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DNT900 - 08/16/10
TxTimeout parameters set to their default values to reduce latency, or above their default values to re-
duce the volume of small packet transmissions.
2.8.3 Multipoint Peer-to-Peer Network Operation
After a remote has joined a point-to-multipoint network, it can communicate with another remote through
peer-to-peer messaging, where the base acts as an automatic message relay. In protocol mode, if a
remote specifies a destination address other than the base address, peer-to-peer messaging is enabled.
In transparent mode, the RmtTransDestAddr parameter sets the destination address. Changing Rmt-
TransDestAddr from the default base address to the address of another remote enables peer-to-peer
messaging. The broadcast address can also be used as a peer-to-peer destination address. In this case,
the message will be unicast from the remote to the base (using ARQ) and then broadcast by the base (no
ARQ). For peer-to-peer broadcasts, no acknowledgement is sent and no TxDataReply packet is reported
to the host.
2.8.4 Tree-Routing Operation
A DNT900 tree-routing system consists of a base, remotes and up to 63 routers. A router is basically a
remote that has been configured with two operating modes - a base mode for its “child” radios and a
remote mode for its “parent” router or the system base. This allows a router to do tree-routing in addition
to normal remote functions. Each router can forward messages to/from a total of 126 child radios. A
DNT900 tree-routing system can cover a much larger area than other DNT900 networks, with the trade-
off that tree-routing increases message transmission latency. Tree-routing systems are well suited to
many industrial, commercial and agricultural data acquisition applications. Tree-routing operation is
supported by CSMA (mode 1) channel access. See Section 2.12 below.
2.9 Full-Duplex Serial Data Communications
From a host application’s perspective, DNT900 serial communications appear full duplex. Both the base
host application and each remote/router host application can send and receive serial data at the same
time. At the radio level, the radios do not actually transmit at the same time. If they did, the transmissions
would collide. As discussed earlier, the base transmits a synchronization beacon at the beginning of each
hop, followed by its user data. After the base transmission, the remotes/routers can transmit. Each
transmission may contain all or part of a complete message from its host application. From an applica-
tion’s perspective, the radios are communicating in full duplex since the base can receive data from a
remote/router before it completes the transmission of a message to the remote/router and visa versa.
2.10 Channel Access
The DNT900 provides three methods of channel access: Polling, CSMA and TDMA, as shown in the
table and figure below. The channel access setting is distributed to all radios by the base, so changing it
at the base sets the entire network or system. Polling refers to an application sending a command from
the base to one or more remote devices and receiving a response from only one remote device at a time.
Polling is suitable for both large and small networks where periodic or event reporting by remotes is not
required. Carrier Sense Multiple Access (CSMA) is very effective at handling packets with varying
amounts of data and/or packets sent at random times from a large number of remotes. Time Division
Multiple Access (TDMA) provides a scheduled time slot for each remote to transmit on each hop. The
default DNT900 access mode is TDMA dynamic mode.
