Abstract:
For providing an interworking function between an N-ISDN and a B-ISDN, the B-TSDN includes a second Signalling System 7 based data communication protocol including an MTP2 level. The functions are split on a first sublevel and a second sublevel.

Description:
This application is a continuation of PCT/SE99/00138, filed on Jan 29, 1997. 
    
    
     The invention relates to data communication networks. In particular it relates to packet-switching networks and to the aspect of such networks interworking. 
     Definitions of some used terms and abbreviations 
     (STALLNDGS, William, Data and Computer Communications; Macmillan Publishing Company; 1991; and DE PRYCKER, Martin, Asynchronous Transfer Mode: Solution for broadband ISDN; Ellis Horwood series in computer communications and networking; 1991; both herein incorporated by reference) 
     ISDN: Integrated Services Digital Network; 
     N-ISDN: Narrowband-ISDN; 
     SS7: Signalling System Number 7 is a layered set of protocols that is used for control communication internal to a digital network, e.g. an N-ISDN, and provides facilities for establishing, maintaining and terminating connections. It comprises in total four levels; 
     N-ISLTD: N-ISDN User Part and the fourth level of SS7. It provides for the control signallin needed in an N-ISDN to deal with N-ISDN subscriber calls and related functions; 
     MTP: Message Transfer Part, lower three levels of SS7, provides a reliable but connectionless service for routing messages through the SS7 network, whereby 
     MTP 1  is the Signalling data link and the first level of SS7; 
     MTP 2  is the Signalling link and the second level of SS7. This level is specified in the ITU-T Recommendation Q.703 (03/93) and is herein incorporated by reference. According to the ITU-T Recommendation Q.703 (03/93) the signalling link functions, together with a signalling data link as bearer, provide a signalling link for reliable transfer of signalling messages between two directly connected signalling points. Signalling messages delivered by superior hierarchical levels are transferred over the signalling link in variable length signal units. A signal unit is constituted of a variable length signalling information field which carries the information generated by a user Part and a number of fixed length fields which carry information required for message transfer control. In the case of link stanus signal units LSSU, the signalling information field and the service information octet is replaced by a status field which is generated by the signalling link terminal. There are three types of signal unit, i.e. the message signal units MSU, link status signal units LSSU and fill-in signal units FISU. The signalling link functions comprise signal unit delimitation, signal unit alignment, error detection, error correction, intitial alignment, signalling link error monitoring and flow control. All these functions are coordinated by the link state control; 
     MTP 3  is the Sionallin, network and the third level of SS7 
     HDLC: HDLC uses synchronous transmission. All transmissions are in frames, and a single frame format suffices for all types of data and control exchanges. The frame has the following fields: Flag, Address; Control; Information; Frame check sequence (FCS); and Flag. Bit stuffing is a procedure which is used for providing data transparency. 
     B-ISDN: Broadband-ISDN is a service or system requiring transmission channels of supporting rates greater than the primary rates; 
     B-ISUP: B-ISDN User part; 
     ATM: Asychronous Transfer Mode (protocol) is a transfer mode solution for implementing a B-ISDN, comprising three layers defined as the physical layer PHY which mainly transports information; 
     the ATM layer which mainly performs switching/routing and multiplexing; and 
     the ATM adaptation layer (AAL) which is mainly responsible for adapting service information to the ATM stream; 
     ST: Signalling terminal; 
     ET: Exchange terminal; 
     IWF/IWU: Interworking function/Interworking unit; 
     NNI: Network to Network interface; 
     Node: to which stations attach, is the boundery of a communication network, e.g. a B-ISDN network, and the node is capable of transferring data between pairs of attached stations. 
     BACKGROUND 
     Both information and parameters are sent from and received by an N-ISDN using the SS7 protocol and are furthermore transmitted in a HDLC (High Level Data Link Control) based frame format, e.g. HDLC or LAP-D. The SS7 and the HDLC based frame format are well known in the art. A B-ISDN, however, sends and receives information and parameters using an ATM protocol which is a specific packet oriented transfer mode based on fixed length cells. The ATM protocol is well known in the art. The difficulty in sending data from one type of network to another resides in the use of different protocols and data formats, frames or cells, required for these protocols. 
