Patent Publication Number: US-5841774-A

Title: Method and system for controlling statistically multiplexed ATM bus

Description:
This application claims benefit of international application PCT/F195/0012 filed Jan. 13, 1995. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a method and system for controlling a statistically multiplexed ATM bus, to the bus being connected a bus controller and interface units for transmission of packets, i.e. cells, over the bus. The invention thus relates to a bus solution applicable to statistical multiplexing and demultiplexing of ATM cells from several interface cards. The interface rates are typically between 0 and 155 Mbit/s. 
     In an ATM (Asynchronous Transfer Mode), data is transmitted in packets, i.e. cells, of 53 octets. One basic ATM rate at which these cells are transmitted and switched is 155 Mbit/s. When interfaces with a lower transmission rate are connected to an ATM switch that switches 155 Mbit/s cell streams, the ATM cells generated by the interfaces must be multiplexed to this rate. Correspondingly, it must be possible to demultiplex a 155 Mbit/s cell stream to a lower rate. 
     Previously used bus solutions, such as the SDH multiplexers of product family SYNFONET and the PCM buses of Nokia DX200, are based on time-shared operation of the bus, in which each interface is allocated a separate time slot. This is a waste of bus capacity and is not suitable to statistical multiplexing since bus capacity is assigned irrespective of transmission need. Because of this, concentration, i.e. serving of interfaces having a greater than nominal cell transmission rate, is impossible. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a method and system by which the above restrictions can be eliminated. The method of the invention is characterized in that after detecting, for each cell to be transmitted to an ATM bus, the address of the interface unit participating in the transmission, the bus controller sets the address on an address bus of the ATM bus, thereby activating the transmission of the cell from the bus controller to the interface unit, or vice versa, over a data bus of the ATM bus; and that during the transmission of the cell, the bus controller fetches the next interface unit address for transmission of the next cell from or to the interface unit concerned. 
     The bus allocation in accordance with the invention enables the use of transmission capacity by only those interfaces that have an actual transmission need. The allocation method allows prioritization of interfaces and makes concentration possible by furnishing the bus with interface units having a transmission capacity of above e.g. 155 Mbit/s. 
     Transmission capacity of a bus controlled as described herein is thus useful in both uplink and downlink directions such that in both directions e.g., a single unit can take the whole transmission capacity if the other units do not have anything to transmit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, the invention will be described by means of an example with reference to the attached drawing, wherein: 
     FIG. 1 illustrates the principle of the invention in the multiplexing direction, and 
     FIG. 2 illustrates the principle of the invention in the demultiplexing direction. 
    
