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Transmission overhead and optimal packet size

 

  
Figure 3: The transmission overhead against packet size for message length 1000.

We define the transmission overhead to be the number of bits, communicated for a message, that do not represent the data bits of the message. In particular, packet headers and acknowledge packets determine the transmission overhead. Most forms of serial communication use encoding, e.g., adding stop bits, to ensure that the receiving side correctly interprets the serial bit stream. DS links use an encoding scheme that extends each byte, 8 bits, to a 10 bit token. The transmission overhead is therefore at least 20 %.

Figure 2 shows the format of data and acknowledgement packets that we use for the DSNIC protocol. It shows that, apart from the DS link routing header, three characters are used for protocol specific information, independent of the payload size. Together with the acknowledgement scheme, this allows us to calculate the transmission overhead for different packet sizes. Figure 3 shows the transmission overhead against packet size, or, more precisely, against payload size, for sending a 1000 byte message. The packet size strongly influences the required number of packets and packet headers, and thereby the transmission overhead.

  
Figure 4: The maximum network throughput versus packet size.

Figure 4 shows the influence of the packet size on the maximum network throughput for a 512 end-node Clos network under random traffic. This graph shows an optimal network throughput for packet size 28. For packets smaller than 28, the network throughput drops due to the domination of the transmission overhead. For packets larger than the optimum, the network throughput becomes worse due to network congestion.

The header of each packet needs to be processed by the DSNIC. This processing requires time. Using a small packet size, such as the optimal 28, requires a lot of processing to achieve full bandwidth communication. To keep this processing from becoming the system's bottleneck, we choose not to fix the packet size, but to make it adaptable so that its influence on the performance of the DSNIC can be investigated. We only support powers of two for the packet size to accommodate the implementation.


next up previous
Next: System design Up: Protocol design Previous: Reliability and end-to-end flow

Marcel Boosten
Wed Mar 11 14:25:07 MET 1998