     Special interworking units/functions IWU/IWF have been developed for solving the problem of interworking between N-ISDN and B-ISDN. The interworking is performed either by an Interworking Unit, IWU which is a unit separate from the B-ISDN or an Interworking Function, IWF which is an integral part of the B-ISDN. IWU/DV are specified in the ITU-T Recommendation I.580 from 03.93: “General Arrangements for Interworking between B-ISDN and 64 kbits/s based ISDN”. Accordingly a standard NNI is the interface between the 64 kbits/s ISDN, i.e. the N-ISDN. and the IWU/IWF and between the B-ISDN and the IWU/IWF. EP-A-0 581 087 discloses an N-ISDN and a B-ISDN interworking by means of an IWU. The advantage of IWU/IWF is that they are standardised. However, they need a large processing capacity. They are furthermore highly dependent on market adaptations. This entails considerable costs. 
     SUMMARY 
     It is an object of the invention to provide an interworking function which needs less processing capacity. 
     It is a further object of the invention to provide an interworking function which is less dependent on the market adaptations. 
     It is yet another object of the invention to provide an interworking function which is low in cost. 
     These and other objects and advantages are obtained according to the invention as disclosed in independent claim  1  and claim  11 . Preferred embodiments of the invention are given in the dependent claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will now be explained in greater detail, referring to the drawings in which 
     FIG. 1 is a schematic view of networks interworking according to the prior art; 
     FIG. 2 is a schematic view of networks interworking according to an embodiment of the invention; 
     FIG. 3 is a schematic drawing of the interworking function according to an embodiment of the invention; 
     FIG. 4 is a flow diagram illustrating the split MTP 2  layer according to an embodiment of the invention; 
     FIG. 5 is a schematic drawing illustrating the functions of the split MTP 2  layer according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 1 is a schematic view of a B-ISDN  10  and an N-ISDN  20  interworking according to the prior art. 
     FIG. 2 is a schematic view of networks signalling interworking according to an embodiment of the invention. An ATM-based network  10 , e.g a B-ISDN, and a SS7-based network  20 , e.g. N-ISDN, are interworking by means of an interworking unit  60  comprised in the B-ISDN, for ensuring that a standard NNI is the only interface between the N-ISDN and the B-ISDN. 
     FIGS. 3 a  and  3   b  show two schematic drawings of a B-ISDN  10  and an N-ISDN  20  interworking. In FIG. 3 a  a B-ISDI  10  can be seen comprising an exchange terminal ET  90  coupled to an ATM-based switch core  70  which is coupled to a signalling link terminal ST  100 . A signalling link terminal refers to the means for performing all of the functions defined at level 2 regardless of their implementation. When interworking with a B-ISDN  10 , the N-ISDN  20  is coupled to the exchange terminal ET  90  of the B-ISDN  10 . A means for achieving the objects of the invention is to provide the B-ISDN  10  with a means for interworking for supporting N-ISDN subscriber calls and related functions within the B-ISDN  10 . FIG. 3 b  describes more specifically the means for interworking. Therein can be seen that the second MTP level of the SS7  80 , i.e. MTP2, is split into two sublevels, MTP 2  lower  92  and MRP2higher  103 . The MTP 2  lower  92  is preferably situated in the exchange terminal ET  90  and the MTP 2  higher  103   103  in the signalling terminal ST  100 . The functions of the MTP 2  are thereby split between the MTP 2  lower  92  and the MTP 2  higher  103 . 