    
     DETAILED DESCRIPTION 
     The principle of the system shown in FIG. 1 and operating in accordance with the invention is the following. 
     The ATM bus of the invention is a `point-to-multipoint` type bus with a transmission capacity of e.g., about 155 Mbit/s. Data is transmitted in cells of 53 octets. The start of a cell is indicated by a cell sync pulse, which occurs at 2.72 μs intervals. The bus divides into a multiplexing (`uplink`) element and a demultiplexing (`downlink`) element, the multiplexing element being shown in FIG. 1 and the demultiplexing element being shown in FIG. 2. A multiplexing element 1 of the bus has its own data bus 1d and address bus 1a. 
     Part 1c of the bus is for timing on the cell and bit level. The operation of the bus is controlled by a bus controller 6, multiplexer, provided with a microprocessor 5 and connected to an ATM switch by a 155 Mbit/s line. The bus further includes a request trace 1r. Each of the data bus, address bus, control bus and request trace, as shown, is provided by physically separate wires or traces. To the bus is also connected a number of interface units 2 1  -2 N , which generate and/or receive ATM cells in accordance with their own transmission need and rate. 
     In the multiplexing direction, the bus controller 6 allocates transmission periods to the interface units 2 1  -2 N  by setting interface unit addresses from a RAM 3 of the controller on the address bus 1a. The controller scan the entire address space within the duration of one cell, which in this example (155 Mbit/s bus) is the above-mentioned 2.72 μs. 
     An interface unit 2 X , i.e. any one of units 2 1  -2 N , that has an ATM cell to transmit, sends a request-to-send signal 1r to the bus controller 6, which stops a counter 4 that increments addresses in the RAM 3 at the corresponding memory location X, maintaining the address of the interface unit 2 X  that sent the request-to-send signal on the address bus 1a. The interface unit that requested transmission receives permission to transmit its cell at the rising edge of the next cell sync pulse (bus signal 1c). The bus controller simultaneously continues the interrupted incrementing of addresses at the next memory location X+1. Those interface units which do not have anything to transmit do not react in any way to the contents of the address bus in the multiplexing direction. The next possible transmitter is looked for simultaneously as the cell of the interface unit 2 X  that has been authorized to transmit is being transmitted. 
     In the multiplexing direction, the interface units thus continuously `listen` to the address bus 1a, but only an interface unit 2 X  with a cell ready for transmission sends a request-to-send signal to the bus controller 6 when it detects its own address on the address bus. 
     By suitably initializing interface unit addresses in the memory locations of the RAM 3, different interface units 2 1  -2 N  can advantageously be given a different position in the allocation of the bus 1. If addresses of interface units are initialized in the memory in proportion to their transmission capacity, bus capacity can be allocated in a congestion in proportion to the transmission rate of the interface, i.e. the number of memory locations per interface unit in the memory of the bus controller is in proportion to the rate of each interface unit. For example, an 8 Mbit/s interface thus has 4 times as many addresses as a 2 Mbit/s interface. 
     It is also possible to assign more addresses to desired interface units than they are actually entitled to in view of their transmission rate, whereby interfaces are prioritized, i.e. the number of memory locations per interface unit in the memory of the bus controller is in proportion to the priority level of the interface unit. 
     Here, addresses of interface units stored in the memory 3 and located on the address bus may mean either addresses in a tabular memory or addresses stored in memory locations according to a rule. In the first case, the address is in a register that is scanned address-by-address, one address at a time; in the second case, the address is fetched from a memory location. Even in the latter case, one memory location is scanned at a time, but not necessarily in the physical order; addresses can thus be prioritized without repeating the same addresses, e.g. by means of prioritization tests varying with the situation. 
     The bus control according to the invention allows concentration of interface units, whereby the bus controller serves a larger number of interface units than the transmission capacity of the bus would allow if the interface units were used at full capacity. 
     In the demultiplexing direction, as shown in FIG. 2, the bus controller 6 selects the address of the ATM cell to be transmitted to the receiver 2 1  -2 N  on the basis of the cell header, the whole or part of the contents of the VPI/VCI field in the cell being used for indicating the memory location of the memory 3 where the bus address of the desired interface unit is located. The address thus obtained is taken to the address bus 1a&#39; of the demultiplexing bus 1&#39;, and the original cell is taken to the data bus 1d&#39;, The multiplexing (FIG. 1) and demultiplexing transmission directions thus have their own address and data buses. 
     The width and detailed timing of the bus (address and data buses 1a, 1d; 1a&#39;, 1d&#39;) on the technology available. The bus may either operate as a backplane bus for one multiplexer, or it may extend from one multiplexer to another. When a twisted twin cable is used for extending the bus from one multiplexer to another, the clock of the bus should be less than 20 MHz (e.g. 19.4 MHz), which leads to 8-bit wide data buses. The width of the address bus depends on the number of units to be multiplexed, but the address space of the bus controller, the memory access rate, and the cell transmission frequency of the bus must be proportioned to one another so that the entire address space can be scanned between two cell sync pulses, i.e. in this example in less than 2.72 μs, by conducting a poll that covers the addresses of the interface units. 
     It is obvious to one skilled in the art that the different embodiments of the invention are not limited by the above-described example but that they may vary within the scope of the attached claims.