Abstract:
A method and system for improving the session establishment or modification performance of a switching system. Statistics associated with the use of session resources within the switching system are collected. Based on these statistics a given session resource is recorded to a connection cache to become reusable for subsequent sessions. In subsequent session establishment or modification within the switching system the connection cache is checked whether there is any matching session resources. If a matching session resource is found, it is used in the establishment of the communication paths pertaining to the session to be established or modified.

Description:
BACKGROUND OF THE INVENTION  
     Field Of The Invention  
       [0001]     The invention relates to telecommunications networks. Examples of such networks are Asynchronous Transfer Mode (ATM) networks, frame relay networks, Internet Protocol (IP) and Synchronous Digital Hierarchy (SDH) networks. Particularly, the invention relates to wireless multimedia switching systems and an adaptive connection cache that improves their performance.  
         [0002]     In the last few years there has been a converging trend in telecommunication network transport technologies. This means that there is a unified technology for transporting various media, for instance, network signaling, voice, video and data. At the transport layer, media is carried using a unified packet format, and only on a higher protocol layer there is a distinction between media types. An example of such a unified media transport technology is the Asynchronous Transfer Mode (ATM).  
         [0003]     Packet switching is a general term for a number of interrelated technologies including, for instance, datagram packet switching and virtual circuit packet switching. Virtual circuit packet switching (VC switching) is a packet switching technique, which is a kind of hybrid of datagram packet switching and circuit switching and combines their advantages. VC switching is a variation of datagram packet switching where packets flow on so-called logical circuits, for which no physical resources like frequencies or time slots or single physical circuits are allocated. Each packet carries a circuit identifier that is local to a link and updated by each switch on the path of the packet from its source to its destination. A virtual circuit is defined by the sequence of the mappings between a link taken by packets and the circuit identifier packets carried on this link. The sequence is set-up at connection establishment time and identifiers are reclaimed during the circuit termination.  
         [0004]     Lately, since the introduction of Internet Protocol (IP) multimedia, Internet Protocol (IP) has also been used as such a unified media transport technology. The ATM uses short, fixed length packets that are referred to as cells. An ATM cell comprises a five byte header, which is used by the network to deliver the cell to the destination, and a 48 byte body that contains data, which may represent part of voice, video or data transmission i.e. communication path being sent across the network. The cell header also provides the network with the ability to control congestion. The cell header also comprises a Virtual Path Identifier (VPI) and a Virtual Channel Identifier (VCI). The VCI and/or VPI are used for cell switching in an ATM switch. A Virtual Channel (VC) is a unidirectional flow of ATM cells between two connecting (switching or end-user) points that share a common identifier number (VCI). A Virtual Channel Connection (VCC) is a concatenation of virtual channel links. Virtual channels in two or more sequential physical circuits can be concatenated to create an end-to-end connection, referred to as a VCC. More information on the ATM can be found in the book “ATM: The New Paradigm for Internet, Intranet and Residential Broadband Services and Applications”, Timothy Kwok, Prentice Hall, 1998.  
         [0005]     The upper layer of the ATM is the ATM Adaptation Layer (AAL). The AAL layer uses currently three different types of adaptation, namely: AAL 1 , AAL 2  and AAL 5 . For instance AAL 1  is used for fixed bitrate circuit emulation whereas AAL 5  is used for connectionless non-real-time undefined bitrate services. AAL 2  is used for variable bitrate real-time services, especially for compressed voice. Only AAL 2  is discussed more closely herein. According to AAL specifications an AAL is divided in two sublayers: the upper sublayer of which is the Convergence Sublayer (CS) and the lower sublayer is the Segmentation And Reassembly (SAR). The convergence sublayer is further divided in the Common Part Convergence Sublayer (CPCS) and the Service Specific Convergence Sublayer (SSCS). The CPCS and SAR are further referred to as the Common Part (CP).  
