Expand Configuration and Management Manual (H06.21+, J06.10+)

Tuning
Expand Configuration and Management Manual — 529522-013
19 - 30
Example 1: Changing Packet Size
because the number of messages and the size of the messages is the same in both
cases.
For messages sent, changing the packet size to 1024 bytes improves the packets-per-
link ratio by about five times. Increasing the packet size to 2048 bytes could further
improve efficiency on the path (but this is not possible for all line types):
Increasing the packet size to 2048 bytes should increase efficiency slightly more
because it would only reduce the number of packets by an additional 40 percent,
possibly reducing Expand subsystem processor cost another 10 to 20 percent.
For each packet saved, 64 bytes of Expand header is no longer needed. The
percentage of bandwidth saved can be estimated. First, choose the busy direction. In
the example, the outgoing direction has the largest average message size (1776
bytes). If in doubt, select the direction where links times message size is greatest.
Calculate the bandwidths required for the 1024-byte packet size and the original 256-
byte size using this formula:
The bandwidth used for 1024-byte packets would be
1776 + (2 * 64) = 1904 = 15323 bits/message
The bandwidth used for 256-byte packets would be
1776 + (10 * 64) = 2416 = 19328 bits/message
If the packet size is changed to 1024 bytes, only 79 percent of the bandwidth that was
previously used would be required. The bandwidth used for the 2048-byte packet size
would be
1776 + (1 * 64) = 1840 = 14720 bits/message
This would be 3 percent less than the bandwid
th required for the 1024-byte packet
size.
1024-Byte Packets 2048-Byte Packets
Messages Sent
(average message size=1776 bytes)
46043/24888 = 1.85 24888/24888 = 1.0
Messages Received
(average message size=638 bytes)
28559/28559 = 1.0 28559/28559 = 1.0
Overall 74602/53477 = 1.4 53477/53477 = 1.0
average_message_size + (number_of_packets * 64)