Fast Ethernet - 1

Fast Ethernet and

Gigabit Ethernet

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).

Fast Ethernet - 2

Fast Ethernet

The goal of the Fast Ethernet is to obtain an order of magnitude increase in speed over 10BaseT Ethernet (IEEE 802.3) while retaining the same wiring systems, MAC method, and frame formats.

In IEEE 802.3, the longest distance between two stations is 2.5km, with a slot time of 51.2us (512 bits for 10Mbps).

One way to increase the speed is to shorten the distance.

In Hub architecture, the distance between station and Hub is at most 100m and the round-trip distance between two stations is 400m.

A faster speed can be used to transmit frames under CSMA/CD protocol while keeping the minimum frame size as 512 bits.

In the standard, the speed is 100Mbps and denoted as 100BaseT.

Fast Ethernet - 3

Fast Ethernet Characteristics

100 Mbps IEEE 802.3 CSMA/CD frame format Medium

Twisted pair -- UTP, STP fiber

CSMA/CD Protocol, do not support priority scheme Do not support guaranteed delay service

Frames collision Bandwidth utilization is not guaranteed to be fair Low bandwidth utilization under heavy load Suitable for multimedia communications under moderate load Good fault tolerance

Hub architecture 100Base4T

Voice-grade Category 3 UTP. Use four pairs of twisted-pair wires. 100BaseX

Category 5 UTP, STP, or Fiber optic.

Fast Ethernet - 4

Communication Architecture

The Convergence Sublayer (CS) provides the interface between CSMA/CD MAC sublayer and PMD layer

IEEE 802.2 LLC

IEEE 802.3CSMA/CD MAC

ANSI X3T9.5 MAC

ANSI X3T9.5 PHY

IEEE 802.3 PLS

ANSI X3T9.5 PMD

CS

IEEE 802.3 MAU

Coaxial CableFiberSTP/UTP

FiberSTP/UTP

實體層

鏈結層

Fast Ethernet - 5

MII Interface Signalling

To support different transmission medium, a Media Independent Interface (MII) is defined between CS and PMD.

The major functions of the CS Convert the transmit and receive serial data streams at the MAC subl

ayer interface into and from 4-bit nibbles for transfer across the MII. Relay the carrier sense and collision detection signals generated by t

he PMD sublayer to the MAC sublayerTransmit clock(TxClk)

Transmit data (TxD{0..3})

Transmit enable (TxEn)

Carrier Sense (CRS)

Collision detect (CD)

Receive clock(RxClk)

Receive data (RxD{0..3})

Receive data enable (RDv)

Receive error (RErr)

Transmit data

Transmit enable

Carrier Sense

Collision detect

Receive data

Receive error

MAC CS PMD

CSMA/CD 次層界面 MII 界面

Fast Ethernet - 6

100Base4T

Category 3 UTP cable contains 4 separate twisted-pair wires. To reduce the bit rate used in each wire, all four wire pairs are

used in 100Base4T. For each direction of transmission, 3 pairs are used to transmit

the frame and the other pair is used for carrier sensing and collision detecting (if any).

載波感測衝撞偵測

載波感測衝撞偵測第一對雙絞線

第二對雙絞線

第三對雙絞線

第四對雙絞線

工作站至集線器集線器至工作站

工作站 集線器

Fast Ethernet - 7

100Base4T -- Line Coding

The bit rate on each pair of wires needs only be 33.33 Mbps. The Manchester encoding used in Ethernet is not used due to a

bit rate of 33.33 Mbps requires a clock rate of 33.33 MHz which exceeds the 30MHz limit set for use with such cables.

To reduce the clock rate, a 3-level (ternary) code is used instead of straight (2-level) binary coding.

8B6T encoding: prior to transmission, each set of 8 binary bits is converted into 6 ternary (3-level) symbols. A symbol signaling rate of (100x6/8)/3 = 25 MHz.

