AFDX NETWORK SUPPORTING A PLURALITY OF SERVICE CLASSES

Abstract

The invention relates to a frame switch for an AFDX network, including a plurality of input ports (615), a plurality of output ports (645), multiplexing means (630) to multiplex the frames arriving at an input port towards one or more of said output ports, control means (620) for controlling said multiplexing means. Each output port is connected by its input to at least two FIFO buffers (640, 641, 642), one of which (640) is dedicated to the deterministic flows of said network, the control means being additionally adapted to determine whether a frame belongs to a deterministic flow and, in the affirmative, to command the multiplexing means to direct said frame to one or more buffers (640) dedicated to the deterministic flows.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in diagrammatic form a simple example of an AFDX network;

FIG. 2 shows in diagrammatic form the structure of a conventional AFDX network switch;

FIG. 3 shows the flow management in a conventional AFDX network switch;

FIG. 4 shows in diagrammatic form the structure of a switch according to a first embodiment of the invention;

FIG. 5 shows in diagrammatic form the flow management in a switch according to a first embodiment of the invention;

FIG. 6 shows in diagrammatic form the structure of a switch according to a second embodiment of the invention;

FIG. 7 shows the flow chart of a flow management method for a switch according to a second embodiment of the invention.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

The idea underlying the invention is to provide for each output port of an AFDX network switch, at least two queues one of which is reserved for the deterministic flows.

In a first embodiment of the switch according to the invention, two FIFO buffers are associated with each output port, one of them being dedicated to the deterministic flows. The queue of the FIFO buffer dedicated to the deterministic flows has priority relative to the other. To be more precise, the buffer relating to the non-deterministic flows is only stripped when the one dedicated to the deterministic flows is empty. In this way, given the frames of the deterministic flows, the network behaves practically as it does in the absence of any non-deterministic flow.

An AFDX network equipped with switches of this kind can support two service classes: a deterministic flow class and a class of the “best effort” type.

For the first service class, a prior check is made analytically, for example using the “network calculus” algorithm, that the routing solution for the virtual links belonging to the service class meets the latency limit and proper routing constraints in full.

On the other hand, for the second service class, neither latency limit nor probability of proper routing of the frames between the source terminal and the destination terminals is guaranteed.

FIG. 4 shows in diagrammatic form the structure of a switch according to a first embodiment of the invention, the elements 415, 430, 445 are identical to the elements 215, 230, 245 in FIG. 2. Unlike the switch in FIG. 2 however, each output port 445 is connected to two FIFO buffers 440, 441, one of them, for example buffer 440 being dedicated to the deterministic flows and the buffer 441 to the other flows. It is important to note that the deterministic flows are thereby physically segregated from the other flows. The outputs of the buffers 440 and 441 are connected to the input of the output port 445. In this way, the control means are adapted to arbitrate the access of the buffers 440 and 441 to the output port by transmitting a stripping order to one or other buffer. It is only when the buffer 440 is empty that the control means authorise the buffer 441 to access the output port.

FIG. 5 shows the management flow in a switch according to the first embodiment. Further consideration is given to the case of the three virtual links in FIG. 3 with the slight difference that only the virtual link VL₁ enjoys a guarantee of determinism.

According to a first variant, the switching table, shown at (B) additionally includes, for each virtual link, an identifier of the service class associated with it, here d for a link with a determinism guarantee and d for a link with no guarantee.

According to a second variant, the switching table is identical to that in FIG. 3 (B), in other words it does not include a field Class_id. In this variant, the header of each frame contains, apart from the virtual link identifier VL_id, the service class identifier Class_id associated with this link. The controller 420 reads on the fly the link identifier and the service class identifier of each incident frame and as a result commands the multiplexing means 430 to direct the frame towards the buffer 440 or the buffer 441 of the output port relative to the virtual link identified by VL_id.

It should be noted that if the virtual link is of the multi-point type, the frames may be switched to a plurality of output ports. In this case, as a function of the service class, the frame will be copied and stored in the buffers 440 or the buffers 441 associated with said output ports. The Output Port field then contains the list of output ports towards which the frame is to be switched.

At (C) it can be seen that the frames of VL₁ are arranged in the order of their arrival in the buffer 540 and the frames of VL₂, VL₃ are arranged in the order of their arrival in the buffer 541. At (D) have been shown the frames emanating from the output port s₃ as a function of time. The buffer 540 is emptied first, and then the frames contained in the buffer 541 are stripped.

In a second embodiment of the switch according to the invention, shown in FIG. 6, a plurality n>2 of FIFO buffers is provided per output port, one buffer being dedicated to the deterministic flows, one or more buffers being dedicated to the flows for which a statistical quality of service is guaranteed and one buffer being dedicated to the flows for which no guarantee is offered.

The elements 615, 630, 645 are identical to the elements 215, 230, 245 in FIG. 2 and will not be further described. Unlike the switch in FIG. 2 however, each output port 645 is connected to at least three FIFO buffers 640, 641, 642. One of them, for example the buffer 640 is dedicated to the deterministic flows. One or more buffers 641 are dedicated to the flows for which a statistical service quality is guaranteed. Lastly, a buffer 642 is dedicated to the flows with no service quality guarantee. The control means 620 are adapted to arbitrate the access of the buffers 640, 641, 642 to the output port 645 giving respective stripping orders to the aforementioned buffers.

