AU672066B2

AU672066B2 - Cell buffer memory management

Info

Publication number: AU672066B2
Application number: AU53829/94A
Authority: AU
Inventors: Michel Henrion
Original assignee: Alcatel NV
Current assignee: Alcatel Lucent NV
Priority date: 1993-01-27
Filing date: 1994-01-18
Publication date: 1996-09-19
Anticipated expiration: 2014-01-18
Also published as: EP0609128A1; EP0609128B1; ATE265709T1; DE69433733D1; FR2700865B1; AU5382994A; FR2700865A1; DE69433733T2

Abstract

This device receives several cell streams characterised by different cell loss rate performances and comprises means for recognising that a cell loss is inevitable when the buffer memory is full, as well as means for deciding that an incoming cell should be recorded in the buffer memory instead of another cell which will be lost. It furthermore comprises means (CF1..., CFi..., CFn) for associating with each cell stream (FC1..., FCi..., FCn) a dynamic indication of cell loss balance (CLF1..., CLFi..., CLFn), which is a function of a cell loss performance (Ni) allotted to this stream and cell losses (RTi) actually noted for this cell stream, and means (SCDA) for selecting a cell stream, especially by comparing the said dynamic indications of cell loss, and for causing as a consequence the incoming cell to be recorded in the buffer memory instead of a cell of the selected cell stream. <IMAGE>

Description

P/00/0O11 28/5/91 Regulation 3.2

AUSTRALIA

Patenits Act 1990 0. 0 000 a

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: "CELL BUFFER MEMORY MANAGEMENT"1 The following statemnent is a full description of this invention, including the best method of S S

S

performing it known to us:- This invention relates to a cell buffer memory management device.

Such a device can be used in data communication systems where data is arranged in packes or cells, in particular in asynchronous time division multiplex systems (ATM) where one of the critical aspects with regard to traffic flow performance level requires ,catistical multiplexing of the various cell flows, this being achieved in the simplest way by means of a buffer memory receiving the input cells of several cell sources and supplying output cells to a single output link. One of the functions of the buffer memory management device is to route the input cells to locations within the buffer memory which are available.

When the total input cell flow is greater than the output cell flow, the i buffer memory is filled. Once full, if an input cell is received, cell loss is unavoidable. This occurrence is rare, since the output link cell rate is at least equal to the sum of the average cell source rates. It occurs at random during simultaneous peaks in the rate of cells from several sources. Therefore, it has an impact on the cell sources in proportion with their rate. If the proportion of cells that can be lost is the same for all sources, it can therefore be accepted.

Nevertheless, the data to be multiplexed may correspond to different services and different performance levels in terms of cell loss that may be o22 assigned to them.

0*The solution consisting of sizing the buffer memory in order to achieve the highest loss performance level with regard to the overall services can be criticised since, in general, it is always advisable to keep the capacity of the °ooeo* buffer memories to a minimum, not to limit the cost, but because the larger the buffer memory, the longer the time taken by some cells. Conversely, for a given buffer memory capacity, it is beneficial to be able to increase the intensity of the cell flow over the output link while achieving the objectives with regard to cell loss rate per service.

More complex solutions have been proposed in the article "Limiting removal depth in the pushout scheme for ATM networks", by R. Beraldi et al, published in IEEE ICC'92.

The first solution called "Threshold" or "Partial Buffer Sharing" consists of defining two cell flows, one with high loss priority, the other with low loss 3 priority, and to define, within a buffer memory having a total capacity K, a smaller space of dimension S. When space S is not filled, the two cell flows can freely access the buffer memory. As soon as space S is filled, only those cells in the high priority cell flow are stored in the buffer memory, while the cells in the low priority cell flow are lost.

The second solution called "Pushout" consists of defining two cell flows as above, and, once the (non partitioned) buffer memory is full, still letting an input cell into this memory, so long as the buffer memory contains at least one cell within a low priority cell flow and so long as the input cell belongs to a high priority cell flow. Once the low priority cell has been pushed out, the queue is eventually rearranged and the input cell can then be accepted into the buffer memory having a location available.

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Both solutions favour the high priority cell flow to the detriment of the low priority cell flow. Nevertheless, they are not satisfactory since they provide for only two classes of cell flow, and buffer memory management becomes difficult as soon as more than two classes are involved. Furthermore, buffer memory sizing can only be based on hypotheses regarding the proportion of cells in the two cell flows in the total input cell flow. Of course, such hypotheses are possible only if it is possible to process predefined cell flows which can be associated with their own rate.

