Abstract:
The present invention describes a method in telecommunication networks where time division multiplexing traffic is transported over packet switched networks comprising one or a number of transmitting parties where the transmitting party will not send idle timeslot data on the transporting network. In one preferred embodiment will the transmitting party provide information regarding which timeslot hat are not used. Said telecommunication network can preferably be a packet switched network, wherein the packet switched network is one of the following: an IP-network, MPLS, ATM or Frame Relay.

Description:
FIELD OF THE INVENTION 
   The present invention is related to telecommunication networks and in particular to methods where time division multiplexing traffic is transported over packet switched networks comprising one or a number of transmitting parties, and one or a plurality of receiving parties. 
   BACKGROUND OF THE INVENTION 
   The invention is applicable where the Time Division Multiplexing technique (TDM) is used on the site and a packet based protocol, i.e. the Internet Protocol (IP), is implemented in the transport network between the sites. Traditionally TDM (Time Division Multiplexing) traffic is today transmitted directly over SDH (Synchronous Digital Hierarchy) networks. However, it is assumed that tomorrow&#39;s telecom transport network will be packet based, but probably with a long reuse time of existing site equipment. 
   Many vendors believe that the future network protocol will be the Internet Protocol (IP). With the introduction of this network technology for real-time applications, telecom companies may get a dilemma. On one hand they want to adapt to the new technology, on the other hand they have a lot of equipment like MSC (Mobile Switching Centre); BSC (Base Station Controller) etc. working very well with circuit switched technology. Buying a complete new packet based solution right away and to phase out circuit switched equipment will be very expensive. Another possibility is to maintain circuit switched and packed switched networks in parallel, but this may also be regarded as too expensive. Facing the fact that the operator on long term probably will implement packet-based transport solutions anyway, i.e. the Internet Protocol (IP), a wanted solution is a mechanism that allows the Telecom Company to use today&#39;s equipment such as MSCs, BSCs etc. towards a packet switched network as an intermediate solution. Then in a later step, when the packet based technology is considered mature for real time applications on site, the change to an “all packet switched” scenario could be regarded reasonable. A problem when transporting TDM over packet based networks is the bandwidth utilization in the transport network. With Time Division Multiplexing, the connections are separated in timeslots. Depending on the traffic load situation, there will be a variable numbers of timeslots not carrying any traffic (they are IDLE). Even though there are timeslots not carrying traffic, these timeslots are today filled with a so-called “Idle Pattern”, “Idle pattern” is a fixed bit pattern and it is used in synchronous systems where there can not be any “holes” in the data stream, and transmitted through the network. 
   A draft to the Internet Engineering Task Force (IETF) describing the concept TDM over IP has been posted. (TDM over IP, Yaakov (Jonathan) Stein et al. March 2003) In this draft there is also a suggestion as to what the protocol stack could look like. The mentioned drafts have no option for dynamic IP bandwidth reduction. There are methods for transporting parts of a trunk (24/32 timeslots), but in a more static way. The described method is rather complicated, introducing a lot of extra overhead etc. 
   This invention describes a solution to avoid the transmission of the timeslots carrying Idle Pattern and hence reducing the average size of IP packets transmitted. The IP bandwidth reduction requires that information is being sent from the transmitting party to the receiving party, about which timeslots are, at a given moment, not carrying traffic. Equipment existing today provides information about whether a timeslot contains traffic or not. 
   The present invention uses this information to avoid transmitting timeslots not carrying traffic. 
   The invention introduces a method for reducing the total amount of data transmitted in a system where traditional TDM circuit switched data are transported over a packet switched network. This will bring a better utilization of the network resources. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a system/device and method that eliminates the drawbacks described above. The features defined in the claims enclosed characterize this method. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to make the invention more readily understandable, the discussion that follows will refer to the accompanying drawings. 
       FIG. 1  shows an example where the invention is applied. 
