Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

174 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
Fixed
Length
Frame
Start 0x10
C
A
A
Checksum
End 0x16
Sing le
Control
Character
0xE5
Figure 7.3
Fixed length frame and single control character
7.5 Addressing
Under IEC 60870-5-101 addressing is both at the link and at the application level. The
link address field may be 1 or 2 octets for unbalanced, and 0, 1 or 2 octets for balanced
communications. As balanced communications are point-to-point the link address is
redundant, but may be included for security. The link address FF or FFFF is defined as
a broadcast address, and may be used to address all stations at the link level.
At the application level, the ASDU contains a 1 or 2 octet common address. This is
defined as the address of the controlling station in the ‘control direction’, and the address
of the controlled station in the ‘monitoring direction’. The common address of the ASDU
combined with the information object address contained within the data itself combine
to make the unique address for each data element.
As in DNP, there may be more than one logical or common address per device. As for
the link level, the address FF or FFFF is defined as a broadcast address. Therefore to send
a broadcast message it is necessary to include this address in both the data link and
application address fields.
Optionally on a per-system basis, originator addresses can also be carried within the
ASDU. This is not shown in Figure 7.2, but is an optional part of the cause of transmission
field.
The information object address is 1 to 3 octets in length, and can be provided either just
once within an ASDU, or for each separate information object within an ASDU. This
allows for efficient transmission of blocks of sequential information.
7.6 Networked version
Under IEC 60870-5 there are two different methods of transporting messages. These are
in effect two different, but closely related protocols. The first is IEC 60870-5-101, or
T101, which provides for bit-serial communications over low-bandwidth communications
channels. This method uses the data link frame shown in Figure 7.2 together with defined
procedures to transport the data over the communications network.
The second method was defined much more recently with the release of the IEC 60870-
5-104, or T104 profile. In this protocol the lower levels of the protocol have been
completely replaced by the TCP and IP transport and network protocols. These protocols
provide for transport of the application service data units (ASDUs) shown in Figure 7.2