Expand Configuration and Management Manual (G06.24+)

Tuning
Expand Configuration and Management Manual—523347-008
20-14
Multi-Line Paths
Processor Type
The processor overhead for serving multiple lines is greater than the processor
overhead for serving one line per path for an equivalent volume of throughput. The
degree of increased cost depends on the processor type, the version of the Expand
software used, and the speed differences (if any) between the lines.
Packets Per Message
While a large message fragmented into small packets might make more efficient use of
the communications bandwidth than a message in a single large packet, it takes more
processor time for the fragmentation and the reassembly.
A single high-speed line might be a better solution than multiple lines, especially from
the standpoint of processor efficiency, when a single important application dominates
the performance considerations. If the variable packet-size feature is used with multiple
lines, the PATHPACKETBYTES modifier value should be configured to use all lines in
the multi-line path equally.
Refer to Variable Packet-Size Configuration on page 20-6 for more information about
the variable packet-size feature.
Window Size
If a line is added to a path using a different protocol or telecommunications network at
Layer 1, the path might have different delay characteristics from the original single line.
It might be necessary to change the Layer 2 window size (TXWINDOW modifier) to
minimize any delays introduced by switches or new protocols.
In some situations, the HDLC Extended protocol might be advantageous on terrestrial
links. Some terrestrial networks, especially those that include private switches and
CBXs, can have variable delays that at times are as great as a 0.25-second satellite
hop. If queuing for an Expand line-handler process occurs in such a network, yet the
process and communications use is low, switching delay might be the cause and using
the HDLC Extended protocol might be advisable.
Variable Packet Size
Variable packet size effectively increases the Expand packet size to 1024 bytes or
greater. Multi-line paths can distribute data across available lines more evenly when
the packet size is smallest. Applications that send messages just below the size of the
configured variable packet size over multi-line paths might notice an increase in
latency since all data is sent across only one line.
For example, a four-line path with a variable packet size (PATHPACKETBYTES
modifier) of 4095 bytes will only use the first line for all requests of 4 Kbytes or less
unless multiple requests are received simultaneously. If the PATHPACKETBYTES
modifier is configured at the default (1024 bytes), requests larger than 4 Kbytes will
utilize all four lines more evenly.