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.
pmp-0047 (March 2014)
1-23
Summary of Contents for PMP 450
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