Abstract:
The present invention relates to methods for changing the bandwidth of a circuit switched channel in a time division multiplexed network, wherein said channel comprises a set of time slots within each recurring frame of a bitstream between a first node and a second node. According to the invention one or more additional time slots within each recurring frame of said bitstream are reserved, including using, during a period of time, only said set of time slots for transmitting payload data pertaining to said channel while providing, during said period of time, information indicating that said one or more additional time slots are currently not used for transferring payload data. After said period of time, said set of time slots is used as well as said one or more additional time slots on said bitstream for transmitting payload data pertaining to said channel.

Description:
TECHNICAL FIELD OF INVENTION 
     The present invention relates to methods changing the bandwidth of a circuit switched channel in a time division multiplexed network, said channel comprising a set of time slots within each recurring frame of a bitstream of said network. 
     BACKGROUND OF THE INVENTION 
     In recent years, the need for network solutions providing quality of service in high bandwidth applications has evolved as a result of the increasing demand for transfer of, e.g., real-time speech, video and multimedia over networks such as the Internet. 
     The use of circuit switched networks, which have the inherent property of providing each host with a guaranteed bandwidth, has been found to provide many advantageous features in this context. 
     A new circuit switched network solution, which has received much interest over the last few years, is known as DTM (Dynamic synchronous Transfer Mode). In a DTM network, circuit switched isochronous channels may be dynamically established, modified, and terminated based upon changes in user capacity requirements. 
     An advantage with DTM is that the bandwidth of an established isochronous channel may be dynamically changed by the allocation or deallocation of time slots to said channel. However, changing the bandwidth of an existing channel requires synchronization between sender and receiver(s), as well as intermediate nodes, to avoid losing data (in case the sender increases the number of slots allocated to a channel before the receiver does), or to avoid receiving garbage data (if the receiver increases the number of slots allocated to said channel before the sender does). 
     Different suggestions have been made of how to arrange the necessary bandwidth change synchronization to avoid these problems. One suggested solution has been to simply stop sending data while the bandwidth change is being performed. However, this means that the channel is unused during a period of time, thus inevitably leading to a waste of bandwidth resources. Another suggested solution has been to ensure that the bandwidth change is being made simultaneously at sender, receiver, and any intermediate nodes, by numbering the network frames and, using control signaling for agreeing on which specific frame to effectuate the change. This of course has the drawback of requiring extra signaling and the provision of an often complicated frame counting mechanism. 
     In view of the above, an object of the invention is to provide a simple, reliable, safe and effective mechanism for synchronizing bandwidth changes in a circuit switched networks of the kind mentioned in the introduction, especially in a DTM network. 
     SUMMARY OF THE INVENTION 
     The above mentioned and other objects of the invention are achieved by the invention as claimed in the accompanying claims. 
     Hence, according to one aspect of the invention, there is provided a method of the kind mentioned in the introduction, said method being used when increasing the bandwidth of a channel and being characterized by: reserving, for said channel, one or more additional time slots within each recurring frame of said bitstream, including using, during a period of time, only said set of time slots for transmitting payload data pertaining to said channel while providing, during said period of time, information indicating that said one or more additional time slots are currently not used for transferring payload data; and using, after said period of time, said set of time slots as well as said one or more additional time slots on said bitstream for transmitting payload data pertaining to said channel. 
     According to another aspect of the invention, there is provided a method of the kind mentioned in the introduction, said method being used when decreasing the bandwidth of a channel and being characterized by: using, during a period of time, only a portion of said set of time slots for transmitting payload data pertaining to said channel while providing, during said period of time, information indicating that the remaining time slots of said set of time slots are curretly not used for transferring payload data; and deallocating, after said period of time, said remaining time slots from said channel while continuing using said portion of said set of time slots for transmitting payload data pertaining to said channel. 
     The invention is thus based upon the idea of not using, during the bandwidth change, those time slots that are to be allocated/deallocated to/from a channel (or at least a number of time slots (within the channel) corresponding to the number of time slots that are to be allocated/deallocated to/from the channel), and to provide the bitstream with information designating them as providing invalid data, i.e. as not payload data, typically by marking these time slots as being idle, while at the same time using those time slots that are to be unchanged (i.e. slot already belonging to the channel in the case of a bandwidth increase, or slots that are not to be deallocated from the channel in the case of a bandwidth decrease) for transferring payload data. 
