Reducing Data Latency

One way of reducing data latency in a switch is to operate at 100 Mbps instead of 10 Mbps. This could reduce latency by a factor of ten. Another approach is to use cut-through operation. With cut-through operation, a switch does not wait for receipt of the complete frame before forwarding. A switch only needs to know the destination address before forwarding, which is available near the beginning of the frame. Therefore, there is no need to wait for the complete frame to be received. The problem with this approach is that the frame could have a failed frame check sequence (FCS) it or could be a runt frame. These frames should not be forwarded. The runt frame problem can be solved by waiting a bit longer before forwarding, but the failed FCS problem is not solvable with cut-through operation.

100 Mbps Operation

If stations have adequate processing power, operating at 100 Mbps should improve network performance. However, the problem at 100 Mbps is that the collision domain diameter decreases by about a factor of ten. Using repeating hubs at this speed is not very practical and certainly not very popular because the reduction in network diameter to about 205 m. Therefore, the use of switches at 100 Mbps instead of repeating hubs is a clear choice.

Full-Duplex Operation

The original Ethernet technology, operated at half-duplex, is called shared Ethernet. Only one station can transmit at a time. With full-duplex and switch technology, a station can both receive and transmit at the same time over a link segment such as twisted-pair or fiber optics. Since the simultaneous transmitting would normally cause a collision, the collision detection circuitry is disabled, eliminating the collision domain. With no collision domain, the only limit to segment lengths is attenuation of the signal over the segment. Therefore, in full-duplex mode, a 2 km fiber optic segment length, or longer, is possible between two switches operating at 100 Mbps whereas it is limited to 412 m in half-duplex mode. Full-duplex further simplifies the expansion rules and could improve performance on some networks. However, with traditional master/slave industrial protocols, you will probably not witness any performance improvement.

Broadcasts and Multicasts

A broadcast is a transmission to all stations and a multicast is a transmission to a selected group of stations. Since the switch does not know all the potential broadcast and multicast stations, it must flood all ports on the switch with these frames. In this situation, a switch functions just like a hub. Modern producer/consumer protocols utilize broadcasts and multicasts and, therefore, network performance may not necessarily improve with the use of switches.

Using Network Analyzers

A network analyzer is a helpful tool when troubleshooting networks. These tools observe all the traffic on the network and capture portions of traffic for analysis purposes. The network analyzer connects to an unused port on one of the switches or repeating hubs. With a repeating hub there is no problem since all traffic can be viewed at any repeating hub port. The problem exists with switches. Since switches filter traffic to its various ports, the network analyzer will not observe the same traffic on all ports. To remedy this problem, the switch could provide a feature called port mirroring where all the switch traffic would be directed to a particular port. This feature is usually only found in higher-cost managed switches.

Issue
Recommendation
Low Cost
Hubs
Reduce data latency
Hubs
Simple application
Hubs
100 Mbps operation
Switches
Full-duplex operation
Switches
Auto-negotiation
Switches
Large distances
Switches
Master/slave protocol
Hubs or Switches
Peer-to-Peer protocol
Switches
Network analyzer
Hubs

This table indicates when to use hubs and switches.

Summary

As you can see there are both advantages and disadvantages in using switches. For simple systems, repeating hubs can be quite effective so their use should not be immediately ruled out.


References

The Switch Book, Rich Seifert, 2000, Wiley Computer Publishing

Ethernet The Definite Guide, Charles E. Spurgeon, 2000, O'Reilly & Association, Inc.

International Standard ISO/IEC 8802-3 ANSI/IEEE Std 802.3, 2000, The Institute of Electrical and Electronic Engineers, Inc.