Solutions to the Bottleneck Problem

Ideally, it would be nice to size the switch according to the number of stations on the network. If there is going to be 16 stations, select a 16-port switch. If there are going to be 20 stations, select a 24-port switch. Of course, there are limits to this approach. A network requiring 28-stations would require a less popular 32-port switch. Even if one is found, there are other issues. A single switch operating at 10, 100 or 1000 Mbps with twisted-pair ports, can only have segment lengths of 100 m each. This means that the network diameter is only 200 m. This distance limitation might be a constraint to the application. Fiber optics would help greatly in increasing network diameter; however, there may only be a limited number of fiber optic ports available on one switch.

The second approach is to have a higher-speed backbone port to directly address the port bottleneck issue. This port would have ten-times the data rate of the other ports dedicated for stations. However, to be fully effective, two backbone ports would be required on each switch. Two ports allow for the daisy-chain connection of more than two switches. One backbone port is fine for end of line applications, but two backbone ports are needed for mid-span applications. (Figure 3) The question then comes up: Is it better to standardize on a single two-port model or allow for two models-one-port for end of line and a two-port for mid-span? The cost of the two-port model must be weighed against the flexibility of having one model to fit all applications.

Figure 3 — Mid-span switches require two backbone ports while end of line switches only require one. In this example, fiber optics are used for the backbone.

If all the stations were 10 Mbps devices, it would make sense to have 100 Mbps backbone ports. However, many of the devices today are capable of 100 Mbps operation and switch ports usually handle 10/100 Mbps selection through the auto-negotiation protocol. Even if a device connects at 100 Mbps, it does not mean it will be swamping the switch with traffic. The traffic from the device may be in spurts which would not burden the backbone ports. Therefore, the backbone ports could also be 100 Mbps. However, to ensure the greatest throughput, the station ports should be 100 Mbps and the backbone ports should be 1000 Mbps. This could be an expensive overkill in some applications. Because 1000 Mbps operation brings with it its own set of issues. The interface is complex, less robust and distance limited and for most control applications unnecessary.

There is still another approach to this problem and it is called link aggregation. With link aggregation there is a compromise between requiring a 10-times performance improvement in backbone port-speed versus port-usage. Plus, there are added benefits as we will see later.

Link Aggregation

The IEEE 802.3ad standard calls it Link Aggregation, but it is simpler to call the concept Trunking. We will use the words interchangeably. If one channel has the capability of sending data at 100 Mbps, why not add another channel to achieve 200 Mbps? If two channels can achieve 200 Mbps collectively, why not add two more channels to achieve 400 Mbps? This is the argument behind link aggregation or trunking. Standard ports on a switch would be configured as a trunk group in order to send data to a distant switch, also configured with a trunk group over a parallel path. The concept of sending data in parallel is not new and is the basis for the 1000BASE-T physical layer. With Gigabit Ethernet, symbols representing a data byte, are sent over four twisted-pairs in order to increase throughput without a significant increase in baud rate which would therefore limit segment length. The four pairs represent a parallel path, although to the user it appears that only one twisted- pair cable is being used. With trunking, there is no change in the physical layer interface. The ports used for trunking are no different, physically, from any other port on the switch. A separate cable is used for each trunk segment. Complete frames are alternately sent down each trunk segment and recombined at the other end. In the case of a 100 Mbps switch, the trunk ports are also 100 Mbps. It must be remembered that the switch used must have the capability for supporting trunking. A regular switch will not work. In fact, establishing a parallel path using standard switches will disrupt the network.

Assume two or more ports are assigned to a trunk group within a switch and the same is done to a second switch. The trunk group will act as one high performance port by sending frames to ports in the trunk group which are available. This usually results in frame transmissions being alternated between the ports within a group. This increases throughput since multiple channels are available for transmitting. On the receiving side, the frames received by the trunk group are treated as if they came from one port.