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- Losses in the medium: losses related with signal's propagation the propagation of the signal in the physical environment. For example, the frame will be lost if the useful signal level is lower than the receiver sensitivity. Losses can also be caused by the physical damage of the interfaces connected to the media or by impulse pickups resulting from poor grounding.
- Losses on the interface: losses during while processing a queue at the incoming or at the outgoing interface. Each interface has a memory buffer, which can be completely filled in case of intensive data stream transmissions. In this case, all the subsequent data entering the interface will be discarded, because it cannot be buffered.
- Losses inside the device: Data discarded by the network device according to the logic of the configuration. If the queues are full and the incoming data cannot be added to the processing queue, the network device will drop it. Also, these losses include the data packets rejected by access lists and by the firewall.
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The parameter that affects the throughput and the state of the queues is the packet performance of the device. Packet performance is the maximum number of data packets of a given length that a device is capable to transmit process per unit of time.
The real throughput depends on both packet performance and on the interface's characteristics, therefore, at the network design stage, pay attention to the coherence of these parameters in order to avoid the situation when one of them becomes a bottleneck for a link or for a network segment.
The packet performance is defined by the hardware capabilities of the central processor and by the amount of internal memory. Network devices process multiple traffic streams with different L2 frame sizes, so the following Ethernet frame size values are used for a performance testingtest:
- minimum size = 64 bytes;
- medium size = 512 bytes;
- maximum size = 1518 bytes.
Due to the limited amount of internal memory amount, better packet performance is achieved for the minimum frame size. Using minimum sized frames assumes a large overhead amount since each data frame has a service header, whose size does not depend on the size of the frame itself.
For example, the service header length for frames 64 bytes long frames (Figure 4b) and 156 bytes frames(Figure 4c) will be the same, but the user data amount will be different. To transmit 138 bytes of user data, three frames 64 bytes long frames or one frame 156 bytes long frame will be required, so in the first case 192 bytes are neededsent, in the second - only 156 bytes. If link has the same For a link having a fixed throughput, large frames will increase the efficiency by rising the useful throughput of the system, but the latency will also increase. The performance of the Infinet devices performance values in various conditions is shown in the "Performance of the InfiNet Wireless devices" document.
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Figure 4 - Examples of Frame structure for various lengths Ethernet frame structurelengths |
Delay
Delay is the data packet transmission time from a source to a receiver. The delay value consists of the following parts:
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