DNT500
2008 by RF Monolithics,
Inc.
9
M-0500-0000 Rev D
CSMA_MaxBackoff, BaseSlotSize and RemoteSlotSize parameters when using this mode.
Note that a CSMA_Persistence parameter setting of 0xFF would lead to collisions if more
than one remote tried to transmit. Applications where more than one remote can receive
serial data to transmit at the same time, or where periodic reporting and/or event report-
ing are enabled should not use this mode.
CSMA Contention (Mode 1) provides classical CSMA channel access, and gives the user
control over both the CSMA_MaxBackoff and CSMA_Persistence parameters. This mode
is well-suited for large numbers of uncoordinated remotes, and/or where periodic/event
reporting is used. In addition to CSMA_MaxBackoff and CSMA_Persistence, the user can
set the BaseSlotSize and RemoteSlotSize parameters when using this mode. The following
guidelines are suggested for setting CSMA_Persistence:
•
For lightly loaded CSMA contention networks, increase CSMA_Persistence to
0x80 or higher to reduce latency.
•
For heavily loaded CSMA contention networks, reduce CSMA_Persistence to
0x20 or lower for better throughput.
CSMA modes can optionally track remotes entering and leaving the network for up to
255 remotes. The base station is operated in protocol mode and is configured to generate
a CONNECT message for its host when a remote registers, and a DISCONNECT mes-
sage when the remote’s registration lease expires.
The base station in a CSMA network can generate CONNECT messages for more than
255 remotes. This allows the host application to track remotes entering and leaving a
large CSMA network by creating a table of MAC addresses and periodically sending a
ping to each remote in the table. Failure to answer the ping indicates the remote is no
longer active in the network.
The CSMA modes work well in many applications, but CSMA does have some limita-
tions, as summarized below:
•
Bandwidth is not guaranteed to any remote.
•
Marginal RF links to some remotes can create a relatively high chance of
collisions in heavily loaded networks.
2.7.2 TDMA Modes
The TDMA modes provide guaranteed bandwidth to some or all of the remotes in the
network. Remotes that register with the base station receive several special parameters,
including ranging information and a specific channel access slot assignment. TDMA reg-
istrations are always leased and must be renewed every 250 hops. The DNT500 provides
three different modes of TDMA access, as discussed below.
TDMA Dynamic Slots (Mode 2) is used for general-purpose TDMA applications where
scaling the capacity per slot to the number of active remotes is automatic. Each remote
that registers with the base receives an equal time slice. As new remotes join, the size of
the TDMA slots shrink accordingly. The number of slots, individual slot start times, and