Patent ID: 6819685
Filing Date: 2004-11-16
Classification: G06F,H04J

Abstract:
A method of controlling a frequency via an asynchronous transmission network, in which method signals received at the output of said asynchronous transmission network have been clocked at a reference frequency and transmitted by said network with respective varying transit delays and the frequency of a local clock is slaved to said reference frequency by means of an error signal formed from selected received signals with minimal transit delays;wherein said error signal is formed from respective time offsets associated with said received signals, each of said offsets occurs between two instants in corresponding relationship, one of which belongs to a received sequence formed of instants respectively marked by said received signals and the other to a theoretical sequence defined by said local clock, said instant of said received sequence is marked by said signal associated with said offset, and a received signal constitutes a selected signal when said offset associated with said received signal is the minimum offset of the offsets associated with a group of consecutive instants of said received sequence; and wherein said method further includes the following steps: sending a series of signals marking respective instants, said sequence, said signals and said instants respectively constituting a transmitted sequence, locking signals and marked instants, said marked instants being separated in said transmitted sequence by successive intervals having the same value constituting a reference interval, injecting said transmitted sequence into said asynchronous transmission network, a receiver at the output of said transmission network receiving said locking signals which form for said receiver a sequence constituting said receive sequence previously referred to, said instants marked by said signals in said sequence constituting receive instants respectively corresponding to said instants marked by said signals in said transmitted sequence, receive intervals being present in said receive sequence and each having a start consisting of one of said receive instants and an end consisting of the instant that follows said start in said sequence, choosing a theoretical interval â€œeâ€  defined relative to the period of said local clock, successively using said local clock to provide a measurement value for each of said receive intervals, said value constituting a measured interval ending at said end of said interval, defining a plurality of calculation ranges each having a start and an end consisting of respective receive instants, and at least the following steps performed for each of said calculation ranges: adding said measured intervals terminating in said range, the result of such addition constituting a cumulative measured interval of said range, the number of said intervals constituting a number of intervals of said range, multiplying said theoretical interval by said number of intervals of said range, the result of said multiplication constituting a cumulative theoretical interval of said range, and calculating an offset of said range having an algebraic value equal to said cumulative measured interval of said range less said cumulative theoretical interval of said range, said method further including action steps for adjusting the frequency of said local clock, each of said steps constituting a terminal step of an adjustment sequence also including an adjustment group of said step, said group including at least one of said calculation ranges, said calculation ranges of said group respectively constituting adjustment ranges, a cumulative offset of said group comprising the algebraic sum of said offsets of said adjustment ranges, an action of said step modifying the frequency of said clock up or down according to whether said cumulative offset of said group is respectively negative or positive, so that said cumulative offset constitutes an error signal, and said method further including the following steps: choosing a succession of search periods each having a start and an end separated by a chosen number of said theoretical intervals, said number being greater than three and constituting a duration of said period, said period including a succession of said receive instants, and search steps respectively executed for said search periods and each including the following steps: choosing a search origin consisting of an instant not posterior to said start of said period, for each of said receive instants of said period, calculating an offset associated with said instant, said offset being said offset of one of said calculation ranges having a start consisting of said search origin and an end consisting of said instant, and defining an adjustment limit, said limit being one of said receive instants of said period and such that said offset associated with said limit is the smallest of said offsets associated with said instants, said starts and ends of said adjustment ranges being said adjustment limits.