Data link layer protocol for wireless TCP

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

4
TCP aware solution
Control connection
user connection

• Same fraction of ACKs received packet loss due
  to congestion
• Acknowledged fraction significantly different,
  random loss

5
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

6
TCP unaware solution airmail
TCP
DLC with strong FEC

• Not efficient

7
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

8
TCP unaware solution I-TCP
TCP
TCP or other protocols

• TCP end-to-end semantics violated
• Huge buffer at base station

9
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

10
TCP unaware solution snoop
TCP
Snoop module

• Function for snoop module
• Buffer new packets from sender
• Suppress duplicated ACK and retransmit lost
  packets

11
Introduction (contd)

• 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
12
Multidimensional Zigzag code
d(1,1)
d(1,2)
d(1,3)
d(1,4)
d(1,5)
d(2,3)
p(1)
d(2,4)
d(2,5)
d(2,1)
d(2,2)
p(2)
d(3,2)
d(3,3)
d(3,4)
d(3,5)
d(3,1)
p(3)
p(I-1)
d(I,2)
d(I,3)
d(I,4)
d(I,1)
d(I,5)
p(I)
13
An example of zigzag code
1
0
1
1
0
p(1)1
1
I5?5
0
1
0
0
p(2)1
1
1
0
1
0
p(3)0
T
1
1
P11010
1
1
p(4)1
1
0
1
0
1
p(5)0
1
14
Multi-dimensional zigzag code
J
J
J
D1
P1
D2
DN
PN
P2
I
(a)
I
I
J
D
I
(b)
P1
P2
PN
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DLC Protocol Description
Mobile terminal
Base station
User process
TCP
IP
IP
Wireless DLC
Wireless DLC
Wireless MAC
Wireless MAC
High-speed radio
High-speed radio
16
Hybrid ARQ for TCP connections
?b
n
(a)
W-DLC header
TCP packet
CRC1
CRC2
Physical preamble
MAC header
(b)
CRC2
Physical preamble
MAC header
Level 2
Level m
Level 1
J
G1
G2
Gm
D
I
P1
P2
Pm1
Pm2
Pm21
Pm11
PN
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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, 1ltiltm1 increase the counter by 1. If
  the retransmission limit is met, end. Otherwise,
  transmit the parity vector in group i.

18
An example for ARQ
?b
12,000
G1
(a)
W-DLC header
TCP packet
CRC1
CRC2
Physical preamble
W-MAC header
(b)
CRC2
Physical preamble
W-MAC header
G2
G1
G2
80
D
150
P1
P2
P7
P8
P9
19
Performance Evaluation
1.544Mbps
1.544Mbps
100 m
200 m

• Additive White Gaussian channel
• Server-client connection 32Mbytes, packet size
  12,000bits
• Goodput measurement

20
Goodput for different SNRs
21
Goodput for different coding schemes SNR 5 dB
22
Automatic Channel Adaptation

• Small number of possible states
• A state suitable for a wide range of channel
  conditions
• Un-match detectable

23
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
k1
k2
I
N
1
2
100
7
9
2
100
5
6
1
3
100
4
6
2
5
2
4
50
7
4
1
5
50
5
2
25
6
5
7
20
4
7
3
7
8
20
4
3
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
• SNR5.4dB, coding state 1 (coding rate 0.36)
• SNR change 0.1dB roughly per 150 packets sent

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state
Nr
I
1
100
9
2
100
6
3
50
7
5
4
50
7
5
25
20
6
7
7
20
4
Table 2 the encoder states for real-time services
27
Encoders 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

29
Thank You