     For example, if a sending node wants to allocate additional resources to a channel, it may at any time reserve additional time slots on the bitstream that it is connected to, as long as it marks the additional time slots as being idle and only transmits payload data in the time slots that already belonged to the channel prior to said allocation. When the sending node later receives information indicating that downstream nodes have allocated the desired extra bandwidth to the channel and/or are “listening”to the additional time slots, the sending node may at any time discontinue transmitting said information designating said additional time slot as being idle and instead start transmitting payload data using said additional time slots as well. 
     Consequently, said period of time will preferably end with, or shortly after, the reception at the sending node of information indicating that concerned downstream nodes switching or receiving said channel have allocated the desired extra bandwidth to the channel and are listening thereto and/or are ready to switch data received therein. However, according to an alternative embodiment, said period of time will be fixed period of time. In such a case, the sending node, after having sent a request for additional capacity to concerned downstream nodes, and after having waited said fixed period of time, will assume that any intermediate nodes switching said channel, and the receiving node listening to said channel, will have had enough time to allocate the necessary resources and to provide the necessary mapping. The sending node will thus simply assume that the extra resources are“up and running” and consequently start transmitting payload data in said additional time slots. This, of course, may in some cases result in loss of data, but will on the other hand provide a simpler mechanism requiring less signaling. 
     Similarily, having received instructions to allocate additional resources to an existing channel, an intermediate node switching said channel may for example at any time allocate additional time slots to said channel on the outgoing bitstream that the channel is switched to as long as the intermediate node provides the outgoing bitstream with information indicating those time slots of the outgoing bitstream that are so far not transferring payload data. When having received instructions as to which time slots that constitute said additional resources slots on said incoming bitstream, and having made sure that concerned downstream nodes have acknowledged the bandwidth increase, the intermediate node will provide the necessary mapping of data with respect to the added time slots and will discontinue transmitting said information indicating that said additional time slots are so far not transferring payload data. 
     In other words, the acknowledgement that the additional bandwidth is accepted is preferrably performed backwards, starting from the receiving node. When having made the neccessary arrangements for listening to the new bandwidth, it will notify the upstream next hop intermediate node handling the channel that it is ready to receive payload data in the new slots. The last intermediate node will then, having made its neccesarry arrangements for switching the added slots, notify its upstream next hop intermediate node similariy, and so on. When, finally, the intermediate node located closest to the sending node notifies the sending node that the added bandwidth is accepted all the way, the sending node may, at any time, start using the additional time slots for transmitting payload data. 
     The invention is also advantageous in the context of multi- or broadcasted channels having several receivers. According to one embodiment of the invention, if wanting to make sure that there is no data loss or the like at any one of the receivers of the channel, before discontinuing sending idle markings in the added time slots, each intermediate node shall make sure to receive information indicating that all downstream nodes receiving or switching said channel have acknowledged the bandwidth change before the intermediate node may go ahead and send an acknowledgement of (and start mapping from/to) the added time slots to the sender or the upstream next hop intermediate node. Thus, when the sender has received one or more acknowledgements indicating that all nodes receiving or switching said channel are “listening” to the added time slot, it may at any time start sending payload data thereon. 
     An advantage of the invention is thus that the need for stringent bandwidth change synchronization between nodes involved in the management of a channel is relaxed. Using the invention, the sending, receiving, and intermediate nodes may, separately and with comparatively relaxed synchronization requirements, allocate additional time slots and start using additional time slots, as long as said additional time slots are for the time being marked as being idle, i.e. not providing payload data. When control signaling between nodes then indicate that the channel from sender to receiver is “up and running”, the sending node may at any time, without any need for notifying downstream nodes, simply switch from marking said additional time slots as idle to transmitting payload data into said additional time slots. 
     Similarly, when decreasing the size of a channel, the sending node may, for example, simply stop transmitting payload data in one or more time slots to be deallocated from the channel, and instead start marking said one or more time slots as being idle, thereby making sure that downstream nodes will not consider data provided in said one or more time slots as valid payload data, while continuing transmitting payload data in those time slots that are not to be deallocated. The sending node may then, at any time, instruct downstream nodes, such as the receiving node or an intermediate node, to deallocate resources corresponding to said one or more time slots from said channel. Each intermediate node, when having received confirmation from downstream nodes that they are aware that the subject resources are no longer to be part of the channel, may then, at any time, completely deallocate said one or more time slots from the channel without any further need for synchronizing the deallocation with other nodes, also informing upstream nodes that it is no longer listening to the deallocated time slots. 
