For serial-only systems, redundancy generally involves keeping spare devices on hand in case of device failure, and ensuring that a reliable backup power supply is available in case of power outages. There rarely is a need for backing up the serial line, since serial connections tend to be more localized compared to the more modern TCP/IP networks. The localized serial connection is seen as a safe and reliable data communication method in most of the industrial sectors for a long time. However, the popularity of serial-to-Ethernet technology has gained its momentum since the release of the first serial device servers in the late 1990's.
There are two major benefits for using Ethernet to transmit data signals between serial devices and host computers.
1. Limited transmission distances of serial-only connections can be extended to essentially any distance with Ethernet..
And then there is the second and major benefit:
2. Centralization; it is possible to route serial signals from an entire factory to one central location where the data can be analyzed, modified, and then acted upon. In other words, it means remote control, cost cutting, new development, fast response time for the company that using Serial-to-Ethernet data connections.
With all this great power of 'Ethernet', many industrial automation engineers or industrial field communication specialist have soon found out the Ethernet networks recovery is much different and much more sophisticated than the Serial networks. A typical Ethernet LAN is designed to support communication between many different network hosts, and as such the topology of such networks can be quite complicated. Furthermore, Ethernet networks use a wide array of hubs, switches and other devices for transmitting data throughout the network. With the industry demand for reliability and safety, since the failure of all or even part of an industrial communications network could cause huge financial losses for companies, setting up reliable redundancy for Ethernet networks is absolutely essential.
Approaches to Serial-to-Ethernet Redundancy
Ethernet networks consist of a collection of links between hosts and switches, and switches and switches, combined to form a tree topology. Ethernet networks could consist of literally hundreds if not thousands of point-to-point wired connections. Since the route taken when sending data from one network host to another is determined by the network and not the sender, a failure of any link in the network could bring the entire operation to a grinding halt. Saying that an Ethernet network is redundant can mean one of several things. In all likelihood, it means that a small portion of the network is kept idle until another part of the network fails, at which point the “redundant” portion is activated to maintain the flow of data. Another possibility is that a large portion of the network, or even the entire network itself, is duplicated to provide redundancy.
Single-network redundancy uses a physical link in the network as the backup path. The redundancy software identifies a particular link in the network that if active could cause data packets to return to the sender before reaching their destination. To avoid broadcast storms, data is blocked from being transmitted across the redundant link during normal operation, and the link is only activated if one of the live links in the network fails.
Two types of single-network redundancy are in common use.
Spanning Tree Protocol (STP) and Rapid Spanning Tree Protocol (RSTP), referred to collectively as STP/RSTP, can be used on very general, mesh-type networks. The other type of redundancy is restricted to networks arranged in a ring topology. Note that for industrial networks, the major determiner of which redundant protocol to use is the “recovery speed.”
Dual-network and Multi-host Redundancies
Dual-network redundancy involves creating a complete backup to the Ethernet network. In general, with dual-network redundancy, the servers and hosts used on the network are not duplicated. With multi-host redundancy, one or more duplicates of the host computers on the network are set up to provide backups in case the primary host crashes. A more complete type of redundancy involves setting up backups for the network and the hosts.
Now, with the above knowledge, you should be well equiped to come out from the stone age of Serial communication and embrace the new Ethernet Technology. So, enjoy all the benefits that Ethernet have to offer without compromising the safety and reliability that importantly need in the mission-critical industrial data communication.