Notwithstanding the above, when these hypotheses are not met, the measures taken do not always yield the expected results. Hence, it can be considered that these two known solutions are both "unfair" when, the low priority cell flow having a relatively low rate, a persisting overflow of the high priority cell flow rate leads to complete filling of buffer memory space S (Threshold solution) or of the overall buffer memory (Pushout solution). Indeed, from this moment on, an excessive total input rate will cause the loss of cells within a single low priority cell flow, although the cause of buffer memory overload is due to the previous excessive rate of the high priority cell flow.

Nevertheless, there are some cases when it is desirable to favour, not a cell flow predefined in relation with others, but a non-predefined cell flow made up of individual cells whose appurtenance to this non predefined cell flow is 4 arbitrary. In this case, it is no longer possible to base hypotheses on the rate of such non-predefined cell flows. The above mentioned known solutions are no longer applicable.

Nlow, in the current state of studies made on broadband systems using ATM techniques, it is indeed a matter of allocating to the cells of individual connections, not only a class of service quality applicable to all cells in the connection, but also a cell loss priority indication inherent to each cell. This is equivalent to making up a non predefined cell flow for each value of the cell loss priority indication, the appurtenance of these cells to such a flow being 10 arbitrary.

Therefore there is a need for a device for the management of cell buffer .o memories receiving several cell flows, which are assigned different performance levels with regard to cell loss rate, and including some means designed to recognise that the loss of a cell is unavoidable when the buffer memory is full, together with some means to then decide that an incoming cell is to be stored in the buffer memory by removing another cell within the buffer memory, the latter cell will be lost while it will then be possible to store the former.

This need is met by the cell buffer memory management device in accordance with the invention, this device includes means of associating each cell flow with a dynamic indication of the cell loss status, based on the cell loss performance level assigned to this flow and the cell losses actually observed for this cell flow; it also includes means of selecting a cell flow, in particular by comparing the said dynamic cell loss indications, and consequently, recording the incoming cell in the buffer memory where space has become available further to the removal of a cell from the selected cell flow.

Hence, for each cell flow, the invention provides for the correlation between a cell loss performance level, i.e. a cell loss rate which should not be exceeded, and the cell losses actually observed; hence we can ascertain whether each cell flow has already been subjected to relatively high or low cell losses; i.e. whether this flow can withstand without problem a further cell loss, or whether it cannot and therefore should not be subjected to such an occurrence.

An advantage of the cell buffer memory management device is that it provides some means of dynamically preselecting the cell flows having at least one cell stored in the buffer memory, those means being coupled with the means used for selecting one cell flow, such that selection can be made from preselected cell flows.

According to a particularly simple method of implementation of the invention, the said dynamic cell loss status indication is obtained by some counting means associated with each cell flow, making it possible to ascertain the actual cell loss rate relating to a given cell flow, this rate being correlated with the said cell loss performance level of the given flow in order to supply the S said dynamic cell loss status indication.

In practice, the said counting means associated with each cell flow include a cell counter incremented by one step each time a cell is presented to the buffer memory and supplying a cell loss credit "token" once a preset number (Ni) of cells has been counted, and a token counter incremented by one step per token and decremented by one step each time a cell flow loses a cell.

The said preset number (Ni) of cells is defined as a function of the said cell flow cell loss performance level, such that the number of tokens counted by the said token counter represents the cell loss credit, i.e. the number of cells a cell flow S can lose in compliance with the said cell loss performance level, and the S positive or negative count generated by the token counter indicates at any given time the positive or negative cell loss status of this cell flow.

As an advantage, the said means of selecting a cell flow include some means which use the said dynamic cell loss status indications to sort the cell flows according to their level of suitability to witchstand a loss of cell; and some means of choosing a flow from the suitable flows or even from only the most suitable flows.

This choice can be made at random or according to an order of priority, whether or not cyclically modified. Preferably, it will be limited to the cell flow(s) best suited to withstand a cell loss.

The various objectives and characteristics of the invention will become clearer further to the following description of examples of implementation of the 6 invention, to be read in conjunction with the attached figure representing the block diagram of an example of implementation of the cell buffer memory management device in accordance with the invention.

The cell buffer memory management device is associated with a cell buffer memory receiving several input cell flows (FC1, FCi, FCn) which are assigned various cell loss rate performance levels, and, following multiplexing by means of a multiplexer (MUX), recording them in a buffer memory space (EMT) from which the cells are then extracted to be transmitted in the form of one or several output cell flows (FS).

The device (or buffer memory includes a cell detector (CD) coupled with the input cell flows (FC1 to FCn), identifying for each input cell o (by means of conventional analysis of its contents, either directly, or via a label translation function), the cell flow to which they belong, and supplying a signal (CR1, CRi, CRn) separately indicating the arrival of each input cell according to the cell flow to which it belongs.

