User guide

Understanding Canopy Networks March 2005
Through Software Release 6.1
Issue 1 Page 65 of 425
Canopy System User Guide
IMPORTANT!
See High Priority Uplink Percentage and Slot Specifications on
Page 191.
7.1.13 Allocations to Downlink and Uplink
The standard and high-priority channels in Canopy PTMP communications are
contrasted in Figure 26 and Figure 27.
Con-
trol
Beacon Data
AP Transmit (Downlink) AP Receive (Uplink)
DataAck
Con-
trol
Ack
Con-
trol
Beacon Data
AP Transmit (Downlink) AP Receive (Uplink)
DataAck
Con-
trol
Ack
Figure 26: Canopy channel, 75% downlink, 0% high priority in uplink
Con-
trol
HP
Beacon Data
AP Transmit (Downlink) AP Receive (Uplink)
DataAck
Con-
trol
Ack
HP
Data
HP
Con-
trol
Ack
Con-
trol
HP
Beacon Data
AP Transmit (Downlink) AP Receive (Uplink)
DataAck
Con-
trol
Ack
HP
Data
HP
Con-
trol
Ack
Figure 27: Canopy channel, 75 % downlink, 40 % high priority (HP) uplink, software scheduling
7.1.14 Software and Hardware Scheduling
In Release 6.0 and later, Canopy provides an alternative to software scheduling for
control of the links in a sector. Hardware scheduling increases throughput and reduces
latency in the link between the SM and AP.
With software scheduling and AP default downlink-to-uplink settings (75% downlink and
25% uplink), if High Priority is set to 25%, then
◦ in the uplink, the total of reserved slots is equivalent to 25% (2 slots in this
example) and
− the bandwidth is 64 bytes per slot, repeated 400 times each second.
− [2 slots/instance] x [64 bytes/slot] x [8 bits/byte] x [400 instances/second]
= 409,600 bps ≈ 400 kbps of uplink bandwidth
◦ in the downlink, the AP
− monitors the Low Latency TOS (Type of Service) bit, Bit 3, in the Ethernet
frame.
− does not reserve slots, but will service all high-priority bandwidth requests.
− can become saturated by attempting to service too much high-priority traffic.