METHOD AND ARRANGEMENT RELATED TO SYNCHRONOUS SWITCHING

Abstract: The present invention relates to a method and an arrangement related to synchronous switching, for example in relation to digital communication between devices being arranged on boards which are housed in racks having a back-plane associated therewith, and for the improvement of minimum delay and configuration flexibility, combined with high capacity and high reliability, it is according to the present invention suggested a decentralised concept, or a decentralised architecture, specifically comprising a novel so-called RSO-bus arranged in the back-plane of said rack.

Description: METHOD AND ARRANGEMENT RELATED TO SYNCHRONOUS SWITCHING Field of the invention The present invention relates to a method related to synchronous switching, for example in relation to digital communication between devices being arranged on boards which are plugged into racks having a backplane associated therewith. The present invention also relates to an arrangement for bringing such a method into practice. Background of the invention Today there are limitations in the flexibility of synchronous digital switches with respect to minimum delay and configuration flexibility combined with a non- blocking concept, multi-slot switching, high capacity and high reliability. Commercial available components use a switching matrix scheme with a centralised architecture. Consequently, such switching matrixes suffer from not being able to achieve high capacity with redundant protection in itself, all of which being tied to the centralised concept. Further, redundancy as mechanisms for protective supervision is complex and causes low MTBF, a fact which has hampered any development suggesting any improvement of the above prior art deficiencies . Prior art US 4 547 880 (De Vita et al . ) discloses a communication control apparatus for digital devices, comprising a com- munication bus for routing data and control signals internally between said digital devices. US 4 955 020 (Stone et al . ) relates to a digital communication system comprising a plurality of modules and a backplane having a bus architecture which is connected between said modules. This prior art bus architecture suggests to a certain degree a solution to the parameter limitation problem, but is silent about introducing a decentralised architecture related to synchronous digital switching. US 5 432 791 (Gancarcik) relates to a digital switching system comprising a system bus transmitting data between interface boards and a control unit, said control unit using a clock system which is synchronised. US 5 161 152 (Czerwiec et al . ) discloses a high-speed synchronous transmission line access terminal, comprising an access module which in turn comprises a plurality of line boards. According to the prior art there is used a multi-link serial bus for connecting a time slot inter- changer to the access module, i.e. for connecting the line boards with the processor unit. Objects of the invention A main object of the present invention is to suggest a method and an arrangement whereby the deficiencies of the prior art are alleviated. Another object of the present invention is to suggest a method and arrangement in connection with synchronous digital switching wherein flexibility is greatly improved and wherein operations may be affected with a further minimum delay. Still further, the method and arrangement according to the present invention have for an objective to leave the centralised architecture solution, and rather suggest a decentralised concept for achieving high capacity and improved redundant protection. Brief disclosure of the invention The above objects are achieved in a method and an arrangement according to the present invention, which is characterized by using a decentralised concept, and more specifically by using a special bus, i.e. an RSO-bus arranged in the backplane of the associated rack. Further features and advantages of the present invention will appear from the following description taken in conjunction with the enclosed drawings, as well as from the enclosed patent claims. Brief disclosure of the drawings Fig. 1 is a schematic diagram illustrating an example of an embodiment according to the present invention. Fig. 2 is a time system illustrating the clock system used according to the present invention. Fig. 3 is a time diagram illustrating an addressing method used in accordance with the present invention. Detailed description of embodiments As stated previously, the present invention suggests a system which in view of a prior art switching matrix scheme with a centralised architecture, suggests a decentralised concept and a decentralised architecture related thereto . In other words, the