     When an application X of the N-ISDN  20  has a message to an application Y of the N-ISLP supported within the B-ISDN  10 , it transfers those data to the N-ISUP  81  of the SS7  80  used in the N-ISDN  20 . Parameters containing the required information for the N-ISUP protocol are appended to those data and is passed as a unit together with the data to the MTP 3  . This process continues down through MTP 2  which generates a unit called frame using e.g. a HDLC based protocol. The frame is then passed by the MTP 1  onto the transmission medium. When the frame is received by the B-ISDN  10  it ascends to the N 1  of the exchange terminal ET  90 . The MTP 1   91  strips off the outermost parameters, acts on the protocol information contained therein, and passes the remainder up to the next layer MTP 2  lower  92 . MTP 2  lower  92  strips off the outermost parameters, acts on parts of the protocol information contained therein in accordance with an embodiment of the invention, and passes the remainder to the AAL  93 . Parameters are appended to the data that contains the required information for the AAL  93  protocol. In substance one can say that the AAL  93  protocol is a protocol for packeting and segmenting data into cells on transmission and reassembling the data from cells on reception. The cells are then passed by the ATM layer  94  onto the transmission medium and switched by the ATM switch core  70  to be received by an ATM layer  101 . The ATM layer  101  strips off the outermost parameters, acts on the protocol information contained therein, and passes the remainder up to the AAL  102 . On reception, the AAL  102  reassembles the data from cells in accordance with its protocol and passes the data up to the MTP 2  higher  103 . MTP 2  higher  103  strips off the outermost parameters, acts on the remaining parts of the MTP 2  protocol information contained therein in accordance with an embodiment of the invention. and passes the remainder to the MTP 3   104 . The MTP 3   104  sizes off the outermost parameters, acts on the protocol information contained therein, and passes the remainder to the N-ISUP  105 . The process continues through N-ISUP  105  for transfering the message of applicantion X to applicantion Y. When applicantion Y has a message for applicantion X, the reverse process occurs. 
     In a preferred embodiment of the invention, the exchange terminal ET  90  comprised within the B-ISDN  10 , comprises the MTP 1  level  91 , the MTP 2  lower  92 , an ATM layer  93  and an ALL  92 , e. g. AAL5 which is the AAL for Variable Bit Rate VBR. The signalling terminal ST  100  comprises an ATM layer  101 , an ATM adaptation layer  102 , e.g. AAL5  101 , MTP 2  higher  103 , MTP 3   104  and N-ISUP  105 . 
     In FIG. 4 the main functions of MTP 2  are shown. In a broad outline: Signal unit delimitation and alignment provide the functions for bit stuffing, insertion and removal of flags and analysis thereof; Error detection provides the functions for analysing the check bit at the end of each signal unit; Error correction provides the function of retransmission; Intitial alignment provides the functions for indicating the alignment status using four different alignment status indications, i.e. status indication “O” for out of alignment (SIO), “N” for normal alignment status (SIN), “E” for emergency alignment status (SIE) and “OS” for out of service (SIOS), all indications being carried in the status field of the link status signal unit LSSU. The alignment procedure passes through a number of states during the initial alignment one of them being proving by which means the signalling link terminal validates the link&#39;s ability to carry signal units correctly by inspecting the signal units; Signalling link error monitoring provides two functions, one which is employed whilst a signalling link is in service and which provides one of the criteria for taking the link out of service, and one which is employed whilst a link is in the proving state of the initial alignment procedure. These are called the signal unit error rate monitor SUERM and the alignment error rate monitor AERM respectively; and lastly flow control for handling a level 2 congestion situation. All the above mentioned functions are coordinated by the link state control. The functions of the MTP 2  and the corresponding procedures are well known in the art. FIG. 4 furthermore shows how the different functions of MTP 2  are split upon two sublevels in accordance with an embodiment of the invention. 
     FIG. 5 shows the MTP 2  split into two sublevels, the two sublevels being separated by a dotted line. It can be seen that the functions of MTP 2  lower  92  relate in general to the handling of the HDLC based frame format and the functions of MTP 2  higher  103  in general to error correction, retransmission and flow control of the transferred data. In particular, bit stuffing, flag detection/insertion and handling of the check sum are performed in the MTP 2  lower  92 , whilst sequence number handling, retransmission and flow control are performed in the MTP 2  higher  103 . 
     More specifically, MTP 2  higher  103  comprises means for error control and correction  120 , 130 , 140  and means for flow control  130 , 150 , 160 , e.g. buffering means. Link control functions are split between the MTP 2  lower  92  and the MTP 2  higher  103  in that regarding alignment it is performed in the MTP 2  higher  103  except the proving part which is performed in the MTP 2  lower  92 . Status control is performed in the MTP 2  higher  103  except for the status detection which is performed in the MTP 2  lower  92 . Furthermore, the reliable exchange of message signalling unit MSU is not effected by the split of the M 2 . The sequence control in MTP 2  is located in the MTP 2  higher  103  and will function in the same way as without the split. 
