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NOV-DEC 2018

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INTECH NOVEMBER/DECEMBER 2018 37 SPECIAL SECTION From the beginning, switch cascading posed a challenge in industrial environments. Apart from the star topology used in the information technol - ogy (IT) field, line, ring, and tree topologies are fre- quently used in automation. These adapted topolo- gies significantly reduce Ethernet installation wiring requirements and costs. Therefore, in industry, two- port switches employing a cut-through strategy, where datagrams are forwarded before being com - pletely received, are integrated into field devices. One size fits it all Because standard Ethernet did not have suf- ficient bandwidth reservation capabilities, au- tomation experts began developing their own Ethernet extensions in 2000. However, paths diverged during development. There is differen- tiation between the following approaches: l Protocols using Ethernet as a transport me- dium for a fieldbus: These protocols claim complete control over the Ethernet medium for themselves. Classic TCP/IP communica- tions are only possible in piggyback style via the fieldbus (EtherCAT and POWERLINK) or through a channel assigned by the fieldbus (Sercos). Bandwidth control is firmly in the hands of the fieldbus. l Protocols that guarantee bandwidth reser- vation through a time-slicing procedure on the Ethernet: PROFINET IRT should be men- tioned here. IRT enables hard determinis- tic, real-time data transmission on the same cable on which soft real-time or background traffic is operated. A precise timing model for the transmission paths is necessary for plan - ning the time slices. l Protocols based on sharing of the Ethernet cable: These protocols use QoS and are at home in factory and process automation ap plications. PROFINET RT and EtherNet/ IP are noteworthy examples. These protocols are limited to the range of soft real time (cycle time greater than or equal to 1 ms). For these standards, special hardware support and, thus, special ASICs are needed. Because PROFINET RT and EtherNet/IP are also based on the embedded two-port switch with cut through, they are not exempt here. Flexible, hardware- based multiprotocol solutions solve the problem in an elegant manner. Enter TSN Breaking free of past limitations, TSN extensions for standard Ethernet in accordance with IEEE 802.1 have successfully been developed. Thus, there is now a standardized layer 2 in the ISO seven- FAST FORWARD l Time-Sensitive Networking in accordance with IEEE 802.1 overcomes the limitations of today's Ethernet and delivers deterministic and highly scalable real-time data transmission. l TSN creates a single network to meet all requirements, including audio, vision, and data with guaranteed speed and determinism for all applications. l TSN is network-protocol agnostic and can be used simultaneously to transport industrial automation protocol traffic, IT data, OPC UA, video, and all other network traffic. Timing model: PHYs, cables, and switches contribute to delays in data transmission. This must be considered with the time-slot method (PROFINET IRT and TSN time aware shaper [TAS]). 8 Bytes 6 Bytes 6 Bytes 4 Bytes 2 Bytes 4 Bytes Minimum 12 Bytes PRE DST SRC TPID PCP DEI DATA Gap Next frame Gap CRC32 Ethertype VLAN ID Drop Eligible point (1 bit) Priority VLAN ethertype Source mac Destination mac Start delimiter (1 bit) Preamble CRC32 VID ETYPE S 46 Bytes to 1500 Bytes Ethernet frame: Data fields relevant to TSN data flow identification are shown in green. Line Ring Kamm Topologies Rx Rx Tx Tx Rx Tx Rx Tx

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