8 位元

0 0 1 0 0 1 1 0

6 符號 字碼

+

+

-

0

Fast Ethernet - 8

100Base4T -- Line Coding

The three signal levels used are +V, 0, -V (+,0,-). The codewords are selected such that the line is d.c. balanced (the

line signal is zero). The 6 ternary symbols have 729 (36) possible codewords. To represent complete set of 8-bit byte combination, 256 codewor

ds are needed. The selection rules are

To achieve DC balance To ensure all codewords that have at least two signal transitio

ns within them (for the purpose of synchronization) For the first case, only those codewords with a combined weight of

0 or +1 are selected. 267 codewords meet this condition. For the second case, those codewords with fewer than two transiti

ons (5 codes) and those staring or ending with four consecutive zeros (6 codes) are eliminated.

267 - 5 - 6 = 256.

Fast Ethernet - 9

8B6T Code Mapping Table

00 - + 0 0 - + 20 - + + - 0 0 40 - 0 0 + 0 + 60 0 + + 0 - 001 0 - + - + 0 21 + 0 0 + - - 41 0 - 0 0 + + 61 + 0 + - 0 002 0 - + 0 - + 22 - + 0 - + + 42 0 - 0 + 0 + 62 + 0 + 0 - 003 0 - + + 0 - 23 + - 0 - + + 43 0 - 0 + + 0 63 + 0 + 0 0 -04 - + 0 + 0 - 24 + - 0 + 0 0 44 - 0 0 + + 0 64 0 + + 0 0 -05 + 0 - - + 0 25 - + 0 + 0 0 45 0 0 - 0 + + 65 + + 0 - 0 0 06 + 0 - 0 - + 26 + 0 0 - 0 0 46 0 0 - + 0 + 66 + + 0 0 - 007 + 0 - + 0 - 27 - + + + - - 47 0 0 - + + 0 67 + + 0 0 0 -08 - + 0 0 + - 28 0 + + - 0 - 48 0 0 + 0 0 0 680 + + - + -09 0 - + + - 0 29 + 0 + 0 - - 49 + + - 0 0 0 69 + 0 + + - -0A 0 - + 0 + - 2A + 0 + - 0 - 4A + - + 0 0 0 6A + 0 + - + -0B 0 - + - 0 + 2B + 0 + - - 0 4B - + + 0 0 0 6B + 0 + - - +0C - + 0 - 0 + 2C 0 + + - - 0 4C 0 + - 0 0 0 6C 0 + + - - +0D + 0 - + - 0 2D + + 0 0 - - 4D + 0 - 0 0 0 6D + + 0 + - -0E + 0 - 0 + - 2E + + 0 - 0 - 4E 0 - + 0 0 0 6E + + 0 - + -0F + 0 - - 0 + 2F + - 0 - 0 - 4F - 0 + 0 0 0 6F + + 0 - - +10 0 - - + 0 + 30 + - 0 0 - + 50 + - - + 0 + 70 0 0 0 + + -11 - 0 - 0 + + 31 0 + - - + 0 51 - + - 0 + + 71 0 0 0 + - + 12 - 0 - + 0 + 32 0 + - 0 - + 52 - + - + 0 + 72 0 0 0 - + + 13 - 0 - + + 0 33 0 + - + 0 - 53 - + - + + 0 73 0 0 0 + 0 014 0 - - + + 0 34 + - 0 + 0 - 54 + - - + + 0 74 0 0 0 + 0 -15 - - 0 0 + + 35 - 0 + - +0 55 - - + 0 + + 75 0 0 0 + - 016 - - 0 + 0 + 36 - 0 + 0 - + 56 - - + + 0 + 76 0 0 0 - 0 +17 - - 0 + + 0 37 - 0 + + 0 - 57 - - + + + 0 77 0 0 0 - + 0 18 - + 0 - + 0 38 +- 0 0 + - 58 - - 0 + + + 78 + + + - - 019 +- 0 - + 0 39 0 + - + - 0 59 - 0 - + + + 79 + + + - 0 -1A - + +- +0 3A 0 + - + - 0 5A 0 - - + + + 7A + + + 0 - - 1B + 0 0 - +0 3B 0 + - - 0 + 5B 0 - - 0 + + 7B 0 + + 0 - -1C + 0 0 +- 0 3C + - 0 - 0 + 5C + - - 0 + + 7C - 0 0 - + +1D - + + +- 0 3D - 0 + + - 0 5D - 0 0 0 + + 7D - 0 0 + 0 0 1E + - 0 +- 0 3E - 0 + 0 +- 5E 0 +++ - - 7E + - - - + +1F - + 0 +- 0 3F - 0 + - 0 + 5F 0 + + - 0 0 7F +- - + 0 0

