wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

WorldFip 34-3
Data producer
Data consumer
Data consumer
Applicat.
layer
Data link
layer
Physical
layer
Logical flow
Physical flow
FIGURE 34.2
The basic producer–consumer data transfer.
LLC sublayer exploits the communication channel made available by MAC and provides two main
kinds of services:
• Services to read/write/transfer process data that are contained into suitable reception/transmission
buffers
• Services to request the confirmed/unconfirmed transmission of message files
Figure 34.2 shows the basic mechanism adopted in WorldFip for data transfer, which is performed
in several steps:
• The user (producer) passes to the AL the information to be transferred to remote consumers.
• The AL entity stores the information inside a buffer that can be read by the DLL entity.
• The DLL entity through the physical medium, transfers such information into the remote buffers
of the various consumers.
• The information is read by the remote ALs and delivered to the consumer processes.
• This sequence is repeated cyclically and is well suited for periodic data transfer.
34.3.1 transmission of Cyclic Traffic
As stated earlier, the task of the DLL is to read a buffer and to transfer its data through the bus. Since
the bus is a shared medium, in order to avoid collisions, it is necessary to organize suitably the various
transmissions. This is the task of the BA, a station that carries out a key role in the system by providing
the authorization for each buffer transfer.
The mechanism adopted for the transmission of cyclic traffic is shown in Figure 34.3 and is executed
through the following steps:
• The BA broadcasts, on the bus, an identifier (ID_DAT) that implicitly specifies all stations
involved in the current transaction. One station will be the producer of the information, whereas
the other ones will be the consumers.
• The identifier is received by both the producer and consumers.
• The producer sends its message (RP_DAT) on the bus.
• The consumers that are involved into the transaction accept the data.
This sequence illustrates clearly the role of BA which, through a single message, identifies the role of all
stations participating to the next phases. All devices receive the identifier but only one will identify itself as
a producer, whereas all the others will be consumers. The architecture of the BA is shown in Figure 34.4. As
we can see, the BA is more than simply a scheduler as it must also cope with asynchronous requests from
nonscheduled traffic. For this reason, the BA must monitor the bus and check for some additional information
that the producer will incorporate in its frame with reference to some additional bandwidth request.
© 2011 by Taylor and Francis Group, LLC