The present invention relates to transmission apparatus of the general type that transmits a stream of unit signals and inserts further unit signals into the stream, more particularly to a method of controlling the points at which the further unit signals can be inserted.
Transmission apparatus of the above type is used in, for example, communication networks employing the asynchronous transfer mode (ATM). The unit signals in these networks are sequences of digital signals referred to as ATM cells, having a standard length of fifty-three bytes.
An example of an ATM transmission apparatus is shown in FIG. 1. The apparatus 10 includes a plurality of interface units 11, 12, 13, 14 and a switching unit 15. Interface unit 11, for example, includes a transmission convergence (TC) layer 11A that terminates a transmission path 21 that may have any of a variety of interface specifications, and an ATM layer 11B that is interconnected to the switching unit 15. Interface unit 13 includes a TC layer 13A that terminates another transmission path 23 and an ATM layer 13B connected to the switching unit 15. The other interface units have a similar structure (not visible). In all there are m interface units, n of which are disposed on the left side of the switching unit 15 in the drawing, where m and n are arbitrary positive integers (m>n).
The transmission paths 21, 23 may carry signals having various bit rates or speeds. The parts of, for example, TC layer 11A that process these signals operate at corresponding speeds. The ATM layer 11B operates at the fastest speed that might be encountered in the TC layer 11A. All of the ATM layers 11B, 13B, etc. have the same internal structure, so that a single type of ATM layer can be employed with transmission paths having different speeds just by changing the TC layer.
The term ‘bandwidth’ is often used as a synonym for bit rate or speed. Thus if TC layer 11A and ATM layer 11B are capable of processing signals at rates of A bits per second and B bits per second, respectively, they will be said to have bandwidths of A and B. In this case, A cannot exceed B (A≦B). If valid user cells are being transmitted at a rate of C bits per second, the user cell traffic will be said to occupy a bandwidth C, which must not exceed either A or B (C≦A≦B). Parts of the bandwidth B not occupied by user cells are filled with idle cells, and with management cells used for the management of system resources.
The user cell traffic is controlled so that bandwidth C is distributed substantially evenly within the bandwidth B of the ATM layer 11B; that is, user cells are kept moving through the ATM layer at a substantially even rate. The reason for this is that if bandwidth C were to be concentrated into one part of bandwidth B (if the user cell traffic were to bunch up), then during the corresponding intervals of time, bandwidth B would be effectively filled, and large buffers would be required for bit-rate conversion from the ATM layer 11B to the TC layer 11A. An even distribution of the user cell bandwidth C within the bandwidth B of the ATM layer reduces the required buffer memory capacity of the apparatus.
The management cells mentioned above are inserted by the ATM layer. A problem that occurs in conventional ATM transmission apparatus is that insertion of these management cells can disturb the even cell distribution, causing the combined non-idle cell traffic to become overconcentrated in certain parts of the ATM-layer bandwidth B. In the worst case, the user-cell bandwidth and management-cell bandwidth may together exceed the bandwidth capability A of the TC layer, forcing user cells to be dropped.