Installation Guide

Table Of Contents
Task 6: Configuring radio parameters PMP 450i and PTP 450i Configuration and User
Guide
Attribute Meaning
Contention Slots
(f.k.a. Control
Slots)
This field indicates the number of (reserved) Contention slots
configured by the operator. The SM uses reserved Contention 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 Contention slots configured to still successfully transmit
bandwidth requests using unused data slots.
A higher number of Contention slots give higher probability that a
SM’s bandwidth request is correctly received when the system is
heavily loaded, but with the tradeoff that sector capacity is reduced,
so there is less capacity to handle the request. The sector capacity
reduction is about 200 kbps for each Contention slot configured in a
20 MHz channel at QPSK MIMO-A modulation. The reduction in
sector capacity is proportionally higher at MIMO-B modulations (2
times at QPSK MIMO-B, 4 times at 16 QAM MIMO-B, 6 times at 64
QAM MIMO-B and 8 times at 256 QAM MIMO-B). If very few
reserved Contention slots are specified, then latency increases in
high traffic periods. If too many are specified, then the maximum
capacity is unnecessarily reduced.
The suggested Contention 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 Contention 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 Contention slots.
In a typical cluster, each AP must be set to the same number of
Contention 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 Contention slots
may provide better results. For APs in a cluster of mismatched
Contention slots setting, or where PMP 450i is collocated with radios
using different technologies, like PMP 430 or FSK, in the same
frequency band, use the frame calculator. To download the PMP 450i
Contention Slots Paper, see
http://www.cambiumnetworks.com/solution-papers/pmp-450-
contention-slots.
pmp-0957 (April 2015) 113