2-8
Theory of Operation
Q u e u e
C o u n t
S i z e
Total Bytes
Input Buffer Queue
2
66
132
Priority Output Buffer Queue
2
66
132
Non-Priority Output Buffer Queue
15
66
990
T O T A L
1 2 5 4
The default size of 66 bytes allows the router to handle packets with maximum
address overhead and data size for any network variable message and explicit
messages with up to 40 bytes of data; this is large enough for any network management
or network diagnostic message. In applications that must route large explicit messages
with more than 40 bytes of data, the buffer size must be increased, and the count of non-
priority buffers decreased. See Chapter 6 of the
Neuron C Programmer’s Guide
to
understand how the network buffer sizes are calculated. The size and count of buffers
can be changed as described in Chapter 8 of this document. They also can be changed
using the
NodeUtil
node utility available on the LonLink
™
bulletin board and Internet
Host. The total memory required by the three buffer queues must not exceed 1254 bytes.
The default buffer configuration places the bulk of the buffers on the output queues of
the router. For example, the standard configuration places two network buffers on the
input queue and 17 buffers on the output queue (2 priority and 15 non-priority) of each
router side. The reasoning behind this configuration is to keep buffered packets on the
output queues, after they have been processed for forwarding. This processing includes
checking for priority packets. Priority packets are sensed and forwarded through the
router's priority output buffers. This assures that priority packets are processed as
quickly as possible, rather than allowing them to be delayed behind non-priority
packages in a large input queue.
There are applications, however, where the network traffic may be 'bursty' where
many packets appear on the network almost at the same time. In these cases the
traffic bursts may cause the input queue to become full and loose the excess packets.
In this case it may be preferable to move more of the packet buffering from the output
queue to the input queue. This can be done by increasing the size of the input queue and
decreasing the size of the output queue. A router with a larger input queue can handle
larger bursts of traffic, at the risk of priority messages being queued behind several
non-priority messages.