資料位元組 字碼 資料

位元組 字碼 資料位元組 字碼

資料位元組 字碼 資料

位元組 字碼 資料位元組 字碼 資料

位元組 字碼 資料位元組 字碼

80 - 0 0 + - + A0 - + + 0 - 0 C0 - + 0 +- + E0 - + + 0 - +81 0 - 0 - + + A1 +- + - 0 0 C1 0 - +- + + E1 + - + - + 0 82 0 - 0 + - + A2 +- + 0 - 0 C2 0 - + +- + E2 + - + 0 - +83 0 - 0 + +- A3 + - + 0 0 - C3 0 - + ++- E3 + - ++ 0 -84 - 0 0 ++- A4 - + + 0 0 - C4 - + 0 ++- E4 - + ++0 -85 0 0 - - + + A5 ++ - - 0 0 C5 + 0 - - ++ E5 + + - - + 0 86 0 0 - +- + A6 ++ - 0 - 0 C6 + 0 - +- + E6 + + - 0 - +87 0 0 - ++- A7 + +- 0 0 - C7 + 0 - + +- E7 + + - + 0 -88 - 0 0 0 +0 A8 - + + - +- C8 - + 0 0 + 0 E8 - + + 0 + -89 0 - 0 + 00 A9 + - ++ - - C9 0 - + +0 0 E9 +- + + - 08A 0 - 0 0 +0 AA + - + - + - CA 0 - + 0 + 0 EA +- +0 + -8B 0 - 0 0 0 + AB + - + - - + CB 0 - + 0 0 + EB + - + - 0 +8C - 0 0 0 0 + AC - + + - - + CC - + 0 0 0 + EC - + + - 0 +8D 0 0 - + 0 0 AD + + - + - - CD + 0 - + 0 0 ED + + - + - 08E 0 0 - 0 0 + AE + + - - + - CE + 0 - 0 + 0 EE + + - 0 +-8F 0 0 - 0 0 + AF + + - - - + CF + 0 - 0 0 + EF + + - - 0 +90 + - - + - + B0 + 0 0 0 - 0 D0 + - 0 + - + F0 + 0 0 0 - +91 - + - - + + B1 0 + 0 - 0 0 D1 0 + - - + + F1 0 +0 - +0 92 - + - + - + B2 0 + 0 0 - 0 D2 0 + - + - + F2 0 +0 0 - +93 - + - + +- B3 0 + 0 0 0 - D3 0 + - + +- F3 0 +0 + 0 -94 + - - + +- B4 + 0 0 0 0 - D4 + - 0 + +- F4 +0 0 + 0 -95 - - +- + + B5 0 0 +- 0 0 D5 - 0 + - + + F5 0 0 + - + 096 - - + +- + B6 0 0 + 0 - 0 D6 - 0 + + - + F6 0 0 + 0 - +97 - - + ++- B7 0 0 + 0 0 - D7 - 0 + + + - F7 0 0 + +0 - 98 +- - 0 +0 B8 +0 0 - + - D8 + - 0 0 + 0 F8 + 0 0 0 + - 99 - + - + 0 0 B9 0 +0 + - - D9 0 + - + 0 0 F9 0 +0 + - 0 9A - + - 0 +0 BA 0 +0 - + - DA 0 + - 0 +0 FA 0 +0 0 + - 9B - + - 0 0 + BB 0 +0 - - + DB 0 + - 0 0+ FB 0 +0 - 0 +9C + - - 0 0 + BC + 0 0 - - + DC + - 0 0 0 + FC + 0 0 - 0 +9D - - + + 0 0 BD 0 0 ++ - - DD - 0 ++ 0 0 FD 0 0 + +- 0 9E - - + 0 + 0 BE 0 0 + - +- DE - 0 + 0 +0 FE 0 0 + 0 + -9F - - + 0 0 + BF 0 0 + - - + DF - 0 +0 0 + FF 0 0 + - 0 +

