SWITCHING SYSTEM

Abstract: A switching system using an asynchronous packet switching technique. The packets may contain voice, data or video signals. The switching speed is increased owing to the processor (SPC) analyzing only some special packets and that it is located out of the transmission path of the packets. It may thus process these special packets at a lower speed than the transmission speed. This system also includes a header correction circuit (HC) which improves the quality of the transmission.

Description: The present invention relates to a switching system for switching digital signals, grouped in incoming packets having a header, applied to any one of a plurality of input terminals thereof to any one of a plurality of output terminals thereof, said switching system including common switching means, input means coupling said plurality of input terminals to said common switching means and including means to detect errors in said incoming packets, and output means coupling said common switching means to said plurality of output terminals. Such a switching system is already known in the art, e.g. from the IEEE PROCEEDINGS, volume 127, Pt. E, No. 6; November 1980, pages 233-240, Old Woking, Surrey, GB; J.A. McDERMID et all.: "Design and use of Comflex - a hardware-controlled packet switch". Therein the error detection means are only able to perform a parity check on each incoming packet. Upon the detection of an error the packet is rejected, as a consequence of which the transmission quality is adversely affected. An object of the present invention is to provide a switching system of the above type but with an improved transmission quality. According to the invention this object is achieved due to the fact that said error detection means are header correction means to correct faulty headers of said incoming packets. Because the faulty headers are not only detected but also corrected the packets need no more to be rejected and the transmission quality of the system is improved. It should be noted that the article "Local Networks - Ethernet controller chip interfaces with variety of 16-bit processors" by Sunsil P. Joshi, published in Electronic Design, volume 30, No. 21, October 1982, pages 193 to 200, also discloses a system provided with error detection means only, i.e. not with correction means. Another characteristic feature of the present invention is that said header correction means include means for providing valid headers, means for detecting if the header of an incoming packet corresponds to one of said valid headers and means which in response to said detecting means having detected a faulty header select one of said valid headers to replace said faulty header in said incoming packet. Moreover, said selected valid header is the valid header which is the nearest to said faulty header. The present invention is also characterized by the facts that the switching system includes a plurality of input means each coupled to one of said input terminals an each including individual header correction means, and that each of said input means further include individual processor means bidirectionnally coupled to individual routing means, said routing means coupling said header correction means either to said processor means or, through said common switching means, to one of said output terminals according to routing information contained in said header of the packet. In the switching system disclosed in the first mentioned document, the routing means are provided in common for all the input means and have therefore to handle the packets supplied to all the input terminals. Moreover in each of these input means all the packets supplied are always processed by the processor means included therein. Hence, in this known system the work loads of both the routing means and the processor means are relatively high. In the present invention, these high work loads are reduced, on the one hand, owing to the individual routing means included in each of the input means and which only handle the packets supplied to the corresponding input terminal, and on the other hand, owing to the routing means which supply the packets either to the switching means or to the processor means, the latter have thus only to handle part of the number of packets supplied to the input means in which they are included. It should also be noted that a switching system of the above type, but not adapted to switch packets, is already known in the art, e.g. from the article "ITT 1240 Digital Exchange - Digital Switching Network" by J.M. Cotton et al, published in "Electrical Communication", Volume 56, No. 2/3 - 1981, pages 148 to 160 or from the UK patent application GB-A-2 016 866. However, none of these known systems presents the above described characteristic feature of the invention. These known systems are able to switch, with a switching quality allowing real time communication, signals (e.g. voice) having a same frequency range and generated by signal sources operating at a fixed bitrate and are not provided with header correction means. Still another characteristic feature of the present switching system is that said common switching means operate at a predetermined clock frequency and that each of said input means include input storage means for storing said signals and having input and output portions, said input portion being coupled to said one of said input terminals and operating at the frequency of the signals supplied to said one of said input terminals, and said output portion being coupled to an input of said routing means and operating at said predetermined clock frequency to supply said stored signals to said common switching means. In this way, incoming signals arriving at said one input terminal at whatever bitrate are loaded in the corresponding input portion of the input storage means. When