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Chapter 4: Network Topologies
At the start of a Fast TDMA Frame, the Master unit sends information to all the units in the TDMA network. Included in this
information is:
1. SYNC Packet—used for system synchronization.
2. TDMA Table/List—This is the list of all the addresses of all the remote units in the network, as well as the order and frequency
in which they are to transmit data. Unit addresses can be listed in any order, and can be listed multiple times to reduce latency.
The Master unit can be included in the table as unit address 1 if data is required to be transmitted from the master to the
remote units.
The TDMA Cycle/period depends on the size of the TDMA table. The current TDMA table used for Fast TDMA operation is
limited to 2048 entries. (128 entries are saved in non-volatile memory and the rest of the entries must be re-entered every time
on power up.)
3. Control Data—Includes various control data, such as the value of the Master units S212 register (Max Expected Packet Size),
and the Master unit’s S112 register (Max Packet Size).
Sending this data to the remote units is important for the operation and timing of Fast TDMA, because it uses this information to
determine the slot duration, and identifies to each unit when, and for how long, it will be allowed to transmit data. Unlike tradi-
tional TDMA, the master unit only sends synchronization data at the very beginning of the TDMA frame. Other user data can be
sent by the master unit, by assigning it a slot in the TDMA table, but at this point it acts as a remote unit, rather than a master
unit.
Register 212 is used to set the expected packet size of the data received by the master from the remote units. This register is only
set in the Master unit, and defines the maximum duration, in bytes, of how long each remote unit is allowed to transmit data.
The slot size for each remote unit is identical, and defined by the S212 register.
The slot size for the user data of the Master as defined by the Master unit’s S112 register; it does not have to match the slot size
of the remote units (S212). The time slot for the Master unit (In a PMP network, the Master always has a unit address of 1), is
dif-ferent than the time slots for the other units.
The S112 register on the remote units is the Max Packet Size of the data transmitted to the master. It has to be equal to or less
than the S212 register of the Master, because the master expects the data to only be this size, or smaller. If S112 of the remote
units is larger than S212, the data will not fit into the allotted slot, and the data will be lost.
The maximum buffer size is 1580 bytes, which limits the values of registers S112 and S212 to 1580 bytes.
The remote units (and the Master) can then use the data above to calculate when it will be allowed to transmit data. By knowing
which unit in the list and how long the remote and master TDMA slots are, each unit in a Fast TDMA network will know precisely
when to transmit its data.
If a remote misses a Master’s SYNC packet the remote unit will not transmit until a new SYNC packet is received. If the remote
keeps missing the sync packet for a duration longer than that defined by register S248 - Sync Timeout (in seconds), the remote
will go into search mode and start looking for a master. This search time can be significant because the master only sends SYNC/
TDMA packets at the very start of a TDMA Frame (time depends on size of the TDMA table).
Fast TDMA assumes single packets are sent during assigned time slots. If multiple packets of varying lengths are required, assign
multiple slots to ensure data overhead is considered. Retransmissions, ACK, and packet fragmentation are not used in this mode.
To calculate the duration of the entire Fast TDMA Frame:
For the link rate 172kbps:
T_tdma_table N_entries) = 2.3076 + 0.0684 * N_entries;
T_data_packet (N_bytes) = 1.9327 + 0.0673 * N_bytes;
T_total = T_tdma T_data_packet_Master* N_M T_data_packet_Slave* N_Slaves, where N_M N_Slaves =
N_entries