       FIG. 2  shows the organization of the transmitted data in ETSI 32 ch standard. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The major achievement for the present invention is the reduced need for bandwidth, due to the fact that a transmitting party will not send idle timeslot data on the transportation network, but only signal to the receiving party which timeslots are not used, hence there will be increased bandwidth utilization in the transport network. 
   In one embodiment of the invention, as depicted in  FIG. 1 , a scenario with TDMoIP is shown. At MSC A, traditional TDM traffic comes from the Group Switch. Today this traffic goes to an Exchange Terminal (ET) before it is sent out on the SDH network. The invention uses the “TDMoIP” Exchange terminal, which can encapsulate TDM traffic into IP packets. From TDMoIP the data is sent to the packet router R 1  communicating with MSC A. Through the IP network, MSC A can communicate with both MSC B and MSC C which are connected through routers R 4  and R 2  respectively. The invention requires that the TDMoIP units receive signalling information on which timeslots are idle. In  FIG. 1 , both MSC A and MSC C have to communicate with MSC B through router R 3 . If the invention is used, it could in this scenario i.e. ease the work for router R 3 , avoiding queue and dropped packets due to fewer/shorter IP packets. In general the invention is particularly useful for the intermediate routers, and the more hops the more favourable is the invention. 
   The principle of the packet based, i.e. IP, bandwidth reduction is that the transmitting party will signal which timeslots are not carrying any traffic. These timeslots would, without this invention, carry the “Idle pattern” and hence block this capacity for other users. The idea is to remove the “idle pattern” data from the payload and only signal that this timeslot is not carrying any user traffic. Arriving at the destination in the transport network, the packet-based bearer will be terminated. The incoming data will, upon some signalling bits, find out which timeslots are not transmitted. The receiving party will then insert the “Idle pattern” into the data stream at the right position. This will happen at the point where i.e. the Internet Protocol (IP) is terminated. The original circuit switched data are then reproduced. 
   To reproduce the original data stream, the receiving site will insert the “Idle Pattern” data into the stream based on the signalling from the transmitting party. Information about which timeslots are transmitting “Idle Pattern” data is i.e. available as an “Idle Pattern Flag” in the DL34 interface from the Ericsson Group Switch on site. It must be noted that according to the standards, the transmitting party cannot remove timeslots from the data stream based on analyses of the data stream itself because the transmitted data could accidentally be identical to the “Idle Pattern”. The unused timeslots should be removed based on information from the signalling in the system (i.e. from Group Switch or ISUP). 
   The signalling part can be solved in different ways depending on the standard used. The 32-channel ETSI (European Telecommunication Standards Institute) standard, timeslot  0  (TS- 0 ) contains some free bits, marked as Sa in the FIG  3 . It is also possible to use other bits in TS- 0  as many of them have fixed values not representing any information value (sync), as it is known that the data arrive when the IP packet has arrived. For this standard, TS 0  will always have to be present in the data stream. 
   For the 32-channel solution depicted in  FIG. 2 , 31 bits are needed to signal which timeslots are idle, it is not necessary to signal for TS- 0 , as this timeslot is always used. Table 1 shows the standardized content of timeslot  0 . There are 20 free bits (Sa-bits) in one sub multi-frame. In order to signal for 31 timeslots, 2 sub multi-frames are needed. This means that Sa4 in Frame  1  in Sub multi-frame  1  will i.e. be used to signal whether timeslot  1  is idle or not. Sa5 in Frame  1  will i.e. be used to signal whether timeslot  2  is idle or not etc. As one cannot signal for all timeslots in every frame, there could be some delay/losses before the far end notices that a given timeslot is idle/not idle anymore. The exact number will be 16 frames, each of 125 microsecond&#39;s duration, totally giving 2 milliseconds delay. Each connection (timeslot) will, theoretically in a “zero delay” system, loose 16 bytes of data. This is normally not critical neither for speech nor for data, as the higher network end-to-end signalling protocols are slower than the process turning Idle Pattern on/off. If it is important not to loose any frames with valid data, it can be implemented with a check for frames containing valid data for a certain timeslot, timeslots with valid data can then be marked as “not idle” before the multi-frame is sent. The drawback is that a delay of 1 ms will be added with such a solution. 
   