         [0006]     The common part for AAL 2  i.e. AAL 2 -CP achieves both low packetization delay and high bandwidth efficiency by allowing variable packet length from 1 octet to 45 octets and by multiplexing several AAL 2  connections in a single ATM virtual channel connection (VCC). An AAL 2 -CP packet consists of a 3-octet header and up to 45 octets of payload. The length of the payload is indicated in the “length indicator” (LI) field. A “user-to-user” (UUI) field has been included for upper layers (users) to transparently convey information (e.g. some SSCS use it to convey a sequence number and/or the type of voice-codec used). An 8 bit Connection Identifier (CID) is used to identify individual AAL 2  connections inside a single AAL 2  link (ATM VCC set up for the transport of AAL 2  traffic). Sometimes a bundle comprising more than one multiplexed AAL 2  connection is referred to as an N-CID. Switching at AAL 2  level is possible by making an association between incoming and outgoing connection CIDs in a switch.  
         [0007]     A prior art AAL 2  capable switching system is illustrated in  FIG. 1 . The AAL 2  switching system comprises an ATM switch core  100  that performs the switching of ATM level packets between connection slots controlled by the switch controller. The switch controller configures the connection slots in order to connect input ports and output ports. Connected to the switch core is a number of multiplexing/demultiplexing units (M)  110 - 113 . The multiplexing/demultiplexing units are herein referred to simply as multiplexers for clarity. There is a number of Network Interface Units (NIU) to which connections  102 - 104  to the external network can be attached. Each NIU is connected to a given multiplexing/demultiplexing unit (M). Slow network interface NIU  120  is connected to the ATM switching core via a multiplexing/demultiplexing unit whereas fast network interface NIU  128  is directly connected to the ATM switching core. There is also a number of AAL 2  switching processors (A 2 SP)  122 ,  126  that perform AAL 2  level switching and are aware of individual AAL 2  connections. The A 2 SPs demultiplex the AAL 2  connections from a given VCC. A 2 SP  122  connects an incoming AAL 2  connection  140  to an ATM level connection  150  comprising user data only from a single AAL 2  connection via an association  160 . In another transfer direction A 2 SP  126  connects an incoming ATM level connection  152  to an outgoing AAL 2  connection  154  via an association  162 . The incoming or outgoing ATM level connections  150 ,  152  can be connected to a tributary processor group  124 , which contains a Digital Signaling Processor (DSP) application. The DSP applications may perform various functions to one or many received media stream(s) and provide one or many altered media stream(s) in output. Examples of such functions are ciphering/deciphering, voice or video transcoding and macro diversity combining.  
         [0008]     A more precise view of the function of the AAL 2  switching system follows in a case where a DSP application is applied to a media stream switched through the AAL 2  switching system. An incoming cell stream  140  associated with a given VCC and AAL 2  connection  140  is received by a NIU. There are a number of AAL 2  connections multiplexed to the VCC. Such a bundle of related AAL 2  connections is referred to herein also as an N-CID connection. The VCC is connected to multiplexer  110  and fed via it to ATM switch core  100 , which takes care of ATM cell switching i.e. ATM level switching. ATM switch core  100  switches the VCC to multiplexer  111 , which in turn connects the VCC to A 2 SP  122 . A 2 SP  122  demultiplexes the AAL 2  connections from the VCC and connects each individual AAL 2  connection, for example AAL 2  connection  140 , to its own ATM level connection, for example ATM level connection  150 . The ATM level connection  150  is switched by ATM switching core  100  to multiplexer  112  that connects it to a TPG and a DSP application in a Computer Unit  130 ,  132  in association with it. The DSP application is manipulating the ATM level connection cell stream and it is fed into ATM level connection  152 . ATM level connection  152  is switched by ATM switch core  100  to multiplexer  113  that connects ATM level connection  152  to A 2 SP  126 . ATM level connection  152  is switched by A 2 SP  126  to AAL 2  connection  142 . AAL 2   142  connection is multiplexed by A 2 SP  126  together with a number of other AAL 2  connections to an outgoing cell stream of an outgoing VCC. The outgoing cell stream thus carries cells belonging to AAL 2  connection  142 .  
         [0009]     ATM and AAL 2  switching is used in wireless multimedia networks such as the Universal Mobile Telecommunications System (UMTS). AAL 2  is used to carry various signaling and media channels in UMTS Radio Access Network (RAN).  