a complete packet is loaded in this input portion, it is transmitted to the output portion thereof and from there to the routing means at the predetermined clock frequency. Hence, the latter frequency, which is also the operating frequency of the common switching means, may be different from the frequency and bitrate of the incoming signals. Also another characteristic feature of the present switching system is that the frequencies of at least two of the signals supplied to distinct terminals out of said plurality of input terminals are different. In this way and because the predetermined clock frequency at which the common switching means operate is independent of the frequencies of the signals supplied at the input terminals, the system operates in a fully asynchronous way. A further characteristic of the present switching system is that said predetermined clock frequency of said common switching means is at least equal to the sum of the values of the frequencies of said signals applied to said plurality of input terminals. Thus the common switching means are non-blocking. In the present switching system, the common switching means are provided with a time division path which comprises a plurality of timeslots each allocated to distinct ones of said input means and said routing means are able to transmit to the allocated one of said timeslots at least a portion of said packet and the address of said one of said output terminals. The above mentioned and other objects and features of the invention will become more apparent and the invention itself will be best understood by referring to the following description of an embodiment taken in conjunction with the accompanying drawing which shows a switching system SW according to the invention. A plurality of such switching systems SW are interconnected to constitute a switching network included in a digital telecommunication exchange to which user stations are coupled via asynchronous time division transmission links. The switching network is able to establish virtual communication paths for various types of signals such as voice, data and video transmitted by the user stations. More particularly, the input signals applied to the network inputs via an asynchronous time division transmission link are digital signals grouped in packets having a fixed length and comprising a header and data. Each packet or a portion thereof is transmitted in a timeslot of the asynchronous time division transmission link. Likewise, the output signals appearing at the network outputs are also digital signals grouped in packets. The rate of both the input and output digital signals may be variable but has an upper limit of e.g. 280 Megabits/second. There may be no relation between the rates of the input signals and these of the output signals, these rates being function of the transmission links and of the devices coupled to the switching network through these links. To be noted that the above input digital signals are accompanied by a clock signal having a frequency corresponding to the input signal rate and which is necessary to synchronize the transmission of these signals to the asynchronous time division transmission link. This clock signal may be supplied either separately on a different link or is preferably included in the input signals. The above switching system SW has 8 input terminals Rx0 to Rx7 and 8 output terminals Tx0 to Tx7 and further includes a common switching bus SB to which each of the input terminals Rx0 to Rx7 is coupled via distinct input means or receive circuits, and which is coupled to each of the output terminals Tx0 to Tx7 via distinct output means or transmit circuits. A combination of a receive circuit with a corresponding transmit circuit in a same switching system is called a switch port. Since the 8 receive circuits and the 8 transmit circuits of the switching system SW are respectively identical, only one of each, i.e. only one switch port comprising the receive circuit RC to which the input terminal Rx0 is connected and the transmit circuit T connected to the output terminal Tx0, is represented in detail. The receive circuit RC includes between the input terminal Rx0 and the switching bus SB the cascade connection of a synchronization circuit SC, an input storage circuit IQ, a header correction circuit HC and a routing circuit RT. RC also includes a processor or switch port controller SPC controlling SC and possibly RT. An output of RT is connected to SPC and the latter is also bidirectionally connected to HC. Furthermore, the input storage circuit IQ comprises an input portion controlled via a control terminal II by the synchronization circuit SC, and an output portion controlled via a control terminal IO by a timing circuit TM coupled to and controlling the switching bus SB. The transmit circuit TC includes an identification circuit IC, an output storage circuit OQ and an output clock OC. The switching bus SB is connected to the identification circuit IC which is controlled by the timing circuit TM. The output storage circuit OQ comprises an input portion to which the switching bus SB is connected and which is controlled by the identification circuit IC via a control terminal OI, and an output portion connected to the output terminal Tx0 and which is controlled by the output clock OC via a control terminal OO. The timing circuit Tm is adapted to generate synchronous Time Division Multiplex (TDM) frames comprising 8 timeslots or channels on the switching bus SB. Each of these timeslots is allocated to one receive circuit and the sampling frequency of the switching bus SB must at least be equal to the sum of the frequencies of the signals at the input terminals. However, in practice this frequency is generally equal to the product of the 8 receive circuits with the above mentioned maximum allowed input rate of the