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Voice transmission in an IEEE 802.11 WLAN based access network. Andreas Koepsel # , Adam Wolisz Telecommunication Networks Group Technical University Berlin www-tkn.ee.tu-berlin.de # Now co-founder of Acticom R&D (Start-up fever in Berlin!) Koepsel@acticom.com.
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Andreas Koepsel#, Adam Wolisz
Telecommunication Networks Group
Technical University Berlin
# Now co-founder of Acticom R&D
(Start-up fever in Berlin!)
Usage of a single technology for voice and data avoids expensive parallel infrastructures
Suitability of IEEE802.11 WLAN for parallel use for data and voice streams
Distributed vs. Centralized coordination schemesMotivation
All mobile nodes (MN) create best-effort traffic
A subset of the wireless terminals generates voice calls to destinations outside of the local environmentArchitectural Scenario
maximum end-2-end delay limitations of ~250-300ms
Maximum loss rate rate (dependent on coding).
Queueing and Dropping:
An audio packet that exceeds its maximum lifetime is dropped hence reducing the overall offered load
The lifetime varies for different audioconnections (different destinations)Audio source modeling and QoS requirements
Packet interarrival times from Pareto distribution
Packet length distribution from packet trace (Harvard university)
Packet length maximum 1500bytes
IEEE 802.11b DSSS system with up to 11Mbit/s
IEEE 802.11a OFDM system with 24Mbit/s mandatory and up to 54Mbit/s optional
Gilbert-Elliott- based channel error model independent channels among every pair of mobilesBest Effort / Physical Layer
Link speeds:2, 11, 24 and 54Mbit/s
Bit error rates from 10-7 up to 10-3Audio transmission with DCF – Delay(1)
Probability density function of Channel Access Delay at 11Mbps PCF
Probability density function of Channel Access Delay at 11Mbps - DCF
An idea: Why not remove the station from the polling cycle if there are no data, and introduce it again after a first packet has been tranmitted in DCF?
But: there might be losses of the polling frame...
RTP fileds SSRF id (for flow identification) and the MORE flag migh be used to decide about the continuation of the flow.Additional Improvement
DCF wastes capacity when used with improved coding schemes (higher bit rates!).
PCF provides lower delay and lower delay variance to audio flows as compared to DCF.
The PCF overhead plays a less significant role for higher data rates.
PCF performance might be further improved by introducing a flow aware scheduling policy.Conclusion
More insight in different load scenarios (best effort vs. RT, number of active stations)
Differentiation of packet importance for improved scheduling ... Papers in ACM MM2001 ans SPIE Denver, August 2001Further Research...
Priority in preserving packets
EMBSD Perceptual Distortion
drop probability p0