Practical Industrial Data Networks:Design, Installation and ...

Industrial Ethernet overview 2271 byte1 byte1 or 2 bytesData or Pad to create minimumof 46 bytes, maximum of 1500DestinationService AccessPointDestinationService AccessPointControlUser Data or PadPreambleSFDDestinationAddressSourceAddressLengthorTypeLLC/DataFrameCheckSequenceFigure 15.7LLC PDU carried in an Ethernet frame15.2.8 Reducing collisionsThe main contributors to collisions on an Ethernet network are:• The number of packets per second• The signal propagation delay between transmitting nodes• The number of stations initiating packets• The bandwidth utilizationA few suggestions on reducing collisions in an Ethernet network are:• Keep all cables as short as possible.• Keep all high activity sources and their destinations as close as possible.Possibly, isolate these nodes from the main network backbone withbridges/routers to reduce backbone traffic.• Separate segments with bridges and switches rather than with repeaters,which do not truncate the collision domain.• Check for unnecessary broadcast packets.• Remember that the monitoring equipment to check out network traffic cancontribute to the traffic (and the collision rate).15.2.9 Design rulesThe following design rules on length of cable segment, node placement and hardwareusage should be strictly observed.Length of the cable segmentsIt is important to maintain the overall Ethernet requirements as far as length of the cableis concerned. Each segment has a particular maximum length allowable. For example,10Base2 allows 185 m maximum length. The recommended maximum length is 80% ofthis figure. Some manufacturers advise that you can disregard this limit with theirequipment. This can be a risky strategy and should be carefully considered.