This means that a FHSS system will degrade gracefully as the band gets noisier, while a DSSS system may exhibit une-
ven coverage or work well until a certain point and then give out completely.
Because it offers greater immunity to interfering signals, FHSS is often the preferred choice for co-located systems. Since
direct sequence signals are very wide, they can offer only a few non-overlapping channels, whereas multiple hoppers can
interleave, minimizing interference. Frequency hopping systems do carry some disadvantages, in that they require an ini-
tial acquisition period during which the receiver must lock on to the moving carrier of the transmitter before any data can
be sent. In summary, frequency hopping systems generally feature greater coverage and channel utilization than compa-
rable direct sequence systems. Of course, other implementation factors such as size, cost, power consumption and ease
of implementation must also be considered before a final radio design choice can be made.
2.0 DNT90M Network Overview
A DNT90M network is referred to as
direct peer-to-peer
network. A transmission from a DNT90M can be directed to any
other peer in its network (unicast), or to all other peers in its network (broadcast). Unless a DNT90M is transmitting or is in
sleep mode, it is constantly scanning all the channels in its frequency hopping sequence for a transmission from another
peer. When a DNT90M has data to transmit, it transmits a beacon that allows the other peers in its network to rapidly syn-
chronize with its phase in the hopping sequence. After sending the beacon, the transmitting radio immediately sends its
data packet. If the data packet is addressed to a specific peer and is received without errors, the destination peer trans-
mits an acknowledgement (ACK) to the originating peer, completing the error-free transmission. If the data packet is being
broadcast to all other peers in the system, it is transmitted several times on different channels to mitigate the chances of a
reception error due to poor propagation or interference on one channel. Most DN90M unicast packets are transmitted and
acknowledged in less than 50 milliseconds. This very low transmission latency is achieved without compromising the ro-
bustness inherent in FHSS communications. Figure 2.0.1 depicts the communication paths available in a network consist-
ing of four DNT90M peers.
Figure 2.0.1
©2012 by Murata Electronics N.A., Inc.
DNT90M Integration Guide (2012/09/17)
Page 9 of 74
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