Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Patent
1997-05-29
1999-02-16
Nguyen, Chau
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
375372, H04L 1256
Patent
active
058727892
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
In ATM communications networks, information is transmitted in the form of packets or cells according to an asynchronous transmission mode (Asynchronous Transfer Mode, ATM). In these ATM networks, jitter of the ATM cells occurs, dictated by the system, during the transmission operation. Jitter is generally produced by buffering of the ATM cells, which is carried out in a network node, and depends on a number of parameters. Thus, for example, the number of nodes in the network, the loading (that is to say how many bits/s are transmitted) and the nature of the traffic (that is to say whether a variable bit rate is transmitted) have a direct influence on the jitter magnitude. The jitter of an ATM cell thus fluctuates between 0 (optimum value, in other words no jitter) and a maximum value. The occurrence of jitter is particularly disadvantageous when the transmitted information takes the form of video signals or voice signals.
Limit values have been specified in the past for the purpose of avoiding excessively high jitter. Thus, for example in accordance with a specification made by the Federal German Post Office, the maximum jitter per network node must not exceed a value of 250 .mu.s. In the case of a connection in which 6 network nodes are traversed (typical value for a national connection), the maximum jitter can consequently amount to up to 1.5 ms. If a transmission bit rate of 4 Mbit/s (corresponding to a rate of 10000 cells/s) is taken as a basis, this means that, in the extreme case, instead of 15 ATM cells, not a single ATM cell arrives in the receiving device.
German Patent Specification DE 41 32 518 C2 discloses a circuit arrangement for the transmission of digital message signals via a broadband communications system operating according to the asynchronous transfer mode. Said document discloses how the effects of jitter in a receiving device can be reduced. This is done by self-synchronization of buffer memory devices, which proves to be complicated in practice.
SUMMARY OF THE INVENTION
The invention is based on the object of specifying a method which enables the effects of jitter to be reduced further using relatively simple means.
The invention has the advantage that the interval between two successive ATM cells is determined prior to storage in the memory device of the receiving device. The intervals determined in this way are then summed over a fixedly defined plurality of ATM cells and averaging is carried out on the basis of these calculations. The average value calculated in this way is then used to read the ATM cells stored in the memory device back out of the memory device and forward them. This is associated with the advantage that the jitter is reduced.
In a development of the present invention, it is provided that the averaging of the intervals between the in each case two successive ATM cells is carried out anew each time an ATM cell arrives.
In a development of the present invention, it is provided that the averaging is carried out in that the last calculated average value of the sum of the intervals between the defined number of ATM cells is subtracted from the last calculated sum of the intervals between the defined number of ATM cells, and the newly determined interval of the ATM cell that arrived last is added. This is associated with the advantage that the respective counter readings do not have to be buffered.
In a development of the present invention, it is provided that a correction quantity is added to and/or subtracted from the averaging result whenever the occupancy of the memory device deviates from a defined value, and that the ATM cells stored in the at least one memory device are read out using the value determined in this way. This is associated with the advantage that the rounding errors which occur unavoidably during the calculation of the average value can at least partially be compensated for again. In particular, the correction quantity can be used whenever deviations from the optimum occupancy of the memory device occur.
In a develo
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Gemmer Thomas
Hintermaier Stefan
Orleth Robert
Steinmaier Anton
Nguyen Chau
Siemens Aktiengesellschaft
Vanderpuye Kenneth
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