         [0010]     A reference is now made to  FIG. 2A , which illustrates the RAN  200  part of a UMTS system. User equipment (UE)  218  has reserved for use a number of radio access bearers  222 , at least when UE  218  is active and has at least one call and/or a packet data context activated. Typically, for a UMTS UE in call and/or active packet data connection state, there can be four different radio access bearers, for example, one for Dedicated Control signaling CHannel (DCCH), one for Adaptive Multirate (AMR) coded speech, one for non-real time data and one for real-time data. There is a number of Base Transceiver Stations (BTS)  210 - 212 . The radio access bearers are carried in RAN  200  within ATM AAL 2  connections  220 . There is one connection for each individual radio access bearer. AAL 2  connections  220  from the BTSes  210 - 212  are connected to Radio Network Controller (RNC)  214 , which takes care of several functions pertaining to the UMTS radio network, for instance, macro diversity combining, ciphering/deciphering, radio channel allocation, handovers etc. RNC  214  has associated with it an AAL 2  switching system comprising an ATM switching core  100 . RNC  214  is connected on the Core Network (CN)  202  side of the UMTS system to a Media Gateway (MGW)  216 . RNC  214  performs the switching of connections between the BTS side connections  220  and connections towards MGW  216 . RNC  214  has TPGs and computer units hosting DSP applications for performing e.g. macro diversity combining and ciphering/deciphering.  
         [0011]     A reference is now made to  FIG. 2B , which illustrates prior art UMTS RNC  214  structure. The RNC comprises Switching Fabric Unit (SFU)  100 , which is equivalent to ATM switch core  100 , multiplexing/demultiplexing units  250 - 256 , network interface units  260 - 263 , control computers  270 - 273  controlling the switching system i.e. RNC. Examples of such control computers are ICSU (signaling unit)  270  that contains, for example, call control applications such as  350 , RRMU (radio resource management unit)  271  and OMU (operations and maintenance unit)  273 . The RNC has also AAL 2  switching units i.e. A 2 SPs  265 ,  266 .  
         [0012]     In ATM switching applications, user plane virtual channels are switched from an ingress interface to an egress interface directly when AAL 2  switching is not used. In the RNC case the situation is somewhat more complex, as RNC also performs digital signal processing functions for the data going through the RNC. The UMTS RNC represents an example of an AAL 2  switching system. However, there are additional components such as the unit for macro diversity combining  268  and the DSP application for it  267 . The figure depicts a case where there are three macro diversity legs i.e. branches  280 ,  281 ,  282  for a given UE. Legs  280  and  281  lead to BTSs under the control of RNC  214 . Leg  282  leads to a BTS under the control of an another RNC i.e. a drift RNC via UMTS lur-interface. A macro diversity combining application  267  performs the combining of user plane data from the UE. The combined user plane data i.e. traffic is forwarded via leg  283  that leads to AAL 2  switching unit  265 . The interface between the RNC and core network is referred to as UMTS lu-interface. Leg  280  is started from a BTS behind the UMTS lub interface and goes via multiplexing/demultiplexing units (MXU)  250 ,  254  and SFU  100  to AAL 2  switching unit  266 , where the switching of AAL 2  packets to internal ATM VCCs is done. Further the leg  280  goes to macro diversity combining unit  268  and is ended there. Leg  281  and Leg  282  are handled in similar way. The leg  284  is started from the core network i.e. MGW behind the lu-interface and goes via multiplexer  254  and switching fabric unit  100  to AAL 2  switching unit  265  and is ended there. Finally, an AAL 2  connection performed to the AAL 2  switching unit  265  connects the leg  283  to the leg  284 .  
         [0013]     Drawbacks of the prior art solutions are that in a switching system such as described, especially when used in the context of a system such as UMTS that uses several parallel AAL 2  connections per one user, the preparing of connections for the user becomes slow and introduces significant delays to, for example, call set-up times. A first type of the delay is due to the structure of an AAL 2  switching system, where the preparing of an AAL 2  connection between an input AAL 2  connection and an output AAL 2  connection via a DSP application hosted in a computer unit in association with a TPG  124  involves four ATM level switching connections  154 ,  150 ,  152 ,  156  via the ATM switching core and two AAL 2  level connections within A 2 SPs  160 ,  162 . The managing i.e. performing of the ATM level switching connections in the ATM switching core and AAL 2  level connections in the A 2 SPs takes a certain minimum time, which cannot be reduced without costly solutions e.g. by increasing the capacity of the ATM switching core. The delay causes problems in cases where rapid connection set-up is needed. One example of such a case is the paging of a UMTS UE, where a radio access bearer has to be set-up for the DCCH in order to carry call set-up signaling. Another type of the delay is due to the fact that there can be several parallel connections associated with a given user via an AAL 2  switching system such as RNC. For instance, if four radio bearers are to be set-up for a MS within a RAN, there are four AAL 2  connections via the RNC to the CN, thereby multiplying the number of ATM level and AAL 2  level switching connections by the factor of four. As explained above in reference to  FIG. 2B , the number of required connections increases in the case of macro diversity combining. In such a case there are also the macro diversity branches i.e. legs, that bring in an additional number of connections.  