Fast Ethernet - 10

8B6T Coding Scheme

DC Balance: All the codewords selected have a combined weight of either 0 or +1. If a string of codewords each of weight +1 is transmitted, then the mean signal level at the receiver will move away rapidly from the zero level, causing the signal to be misinterpreted. This is known as DC Wander.

To overcome this, whenever a string of codewords with a weight of +1 is to be sent, the symbols in alternate codewords are inverted prior to transmission.

For example: + + 0 + - - => + + 0 + - - , - - 0 - + +, + + 0 + - - , - - 0 - + +,... State Machine:

總和 = 0 總和 = 1

比重 = 1, 字碼不變

比重 = 1, 字碼反相比重 = 0字碼不變

比重 = 0字碼不變

開始

Fast Ethernet - 11

8B6T Coding Scheme

To reduce the latency during the decoding process, six ternary symbols corresponding to each encoded byte are transmitted on three wire pairs.

11000101 11110001 00101011 01011000 00110011 10011111

+ 0 - - + + 0 + 0 - + 0 + 0 + - - 0 - - 0 + + + 0 + - + 0 - - - + 0 0 +

......

A1(w=1) B1(w=1) C1(w=0) A2(w=1) B2(w=0) C2(w=0)

A1(w=1) A2(w=1)

B1(w=1) B2(w=0)

C1(w=0) C2(w=0)

IEEE 802.3 位元串

6 符號字碼

傳送至三條 雙絞線上 + 0 + - - 0 - - + 0 0 +

0 + 0 - + 0 0 + - + 0 -

+ + 0 - - - ......

...雙絞線

+ 0 - - + +

Fast Ethernet - 12

8B6T Coding Scheme

The transmission procedure adopted enables further error checking to be added to the basic CRC. At the end of each frame transmission (after the four CRC bytes have been transmitted) one of two different End-Of-Stream (EOS) codes is transmitted on each of the three pairs.

雙絞線

CRC-3

CRC-4

E1 - - - - - - ++++++E2 - - - - ++++E3 - - ++

E3

E2

總和 = 0 總和 = 1

E1 +- +- +- +- +- +-

+- +- +- +-

+- +-

EOS 功能結束

線路重新達成平衡

Fast Ethernet - 13

Collision Detection

A DTE detects a collision by detecting a signal on pair 2 while it is transmitting. The hub detects a collision by the presence of a signal on pair 1.

The strong signals transmitted on pairs 1, 3, and 4 from the DTE side each induce a signal into the collision detect wire (pair 2). This is known as Near-End CrossTalk (NEXT) which is interpreted by the DTE as a collision signal being received from the hub.

載波感測衝撞偵測

載波感測衝撞偵測第一對雙絞線

第二對雙絞線

第三對雙絞線

第四對雙絞線

工作站 集線器

NEXT

Fast Ethernet - 14

Collision Detection

To minimize NEXT the preamble at the start of each frame is encoded as a string of 2-level (as opposed to 3-level) symbols. This increases the signal-level amplitude variations which helps the DTE/hub to distinguish between an induced NEXT signal and the preamble of a colliding frame.

