GB2361147A - A packet scheduler and method - Google Patents

Abstract

In a CDMA cellular communication network, packet data may be broadcast at a discontinuous rate using a packet scheduler 7 which utilises available communication resources in each cell more efficiently than for continuous transmission. To maintain quality of service requirements, each packet scheduler is associated a power bin (9,10,11) which represents the total power available for transmission and reception in the cell. The maximum power level for each power bin may be adjusted in dependence of the quality of service experienced in each cell. A cell experiencing high levels of traffic may have its power bin increased to maintain acceptable carrier to interference levels at mobile stations 4,5, this is achieved by reducing the power bins of neighbouring cells which reduces intercell interference. Cell power levels are calculated using a genetic algorithm.

Description

2361147 A PACKET SCHEDULER AND METHOD

Field of the Invention

This invention relates to a packet scheduler and method t herefor, and in particular to a packet scheduler for a cellular communication system for mobile communication.

Background of the Invention

In a cellular communication system remote terminals (typically mobile stations) communicate with a fixed base station. Communication from the mobile stations to the base station is known as uplink and communication from the base station to the mobile stations is known as downlink. The total coverage area of the system is divided into a number of separate cells, each covered by a single base station.

The cells are typically geographically distinct with an overlapping coverage area with neighbouring cells.

As a mobile station moves from the coverage area of one cell to the coverage area of another - cell, the communication link will change from being between the mobile station and the base station of the first cell, to being between the mobile station and the base station of the second cell. This is known as a handover or handoff.

All base stations are interconnected by a fixed network. This fixed network comprises communication lines, switches, interfaces to other communication networks and various controllers required for operating the network. A call from a mobile station is routed through the fixed network to the destination specific for this call. If the call is between two mobile stations of the same communication system the 2 call will be routed through the fixed network to the base station of the cell in which the other mobile station currently is. A connection is thus established between the two serving cells through the fixed network. Alternatively, if the call is between a mobile station and a telephone connected to the Public Switched Telephone Network (PSTN) the call is routed from the serving base station to the interface between the cellular mobile communication system and the PSTN. It is then routed from the interface to the telephone by the PSTN.

A cellular mobile communication system is allocated a frequency spectrum for the radio communication between the mobile stations and the base stations. This spectrum must be shared between all mobile stations simultaneously using the system.

One method of sharing this spectrum is by a technique known as Code Division Multiple Access (CDMA). In a Direct Sequence CDMA (DS-CDMA.) communication system, prior to being transmitted the signals are multiplied by a high rate code whereby the signal is spread over a larger frequency spectrum. A narrowband signal is thus spread and transmitted as a wideband signal. At the receiver the original narrowband signal is regenerated by multiplication of the received signal with the same code. A signal spread by use of a different code will at the receiver not be despread but will remain a wide band signal. In the receiver the majority of interference caused by interfering signals received in the same frequency spectrum as the wanted signal can thus be removed by filtering. Consequently a plurality of mobile stations can be accommodated in the same wideband spectrum by allocating different codes for different remote terminals. Codes are chosen to minimise 3 the interference caused between mobile stations typically by choosing orthogonal codes when possible. A further description of CDMA communication systems can be found in .Spread Spectrum CDMA Systems for Wireless Communications', Glisic & Vucetic, Artech house Publishers, 1997, ISBN 0 89006-858-5. Examples of CDMA cellular communication systems are IS 95 standardised in North America and the Universal Mobile Telecommunication System (UMTS) currently under standardisation in Europe.

Traditional traffic in mobile cellular communication systems has been circuit switched voice data where a permanent link is set up between the communicating parties.

In the future it is envisaged that data communication will increase substantially and typically the requirements for a remote terminal to transmit data will not be continuous but will be at irregular intervals. Consequently it is inefficient to have a continuous link setup between users and instead a significant increase in packet based data traffic is expected, where the transmitting remote terminal seeks to transmit the data in discrete data packets when necessary. An example of a packet based system is General Packet Radio Service (GPRS) introduced to the Global System for Mobile communication (GSM). Further details on data packet systems can be found in 'Understanding data communications: from fundamentals to networking, 2nd ed.1, John Wiley publishers, author Gilbert Held, 1997, ISBN 0 471-96820-X.

