1
Data link layer protocol for wireless TCP
K.S. Chan
EEE Department
The University of Hong Kong
2
Outlines
• Introduction
• Multi-dimensional zigzag code
• Data link layer protocol
• Performance evaluation
• Automatic adaptation
• Conclusions
3
Introduction• Packet loss in wireless TCP:
– congestion: TCP reacts properly– Random loss: reducing congestion window will cause
performance degradation• New schemes differentiating congestion packet loss and random loss
needed
• Existing schemes:– Wireless aware TCP: two connections
• TCP modification needed
– Wireless unaware TCP:• Link layer: airmail, unlimited ARQ• Split connection: I-TCP, M-TCP• Proxy: snoop, new snoop
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TCP aware solution
• Same fraction of ACKs received: packet loss due to congestion
• Acknowledged fraction significantly different, random loss
Control connection
user connection
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Introduction• Packet loss in wireless TCP:
– congestion: TCP reacts properly– Random loss: reducing congestion window will cause
performance degradation• New schemes differentiating congestion packet loss and random loss
needed
• Existing schemes:– Wireless aware TCP: two connections
• TCP modification needed
– Wireless unaware TCP:• Link layer: airmail, unlimited ARQ• Split connection: I-TCP, M-TCP• Proxy: snoop, new snoop
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TCP unaware solution: airmail
• Not efficient
TCP
DLC with strong FEC
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Introduction• Packet loss in wireless TCP:
– congestion: TCP reacts properly– Random loss: reducing congestion window will cause
performance degradation• New schemes differentiating congestion packet loss and random loss
needed
• Existing schemes:– Wireless aware TCP: two connections
• TCP modification needed
– Wireless unaware TCP:• Link layer: airmail, unlimited ARQ• Split connection: I-TCP, M-TCP• Proxy: snoop, new snoop
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TCP unaware solution: I-TCP
• TCP end-to-end semantics violated
• Huge buffer at base station
TCP TCP or other protocols
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Introduction• Packet loss in wireless TCP:
– congestion: TCP reacts properly– Random loss: reducing congestion window will cause
performance degradation• New schemes differentiating congestion packet loss and random loss
needed
• Existing schemes:– Wireless aware TCP: two connections
• TCP modification needed
– Wireless unaware TCP:• Link layer: airmail, unlimited ARQ• Split connection: I-TCP, M-TCP• Proxy: snoop, new snoop
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TCP unaware solution: snoop
• Function for snoop module:– Buffer new packets from sender
– Suppress duplicated ACK and retransmit lost packets
TCP
Snoop module
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Introduction (cont’d)• Our scheme:
– Link layer– Hybrid ARQ with limited re-transmission times– No transport layer activities– Adaptive to time-varying channel conditions
TCP
DLC with limited retransmission of hybrid ARQ
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Multidimensional Zigzag coded(1,1)
d(1,3)d(1,4) d(1,5)
d(1,2)
p(1)
d(3,1)
d(3,2) d(3,3)d(3,4) d(3,5)
d(I,1)
d(I,2)d(I,3) d(I,4)
d(I,5)
d(2,2)
d(2,3)d(2,4)d(2,5) d(2,1)
p(I-1)
p(2)
p(3)
p(I)
Iiipjidip
jdp
J
j
J
j
,,3,2 2, mod )1(),()(
2 mod ),1()1(
1
1
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An example of zigzag code1
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0
0
p(1)=1
01
1 10
1 01
01
010
0 1
p(4)=1
p(2)=1
p(3)=0
p(5)=0
1 1
11
1P=[11010]
T
I=55
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Multi-dimensional zigzag code
D1
P1
J
ID2
P2
J
I DN
PN
J
I
D
J
I
P1 P2 PN
(a)
(b) )(DD nn
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DLC Protocol Description
User process
TCP
IP
Wireless DLC
Wireless MAC
High-speed radio
IP
Wireless DLC
Wireless MAC
High-speed radio
Mobile terminal Base station
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Hybrid ARQ for TCP connections
D
J
I
P1 P2 Pm1 Pm2 Pm2+1Pm1+1 PN
G1 G2 Gm
Level 1 Level 2 Level m
W-DLC header TCP packet CRC1 CRC2Physical preamble(a)
(b)
MAC header
CRC2Physical preamble MAC header
b n
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Operation• Estimate the allowed retransmission times• When a new packet is received, encode it N-
dimensional zigzag codeword, and divide the N parity vectors into m groups
• Transmission counter set to 0• Level 1 transmission: increase the transmission counter
by 1, and transmit the information matrix with parity vector group 1
• Level i, 1<i<m+1: increase the counter by 1. If the retransmission limit is met, end. Otherwise, transmit the parity vector in group i.
