User's Manual

AP_CMM2 User Manual Issue 4 Draft Pa
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e 21 of 48
BANDWIDTH MANAGEMENT
Subscriber Module bandwidth management is set per Access Point. All Subscriber Modules which
register to an Access Point module will receive and use the same bandwidth management
information.
The software uses “token buckets” to manage each subscriber’s bandwidth. Each subscriber’s
bucket (actually two buckets, one for uplink and one for downlink) is constantly being filled with
tokens at the Sustained rate, up to the Burst size (the size of their bucket). When they use the
internet, they have full bandwidth until they “drain” their bucket, then they only have the Sustained
rate, until they quit draining their bucket, and let it refill a bit.
After a burst is fully or partially used, it then “recharges” at the Sustained rate. Short bursts
recharge quickly, often before the next burst. Large bursts take longer to recharge.
The way bandwidth management appears to the subscriber is that as long as they are doing
normal web browsing and e-mail handling, small file transfers, and short streaming video, they will
rarely be speed limited, depending on what the bandwidth management is set to. If they do large
downloads (software upgrades, streaming video, and so on), or a series of medium-size
downloads, they will have high bandwidth until they hit the burst limit, then drop down in speed to
the sustained setting. When they are idle, the burst limit will then “recharge” at the sustained rate.
To manage bandwidth separately for each SM, Canopy offers the Bandwidth and Authentication
Manager (BAM). The BAM supports per-SM setting of Sustained Uplink, Sustained Downlink,
Uplink Burst, and Downlink Burst, as well as SM authentication and user-specification of DES keys.
The BAM is a Canopy software product running on a networked Linux PC.
HIGH PRIORITY BANDWITH
To support traffic with a low latency requirement such as VoIP (voice over IP), the Canopy System
implements a high priority data pipe. This implementation does not affect the inherent latencies in
the Canopy System but allows high priority traffic to be serviced immediately. The high priority pipe
separates low latency trafiic from traffic that is latency tolerant such as standard web surfing and
file downloads. This traffics is separated by the Canopy System via the IPv4 TOS (type of service)
Low Latency bit. If this bit is set, the packet will be sent on the high priority pipe. This pipe is
serviced before any normal priority traffic.
The high priority system is enabled via four fields found in the Configuration web page. The fields
are:
• High Priority Uplink Percentage
• Uacks Reserved High
• Dacks Reserved High
• NumCtrlSlots Reserved High
The High Priority Uplink Percentage parameter describes the percentage of the uplink bandwidth
that will be dedicated to low latency traffic. When set, this percentage of RF link bandwidth will be
permanently allocated to low latency traffic regardless of the amount of this kind of traffic present.
There is no corresponding downlink parameter as this bandwidth is allocated on as-needed basis
by the scheduling algorithms.
The UAcks (Uplink ACK) Reserved High parameter describes the number of slots used to
acknowledge high priority data that is received by a subscriber module. The Canopy team
recommends that this parameter be set to 3 and then the TotalNumUAcksSlots parameter should
be set to 6.