A conventional buffer memory addressing device (DAMT) associated with the buffer memory or included in the device supplies the addresses of the vacant locations within the buffer memory space (EMT) and lists the occupied locations, separately, for the various input cell flows. It also supplies a °:2I signal (MTP) indicating how full the buffer memory is, one value of this signal (MTP) indicating that the buffer memory is full and that the loss of a cell is unavoidable; it also supplies one signal per cell flow (CP1 CPn) indicating the presence or absence of at least one cell, separately, for each of the cell flows in the buffer memory, this is easily deduced from the list of locations occupied by each cell flow (list empty or not empty).

These signals are transmitted to some selection means (SCDA) designed to decide, when signal IMTP indicates that the cell buffer memory is full, whether an incoming input cell should be stored in the buffer memory instead of another cell which will be lost, while the former can then be preserved, i.e. in general, to select the cell flow which, under similar circumstances, shall withstand the loss of a cell.

To this end, the device is also fitted with some means (CF1 CFi...

CFn) of associating each cell flow with a dynamic cell loss status indication (CLF1 CLFi CLFn) based on the cell loss performance level assigned to this flow and the cell losses actually observed for this cell flow, and of supplying these indications to the said selection means (SCDA).

In general, for each cell flow, the invention provides for the correlation between a cell loss performance level, i.e. a cell loss rate which should not be exceeded, and the cell losses actually observed; hence it can be ascertained whether each cell flow has already been subjected to relatively high or low cell losses; i.e. whether this flow can withstand without problem a further cell loss, or whether it cannot and therefore should not be subjected to such an occurrence.

In the method of implementation shown in the attached figure, the selection means (SCDA) are designed to dynamically preselect the cell flows having at least one cell stored in the buffer memory by reading the signals (CP1 CPn) such that the said selection is made from preselected cell flows.

The dynamic cell loss status indication (CLFi) for instance, is obtained by counting means associated with each cell flow (CCi, CLCAi) in this instance, making it possible to ascertain the actual cell loss rate relating to the cell flow, :2"1 this rate being correlated with the said cell loss performance level relating to this flow in order to supply the said dynamic cell loss status indication.

More precisely, the said counting means (CFi) associated with the cell flow (FCi) include a cell counter (CCi) incremented by one step each time a cell of the cell flow (FCi) is presented to the buffer memory and indicated by the signal (CRi). It supplies a cell loss credit "token" once a preset number (Ni) of cells has been counted, To this end, in this example of implementation it is complemented by a register (Ni) containing the value (Ni) and by a comparator (COMPi) which, when the counter (CCi) indicates the value supplies the token (JETi), this token controls the reset of the counter (CCi). The token (JETi) is transmitted to a token counter (CLCAi) which is incremented by one step per token and decremented by one step each time a cell flow (Fci) loses a cell as will be described later, 1 i 8 The said preset number (Ni) of cells is defined as a function of the said cell flow cell loss performance level, such that the number of tokens counted by the said token counter (CLCAi) represents the number of cells a cell flow can lose in compliance with the said cell loss performance level, and the positive or negative count generated by the token counter, which is the said positive or negative dynamic cell loss status indication, indicating at any given time the cell loss status of this cell flow in relation with this number.

The Said selection means (SCDA) include some means using the said dynamic cell loss status indications (CLF1 CLFi..., CLFn) to sort the cell flows according to their level of suitability to withstand the loss of a cell; and some means of selecting a flow from the suitable flows or even from only the most suitable flows. The selection thus made occurs in two ways: a signal is sent over one of the conductors (RT1 RTi... RTn) to remove one unit (one token) from the token counter (CLCAn for instance) associated with the selected cell flow (FCn), and a signal is sent over a set of conductors (FCS), identifying the selected cell flow, in order to trigger the storage of the incoming oa.o cell of the cell flow (FCi) in the buffer memory. Depending on the queue management method used (which is outside the scope of the invention), for s:oe*: instance first-in, first-out, etc., the cell is either recorded at the location that was freed or at another location after the queue has been rearranged. It should be noted that the selection means (SCDA) may very well select the cell flow (FCi). The effect will be the same: this cell flow will lose a cell. In this particular case, an equivalent solution consists of not accepting the input flow belonging to flow FCi, this cell will be lost.

The operation to be performed in the selection means (SCDA) includes a choice when several cell flows have all at least one cell in the buffer memory and the same token count which is the highest, or which is simply positive.

This choice can for instance be made: at random, or according to a cyclically modified order of priority, according to a non-modifiable order of priority, i.e.

preset; or according to the cell loss performance levels assigned to the relevant cell flows.

