Murata Electronics North America 5811M Manuel D’Utilisation
© 2003 Cirronet
Incorporated
8
M-5811-0008 Rev -
amount of data it has to send. Up to three remotes can request time slots during a single
contention period. On the next hop, the base radio assigns some transmit slots to the remote.
On that same hop, the remote transmits as much data as will fit in the assigned time slots and
request time slots for the next hop. The requests for time slots by remotes currently assigned
time slots do not occur in the contention period and thus do not count against the three
remotes that can request slots during this period. On the hop when the remote exhausts the
data it has to send, the remote indicates to the base that it has no data to send. The base adds
those slots back into the pool of unused slots. There are a total of 26 remote to base transmit
slots. Thus a maximum of 26 remotes can be transmitting to the base on a single hop with
each remote assigned a single slot. Remotes will be assigned as many slots as are available
up to the number requested by the remote. A remote can request a maximum of 26 slots. The
number of slots requested by a remote is calculated by the remote based on the amount of
data it has to send. The calculation is performed to send the data in as few hops as possible.
When setting up a network, keep in mind that time slot length, maximum packet size and hop
duration are all interrelated. The hop duration parameter will determine the time slot size
and the maximum amount of data that can be transmitted per hop by the remotes. The base
station requires 1.7 ms overhead for tuning, the synchronization signal and parameter
updating, as well as a guard time of 150
contention period. On the next hop, the base radio assigns some transmit slots to the remote.
On that same hop, the remote transmits as much data as will fit in the assigned time slots and
request time slots for the next hop. The requests for time slots by remotes currently assigned
time slots do not occur in the contention period and thus do not count against the three
remotes that can request slots during this period. On the hop when the remote exhausts the
data it has to send, the remote indicates to the base that it has no data to send. The base adds
those slots back into the pool of unused slots. There are a total of 26 remote to base transmit
slots. Thus a maximum of 26 remotes can be transmitting to the base on a single hop with
each remote assigned a single slot. Remotes will be assigned as many slots as are available
up to the number requested by the remote. A remote can request a maximum of 26 slots. The
number of slots requested by a remote is calculated by the remote based on the amount of
data it has to send. The calculation is performed to send the data in as few hops as possible.
When setting up a network, keep in mind that time slot length, maximum packet size and hop
duration are all interrelated. The hop duration parameter will determine the time slot size
and the maximum amount of data that can be transmitted per hop by the remotes. The base
station requires 1.7 ms overhead for tuning, the synchronization signal and parameter
updating, as well as a guard time of 150
µs between each remote slot. Thus the amount of
time allocated per remote slot is roughly:
hop duration – base slot – 1.7ms – 25 transmit slot guard bands·150µs
26 remote transmit slots
For the default settings of base slot size of 160H and hop duration of 240H, the amount of
data that can be transmitted by remotes per hop is calculated by:
The hop duration is set in 52.1
µsec increments. Thus a hop duration of 240H becomes:
576 x 52.1µsec = 30msec
The base slot size is set in increments of 8 bytes. A base slot size of 160H is:
160H = 352 Decimal x 8bytes = 2,816 bytes
With a 1,228,800 bps data rate, the time it takes to transmit 2,816 bytes of data is:
2,816 bytes x 8bits per byte/1228800bps = 18.3msec
Adding the 1.7msec of base overhead gives a total base time of:
18.3msec + 1.7 = 20.0msec
Subtracting the guard band time of 25 x 150
µsec or 3.75msec leaves
30msec – 20.0msec – 3.75msec = 6.25msec
Dividing this time by 26 slots yields a remote transmit slot time of
6.25msec/26 slots = 0.240msec per slot
Converting that time to bytes of data yields:
0.240msec x 1228800bps/8bits/byte = 36 bytes per slot