GB2309863A

GB2309863A - Cell aligners

Info

Publication number: GB2309863A
Application number: GB9626386A
Authority: GB
Inventors: Geoffrey Chopping
Original assignee: GPT Ltd; Plessey Telecommunications Ltd
Current assignee: GPT Ltd; Plessey Telecommunications Ltd
Priority date: 1996-02-02
Filing date: 1996-12-19
Publication date: 1997-08-06
Anticipated expiration: 2016-12-19
Also published as: UA47457C2; GB9626386D0; GB2309863B; GB9602078D0

Abstract

For terminating an Asynchronous Transfer Mode (ATM) multiplexed data stream at a functional unit, where virtual channel data not including a frame alignment signal is carried in cells each having a message sequence number; a cell aligner having a plurality of cell buffers each storing the payload data contained in a single cell, the cell buffers being conditionally loaded in accordance with lower significant bits of the message sequence number provided the message sequence number does not correspond to the message sequence number of the cell buffer being depacketised and is not more than a pre-determined number of message sequence numbers in advance of the cell buffer being depacketised or an empty cell buffer being replaced by a fill-in cell and the cell buffers being emptied according to the conditions of a combined depacketiser and slip algorithm to form a continuous data stream aligned to the clock and frame start signal of the functional unit; the cell aligner further having a fill-in cell format generator which on the failure of a valid cell to arrive causes the replacement of the missing cell by a fill-in cell in the continuous data stream and a time-out function which is cleared and restarted after each valid cell is received in order to cause a restart of the combined depacketiser and slip algorithm if it should mature.

Description

CELL ALIGNERS Aligners have been used when terminating a synchronous multiplex, such as a framed 2048 kbit/s link, onto a clocked functional unit, where the clock of the functional unit and the synchronous multiplex cannot be assumed to be of the same frequency and phase.

The use of aligners implies a synchronous service which will tolerate (although reluctantly) some slips. The design characteristic of an aligner is that if the limit of its buffering is reached then a controlled slip is performed. Aligners not only must be able to accept any phase of the incoming stream, but also must include sufficient hysteresis to cope with delay variations introduced by the network.

The use of aligners can also be extended to data carried by a cell based Asynchronous Transfer Mode (ATM) Virtual Channel, so that traffic of a nominal constant bit rate can be transferred from an ATM multiplex onto functional units which are expecting traffic of the same nominal constant bit rate. Such a clocked functional unit could be a synchronous switch or a digital to analogue to decoder.

Asynchronous traffic is characterised by being carried in packets, frames or cells, cells being the term commonly used for ATM traffic. ATM cells are of a constant size and can carry a fixed amount of constant bit rate traffic data When an aligner slips it must miss out a fixed amount of data if the traffic is arriving too quickly. If the traffic is arriving too slowly, then either a fixed amount of data is repeated or a fixed substitution set of data is inserted.

A Virtual Channel ATM cell aligner described is suitable for cell streams where the cell delay variation is not greater than the average cell arrival rate of the cell stream. Consequently cell aligners are appropriate for low bit rate circuits such as 64 kbit/s.

According to the present invention in terminating an Asynchronous Transfer Mode (ATM) multiplexed data stream at a functional unit, where virtual channel data not including a frame alignment signal is carried in cells, each of the cells having a message sequence number, a cell aligner comprises a plurality of cell buffers each storing the payload data contained in a single cell, the cell buffers being conditionally loaded in accordance with lower significant bits of the message sequence number provided the message sequence number does not correspond to the message sequence number of the cell buffer being depacketised and is not more than a predetermined number M of message sequence numbers in advance of the cell buffer being depacketised or an empty cell buffer being replaced by a fill-in cell; and the cell buffers being emptied according to the conditions of a combined depacketiser and slip algorithm to form a continuous data stream aligned to the clock and frame start signal of the functional unit; a fill-in cell format generator which on the failure of a valid cell to arrive causes the replacement of the missing cell by a fill-in cell in the continuous data stream; and a time-out function which is cleared and restarted after each valid cell is received in order to cause a restart of the combined depacketiser and slip algorithm if the time-out should mature.

The depacketiser contains sufficient cell buffers to handle the expected delay variation, 2 cell buffers for less than 2 millisecond and 4 cell buffers for up to 6 milliseconds of delay variation.

When a cell is loaded into a cell buffer within the depacketiser, a full/empty binary flag is set to full and the time-out is cleared and re-started. When a cell buffer has just finished being emptied, its full/empty binary flag is set to empty.

