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CCT24
One disadvantage of direct sequence systems is that due to design issues related to broadband transmit-
ters and receivers, they generally employ only a minimal amount of spreading, often no more than the
minimum required by the regulating agencies. For this reason, the ability of DSSS systems to overcome
fading and in-band jammers is relatively weak. By contrast, FHSS systems are capable of hopping
throughout the entire band, statistically reducing the chances that a transmission will be affected by
fading or interference. This means that a FHSS system will degrade gracefully as the band gets noisier,
while a DSSS system may exhibit uneven 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 chan-
nels, whereas multiple hoppers can interleave, minimizing interference. Frequency hopping systems do
carry some disadvantages, in that they requires an initial acquisition period during which the receiver
must lock onto the moving carrier of the transmitter before any data can be sent, which typically takes
several seconds. In summary, frequency hopping systems generally feature greater coverage and chan-
nel utilization than comparable 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 CCT24 Radio Operation
2.1 Network Synchronization and Registration
As discussed above, frequency hopping spread spectrum radios such as the CCT24 periodically change
their transmit frequency. In order for the other radios in the network to receive the transmission, they must
be listening to the frequency on which the current transmission is being sent. To do this, all the radios in
the network must be synchronized to the same hopping pattern.
In all CCT24 networks, one radio is designated as the base. All other radios are designated as remotes or
routers. The base transmits a beacon each time it hops to a different frequency, which allows the other
radios in its network to synchronize with it. Since all radios in the network know the hopping pattern, once
they are synchronized with the base, they know which frequency to hop to and when.
When a remote or router is powered on, it rapidly scans the frequency band for the synchronizing signal.
Since the base is transmitting on up to 38 frequencies and the remotes and routers are scanning up to 37
frequencies, it can take several seconds to synchronize with the base.
Once a radio has synchronized with the base, it will request registration information to allow it to join the
network. Registration is handled automatically by the base. When a radio is registered, it receives several
network parameters from the base, including
HopDuration
,
InitialNwkID
,
FrequencyBan
d and
Nwk_Key
(see Section 4.2 for parameter details). Note that if a registration parameter is changed at the base, it will
update the parameter in the remotes or routers over the air.
When leasing is enabled, registration also allows the base to track radios entering and leaving a network,
up to a limit of 126. The base builds a table of serial numbers of registered radios using their three-byte
MAC addresses. To detect if a radio has gone offline or out of range, a registration is leased and must be
renewed within the configured lease interval. CCT24 radios automatically send lease renewal request to
the base. There is nothing a remote host needs to do to keep the lease renewed. Note that more than
126 radios can join a network, but base-managed leasing cannot be used. In this case, the base can be