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An example for ARQ
D
80
150
P1 P2 P7 P8 P9
G1 G2
W-DLC header TCP packetCRC1 CRC2Physical preamble(a)
(b)
W-MAC header
CRC2Physical preamble W-MAC header
b 12,000 G1
G2
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Performance Evaluation
• Additive White Gaussian channel
• Server-client connection: 32Mbytes, packet size: 12,000bits
• Goodput measurement
100 m200 m
1.544Mbps1.544Mbps
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Goodput for different SNRs
Fig. 1 Comparison of throughtput of vary SNR values
0
200,000
400,000
600,000
800,000
1,000,000
1,200,000
1,400,000
1,600,000
21.0 22.0 23.0 24.0 25.0 26.0 27.0 28.0 Time (min)
Goodput(bit/sec)
Error FreeDLC at SNR2DLC at SNR4DLC at SNR5no DLC at SNR 14
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Goodput for different coding schemes SNR: 5 dB
Fig.2 Comparison of throughput of different coding rate
0
200,000
400,000
600,000
800,000
1,000,000
1,200,000
1,400,000
1,600,000
21.0 22.0 23.0 24.0 25.0 26.0 27.0 28.0
Time (min)
Goodput(bit/sec)
Coding Rate1: 600row*20colCoding Rate2:300row*40colCoding Rate3:150row*80colCoding Rate4:500row*24col
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Automatic Channel Adaptation
• Small number of possible states
• A state suitable for a wide range of channel conditions
• Un-match detectable
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Adaptation
• 8 states• Two level transmission• criteria for state change:
– If consecutively 100 information packets can be correctly decoded by only (L1-1) parity check vectors, increase the state by 1
– Information part of 5 packets out of 100 consecutive packets needed to be retransmitted, decrease the state by 1
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state
1
2
3
4
5
6
7
8
I k1 k2 N
100
100
100
50
50
25
20
20
5
7
4
5
4
5
9
3
4
7
5
7
6
6
2
3
1 4
7
2
2
1
2
1
Table 1: the encoder states for non-real-time services
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Example for adaptation
• A FEC connection• if 3 packets out of consecutively 60 packets are
discarded, state is decreased by 1• if consecutively 40 packets can be decoded with
less than Nr parity vectors, state is increased by 1• simulation condition:
– SNR=5.4dB, coding state: 1 (coding rate: 0.36)
– SNR change 0.1dB roughly per 150 packets sent
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state
1
2
3
4
5
6
7
I Nr
100
100
50
50
25
20
20
6
9
7
7
7
5
4
Table 2: the encoder states for real-time services
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-2
-1
0
1
2
3
4
5
6
1 2001 4001 6001 8001 10001 12001 14001
time (number of packets)
sig
na
l-to
-no
ise
ra
tio
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
co
din
g r
ate
SNR
coding rate
Encoder’s adaptation in the time-varying channel.
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Conclusions
• Data link layer protocol for TCP over wireless links proposed– Hybrid ARQ with limited retransmission times– Adaptive
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Thank You
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