Abstract:
A system for expanding a parameter encoding field in a new communications protocol that is compatible with an old protocol. An unused command parameter in an old protocol is used to indicate an expanded parameter encoding field in a new protocol. Parameter encodings from the old protocol are unchanged in the new protocol. A system communicating using the old protocol recognizes all parameters from the old protocol and ignores parameters from the new protocol. A system communicating using the new protocol recognizes parameters from both the old and new protocols.

Description:
FIELD OF THE INVENTION 
     This invention relates to expanding the number of encodings that can be used to represent parameters in a parameter field of messages transmitted between a first and a second communications device. More particularly, this invention relates to an indicator in the parameter field that signals that the message contains an expanded parameter encoding. 
     1. Problem 
     It is common for a first device and a second device, such as connected computer systems, to communicate with one another via a communications path. Most communications between devices involve transmitting message packets between the first and second devices. The message packets are streams of binary data that are divided up into different fields representing different pieces of information being transmitted in the message. In order to communicate, the devices must use the same protocol to generate and read the message packets. A protocol is a set of rules that defines the structure of the fields in message packets as well as the encodings of bits in a field. 
     Packets transmitted between the devices may contain a parameter which may be a command that a receiving device perform a specified function as well as the data required to perform the function or a specific type of data. A parameter is one data field in a message. Each parameter is divided into several sub-fields including a data length sub-field which indicates the length of the parameter in number of bits, a parameter identification field identifying the requested function, and a data field. A unique encoding in the parameter identification field is used to indicate each different parameter supported in a protocol. For example, the parameter identification field may be 8-bits long or one-byte long and each different 8-bit encoding represents a different command. A first parameter could be represented by the encoding 00000000 and a second parameter may be represented by the encoding 00001111. A device receiving a message would read the parameter identification field from a parameter in the message and determine the command to be performed. 
     It is a problem to add additional commands to a protocol system having a parameter identification field of a predetermined length. The predetermined length of encodings limits the number of encodings that can be used to represent parameters. For example, a one byte or 8-bit encoding field can only hold 256 different encodings. After the 256 different encodings are assigned to different parameters, no more parameters can be added to the protocol. 
     One method for adding additional parameters to a protocol would be to change the length of the identification field in a parameter to increase the number of available encodings available to represent parameters. However, lengthening of the identification sub-field is unacceptable. If the identification field is lengthened, each device that receives messages using the protocol must receive an updated version of the protocol to recognize the new length of the identification field in a parameter. Otherwise, systems with the older version of the protocol do not recognize the new encodings with the new length and are not able to correctly read a message transmitted in the new protocol. In many systems, expenses and logistics make it impossible to update the protocol used by every device in the system to communicate. A common example of a system that has the above problem is a telephone system. In telephone systems, controllers of switching systems communicate with one another via trunks connecting the switching systems. Message packets are transmitted between the switching systems to establish and maintain connections between calling stations. The message packets include a message type identification, a field indicating the number of bits in the packet and multiple parameters. Each parameter in a packet includes a length field giving the length of the parameter in bits, an identification field of one-byte containing the parameter encoding, and data needed to perform the operation represented by the parameter. 
     In a telephone system, it is common to reserve specific encodings for parameters representing common operations and data needed to provide telephone service. There remaining encodings can be used to represent other commands that are required to provide custom calling features. However, many telephone companies are expanding the telephone service provided to customers and require more parameter types than there are available encodings. It would be expensive and logistically impossible to install a new protocol in every switching system in a telephone network to add more parameters for the provision of expanded services. Therefore, there is a need to provide a protocol that has more encodings to represent new parameters and is compatible with the existing protocol to allow systems communicating with the older version of the protocol to remain in use. 
     2. Solution 
     The above and other problems are solved and an advance in the arts is made by the provision of a system for increasing the number of parameter identifications that can be encoded in a parameter of a message packet while maintaining existing encodings for existing parameters in a protocol. The system of present invention allows a device that communicates using an older version of a protocol to receive messages that contains parameters having expanded encoding fields. The device reads and processes parameters having encodings that represent commands in the older protocol. Parameters having an expanded parameter identification field of the new length are ignored by a device communicating with the older protocol. A device communicating using the new protocol reads and processes parameters having identification field that is either the length expected by the new or old protocol. This invention allows devices that operate using the new protocol to communicate with devices using the old protocol. 
     In a new protocol, the identification field of a parameter is one of two lengths. The first length of the identification field is the length of encodings in the old protocol. Encodings for parameters supported by the old protocol are written into the identification field of a parameter. If the parameter is a new parameter that is supported by the new protocol, the encoding is the second, expanded length. An encoding that does not represent a parameter in the old protocol is written into the identification field of a new parameter. The unused encoding indicates that an expanded encoding is written in an identification sub-field at the beginning of the data sub-field. 