In apacket based system where a high number of mobile stations may require resources for packet transmissions at unknown and irregular intervals it is important for optimal utilisation of the limited resource to schedule the order and time for transmission of the individual packets. This 4 becomes even more important when different data packets have different requirements with respect to delay, bit error rate etc. Therefore most packet based systems contain schedulers which control when the individual data packets are transmitted and therefore share the available resource, whether time-slots in a TDMA system or power and codes in a CDMA system. An introduction to schedulers can be found in 'Service discipline for guaranteed performance service in packet-switching networks', Hui Zhang, Proceedings of the

IEEE, volume 83, no. 10, October 1995.

However, known schedulers have been optimised for different environments than CDMA systems. For example, scheduling algorithms used for GPRS are optimised for a Time Division Multiple Access (TDMA) system and therefore not optimal for CDMA systems where codes and power must be shared.

This invention aims to provide a packet scheduler suitable for use in a CDMA system and which enables an efficient sharing of network resource.

Summary of the Invention

According to a first aspect, the present invention comprises apparatus for scheduling queued data packets for transmission between a plurality of base stations, each serving a respective cell in a cellular communications network, and a plurality of mobile stations located therein, the apparatus comprising:

means for assigning an associated power bin to each cell into which at least some of the queued data packets are to be placed, means for setting a maximum power level value for each power bin and for adjusting the maximum power level value for each power bin dependent on the quality of service experienced in the respective cell.

According to a second aspect, the present invention comprises a method for scheduling queued data packets for transmission between a plurality of base stations, each serving a respective cell in a cellular communications system, and a plurality of mobile stations located therein, the method including the steps of:

(a) assigning an associated power bin to each cell into which at least some of the queued data packets are to be placed, and setting a maximum power level value for each power bin and for adjusting the maximum power level value for each power bin dependent on the quality of service of experienced in the respective cell.

Preferably application of a genetic algorithm allows improved adaptation to changes in operating conditions.

Brief Description of the Drawings

Some embodiments of the invention will now be described, by way of example only, with reference to the drawings of which:

Fig.1 is a schematic block diagram of a cellular communications system operating in accordance with the invention; 6 Figs. 2, 3 & 4 are schematic diagrams illustrating the operation of a packet scheduler in accordance with the invention; Fig. 5 is a flow chart illustrating operation of a specific embodiment of the invention; and Fig. 6 shows a table illustrating a 'bottom up' adaptive technique used to determine the maximum power budget that can be used for scheduling packets within a distributed uplink scheduler.

Detailed Description of the Preferred Embodiments

The following embodiments are described within the context of the current approach for the standardisation of UMTS but it will be apparent that the invention is not limited to this application.

FIG 1 shows a schematic diagram of an embodiment of a CDMA communication system in accordance with an embodiment of the invention. The communication system has a number of base stations 1, 2, 3 each covering a geographical area and thereby defining a cell. A number of mobile stations 4, 5 are associated with the communication system and communicate to each other or to other systems via the base stations 1, 2, 3.

The base stations are connected to a common Radio Network Controller (RNC) 6. The RNC further provides gateways to other communication systems such as the fixed public telephone system (not shown) and contains a scheduler 7 7 U Each of the mobile stations 4, 5 have independent communication needs and communicate by use of data packets. The mobile stations 4, 5 may require different services and can for example be Internet browsers, telephones or data terminals. Each remote terminal may also request different services at different times.

The resource requirement for each individual mobile station may vary significantly over time so that a mobile station may sometimes require no transmissions and at other times require long transmissions at high data rate. The resource requirement for each mobile station from the communication network can thus vary significantly and in order to ensure that the available network capacity is used optimally an efficient scheduling of packets for the different mobile stations is required. This task is performed by the scheduler 7.

In a frame based communication system such as UMTS the communication is divided into discrete time intervals or f rames and the communication resource is allocated on a per frame basis. In UMTS, packets to be transmitted are scheduled during one time frame and transmitted during a subsequent frame.

In CDMA systems the total data throughput is affected by the interference at a mobile station's receiver caused by transmissions from base stations other than its serving base station. A successful sharing of the available resources of the system requires a knowledge of these interference levels so that the signal to noise ratio for each connection is appropriate for the desired quality of service (QoS). In order to perform an optimal allocation, transmissions from non-serving base stations must therefore 8 be taken into account and scheduling in all cells must be done simultaneously.