The preamble pattern: Start-of-Stream (SOS) is made up to two 2-level codewords, SOS-1 and SFD.

第二對雙絞線

SOS-1 SOS-1 SFD 資料

SOS-1 SOS-1 SFD 資料

SOS-1 SFD 資料

第三對雙絞線

第四對雙絞線

工作站 集線器

Fast Ethernet - 15

Ether-Switch

To speed up the transmission rate of Ethernet Hub without changing the interface cards on stations.

Ether-Switch Architecture Each pair of Ethernets can have a transmission

simultaneously. For example, (Ethernet 1 - Ethernet 4) and (Ethernet 2

- Ethernet 3)

Ether-Switch

A B

...C D

E F

...

...

G H

... Ethernet 4(10Mbps)

Ethernet 3(10Mbps)

Ethernet 2(10Mbps)

Ethernet 1(10Mbps)

Fast Ethernet - 16

Ether-Switch Block Diagram

埠 4埠 3埠 2

控制處理機

工作站 埠

埠 1

A

B

... C

D

... E

F

... G

H

...

交換元件

緩衝 器

緩衝 器

緩衝 器

緩衝 器

衝撞偵測

A 1

C 2

F 3

G 4

位址對照表

Fast Ethernet - 17

Gigabit Ethernet Network Structure

PCSPMAPMD

Reconcilation

Medium

PCSPMAPMD

Reconcilation

Medium

PLS

Reconcilation

Medium

PMA

PLS

Medium

PMA

MACMAC Control (Option)

LLC

Higher Layers

MDI MDI MDI MDI

AUI AUI

MII MII GMII

1Mbps, 10Mbps 10 Mbps 100 Mbps 1000 Mbps

PHYSICAL

DATA LINK

NETWORK

TRANSPORT

SESSION

PRESENTAION

APPLICATION

PHYMAU

Fast Ethernet - 18

Frame Format (Carrier Extension)

Preamble SFD DA SA LEN LLC PAD FCS Extension

7 1 2, 6 2, 6 2 46 <= N <= 1500 4 0 <= M <= 448 位元組

最短訊框尺寸（ >= 64 位元組）

最短訊框尺寸 + 擴充尺寸（ >= 512 位元組）

FCS 錯誤檢查碼涵蓋範圍

訊框之載波期間

Fast Ethernet - 19

Frame Bursting Example

訊爆週期 時槽時間(512 位元組）

載波偵測

傳送訊框 訊框 1 訊框 2 訊框 3 訊框 4

訊框間隔（ 96 位元時間）延伸載波

Preamble SFD DA SA LEN LLC PAD FCS

Fast Ethernet - 20

Gigabit Ethernet Repeater Set

MAC

MAC Control (Option)

LLC

Higher Layers

1000 Mbps Link Segment

PHYSICAL

DATA LINK

NETWORK

TRANSPORT

SESSION

PRESENTAION

APPLICATION

PHYPCSPMAPMD

Medium

MDI

GMII

PCSPMAPMD

Reconcilation

Medium

MDI

GMII

PHYPCSPMAPMD

MDI

GMII

PHY

1000 Mbps 基頻集線器

1000 Mbps Link Segment

1000 Mbps 基頻集線器組

ISO 參考模式 CSMA/CD 網路

Fast Ethernet - 21

Gigabit Ethernet Communication Structure

1000BASE-LX1270-1355 nm 光傳送接收器

1000BASE-SX 770-860 nm 光傳送接收器

1000BASE-CX STP 傳送接收器

1000BASE-T 4-Pair 傳送接收器

SMF MMF MMF50 um

MMF62.5 um

Balance Shielded Copper

Cat-5 UTP

3 km 550m 550m 300m 25m 100m

8B/10B 編碼 / 解碼 1000BASE-T 編碼 / 解碼

Gigabit Media Independent Interface (GMII)