         [0014]     Similar drawbacks can be observed in a switching system utilizing multiple connections either due to switching on multiple protocol layers or due to multiple parallel connections per user or endpoint.  
       SUMMARY OF THE INVENTION  
       [0015]     The invention solves the problems discussed before. Particularly, the invention speeds up the set-up of connections via a switching system.  
         [0016]     The invention is a method for improving the session establishment or modification performance of a switching system. In the method statistics associated with the use of session resources pertaining to the switching system are collected; based on the statistics information pertaining to a session resource is recorded to a connection cache; in the processing of a session establishment or modification request the connection cache is checked for at least one matching session resource; and the matching session resource is used in the establishment of at least one communication path pertaining to the session establishment or modification request.  
         [0017]     The invention is also a system for improving the session establishment or modification performance of a switching system. The system comprises means for switching communication paths; means for receiving session establishment or modification requests; means for collecting statistics of session resources used by the sessions pertaining to the session establishment or modification requests; a connection cache for recording information pertaining to the session resources based on the statistics; means for reusing a session resource, and the information of which has been stored in the connection cache, in the context of a new session establishment or modification request.  
         [0018]     The invention is also a node for improving the session establishment or modification performance of a telecommunications system comprising one or many switching systems. The node further comprising: means for receiving session establishment or modification requests; means for collecting statistics of session resources sources used by sessions pertaining to the session establishment or modification requests; a connection cache for recording information pertaining to the session resources based on the statistics; and 
        means for reusing a session resource, the information of which has been stored in the connection cache, in the context of a new session establishment or modification request.        
 
         [0020]     In one embodiment of the invention the session resources comprise cross-connections in a switch, which can particularly be an ATM switch. The cross-connections are on the ATM level, that is, on cell switching level or on AAL 2  level. If media manipulation by means of a DSP application is desired for the communication path, the session resources also comprise connections to computer units hosting DSP applications. The computer units and their DSP applications are connected to the communication path via ATM cross-connections. One or several media stream manipulation computer units or other equivalent circuitry is referred to hereinafter also as media stream processing means.  
         [0021]     In one embodiment of the invention the matching utilizes at least one quality of service parameter pertaining to the session request. One typical example of such a parameter is the bitrate associated with a communication path pertaining to the session. In one embodiment of the invention the disclosed node is a radio network controller. In yet another embodiment of the invention, the computer units are grouped into at least one computer unit group, computer units from the computer unit group being preferred for sessions associated with predefined incoming or outgoing connections, that is, connections associated with a certain VCC.  
         [0022]     The benefits of the invention are related to the performance of the switching system. The performance is increased due to the fact that the caching of connection information avoids the time consuming task of creating connections from starting at, for instance, ATM switching core, higher protocol level switches such as AAL 2  switching units or other type of switching matrix. Instead, ready-made connections from previous calls can be reused at call control level. In the case of UMTS radio network controller (RNC), the invention is further improved by grouping computer units into computer unit groups so that a given group is associated with a group of incoming or outgoing connections. In this case the number of possible connections connecting different NIUs, A 2 SPs and TPGs is restricted. By forcing the connections to follow generally same paths and patterns in the switching system, the probability of creating reusable connections and the probability of finding them in the connection cache during subsequent use is increased.  
         [0023]     It should be noted that throughout this disclosure by a switching system is meant any system capable of processing set-up requests pertaining to calls, packet data connections and IP multimedia sessions, and managing of cross-connections in one or many switching elements in order to establish communication paths pertaining to set-up request via said switching system.  
