User Manual

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CCT24
immediately as remotes join or leave the network. When running in protocol mode on a remote, care must
be taken not to generate messages too long to be sent in a single hop due to automatic RemoteSlotSize
reduction.
TDMA Fixed Slots (mode 3) is used for applications that have fixed data throughput requirements, such
as isochronous voice or streaming telemetry. The slot start time and the RemoteSlotSize are computed
automatically by the CCT24 network in this mode. The user must set the number of slots using the
MaxSlots parameter. The base radio will allocate remote slot sizes as if MaxSlots number of radios are
linked with the base, even when fewer remotes/routers are actually linked. In this mode, the remote slot
sizes are constant.
TDMA with PTT (mode 4) supports remotes with a "push-to-talk" feature, also referred to as "listen-
mostly" remotes. This mode uses fixed slot allocations. Remotes can be registered for all but the last slot.
The last slot is reserved for the group of remotes that are usually listening, but occasionally need to
transmit. In essence, the last slot is a shared channel for this group of remotes. When one of them has
data to send it keys its transmitter much like a walkie-talkie, hence the name push-to-talk (PTT). There is
no limit to the number of remotes that can listen to the last slot.
The slot start time and the RemoteSlotSize are computed automatically by the CCT24 network in mode 4.
The user must specify the number of slots using the MaxSlots parameter. The last slot is reserved for the
PTT remotes. The user must configure PTT remotes individually to select mode 4 operation. The user's
application must ensure that only one PTT remote at a time is using the slot. Mode 4 does not support
tree-routing operation.
2.11 Transmission Configuration Planning
Because frequency hopping radios change frequency periodically, a single message may be sent in one
or more RF transmissions. The length of time the radio stays on a frequency, the hop duration, impacts
both latency and throughput. The longer the radio stays on a single frequency, the higher the throughput
since the radio is transmitting for a higher percentage of the time, but latency is also higher since radios
may have to wait longer to transmit. So latency and throughput trade off against one another. The CCT24
has several configuration parameters that allow latency and throughput to be optimally balanced to the
needs of an application.
2.11.1 TDMA Throughput
For TDMA channel access without routers, throughput and latency are controlled by the RF data rate, the
serial port baud rate, the BaseSlotSize, the HopDuration, and the number of remotes. A wide range of
throughput and latency combinations can be obtained by adjusting these parameters. The throughput of a
radio in a TDMA network is simply:
Number of bytes per hop/Hop Duration
For the base, the number of byes per hop is controlled by the BaseSlotSize parameter so the throughput
of the base radio is:
BaseSlotSize/HopDuration
Note that if fewer bytes than the BaseSlotSize limit are sent to the base radio by its host during the hop
duration time in transparent mode, the observed throughput of the base radio will be reduced. If the base
is in protocol mode, it will wait until a protocol formatted message is completely received from its host