Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

8.4 Data link layer
Fundamentals of IEC 60870-5 187
The data link layer is responsible for the passing of data across the communications
channel, and ensuring that the data is received in full and uncorrupted by errors. It does
this using a unit of data known as a frame, combined with procedures to govern its transmission
and reception. The frame is made up of an amount of data that is large enough to
carry control information such as a destination address, checking information used to
detect errors, and a payload of data, if required. It is also an amount of data that is not
too large, so that a transmission error will not cause the loss of too much data, or so that
timing discrepancies between transmitter and receiver can lead to loss of synchronism.
IEC 60870-5-101, or T101, specifies the operation of the data link layer by referring
to and making selections from the standards identified in Table 8.1, repeated in the
following extract:
Link Layer IEC 60870-5-2 Transmission procedures
IEC 60870-5-1
Frame formats
In this section the operation of the data link layer is explained in detail, commencing
with the data frame structure and then looking at the transmission procedures.
8.4.1 Frame format
The frame format used by T101 is referred to as the FT1.2 format. There are two forms of
this, one of fixed length and the other of variable length. The fixed length frame is
restricted to use for frames carrying no user data, and therefore is used only for data link
control command and acknowledgment frames. In addition to the fixed and variable length
frames, there is a ‘single control character’ frame which consists of a single byte. This may
be used for acknowledgment only.
These frames are shown in Figures 8.5 and 8.6. Figure 8.5 shows the actual bit pattern
that would be seen on the physical channel, interpreted from left to right. This representation
includes the start and stop bits that are transmitted with every byte or octet of the
frame. The overall frame construction is shown in Figure 8.6. This does not represent the
bit pattern, but shows only the information content down to the octet level. For consistency
with the applicable standards, these are presented vertically in octet order. Thus, the first
octet is shown at the top, and following octets are shown below. Clarification of the order
of bits and octets is included in the following section.