     According to a preferred embodiment of the invention, the herein discussed bandwidth change instructions and/or bandwidth change acknowledgements are preferably transmitted between nodes using control channels that are established to comprise one or more time slots on the bitstreams of the network but are separated from the payload carrying channels, thereby not interfering with the ongoing traffic. 
     A very advantageous aspect of the invention is thus that the decisions at different nodes as to when to allocate and start using additional time slots, or when to stop using and deallocate time slots, may be made more independently than compared to prior art solutions. 
     Another important advantage of the invention is that payload data are transmitted (at least if so desired) in the unaffected time slots of the channel while the bandwidth change is being executed, thereby efficiently limiting the degree of temporary bandwidth blocking during the change. 
     According to a preferred embodiment of the invention, said information indicating that a time slot is not used for transferring payload data is provided by the step of marking said time slot as being idle, typically by sending an idle time slot pattern, for example, using an idle code word if using 8B/10B encoding, into said time slot or by setting a one-bit flag of the time slot to indicate that the time slot is invalid, i.e is not transferring payload data. In such cases, the information stating that a time slot does not transfer payload data is advantageously provided within the time slot itself. 
     However, the manner in which one or more time slots are marked as not providing payload data may be selected in many other ways. For example, the information indicating that a time slot is not providing payload data need not be provided within the time slot as such, but may be derived from another location or part of the associated bitstream, i.e. using some kind of out of band signaling. 
     In this context, it is to be noted that the actual writing of a code word, setting of a flag, or out of band signaling may just as well be provided by a link head end node marking by default essentially all slots on the bitstream as idle, thus leaving it up to downstream nodes, such as the transmitting node, to “unmark” those slots that will transfer payload data. 
     Of course, the degree of dynamics by which the features of the invention may be utilized at the nodes handling a channel will depend upon in which degree the nodes handling the channel are equipped with capability to detect, interpret, and generate said information indicating that a time slot is not providing payload data. However, even if only the receiving node is capable of detecting and discarding idle time slots and only the sending node, and preferrably also intermediate switch nodes, are capable of generating idle data, i.e. of designating time slots not providing payload data as being non-valid, the invention will still come to advantageous use. 
     Furthermore, if said information designating that a time slot is not providing payload data is provided by the writing of an idle block or code word into said time slot, this information is easily propagated to a next hop bitstream by the simple mapping of the idle block from said time slot to a time slot of the channel on the next hop bitstream. Consequently, no extra processing in addition to mere time slot mapping is in such a case needed at the intermediate node to detect the presence of an idle time slot and to provide the next hop bitstream with corresponding information. 
     A problem which may arise when allocating additional time slots to a channel (or deallocating time slots from said channel) and when said channel extends over more than one bitstream, is that whereas one node may want to allocate, for example, an additional time slot occupying a time slot position after the last one of the time slots already allocated to the channel on the respective bitstream, another node may want to allocate a time slot position in the middle of the time slots already allocated to the channel, on the respective bitstream, thus potentially cauting re-ordering of data. (Generally, this problematic situation will not occur at nodes that use channel based mapping, since such a node will typically make sure that the sequential order data received within a channel is preserved.) 
     One way to avoid this problem would be to only allow intermediate nodes to allocate new slots to a channel in the same time slot order as the sending node. However, this would drastically decrease the possibility to set up the channel and thus result in unnecessary blocking. 
     Another way of solving this problem is to re-configure (referred to herein as “re-mapping”) the mapping instructions at each concerned intermediate node so as to ensure that the order of time slot data is preserved even if the position of the “new” time slot with respect to the other time slots allocated to the channel is different on different bitstreams. Such a re-mapping need then to take place before upstream nodes starts to sending or forwarding payload data instead of idle data blocks in said additional time slots. 