signals at the input terminals Rx0 to Rx7, i.e. 280 Megabits/second, the sampling frequency of the switching bus SB is then equal to 2,240 Megabits/second. In this way, the maximum possible delay for a packet present in the receive circuit RC to be loaded on the switching bus SB is mainly equal to the time spent in waiting until the timeslot corresponding to this particular receive circuit RC appears on the switching bus SB. Each of the above input/output terminals of the switching system SW is coupled by an asynchronous time division transmission link to an upstream/downstream located device which may either be another similar switching system or a user station. The upstream located device is able to feed digital signals to the input terminal Rx0 to Rx7 of the switching system SW under the form of packets containing a header and data. These packets have, e.g., a fixed length of 16 bytes two of which being used as header and may be : control packets which are transmitted to establish and/or to update a virtual communication path in the switching system SW and contain information about this path; synchronization packets which are used to synchronize the operations of the upstream located device and the switching system SW. At least one of these packets is transmitted within an allocated predetermined time interval; normal communication packets which contain communication data. To be noted that the switching system SW also includes input serial-to-parallel and output parallel-to-serial conversion circuits (not shown) in order to first convert the serial input signals in a parallel form and to then convert the parallel form into output serial signals. The operation of the above switching system SW is described hereinafter. When the above input digital signals are applied, under the form of input packets, to the input terminal Rx0 they are loaded in the synchronization circuit SC. The latter includes a clock extraction circuit (not shown) which extracts the clock signal from the input digital signals and applies it to the control terminal II of the input portion of the input storage circuit IQ. Under the control of this clock signal the digital input signals are then loaded into IQ and when the latter contains the digital signals of a complete packet, e.g. 16 bytes, this packet is fed to the header correction circuit HC under the control of the timing circuit TM which operates via the control terminal IO. The header of a packet applied to HC might be modified due to errors, e.g. occurring during the transmission of the input digital signals from the upstream located device to the switching system SW. As a result, the transmission quality of the virtual communication path used is adversely affected. It may even happen that the faulty header is equal to the header of a packet belonging to another virtual communication path so that also the transmission quality of the latter path is adversely affected. The purpose of the header correction circuit HC is to improve the overall system quality by correcting the received faulty headers, and to collect information which is indicative of the reliability of the system. To this end, HC includes means (not shown) to process the header of a received packet, the data of this packet being directly transmitted to the routing circuit RT. Normal communication packets are then transmitted to the switching bus SB, whereas synchronization or control packets are supplied to the SPC. The processing of the header consists of comparing the incoming header with valid headers contained in a table (not shown) or calculated by means of an algorithm. If the incoming header is equal to one of the valid headers, it is correct and therefore is sent to the routing circuit RT. On the contrary, if the incoming header is not equal to one of the valid headers, this incoming header is faulty and therefore it is replaced, e.g. by the valid header which has the nearest value. This new header is then sent to the routing circuit RT. Each time a correction is performed, a correction counter (not shown) included in HC is incremented so that the value contained in this counter is indicative of the reliability of the system. The number of corrections stored in the correction counter is permanently known by the switch port controller SPC owing to the bidirectional link between HC and SPC. When this number exceeds a predetermined value, SPC is able to take appropriate actions, e.g. to inform the telecommunication exchange of a faulty operation of the receive circuit RC or of the upstream located device, and to resist the value of the correction counter to zero. As mentioned above control packets and synchronization packets supplied to input terminal Rx0 of the receive circuit RC are fed through SC, IQ, HC and RT and are finally stored in SPC. The latter includes means (not shown) to analyze the packets received and to thereby distinguish between control packets and synchronization packets. In the case of control packets, which for instance contain information about a virtual communication path to be established, the SPC communicates to the routing circuit RT an outgoing header to be used to replace the incoming header of the incoming packets to route further these packets on the virtual communication path. More particularly, the incoming header, the outgoing header and also the address of the transmit circuit of SW forming part of the virtual communication path are stored, as long as this path remains established, in a routing table (not shown) included in the routing circuit RT. In this way, when a virtual communication path has been established the routing circuit RT replaces the incoming header of each incoming packet of that path by the associated outgoing header and transmits the modified packet and the address of the associated transmit circuit to the switching bus SB. In