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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 Outline Introduction Self-synchronised Packet Transfer mechanism Performance evaluation Summary PGTS’2006 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 PGTS’2006 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. D D Buffer M U X D PHY M U X wired system D1 PHY Buffer MAC MAC Buffer MAC MAC Buffer MAC MAC PGTS’2006 Buffer MAC MAC D2 WLAN system Introduction (3) Other solutions: 1. 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) 2. To tune MAC protocol parameters (e.g. congestion window, inter-frame space) to differentiate packet handling (EDCF) PGTS’2006 delay variation can be limited but not eliminated gives only „relative” QoS not available in existing devices (possible in 802.11e) 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 station 1 station 2 station n Access Point CBR application CBR application CBR application n CBR applications IP Layer IP Layer IP Layer IP Layer SPT MAC layer SPT SPT SPT SPT MAC layer MAC layer MAC layer SPT PHY layer PGTS’2006 Uplink transfer Downlink transfer 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 n 1 CONF d initial T n TT D T TCONF – time when previous packet was confirmed TT –packet transmission time in WLAN n ARRIV D- CBR inter-arrival time TARRIV – packet arrival time 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 PGTS’2006 SPT mechanism (3) When all stations are synchronised we observe that: D D1 PHY Buffer MAC MAC Buffer MAC MAC Buffer MAC MAC Buffer MAC MAC D WLAN system D PHY SPT MAC MAC SPT MAC MAC SPT MAC MAC SPT MAC MAC D WLAN system with SPT mechanism D Buffer PGTS’2006 D2 M U X PHY M U X wired system Performance evaluation (1) Objective: Evaluate packet delay characteristics Evaluate time that required for SPT synchronisation Simulated system: PGTS’2006 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 Performance evaluation (2) Packet transfer delay characteristics (IPTD, IPDV) collected in standard and SPT enhanced WLANs collected after passing synchronisation phase 70 20 60 Standard 50 STP IPDV (ms) IPTD (ms) 15 10 Standard STP 40 30 20 5 10 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 1 3 5 7 9 11 13 15 17 Number of connections Number of connections Conclusions: SPT assures constant packet transfer delay (IPDV=0) Profit is especially visible in heavy load conditions PGTS’2006 SPT can handle a few more VoIP connection then standard WLAN Performance evaluation (3) SPT synchronisation time for N-th connection that arrived to the system with N-1 already synchronised connections Synchronisation time (ms) 500 95% limit (approx) 400 Average 300 200 100 0 0 5 10 15 Number of arriving connection Conclusions: In most cases SPT synchronisation time is lower then 100ms PGTS’2006 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: PGTS’2006 To analyse non-homogenous CBR sources To analyse impact of packet retransmissions