BS EN 62439-7:2012 pdf download – Industrial communication networks — High availability automation networks Part 7: Ring-based Redundancy Protocol (RRP)
4 RRP overview
4.1 General The RRP specifies a recovery protocol, based on a ring topology. All links in an RRP network shall be full duplex through the use of an internal hardware Ethernet switch. Thus, RRP provides a collision-free transmission mechanism between two nodes. Every RRP device detects link failure and link establishment using the rules specified in ISO/IEC 8802-3:2000 and shares this information with other RRP devices so that fast connectivity recovery time is also guaranteed in the ring network. A RRP device is a dual-port switching device that receives and transmits standard ISO/IEC 8802-3:2000 Ethernet frames. It is intelligent and can control directional frame forwarding between its dual ports according to the network status and device status. RRP uses a special network management scheme specified in this standard. RRP also uses a network control based on device address and MAC address, and thus general bridge hub or switch might not be suitable for RRP network. However, when connecting a general Ethernet device to RRP network, Gateway Device (GWD) should be used.
4.2 Frame forwarding and receiving control
4.2.1 General RRP provides a collision-free transmission mechanism with an internal full-duplex hardware switch with switching queue and dual MACs in a device. The switching priority method between Tx and Forwarding can be Round-Robin, Tx-First or Forwarding-First scheme. However RRP does not specify the switching method. Thus, a RRP device transmits frames without the restriction of medium access, as soon as they appear in the transmit queue for each MAC. Figure 1 shows the forwarding and receiving control of the RRP device.
4.2.2 Normal Device (ND) and Gateway Device (GWD) RRP is operated in a dual-port ring topology. A general Ethernet device can send standard Ethernet frames through RRP ring network with GWD. Multi-ring network can also be established using GWD. GWD is responsible for switching Ethernet frames between RRP network and external Ethernet networks through application layer using a dynamic table. The dynamic table maps addresses to external Ethernet ports automatically. The dynamic table is automatically made by learning frame movements in the network. The GWD inspects both the destination and the source addresses. The destination address is used for the forwarding decision; the source address is used for adding entries to the table and for updating purposes. When an Ethernet frame is received at the media access control (MAC) layer through the physical interface transceiver (PHY), a GWD handles the received frame by taking one of the following actions, depending on the destination MAC address and the source MAC addresses in the received frame:
• for a broadcast or multicast frame, accept and deliver the frame to the data link layer entity (DLE), and forward the frame to the other RRP port and external Ethernet ports;
• for a frame designated for the device itself, accept and deliver the frame to the DLE without forwarding;
• for a frame designated for another device, accept the frame to its application layer and inspect both the destination and the source addresses. When the destination address of the frame is in the dynamic table, the GWD delivers the frame to the corresponding port in the dynamic table without forwarding to other ports. Otherwise, the GWD delivers the frame to all other ports. The GWD adds this entry to the dynamic table with source MAC address and port number information. NOTE Dynamic table entries are automatically removed after the Ageing Time which is specified in IEEE 802.1 D. Figure 2 shows different structures of ND and GWD. In GWD, external Ethernet connection is connected to RRP ring network through MAC_E and PHY_E.