Consider sharing the power in a single cell. The scheduler's task is to divide the available transmit power of the base station (BS) between data packets removed in sequence from the queue. For each transmission the signal to noise ratio (SNR) at the destination mobile station must equal that necessary to achieve the agreed QoS. The SNR at 10 mobile station 1 is given by the following equation:

SATR:-' P00i / Li aPO (1 - 0) / Li + +Pthermal where P =B S transmit power Oi = fraction of power assigned to user i Li = path loss from BS to user i a = loss of orthogonality factor I interecit,i intercell interference at user i PthenmI thermal noise power added before the receiver To determine the transmit power to a mobile, equation (1) must be solved for TI, giving SM, (P,, Li + Iintercell,iLi +ap,) P,, (1 + SNRia) where Oi Is equivalent to one or more data packets.

.. (1) The path loss to a mobile station, the loss of orthogonality factor and the thermal noise power can all be measured by known techniques and reported to the scheduler.

.. (2', 9 P,can be set by the network operator and so can a target SNR. Intercell interference can be calculated from knowledge of transmit power of the other base stations in the network and from the path loss.

Thus, by substituting the relevant values into equation (2), a value for fractional power of every mobile station operative in a cell can be found. The total power transmitted (during a frame) from a base station will then be the sum of the individual (Di values (DD,) multiplied by p 0 However, the total transmit power of any base station is limited for practical reasons (e.g., depending on the capabilities of the power amplifiers). Say that this limit is the same for all base stations and has a value P,,max.

Hence, the number of packets that can be transmitted by a particular base station during a frame is similarly limited.

It will be appreciated, however, from inspection of equation (2) that if the intercell interference (for a particular cell) should decrease, then for a fixed target SNR, (D1 will also decrease and hence so will DD i. This means that more packets can be scheduled to and transmitted from the base station serving that cell without exceeding P,max.

Fig. 2 shows a schematic representation of the constituent parts of the scheduler 7. A single queue of packets 8 is allocated as necessary to one of three power bins 9, 10, 11 which relate respectively to base stations 1, 2 and 3.

Packets labelled 1 are destined for transmission by base station 1, packets labelled 2 are destined for base station 2 and packets labelled 3 are destined for base station 3. A maximum value for the power bins 9, 10, 11 is set at 5 P,,max.

A power bin represents the total available power at a base station, for either reception or transmission, which is to be shared by i users. The height of a data packet shown in the power bins of Figs. 2, 3 and 4 represents the amount of power required by a user per frame/scheduling period.

Fig. 3 shows the situation part-way through a frame/scheduling period. The scheduler has scheduled and calculated the values for (D1 for a number of packets destined for base stations 1, 2 3 and begun to fill up the power bins 9, 10, 11 accordingly. In order to calculate a value for intercell interference, the value for power transmitted by every base station is taken to be P,max. In Fig. 3 the power bin 9 for base station 1 is completely full with some packets in the queue 8 still remaining to be scheduled. On the other hand, power bins 10 and 11 are not yet full, so more packets are required to be scheduled for base stations 2 and 3 during this frame. Thus base stations 2 and 3 have spare resource which could be utilised by the over- loaded base station 1.

Heretofore, when a power bin of a certain base station became filled (it reached Pmax) then further packets directed this base station would have to wait until the next scheduling period/frame.

11 Heretofore, at the end of the scheduling, base stations 2 and 3 would transmit at a lower power because there would not be sufficient packets for the cells to reach P,,max (no packets queued for the cells would remain in the single queue.) Therefore, there resulted some unused capacity in the network.

Furthermore, heretofore, as packets were skipped once the p ower bin for a base station became filled, coverage of hot spots (which occur when there is a lot of high priority traffic in a single cell) would be prevented.

The present embodiment can utilise the unused capacity and further accommodate hot spots as follows.

In the present embodiment, P,max for power bins 10 and 11 is lowered to a new value P.max (new) [chosen by one of several methods to be discussed below]. The scheduler 7 then re-calculates the values for 0, for the base station 1 based on values for intercell interference determined from the adjusted values P,,max (new). The sum of the new 0, values will now have a value of less than unity. This means that more packets (remaining in queue 8) can be scheduled to base station 1 into power bin 9) yet still keeping total transmit power within the Pmax limit.

