資料通訊
Week 2 (9/20)
Instructor: Ling-Jyh Chen
General Information
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Time: Tuesday 2:10pm~5:00pm
Place: S203
Instructor: 陳伶志 Ling-Jyh Chen
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TA: 李威賢 walker1202@gmail.com
Course Website:
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Phone: 02-27883799 ext. 1702
Email: cclljj@iis.sinica.edu.tw
http://www.iis.sinica.edu.tw/~cclljj/teaching/
Google Groups: NTNU-2011-data-communication
Textbook
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Required:
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Data Networks, 2nd edition, Dimitri
Bertsekas and Robert Gallager,
Prentice Hall, 1992. (新月圖書代理)
Recommended:
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
An Engineering Approach to
Computer Networking,
S. Keshav, Addision-Wesley.
Computer Networks, 4th ed.,
Andrew S. Tanenbaum,
Prentice-Hall.
Schedule
Grading
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In-Class Participation: 5%
Homework: 30%
Midterm (closed book): 30%
Final (open book): 35%
Week 2
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Textbook: Section 2.3, 2.4
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Additional materials
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FEC
Adaptive ARQ
6
Sec 2.3 Error Detection
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Question: How to detect errors?

Error Detection Techniques
1. Single Parity Checks
The parity check bit (c) is the sum k, modulo j, of the bit values
in the original bit string.
2. Horizontal and Vertical Parity Checks


Arrange a string of data bits in a two-dimensional array.
Apply single parity checks to each row and column of the 2D
array.
Ex: Cyclic Redundancy Checks (CRC)
7
Sec 2.3 Error Detection (cont.)
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
code: refers to the transmission itself
code word: refers to an encoded bit string

(data K + parity checks L)
K + L = frame length

The effectiveness of a code is measured by
1. The min distance d of the code (ps: 2/4 for
method 1/2) => error correction can be possible
if there are fewer than d/2 errors
2. The burst-detecting capability
3. The prob. that a completely random string will be
accepted as error-free
8
Sec 2.4 ARQ
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Question: how to deal with errors?
ARQ: Automatic Retransmission reQuest
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retransmit the erroneous packets/frames
There is always a trade-off for data
transmission: correctness vs efficiency
Three types of ARQ
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Stop and wait ARQ
Go back n ARQ
Selective repeat ARQ
9
Sec 2.4 ARQ (cont.)
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Stop and wait ARQ (1/4)
Time at A
Node A
Ack
Node B
Packet 0
Time at B
Packet 0 or 1?
* We need Sequence Number (SN) in the data
packets.
10
Sec 2.4 ARQ (cont.)

