IEEE 802.3
Carrier Sense Multiple Access with
Collision Detection (CSMA/CD)

All rights reserved. No part of this publication and file may be reproduced, stored in a retrieval system,
or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise,
without prior written permission of Professor Nen-Fu Huang (E-mail: nfhuang@cs.nthu.edu.tw).
Ethernet - ‹#›/21
Typical CSMA/CD Network
B
收發器
D
F
終端器
同軸電纜
收發器電纜
A
C
E
終端器
Ethernet - ‹#›/21
IEEE 802.3 Architecture
OSI參考模式
CSMA/CD
應用層
表達層
高層通訊協定
會議層
傳輸層
網路層
邏輯鏈結控制
LLC (IEEE 802.2)
媒介擷取控制
MAC
實體層訊號處理 PLS
鏈結層
實體層
PMA
AUI
MAU
MDI
傳輸媒介
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CSMA/CD Protocol
Carrier Sense before transmission
Carrier Sense while transmission
Collision: Two or more stations transmitting
simultaneously
Backoff: Random delay after collision
Deference: Defers transmission if channel is
sensed busy
Collision Window (Slot time): Round-trip
propagation delay time plus some carrier sense
time. In IEEE 802.3, this value is defined to be
51.2 us.
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CSMA/CD Collision Handling
Collision Signal is generated by Physical
layer.
Jam signal (collision enforcement): To make
sure that all stations involved in the collision
will detect collision. A pattern of 32 bits.
Collision backoff and retransmission method
(Truncated Binary Exponential Backoff
Algorithm, BEBA):
 n : number of collisions experienced (n <= 16)
 k : Min (n,10) -- Truncation
 r : Random delay time (unit: slot time) between 0
<= r < 2k
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CSMA/CD Collision Handling
Slot time = 51.2 us.
Disadvantage of BEBA:
Last-in-First-out effect: Stations with no
or few collisions will have a better chance
to transmit before stations that have
waited longer.
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IEEE 802.3 Frame Format
7
1
Preamble SFD









2, 6
DA
2, 6
SA
2
LEN
4
LLC
PAD
位元組
FCS
Preamble: (101010...1010) for Synchronization
I/G
15-位元地址
SFD: Start Frame Delimiter (10101011)
DA: Destination Address
I/G U/L
46-位元地址
SA: Source Address
Length: Length of LLC-Frame
I/G = 0 Individual Address （個別地址）
LLC-Frame: Up to 1500 bytes
I/G = 1 Group Address （群體地址）
PAD: Padding when LLC-Frame < 46 bytes
U/L = 0 Globally Administered Address （整體性地址）
U/L = 1 Locally Administered Address （區域性地址）
FCS: Frame Check Sequence (CRC-32)
MAC-frame size -- from DA to FCS
 Min 64 bytes to distinguish from collision
 Max 1518 bytes to prevent dominating bandwidth
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Cyclic Redundancy Check
(CRC32) Hardware
Implementation
X0
X1
X8 X9
X7
17
X 16 X
X 18 X19 X20 X 21
X 26 X 27 X 28 X
29
MUX
B
X4
X5 X6
X 10
X 11
X 12 X 13 X 14 X 15
X 22
X 23 X24 X 25
.
X 30 X 31
.
A
C
X2 X3
= AND
= XOR
= NOT
MUX
O = A, C = 0
輸出
O = B, C = 1
控制訊號
輸入
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Flow
Chart
Y
傳送訊框
接收訊框
組成訊框, i=1
開始接收
線上有
訊號
接收完畢
Y
N
傳送訊框
發生衝撞
訊框過短
（衝撞）
送出擾亂訊號
N
傳送完畢
Y
N
Y
Y
N
N
i=i+1
位址辨識
成功
N
Y
Y
i > 16
N
N
尚有多餘
位元
Y
計算延遲時間
檢查碼
正確
Y
長度欄位
內容正確
N
Y
刪除訊框頭尾
等待延遲時間
傳送成功
傳送失敗
訊框錯誤
對齊錯誤
接收成功
長度錯誤
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Collision Detection Window
for Baseband CSMA/CD (=2a)
a = 0.5, 傳輸時間 ＝ 1
t0
A
B
Ａ 開始傳送訊框
t0+a-e
A
Ｂ 開始傳送訊框
B
衝撞
t0+0.5
A
Ｂ 偵測出衝撞
B
t0+1-e
A
Ａ 偵測出衝撞
B
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Cable Signaling
(Manchester Encoding)
Idle
1
0
1 ... 0
Idle
Data
Preamble
1
1
1 ... 0
0
Coaxial Cable
0V
-0.225V
-1.825V
Idle
1
0
1 ... 0
Idle
Data
Preamble
1
1
1 ... 0
0
Transceiver Cable
+0.7V
0V
-0.7V
100ns
50ns
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Network Configuration
Example 1 (Single segment)
同軸電纜區段（最長 500 公尺）
C
B