     According to a preferred embodiment of the invention, in order to make sure that said re-mapping doesn&#39;t result in that payload data is re-mapped to a slot not yet recognized by a downstream receiving node as belonging to the channel, any re-mapping to be performed among the time slots forming the channel at an intermediate node, as a result of the change of the amount of resources allocated to said channel, is carried out after the acknowledgement of the bandwidth change from downstream intermediate nodes located closer to a receiver of the channel (which nodes as such may also have had to perform re-mapping among the time slots forming the channel at their respective locations), and preferrably also before use of the added slots is acknowedge to upstream node being the sender of or switching said channel. In other words, any necessary mapping or re-mapping is preferrably performed backwards along with the bandwidth change acknowledgement discussed above, starting from the intermediate node located closest to the receiving node. Preferrably, when having received acknowledgement that the receiving node is aware that the additional slots now belongs to the channel, the last intermediate node will perform it&#39;s mapping (or re-mapping), and will then notify the upstream next hop intermediate node that the new time slots are in use up to that location, said upstream next hop intermediate node thereby being allowed to perform its own re-mapping, if so desired, and so on. When, finally, the intermediate node located closest to the sending node notifies the sending node that mapping has been established up to that location, the sending node may, at any time, start using the additional time slots for transmitting payload data. 
     As is understood by those skilled in the art, the way in which the invention is realized, for example with respect to re-mapping issues, will partly, but not necessarily, depend on whether or not channel or slot based mapping is used at the nodes switching said channel. 
     Even though applicable in many different types of circuit switched time division multiplexed networks using isochronous channels, the most preferred use of the invention is in a so called Dynamic synchronous Transfer Mode (DTM) network, wherein the possibility to dynamically change bandwidth is a very advantageous feature. 
     For definition, when used herein, the term “isochronous channel” refers to a channel carried on a bitstream wherein the frames of the bitstream occur at a regular pace and wherein the set of time slots defining the channel on that bitstream occupies the same time slot positions within each frame on that bitstream. 
     Further description of ways to incorporate and detect as such the presence of information designating time slots as not providing payload data may for example be found in the co-pending patent application SE 9703449-0. For specific details on a preferred way of implement idle markings in for example a DTM network, reference may be made to currently ongoing DTM standardization procedure (Ref: ETSI ES 201 803) within the European Telecommunications Standards Institute, ETSI. 
     Also, even though the invention is described more i detail herein with respect to embodiments wherein signaling takes place between the sending node, the receiving node and posible intermediate nodes switching said channel, embodiments of the invention could just as well be realized using for example signaling to/from one or more “master” nodes that handle all signaling with respect to one or more of the involved channel handling nodes. 
     The above-mentioned and other aspects, features, and advantages of the invention will be evident from the accompanying claims and from following description of exemplifying embodiments thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplifying embodiments of the invention will now be described with reference to the accompanying drawings, wherein: 
         FIG. 1  schematically shows a DTM network; 
         FIG. 2  schematically shows allocation of time slots to channels on a bitstream; 
         FIGS. 3   a – 3   f  schematically illustrates a DTM network and its bitstreams and time slots during a change of channel bandwidth according to an embodiment of the invention; 
         FIGS. 4   a – 4   c  schematically illustrates a DTM network during a change of channel bandwidth requiring re-mapping of time slots according to another embodiment of the invention; and 
         FIG. 5  schematically shows an example of the exchange of messages between nodes during a change of bandwidth according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A time division multiplexed network, in this example being a DTM (Dynamic synchronous Transfer Mode) network, will now be described with reference to  FIG. 1 . In  FIG. 1 , five nodes  111 – 115  are connected via three multi-access bi-directional links, a first one formed by bitstreams  101   a  and  101   b , a second one formed by bitstreams  102   a  and  102   b , and a third one formed by bitstreams  103   a  and  103   b . Nodes  111  and  112  are connected to the first link, nodes  112  and  113  to the second link, and nodes  113 – 115  to the third link. Consequently, node  112  provides switching between the first and second link and node  113  provides switching between the second and third link. 
     The data transport structure of the bitstreams in  FIG. 1 , using bitstream  101   a  as an example, will now be described with reference to  FIGS. 2   a – 2   b.    
     As shown in  FIG. 2  at (a), the bitstream  101   a  is divided into recurrent, essentially fixed sized, e.g. 125 μs, frames. In turns as illustrated at (b), each frame is divided into a plurality of fixed sized, e.g. 64 bit, time slots. As shown at (a), the start of each frame is defined by a frame synchronization time slot F. 