     The new protocol is completely backwards compatible with the old protocol. The two protocols are compatible because none of the encodings for commands in the old protocol is changed in the new protocol. A device communicating using the old protocol can read a message packet transmitted by a device using the new protocol and is able to recognize all of the encodings for parameters from the old protocol. When a device communicating with the old protocol reads the unused encoding indicating an expanded encoding, the unused encoding is not recognized as a valid parameter identification in the old protocol and the device does not read the rest of the parameter. The use of the unused encoding ensures that a device communicating in the old protocol does not read the expanded identification field in the data field of a parameter. There is no chance of a device using the old protocol will misinterpret a new parameter encoding as an old parameter and data. 
     New parameter encodings are recognized by a device using the new protocol in the following manner. The device reads the unused encoding from the identification field of a parameter. The unused encoding signals that is an expanded encoding has been written into the data field. The device then reads the first n-bits of a data field which contain the new encoding for the new parameter. The addition of the new encodings only increases packet length by n bits per parameter containing a new command. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The above and other features of this invention can be understood by reading the detailed description below in conjunction with these drawings: 
     FIG. 1 illustrates a first and a second exemplary device communicating using a message packet protocol; 
     FIG. 2 illustrates a common processing unit of the exemplary devices; 
     FIG. 3 illustrates a message packet generated using the message packet protocol of this invention; 
     FIG. 4 is a flow diagram of a process executed when a new command parameter is read by a device using an old protocol; and 
     FIG. 5 is a flow diagram of a process executed when a new command is read by a device using the new protocol. 
    
    
     DETAILED DESCRIPTION 
     This invention relates to a protocol used to encode parameters in message packets transmitted between devices. One example of a system that can use the protocol of the present invention is illustrated in FIG. 1. A portion of telephone system  100  is illustrated in FIG. 1 . Telephone system  100  has a first switching system  101  and a second switching system  111 . Switching system  101  provides telephone service to n calling stations  131 - 132  via lines  141 - 142  and switching system  111  provides telephone service to telephone stations  133 - 135  via lines  143 - 145 . Switching systems  101  and  111  may be switching systems that are well known in the art such as 5-ESS switching system produced by Lucent Technologies. Trunk  102  connects switching systems  101  and  111  to complete call circuits between the systems. Switching systems  101  and  111  can also transmit message packets between one another via trunk  102 . 
     Switching system  101  has a controller  103 . Controller  103  is a processing system, as illustrated in FIG. 2, which controls the operations performed by switching system  101 . Controller  103  transmits control signals over path  104  to switching system  101 . Switching system  111  has a controller  113 . Controller  113  is a processing system, as illustrated in FIG. 2, which controls the operations performed by switching system  111 . Controller  113  transmits control signals over path  114  to switching system  111 . Controller  103  and  113  communicate by transmitting message packets over trunk  102  to coordinate operations of switching systems  101  and  111  to provide telephone service. 
     FIG. 2 illustrates a typical processing unit  200  that executes applications stored in a memory to perform the functions of controllers  103  and  113 . Central Processing Unit (CPU)  201  is a processing unit which reads instructions from memory and executes the instructions. CPU  201  reads and writes data to memory via bus  202  and memory bus  203 . Read Only Memory (ROM)  206  is connected memory bus  203  via path  204 . ROM  206  stores instructions needed by CPU  201  to execute the basic processing instructions needed to operate processing system  200 . Random Access Memory (RAM)  207  is connected to memory bus  203  via path  205 . RAM  207  stores data and instructions needed to perform an application. 
     I/O bus  208  is connected to bus  202  to allow CPU  201  to transfer data to I/O devices connected to the bus. Disk drive  212  is an example of an I/O device that is connected to I/O bus  208 . Disk  212  is connected to I/O bus  208  via path  209  and is to store data on a storage media for purposes of communicating with other processing systems to transfer data, processing system  200  has a network interface  213  such as an Ethernet card or a modem. Network interface  213  receives and transmits data from the network via path  215  and is connected to I/O bus  208  via path  210 . Other I/O devices  214  may be connected to I/O bus via path  211 . 
     The present invention is a system for providing a new protocol having expanded parameter encodings that are backwards compatible with older protocols. FIG. 3 illustrates a message packet  300  used to transmit data between switching systems using a call protocol similar to the SS 7  call protocol for providing telephone service. It is understood that the system for expanding parameters of the present invention can be used in message packets of other protocols and in other systems to expand parameter encoding fields. 
     Message packet  300  includes standard overhead data well known in the art including message identification field  301 , data field  302 , message length field  303 , Cellular Processing Module ID (CPMID) field  304 , Executive Cellular Processor Call ID A (ECID A) field  305 , and Executive Cellular Processor Call ID B (ECID B) field  306 . Field  307  is the start of the parameter fields encoded in packet  300 . For exemplary purposes, message  300  contains a parameter  310  having a parameter encoding for a parameter from the older protocol and a parameter  320  having a new parameter encoding from the new protocol. A typical packet  300  has at least one parameter and may contain a multiple parameters in one packet. The number of parameters in a message is left to a designer of the protocol. 
