SCA Transport Protocol Driver

Transcription

1 SCA Transport Protocol Driver No: Introduction This report provides a detailed, technical description of the wireless modem s transport protocol driver developed for Mobile Data Computer Systems (MDCS). This module supports the Software Corporation of America (SCA) server/client transport protocol or can be adapted into any data transport protocol requiring high level, end-to-end data integrity. The purpose of this data transport method is to guarantee that the data sent between a client computer, usually a laptop in a public safety vehicle, and the server computer is checked between all components in the system. The series of radio modem uses a 32-bit Cyclic Redundancy Check (CRC) error detection on all radio packet that grantees the data accuracy on the radio link but, it was found that the integrity of the data could be compromised between the laptop and the wireless modem in the vehicle by electrical noise, power quality issues or other data collisions on the serial port of the computer. If the data packet structure was intact but the internal data corrupt, this could then be passed over the radio link to the server which could have trouble interpreting this data. To eliminate this as a potential problem, all data packets are checked between the laptop and the before transport and checked again between the and the server. Software Commands A new software command (SCA ON/OFF) has been added to the operating system in version Listed below are the discrete commands that are enabled in the when this software command is enabled: SCA = ON Enables the Transport Protocol and Programs the following commands. SWITCH 2 or 8 = UP Enables autoconnect (depends upon modem case style AUTOLF = OFF Disables auto line feeds ECHO = OFF Disables echo EDIT = OFF Disables edit functions from converse SENDPAC = 255 Disables sendpacket COMMAND = 0 Disables command mode return TERMC = ON Enables receive termination timer MESS = ON Enables formatted status messages Structure The following diagrams outline the required packet format for the MDCS Transfer Protocol: The sending packet is originated from either the or Server computer. Start Repeater Dest. Start Data End TX Error Header Route(s) Address Length Data Check SOH 3-Bytes 1-Byte 2-Bytes STX Bytes EOT 1-Byte 415 N. QUAY STREET KENNEWICK, WA Page 1 of 6

2 Description SOH (SOH) ASCII Start of Header (0x01) Repeater The client/server software driver controls all repeater routing. The repeater route fields are in Route bytes 2 through 4. If only 1 repeater is used, the first address in the route field will have the address of the repeater and the remaining 2 fields must be 0. The repeater routing fields will be loaded in successive bytes 2-4 with no repeater designated by inserting a 0 in that field. Destination Single byte address of remote. Valid addresses are (Decimal). Address Two byte field of total packet size including the BCC error check. Length Start Data (STX) ASCII Start Text (0x02) Data The modem can accept a data field from Bytes in length. Through testing, a maximum packet size of 512 Bytes provided best results. End of TX Error Check (EOT) ASCII End of Text (0x04) Single Byte Block Check Code (BCC) to validate packet information. Calculations for BCC listed below. The receiving packet is originated in the remote and sent to either the or Server computer. Start Repeater Source Start Data Data End TX Error Check Header Route(s) Address Length SOH 3-Bytes 1-Byte 2-Bytes STX Bytes EOT 1-Byte SOH Repeater Route Source Address Length Start Data Data End of TX Error Check Description (SOH) ASCII Start of Header (0x01) The repeater route fields are in bytes 2 through 4. For example, if only 1 repeater is used, the first address in the route field will be the address of the repeater and the remaining 2 fields will be 0. The addressing in the route order will not be modified from the send packet. Single byte address of that sent the data. Valid addresses are (Decimal). Two byte field of total packet size including the BCC error check. (STX) ASCII Start Text (0x02) The modem can accept a data field from Bytes in length. Through testing, a maximum packet size of 512 Bytes provided best results. (EOT) ASCII End of Text (0x04) Single Byte Block Check Code (BCC) to validate packet information. Calculations for BCC listed below. Reply s The reply packet will be sent from either the computer or to acknowledge a correct data packet. Acknowledge Not-Acknowledge ACK NAK (1-Byte) (1-Byte) Reply Description Acknowledge (ACK) ASCII Acknowledge (0x06) Not-Acknowledge (NAK) ASCII Not- Acknowledge (0x15) 415 N. QUAY STREET KENNEWICK, WA Page 2 of 6