present invention suggests the intro- duction of a special bus, a so-called RSO-bus arranged in the backplane of the rack wherein a plurality of devices communicating digitally with each other, are located. A detailed example of how such an RSO-bus can be imple- mented, appears from Fig. 1. With a good designed timing and by controlling the physiological parameters, it is possible to build a synchronous digital switch without having active switching matrix components . We describe a new bus structure, named the RSO-bus, which overcome all the constraints mentioned earlier. This bus is used on a backplane which sits in a rack. In this rack there are several boards connected together through the backplane with the RSO-bus. This backplane bus use an 8 bit parallel interface for traffic data (timeslots) connections. Each daughter/line board carries an internal address mechanism which is synchronised through the backplane. Both write clock read clock is generated locally on each line board, synchro- nised to a common 125 μs framing. With this scheme it is possible to switch for example 2048 duplex timeslots within 125 us. Here is more thorough look equivalent to 128 2 Mbit/s interfaces. Internal interfaces The system where the backplane structure is used, is composed of four types of functional units: line boards, a CPU board, a clock board and 2 Mbit/s transmission boards with an arbitrarily number of channels, but typically 8 channels in a practical switch. The Traffic Interface The traffic interface is an 8 bit parallel data bus and a 125 μs framing pulse. The Backplane interface has to be seen in conjunction with the line boards which hold the interface circuit to the backplane and the addressing mechanism for giving each line board access to the backplane and then to a certain set of timeslots. The traffic is being sent between the different line boards in a thorough and well defined way. The bus drivers used in this implementation is a GTL type, but others could be used as long as they can handle the speed requirements . The clock system and the internal addressing system are what gives the thorough timing for how and when each line board sends and receives traffic data. The definition follows. Clock system This system uses 2 clocks. Both clocks are generated locally on the LT-board. The read clock is adjusted for optimised reading of the data from the bus and into the line boards. This is illustrated in Fig. 2. Redundancy A bus system is a common resource and is a single point of failure. By using a duplicated bus, there can be established redundancy against bus errors. One bus is ac- tive and one stand-by. Writing is done to both buses. By reading back and comparing reading with writing to the buss, a bus error can be detected and the standby bus can be activated. Addressing method The address method comprises letting the line boards itself have its own address counter. The counter is reset/synchronised from the clock board. With the counter you can have a small RAM of 4096 bits. Each 2 bit position corresponds to a timeslot position and whether the board shall have read or write on the buss. The internal address counter gives addresses to this RAM and when a 1 is read from any of the timeslot positions, the line board will read or write in that timeslot according to how the second bit is set. The bits have the following meanings: When the bits are 1 then a read or write occur. If the bits are 0 no action will occur, see Fig. 3. Traffic types handled in RSO bus structure This bus can in a non-blocking, synchronous way handle the following traffic types. Bidirectional When 2 boards are to switch timeslots of 64 Kbit/s (1 each) they have to share the same bus in one 61 ns period. The traffic interface will belong to one board in the first 30.5 ns transmitting and to the second board transmitting in the next every 30.5 ns . This means that the buss has to change transmit direction every 30.5 ns . Two boards can exchange 1 to 2048 switching cases. Undirectional Read and write in different directions. In the 61 ns period there can be different read/write addresses. Broadcasting One or more timeslots are transmitted (write) and can be received (read) by an arbitrary number of boards. If multidrop is used only one of the reading addresses will be writing back. Advantages The advantages of this solution is a cost-effective solu- tion with a potential of very high reliability due to the distributed concept. The distributed concept also offers the highest degree of flexibility in switching. Redundancy can be implemented with a minimum of complexity. The decentralised architecture of the system also makes it possible to address an arbitrary number of timeslots up to maximum capacity, and by this make it