The re-configuration of the power bins at the end of the re-calculation procedure is shown in Fig. 4. At the end of the frame, base station 1 transmits all the queued packets for its cell at a total power of P,,max. Base stations 2 and 3 transit at a total power level of Pand P, both being 12 less than P,,max (new). In Fig. 4, all packets in queue 8 have been successfully scheduled during the one frame.

Hence to accommodate hot spots, the embodiment permits the 5 borrowing of downlink capacity from neighbouring cells.

One option for selecting which cells should have their maximum power reduced, is to choose those cells which generate the greatest interference to mobile stations lying within the hot spot cell.

In a further embodiment, the scheduler can be configured to redistribute any unused capacity within the network based on a re-calculation of the 01 values (and of 10,) in one or more neighbouring cells or all cells in the network.

A hot-spot is defined as a cell in which there is high priority traffic queued which cannot all be serviced if the network capacity is shared equally amongst all cells (P,,max equal in all cells). Hot-spots have limited lifespans, otherwise the network operator would deploy greater infrastructure in these areas to handle the greater demand. Examples include cells around a football ground (such a cell becomes 'hot' at half-time and the end of play as people phone home); and cells covering a stretch of motorway when there has been an accident and a queue of cars develops. It is of considerable benefit to an operator if hot-spots can be handled efficiently by careful radio resource management over a number of cells.

To handle a hot-spot it is necessary to be able to allocate packets from the single queue 8, which conventionally is ordered in terms of transmission priority, without skipping 13 packets. This is possible by borrowing capacity from other cells, in the manner described above with reference to Fig.

In order to choose the appropriate values for P,max (new) for any underused base station(s), the scheduler may keep reducing P,,max of the appropriate base station(s) and carry out the re-calculation of 0, values procedure until enough space has been created in the over-subscribed power bin to accommodate the queued packets. It is preferable to put a lower limit on P,,max (new) so that the capacity of the reduced power bins is not disadvantaged, i.e., there should be enough resource left available in the reduced bins so that their own packets which are already scheduled can be transmitted.

Different strategies may be employed in determining how to arrive at the best Pmax (new) value. For example, it is possible to reduce P.max in cells for which the path loss from the base station to the mobile stations in the overloaded cell is smallest. This will require a smaller reduction than for cells which are 'further' (in the path loss sense) from the overloaded cell. In other words, it is best to borrow capacity from the neighbouring cells.

Alternatively, it is possible to reduce P,max in proportion to the remaining available transmit power (designated Pr in Fig. 3) in the cells which have the greatest spare capacity.

As an alternative to this technique, the buffered traffic in queue 8 for each cell may be determined by the scheduler 14 7 prior to filling the power bins. Using this information the scheduler 7 can more accura tely determine which cells will have spare capacity and therefore should have their P,,max re-defined.

Making the changes to a minimum number of base station P,,max values minimises complexity of the scheduling process. The process is simpler if only P,max in one cell needs to be changed.

Using equation (2) the scheduler 7 forms an equation for the sum of Ti (including an additional packet [s]) in the overloaded cell, and solves this for a P,max (new) value of the single cell whose maximum power has been adjusted. If the scheduler were to adjust the P,,max in a number of cells simultaneously the mathematics is more complex and an iterative approach as outlined in Figure 5 (see below) is more appropriate. Following the accommodation of the additional packet(s) the scheduler recalculates the Ti values allocated in cells other than the over-loaded cell (since the intercell interference may have changed). Alternatively, it may be less complex to recalculate the (pi values for packets in a cell only when the scheduler is arranging for the addition of a packet to that same cell.

It is also possible to calculate the new (pi values for all packets in the one cell in one calculation.

Optionally, a safety margin may be introduced so that the sum of the desired recalculated (pi values is E(pi 1- margin.

Figure 5 shows a flow chart illustrating the adjustment of the maximum transmit power (P,max) in one or more cells to accommodate the allocation of an additional packet to a cell "A!' which is a "'hot spot". All steps are performed in the scheduler 7.

In Fig. 5, in step 12, P,,max is reduced for one or more neighbouring base stations. In step 13, the scheduler 7 re-calculates E(D1 including an additional data packet in cell A. In step 14 it tests the value of DD, for cell A to see if it lies within a pre-defined margin of value 1. if it does, then the scheduler 7 adds the additional data packet to the power bin of cell A (step 15). (It may also go on to calculate 0, values in all other cells [step 61).