Media Access Control (MAC)

Logical Link Control (LLC)

Ethernet Upper Layers

Fast Ethernet - 22

Gigabit Ethernet Applications--Multi-port Bridge (Single bandwidth)

集線器(Repeater)

工作站 (DTE)

工作站 (DTE)

工作站 (DTE)

工作站 (DTE)

集線器(Repeater)

工作站 (DTE)

工作站 (DTE)

工作站 (DTE)

工作站 (DTE)

集線器(Repeater)

工作站 (DTE)

工作站 (DTE)

工作站 (DTE)

工作站 (DTE)

多埠橋接器(Multi-port Bridge)

衝撞網域 1

衝撞網域 2 衝撞網域 3

Fast Ethernet - 23

1000 Mbps 專屬線路工作站 (DTE)

工作站 (DTE)

100BASE-T 集線器

工作站 (DTE)

工作站 (DTE)

工作站 (DTE)

多埠橋接器(Multi-port Bridge)

100 Mbps 分享網路

100 Mbps

1000 Mbps 專屬線路

1000 Mbps

1000 Mbps

1000BASE-T 集線器

工作站 (DTE)

工作站 (DTE)

工作站 (DTE)

1000 Mbps 分享網路

1000 Mbps

10 Mbps 專屬線路

工作站 (DTE)

工作站 (DTE)

100 Mbps 專屬線路

10 Mbps100 Mbps

Gigabit Ethernet Applications--Multi-port Bridge (Multiple Bandwidth)

Fast Ethernet - 24

CSMA/CD 網路的實際大小受限於各個網路元件的特性。這些特性包括：

傳輸媒介（線路）長度及其傳遞延遲。 集線器元件延遲（啟動，穩定，結束）。 MAUs 及 PHYs 延遲（啟動，穩定，結束）。 集線器所造成的訊框間隔萎縮。 DTEs 執行 CSMA/CD 演算法的延遲。 MAUs 及 PHYs 的衝撞偵測延遲。

Fast Ethernet - 25

1000Mbps 網路區段時間延遲

媒介種類每區段最多

連接點數

每區段最

長（公尺）

每區段最大來

回傳遞延遲

（位元時間）

平衡纜線區段（1000BASE-T） 2 100 1112

雙絞纜線區段（1000BASE-CX） 2 25 253

光纖區段

（1000BASE-SX, 1000BASE-LX）2 316 3192

Fast Ethernet - 26

Transmission System Model (TSM -1 )

集線器組

工作站 (DTE)

工作站 (DTE)

A+B = 衝撞網域直徑

工作站 (DTE)

工作站 (DTE)

A B

以下四種網路架構的限制適用於使用傳輸系統模式一的網路：

(1) 只能單一集線器。

(2) 所有平衡纜線（銅線）無遮蔽區段長度都小於或等於 100 公

尺。

(3) 所有雙絞線區段 (TW-style) 長度都小於或等於 25公尺。

(4) 所有光纖區段長度都小於或等於 330公尺。

Fast Ethernet - 27

Transmission System Model (TSM -2 )

集線器工作站 (DTE)

工作站 (DTE)PHY PHY PHY PHY

每一條欲驗證的路徑都應該以下列公式計算其 PDV：

PDV = 鏈路延遲（LSDV）+ 集線器延遲+工作站延遲+安全差數

其中鏈路延遲（Link Segment Delay Value, LSDV）表示網路區段的

來回時間延遲，計算公式為

LSDV = 2 x區段纜線長度 x纜線時間延遲

Fast Ethernet - 28

8B/10B Encoding

0 1 2 3 4 5 6 7 8 9

8B/10B編碼器

a b c d e i f g h j

H G F E D C B A

10

8+Control

7 6 5 4 3 2 1 0 GMII(125 Mbytes/s)