Of course, the means described above are given as an example only. In 1 S 9 practice, it is obvious that an implementation could be envisaged in the form of a set of logic circuits integrated in the same semi-conductor chip. In addition, at least part of the operations to be performed can be handled by a programmed processor, Depending on the technology used, the expert may have to modify the above mentioned implementation in order to achieve the best cost/performance ratio, in particular at the stage of splitting into functional blocks, the choice of connections between blocks which may be for serial or parallel transmission, the order according to which operations are to be performed, the inversion of some operations being possible, etc..

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Claims

1. A cell buffer memory management device for the management of cell buffer memories receiving several cell flows, which are assigned different performance levels with regard to cell loss rate and including means arranged to recognise that the loss of a cell is unavoidable when the buffer memory is full, together with means to then decide that an incoming cell is to be stored in the buffer memory by getting rid of another cell within the buffer memory, the latter cell will be lost while it will then be possible to store the former, wherein said device includes: means of associating each cell flow with a dynamic cell loss status indication based on the cell loss performance level assigned to this flow and the cell losses actually observed for this cell flow, and selection means to select a cell flow from one or more cell flows capable of sustaining the loss of a cell by comparing the dynamic cell loss indications, and substituting the incoming cell into the buffer memory in place of a cell of the selected cel flow, wherein the selected cell flows having a cell loss e rate lower than the individual predetermined cell loss rate allocated to each of the cell flows.

2. A cell buffer memory management device as claimed in claim 1, including means of dynamically preselecting the cell flows having at least one cell stored in the buffer memory, and characterised in that, the said means of So.° selecting a cell flow are coupled with the means of dynamically preselecting the cell flows, such that the said selection can be made from preselected cell flows. oo

3. A cell buffer memory management device as claimed in claim 1 or claim 2, wherein said dynamic cell loss status indication is obtained by counting means associated with each cell flow, making it possible to ascertain the actual cell loss rate relating to a given cell flow, this rate being correlated with the said cell loss performance level of the given flow in order to supply the said dynamic cell loss status indication 11

4. A cell buffer memory management device as claimed in claim 3, wherein said counting means associated with each cell flow include a cell counter incremented by one step each time a cell is presented to the buffer memory and supplying a token once a preset number of cells has been counted, and a token counter incremented by one step per token and decremented by one step each time a cell flow loses a cell.

A cell buffer memory management device as claimed in claim 4, wherein said preset number of cells is defined as a function of a cell flow cell loss performance level, such that the number of tokens counted by the said token counter represents the number of cells a cell flow can lose in compliance with the said cell loss performance level, and the positive or negative count generated by the token counter indicates at any given time the positive or negative cell loss status of this cell flow.

6. A cell buffer memory management device as claimed in any one of claims 2 to 5, wherein said means of selecting a cell flow include means using the said dynamic cell loss status indications to sort the cell flows according to their level of suitability to withstand a loss of cell; and means of choosing a flow from the suitable flows.

7. A cell buffer memory management device as claimed in claim 6, wherein said choice can be made at random.

8. A cell buffer memory management device in accordance with claim 6, wherein said choice can be made according to a cyclically modified order of priority.

9. A cell buffer memory management device in accordance with claim 6, 23 wherein said choice can be made according to a preset order of priority.

10. A cell buffer memory management device in accordance with claim 6, wherein said choice can be made according to the cell loss performance levels assigned to the relevant cell flows.

11. A cell buffer memory management device in accordance with claims 7, 8, 9 or 10, wherein said choice is limited to the cell flow(s) best suited to withstand the loss of a cell. 12

12. A cell buffer memory management device, substantially as herein described will reference to the figure of the accompanying drawing. DATED THIS NINETEENTH DAY OF JULY 1996 ALCATEL N.V S** S** S* **o *o ABSTRACT A cell buffer memory management device which receives several cell flows which are assigned different performance levels with regard to cell loss rate, and includes some means designed to recognise that the loss of a cell is unavoidable when the buffer memory is full, together with some means to then decide that an incoming cell is to be stored in the buffer memory in the place of another which will be lost. The device also includes some means CFn) of associating each cell flow FCn) with a dynamic cell loss status indication (CLF1..., CLFi..., CLFn) based on the cell loss performance level (Ni) assigned to this flow and the cell losses (RTi) actually observed for this cell flow, and some selection means (SCDA) used to select a cell flow, in particular by comparing the said dynamic cell loss indications, and consequently, recording the incoming cell in the buffer memory in the location of a cell from the selected cell flow. Figure to be published: Single figure.