     
       
             
             
           
             
             
             
           
             
             
             
             
             
             
             
             
             
             
           
         
             
                 
             
             
               Sub 
                 
             
           
        
         
             
               Multi 
                 
               Bit 1 to 8 in Timeslot 0 
             
           
        
         
             
               Frame 
               Frame 
               1 
               2 
               3 
               4 
               5 
               6 
               7 
               8 
             
             
                 
             
             
               1 
               0 
               C1 
               0 
               0 
               1 
               1 
               0 
               1 
               1 
             
             
                 
               1 
               0 
               1 
               A 
               Sa4 
               Sa5 
               Sa6 
               Sa7 
               Sa8 
             
             
                 
               2 
               C2 
               0 
               0 
               1 
               1 
               0 
               1 
               1 
             
             
                 
               3 
               0 
               1 
               A 
               Sa4 
               Sa5 
               Sa6 
               Sa7 
               Sa8 
             
             
                 
               4 
               C3 
               0 
               0 
               1 
               1 
               1 
               1 
               1 
             
             
                 
               5 
               0 
               1 
               A 
               Sa4 
               Sa5 
               Sa6 
               Sa7 
               Sa8 
             
             
                 
               6 
               C4 
               0 
               0 
               1 
               1 
               1 
               1 
               1 
             
             
                 
               7 
               0 
               1 
               A 
               Sa4 
               Sa5 
               Sa6 
               Sa7 
               Sa8 
             
             
               2 
               0 
               C1 
               0 
               0 
               1 
               1 
               0 
               1 
               1 
             
             
                 
               1 
               0 
               1 
               A 
               Sa4 
               Sa5 
               Sa6 
               Sa7 
               Sa8 
             
             
                 
               2 
               C2 
               0 
               0 
               1 
               1 
               0 
               1 
               1 
             
             
                 
               3 
               0 
               1 
               A 
               Sa4 
               Sa5 
               Sa6 
               Sa7 
               Sa8 
             
             
                 
               4 
               C3 
               0 
               0 
               1 
               1 
               1 
               1 
               1 
             
             
                 
               5 
               0 
               1 
               A 
               Sa4 
               Sa5 
               Sa6 
               Sa7 
               Sa8 
             
             
                 
               6 
               C4 
               0 
               0 
               1 
               1 
               1 
               1 
               1 
             
             
                 
               7 
               0 
               1 
               A 
               Sa4 
               Sa5 
               Sa6 
               Sa7 
               Sa8 
             
             
                 
             
           
        
       
     
   
   This opens for another way of signalling which timeslots are IDLE. If this new protocol shall be able to handle the bandwidth reduction solution described in this document, some bits must be reserved for this purpose. In the long term this will probably be the best solution and it will be usable for both 24 channel and 32 channel systems. 
   The invention can be used for all kinds of traffic based on TDM technology that needs to be transported through a packet-based network. Examples of packet based transport networks are IP, MPLS, ATM, and Frame Relay etc. To be able to use the bandwidth reduction, there must be some indication (i.e. from the Group Switch or other signalling) available as to which timeslots are filled with Idle Pattern. 
   It is also possible to add an extra field in the protocols used for signalling which timeslots are filled with Idle Pattern. The drawback is that whenever there are no idle timeslots, the packet size will be increased due to the introduction of the extra signalling bits. 
   This principle could be used for any known fixed bit pattern, not only for the Idle Pattern used in the standard TDM telephone systems. 
   List of Abbreviations 
   
       
       ATM Asynchronous Transfer Mode 
       BSC Base Station Controller 
       ETSI European Telecommunication Standards Institute 
       IETF Internet Engineering Task Force 
       IP Internet Protocol 
       ISUP ISDN User Part 
       MPLS Multi Protocol Label Switching 
       MSC Mobile Switching Centre 
       RFC Request for Comments 
       SDH Synchronous Digital Hierarchy 
       TDM Time Division Multiplex 
       PSN Packet Switched Network