There is further provided a method of aligning one ATM Virtual Channel cell Stream, of an ATM multiplex which includes a depacketiser wherein the combined depacketiser and slip algorithm is defined as: if on two successive occasions during the depacketisation of cell (n), or the generation of a fill-in cell to replace cell (n), the arrival of cell (n+ M) occurs before B bytes of cell (n) have been depacketised, then the first S bytes of the next cell to be depacketised, or replaced by a fill-in cell, are discarded in order to introduce a forward slip of S bytes; otherwise if after the completion of the depacketisation of cell (n) or the generation of a fill-in cell to replace a missing cell (n), cell (n+l) has been received then cell (n+l) is fully depacketised and becomes the new cell (n); or if after the completion of the depacketisation of cell (n) or the generation of a fill-in cell to replace a missing cells), cell (n+l) has not been received then cell (n+1) is replaced by a fill-in cell and becomes the new cell (n); unless if within S byte periods of the starting of a fill-in cell to replace cell (n), the missing cell (n) is received, then a backward slip of S bytes is achieved by halting the fill-in cell after S bytes and depacketising the received cell (n).

When M = 3, B = 47 bytes and S = 47 bytes; When M = 2, B = 47 bytes and S = 23 bytes; When M = 1, B = 15 bytes and S = 16 bytes.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a typical deployment of depacketiser and cell aligner functions; relative to packetisers, analogue to digital converters and digital to analogue converters; Figure 2 illustrates a 47 (or 23) byte adjustment cell aligner; Figure 3 illustrates a 16 byte adjustment cell aligner; Figure 4 illustrates the combined depacketiser and slip algorithm for a 47 byte adjustment cell aligner; Figure 5 illustrates the combined depacketiser and slip algorithm for a 23 byte adjustment cell aligner; Figure 6 illustrates the combined depacketiser and slip algorithm for a 16 byte adjustment cell aligner, Figure 7 provides the key for Figures 4 to 6.

Figure 8 shows a comparative timing chart for the algorithms of Figures 4 to 6.

The object of the present invention is to minimise the consequences of the extra delay and delay variation incurred by 64 kbitls circuits when traversing Asynchronous Transfer Mode (ATM) networks as shown in Figure 1.

The delay variation introduced to the 64 kbit/s circuits will be removed but at the expense of adding some further delay.

There are many cases where the external interface will have a common clock for more than one 64 kbit/s circuit (eg, 144 kbit/s ISDN links and 2 Mbit/s links) and therefore aligning techniques have to be used.

The unpacking of the 47 bytes of data from a cell will be performed using a cell aligner arrangement working to one of the combined depacketiser and slip algorithms shown in Figures 4,5 and 6 depending on the Cell Delay Variation that has to be catered for.

In order to performance monitor a circuit across the network additional Operational and Maintenance (OAM) cells may also be occasionally sent across the network, This means the ATM network must be dimensioned to carry 128 kbit/s bursts of traffic without causing cell loss or exceeding the total network delay variation figure. The packetisation function must generate an ordinary cell (non OAM cell) regularly every 5.875 ms.

Consequently the total one way end to end network delay variation, even in the presence of OAM cells needs to be less than 5.8 ms, 2.8 ms or 1.8 ms, (depending on the combined depacketiser and slip algorithm used) if slips are to be avoided.

Slips are far more service affecting than corrupted data: especially for modem traffic, encrypted data and video coding. Consequently these combined depacketiser and slip algorithms aim to minimise the risks of slips occurring.

Slips can consist of the removal of 47 (23 or 16) bytes of data, or the addition of a fillin sequence of 47 (23 or 16) bytes of data.

If a cell slip should occur, a further cell slip should not happen until a further phase shift of at least + or - 5.8 ms (2.8 ms or 1.8 rns) has occurred.

If the local reference clock is not phase related to the original 64 kbit/s data source, then regular slips may occur. However, they will occur 47 (23 or 16) times less often than frame aligners as disclosed in Patent Numbers GB 2063624 and GB 2151437 corresponding to US 4,368,531 and US 4,617,659 respectively. The cell aligners ensure that unnecessary cell slips do not occur, whether due to initialisation, delayed cells, missing/corrupted cells or additional cells. The total loop delay added by the ATM Adaptation Functions, namely; packetisation, depacketisation and cell aligning functions, should in practice remain at a constant of approximately 24 ms (18 ms or 16 ms).

This extra adaptation delay value of 24 ms (18 ms or 16 ms) should apply for a synchronised network from initialisation until a break of at least 15 ms occurs, provided the delay variation figure of 5.8 ms (2.8 ms or 1.E ms) is not exceeded.

The 24 rns (18 ms or 16 ms) of adaptation delay is in addition to the initial path loop delay; the initial path loop delay results from transmission propagation delays, transmission multiplexing delays, ATM switching delays, ATM buffering delays, analogue to digital and digital to analogue delays (if applicable) and private network delays ( if applicable). The ATM switching and buffering delays being those ATM switching and buffering delays that delayed the first cell in each direction after a break.

