Abstract:
There is disclosed a network node, and a method of operation thereof, for use in, for example, an ISDN network. The node receives messages which include mandatory elements (without which the message cannot be processed), and optional elements (without which the message may still be processed). The mandatory elements are decoded and checked for errors and, only if they are present and error-free are the optional elements decoded and checked for errors.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates to a control system for use in a network, and more particularly an ISDN data and telecommunications network. 
     DESCRIPTION OF RELATED ART 
     The invention relates in specific embodiments thereof to the B-ISDN user-network protocol Q.2931, as standardized by the ITU-T. 
     This protocol specifies the required form of each message, and a control system at a network node must be able to decode an incoming signalling message and then perform error checking. The required error checks are specified in Chapter 5.6 of the Q.2931 specification. 
     Thus, in accordance with the prior art, the incoming stream of bits which represents the signalling message is decoded and converted into a particular format, and error checking is then performed. 
     SUMMARY OF THE INVENTION 
     An incoming signalling message will consist of a number of different components, some of which are essential, that is they must be present for the message to be able to be processed at all, and others of which are optional, that is the message can be processed even if those elements are not present. However, in accordance with the prior art, it is necessary to decode the whole of each incoming message, before performing error checking. If one or more of the essential message components is absent or contains an error, the computational effort required to decode the remaining elements of the message will have been wasted. 
     In embodiments of the present invention, therefore, the message is decoded in stages. More specifically, the elements which must be present can be decoded and checked for errors before the optional elements are decoded. Then, if there is an error in one of the compulsory elements, or if such an element is missing entirely, it will not have been necessary to decode the remainder of the message. 
     This has the advantage that less computational effort is wasted on messages which cannot be processed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a representation of a message in accordance with a particular signalling protocol. 
     FIG. 2 is a block schematic diagram of a network node in accordance with one aspect of the invention. 
     FIG. 3 is a flow chart illustrating a method in accordance with another aspect of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 shows a message  2  in accordance with the Asynchronous Transfer Mode (ATM) User Network Interface (UNI) signalling protocol, used in the B-ISDN network. Each message  2  is made up of a message head  4  and a message body  6 . The message head  4  contains data which acts as: a protocol discriminator; a call reference; an indication of the message type, including a message compatibility instruction indicator; and an indication of the message length. In accordance with the protocol, each of these elements must be present in every valid message. Moreover, each of these components of the message head has a fixed size. 
     The message body  6  is made up of a number of information elements  8 . 1 ,  8 . 2 , . . . ,  8 .n. For example, there may be up to 30 such information elements in a single message, and the order of arrival of these information elements is irrelevant. 
     In the ATM UNI signalling protocol, each information element  8  is made up of an information element head  10  and an information element body  12 . Each information element head  10  includes: an information element identifier; an information element instruction field; and an indication of the length of the contents of the information element. 
     Some of these information elements are mandatory, that is the message may not be processed unless these information elements are present. For example, these elements may be absent, or may contain errors which make them unprocessable, if received from a faulty or incompatible terminal. Other information elements are optional, that is, if there is an error in one of these information elements, then the message should still be processed. In a typical message, the optional elements might have a length five times that of the mandatory elements. 
     FIG. 2 is a schematic representation of a network access node embodying the present invention. Received signalling messages are supplied to an input device  22 , and then to a memory  24 , where they are stored. From the memory  24 , data can be sent to a decoding device  26 , and then to an error checking device  28  and an output device  30 , the memory  24 , decoding device  26  and error checking device  28  being under the control of a processor  32 . 
     The form of the input device  22 , memory  24 , decoding device  26 , error checking device  28  and output device  30  are not described further, because they are generally conventional. Moreover, the processor  32  is of a generally conventional type, but is programmed to handle incoming signalling messages in accordance with a procedure described in more detail with reference to FIG.  3 . In the procedure of FIG. 3, the first step, step  51  is the start. At step  52 , an incoming signalling message is received in the input device  22  and memory  24 . Under the control of the processor  32 , the data representing the message head are sent to the decoding device  26  and decoded, in step  53  of the procedure in FIG.  3 . The decoded message head is sent in step  54  to the error checking device  28 , which performs error checking according to the standards set out in paragraphs 5.6.1-5.6.5 in the ITU-T Q.2931 protocol. If the message head is correct, it can be stored in the output device  30  ready for further transmission. If, however, the message head is incorrect, the entire message can be rejected, without requiring any further processing. 
     If the message head is correct, the information element heads can be retrieved from the memory  24  and decoded in the decoding device  26 , and the decoded information element heads, together with the associated undecoded information element bodies, are sorted into mandatory information elements and optional information elements, in step  55  of the procedure in FIG.  3 . In step  56  of the procedure, it is first checked that all of the information element heads are correct, based on the standards set out in Chapters 5.6.6-5.6.7.1, 5.6.8.1 and 5.6.8.3 in the ITU-T Q.2931 protocol. Any incorrect information element heads, together with their associated bodies, can be removed. Following this, it is then checked that all mandatory information element heads are present. If any mandatory information element heads are not present, the entire message can be rejected. 
     If all of the mandatory information elements are present then, in step  57 , the mandatory information element bodies are decoded in the decoding device  26 . Then, in step  58 , it is determined in the error checking device  28  whether all of the mandatory information element bodies are correct according to the standards defined in paragraphs 5.6.7.2 and 5.6.8.2 in the ITU-T Q.2931 protocol. Again, if they are not correct, the entire message can be rejected. If all of the mandatory information element bodies are found to be correct, then, in step  59 , the decoding device  26  decodes the optional information element bodies and the message is processed further. 
     Thus, there is disclosed a procedure which allows incorrect messages to be dealt with in a particularly efficient way, by avoiding the need to decode the optional elements of such messages.