編碼器輸入

編碼器輸出

PMA 服務界面(125 M code_groups/s)

PMD 服務界面 (1250 Mbps) 位元 0 先傳送

tx_code_group<9:0>0 1 2 3 4 5 6 7 8 9

8B/10B解碼器

a b c d e i f g h j

H G F E D C B A

10

8+Control

7 6 5 4 3 2 1 0

0 0 1 1 1 1 1 x x x

GMII(125 Mbytes/s)

解碼器輸入

解碼器輸出

PMA 服務界面(125 M code_groups/s)

PMD 服務界面 (1250 Mbps) 位元 0 先接收

rx_code_group<9:0>

Comma + Symbol 對齊

PCS 編碼功能PCS 解碼功能

TXD<7:0> RXD<7:0>

Fast Ethernet - 29

子區塊偏差值的計算方式

子區塊偏差值的計算方式如下：

任何一個子區塊後的偏差值為正，如果該子區塊包含 ” 1” 的數量大於 “0” 的數量。如果是在一個值為 000111 的六位元子區塊後，則偏差值仍然為正。相同的，如果是在一個值為 0011 的四位元子區塊後，則偏差值仍然為正。

任何一個子區塊後的偏差值為負，如果該子區塊包含 ” 1” 的數量小於 “0” 的數量。如果是在一個值為 111000 的六位元子區塊後，則偏差值仍然為負。相同的，如果是在一個值為 1100 的四位元子區塊後，則偏差值仍然為負。

在其他狀況下（包含 ” 1” 的數量等於 “ 0” 的數量），則偏差值保持不變。

Fast Ethernet - 30

偏差值計算與錯誤偵測範例（一）

字元串 群碼 群碼 群碼

RD RD RD RD RD RD RD

傳送群碼 - D21.1 - D10.2 - D23.5 +

傳送位元串 - 101010 - 1001 - 010101 - 0101 - 111010 + 1010 +

接收位元串 - 101010 - 1011a + 010101 + 0101 + 111010 +b 1010 +

接收群碼 - D21.0 + D10.2 + 錯誤群碼b +c

a: 位元發生錯誤 (1001 1011)b: 非零偏差的區塊應該交換其前後偏差值之極性 (+ -)。c: 無論接收群碼是否正確，偏差值仍須繼續計算。非零偏差的區塊可避免錯誤蔓延。

Fast Ethernet - 31

偏差值計算與錯誤偵測範例（二）

字元串 群碼 群碼 群碼

RD RD RD RD RD RD RD

傳送群碼 - D21.1 - D23.4 - D23.5 +

傳送位元串 - 101010 - 1001 - 111010 + 0010 - 111010 + 1010 +

接收位元串 - 101010 - 1011a + 111010b + 0010 + 111010 + 1010 +

接收群碼 - D21.0 + 錯誤群碼b - D23.5 +

a: 位元發生錯誤 (1001 1011)b: 非零偏差的區塊應該交換其前後偏差值之極性 (+ -)。

Fast Ethernet - 32

偏差值計算與錯誤偵測範例（三）

字元串 群碼 群碼 群碼

RD RD RD RD RD RD RD

傳送群碼 - D3.6(FCS3) - K29.7(/ T/ ) - K23.7(/ R/ ) -

傳送位元串 - 110001 - 0110 - 101110 + 1000 - 111010 + 1000 -

接收位元串 - 110001 - 0111a +b 101110 +c 1000 - 111010 + 1010 -

接收群碼 - 錯誤群碼d + 錯誤群碼e - K23.7(/ R/ ) -

a: 位元發生錯誤 (0110 0111)b: 非零偏差的區塊應該交換其前後偏差值之極性 (- +)。c: 非零偏差的區塊應該交換其前後偏差值之極性 (+ -)。d: 接收群碼在表7-9, 表7-10中找不到（檢查出錯誤群碼）。e: 非零偏差的區塊可避免錯誤蔓延。