Stop and wait ARQ (2/4)
0
0
1
?
? Time at A
2
Node A
Ack
Node B
Packet 0
Ack
Nak
Time at B
* We need Request Number (RN) in the ACK
packets.
11
Sec 2.4 ARQ (cont.)
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Stop and wait ARQ (3/4)
0
0
1
2
3
Node A
Node B
1
0
1
2
1
3
2
3
12
Sec 2.4 ARQ (cont.)
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Stop and wait ARQ (4/4)
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
S&W ARQ is safe: it never produces
incorrect results.
S&W ARQ is live: it can continue forever
to produce results.
0,0
Even numbered
packet received at B
Request for even
packet received at A
1,0
0,1
Odd numbered packet
received at B
Request for odd
packet received at A
1,1
state: (SN mode 2, RN mode 2) 13
Sec 2.4 ARQ (cont.)
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Go back n ARQ (1/6)
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S&W ARQ is simple, but ineffective.
Go back n ARQ is the most popular
scheme.
Idea: several successive packets can be
sent w/o waiting for the next packet to be
requested.
a.k.a. sliding window ARQ
14
Sec 2.4 ARQ (cont.)
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Go back n ARQ (2/6)
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No errors (n=7)
[0,6]
0
1
2
3
[1,7]
[2,8]
4
5
[3,9]
[5,11]
6
Node A
Node B
0
0
0
1
1
2
2
5
3
3
4
5
5
15
Sec 2.4 ARQ (cont.)
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Go back n ARQ (3/6)
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Errors in the forward direction (n=4)
timeout
[0,3]
0
1
[1,4]
2
3
[2,5]
4
1
2
3
4
Node A
Node B
0
1
0
1
1
1
1
2
1
3
2
3
16
Sec 2.4 ARQ (cont.)
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Go back n ARQ (4/6)
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Errors in the backward direction (n=4)
timeout
[0,3]
0
1
[2,5]
2
3
4
[4,7]
5
2
[5,8]
4
5
Node A
Node B
0
1
0
2
1
3
2
4
3
5
4
5
6
17
Sec 2.4 ARQ (cont.)
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Go back n ARQ (5/6)
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Long delay in the backward direction (n=4)
timeout
[0,3]
0
1
[1,4]
2
3
[3,6]
4
1
3
4
[4,7]
3
4
5
Node A
Node B
0
1
0
1
2
3
4
5
18
Sec 2.4 ARQ (cont.)
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Go back n ARQ (6/6)
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Go back n ARQ is safe and live when SN
and RN are integers modulo m, for m > n.
The timeout value should be chosen long
enough to include round-trip propagation
and processing delay plus transmission
time for two MAX length packets in the
reverse direction.
Go back n ARQ with buffers -> Selective
repeat ARQ
19
Sec 2.4 ARQ (cont.)
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Selective repeat ARQ
0
1
…
b
0
b+1
…
2b
0
b+1
Node A
Node B
0
…
0
0
…
0
1
0
1
0
…
0
0
…
0
b
…
b
0
0
b+1
…
2b
2b+1
Node A
Node B
0
0
1
…
0
b
0
0
0
b+1
…
20
FEC: Forward Error Correction
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FEC: using redundancy to provide capabilities of
correcting minor bit errors in a packet
 Example:
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IEEE 802.11a, Bluetooth
Pros: using FEC to correct minor errors in a
packet -> reduce the number of retransmissions
Cons: FEC causes overhead -> reduce
effectiveness
Solution: adaptation!!
21
Adaptive ARQ
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We use Bluetooth as the examples.
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Example 1: Adaptive Packet Type (ICC’04)
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Example 2: Adaptive ARQ Timeout (MNSA’04)
22
Enhancing Bluetooth TCP Throughput
via Packet Type Adaptation
Ling-Jyh Chen, Rohit Kapoor, M. Y. Sanadidi, Mario Gerla
Dept. of Computer Science, UCLA
Outline of the Talk
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The problem: Wireless interference and bit errors
severely affect TCP efficiency.
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The opportunity: Bluetooth offers multiple packet type
options with different FEC and packet lengths. Moreover,
the link layer API provides link error quality information.
Opportunity for cross-layer adaptation.
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Key idea: dynamically select packet type based on
measured link quality.
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The results: we show that the “Adaptive Packet Type”
approach in Bluetooth can effectively enhance TCP
performance.
24
Who still remembers Bluetooth?
Personal Area Network
Designed for “cable”
replacement
Application Examples
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Automatic synchronization of calendars, address
books, business cards
Proximity operation (camera to cellphone, etc)
25
Bluetooth Overview
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The “enabler” of Personal Area Network (PAN)
Low power, low cost, and smaller chips
Packet
Mode
FEC
Size
(bytes)
Length
(slots)
Symmetric
Throughput
(kbps)
Asymmetric
Throughput
(kbps)
DM1
yes
17
1
108.8
108.8
108.8
DM3
yes
121
3
258.1
387.2
54.4
DM5
yes
227
5
286.7
477.8
36.3
DH1
no
27
1
172.8
172.8
172.8
DH3
no
183
3
390.4
585.6
86.4
DH5
no
339
5
433.9
723.2
57.6
26
Bluetooth packet types
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DH: Stop and Wait ARQ
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DM: ARQ as well as 2/3 FEC codes to correct single bit errors
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FEC coding scheme:
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(15, 10) Hamming code,
each block of 10 information bits is encoded into a 15 bit codeword
can correcting a single bit error in each block.
27
Throughput Analysis
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DH mode: (ARQ)
PER: P  1  (1  B)
S
T '  T  (1  P)  T  ( 1  B)S kbps
1 hop Throughput:
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DM mode: (ARQ+FEC)
PER: P  1  (1  B)  15B(1  B) 
14 S / 15
15
1 hop Throughput:

T '  T (1  B)15  15B(1  B)14

S / 15
kbps
P: Packet Error Rate, B: Bit Error Rate, S: Packet Size, T: Max Throughput
28
PER vs BER
29
Bluetooth Throughput
Mode
BER range
DH5
<0.0001529
DM5
>0.0001529, and <0.0060795
DM3
>0.0060695, and <0.0157813
DM1
>0.0157813
30
Proposed Approach
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Adaptive Packet Type (APT):
 In
BT specs, the function call, Get_Link_Quality,
returns the Quality of the specified Link.
 We
read the returned link Quality Value, and
adapt packet type so as to optimize throughput.
31
Simulation 1: Fixed BER
Time: 600 seconds
TCP Packet Size: 500 bytes
Buffer Size: 9000 bytes
32
Simulation 2: Varying BER
Time: 600 seconds
TCP Packet Size: 500 bytes
Buffer Size: 9000 bytes
BER: changes between 0.0001 and
0.0005 every 1 second
33
Conclusions
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In Bluetooth, TCP throughput collapses with BER above
0.03% (eg, BER caused by near 802.11 interference)
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APT (Adaptive Packet Type) approach can restore TCP
throughput to acceptable values for much higher BER
(we tested up to .3%)
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APT technique can be applied to any wireless link with
packet length and FEC options, and with link quality (ie
BER) feedback.
34
Audio Streaming over Bluetooth:
An Adaptive ARQ Timeout Approach
Ling-Jyh Chen, Rohit Kapoor, Kevin Lee,
M. Y. Sanadidi, Mario Gerla
Department of Computer Science, University of California at Los Angeles
Introduction
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Wireless PAN is becoming popular nowadays.
Digital media, such as audio/video streaming, is
becoming more desired than before.
The varying nature of the wireless link makes
audio streaming over wireless become a
challenging problem.
A well-designed solution should minimize the
packet loss rate and delay of perceived
streaming data.
36
Retransmission - ARQ
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Stop-and-wait ARQ: retransmit a packet
until either the acknowledgement of a
successful reception is received or the
retransmission timeout is exceeded.
Decrease delay
Low
Increase reliability
Retransmission timeout
More packets will be dropped, and the
streaming quality will become poor
High
Enlarge the delay, and degrade
the streaming quality
37
Proposed Approach
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Adaptive ARQ Timeout in Bluetooth
 Problem:
The deployed stop and wait ARQ is harmful
to real-time streaming audio over Bluetooth,
especially in the error prone environment.
 Aim: to reduce the audio packet delay and loss rate
 Ideas:
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If we spent too much time on sending packet n, then we
should decrease RTO on packet n+1
If we save some time on sending packet n, then we should
increase RTO on packet n+1
If we drop at least two of the last 5 packets, then we should
reset RTO
38
RTO Update Equation
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Initialization
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Update
SRTT  RTT
RTO  80 Max((MaxBufferSize  UsedBufferSize)  75%,2)
SRTT'  (1   )  SRTT    RTT
  RTO ; if RT T  SRT T
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RTO'     RTO; if RT T  SRT T
 RTO; if packetdropped
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min  RTO  80 ( MaxBufferSize  UsedBufferSize)  75%
  0.25 ;   1.1 ;   0.9
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Reset
RTO  80 Max((MaxBufferSize  UsedBufferSize)  75%,2)
39
Implementation
Audio Client
RTP Packet
Average Delay
RTP Packet Loss Rate
BlueZ
Audio Server
128kbps MP3
80 ms/RTP packet
BNEP
BNEP
L2CAP
L2CAP
HCI
HCI
RTP Packet Buffer
Flush Timeout Packets
Bluetooth
Device
Baseband
Baseband
RTP Packet RTT
DH5 packet
Link Quality
40
Experiments
41
1400
RTT
RTO
drop
1200
1000
800
600
400
200
0
400
450
500
550
600
RTP seqno
42
100
90
80
70
60
50
40
30
Adapt pkt
Fixed 160 pkt
20
Fixed 400 pkt
Fixed 1200 pkt
10
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0.0045
Bit Error Rate
43
400
Adapt delay
Fixed 160 delay
Fixed 400 delay
350
Fixed 1200 delay
300
250
200
150
100
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
0.0045
Bit Error Rate
44
Conclusion
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Such cross-layer optimization can
significantly improve application
performance in PAN.
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Our proposed approach is simple and
applicable to other wireless technologies.
45
Summary
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In today’s lecture, we have studied
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Error detection schemes
Three basic ARQ schemes
Two adaptive schemes by combining ARQ
and FEC
What’s next?
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We will discuss “delay model” in the next
three weeks.
46