a preferred embodiment there may be a fixed relation between the incoming and the outgoing headers, e.g. a rotation of the digits included in the header. The routing table RT is then able to substitute the outgoing header for the incoming header without the control of the processor SPC. It is to be noted that control packets may also be received when a virtual communication path is already established. These packets contain then information to update that virtual communication path, i.e. the outgoing header and/or the address stored in the routing table of the routing circuit RT. When the transmission of the packets on the virtual communication path occurs in a synchronous manner at least one synchronization packet will be received by SPC within the predetermined time interval allocated to the synchronization packet and no action is taken by SPC. On the contrary, when the SPC does not receive such a synchronization packet or a signal from the SC indicating that a synchronization packet has arrived within this time interval, it orders the synchronization circuit SC to delay by one or more bits the signals received at the input terminal Rx0. As a consequence, the packets transmitted by the output portion of the input storage circuit IQ to the routing circuit RT are shifted by as much bits. This operation is repeated until the routing circuit RT recognizes the header of a synchronization packet which is then transmitted to the SPC. It is to be noted that the header and the data of a same normal communication packet should arrive simultaneously at the switching bus SB. But, since only the header is processed by HC and by RT, it may be subjected to a small delay during its transmission from IQ to SB. Therefore, the data of a packet are latched in IQ for a longer time than the header. A possibility to reduce the number of parallel links and thus the dimensions and the control of the switching bus SB is to split the packets of 16 bytes which arrive thereat into four portions of 4 bytes each comprising a part of the header and a part of the data. These four portions are then regrouped in a complete packet of 16 bytes in the output storage circuit OQ. The operation of the transmit circuit TC is considered hereinafter. The identification circuit IC forming part of the transmit circuit TC shown is synchronized to the timeslots of the switching bus SB by the timing circuit TM. It is able to read the various addresses of the transmit circuits which have been loaded in these timeslots by the routing circuit RT in the way described above. When IC, among the addresses read, recognizes the address of its own transmit circuit TC, the header and the data of the packet also loaded in that same timeslot are transferred from the SB to the output storage circuit OQ. This transfer is controlled by IC via the control terminal OI. From there, the packets are transmitted on the asynchronous time division transmission link connected to Tx0. Because the transfer of packets occurs at the rate of the timeslots of SB whereas the transmission from Tx0 is performed at the rate of the output clock OC which is much lower that the first mentioned rate, OQ is sufficiently large to store a queue of packets such that in practice no packets can be lost. The maximum length of this queue obviously depends on the expected maximum communication traffic able to flow through the transmit circuit TC. Furthermore, the delay introduced in the virtual communication path by a switch port mainly depends on the current length of this queue. It is to be noted that, as for the rate of the input signals, the rate of the output signals and therefore the frequency of the output clock OC may also be variable. The maximum value of the rate of the output signals is for instance also 280 Megabits/second. The ability to change the transmission rate of the input and of the output signals is useful to adapt these rates to the properties of the transmission links and of the coupled devices so that a same switching system SW may be used to switch a wide frequency range of signals such as voice, data and video as mentioned above.

Claims: Switching system (SW) for switching digital signals, grouped in incoming packets having a header, applied to any one of a plurality of input terminals (Rx0 to Rx7) thereof to any one of a plurality of output terminals (Tx0 to Tx7) thereof, said switching system including common switching means (SB, TM), input means (RC) coupling said plurality of input terminals to said common switching means and including means to detect errors in said incoming packets, and output means (TC) coupling said common switching means to said plurality of output terminals, characterized in that said error detection means are header correction means (HC) to correct faulty headers of said incoming packets. Switching system according to claim 1, characterized in that said header correction means (HC) include means for providing valid headers, means for detecting if the header of an incoming packet corresponds to one of said valid headers and means which in response to said detecting means having detected a faulty header select one of said valid headers to replace said faulty header in said incoming packet. Switching system according to claim 2, characterized in that said selected valid header is the valid header which is the nearest to said faulty header. Switching system according to claim 1, characterized in that it includes a plurality of input means (RC) each coupled to one of said input terminals (Rx0 to Rx7) and each including individual header correction means (HC). Switching system according to claim 4, characterized in that each of said input means (RC) further include individual processor means (SPC) bidirectionnally coupled to individual routing means (RT), said routing means coupling said header correction means (HC) either