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

• Same fraction of ACKs received: packet loss due to congestion

• Acknowledged fraction significantly different, random loss

Control connection

user connection

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

• Not efficient

TCP

DLC with strong FEC

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 end-to-end semantics violated

• Huge buffer at base station

TCP TCP or other protocols

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

• Function for snoop module:– Buffer new packets from sender

– Suppress duplicated ACK and retransmit lost packets

TCP

Snoop module

11

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

12

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

13

An example of zigzag code1

11

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

14

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

15

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

16

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

17

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.

18

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

19

Performance Evaluation

• Additive White Gaussian channel

• Server-client connection: 32Mbytes, packet size: 12,000bits

• Goodput measurement

100 m200 m

1.544Mbps1.544Mbps

20

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

21

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

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

24

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

25

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

26

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

27

-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.

28

Conclusions

• Data link layer protocol for TCP over wireless links proposed– Hybrid ARQ with limited retransmission times– Adaptive

29

Thank You