It is known to make use of special time-stamped frames for measuring the time required for transfer between a piece of computer equipment and a remote piece of equipment to which it is connected, e.g. by a communications network, and in particular the Internet.
In known techniques, the time stamp of such a frame corresponds to the instant at which it was generated.
Frames generated under such conditions do not enable transmission time to be measured accurately, and in any event transmission time is defined in standards as the time between the last bit of the frame being sent and the first bit of the frame being received by the remote equipment.
Reference can be made to the following provisional documents of the Network Working Group:
A one-way delay metric for IPPM, by G. Alnes et al. (Request for Comments, September 1999); and
Instantaneous packet delay variation metric for IPPM, by Demichelis and Chimento (Internet draft, December 1999).
In the present state of network performance, e.g. Internet performance, this lack of accuracy is without consequence since the transmission time being measured can be as long as several hundreds of milliseconds, or even several seconds.
Nevertheless, there is a very clear trend at present towards higher data rate networks and towards the creation of services in which transmission time is guaranteed, and this makes it necessary to have a higher-performance measurement tool for quality control of high-quality services.
It should also be observed that the above-mentioned lack of accuracy is associated with lack of accuracy concerning the absolute time value generated by the system clock of the computer for time stamping frames. As is well known, system clocks are subject to phenomena of drift and of jitter which give rise to uncertainty about the absolute time value which at best is of the order of about 10 milliseconds.
The invention makes it possible to avoid at least one of the above-mentioned drawbacks by a method of using computer equipment to send a time-stamped frame, the method performing the steps of:
generating at a given instant TS a frame whose time stamp TO is equal to TS+xcex4, where xcex4 is a time interval required by the computer equipment between generating a time-stamped frame and sending the last bit thereof;
sending said time-stamped frame as generated at the instant TS when an absolute time clock of the computer equipment reaches a value equal to TS+xcex4xe2x88x92"sgr"where "sgr" is the duration required for sending the bits of said frame, such that the last bit of the frame is sent at instant TS+xcex4.
It is advantageous for said absolute time clock to be clocked at a frequency equal to a frequency at which frame bits are sent by the computer equipment, or indeed to a multiple or a submultiple thereof.
Preferably, the given instant TS is generated by a reference counter based on an oscillator which is updated by a time reference, the oscillator being, for example of the temperature-compensated crystal oscillator type.
Said time reference may be generated by a receiver for receiving a universal time reference, in particular a GPS receiver.
The absolute time clock may present a first counter constituted by said reference counter and a second counter clocked by a frame send clock whose frequency is equal to the frequency at which frame bits are sent by the computer equipment, or indeed a clock whose frequency is equal to a multiple or a submultiple of the frequency of the frame send clock, and which is reset to zero by said first counter.
Advantageously, the method stores said time-stamped frames in a first register and stores other frames that are not time-stamped in a second register, and sends a time-stamped frame on a priority basis from the first register at least whenever the computer equipment is not capable of sending an entire frame contained in the second register before instant TS+"sgr"xe2x88x92xcex4 which corresponds to the instant at which the first bit of said time-stamped frame should be sent.
In a preferred implementation, the method performs the steps of:
a) analyzing the content of the first and second registers by first and second respective frame identification pointers;
b) if at least one frame is contained in the second register, calculating the time it will take to send and calculating an ideal sending time TE which is the sum of the absolute time and the time taken to send said frame;
c) if the ideal time TE is less than TS+"sgr"xe2x88x92xcex4, triggering sending of said frame contained in the second register;
d) otherwise, causing the second pointer to wait;
e) where appropriate, repeating steps b) to d) until the second pointer is caused to wait;
f) if the second pointer is caused to wait or if the test performed in a) indicates that the second register does not contain a frame, sending said time-stamped frame contained in the first register; and
g) sending any frames contained in the second register and returning to a).