[ 0021 ] fig1 illustrates the known protocol stacks by means of which signalling at the uni interface ( user - network interface ) and nni interface ( network - network interface ) of a b - isdn network is carried out . as can be seen from the figure , in both cases the three lowermost layers comprise a physical layer phy , an atm layer atm , and an saal layer ( signalling atm adaptation layer ), the last - mentioned being the layer corresponding to the ml layer on the control plane , i . e ., constituting the signalling aal . in the b - isdn protocol reference model , the control plane is the plane containing the functions of call and connection control and the functions of the signalling relating thereto . the signalling protocol for uni interfaces is defined in itu - t specification q . 2931 ( digital subscriber signalling system no . 2 ). these operations attend to the connection set - up and release . at nni interfaces , on the other hand , the signalling application protocol is provided by b - isup ( broadband - isup ), beneath which there is an mtp3 layer . both of these have been modified from the relevant parts of the ss7 standard ( mtp3 and isup ). b - isdn uses the third level of the message transfer part ( mtp ) only , since the functionalities of mtp1 and mtp2 have been implemented with the saal layer and the layers beneath . the saal layers are also differently configured depending on whether they are located at the uni interface or the nni interface . b - isup is responsible for processing messages relating to call and connection control and also for the new functions of connection control . transaction capability application part tcap and service connection control part sccp , which are today widely used e . g . in intelligent network applications , are used to implement the new functions of call control . since all of the protocol layers described above are known as such , they will not be described in detail in this context . in the following , the invention will be described first with reference to examples relating to nni interfaces , since the advantage afforded by the method of the invention will be greatest with nni interfaces . first the known procedures upon which the invention is based will be described . [ 0027 ] fig2 illustrates message exchange between two adjacent exchanges ( a and b ) in typical connection set - up . to start with , exchange a sends an initial address message iam . at this stage , exchange a assigns an identifier to this side of the connection . this identifier called osida ( origination signalling id a ) is included , among other things , in the initial address message to be sent . when the iam message arrives at exchange b , the latter also assigns a corresponding identifier osidb ( origination signalling id b ) to the connection . by means of these identifiers , the exchanges can identify messages pertaining to said connection . thereafter , exchange b sends to exchange a an acknowledgement message im ( iam acknowledgement message ), indicating that the iam message has been accepted and resources are available . exchange b incorporates into this message , among other things , identifier dsida ( destination signalling id a ) which is the same as osida assigned by exchange a and identifier osidb . if the iam message is complete and contains all data needed for processing the call , exchange b additionally sends to exchange a a message acm ( address complete message ), including e . g . identifier dsida . thereafter exchange a knows that connection set - up is being proceeded with in the direction of the called subscriber . when the called subscriber responds , exchange b sends to exchange a a message anm ( answer message ). also this message includes the identifier dsida . after exchange a has received said message , a bidirectional virtual connection has been established and the calling and called subscriber can communicate with one another . when any of the subscribers terminates the connection , the exchange associated with the terminating subscriber sends a message rel ( release message ), which initiates the release of the connection . ( the message includes the identifier dsidb , which is the same as osidb ). having received such a message , the exchange at the opposite end releases the identifier assigned to the connection for new use and sends to the opposite end a message rlc ( release complete message ), indicating to the opposite end that the relevant virtual connection has been released . ( the message includes the identifier dsida , which is the same as osida ). as a result , also the opposite end releases the identifier of the connection for new use . in the present invention , the identifiers described above will be utilized in the manner to be described hereinbelow . as stated above , a modification procedure wherewith the traffic parameters of a user connection can be modified has been defined for user connections in the current recommendations ( itu - t recommendation q . 2725 . 2 for an nni interface and recommendation q . 2963 . 1 for a uni interface ). [ 0032 ] fig3 illustrates an example of how the bandwidth is successfully changed on the virtual connection between exchanges a and c when there is one transit exchange between the exchanges . the exchange initiating the modification sends to the next exchange a special modification message modreq ( modify request ), including e . g . identifier dsid ( destination signalling id ), the value ( b 1 ) of which will identify at the next exchange ( b ) the connection to which the request relates . in addition to this identifier , the request message also contains e . g . information on the requested atm cell rate . upon receipt of this message , transit exchange b allocates resources as indicated in the message on the incoming and outgoing side of the exchange and forwards the modreq message to exchange c . in this message , the value ( c 1 ) of the identifier dsid is naturally the one by which exchange c will identify the virtual connection concerned . since exchange c is a terminal exchange , it allocates the resources on its incoming side . in response to successful resource allocation , it sends to exchange b an acknowledgement message modack ( modify acknowledge ), including e . g . an identifier by which exchange b can identify the virtual connection that the acknowledgement relates to . exchange b forwards the acknowledgement to exchange a , using the identifier assigned by exchange a to said connection as the value ( al ) of identifier dsid . thereafter , the new bandwidth is available . it should be noted that since exchange b is a transit exchange , two values ( b 1 