Specifications

The maximum SNAP packet size is 123 bytes. If you set this parameter to a value greater than 123, the
system will simply substitute a value of 123. If you set this parameter equal to or less than the packet
header size, SNAP will construct packets with a complete header and one byte of data.
Like parameters #13 and #15, larger values can result in larger (more efficient) packets, at the expense
of greater latency. Also, at higher baud rates, setting this value too high can result in dropped
characters if the packet buffer gets overfilled between SNAP checks.
ID 15 – Inter-character Timeout
This lets you tune inter-character serial data timeout. This value is in milliseconds, and defaults to 0
(in other words, disabled).
This timeout is similar to NV Parameter #13, but this one refers to the time between individual
characters. One way of thinking of it: this timeout restarts with every received character – the other
timeout always runs to completion.
Larger inter-character timeouts can give better MULTICAST TRANSPARENT MODE reliability, at
the expense of greater latency.
Note that either timeout #13 or #15 (if enabled) can trigger the transmission of the buffered data before
the Buffering Threshold (#14) is reached. Conversely, if the timeouts are high (or disabled) to the
extent that enough data is buffered to reach the Buffering Threshold before the timeouts are reached,
that threshold will trigger the transmission of the buffered data before either of the timeouts are
reached.
ID 16 – Carrier Sense
Basically, this instructs the radio to “listen before you transmit.”
This value defaults to False. Setting this value to True will cause the node to do what is called a Clear
Channel Assessment (CCA). Basically this means that the node will briefly listen before transmitting
anything, and will postpone sending the packet if some other node is already talking. This results in
fewer collisions (which means more multicast packets make it through), but the “listening” step adds a
small delay to the time it takes to send each packet.
If in your network the probability of collisions is low (you don’t have much traffic), and you need the
maximum throughput possible, then leave this value at its default setting of False. If in your network
the probability of collisions is high (you have a lot of nodes talking a lot of the time), then you can try
setting this parameter to True, and see if it helps your particular application.
ID 17 – Collision Detect
Basically, this instructs the radio to “listen after you transmit.”
This value defaults to False. Setting this value to True will cause the node to do a CCA after sending a
multicast packet. This will catch some (but not all) collisions. If the node detects that some other node
SNAP Reference Manual Document Number 600-0007K Page 97 of 202