Different types of switches FRNT and Spanning Tree - … · Different types of switches ... RSTP is...
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128 Theoretical and general applications www.westermo.com Different types of switches Depending on the application and installation requirements there are a number of dif- ferent switches. First we differentiate between the interfaces, where there are TX (copper) and FX (fibre). Other variants are unmanaged/managed switches, this means you either have or do not have the possibility of communicating with and monitoring the switch using SNMP . Finally we differentiate between ring and time synchronised switches which are used when you intend to build a ring network with redundancy or a network where demands on time synchronisation are made. FRNT and Spanning Tree Complex networks with requirements on redundancy must be possible to reconfigure should a network error occur. Reconfiguration is handled by the switch, that is to say, the switch must identify that a link error has occurred. This can be done in different ways, of which standardised solutions are, IEEE Spanning Tree Protocol (STP) or Rapid Spanning Tree Protocol (RSTP). The Spanning Tree Protocol creates a connection through the network at the same time as it eliminates unwanted loops in the network. Redundancy is created by the protocol keeping the tree structure in the network in order, where some connec- tions are blocked (set in standby mode). When a segment can not be reached, the network is reconfigured using the Spanning Tree algorithm, which results in connec- tions set to standby becoming active. Reconfiguration of a STP network can take up to 30 seconds, as new conditions must be calculated and switches updated. This calcula- tion is complex as the network does not have a determined topology. RSTP is a deve- lopment of STP with faster reconfiguration, from the earlier 30 seconds a network can be reconfigured in 5 seconds. There are also specially developed solutions available, for example, Fast Recovery Network Topology (FRNT), which is used in our ring switch R200 and time synchro- nised switch T200. FRNT is a patented solution that reconfigures the network extremely quickly, <30 ms. This is achieved through the switches knowing the network configuration, which is also a ring topology. In addition, reconfiguration is event con- trolled, ”idle traffic” is sent between each device in the ring to check whether the link is up. When an error is detected, information is sent immediately to the ring’s focal point (ringmaster) which reconfigures the network.
Transcript of Different types of switches FRNT and Spanning Tree - … · Different types of switches ... RSTP is...
128 Theoretical and general applications www.westermo.com
Different types of switchesDepending on the application and installation requirements there are a number of dif-ferent switches. First we differentiate between the interfaces, where there are TX(copper) and FX (fibre). Other variants are unmanaged/managed switches, this meansyou either have or do not have the possibility of communicating with and monitoringthe switch using SNMP. Finally we differentiate between ring and time synchronisedswitches which are used when you intend to build a ring network with redundancy ora network where demands on time synchronisation are made.
FRNT and Spanning TreeComplex networks with requirements on redundancy must be possible to reconfigureshould a network error occur.
Reconfiguration is handled by the switch, that is to say, the switch must identify thata link error has occurred. This can be done in different ways, of which standardisedsolutions are, IEEE Spanning Tree Protocol (STP) or Rapid Spanning Tree Protocol(RSTP). The Spanning Tree Protocol creates a connection through the network at thesame time as it eliminates unwanted loops in the network. Redundancy is created bythe protocol keeping the tree structure in the network in order, where some connec-tions are blocked (set in standby mode). When a segment can not be reached, thenetwork is reconfigured using the Spanning Tree algorithm, which results in connec-tions set to standby becoming active. Reconfiguration of a STP network can take up to30 seconds, as new conditions must be calculated and switches updated. This calcula-tion is complex as the network does not have a determined topology. RSTP is a deve-lopment of STP with faster reconfiguration, from the earlier 30 seconds a network canbe reconfigured in 5 seconds.
There are also specially developed solutions available, for example, Fast RecoveryNetwork Topology (FRNT), which is used in our ring switch R200 and time synchro-nised switch T200. FRNT is a patented solution that reconfigures the networkextremely quickly, <30 ms. This is achieved through the switches knowing the networkconfiguration, which is also a ring topology. In addition, reconfiguration is event con-trolled, ”idle traffic” is sent between each device in the ring to check whether the linkis up. When an error is detected, information is sent immediately to the ring’s focalpoint (ringmaster) which reconfigures the network.
www.westermo.com Theoretical and general applications 129
RingswitchOur ringswitches are available in two variants, for basic ring networks and for bridgedring networks. The models feature different software for reconfiguration FRNT0 andFRNT1.
FRNT0There are always two alternative directions for traffic in a ring, a right-handed or left-handed circuit. A ringswitch utilises this and in doing so eliminates network errors.Should an error occur the switch, which is configured as the focal point, is notified. Thisreconfigures the network so that everyone can communicate with each other.
FRNT1Some switches have the capability to connect together several rings whereby furtherreliability is achieved. These rings are bridged using a primary and secondary link toother rings in the network. When an error occurs on the primary link, the focal pointis notified. This then reconfigures the network and connects the secondary link to theunderlying ring. When a cable failure occurs this must be rectified and with redundan-cy this error will not be detected unless an alarm is generated at the same time.
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130 Theoretical and general applications www.westermo.com
Time switchesEthernet through design is not deterministic, i.e. you can not guarantee the transfertime of a data packet from one occasion to another. This previously made it impossibleto use Ethernet for real time applications, such as monitoring transformer stations orcontrolling complex machinery, but these limitations no longer exist. In a real time sys-tem all links must communicate with full duplex while flow control (on the Ethernetlevel) must be shutoff, furthermore, it must be possible to prioritise data. All data witha high priority will be placed at the front of the queue and be communicated with pri-ority to the recipient. Combined with time synchronisation this creates the possibilityof designing real time applications with Ethernet, also see pages 136 to 137.
What can cause problems for real time applications in a switched network?A switched network is subject to delays due to the load, speed of the drop link, packetsize, switch architecture and the number of switches between the server and client. Adelay can vary from ten µs to several ms. Most switches are based on the “store andforward” technology, which receives and checks the entire packet before it is forward-ed on. Assume that the switch has a drop link speed of 10 Mbit/s (receiving port onthe switch), the packet size is 1522 bytes, this results in a maximum delay of 1.2 msdue to “store and forward”. However, if you have 100 Mbit/s the maximum delay willbe 1.2 µs. To choose the right technology supplemented with time synchronisationgives the prerequisites for Ethernet in real time applications.
