User Guide

PMP 450 Planning Guide
Attribute Meaning
Control Slots This field indicates the number of (reserved) control slots configured by the
operator. The SM uses reserved control slots and unused data slots for
bandwidth requests
Uplink Data Slots are used first for data. If they are not needed for data in a
given frame, the remaining data slots can be used by the SMs for bandwidth
requests. This allows SMs in sectors with a small number of control slots
configured to still successfully transmit bandwidth requests using unused data
slots.
A higher number of control slots give higher probability that an SM’s
bandwidth request will be correctly received when the system is heavily
loaded, but with the tradeoff that sector capacity is reduced, so there will be
less capacity to handle the request. The sector capacity reduction is about 200
kbps for each Control Slot configured in a 20 MHz channel at QPSK SISO
modulation. The reduction in sector capacity is proportionally higher at
MIMO modulations (2 times at QPSK MIMO, 4 times at 16 QAM MIMO, 6
times at 64 QAM MIMO and 8 times at 256 QAM MIMO). If too few reserved
control slots are specified, then latency increases in high traffic periods. If too
many are specified, then the maximum capacity is unnecessarily reduced.
The suggested Control Slot settings as a function of the number of active VCs
in the sector are shown in the table below.
Number of VCs
Recommended Number of
Control Slots
1 to 10
3
11 to 50
4
51 to 150
6
151 and above
8
Note that each SM uses one or two VCs. All SMs have a Low Priority Channel
that uses one VC; if the High Priority Channel is enabled for the SM, then the
SM uses a second VC. Therefore the number of active VCs in a sector is greater
than or equal to the number of SMs registered to the AP in the sector. For
example, a network including 20 SMs with High Priority Channel disabled and
20 SMs with High Priority Channel enabled has 60 active VCs and may be
configured with 6 Control Slots.
In a typical cluster, each AP should be set to the same number of control slots
to assure proper timing in the send and receive cycles. However, where high
incidence of small packets exists, as in a sector that serves several VoIP
streams, additional control slots may provide better results. For APs in a
cluster of mismatched control slots setting, or where PMP 450 is collocated
with radios using different technologies, like PMP 430 or FSK, in the same
frequency band, use the frame calculator.
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pmp-0047 (March 2014)