3 Data Flow When the receives a packet from the server or client computer () it checks that the data was received correctly by calculating the checksum and comparing it to the BCC error check byte at the end of the packet (Figure 1). If the BCC does not match, the sends a NAK to the computer and discards the packet (Figure 2). If the checksum matches, the sends an ACK (Reply ) to the computer and forwards the data to the RF transmitter. 1st Successive s SS01-99 * (0x2A) * (0x2A) (ADD 10) RF Link (ADD 99) Server Time Figure 1: Transport Protocol Diagram Bad Data Re-send Bad Data Re-send (3X Max) NAK (0x15) * (0x2A) (ADD 10) 10ms 10ms (ADD 99) NAK (0x15) Server Time Figure 2: Error Handling Diagram The then sends the packet to the remote over the RF link using the route and destination address fields in the packet header. If the RF packet is the initial radio communication between the modems, the will return a connection status message (SS01- Destination Address ) to the computer. The remote will send a radio acknowledge to the transmitting that the packet was received correctly. After the receives the acknowledge from the remote it will send back an asterisk (* - 0x2A) to the originating computer indicating the data was received by the remote. If the sending does not receive an acknowledgement from the remote the packet is retransmitted the number of time indicated by the Retry command (6 Retries by default). If the packet fails to be passed over the RF link, the will send back a status message indicating the link is down (SS03) and the data discarded (Figure 3). An asterisk follows the status message. 1st RF Link Time RF Link Retry (6 Times) Failed to Connect Message SS05 If RF Link Already Established RF Link Retry (6 Times) Link Down Message SS03 (ADD 10) (ADD 99) Server Figure 3: RF Error Message Diagram 415 N. QUAY STREET KENNEWICK, WA Page 3 of 6

4 At the remote, the data is sent to the server/client computer ( Using a predetermined time delay (termt time), the will wait for an ACK or NAK and re-send the data if NAKed or if nothing is received (Figure 2). The will re-send the serial packet up to 3 times and then discard the data packet. Special Software Commands To return the to the Command mode and change software parameters, send the a data packet with 00 as destination. The will return to the command mode and will to be sent a Reset command to return to converse mode. Example Code for Calculating BCC The BCC will encompass all bytes including control characters. /*========================================= Block Check Code (BCC) Return BCC byte ========================================*/ BYTE Block_Check_Code( BYTE *block, WORD len ) { WORD i; WORD bcc = ZERO; For (i = ZERO; i < len; i++) { bcc^= string [i] ; /*XOR byte*/ bcc << = one ; /*rotate left by 1*/ bcc= bcc+((bcc>>8) & 0X01; /*Wrap MSB into LSB*/ } return (byte) bcc; } 415 N. QUAY STREET KENNEWICK, WA Page 4 of 6

5 Example Data Flow Diagrams The following diagrams list examples of the data flow from the client/server computer though the modems: Computer (Address 10) Server Computer RF Link (Address 99) Figure 4: Example Link Diagram Layout without repeaters Figure 4 (All Numbers in Hex) XX (Calculated) SOH Repeaters DA Length STX Data (Example) EOT BCC Layout without repeaters Figure 4 (All Numbers in Hex) A XX (Calculated) SOH Repeaters SA Length STX Data (Example) EOT BCC 415 N. QUAY STREET KENNEWICK, WA Page 5 of 6

6 Example Data Flow Diagrams (Cont.) Address 10 Address 101 Repeater Node 2 Mobile Node () Address 99 Address 100 Repeater Node 1 RF Server Computer Base System (Server) Figure 5: Example Repeater Diagram Layout Using Repeaters 100 and 101 Figure 5 (All Numbers in Hex) XX (Calculated) SOH Repeaters DA Length STX Data (Example) EOT BCC Layout Using Repeaters 100 and Figure 5 (All Numbers in Hex) A XX (Calculated) SOH Repeaters SA Length STX Data (Example) EOT BCC 415 N. QUAY STREET KENNEWICK, WA Page 6 of 6