referring firstly to fig6 , each satellite ground station 70 is associated with one or more terminals 71 which operate to transmit packet data to the satellite 72 , which relays them to other ground stations ( not shown ). the allocation of slots in the packet frame is controlled by a sequencer 73 located in the satellite 72 . a conventional allocation scheme is shown in fig1 . this consists of a series of data transmission slots d , f interleaved with dama request slots r . the request algorithm is given in fig3 . ground terminals 70 transmit requests for bandwidth with their uplink packet transmissions , using the adjacent request slots r as and when required . as has been stated , in the conventional arrangement the ground terminals 71 each request a number of slots d based on the instantaneous ground terminal total queue size ( irrespective of whether they form part of one burst or several ), and the number of outstanding slot requests . the frame format used in the prior art system described in international patent specification wo03 / 017589 is shown in fig2 , and is similar to the conventional arrangement shown in fig1 in that it consists of a series of data transmission slots d , f interleaved with dama request slots r . the algorithm used in this prior art system is shown in fig4 . the requests take the form of bandwidth signalling on a burst - by - burst basis . terminals 71 are flagged in the scheduler 73 as existing in one of two states , on or off . when a packet in a burst is transmitted on the uplink ( step 40 ) the terminal 71 determines the position ( pos ) of the packet in the burst ( step 41 ). the terminal 71 labels the first and / or last packets in order to indicate their position in the burst to the scheduler 73 . the first packet of a burst may carry a label not only indicating to the scheduler 73 that it is the first packet , but also indicating the length of the burst . in this case the scheduler 73 need only count the packets to identify the last one in the burst , and thus can re - set the terminal status without a further specific signal being transmitted . however , this arrangement is only possible if the length of the burst is already known to the terminal , so it is not possible to use this technique unless the entire burst is already in the queue at the terminal 71 . alternatively , the last packet of each burst may be labelled , allowing the scheduler to then recognise the next packet to arrive from the same terminal as being the first packet of the next burst . as seen in the truth table 42 ( fig4 ), when the scheduler 73 identifies a packet as the first of a burst , it changes the setting for the originating terminal to the on state ( step 43 ), and sets its status flag to on ( step 44 ). when the last packet of each burst is transmitted , the scheduler 73 changes the status of the relevant terminal 71 back to the off state ( step 45 ) and returns its status flag to off ( step 46 ). if the packet is neither the first nor the last , but some intermediate packet , the signalling and status flag remain unchanged . ( the truth table also shows two other conditions , when a packet labelled “ first ” is received when the terminal state is already on , or a “ last ” packet is received when the terminal state is off . both conditions can occur as the result of loss or corruption of an earlier packet . in each case the terminal state is already in the desired condition , so steps 43 - 46 are again omitted ) a process according to the present invention is illustrated in fig5 . those steps common with the prior art process of fig4 are indicated by the same reference numerals 40 , 43 , 44 , 45 , 46 . in this process no position indicators are transmitted . instead , the scheduler 73 detects the presence or absence of data ( step 51 ) to identify the beginnings and ends of data bursts . according to the truth table 52 , if the terminal status is off when data is detected , the flag for that terminal is set to on ( steps 43 , 44 ). it is maintained at on whilst data is present . if data ceases a counter is started which measures a time “ t ” ( step 520 ). if the value “ t ” reaches a threshold value “ t ” before any further data bursts are received , thereby signifying the end of a burst , the flag is reset to off ( step 45 , 46 ) as in the prior art system , the scheduler 73 in the satellite 72 maintains two lists : one containing the terminals whose status in on and one containing the terminals whose status is off . each frame consists of a variable number of free assigned slots determined by : number ⁢ ⁢ of ⁢ ⁢ free ⁢ ⁢ slots = ⁢ [ n off n tot ] * s tot * free max ⁢ n off & lt ; n tot = ⁢ s tot ⁢ n off = n tot number ⁢ ⁢ of ⁢ ⁢ demand ⁢ ⁢ slots = s tot - number ⁢ ⁢ of ⁢ ⁢ free ⁢ ⁢ slots n tot = total number of terminals s tot = total number of slots in the frame free max = maximum proportion of free slots thus , provided that at least one terminal is in the “ on ” state , a proportion of the slots ( at least 1 − free max ) are demand - assigned . the value of free max is set such that when demand is low ( but non - zero ) demand - assigned slots are not delayed by an excessive number of free - assigned slots . this proportion increases as the number of terminals in the “ on ” state increases . similarly , provided that at least one terminal is in the “ off ” state , the number of free slots is non - zero . the scheduler then allocates bandwidth to each terminal on a frame - by - frame basis , with the demand - assigned and free - assigned slots allocated to the on and off terminals respectively . there is no need to request capacity based on queue size . the number of available demand - assigned slots d and free - assigned slots f changes dynamically to suit the instantaneous requirements , by changing the position of the boundary b shown in fig2 . when all the nodes are off , the entire frame is free - assigned to minimise the signalling delay for terminals following the start of a burst . if the labelling of the packets includes an indication of burst length , the scheduler 73 can be arranged to distribute the demand - assigned slots between the bursts currently in progress in accordance with burst size . this may be done in several different ways , for example in order to weight the allocation in favour of the larger bursts so that subsequent bursts from the same terminal are not delayed unduly , or to give preference to any bursts which can be completed in the current frame , thereby releasing capacity for other bursts from the same or other terminals 71 . it should be noted that it is burst size , and not queue size , which determines the allocation of capacity in these embodiments . the size of a queue is constantly changing , requiring much more frequent signalling from the terminal 71 to the scheduler 73 than is the case when burst size is the determining factor . however , the size of an individual burst is fixed . with conventional dama schemes for data , traffic requests have to be made for transmission of each packet in the queue , resulting in delays whilst such requests are processed . in a satellite system such delays can be significant because of the distance of the satellite from the terminals . in the present invention , delays are minimised because once a terminal has signalled on , slots are continually provided without the need for repeated requests . in effect , the connectionless packet bursts are treated like a individual connection - oriented ( circuit - switched ) applications by providing a continual supply of capacity to a terminal for as long as necessary to transmit an individual burst . the minimum delay for each slots is reduced to one satellite hop instead of two , since no further requests for capacity have to be made whilst the status is set at on . the invention provides terminals with access rights to the demand assigned bandwidth , which is shared equally between them . the maximum channel throughput is dependent on the number of terminals currently being supported with no hard limit on the number of terminals that can be supported , simply a gradual reduction in the bandwidth available to each terminal as the number of active terminals increases .