     Parameter  310  exists in the old protocol and has a parameter encoding that is recognized by systems that communicate using either the old or the new protocol. The first data field  311  indicates the length in bytes of parameter  310 . A system will group this amount of bytes with this parameter when the packet is read. Parameter identification field  312  is a byte of data containing an encoding that is unique for the encoded command in the older protocol. Fields  313 - 315  are fields of n bytes of data that are transmitted in the parameter for use in performing the operation represented by the parameter. A switching system operating using either the old or new protocol will recognize parameter  310  and will process parameter  310 . 
     Parameter  320  is from the new protocol and has an expanded parameter identification field. Parameter  320  is not supported by the old protocol and a system using the old protocol will not recognize the parameter encoding. A system communicating using the old protocol will skip new parameter  320  because the encoding is not recognized. In parameter  320 , length field  321  indicates the length of parameter  320  in bytes. In the preferred embodiment, one byte representing the new parameter encoding is added to the data length. It should be recognized by one skilled in the art that any number of bytes can be added to be used for parameter encodings. 
     In command encoding field  322 , an unused parameter encoding from the old protocol is written. The unused encoding indicates to a system operating communicating with the new protocol that a new encoding is contained in parameter  320 . The unused encoding is not recognized by a system communicating using the old protocol and the system skips parameter  320 . In the preferred embodiment, second parameter identification field  323  is contained in the first byte of data. It should be recognized by one skilled in the art that the location of the second parameter encoding field may be placed at any location in the data field and any number of subsequent parameter encoding fields can be added to the data of parameter  320  to increase the number of parameters that can be represented. Fields  324 - 326  contain the n bytes of data needed to execute parameter  320 . 
     The advantage of the encoding scheme in the new protocol used to generate message packet  300  is that the message packets from the new protocol are readable by systems operating using the older protocol. This allows the new protocol to be implemented without having to incorporate the new protocol in every system in a network. Systems using the old protocol simply ignore new parameter encodings as unidentified parameters. FIGS. 4 and 5 describe processes for reading messages that are executed by systems communicating in the old protocol and systems communicating in the new protocol. 
     FIG. 4 illustrates process  400  which is an overview of the steps executed by system that communicates using the old protocol to read packets received from a system communicating in the new protocol. Process  400  begins in step  401  with the system using the old protocol receiving packet  300 . In step  402 , the system reads message packet information from fields  301 - 306  of packet  300 . In step  403 , parameter length is read and a counter is set. In step  404 , the parameter identification field command encoding field of the parameter is read. 
     It is then determined whether the parameter encoding is recognized in step  405 . If the parameter encoding is recognized, the number bytes of data identified in a data length field  311  are read in step  406  and the system proceeds by processing the parameter in step  407 . If the parameter encoding is not recognized, the parameter length in bytes from the data length field  311  or  321  is read and discarded in step  408 . After step  407  or  408 , it is determined if there are more parameters in message packet  300  in step  409 . If there are more parameters, steps  403 - 409  are repeated. Otherwise, process  400  ends. 
     FIG. 5 illustrates process  500  which is an overview of the operation of receiving and reading a message packet performed by a system using the new protocol. Process  500  begins in step  501  with the system using the new protocol receiving packet  300 . In step  502 , the system reads message packet information from fields  301 - 306  of packet  300 . In step  503 , command message length for a command is read and a counter is started which determines the amount of bytes read. In step  504 , the parameter identification field  312  or  322  of the parameter is read. 
     It is then determined whether the read encoding is an encoding indicating a new parameter in step  505 . A new encoding is indicated by a predetermined unused command encoding from the old protocol. If the encoding indicates a new parameter, the second identification field containing the new encoding is read from the expanded command encoding field in step  506 . In step  507 , it is determined whether the read new encoding is valid identification of a parameter. 
     If the new encoding is valid, the number of n data bytes indicated in the data length field  311  or  321  are read in step  508 . The parameter is then processed in step  509 . In step  510 , it is determined if message packet  300  contains another parameter. If message packet  300  does not contain another parameter, process  500  ends. If message packet  300  contains another parameter, process  500  is repeated starting from step  503 . If it is determined that the new parameter encoding is not a valid parameter identification in step  507 , the next n bytes of data are read and ignored in step  511 . The system then proceeds to step  510 . 
     If it is determined that parameter encoding field  312  or  322  contains an encoding from the older protocol, it is determined whether the encoding is a valid parameter identification in step  507 . If the encoding is not valid, the number of data bytes in data length field  311  or  321  is read and ignored in step  511 . Process  500  proceeds to step  510  after step  511 . If the command is valid, steps  508  through  510  are executed. 
     The above description is one exemplary of a command parameter of a new protocol that is compatible with command parameters of an old protocol. It is expected one skilled in the art can and will design alternative compatible command parameters that infringe on the invention as set forth in the claims below either literally or through the Doctrine of Equivalents.