A Method for Improving Transfer Quality of CBR Streams over Wireless LANS

1. A Method for Improving Transfer Quality of CBR Streams over Wireless LANS Jarosław Śliwiński, Wojciech Burakowski and Andrzej Bęben Telecommunication Network Technologies Group Institute of Telecommunications, Warsaw University of Technology, Poland

2. Outline • Introduction • Self-synchronised Packet Transfer mechanism • Performance evaluation • Summary

3. Introduction (1) Handling of constant bit rate (CBR) streams in Wireless LAN (WLAN) network is not solved in a satisfactory way • CBR streams (e.g. VoIP, VTC) require: • Low packet transfer delay (< 150ms) • Low delay variation (<50ms) • However, packets transferred in WLANs experience large delay variation about 20-30 ms

4. wired system WLAN system Introduction (2) What is the reason of large delay variation? • MAC (Medium Access) protocol that uses random access to the medium with the backoff procedure As a consequence packet multiplexing in WLANs is different comparing with a wired systems.

5. Introduction (3) Other solutions: • To extend MAC protocol with the polling mechanism that emulates TDMA system (PCF, HCCA) • needs complicated control mechanism for scheduling transfer of particular stations • in fact not available in existing devices (access point and WLAN cards) • To tune MAC protocol parameters (e.g. congestion window, inter-frame space) to differentiate packet handling (EDCF) • delay variation can be limited but not eliminated • gives only „relative” QoS • not available in existing devices (possible in 802.11e)

6. SPT mechanism (1) • Self-synchronised Packet Transfer (SPT): • is aimed to avoid transmission backofffs by synchronisation moments when stations submit packets to the MAC layer • is implemented on the top of MAC layer • in each station SPT introduces „initial delay” in order to assure that packets are injected to MAC only when medium is idle

7. TCONF – time when previous packet was confirmed D- CBR inter-arrival time TT –packet transmission time in WLAN TARRIV – packet arrival time SPT mechanism (2) • How SPT entity fixes its „initial delay” • Each SPT entity fixes its „initial delay” independently form the other stations • The SPT observes „the last transmission” and it fixes initial delay in such a way to start new transmission exactly one inter-arrival time (D) after the previous one • When initial delay does not change for a few consecutive packets SPT is regarded as synchronised • However, SPT may lose synchronisation when new call arrives

8. WLAN system WLAN system with SPT mechanism wired system SPT mechanism (3) When all stations are synchronised we observe that:

9. Performance evaluation (1) • Objective: • Evaluate packet delay characteristics • Evaluate time that required for SPT synchronisation • Simulated system: • One Access Point (802.11b) dedicated for handling VoIP traffic • A number of VoIP connections (G.729 codec, sending 60B packets every 20ms) • each connection is handled by single station

10. Performance evaluation (2) Packet transfer delay characteristics (IPTD, IPDV) collected in standard and SPT enhanced WLANs collected after passing synchronisation phase • Conclusions: • SPT assures constant packet transfer delay (IPDV=0) • Profit is especially visible in heavy load conditions • SPT can handle a few more VoIP connection then standard WLAN

11. Performance evaluation (3) SPT synchronisation time for N-th connection that arrived to the system with N-1 already synchronised connections • Conclusions: • In most cases SPT synchronisation time is lower then 100ms

12. Summary • The SPT mechanism: • assures constant packet transfer delay after passing synchronisation phase • synchronise in acceptable time (usually less then 100ms) • does not require modification of MAC layer • Further works: • To analyse non-homogenous CBR sources • To analyse impact of packet retransmissions