         [0024]     It should also be noted that throughout this disclosure the term session refers to audio or multimedia calls, packet data connections, IP multimedia sessions or any kind of data transmission, which involves the setting up of switching connections in order to establish communication paths, for example AAL 2  connections carrying e.g. a voice or video stream. 
     
    
     BRIEF DESCRIPTION OF TEH DRAWINGS  
       [0025]     The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings:  
         [0026]      FIG. 1  (PRIOR ART) is a block diagram of prior art showing AAL 2  switching system,  
         [0027]      FIG. 2A  (PRIOR ART) is a block diagram of prior art showing a UMTS radio access network,  
         [0028]      FIG. 2B  (PRIOR ART) is a block diagram of prior art UMTS radio network controller,  
         [0029]      FIG. 3  is a block diagram depicting a system utilizing the connection cache of the invention in order to control an AAL 2  switching system of  FIG. 1 ,  
         [0030]      FIG. 4  is a flow chart depicting one embodiment of connection set-up in the system of  FIG. 3 , in accordance with the invention.  
         [0031]      FIG. 5  is a flow chart depicting one embodiment of connection release in a system of  FIG. 3 , in accordance with the invention.  
         [0032]      FIG. 6  is a block diagram depicting one embodiment of the use of the system of  FIG. 3  in a UMTS radio access network. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0034]      FIG. 4  illustrates a flow chart depicting one embodiment of connection set-up in the system of  FIG. 3 . The method applies to the establishment of any kind of sessions. Herein both are referred to as calls for simplicity. In turn the system of  FIG. 3  is controlling a switching system of  FIG. 1 .  
         [0035]     In step  400  a call set-up request is received by a call control application  350 . It should be noted that by a call set-up request is meant throughout this disclosure any kind of connection request requiring the performing of a number of connections within the switching system in order to establish one or many communications paths via the switching system from a given origin to a given destination. Therefore, the term call set-up request is also used to refer to packet data connection and IP multimedia session set-up requests. Thus, call set-up requests could also be referred to as session set-up i.e. establishment requests. In one embodiment of the invention, the communication paths are AAL 2  connections. In the context of a UMTS radio access network  200 , the call set-up request may typically be a radio resource request issued from an idle mode UE to RNC  214 . Similarly, it may be a paging request received from the core network side requiring the set-up of a signaling channel from the core network towards the UE via RNC  200 . In an another type of multimedia system the call set-up request may be a request to set-up a multi-stream connection from user equipment via the switching system to an another user equipment.  
         [0036]     In step  402  call control application  350  sends routing information carried in the call set-up request to route analyzer  354  to be analyzed. The routing information can be an address, which is used by the route analyzer  354  to determine the next hop for the call. Route analyzer  354  responds to call control application  350  to provide the route information to determine the next node in call routing. In the case of UMTS UE  218  paging, the route analysis can only determine the set of BTSes  210 ,  212  via which paging of UE  218  is to be performed. Only after the UE  218  has responded to the paging request, it is determined in RNC  200 , under which BTS  210  the UE is currently camping and to which AAL 2  connections  220  from RNC  200  are to be allocated.  
         [0037]     In step  404  the required call resources are determined. This is possible after the routing information analysis at step  402  and the call control application have determined the next node, to which an outgoing AAL 2  connection is to be allocated. This determination may also involve signaling to other network nodes. In the case of UMTS UE paging, this node is BTS  210 , under of which the UE is camping currently. In one embodiment of the invention only those call resources that are to be accessed via the ATM switch core and that thus require connection allocation are of interest. It should be noted that throughout the disclosure by a call resource is meant any connection, circuit, virtual circuit or device input/output connection, which is connected to call or packet data connection user plane via a switching matrix, preferably ATM switching core  100 . The call resources include at least the suitable VCC, to which an outgoing AAL 2  connection  154  is going to be accommodated, and the actual outgoing AAL 2  connection itself. Similarly, if the incoming and outgoing AAL 2  connections are going to be connected to each other via a DSP application in order to implement a media stream conversion, for example ciphering, the ATM connections  150 ,  152  have to be allocated that lead to TPG  124  that has plugged in it computer units  130 ,  132  hosting the required application. TPG  124  will take care of the routing of media streams between the computer unit hosting the required application and the incoming and outgoing ATM level connections  150 ,  152 .  
