GB2474269A

GB2474269A - Control and rate shaping of insertion of management packets into a data stream

Info

Publication number: GB2474269A
Application number: GB0917659A
Authority: GB
Inventors: Con Cremin; Anne Geraldine O'connell
Original assignee: Mingoa Ltd
Current assignee: Mingoa Ltd
Priority date: 2009-10-09
Filing date: 2009-10-09
Publication date: 2011-04-13
Also published as: GB0917659D0; WO2011042068A1

Abstract

A system for controlling the insertion of management packets into a stream of data packets, comprises an arbiter 12 for forming a mixed data stream of data packets and management packets; at least one FIFO 14 for controlling the supply of management packets for arbitration by the arbiter; a credit bank 18; means 17 for supplying indications of numbers of management packets to the FIFO and corresponding numbers of credits to the credit bank; an accumulator 19 of available credits; and a timer 15 for controlling a rate of supply of credits from the credit bank to the accumulator. The system allows insertion of management packets into the data stream when the number of available credits indicated by the accumulator is above a threshold. Particularly for use in inserting Operations, Administration and Management (OAM) packets into data streams.

Description

Control and rate shaping of insertion of-management packets into a data stream The present invention r&ates to the control of the insertion of management packets into a data stream. The main objects of the invention are to achieve a fair arbitration between packets from the data stream and the management packets, preferably in a gate-efficient manner and avoiding undue jitter in the transmission of the management packets.

The invention is particularly concerned with, and will be described in relation to, the transmission of OAM (operation, administration and management) packets. Their use in Ethernet networks is well established and is subject of specified standards, particularly IEEE 802.lag and ITU-T Y.1731.

Summary of the invention

The present invention provides a system for controlling the insertion of management packets into a stream of data packets, comprising: an arbiter for forming a mixed data stream of data packets and management packets; at least one FIFO for controlling the supply of management packets for arbitration by the arbiter; a credit bank; means for supplying indications of numbers of management packets to the FIFO and corresponding numbers of credits to the credit bank; an accumulator of available credits; and a timer for controlling a rate of supply of credits from the credit bank to the accumulator; wherein the system allows insertion of management packets into the data stream when the number of available credits indicated by the accumulator is above a threshold.

The said means is preferably an interval timer. It may comprise a plurality of interval timers each supplying a respective FIFO. The system may accordingly include a priority scheduler which controls the supply of management packets by the FIFOs to the arbiter.

Preferably the or each FIFO is constituted by a respective counter value which represents an accumulated count of management packets which are to be transmitted, the count being augmented by a predetermined number when a timer interval expires and being decremented by unity each time a management packet is transmitted.

The system preferably also includes a packet generator which generates the management packets from a template addressed by a pointer generated by reading a transmit list associated with each timer interval.

In order to explain the background to the invention and its significance reference will be made to the drawings.

Brief Description of the Drawings

Figure 1 is an explanatory diagram illustrating one option of insertion of OAM packets in a data stream.

Figure 2 is an explanatory diagram showing a second option for the insertion of OAM packets in a data stream.

Figure 3 is an explanatory diagram illustrating a third option for the insertion of OAM packets in a data stream.

Figure 4 is a diagram illustrating one embodiment of the invention.

Figure 5 is a diagram illustrating a more complex embodiment of the invention.

Detailed Description

Figures 1, 2 and 3 illustrate options which are of increasing complexity but which all present difficulties which are intended to be overcome by the present invention.

In Figure 1, data packets from a source 10 and OAM packets from an QAM packet generator 11 are subject to arbitration in an arbiter 12 and formed into a mixed stream 13.

OAM packets are generated at regular intervals, for example at the rate of 1000 packets every 3.3 microseconds. In the scheme shown in Figure 1 therefore, a single OAM packet is sent every 3.3 microseconds. If the data packet and the OAM packet collide, as detected by the arbiter 12, the current data packet must be stored until the OAM packet is sent. For the scheme shown in Figure 1 it is presumed that the arbiter 12 gives priority to OAM packets. That ensures that the OAM packets are sent precisely at specified rate within the tolerance allowed by the jitter specification.

However, if the data stream is busy but is interrupted by the presence of OAM packets, substantial storage space is required to store the delayed data packets.

Moreover, difficulty arises if an OAM packet is due for transmission according to the arbiter in the middle of a large data packet.

Figure 2 illustrates a modified scheme in which OAM packets from the OAM packet generator 11 are loaded into a FIFO 14. For example, 1000 OAM packets may be loaded into the FIFO every 3.3 microseconds. The arbitration performed by arbiter 12 may be changed to give priority to the data stream so that OAM packets are inserted into gaps as they occur in the data stream. In such a scheme, OAM packets left over from one interval can be sent to the next interval.

Although in Figure 2 the FIFO is shown (for simplicity) as actually storing OAM packets, as is well known this is not required or even customary. A preferred form of FIFO is described with reference to Figure 4.

There are several problems with the second option shown in Figure 2. The OAM packets are bunched and owing to the delay in sending of QAM packets, there may be a large jitter between successive OAM packets. It may be noted that the permissible jitter is normally a quarter of an interval time, for example 8.25 microseconds for a 3.3 microsecond interval.