The adaptation loop delay is mainly made up of a packetiser and depacketiser delays of 5.875 ms each way and each aligner having 5.875 ms (2.875 ms or 2.000 ms) worth of data in hand on initialisation. If cell slips occur because of a free running local reference then the loop delay can be increased or decreased by 5.875 ms (2.875 rns or 2.000 ms). If the delay variation exceeds 5.8 ms (2.8 ms or 1.8 ms) then both aligners could increase or decrease by 5.875 ms (2.875 rns or 2.000 ms).

Three compatible combined depacketiser and slip algorithms for the cell aligners are available to meet future network requirements.

Normal additional constant loop delay is approximately: 24 ms for both ends using 47 byte adjustments as shown in Figure 2; 18 ms for both ends using 23 byte adjustments as shown in Figure 3; 16 ms for both ends using 16 byte adjustments as shown in Figure 4; an average value results if the two ends use different algorithms.

Any slips which occur introduce a phase shift of 5.875 ms or 2.875 ms or 2.000 ms depending on which algorithm is in use.

After initialisation there should be no slips, if the ATM Network Cell Delay Variation is less than 5.8 ms, 2.8 ms or 1.8 ms.

An ATM cell aligner containing 4 cell buffers each of 47 bytes can be used for 47, 23 and 16 byte adjustment algorithms, as shown in Figures 2 and 3.

These algorithms have been designed to minimise the chance of cell slips. The algorithms initialise to the central position of the operational range of the buffer. They do not do individual byte slips, but do slips of 47 (23 or 16) bytes when really necessary.

Because the cell aligner initialises to its central position, this results in 24 ms (18 ms or 16 ms) of additional loop delay. On end to end network connections this additional delay should normally remain at that value, unless there is significant Cell Delay Variation.

The conditions for trying to initialise are that: all the cell buffers are empty; and that 2.5 nominal cell periods have elapsed (15 ms) since a valid cell was received.

(A valid cell has to have an appropriate sequence number).

Once the "trying to initialise" state has occurred, if a cell is then received with any sequence number, it is accepted and after a wait of 5.875 ms (2.875 ms or 2.000 ms) then depacketisation starts.

The cell aligner with 4 cell buffers, as shown in Figure 2, can be controlled so that it performs one of three algorithms which are defined by the three tables in Figures 4, 5 and 6 and the following explanatory notes.

A cell aligner with only 2 cell buffers, as shown in Figure 3, can only operate the 16 byte adjustment algorithm defined by the table shown in Figure 6.

When a cell is accepted into one of the four cell buffers it is written into the buffer which corresponds to the least significant 2 bits of the cell sequence number. Consequently four cell buffers are used even though only three are shown in the 23 byte adjustment table.

For the 16 byte adjustment algorithm, used by the cell aligners as shown in Figure 3 the least significant bit of the cell sequence number is needed to define the cell buffer.

If a cell is to be depacketised then it is defined as cell n. Normally cell n+l becomes the next cell n. A cell is depacketised during an internal cell cycle.

Cells are only accepted, if its cell sequence number satisfies the conditions specified in the table, the sequence number of the current cell n being known.

The fully empty states at the start of each internal cell cycle can be one of the A to F, (A to E) or (A to D) states shown.

For the 47 and 23 byte adjustment algorithms, during an internal cell cycle up to two cells can be accepted for some of the full/empty states. If two are accepted then they must have arrived in the correct sequence.

The "go to" indication confirms the full/empty states for the start of the next internal cell cycle.

For the 16 byte adjustment algorithm an early cell is defined as a cell arriving when less than 15 bytes of the previous cell or fill in cell have been depacketised. If it arrives early by 5.875 ms (47 bytes) or more then it is rejected as being out of sequence.

Claims

1. In terminating an Asynchronous Transfer Mode (ATM) multiplexed data stream at a functional unit, where virtual channel data not including a frame alignment signal is carried in cells, each of the cells having a message sequence number, a cell aligner comprises a plurality of cell buffers each storing the payload data contained in a single cell, the cell buffers being conditionally loaded in accordance with lower significant bits of the message sequence number provided the message sequence number does not correspond to the message sequence number of the cell buffer being depacketised and is not more than a pre-determined number M of message sequence numbers in advance of the cell buffer being depacketised or an empty cell buffer being replaced by a fill-in cell; and the cell buffers being emptied according to the conditions of a combined depacketiser and slip algorithm to form a continuous data stream aligned to the clock and frame start signal of the functional unit; a fill-in cell format generator which on the failure of a valid cell to arrive causes the replacement of the missing cell by a fill-in cell in the continuous data stream; and a time-out function which is cleared and restarted after each valid cell is received in order to cause a restart of the combined depacketiser and slip algorithm if it should mature.

