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RS‐232/485 RF Modem Product Manual – MAN0782‐02
Multi-Transmit Mode
Attributes:
Required Parameter Values (TX Modem): MT (Multi-Transmit) >= 1
Other Related Commands: Networking (DT, MK, MY, RN, TT), Serial Interfacing (BR, PK, RB,
RO), RF Interfacing (FS)
Recommended Use: Use for applications that require Reliable Delivery without using retries and
acknowledgements.
Multi-Transmit Mode Connection Sequence
Events up through the 'Transmit Packet'
process are common to all three RF
Communication Options. Refer to the Transmit
Mode section [p 18] for more information.
In Multi-Transmit mode, each packet is retransmitted
MT times, for a total of (MT+1) transmissions. There
is no delay between retransmissions, and the
transmitting modem will never receive RF data
between retransmissions. Each retransmission
includes an RF initializer. A transmission event may
include follow-on packets, each of which will be
retransmitted MT times. The Forced Sync (FS)
parameter is ignored in multiple-transmission mode.
The RB and RO parameters are not applied to follow-
on packets, meaning that once transmission has
begun, it will continue uninterrupted until the DI
buffer is empty or the streaming limit (TT) has been
reached. As with the first packet, the payload of each
follow-on packet includes up to the maximum packet
size (PK) bytes, and the transmitting modem checks
for more pending data near the end of each packet.
Follow-on packets are not sent until all
retransmissions of the previous packet are finished.
The streaming limit (TT) is specified at the
transmitting modem as the maximum number of
bytes that the transmitting modem can send in one
transmission event, which may consist of many
packets. If the TT parameter is reached, the
transmitting modem will force a random delay of 1 to
RN delay slots (exactly 1 delay slot if RN is zero). In
Multi-Transmit mode, each packet is counted only
once when tracking the streaming limit (TT), no
matter how many times it is retransmitted.
When a receiving modem receives a Multi-Transmit
packet, it calculates the amount of time remaining in
the Multi-Transmit event, and inhibits its own
transmissions for the duration of the Multi-Transmit
event, plus a random number of delay slots between
0 and (RN-1). If the local RN parameter is zero, the delay is only for the calculated duration of
the Multi-Transmit event. Thus, a receiving modem need only receive one of the transmissions,
and it will keep off the channel until the transmitting modem is done. If follow-on packets are
coming, the receiving modems will move to the new frequency and listen for the follow-on packet
for a specific period of time.
Reliable Delivery through forced transmission of every RF packet
Every RF packet is sent exactly (MT + 1) times with no delays between packets
Diminished throughput and increased latency
Figure 3.14. Multi‐Transmit Mode State Diagram
Page 27
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