         [0038]     In one embodiment of the invention there is direct correspondence between the processing capacity of means for media stream processing and the total connection capacity permitted to them in the ATM switch core. Hereinafter, the media stream processing means are referred to as processing means for brevity. An example of such processing means are the DSP applications hosted in association with one TPG  124 . In other words, a capacity slice from the processing means is allocated as a call resource by allocating connections of required bandwidth leading to the processing means from the switching core. In this sense only the caching of connections leading to the processing means is required in order to ensure the availability of such means for subsequent calls. The connection admission control of the switching system keeps track of the total allocated bandwidth leading to the processing means. Thus, only a limited number of connections to the processing means are admitted simultaneously. In other embodiments separate allocation means for the processing means is required.  
         [0039]     After step  404  resource selector  352  checks for each call resource whether it is available in a connection cache  370 .  
         [0040]     In step  405  resource selector  352  checks the properties of a required call resource, and checks connection cache  370  for call resources with matching properties. If the switching system has performed a startup from scratch and no existing connections are available, connection cache  370  will be empty. The properties of the call resources preferable include for AAL 2  connections the quality of service parameters, for example bitrate. Similarly, the properties may include information about the units or ports to which the connections start and end. The information about connection start and endpoint may also be in the form of an ATM VCI/VPI.  
         [0041]     If connection cache  370  has a record of a call resource with matching properties, then in step  410  a decision to reuse a cached call resource and its accompanying connections is made by resource selector  352 . If connection cache  370  does not have a record of a resource with matching properties, in step  408  the resource selector  352  prepares to build a new connection and returns information of the required call resources and their properties to the resource broker.  
         [0042]     In step  412  resource selector  352  updates connection statistics stored by connection cache  370 . The connection statistics are updated to include information about the required call resource. The connection statistics include preferably information such as the quality of service parameters for AAL 2  connections. Necessarily, also the connection statistics specify all the needed information to determine whether the connection is of correct type, that is, what units it is connecting. For example, for an AAL 2  connection there may be recorded the A 2 SP from which it starts and the TPG where it ends. Preferably, only those quality of service parameters are stored in statistics that are necessary for the matching of required AAL 2  connections to suitable existing connections that have been cached to connection cache  370 . One such quality of service parameter to be explicitly mentioned is the bitrate i.e. bandwidth of the AAL 2  connection. In one embodiment of the invention, the statistics on AAL 2  connections are grouped according to the VCC that is used to carry them. In other words, the statistics are VCC i.e. N-CID specific. The principle of the statistics is to gather information about most frequently used AAL 2  connection types e.g. the most frequently used AAL 2  connection bitrates. The statistics affect the functioning of connection cache  370  e.g. in the way that records on only those AAL 2  connections that are used frequently enough are cached to connection cache  370 . The procedure is similar relating to any other call resources and their accompanying connections. The call resource statistics are used to determine, whether or not it is probable that the kind of call resource is going to be used in the near future, that is, in a time frame specified.  
         [0043]     In association with step  412  a decision whether or not it is necessary to retain the call resource at hand in connection cache  370  after the call has been released. In one embodiment of the invention, the call resource and its accompanying connections are recorded to the cache at this phase. In one embodiment of the invention, the information recorded to the connection cache includes for AAL 2  connections the bitrate QoS parameter, CID and the VCC carrying the CID. For connections ending or starting in a TPG the information includes the ATM VCI and VPI to access the TPG in question, and optionally other computer unit addressing information. In those embodiments of the invention where the computer units plugged to a given TPG host only a limited set of DSP application types, the type of application must be also recorded. For ATM level connections, the information recorded includes the bitrate QoS parameter and the VCI and VPI. It should be noted that the recorded information must generally include the information necessary to reuse the call resources in association with a new call request and to connect the call resources to newly created non-cached call resources.  
         [0044]     In step  414  it is checked whether more call resources and accompanying connections are needed for the call at hand. If more call resources are needed, processing continues from step  405 . If more call resources are not needed, the processing continues at step  416 .  
