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Transcript of Fast Ethernet - 1 Fast Ethernet and Gigabit Ethernet All rights reserved. No part of this...
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: [email protected]).
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。