Content

| 3.2 |  | Characteristics of Physical Layer Optimization Problems |
| 3.2.7 |  | Utilization |

A component may be operating sub-optimally at the physical layer because it is being utilized at a higher average than it is configured to operate. When troubleshooting this type of problem, it will become evident that resources for the device are operating at or near the maximum capacity and there is an increase in the number of interface errors. Gathering symptoms reveals excessive runts, late collisions, or an increase in the number of buffer failures. The output from a ping or traceroute command results in excessive packet loss or latency.

How Much Utilization is ok?
Shared Ethernet networks are believed to suffer from throughput problems when average traffic loads approach a maximum average capacity level of 40 percent. This percentage is actually conservative. Higher average percentages are certainly possible. The classic solution to excessive traffic is to micro-segment LANs by installing switches. This solution works well until the amount of broadcast traffic grows too large. Since bridges and switches always forward all broadcast traffic to all ports, even infrequent broadcasts from each station will eventually be too many when the station count for the broadcast domain goes up.

If a single station is connected to a half duplex switch port acceptable utilization is best learned by monitoring switch port statistics. There will be some level of collisions, but since there are only two devices on that link (the switch and the station) the utilization level should be capable of averaging quite high. There will likely be excessive collisions errors reported by the switch periodically, resulting from the Ethernet capture effect. This problem is not terribly significant overall, and will result in a slight reduction in performance. If the link is allowed to negotiate to full duplex instead of half, the connection should be capable of approaching theoretical limits for full line rate Ethernet. This will depend on the processing power of the attached station. The current generation of switches is quite capable of sustaining line-rate traffic at the minimum frame size.

Another problem can occur when access to servers or services is reached through a single switch uplink path. Unless the bandwidth of the uplink path is bigger than the total of simultaneous station requests, the uplink itself becomes a bottleneck. This scenario arises when a network is designed with all servers collected in a server farm, separated from the VLANs they serve.

Network bottlenecks or congestion typically manifests itself to users with the following symptoms:

- Highly variable response times
- Network time-outs or server disconnects
- Inability to establish network connections
- Slower application loading and/or running

Even when network resources are on the same LAN segment as the users, it is still necessary to examine the network architecture to see if bottlenecks exist. If too much traffic is required to pass through an inadequate aggregation path then a potentially useful architecture design is defeated. To prevent this, the network administrator should limit inter-segment traffic by carefully considering which nodes should attach to each segment. The process involves an investment of time and may need to be repeated regularly on extremely dynamic networks. Monitor uplink paths as a part of routine maintenance in order to detect impending saturation of any one path.