If not, then it cheeks to see if further changes in P,,max for neighbours is possible (step 16). If so, the process repeats. If not, the process ends.

Employing the method for dealing with hot spots as described above has a limitation in that the overall capacity of the system can be-less than if the capacity is shared equally amongst all cells. This compromise may be addressed by considering a hybrid method which employs a priority obedience factor, gamma. In such a hybrid method the extent to which maximum transmit power may be reduced below the nominal value of Pmax is controlled by gamma. When gamma is set to 1, this is the hot spot method as described above and the transmit power P, max (new) may be set to zero if necessary. In addition, gamma values of between zero and 1 can be used where gamma controls either the maximum reduction of the transmit power below Pmax (i.e., this is a hard limit), or aspects of a probability 16 density function for the transmit power reduction, for example, the standard deviation (i.e., this is a soft limit). In this configuration there is both an attempt to follow the strict priority order and also to achieve good utilisation of the network.

In an alternative embodiment with reference again to Fig. 4, the P,,max (new) values of the under-utilised power bins (e.g., bins 10 and 11) are set at P 2 and P3 respectively.

The recalculation of 0, values and further allocation of queued packets to power bin 9 is then performed as above. In general terms, in this embodiment which permits sharing of capacity among cells, the scheduler freezes the value of P,,max (in a power bin which is unfilled) to the level reached at some pre-determined point during the frame, and then considers re-allocation of the further queued packets to cells whose power bins are already full. A procedure for this embodiment is as follows. At the pre- determined point in a frame, the scheduler sets P,max (new) for each power bin to the filled power level in each bin. For power bins thus having P,,max (new) less than P,max, these levels are effectively "'frozen'.. Next, the scheduler 7 recalculates (D1 values in cells which have P,max (new) equal to P,,max (i.e., full bins). The scheduler then continues scheduling and adds more packets where possible to the power bins not "frozen"'.

While the preferred embodiments have been described with respect to scheduling data packets on a downlink, it is to be understood that similar principles can be employed on an uplink, thus allowing system capacity sharing between cells.

17 In this case, a power bin now represents received power at a base station, and this includes both useful power and intercell interference. The total uplink power resource in a cell Rt,tal) is a constant based on the characteristics of the UMTS uplink. The capacity of the uplink is limited by the need to meet the signal to noise ratio for every transmission (SNR). Pt,t,l is set to such a level as to ensure a high sustainable throughput.

At the beginning of each frame, a maximum useable power for resource scheduling in any cell (P.a,) is set to an appropriate level based on the mean ratio of the received power at a base station from the mobile station it serves to the received power from all other mobile stations. (This ratio is set a approximately 1:0.6). This ensures that there is adequate headroom within any cell to allow for the neighbouring cells to be fully loaded and thus use their power resource fully.

The techniques as described above in the downlink case can be similarly applied in the uplink case in order to transfer power resource between cells whilst maintaining the Signal to Noise Ratio (SNR) and therefore the desired QoS. Again, the principle employed here is to reduce the P.. a,,in cells neighbouring a highly loaded cell. This means that while Ptotal remains constant, the split between usable uplink power Pm, at the receiver of a highly loaded base station and intercell interference is altered by lowering the amount of allowed intercell interference. This reduces the interference expected in the highly loaded cell, allowing Pm, to be increased, and thus allowing additional packets to be scheduled.

18 Referring now to Fig. 6, an alternative resource allocation strategy uses a 'bottom up' adaptive approach, in which 4 19 every cell decides its own P,,,,, based upon its quality of service (QoS) in the previous few frames. The problem of uplink power allocation across cells can be seen as a problem of managing a scarce, common resource or good. This type of problem is tackled using an adaptive approach. Because the problem is iterated (i.e. the same agents must split the resource repeatedly), adaptive strategies can lead to stable, efficient solutions.

In particular, the problem of determining Pmax. is efficiently tackled by a genetic algorithm which in effect holds a series of decision rules for setting the P,,,x of a cell. These rules are updated at regular intervals using a genetic algorithm. The result is an adaptive ruleset capable of reacting to variations in load and traffic statistics.

Thus, an adaptive algorithm is used to determine the maximum power budget that can be used for scheduling packets within a distributed uplink scheduler.