to said processor means or, through said common switching means (SB, TM), to one of said output terminals (Tx0 to Tx7) according to routing information contained in said header of the packet. Switching system according to claim 5, characterized in that, in response to at least one packet received, said processor means (SPC) are able to indicate to said routing means (RT) which one of said output terminals (Tx0 to Tx7) has to be selected when a packet is received in said routing means. Switching system according to claim 5, characterized in that said individual header correction means (HC) are coupled between said one of said input terminals (Rx0 to Rx7) and said routing means (RT) and are controlled by said processor means (SPC). Switching system according to claim 5, characterized in that said routing means (RT) are able to replace the headers of incoming packets received from said header correction means (HC) by new headers prior to route said packets with said new headers to said common switching means (SB, TM). Switching system according to the claims 6 and 8, characterized in that the value of said new headers are provided to said routing means (RT) by said processor means (SPC) in response to at least one control packet supplied to said system (SW) via said one of said input terminals (Rx0 to Rx7) and transmitted to said processor means via said routing means. Switching system according to claim 1, characterized in that said common switching means (SB, TM) operate at a predetermined clock frequency and in that each of said input means (RC) include input storage means (IQ) for storing said signals and having input and output portions, said input portion being coupled to said one of said input terminals (Rx0 to Rx7) and operating at the frequency of the signals supplied to said one of said input terminals, and said output portion being coupled to an input of said routing means (RT) and operating at said predetermined clock frequency to supply said stored signals to said common switching means. Switching system according to claim 10, characterized in that the frequencies of at least two of the signals supplied to distinct terminals out of said plurality of input terminals (Rx0 to Rx7) are different. Switching system according to claim 11, characterized in that said predetermined clock frequency of said common switching means (SB, TM) is at least equal to the sum of the values of the frequencies of said signals applied to said plurality of input terminals (Rx0 to Rx7). Switching system according to claim 11, characterized in that said predetermined clock frequency of said common switching means (SB, TM) is equal to the product of a maximum allowed value of frequency of said signal and of said plurality of input terminals (Rx0 to Rx7). Switching system according to claim 10, characterized in that said signal frequency is variable. Switching system according to claim 10, characterized in that each of said input means (RC) further include synchronization means (SC) coupled between said one of said input terminals (Rx0 to Rx7) and said input portion of said input storage means (IQ) and able to provide to a control input (II) of said input portion of said input storage means said signal frequency in order to control the operation of said input portion. Switching system according to claim 15, characterized in that, within a predetermined time interval relatively larger than the output rate of said packets of said routing means (RT), at least one of said packets is a synchronization packet containing a synchronization pattern and is transmitted to said processor means (SPC) via said routing means, said processor means controlling said synchronization means (SC) to obtain packet synchronization in response to the reception or to the non-reception of said synchronization packet by said processor means within said time interval. Switching system according to claim 10, characterized in that it includes a plurality of output means (TC) each coupled to one of said output terminals (Tx0 to Tx7) and in that each of said output means (TC) include output storage means (OQ) for storing at least a portion of said packets and having an input portion coupled to said common switching means (SB, TM) and operating at said predetermined clock frequency for entering signals from said common switching means, and an output portion coupled to said one of said output terminals (Tx0 to Tx7) and operating at the frequency of an output clock (OC) also included in said output means to convey said packets to said one of said output terminals. switching system according to claim 17, characterized in that said frequency of said output clock (OC) is variable. Switching system according to claim 1, characterized in that said common switching means (SB, TM) are provided with a time division path (SB) which comprises a plurality of timeslots each allocated to distinct ones of said input means (RC) and that said routing means (RT) are able to transmit to the allocated one of said timeslots at least a portion of said packet and the address of said one of said output terminals (Tx0 to Tx7). Switching system according to the claims 17 and 19, characterized in that each of said output means (TC) further include identification means (IC) coupled to said time division path (SB) and able to read and to recognize said address of said one of said output terminals (Tx0 to Tx7) amongst said addresses transmitted by said routing means (RT) within one of said timeslots and then to order via a control terminal (OI) said input portion of said output storage means (OQ) to store said packet associated to said one of said timeslots.

Inventors: BAUWENS, JAN GASTON, DE SOMER, MICHEL PHILEMON MADELEINE, DEBUYSSCHER, PIERRE LEON
Applicants: ALCATEL N.V.
Application year: 1987
Main classification: H04L12
IPC codes: H04L29, H04Q11, H04L12