     The time slots of a frame are generally divided into control slots and data slots. The control slots are used for signaling between the nodes of the network, whereas the data slots are used for the transfer of payload data. The write access to both control slots and data slots are distributed as desired to the nodes having access to the respective bitstream. The nodes uses the data slots to define channels CH 1 , CH 2 , CH 3 , CH 4  on the bitstream. In  FIG. 2  at (b), it is for example assumed that channel CH 1  has been established from node  111  to node  112 . As shown, each channel is allocated a respective set of slots. In the example, the transfer capacity of channel CH 1  is larger than the transfer capacity of channel CH 2 , since the number of time slots allocated to channel CH 1  within the frame is larger than the number of time slots allocated to channel CH 2 . The time slots allocated to a channel occupy the same time slot positions within each recurrent frame of the bitstream. As is understood, a channel may be defined over more that one bitstream, then comprising a set of slots on each of the bitstrems that it is defined over. In such a case, switch nodes will perform the necessary mapping of slots from time slot positions on one bitstream to time slot positions on another. 
     Exemplifying procedures for changing the bandwidth of a channel according to an embodiment of the invention will now be described with reference to  FIGS. 3   a – 3   f , all schematically illustrated a simplified view of the DTM network of  FIG. 1 . 
     In  FIG. 3   a , it is assumed that a circuit switched channel has been established from node  111  to node  114  via nodes  112  and  113 . As schematically illustrated in  FIG. 3   a , the channel is defined to comprise two time slots (marked black in  FIG. 3   a ) within each frame of bitstreams  101 ,  102 , and  103 . More specifically, the channel is defined to comprise the first and second time slot within each frame of bitstream  101 , the first and third time slot within each frame on bitstream  102 , and the third and fourth time slot within each frame on bitstream  103 . Consequently, node  112  is arranged to map the content of the first and second time slot on bitstream  101  into the first and third time slot, respectively, on bitstream  102 . Likewise, node  113  is arranged to map the content of the first and third time slot on bitstream  102  into the third and fourth time slot, respectively, on bitstream  103 . 
     In this embodiment, node  111  manages allocation of time slots to said channel on bitstream  101 , node  112  manages allocation of time slots to said channel on bitstream  102 , and node  113  manages allocation of time slots to said channel on bitstream  103 . 
     It is now assumed that, based upon an end user request or for some other reason, node  111  decides to increase the bandwidth of the channel, in this example by one time slot per frame. Preferably using its own pool of free slots, node  111  allocates the desired bandwidth, in this case one slot per frame, to the channel on bitstream  101 , in this example the third time slot within each frame. 
     Using control signaling in a channel defined by a control time slot position on bitstream  101 , node  111  sends a bandwidth change request to node  112  requesting an increase in the bandwidth allocated to said channel corresponding to one slot per frame and informing node  112  that time slot three on bitstream  101  is now to be part of the channel. At the same time, node  111  continues transmittin payload data in the previously already allocated first and second time slots, and starts transmitting idle markings into the third time slot within each frame of bitstream  101 .  FIG. 3   b  illustrates this situation. In  FIG. 3   b , node  111  has allocated an additional third time slot to said channel on bitstream, and is transmitting payload data into slots one and two and idle markings (as illustrated with an x-marking) into slot three, whereas node  112  and  113  have not yet allocated any additional time slots to said channel on bitstreams  102  and  103 . 
     Since node  112  in the situation shown in  FIG. 3   b  has not yet allocated any additional time slots to the channel on bitstream  102 , it has not yet started to map the third time slot from bitstream  101  into bitstream  102 . 
     In a similar manner, preferably using their own pools of free slots, nodes  112  and  113  will correspondignly allocate the desired bandwidth, in this example one slot per frame, to the channel on bitstreams  102  and  103 , respectively. It is thus assumed that node  112  allocates the fourth time slot within each frame of bitstream  102  to said channel, and that node  113  allocates the sixth time slot within each frame of bitstream  103  to said channel. Also, node  112  will transmit an announcement message to node  113  informing which time slot (in this case time slot four) on bitstream  102  is now to be part of the channel, and node  113  will transmit an announcement message to node  114  informing that time slot six on bitstream  103  is now to be part of the channel. 