2. A cell aligner as claimed in Claim 1, wherein the combined depacketiser and slip algorithm is defined as: if on two successive occasions during the depacketisation of cell (n), or the generation of a fill-in cell to replace cell (n), the arrival of cell (n+M) occurs before B bytes of cell (n) have been depacketised, then the first S bytes of the next cell to be depacketised, or replaced by a fill-in cell, are discarded in order to introduce a forward slip of S bytes; otherwise if after the completion of the depacketisation of cell (n) or the generation of a fill-in cell to replace a missing cell (n), cell (n+l) has been received then cell (n+l) is fully depacketised and becomes the new cell (n); or if after the completion of the depacketisation of cell (n) or the generation of a fill-in cell to replace a missing cell (n), cell (n+l) has not been received then cell (n+l) is replaced by a fill-in cell and becomes the new cell (n); unless if within S byte periods of the starting of a fill-in cell to replace cell (n), the missing cell (n) is received, then a backward slip of S bytes is achieved by halting the fill-in cell after S bytes and depacketising the received cell (n).

3. A cell aligner as claimed in Claim 2, wherein: the depacketiser contains at least 2 cell buffers; and slips are of 16 bytes so that the combined depacketiser and slip algorithm is defined as: if on two successive occasions during the depacketisation of cell (n), or the generation of a fill-in cell to replace cell (n), the arrival of cell (n+l) occurs before 15 bytes of cell (n) have been depacketised, then the first 16 bytes of the next cell to be depacketised, or replaced by a fill-in cell, are discarded in order to introduce a forward slip of 16 bytes; otherwise if after the completion of the depacketisation of cell (n) or the generation of a fill-in cell to replace a missing cell (n), cell (n+1) has been received then cell )n+1) is fully depacketised and becomes the new cell (n); or if after the completion of the depacketisation of cell (n) or the generation of a fill-in cell to replay a missing cell (n), cell (n+l) has not been received then cell (n+l) is replaced by a fill-in cell and becomes the new cell (n); unless if within 16 byte periods of the starting of a fill-in cell to replace cell (n), the missing cell (n) is received, than a backward slip of 16 bytes is achieved by halting the fill-in cell after 16 bytes and depacketising the received cell (n).

4. A cell aligner as claimed in Claim 2, wherein: the depacketiser contains at least 4 cell buffers and slips are of 23 bytes so that the combined depacketiser and slip algorithm is defined as: if on two successive occasions during the depacketisation of cell (n), or the generation of a fill-in cell to replace cell (n) , the arrival of cell (n+2) occurs before all the bytes of cell (n) have been depacketised, then the first 23 bytes of the next cell to be depacketised, or replaced by a fill-in cell, are discarded in order to introduce a forward slip of 23 bytes; otherwise if after the completion of the depacketisation of cell (n) or the generation of a fill-in cell to replace a missing cell (n), cell (n+l) has been received then cell (n+l) is fully depacketised and becomes the new cell (n); or if after the completion of the depacketisation of cell (n) or the generation of a fill-in cell to replace a missing cell (n), cell (n+l) has not been received then cell (n+l) is replaced by a fill-in cell and becomes the new cell (n); unless if within 23 byte periods of the starting of a fill-in cell to replace cell (n), the missing cell (n) is received, then a backward slip of 23 bytes is achieved by halting the fill-in cell after 23 bytes and depacketising the received cell (n).

5. A cell aligner as claimed in Claim 2, wherein: the depacketiser contains at least 4 cell buffers and slips are of 47 bytes so that the combined depacketiser and slip algorithm is defined as: if on two successive occasions during the depacketisation of cell (n), or the generation of a fill-in cell to replace cell (n), the arrival of cell (n+3) occurs before all the bytes of cell (n) have been depacketised, then the next cell to be depacketised, or replaced by a fill-in cell, is discarded in order to introduce a forward slip of 47 bytes; otherwise if after the completion of the depacketisation of cell (n) or the generation of a fill-in cell to replace a missing cell (n), cell (n+1) has been received then cell (n+1) is fully depacketised and becomes the new cell (n); or if after the completion of the depacketisation of cell (n) or the generation of a fill-in cell to replace a missing cell (n), cell (n+l) has not been received then cell (n+1) is replaced by a fill-in cell and becomes the new cell (n); unless if during a fill-in cell to replace cell (n), the missing cell (n) is received, then a backward slip of 47 bytes is achieved by completing the fill-in cell and then depacketising the received cell (n).

6. A cell-frame aligner as claimed in Claim 1 and substantially as hereinafter described with reference to and illustrated in the accompanying drawings.