User Manual

Rev: 29 April 2005
3.7 Performance parameters
When a successful Bluetooth connection has been established to a remote device; the serial port plug can
be optimized for either throughput or latency mode. The throughput/latency mode configuration allows the
user to fine-tune the low-level settings associated with the UART.
Throughput mode attempts to maximize the throughput at the cost of moderate latency. If low latency is
more important than maximizing throughput, then latency mode may be selected instead.
When optimized for latency - the UART driver is more aggressive at looking for subsequent data bytes at
the cost of an increased CPU load; hence the tradeoff against throughput.
Optimization of the plug for latency or throughput was added in the 3.0 version of the serial port plug
software.
The Link Manager of the Serial port plug firmware also provides Quality of Service (QoS) capabilities. “Best
effort” and “Guaranteed” service types are supported.
A poll interval, which is defined as the maximum time between subsequent transmissions from the master
to a particular slave on the ACL link, is used to support bandwidth allocation and latency control. The poll
interval is guaranteed in the active mode except when there are collisions with page, page scan, inquiry and
inquiry scan. The poll interval is also known as T
poll.
Connections are given a “Best effort” service type with a Tpoll value of 40 slots (25msec), by default, when
created by serial port plug running as master (connecting).
If a different service type and/or Tpoll value for the established connection is desired, a QoS_Setup
command must be sent to request other parameters to be used instead. The Quality Of Service parameters
in the configuration software specify the parameters that will be used in the QoS_Setup command. The
QoS_Setup command (if enabled) will be sent when a successful connection is established and can be sent
by the master as well as the slave. The requested parameters can be rejected, e.g., if QoS_Setup is sent
on a slave the master may reject the change. The user can only set the requested parameters for the link, it
does not know the actual parameters negotiated.
Being able to adjust the link's Tpoll value allows reduction of the maximum latency for starting data
transfers from slave to master at the cost of extra polling (and thus power consumption) on the master.
The service mode and latency parameters of the QoS_Setup command can be configured, the other
parameters are not configurable and are set to its default values.
The latency parameter is translated from microseconds to piconet slots (rounded down) and used in an
attempt to set the connection's Tpoll value. The Tpoll value is also subjected to some limits.
The service mode parameter indicates the level of service required.
If “Best effort” is selected, the service type does not require any guarantees. The fields of the QoS_Setup
should be treated as hints by the remote device. The remote device may choose to ignore the fields, try to
satisfy the hint or respond with the settings it will try to meet.
If “Guaranteed” is selected, the remote device will “guarantee” the latency. This allows the master to boost
the connection's priority when using sniff mode, allowing for short, tight sniff timing.
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