and b 2 ) have been allocated for said virtual connection therein , one in each direction . the bandwidth of the connection can be changed in one or both transmission directions , and the bandwidth of each transmission direction can be decreased or increased independently of the reverse direction . the modification of the user connections is carried out in the manner described above link by link from the initiating terminal exchange to the opposite terminal exchange . the modification will only be successful if it is successful in every link span of the connection . the modification procedure can be performed only when connection set - up and release processes are not in progress , that is , at the point indicated in fig2 for example . since the modification procedures are known as such , they will not be described more closely in this connection . if the reader so desires , he can find more background information in the recommendations referred to above . the formats of b - isup messages have been specified in itu - t recommendation q . 2763 , wherein more background information is to be obtained . in the present invention , the modification procedure described above is utilized to change the capacity of a signalling link , employing the connection identifiers described in the context of fig2 . the modification takes place between two adjacent exchanges only in those parts of the network in which a need for changing the capacity of the signalling link is detected . as stated in the above , an identifier of the kind described above for identifying a connection does not exist for permanently defined connections , such as signalling connections . however , in order that modification messages may be identified as pertaining to a specific signalling link , in the present invention one or more values are reserved in advance for the signalling links from the sid space , and this value cannot be used for the signalled connections . in the following , the latter alternative ( reservation of more than one value ) will first be described , since it is regarded as the primary alternative . in the primary embodiment of the invention , a dedicated , unique sid value is assigned to each signalling link within the network or a part thereof . fig4 illustrates this principle by depicting a network or sub - network comprising nodes a , b , c and d , wherein the signalling links between the nodes are denoted by lines between the nodes . in this example , the signalling links are assigned values sid = 1 . . . 8 , said values not being available for identifying signalled connections . it should further be noted that the signalling links are signalling channels between two adjacent signalling points , and the figure does not discuss how they are located on the physical transmission medium . each signalling link has its dedicated , unique value either within the area of the entire network or in its own sub - network . what is essential to the invention is that two adjacent network elements are provided with values by means of which the signalling links can be unequivocally identified . hence , reuse of the sid values used by the signalling links in other parts of the network is possible . [ 0039 ] fig5 shows an example of message exchange in connection with a successful change of the signalling link capacity in accordance with the invention , when it is presumed that exchange a in fig4 detects a need for additional capacity in the direction of exchange b . the modification - initiating exchange a sends a modification message modreq in accordance with the recommendations to exchange b at the opposite end of the signalling link . however , in this message identifier dsid has a value that has been configured in advance to point to a specific signalling link in the network . the receiving exchange finds on the basis of this value that a modification request relating to a signalling link between said exchanges is involved , upon which it initiates a process for allocating the requested bandwidth to said link . when the desired resources have been allocated , exchange b sends an acknowledgement message modack back to exchange a . the message exchange is thus fully in accord with the recommendations , but the messages include a dsid value that points to a specific signalling link between said exchanges . [ 0040 ] fig6 illustrates the modification process within the modification - initiating exchange a . the different steps of the process are denoted by circled numbers . initially the exchange detects the need to increase the signalling capacity in the direction of exchange b . the need for change can be monitored for example at the mtp3 layer , since the tasks of said layer include management functionalities of the signalling network , e . g . monitoring of the signalling traffic to detect overload and congestion situations . hence , the observation regarding the need to add signalling capacity is obtained from the mtp3 layer ( step 1 ), which informs the modification control block mcb of the detected need by sending it an internal message ( step 2 ) including e . g . the signalling point code of the exchange in the direction of which congestion or an excess load has been detected . having received said message , the control block mcb sends ( step 3 ) a request for additional capacity to block cac , which performs the call admission control ( cac ) procedure in the exchange . the request relays the above signalling point code and information on the new capacity need . the need for additional capacity is defined in control block mcb . cac examines on the basis of the message which signalling link in said direction ( to exchange b ) can be assigned additional capacity , so that the desired total capacity is reached . cac allocates the relevant capacity and informs the control block mcb as to the sid value of said signalling link . ( it should be noted that in this context allocation denotes logical distribution of resources among virtual connections , as in reality the connections utilize common resources , such as buffers and the bandwidth of the physical transmission medium . logical distribution is necessary in view of managing the degree of utilization of the resources .) thereafter the modification control block sends a modification request to b - isup ( step 5 ). this request includes the signalling point code for exchange b , the sid identifier of said signalling link , and the necessary traffic parameters , such as the capacity allocated . having received this internal message , the signalling protocol performs a modification procedure of the kind described above , that is , sends a modreq message to exchange b ( step 6 ). the next step ( step 7 ) is to receive a modack message from the network ( cf . fig5 ). having received this acknowledgement message , b - isup informs the control block mcb of the fact that the modification was successful ( step 8 ). the control block reports the successful operation to cac ( step 9 ), which carries out the change of capacity in a known manner similarly as in the case of user connections . [ 0042 ] fig7 illustrates the internal process in exchange b receiving the modification request message . on the receiving side , b - isup first receives a modreq message , including e . g . a special value for sid ( step 1 ). b - isup is aware of the sid values reserved for the signalling links , and thus it recognizes that the message concerns modification of a signalling link and informs the modification control block mcb of the arrival of a modification request ( step 2 ). this message relays the sid value and the traffic parameters received in the message as well . the control block mcb relays the request to cac ( step 3 ) which checks whether there is existing capacity , i . e ., whether the request can be granted . if this is the case , cac accepts the modification and informs the control block mcb of the acceptance ( step 4 ). the control block relays the acceptance message to b - isup ( step 5 ), which sends the modack message to the network ( step 6 ) in response to said message . thereafter cac changes the capacity in the known manner similarly as in the case of user connections . instead of cac allocating the entire capacity to one signalling link , it can also allocate additional capacity in such a way that the desired total capacity is divided between more than one signalling link in said direction in the desired proportion . in such a case , the modification procedure described above is performed separately on each signalling link to which capacity is allocated . in the first embodiment described above , unique sid values were used for all signalling connections in the network or a part thereof . in another embodiment of the invention , the same predetermined sid value ( e . g . the value 1 ) is always used in the entire network ( or sub - network ). in such a case , the modification request is sent , in order to identify this link , specifically on that signalling link that one wishes to modify . since mtp3 makes the decision as to which signalling link the message is sent to , mtp3 must be separately informed within the node as to which signalling link is concerned . on the transmitting side , this is easy to implement . on the receiving side , however , the required changes are more difficult to carry out , since the b - isup layer must interrogate “ downwards ” which link this message came from when it detects a predetermined sid value in the message . this is due to the fact that the mtp3 layer does not examine the received message and neither do the layers beneath , but the mtp3 layer relays the message arriving at the node directly to the b - isup layer . hence , upon receipt of the message the b - isup layer must interrogate “ downwards ” which signalling link said message came from . as stated previously , a corresponding modification procedure pertaining to user connections is specified in itu - t recommendation q . 2963 . 1 for dss2 signalling ( digital subscriber signalling system no . 2 ) for use at a uni interface . dss2 signalling employs a call reference field of 4 octets in the messages to be sent for identifying the connection . this field is made use of in the present invention in accordance with the first or second embodiment described above . in other words , one or more call reference values that cannot be used for the signalled connections are reserved for the signalling links . the call reference field includes a one - bit call reference flag , identifying which end has assigned the call reference value . in the modification of a signalling link in accordance with the invention , this flag is used in a similar way : the end transmitting the modification request message assigns the call reference value and uses the value zero for the flag . the opposite end uses the same call reference value and value one for the flag . fig8 illustrates message transfer across a uni interface in an exemplary case where the characteristics of the signalling link are successfully changed . thus , the existing recommendations are made use of at both uni and nni interfaces to increase the capacity of a signalling link . in the case of a uni interface , the saal - uni layer is the entity making known the need for adding signalling capacity ( cf . fig1 ). even though the procedure in accordance with the invention is implemented between two adjacent network elements , in an end - to - end connection the above procedure can be initiated at any ( one or more ) network element in which a need for modifying the capacity of a signalling link is detected . the signalling point detecting the need for additional capacity is capable of increasing the capacity on one or more signalling links . the restoring of the capacity of the signalling link back to its original value can be realized in a variety of ways . for example , the mtp3 layer can continuously send information on the current load situation to the control block . on the other hand , the control block can interrogate the mtp3 layer about the load situation at given intervals and initiate retrieval if the response received meets predetermined criteria the restoration can also take place in slightly different ways depending on how vital the restoration is for the other connections . the control block can for instance interrogate cac how vital it is to restore the additional capacity to the use of the user connections and adjust the intervals of the interrogations made to the mtp3 layer accordingly , so that vital resources are restored as soon as possible to the use of the user connections . the restoration can thus be dependent on , besides the quantity of signalling traffic , also on the quantity of user traffic . even though the invention has been explained in the foregoing with reference to the examples in accordance with the accompanying drawings , it is evident that the invention is not so restricted , but it can be varied within the scope of the inventive idea set forth above and in the appended claims . for example , although the invention has been described using as examples the current signalling protocols of the atm - based network , it is evident that the invention can also be applied to future networks whose signalling methods or protocols support a modification procedure of the kind described above .