Statistical quality of service is defined as a probability of proper routing or, in an equivalent way as a loss rate. Another potential type of statistical quality of service is that of a median bandwidth. As far as the loss rate is concerned, a frame will be lost when the output buffer in which it is to be stored has reached saturation. The loss rate may be estimated in different ways particularly by statistical calculation, by Monte Carlo type simulation or by using measurements on representative networks. In the event of frame loss, a error recovery mechanism will be provided at a higher protocol level.

FIG. 7 shows the flow management method for the switch in FIG. 6. It is applied to the output ports in parallel, so that they are processed equitably.

A check is made at 710 as to whether the buffer 640 is empty and, in the negative, it is used at 720, in other words its oldest frame is stripped. The process is iterated until the buffer 640 is empty. The next stage is testing 730 where a check is made as to whether the or all the buffers 641 are empty. In the affirmative, the testing stage 750 is implemented. In the negative, a distinction is made between a single buffer 641 and a plurality of buffers 641. In the first case, the single non-empty buffer is served at 740. In the second case, a selection strategy explained in detail below is applied. Then go back to testing 710.

At 750, a check is made as to whether the buffer 642 is empty. In the affirmative go back to testing 710. In the negative, this buffer is used at 760 then go back to testing 710.

According to a first strategy, the buffer 641 to be used at 740 is selected as a function of a priority level assigned to each buffer: a non-empty buffer of given priority level is only used if the non-empty buffers of higher priority levels have all been used. To do this, a service flag is used for each buffer and when all the buffers 641 have either been used or are empty, the flags are re-initialised.

According to a second strategy, the buffer to be stripped is selected according to a cyclical or pseudo-random sequence, each buffer 641 having on average a probability p_i of being used at stage 740, with of course

$\sum _{i=1}^Np_i=1.$

The probabilities p_i are advantageously selected different so as to offer different statistical service quality classes.

Among these service quality classes, only the deterministic flow class allows a virtual circuit to be established between any two terminals of an AFDX network. On the other hand, the statistical service quality classes and all the more so the class with no service quality guarantee are not able to guarantee the proper routing of the frames. They do however allow the network resources left available by the deterministic flows to be used effectively.

Claims

1. Frame switch for an AFDX network, including a plurality of input ports (415, 615), a plurality of output ports (445, 645), multiplexing means (430, 630) to multiplex the frames arriving at an input port towards one or more of said output ports, control means (420, 620) for controlling said multiplexing means, characterised in that each output port is connected by its input to at least two FIFO buffers, one of which is dedicated to the deterministic flows of said network, the control means being additionally adapted to determine whether a frame belongs to a deterministic flow and, in the affirmative, to command the multiplexing means to direct said frame to one or more buffers dedicated (440, 640) to the deterministic flows.

2. Switch according to claim 1, characterised in that each output port (445) is connected by its input to two FIFO buffers (440, 441), a first buffer (440) being dedicated to the deterministic flows and a second buffer (441) being dedicated to the other flows, the control means being adapted to arbitrate the transfer of frames from the first and second buffers towards said output port such that a frame from the second buffer is only transferred towards said port when the first buffer is empty.

3. Switch according to claim 1, characterised in that each output port (645) is connected by its input to a first FIFO buffer (640) dedicated to the deterministic flows, at least one second buffer (641) dedicated to flows that have a statistical service quality guarantee and a third buffer (642) dedicated to the other flows, the control means (620) being adapted to arbitrate the transfer of frames from the first, second and third buffers towards said output port such that a frame from a second buffer is only transferred towards said port when the first buffer is empty and that a frame from the third buffer is only transferred towards said port when the first buffer and the second buffer or buffers is or are empty.

4. Switch according to claim 3, characterised in that each output port is connected to a plurality of second buffers (641), each second buffer being associated with a preset priority level, the control means being adapted to transfer a frame from a second buffer of given priority level towards said output port only if each second buffer of said plurality associated with a higher priority level has itself been stripped of a frame or is empty.

5. Switch according to any one of the previous claims, characterised in that it additionally includes a switching table indicating, for each virtual link to be switched, the output port or ports towards which the link is to be switched as well as the service class to which it belongs.

6. Switch according to claim 5, characterised in that the control means are adapted to extract from each incident frame a virtual link identifier and to deduce therefrom based on the switching table the output port or ports towards which the frame is to be switched, and, for each of these output ports, the FIFO buffer in which it is to be stored.

7. Switch according to any one of claims 1 to 4, characterised in that it includes a switching table indicating, for each virtual link to be switched, the output port or ports to which the link is to be switched, the control means being adapted to extract from each incident frame a virtual link identifier as well as a service class identifier to which it belongs and to deduce therefrom based on the switching table the output port or ports towards which the frame is to be switched, and, for each of these output ports, the FIFO buffer in which it is to be stored.

8. AFDX network characterised in that it includes a plurality of frame switches according to one of the previous claims.

9. Aircraft characterised in that it includes an AFDX network according to claim 8.