Download On Performance of EQ-BGP in Providing end-to-end

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