Motorola V186 User Manual - Page 98

Communications RTU to simultaneously run several kinds of communication applications, such as reporting alarms by contention, on-line monitoring, performing diagnostics checks, etc. The MDLC protocol is discussed below. MDLC Protocol The MDLC protocol is a Motorola SCADA protocol that is based on the Open System Interconnection (OSI) model recommended by the International Organization for Standardization. MDLC utilizes all seven layers of the OSI model. This protocol is designed for optimum operation in SCADA systems which operate with diverse communication media such as two-way radio, line, LAN, etc. Each RTU, FEP, or ToolBox has all seven layers of the MDLC protocol available to them. The functions of the seven layers are summarized below. Layer Layer 1: Physical Layer 2: Link Layer 3. Network Layer 4. Transport Layer 5. Session Layer 6. Presentation Layer 7. Application Function This layer caters to communications over conventional radio, trunked radio, data radio, serial data channels, modems, Ethernet or telephone lines. The layer is also responsible for channel access and collision control on shared media. This layer ensures proper communications over a physical link. The layer arranges the data in variable-length frames and attaches addresses, frame sequence numbers, and Cyclic Redundancy Code (CRC) to the frames. This layer is responsible for the establishment of end-to-end communication paths in a network. This is necessary since communications may take place on more than one link and a message may travel through several nodes before reaching the final destination. This layer ensures end-to-end integrity of the information flow between two nodes in the network. This is achieved by remote-end acknowledgement that data has been received completely and passed in the correct order to the next layer. This layer allows the definition of any number of entities capable of conducting simultaneous sessions with an equivalent entity in some remote unit. This enables transparent communications among multiprocessing machines without interference in their applications. This layer structures the information to/from various applications. This layer may also perform format conversion, data authentication, etc. if implemented. This layer interfaces to the various applications such as data transfer, configuration downloading, application software monitoring, remote diagnostics, etc. The MDLC protocol is intended for operation in point-to-multipoint links, such as twoway radio or multidrop wireline, as well as in point-to-point communication networks. The protocol facilitates communications among all sites in the system, including extensive diagnostic messaging. MDLC is transparent and liberates the system engineer 94