A genetic algorithm (GA) is an optimisation technique particularly suited to rugged multipeaked search spaces (full details of a GA are well known see Holland, "Adaptation in Natural and Artificial Systems", MIT Press, 1975). It is a simplification and abstraction of the biological processes of sexual reproduction and mutation under a selective pressure. It is particularly applicable in a situation such as the one presented, where there is naturally a population of agents (in this case base stations, one per cell), and a well defined measure of agent utility (usually referred to as fitness) exits; in this case a measure of attained QoS within any cell. It uses an analogue of the Darwinian principles of selection plus sexual reproduction to maintain a population of possible solutions. The genetic algorithm consists of a population of possible solutions, each encoded as a genotype. The genotype usually consists of a fixed length string, each position on the string being drawn from a limited alphabet (usually binary). A problem specific decoding algorithm is used to translate the genotype into a phenotype (a solution) whose utility may then be evaluated. In this case the genotype is an encoding of a rule-set used to alter the usage of the uplink bandwidth within a given cell.

The coding that has been used to encode the rule-set is based on the recent history of QoS within a given cell over the past three frames. In each frame the QoS is assessed as adequate/inadequate. Interpretingadequate as 1, and inadequate as 0, the history of the QoS over the past 3 frames can be written as a 3 digit binary number, i.e. within the range [0,7]. This is used as a key to access a lookup table of alterations (e.g., by 5%) to the power budget within a cell: (I)ncrease, (U)nchanged, or (D)ecrease. Hence the lookup table can be written as an 8digit string, as demonstrated in the table of Fig. 6.

A static lookup table is not intelligent; however, by dynamically altering this lookup table to suit the prevailing circumstances, both individual cells and the entire network can be made adaptive. A Genetic Algorithm is used for this purpose.

The GA acts to regularly (e.g., every 100 frames) replace the weakest rulesets (the replacement of at most 5% of the population is recommended). The strength of the ruleset used by each Base Station is defined as the QoS achieved by 21 that base-station since the GA was last run. Thus, an update regularly occurs of the lookup tables of cells that are performing badly relative to the population of cells as a whole. Within these cells a new lookup table is formed by combining aspects of the lookup tables of other, better performing cells. A background element of random variation is also introduced. This ensures that the system does not converge completely, and thus can respond better to a changing environment.

This type of coding has been shown to give impressive results in noisy environments. Not only can such a schema achieve high levels of cooperation between base stations with conflicting priorities, it has also been shown to be extremely stable to noise in the system, resulting from operating conditions such as external conditions (such as a rise in the background noise levels), or changes to the system (e.g. changes in the cell topography, or malfunction in a cell Base Station).

It will of course be appreciated that the examples described above may be embodied in and performed by s. oftware, which may reside in a data carrier such a magnetic or optical medium or programmed semiconductor memory device.

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Claims (7)

1. Apparatus for scheduling queued data packets for transmission between a plurality of base stations, each serving a respective cell in a cellular communicati.ons network, and a plurality of mobile stations located therein, the apparatus comprising: means for assigning an associated power bin to each cell into which at least some of the queued data packets are to be placed, means for setting a maximum power level value for each power bin and for adjusting the maximum power level value for each power bin dependent on the quality of service experienced in the respective cell.

2. Apparatus as claimed in claim 1 further comprising means for periodically applying to the cells a genetic algorithm dependent on quality of service to allow the maximum power level values to adapt to operating conditions.

3. A method for scheduling queued data packets for transmission between a plurality of base stations, each serving a respective cell in a cellular communications system, and a plurality of mobile stations located there the method including the steps of:

(b) assigning an associated power bin to each cell into which at least some of the queued data packets are to be placed, and setting a maximum power level value for each power bin and for adjusting the maximum power level value for each power bin dependent on the quality of service of experienced in the respective cell.

(c) 23

4. A method according to claim 2 further comprising the s tep of applying to the cells a genetic algorithm dependent on quality of service to allow the maximum power level 5 values to adapt to operating conditions.

5. A computer program product comprising a medium on or in which is recorded a program which, when executed in a computer-controlled system, will perform the method of claim 3 or 4.

6. A data packet scheduler substantially as hereinbefore described with reference to Fig. 6 of the accompanying drawings.

7. A method for scheduling queued data packets substantially as described with reference to Fig. 6 of the accompanying drawings.