User guide

Canopy System User Guide
pmp-0229 (Mar 2013)
99
Chapter 8: Meeting Link Requirements
AP-SM Links
APs communicate with SMs using a point-to-multipoint protocol. An AP-SM link has lower
throughput and higher latency than a backhaul link for two reasons:
• Many endpoints are involved.
• The bandwidth request and reservation process consumes bandwidth.
In the 900-MHz frequency band range, round-trip latency is typically 15 msec. In all other
frequency band ranges, round-trip latency is typically 6 msec.
At range settings of greater than 40 miles (64 km) in the 900-MHz AP, more time elapses between
transmit and receive cycles to compensate for greater air delay. In each frame, this reduces the
number of data slots, which slightly reduces the aggregate throughput of the link. However, the
throughput is as predictable as in other point-to-multipoint links.
Throughput is a factor of the Max Range parameter in the AP and is effective for all SMs,
regardless of their distance from the AP. Throughput includes all downlink data to all SMs and all
uplink data from all SMs that link to the AP. For throughput, see Table 14 on Page 63.
End user perspective of throughput is based on both bandwidth in the sending direction and the
return of TCP acknowledgement packets in the other. Where sufficient downlink bandwidth exists
to support downlink traffic and overhead, transient traffic congestion in the uplink can cause some
TCP acknowledgement packets to be dropped, and the end user to perceive a reduction in
throughput. This can also occur with sufficient uplink bandwidth and dropping acknowledgment
packets in the downlink.
However, a network operator can optionally enable the Prioritize TCP ACK parameter in the AP
and BHM, giving these packets priority over other packet types. This results in fewer of them
being dropped.
The effects of changing network conditions on PTMP throughput are indicated in
Table 24.