     FIG. 5 shows the functions of the MTP 2  layer and how they are distributed between the MTP 2  lower  92  and the MTP 2  higher  103 . When transferring data from lower to higher layers. MTP 2  lower  92  receives data from MTP 1  , strips off the outermost parameters and acts on the protocol information contained therein. It removes the bit stuffing and detects the flags  108 . The check bits are tested and removed  110 . If errors are detected a signal is sent to the MTP 2  higher for error correction. When transferring data from higher to lower layers, the reverse process occurs  180 ,  190 . Furthermore, there is in the MTP 2  lower  92  a signal unit error rate monitor SUERM for monitoring the status of the signalling link, e.g. the link status signal unit LSSU “in service”. Upon detection of excessive error rate, an internal error signal is generated for being transferred to the MTP 2  higher  103 . Moreover, there is an alignment error rate monitor AERM for monitoring the alignment of the signalling link, whereby only the proving state of the alignment monitoring is performed in the MTP 2  lower  92 . If a counter reaches the error threshold during the proving period, an internal error signal is generated for being transferred to the MTP 2  higher  103 . If the the MTP 2  receives a link status signal unit LSSU ocher than SIN/STE, then the proving part of the alignment monitoring is terminated and the LSSU is passed on to the MTP 2  higher  103 . The generated internal signals may be transferred in the control field. The error thresholds for the SUERM and AERM are parameters which are initialized at restart. 
     A preferred embodiment regarding error detection and link control may be implemented as following: 
     In the direction of transferring data from the N-ISDN  20  to the B-ISDN  10 , MTP 2  lower  92  filters fill in signal units FISU by firstly detecting them and if a FISU is equal to the preceding FISU, then it is discarded; otherwise it is passed through to the MTP 2  higher  103 . In addition, a FISU may regularly be passed to the MTP 2  higher  103  as an “I&#39;m alive” signal. In the direction of transferring data from the B-ISDN  10  to the N-ISDN  20 , the fill in signal units FISU will be inserted. To do this the MTP 2  lower  92  must keep the sequence numbers from the previous fill in signal unit FISU and use it for the generation of the fill in signal units FISU. The result of the filtering is that only fill in signal units FISU with relevant/new information is passed through to the MTP 2  higher  103 . 
     The proving part of the alignment monitoring is handled by the MTP 2  lower  92 , initiated by the MTP 2  higher  103 . Proving signal units are generated by the MTP 2  lower  92  and the result of the proving is transferred to the MTP 2  higher  103 . 
     The alignment status indications, e.g. SIOS/SIO/SIN/SIE, carried by the link status signal unit LSSU are filtered in the MTP 2  lower  92 . When the MTP 2  lower  92  receives one of these link status signal units LSSU, it will repeat it until another one is received. This filtering is valid for both directions and will thus be unburdening the MTP 2  higher  103 . 
     Hence, the MTP 2  higher  103  will be unburdened from continuous fill in signal units FISU and continuous link status signal units LSSU. However, for security in case a Link signal is lost, fill in signal units FISU and link status signal units LSSU will regularly be transferred from the MTP 2  lower  92  to the MTP 2  higher  103 . MTP 2 higher  103  sends a signal to the MTP 2 lower  92  when proving of alignment should be initiated by the MTP 2 lower  92 . A counter sudervises the MTP 2 lower  92  when proving. 
     The MTP 2 higher  103  is informed by the MTP 2 lower  92  of excessive error rate during the signalling link error detection by an internal error signal. 
     The functions of the MTP 2 lower  92  can be implemented in a access processor of the exchange termination board  90  and the MTP 2  upper in a central processor, the signalling terminal ST  100 . N-ISUP termination and/or message mapping to the B-ISUP can be handled in the B-ISDN node. By N-ISUP termination in the B-ISDN node, real N-ISDN functionality can be provided within the B-ISDN. 
     The invention is also applicable whenever two CCSS 7  based networks are interworking, i.e. MTP 2  interworking with other layer  2  protocols.