     Having been informed of the new time slot, the receiving node  114  will start listening thereto and will acknowledge use of the new slot using an acknowledge message sent in a control channel to node  113 . As a result, node  113  may stop sending idle into time slot six on bitstream  103  and instead start mapping data from all three slots on bitstrems  102  to all three slots on bitstream  103 . 
     Having done so, node  113  will acknowledge use of the added time slot to node  112 , whereby node  112  may stop sending idle markings into time slot four on bitstream  103  and instead start mapping data from all three slots on bitstrems  101  to all three slots on bitstream  102 . Resulting in the idle-mapping situation illustrated in  FIG. 3   c.    
     Node  112  will then acknowledge use of the added time slot to node  111 , whereby node  111  will determine that it may now go ahead and, at any time, start using the additional time slot for transferring payload data, which will then be mapped by nodes  112  and  113  to reach node  114 , as is illustrated in  FIG. 3   d.    
     As is understood, if an intermediate node, e.g. node  112  or  113 , allocates the requested additional time slot to the channel before the upstream next hop node has had time to allocated a time slot to said channel for said intermediate node to map data from, said intermediate node may still inform a downstream next hop node that said additional time slot is allocated to said channel as long as said intermediate node provides the additional time slot with idle data blocks designating the additional time slot as not providing payload data. 
     Starting again from  FIG. 3   a , wherein said circuit switched channel has been established from node  111  to node  114  via nodes  112  and  113 , it is now assumed that node  111 , based upon an end user request or for some other reasons has decided to decrease the bandwidth of the channel, in this example by one time slot per frame. 
     Having decided which time slots to deallocate from said channel, node  111  will start marking the time slot selected to be deallocated as idle, but will continue to transmit payload data into the time slot not selected to be deallocated. In  FIG. 3   e , node  111  has decided to deallocate the second time slot within each frame on bitstream  101  from said channel and is thus transmitting payload data into slot one and idle markings into slot two (as illustrated with an x-marking), which is then mapped by node  112  and  113  to reach node  114  in time slot four within each frame on bitstream  103 . Hence, at this point, the idle marked time slots arriving at node  114  in time slot four of bitstream  103  are consequently discarded by node  114 . 
     Using control signaling, node  111  now instructs node  112 , and indirectly (via node  112 ) node  113 , to deallocate the time slot corresponding to time slot two on bitstream  101  from said channel on their respective bitstreams, and also indirectly (via node  113 ) node  114  that time slot four on bitstream  103  is no longer to be considered part of the channel. 
     Having received such instructions, and having stoped listening to the so identified time slots, each one of the nodes  112 ,  113 , and  114 , will, starting from the receiving node and going down-up in similar to what has been described above, stop using (deallocate) the concerned time slot and send a message acknowledging the bandwidth change confirming that they have performed the requested deallocation. Receiving the final confirmation from node  112 , node  111  may at any time deallocate said second time slot on bitstream  101  from said channel, thus leaving only one time slot allocated to said channel, as illustrated in  FIG. 3   f.    
     An exemplifying procedure for changing the mapping of time slots when performing a change of bandwidth will now be described with reference to  FIGS. 4   a – 4   c.    
       FIGS. 4   a – 4   c  schematically show a simplified view of the DTM network of  FIG. 1 . In  FIG. 4   a , in similar to  FIG. 3   a  above, it is assumed that a circuit switched channel has been established from a sending node on bitstream  101  to a receiving node on bitstream  103  via two intermediate nodes, wherein the first intermediate node provides time slot mapping between bitstream  101  and bitstream  102  and wherein the second intermediate node provides mapping beteen bitstream  102  and  103 . For simplicity, none of these nodes are showed in  FIGS. 4   a – 4   c.    
     As schematically illustrated in  FIG. 4   a , the channel is defined to comprise the first and second time slot within each frame of bitstream  101 , the fourth and sixth time slot within each frame on bitstream  102 , and the eighth and ninth time slot within each frame on bitstream  103  (as indicated by full line squares in  FIG. 4   a ). Consequently, the first intermediate node is arranged to map the content of the first and second time slot on bitstream  101  into the fourth and sixth time slot, respectively, on bitstream  102 . Likewise, the second intermediate node is arranged to map the content of the fourth and sixth time slot on bitstream  102  into the eighth and ninth time slot, respectively, on bitstream  103 . 
