measured by the second clock at the second network node; obtaining the timestamp of the one packet recorded at the first clock; computing a time delay between a time of transmitting the one packet and a time of receiving the one packet; computing a jitter between the time delay for the one packet and the time delay for a next packet consecutively transmitted after the one packet; averaging jitters after transmitting the plurality of packets; averaging inter-packet times, each of the inter-packet times being a time interval between transmitting two consecutive of the plurality of packets at the first network node; and determining the clock skew by dividing an average of the jitters by an average of the inter-packet times.
Further in one embodiment, the network is a packet based communication network.
Still in one embodiment, the time recordings by the first and second clocks are relative to a global clock.
The present invention is also a system of determining a clock skew between at least a first and second clocks in a network. In one embodiment, the system includes means for transmitting a plurality of packets from a first network node to a second network node, each of the packets having a timestamp which records time measured by the first clock at the first network node; means for receiving one of the packets at the second network node and recording time measured by the second clock at the second network node; and means for determining a clock skew between the first and second clocks based on the timestamps and the time measured by the second clock.
Further in one embodiment, the clock skew determining means includes means for obtaining the timestamp of the one packet recorded at the first clock; means for computing a time delay between a time of transmitting the one packet and a time of receiving the one packet; means for computing a jitter between the time delay for the one packet and the time delay for a next packet consecutively transmitted after the one packet; a first averager averaging jitters after transmitting the plurality of packets; and a second averager averaging inter-packet times, each of the inter-packet times being a time interval between transmitting two consecutive of the plurality of packets at the first network node; wherein the clock skew is determined by dividing an average of the jitters by an average of the inter-packet times.
Further, the present invention provides an article of manufacture for a computer-based data processing system, the article of manufacture including a computer readable medium having instructions for causing a computer to perform a method for determining a clock skew between at least a first and second clocks in a network, includes: transmitting a plurality of packets from a first network node to a second network node, each of the packets having a timestamp which records time measured by the first clock at the first network node; receiving one of the packets at the second network node and recording time measured by the second clock at the second network node; obtaining the timestamp of the one packet recorded at the first clock; computing a time delay between a time of transmitting the one packet and a time of receiving the one packet; computing a jitter between the time delay for the one packet and the time delay for a next packet consecutively transmitted after the one packet; averaging jitters after transmitting the plurality of packets; averaging inter-packet times, each of the inter-packet times being a time interval between transmitting two consecutive of the plurality of packets at the first network node; and determining the clock skew by dividing an average of the jitters by an average of the inter-packet times.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.
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“On Assessing Uni
Altera Law Group LLC
Nokia Telecommunications, Inc.
Olms Douglas
Vanderbuye Ken
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