User's Manual

Radio Operation
Crescendo UHF Half-Duplex User Manual Page 21 of 78
5. Radio Operation
This section describes the two modes of Crescendo operation: data and packet-driven. These modes underlie
the different protocols supported, described in section 6.
Regardless of the mode used serial characters are assembled in small blocks for the purpose of Forward
Error Correction (FEC). These blocks are then subject to error coding / interleaving, and protected by a
CRC. This operation is transparent to the user, with the advantage of added robustness in multipath fading
and noisy environments.
5.1 Data-Driven Mode
In data-driven mode many of the features that are available in the Crescendo are not utilised in order to
provide a serial transfer with minimum delay. In particular data driven mode does not utilise:
 Unit addressing / repeaters
 Retries / routing
 Complex network structures / network address
Data driven mode provides a low latency broadcast network where any data presented on the main port of a
unit is immediately transmitted over the air, received by all units in range and transmitted out their main
ports.
Data-driven mode is used when the data driven protocol (see section 6.2) is selected on the main port. For all
other protocols the packet-driven mode is used.
5.2 Packet-Driven Mode
Packet-driven operation is based upon Automatic Repeat Requests (ARQ) with retries and exponential back-
off.
A transmission consists of a packet transmitted from source to destination, followed by an acknowledgement
from destination to source. Acknowledgements are done on an end-to-end basis, so intermediate repeaters
simply pass the packet and acknowledgement on. This is shown in Figure 3.
Repeater
Remote
Payload Packet
Base
Payload Packet
ACK
ACK
Figure 3: Packet driven with ARQ operation