         [0045]     In step  416  a resource broker  356  receives information on pending call resources and accompanying properties associated with them from the resource selector that were not available in connection cache  370 . The resource broker forwards information on the pending call resources and the call resources that were available to a connection performer  360 . Connection performer  360  issues ATM connection requests corresponding to the required ATM level connections to ATM switch core  368 , AAL 2  connection requests to the A 2 SP  364  and start requests to DSP applications in TPG  366 . It should be noted that there may be several TPGs and A 2 SPs to which requests must be sent.  
         [0046]     The method depicted in  FIG. 4  and explained above can also be used in call i.e. session modification requests. The difference is that at step  400  a session modification request is received for an existing session instead of a call set-up request. The session modification request could typically provide information about a new communication path to be associated with the session. In the UMTS RAN the new communication path could be an AAL 2  connection associated with CDMA macro diversity combining. In the case of an IP multimedia system, the new communication path could be a new parallel user-to-user communication path pertaining to a new media type to be associated with the session.  
         [0047]     In  FIG. 5  is illustrated a flow chart depicting one embodiment of connection release in a system of  FIG. 3 . The method applies as well to the set-up of calls and packet data connections.  
         [0048]     In step  502  a call release request is received by a call control application  350 . At this step call control application  350  issues a resource release request to a resource broker  356 . After step  502 , the reuse of each call resource associated with the call to be released is checked.  
         [0049]     In step  504  resource broker  356  checks whether or not a given call resource and its accompanying connections have been determined during the connection setup step  412  as a call resource that must be retained in a connection cache  370  after the release of the call. If the call resource and its accompanying connections are to be reused, they are not released. In a one embodiment of the invention no messaging with connection cache  370  is required at step  508 , since information on the call resource has already been recorded in step  412 . In other embodiments the call resource information may be forwarded in step  508  to connection cache  370 .  
         [0050]     If the call resource and its accompanying connections are to be released, resource broker  356  issues in step  510  a request to a connection performer  360  and resource selector  352  to make the call resource and the accompanying connections available for other calls. For example, if a DSP application is to be released, also the ATM level connections connecting to it are as well released. In  FIG. 1  such ATM level connections are  150  and  152 . In one embodiment of the invention the resource selector  352  marks the call resources and the connections as available. Connection performer  360  may also issue, for example, requests to ATM switch core  368  to clear information pertaining to the released connections.  
         [0051]     The method depicted in  FIG. 5  and explained above can also be used in the call i.e. session modification requests. The difference is that at step  500  a session modification request is received instead of a session release request. The release request carries information about a communication path to be released from the session.  
         [0052]     In  FIG. 6  is illustrated a block diagram depicting one embodiment of the invention where the use of connection cache  370  is made more effective when applied in a UMTS RNC  606 . In this embodiment the tributary processor groups, TPGs  620 - 625  that have associated with them computer units hosting DSP applications, are grouped in TPG groups  630 ,  631 ,  632 . The TPG groups  630 ,  631 ,  632  are dedicated to given VCCs i.e. N-CIDs. Each N-CID  640 ,  642 ,  644  has associated with it a given TPG group  630 ,  631 ,  632 . Each N-CID terminates on the one end to a BTS  600 - 604  and on the other end to an A 2 SP  610 ,  612 . Therefore, a given N-CID  640 ,  642 ,  644  is associated with a given cell or a small group of cells provided by one BTS  600 - 604 . By having a given TPG group  630 ,  631 ,  632  associated with a given N-CID  640 ,  642 ,  644 , it is possible to increase the probability of finding a suitable cached connection and a call resource that is already associated with the correct A 2 SP. For example, it is possible to find a suitable TPG and the accompanying ATM level connections  150 ,  152  from connection cache  370  that is readily connected to the correct A 2 SPs i.e. the A 2 SP, which has associated with it a call request that is being processed. The probability of finding a suitable cached connection and a call resource is inversely proportional to the ratio of the number of TPGs in TPG group  630 ,  631 ,  632  and the total number of TPGs in RNC  606 . In one embodiment of the invention, the TPG groups  630 ,  631  associated with the N-CIDs  640 ,  642  leading to two neighboring BTSs  600 ,  602  are partially overlapping, that is, some TPGs belong to both TPG groups  630 ,  631 . In this embodiment, the benefit is that connections between the neighboring BTSs via a TPG are likely to be cached and available for subsequent soft handovers. The TPG will then be required to access its plug-in computer unit to run the DSP application for macro diversity combining.  
         [0053]     It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.