The Time-Triggered Protocols
Kluwer Academic Publishers eBooks · 2006
The Time-Triggered Protocols (TTP) form a new protocol class that has been designed at the Technische Universität Wien to accommodate the specific requirements of fault-tolerant distributed real-time systems. The chapter starts with a statement of the protocol objectives, and explains the rationale that governed the protocol design. There are two different variants of TTP, TTP/C for the implementation of a fault-tolerant intra-cluster communication system, and the lowcost TTP/A version for the implementation of a field bus. Section 8.2 describes the layers of TTP. Apart from the physical and data link layer, the TTP layers are different from those of the OSI model. The smallest replaceable unit (SRU) layer provides the consistent SRU membership service. The redundancy management layer is responsible for the startup and reconfiguration of a TTP system. The fault-tolerant unit (FTU) layer groups nodes into FTUs, and provides an FTU membership. The most important interface of a TTP system is the node-internal communication-network interface (CNI) that acts as a temporal firewall between the host computer and the communication network. The structure of the CNI is explained in Section 8.3. Section 8.4 outlines the internal logic of TTPK. The membership service of TTP/C is explained, the novel method for CRC calculation that guarantees the consistency between the protocol state of the sender and the receiver is presented, and the message formats are depicted. Section 8.5 is devoted to the time-triggered field bus protocol TTP/A. TTP/A is intended for the interconnection of intelligent sensors and actuators to an interface node. The implementation of this low-cost protocol requires only a standard UART channel and a local timer that can be found on almost all single chip microcontrollers.