Figure 3 shows a further option in which a credit timer 15 and a coincidence gate 16 have been added. The credit timer 15 accumulates credits at a programmable rate, for example one credit for every two microseconds. The coincidence gate 16 controls the sending of an OAM packet to the arbiter and will allow such sending only when an OAM packet exists and a credit is available.

This scheme regulates the rate of OAM insertion, helps to stop bunching, and reduces jitter. OAM packets can catch up during bursty data. Although the transmission of OAM packets may be held off, it can catch up when data stops by rapidly using the accumulated credits.

However, the accumulation of credits every set interval, such as two microseconds, can cause the accumulation of too many credits, For example, if a particular number (e.g. 1000) of OAM packets are to be sent in each interval, one should accumulate only the same number (1000) of credits per interval.

Figure 4 illustrates an embodiment of the invention. An interval timer 17 which supplies the FIFO 14 with accumulated counts (as described below) and also controls a credit bank' 18. A coincidence gate 16 is provided for the credit bank 18 and the credit timer 15. The coincidence gate 16 provides an output to credit accumulator 19 which contains available credits' and provides an indication of credit status to the arbiter 12. Here, the arbiter will allow transmission of packets as released by means of the FIFO into the data stream only when an appropriate credit status indication is received from the available credit accumulator 19.

As previously indicated, the FIFO does not itself store management packets.

Considerable efficiencies in the utilisation of gates can be achieved with the use of a virtual' FIFO. In particular, the FIFO is constituted by a single counter value (queueDepth), instead of a number of entries. The counter represents the accumulated count of management packets to be transmitted. The count is updated (augmented), each time the timer interval expires, with the number of packets that should be transmitted within the timer interval. This number will be added to any number currently held in the counter. Normally the count should be zero when the timer interval expires, indicating that all the previously' virtually stored' OAM packets had been transmitted. If not all such packets have been transmitted there will be a residue, now augmented by a new predetermined number.

The management (OAM) packets themselves are preferably generated from a template. This may be addressed by a pointer which can be generated by reading a transmit list 24 associated with each timer interval. If a given number (e.g. 1000) of OAM packets are to be transmitted in an interval (as above), the list would contain the same number (1000) of entries. The entries are consecutive, have a start and end location in memory, and each contain a pointer to an individual template. If the queueDepth' (as above) is greater than zero and a credit exists, then the list continues to be read one entry at a time. Appropriate hardware reads the OAM packet from the template indicated by the pointer and queues the packet for transmission.

In this scheme, when the timer interval expires, the FIFO 14 is loaded with an indication of a number (such as 1000) of packets and the credit bank 18 is loaded with an equivalent number of credits. When the credit timer 15 expires, a credit is moved from the credit bank 18 to the available credits' accumulator. An OAM packet will be sent by scheduler 20 only if a packet is available and credit is ready. This will regulate or rate shape the transmitted stream and there will be a one-to-one correspondence between OAM packets and credits.

When the count of available credits (in accumulator 19) is in excess of some appropriate threshold, for example 200 credits, the arbiter 12 is forced to give bandwidth to OAM packets. This can maintain the OAM jitter specification but also can optimise the opportunity to find gaps in the data stream, before the arbiter changes priority. The use of the threshold extends the duration in which the data stream has priority, and thereby optimises the opportunity for gaps to appear in that stream.

Figure 5 illustrates the extension of the scheme of Figure 4 to multiple OAM rates, there being an interval timer (17a, 17b...17N), a FIFO (14a, 14b... 14N) and transmit list (24a, 24b. ..24n) for each of N possible OAM rates. The credit bank 18 sums all the credits. When each interval timer expires, a number of OAM packets is added to the respective FIFO and a corresponding number of credits is added to the credit bank. The credit timer 15 moves credits from the credit bank 18 to the available credits accumulator 19.

The priority scheduler 20 will allow OAM packets from one or other of the FIFO-controlled queues to the arbiter 12 only if sufficient credit is available, It allots priority to the queues and of course services the higher priority queues before lower priority queues.

Claims

Claims 1. A system for controlling the insertion of management packets into a stream of data packets, comprising: an arbiter for forming a mixed data stream of data packets and management packets; at least one FIFO for controlling the supply of management packets for arbitration by the arbiter; a credit bank; means for supplying indications of numbers of management packets to the FIFO and corresponding numbers of credits to the credit bank; an accumulator of available credits; and a timer for controlling a rate of supply of credits from the credit bank to the accumulator; wherein the system allows insertion of management packets into the data stream when the number of available credits indicated by the accumulator is above a threshold.
2. A system according to claim 1 in which the said means is an interval timer.
3. A system according to claim 1 or claim 2 in which the said means comprises a plurality of interval timers each supplying a respective FIFO.
4. A system according to claim 3 in which a priority scheduler controls the supply of management packets by the FIFOs to the arbiter.
5. A system according to any foregoing claim in which the management packets are Ethernet OAM packets.
6. A system according to any foregoing claim in which the or each FIFO is constituted by a respective counter value which represents an accumulated count of management packets which are to be transmitted, the count being augmented by a predetermined number when a timer interval expires and being decremented by unity each time a management packet is transmitted.
7. A system according to claim 6 and including a packet generator which generates the management packets from a template addressed by a pointer generated by reading a transmit list associated with each timer interval.