possible to connect a 155 Mbit/s SDH type of transmission to the switching backplane. This is beneficial since this makes it possible to switch the overhead capacity (TU-12/VC-12 ) following the 2 Mbit/s circuits in SDH. Switching of this overhead makes it possible through transmission systems (HDSL) to terminate the overhead at a remote customer interface and by this having end to end supervision of a 2 Mbit/s circuit through a network. Higher switching capacity can be achieved as electronic components and mechanical building practice are devel- oped. The 2048 timeslots in the present solution can easily be extended. Abbreviations MTBF - Mean Time Between Failure RSO - Inventor's initials CPU - Central Processing Unit GTL - Gate Transistor Logic OCTO - Eight Channel Link Board PRI - PCM-Primary Rate SDH - Synchronous Digital Hierarchy LT-board - Line Terminating Board

Claims: P a t e n t c l a i m s 1. Method related to synchronous switching, for example in relation to digital communication between devices be- ing arranged on boards which are housed in racks having a backplane associated therewith, said method comprising the use of a decentralised concept, c h a r a c t e r i z e d i n that there is used a special bus, i.e. an RSO-bus arranged in the backplane of said rack, by arranging on the same line boards the address mechanism of the line boards in question, and by the use of line boards or data boards having their own address counters. 2. Method as claimed in claim 1, c h a r a c t e r i z e d by mounting the interface circuits of said backplate on the respective line boards of said rack. 3. Method as claimed in claims 1 or 2 , c h a r a c t e r i z e d i n that the decentralised concept is implemented by introducing an interface comprising an 8 bit parallel data bus and a 125 μs framing pulse. 4. Method as claimed in any of the preceding claims, c h a r a c t e r i z e d by using an RSO-bus based on a bi-directional bus, i.e. a read clock and a write clock. 5. Method as claimed in claim 4, c h a r a c t e r i z e d i n that read and write clocks are generated on each line board. 6. Method as claimed in any of the preceding claims, c h a r a c t e r i z e d by using four types of functional units: line boards, a CPU board, a clock board and 2 Mbit/s transmission boards with an arbitrary number of channels, typically 8 channels in a practical switch. 7. Method as claimed in any of the preceding claims, c h a r a c t e r i z e d by using a duplicated bus, one bus being an active bus and the other being a standby bus, possibly by letting writing and reading being done to both buses. 8. Arrangement related to synchronised switching, for example in relation to digital communication between devices being arranged on boards which are housed in racks having a backplane associated therewith, said arrangement being based on a decentralised or distributed concept, c h a r a c t e r i z e d by a special bus, i.e. an RSO-bus arranged in the backplane of said rack, by address mechanisms means of the line boards in question ar- ranged on the same line boards, and by line boards or data boards equipped with their own address counters. 9. Arrangement as claimed in claims 8, c h a r a c t e r i z e d i n that said backplate comprises interface circuits on the respective line boards of said rack. 10. Arrangement as claimed in claims 8 or 9 , c h a r a c t e r i z e d i n that an interface com- prises an 8 bit parallel data bus and a 1 bit address bus. 11. Arrangement as claimed in any of the claims 8-10, c h a r a c t e r i z e d by an RSO-bus comprising a bi-directional bus, i.e. a read clock and a write clock. 12. Arrangement as claimed in claim 11, c h a r a c t e r i z e d i n that a clock board is arranged to generate a master clock, and that each line board is implemented to generate locally read and write clocks. 13. Arrangement as claimed in any of the claims 8-12, c h a r a c t e r i z e d i n that the backplane structure is composed of four types of functional units: line boards, a CPU board, a clock board and 2 Mbit/s transmission boards with an arbitrary number of channels, typically 8 channels in a practical switch. 14. Arrangement as claimed in any of the claims 8-13, c h a r a c t e r i z e d i n that the arrangement comprises a duplicated bus, one bus being active and the other being stand-by, and possibly both buses being adapted for receiving writing.

Inventors: SCHUMANN-OLSEN REIDAR, SOERENSEN BJOERN ERIK, SCHUMANN-OLSEN, REIDAR, SOERENSEN, BJOERN, ERIK
Applicants: ERICSSON TELEFON AB L M, SCHUMANN OLSEN REIDAR, SOERENSEN BJOERN ERIK, SCHUMANN-OLSEN, REIDAR, SOERENSEN, BJOERN, ERIK, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 1999
Main classification: H04J
IPC codes: G06F13, H04L12