E
終端器
同軸電纜
（最長 50 公尺）
收發器電纜
A
（每區段最多接 100 個）
收發器
D
終端器
Ethernet - ‹#›/21
Network Configuration
Example 2 (Two segments)
同軸電纜區段（最長 500 公尺）
B
A
C
第一段同軸電纜
收發器電纜（最長 50 公尺）
D
訊號增益器
E
F
第二段同軸電纜
G
H
I
同軸電纜區段（最長 500 公尺）
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Network Configuration Example
3 (Five segments, maximum)
A
1
D
區段 1
區段 2
B
E
C
F
訊號增益器
2
H
G
3
區段 3
4
L
半訊號增益器
5
I
J
區段 4
K
半訊號增益器間電纜
（最長 1000 公尺）
區段 5
M
N
6
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Architecture, Functions, and
Implementation
工作站界面
高層通訊協定
網路控制卡
訊框包裝
鏈結管理
鏈結層
編碼及解碼
實體層
收發器電纜
收發器
傳送及接收
銅軸電纜
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Calculation
of Slot time
(Example 3)
元件
元件
穩定延遲
編碼器
0.1us
收發器電纜
前進路徑 回程路徑
元件
元件數
元件數
啟動延遲
0.1us
整體延遲
5
5
2.0us
5.13ns/公尺 0
300公尺
300公尺
3.08us
收發器（傳送路徑）
0.05us
0.3us
3
3
2.10us
收發器（接收路徑）
0.05us
0.6us
3
0
1.95us
收發器（衝撞路徑）
0
0.9us
0
3
2.70us
同軸電纜
4.33ns/公尺 0
1500公尺
1500公尺
12.99us
半訊號增益器間電纜
5.13ns/公尺 0
1000公尺
1000公尺
10.26us
電纜驅動器
0.1us
0
2
2
0.40us
電纜接收器
0.1us
0
2
2
0.40us
訊號增益器（增益路徑）
0.2us
0.4us
2
0
1.20us
訊號增益器（衝撞路徑）
0.2us
0.2us
0
2
0.80us
載波感測
0
0.2us
5
0
1.00us
衝撞偵測
0
0.2us
0
5
1.00us
訊號上升時間
0
0.1us
3
0
0.30us
0
2.0us
0
3
6.00us
0
0.2us
0
1
0.20us
（至 70% 於500公尺處）
訊號上升時間（由 50%
至 94% 於 500公尺處）
衝撞分割
訊號來回傳遞最長延遲
46.38us
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10BaseT Ethernet Examples
10BASE5 界面 10BASE2 界面
10BASE5 界面 10BASE2 界面
10BASET
集線器（ＨＵＢ）
10BASET
集線器 (HUB)
H
A
B
F
D
C
G
串聯線
A
B
D
C
E
無遮蔽式雙絞線(UTP)
最長 100 公尺
E
F
G
H
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Half-duplex MAC
 Ethernets have always used a half-duplex MAC.
 Efficient bidirectional communications is effected by
rapidly changing the direction of communication on
half-duplex channel.
 For Ethernet, this is not a problem due to stations can
quickly arbitrate for the ability to send their frames.
 However, the extension of the Ethernet MAC algorithm
to gigabit data rates does strain the ability to efficiently
operate in this mode.
 When the arbitration time (equal to the round-trip
delay, in the worst-case) approaches or exceeds the time
to transmit a typical frame, the efficiency of the
algorithm suffers.
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Limitations of Half-duplex
Operation
 CSMA/CD implies an intimate relationship between the
minimum length of a frame (L, measured in bit-times,
not absolute time) and the maximum round-trip
propagation delay (2a) of the network:
L > 2a
 If we want to allow very long networks, we can make
the minimum frame very long. Padding needs if a
station wishes to send less data than this minimum
frame. Reduces the transmission efficiency.
 If we want to avoid the overhead of padding, we must
reduce the extent of the network so that collision can be
detected in all cases.
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Limitations of Half-duplex
Operation
 For the original 10 Mbps Ethernet, a compromise was struck.
 Minimum frame = 512 bits (64 bytes), not including the preamble and
Physical Layer overhead.
 Minimum data field = 46 bytes rarely imposes a significant padding
overhead (IP header + TCP header = 40 bytes).
 At 10 Mbps, 512 bit-times is 51.2us. Depends on the type of cable used
and the network configuration, the extent of a 10 Mbps Ethernet can be
on the order of from 2-3 Km.
7
1
Preamble SFD
6
6
2
DA
SA
LEN
46
4
Data
FCS
bytes
Minimum Frame Length (512 bits)
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Network Extent
 For a given minimum-length frame, the extent of a
network scales inversely with data rate.
10,000 m
~ 2800m
1,000 m
~ 205m
100 m
~ 20m
10m
10Mbps
100 Mbps
1000 Mbps
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