     It is now assumed that the sending node decides to increase the bandwidth of the channel, in this example by one time slot per frame. Using control signaling the sending node sends a request to the intermediate nodes requesting an increase in the bandwidth allocated to said channel corresponding to one slot per frame. 
     In this example, it is assumed that the sending nodes decides to allocate the third time slot within each frame on bitstream  101  to the channel and start sending idle markings therein (as indicated by the broken line square in  FIG. 4   a ). However, it is also assumed that the second intermediate nodes decides to allocate the fifth time slot within each frame on bitstream  102  and that the second intermediate node decides to allocate the seventh time slot within each frame on bitstream  103 . Thus, a slot mapping inconsisteny problem is present, since the allocated additional time slots occupy the last position within the channel on bitstreams  101  and  103 , but the second last position on bitstream  102  and the first position on bitstream  103 . 
     Thus remapping is needed. In this embodiment the remapping is performed starting from the last intermediate node. Hence, before the second intermediate node acknowledges the bandwidth change, and after it has received information from the downstream receiving node that it is listening to the added time slots, the second intermediate node will switch to map data so that the fourth time slot on bitstream  102  is mapped to the seventh, newly allocated time slot on bitstream  103  and the newly allocated fifth time slot on bitstream  102  is mapped to the eighth time slot on bitstream  103 , as illustrated in  FIG. 4   b . Only then will it send the acknowledgement to the first intermediate node. 
     As a next step, having received the acknowledging message from the second intermediate node informing that the mapping at the second intermediate node is complete, the second last, i.e. the first intermediate node may similarity at any time make its mapping decision and will then select to map data in such a way that the second time slot on bitstream  101  is mapped to the fifth, newly allocated time slot on bitstream  102  and the newly allocated third time slot on bitstream  101  is mapped to the sixth time slot on bitstream  102 , see  FIG. 4   c.    
     When the first intermediate node then acknowledges to the sending node that allocation of resources and the necessary mapping has been accomplished, the sending node may at any time start using the additional time slots for transmitting payload data, as dicussed above. 
       FIG. 5  schematically shows an example of the exchange of messages between the nodes of  FIG. 1  during a change of bandwidth according to an embodiment of the invention, much in similar to what has already been described above with reference to  FIGS. 3   a – 3   d.    
     In  FIG. 5 , to increase the bandwidth of a multicast channel established with node  111  as sender, nodes  114  and  115  as receivers, and nodes  112  and  113  as intermediate switching nodes, node  111  reserves (RES) the required additional slots on its bitstream  101  and, in doing so, starts transmitting idle markings therein (while still transmitting payload data in the channel&#39;s original slots). Node  111  then sends a change request message (BC) in a control channel to node  112 . Receiving this message, node  112  similarly reserves the required additional slots on its bitstream  102 , starts transmitting idle markings therein (while still mapping data from/to the channel&#39;s original slots), and sends a similar change request message in a control channel to node  113 . Receiving this message, node  113  similarly reserves the required additional slots on its bitstream  103 , starts transmitting idle markings therein (while still mapping data from/to the channel&#39;s original slots), and sends a similar change request message in a multicasted control channel to nodes  114  and  115 . 
     When the change bandwidth request message from node  113  reaches the receiving node  114 , it starts listening (USE) to the added time slots (as well as the channel&#39;s original slots) and sends an acknowledging message (ACK) in a control channel to node  113 . Similarity, when the change bandwidth request message from node  113  reaches the receiving node  115 , it also starts listening (USE) to the added time slots (as well as the channel&#39;s original slots) and sends an acknowledging message (ACK) in a control channel to node  113 . Having received acknowledgements from both receiving nodes  114  and  115 , node  113  performs any necessary re-mapping to start mapping (MAP, USE) data from the new slots as well as the original slots on bitstream  102  into bitstream  103 . It then sends an acknowledging message to node  112 , and so on. Finally, when the sending node  111  receives an acknowledging message from node  111 , it may at any time start using the new time slots for transmitting payload traffic. 
     The scope of the invention is not to be limited by the exemplifying embodiments thereof disclosed herein, while combinations and modifications thereof, as will be evident for those skilled in the art, may be perforemed within the scope of the invention, which is defined by the accompanying claims.