IEC 60870-5-104
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IEC 60870-5-104 (IEC 104) protocol is an extension of IEC 101 protocol with the changes in transport, network, link & physical layer services to suit the complete network access. The standard uses an open TCP/IP interface to network to have connectivity to the LAN (Local Area Network)and routers with different facility (ISDN, X.25, Frame relay etc) can be used to connect to the WAN (Wide Area Network). Application layer of IEC 104 is preserved same as that of IEC 101 with some of the data types and facilities not used. There are two separate link layers defined in the standard, which is suitable for data transfer over Ethernet & serial line (PPP - Point-to-Point Protocol).
[edit] Additional Synchronization Mechanisms
Control field data of IEC104 contains various types of formats /mechanisms for effective handling of network data synchronization
1. I Format – It is used to perform numbered information transfer. It contains send-sequence number and receive-sequence number. The transmitter station increases send-sequence number when it sends any data and receiver increases receive-sequence number when it receives any data. The sending station has to hold the send APDUs in the buffer until it receives back the send sequence numbers as the receive sequence number from destination station.
2. S Format – It is used to perform numbered supervisory functions. In any cases where the data transfer is only in a single direction, S-format APDUs has to be send in other direction before timeout (t2), buffer overflow or when it has crossed maximum number of allowed I format APDUs without acknowledgement (w).
3. U Format – It is used to perform unnumbered control functions. This is used for activation and confirmation mechanisms of STARTDT (start data transfer) & STOPDT (stop data transfer) & TESTFR (test APDU).
4. Test Procedure – Open but unused connections must be tested periodically (when it has crossed ‘t3’ after the last message) by sending TESTFR frames, which need to be acknowledged, by the destination station. The connection needs to be closed when there is no reply for the test message after timeout (t1) or when there are more numbers of I-format APDUs than the specified ‘k’.
