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Industrial Ethernet Book Issue 75 / 35
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Redundant Ethernet and WLAN network for oil refinery

Using a Parallel Redundancy Protocol (PRP) that requires fewer units and delivers zero fail time, an oil refinery created an effective networking infrastructure using PRP over WLAN and Industrial Ethernet. The network featured improved security and a lower risk of both data theft and possible network failures. Remote access and mobile monitoring capabilities are delivered by implementing SNMP and PROFINET I/O diagnostics.

Dexter System Solutions is an operations and solutions integrator for the industrial process industry. The company has a global reputation by producing mission critical networking solutions for the oil and gas industry, within a wide array of hazardous environments according to the latest safety requirements such as ATEX, IEC-Ex and NEN.

To meet the needs of one of the world's largest petrochemical companies, Dexter Solution Systems and Belden partnered together to provide a redundant industrial Ethernet and WLAN network according to IEC 61850 specifications for an oil refinery location. This system required explosion proof certifications (UL and ATEX) and shipboard approvals (ABS and DNV) and a high level of industrial security.

Advanced Switching Technology

Using Hirschmann RSR(S) and OpenBAT-F technologies combined with ruggedized industrial backbone switches, Belden and Dexter were able to deploy a comprehensive and redundant WLAN and Ethernet Parallel Redundancy Protocol solution that can withstand the environment and strict standardization requirements commonplace within the oil industry.

Use of PRP is relatively new in Ethernet via the RSP, and this installation also needed to be created in accordance with series DIN EN/IEC 62439. But PRP executed over WLAN via the RSPS is a new development. Both networks are managed centrally using Hirschmann MACH1040 switches. In addition, both networks are connected centrally to the plant control room and managed by a MACH4000 switch. Industrial security is provided by the EAGLE30 which segments each independent network into subnets. According to Belden, for the first time, networks are able to guarantee uninterrupted data communication over WLAN and Ethernet.

Refinery network using Parallel Redundancy Protocol

System Requirements

Because this application transmits video, the project required stable and reliable wireless links demanding high bandwidth and information technology equipment that conforms to the EN 60950 safety standard. Additional requirements included a transmission rate is 450 Mbps using an IEEE 802.11n for WLAN network, operating temperatures between -40 and 70oC, plus high availability Ethernet in accordance with IEC 62439. The system implemented a multimode fiber optic network and certification for Atex Zone 2 hazardous environment was also required.

Belden and Dexter System Solutions were able to meet the customer's wireless and Ethernet network demands with a minimalist solution that surpassed the competition in redundancy features, technical capability and overall cost-effectiveness. The Belden solution also provided the customer with access to network design and integration support.

Belden has a broad portfolio of Hirschmann Ethernet switches and wireless modules that meet the strict EX and shipboard regulatory standards commonplace within the process automation sector.

Advanced network architecture enables an oil refinery to implement a redundant Ethernet and wireless LAN solution.

Parallel Redundancy Protocol

Although a fast MRP ring is now able to cover a very large number of requirements, there are applications that cannot tolerate any switchover time at all. To fulfill such requirements, the application needs to take an entirely new approach to the question of guaranteed high availability.

The basis of this new approach to network redundancy is to have two independent active paths between two devices. The sender uses two independent network interfaces that transmit the same data simultaneously. The redundancy monitoring protocol then makes sure that the recipient uses only the first data packet and discards the second. If only one packet is received, the recipient knows that a failure has occurred on the other path.

Using the principle employed by the parallel redundancy protocol (PRP), as described in the IEC 62439-3 standard, PRP technology uses two independent networks with any topology and is not limited to ring networks. The two independent parallel networks may be MRP rings, RSTP networks and even networks without any redundancy at all. The principal advantage of PRP is its interruption-free switchovers, which take no time at all to switch over in failure situations and thus offer the highest possible availability. Naturally this applies only provided both networks do not fail simultaneously.

PRP is implemented in the end devices, while the switches in the networks are standard switches with no knowledge of PRP. An end-device with PRP functionality is called a double attached node for PRP (DAN P) and has a connection to each of the two independent networks. These two networks may have an identical structure or may differ in their topology and/or performance. A standard device with a single network interface (single attached node, SAN) can be connected directly to one of the two networks. Naturally, in this case, the device will have no redundant path available in the event of a failure. A SAN can alternatively be connected to a redundancy box (RedBox) that connects one or more SANs to both networks. SANs do not need to know anything about PRP, they can be standard devices. In many applications only critical equipment will need a dual network interface and less vital devices can be connected as SANs, with or without a redundancy box.

A DAN P implementation controls the redundancy and deals with duplicates. When the upper layers receive a packet for transmission, the PRP unit sends this frame to the network via both ports simultaneously. When these two frames traverse the two independent networks they will normally be subject to different delays on their way to the recipient. At their destination the PRP unit passes the first packet to arrive to the upper layers, i.e. to the application, and discards the second one. The interface to the application is identical to any other Ethernet network interface.

The redundancy box implements the PRP protocol for all the attached SANs and operates as a kind of redundancy proxy for all types of standard equipment. Duplicates are recognized by means of the redundancy control trailers (RCT) introduced into each frame by a PRP connection or RedBox. In addition to a network identifier (LAN A or B) and the length of the user data contained in the frame, these 32-bit identification fields also contain a sequence number that is incremented for each frame sent by a node. A RedBox or DAN P connection can recognize duplicates, and if necessary discard them, on the basis of the clearly identifiable features contained in each frame (physical MAC source address and sequence number).

Since the RCT is inserted at the end of the frame, all the protocol traffic can still be read by SANs, which interpret the RCT merely as additional padding with no significance. This means that a SAN that is connected to a PRP network directly, i.e. without a RedBox, is able to communicate with all the DAN Ps and with any SANs in the same network (either A or B). It lacks only connections to the nodes of the other network, because a DAN P does not pass any frames from one LAN to the other one. PRP switchover times fulfill the very highest demands, and it is also extremely flexible as regards network structure and possible topologies, but it does always need twice the installed infrastructure of switches and other network components.

Application article by Belden and Dexter System Solutions.

Source: Industrial Ethernet Book Issue 75 / 35
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