Ethernet

Transcript Ethernet

IEEE 802.3 Carrier Sense Multiple Access with Collision Detection (CSMA/CD)



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Ethernet ‹#›/21

Typical CSMA/CD Network

收發器終端器

收發器電纜

同軸電纜

D F E

終端器 Ethernet ‹#›/21

IEEE 802.3 Architecture

CSMA/CD OSI

參考模式應用層表達層會議層傳輸層網路層鏈結層實體層高層通訊協定邏輯鏈結控制媒介擷取控制實體層訊號處理

LLC (IEEE 802.2) MAC PLS PMA AUI MAU MDI

傳輸媒介 Ethernet ‹#›/21

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.

Ethernet ‹#›/21

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 < 2 k

Ethernet ‹#›/21

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.

Ethernet ‹#›/21

IEEE 802.3 Frame Format

7 1 2, 6 2, 6 2 4

位元組

Preamble SFD DA SA LEN LLC PAD FCS

        

Preamble: (101010...1010) for Synchronization SFD: Start Frame Delimiter (10101011) DA: Destination Address SA: Source Address Length: Length of LLC-Frame I/G I/G U/L 15-

位元地址

46-

位元地址

LLC-Frame: Up to 1500 bytes PAD: Padding when LLC-Frame < 46 bytes FCS: Frame Check Sequence (CRC-32) MAC-frame size -- from DA to FCS

  I/G = 0 Individual Address （個別地址） I/G = 1 Group Address （群體地址） U/L = 0 Globally Administered Address （整體性地址） U/L = 1 Locally Administered Address （區域性地址）

Min 64 bytes to distinguish from collision Max 1518 bytes to prevent dominating bandwidth

Ethernet ‹#›/21

Cyclic Redundancy Check (CRC32) Hardware Implementation

X 0 X 1 X 2 X 3 X 4 X 5 X 6 X 7 X 8 X 9 X 16 X 17 X 18 X 19 X 20 X 21 X 10 X 22 X 11 X 23 X 24 X 25 X 12 X 13 X 14 X 15 X 26 X 27 X 28 X 29 X 30 X 31 A .

C MUX O = A, C = 0 O = B, C = 1 B

控制訊號輸入

MUX

輸出

= AND = XOR = NOT

Ethernet ‹#›/21

Flow Chart

傳送訊框組成訊框

, i=1

接收訊框開始接收

Y N

線上有訊號

傳送訊框接收完畢

Y N

訊框過短（衝撞）

N Y

發生衝撞

送出擾亂訊號

傳送完畢

Y Y i = i + 1 i > 16 N

計算延遲時間

Y N

位址辨識成功

檢查碼正確

尚有多餘位元

長度欄位內容正確

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 Ethernet ‹#›/21

Collision Detection Window for Baseband CSMA/CD (=2a)

a = 0.5,

傳輸時間＝

1 t 0 A B

Ａ開始傳送訊框

t 0 +a-

Ｂ開始傳送訊框

衝撞

t 0 +0.5

A t 0 +1-

Ａ偵測出衝撞Ｂ偵測出衝撞

B B

Ethernet ‹#›/21

Cable Signaling (Manchester Encoding)

Idle 0V -0.225V

-1.825V

Preamble Idle +0.7V

0V -0.7V

Preamble 100ns 50ns Data Data Idle Idle Coaxial Cable Transceiver Cable

Ethernet ‹#›/21

Network Configuration Example 1 (Single segment)

終端器

同軸電纜區段（最長

500

公尺）

同軸電纜

A 50

公尺）

100

個）

D E

終端器 Ethernet ‹#›/21

Network Configuration Example 2 (Two segments)

同軸電纜區段（最長

500

公尺）

第一段同軸電纜

收發器電纜（最長

公尺）訊號增益器

第二段同軸電纜

D F G H

同軸電纜區段（最長

500

公尺）

Ethernet ‹#›/21

Network Configuration Example 3 (Five segments, maximum)

A 1

區段

1 B C

訊號增益器

2 G 3

區段

3 4 H D

區段

2 F J I

區段

4 K E

半訊號增益器間電纜（最長

1000

半訊號增益器公尺）

區段

5 N 6 L M

Ethernet ‹#›/21

Architecture, Functions, and Implementation

工作站界面高層通訊協定訊框包裝網路控制卡鏈結層鏈結管理實體層編碼及解碼收發器電纜收發器傳送及接收銅軸電纜 Ethernet ‹#›/21

Calculation of Slot time (Example 3)

元件元件穩定延遲編碼器收發器電纜收發器（傳送路徑）收發器（接收路徑）收發器（衝撞路徑）同軸電纜半訊號增益器間電纜電纜驅動器電纜接收器訊號增益器（增益路徑）訊號增益器（衝撞路徑）載波感測衝撞偵測訊號上升時間（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

0 0

元件啟動延遲

0.1us

5.13ns/

公尺

0.05us

0 4.33ns/

公尺

5.13ns/

公尺

0.1us

0 0 0.2us

0.2us

0 0.1us

0 0.3us

0.6us

0 0 0.9us

0 0 0.4us

0.2us

0.1us

2.0us

0.2us

前進路徑元件數回程路徑元件數

0 3 2 0 5 3 3 5 300

公尺

2 2 0 1500

公尺

1000

公尺

0 0

訊號來回傳遞最長延遲

5 0 0 2 0 3 0 5 300

公尺

2 2 3 1500

公尺

1000

公尺

3 1

整體延遲

6.00us

0.20us

46.38us

2.0us

3.08us

2.10us

1.95us

2.70us

12.99us

10.26us

0.40us

0.40us 1.20us

0.80us

1.00us

1.00us 0.30us

Ethernet ‹#›/21

10BaseT Ethernet Examples

10BASE5

界面

10BASE2

界面

10BASET

集線器

(HUB) A B C D F G H E

無遮蔽式雙絞線

(UTP)

最長

100

公尺

10BASE5

界面

10BASE2

界面

10BASET

集線器（ＨＵＢ）

A B C D E F G H

串聯線 Ethernet ‹#›/21

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.

Ethernet ‹#›/21

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.

Ethernet ‹#›/21

Limitations of Half-duplex Operation

   

For the original 10 Mbps Ethernet, a compromise was struck.

Minimum frame = 512 bits (64 bytes), Physical Layer overhead.

not including the preamble and Minimum data field = 46 bytes rarely imposes a significant padding overhead (IP header + TCP header = 40 bytes).

At 10 Mbps, 512 bit-times is on the order of from 2-3 Km .

51.2us

. Depends on the type of cable used and the network configuration, the extent of a 10 Mbps Ethernet can be 7 1 6 6 2 46 4 bytes Preamble SFD DA SA LEN Data FCS Minimum Frame Length (512 bits)

Ethernet ‹#›/21

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

Ethernet ‹#›/21

Ethernet

Transcript Ethernet

Typical CSMA/CD Network

IEEE 802.3 Architecture

CSMA/CD Protocol

CSMA/CD Collision Handling

CSMA/CD Collision Handling

IEEE 802.3 Frame Format

Cyclic Redundancy Check (CRC32) Hardware Implementation

Flow Chart

Collision Detection Window for Baseband CSMA/CD (=2a)

Cable Signaling (Manchester Encoding)

Network Configuration Example 1 (Single segment)

Network Configuration Example 2 (Two segments)

Network Configuration Example 3 (Five segments, maximum)

Architecture, Functions, and Implementation

10BaseT Ethernet Examples

Half-duplex MAC

Limitations of Half-duplex Operation

Limitations of Half-duplex Operation

Network Extent

Directory