NTRON技术白皮书:网络自动协商机制-USB迷|专注于互联网分享

2024年7月23日发(作者：辛天曼)

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Ethernet Auto-negotiation

In today’s world of computer networks auto-negotiation is an important plug-and-play

technology. Auto-negotiation as an algorithm was defined by Section 28 of the IEEE 802.3

standard and first introduced in 1997 as part of the IEEE 802.3u standard on Fast Ethernet.

Auto-negotiation was designed to be backward compatible with original Ethernet networking

standards as well. Auto-negotiation was further enhanced in 1999 by the IEEE standard

802.3ab with the introduction of Gigabit Ethernet. Auto-negotiation is best defined as the mutual

agreement by two network devices sharing a wire on the speed, duplex, and controls to govern

the use of that wire. As a protocol auto-negotiation exists strictly at the PHY (physical) layer of

the OSI (Open System Interconnection Reference Model) and is implemented by software,

hardware, or a mixture of both. Specifically this white paper will detail how the protocol

negotiates speed, duplex, Auto-MDIX (cable termination), and flow control.

As you will see in the technical discussions that follow, auto-negotiation is an extremely

important setting on today’s wired Ethernet networks. For a link to function properly the devices

on either side of the wire must be configured in the same manner; either both set to auto-

negotiation or both set to the same hard-coded speed and duplex settings. In an environment

where one device is set to auto-negotiate and the other device is set to a hard-coded speed and

duplex the auto-negotiate algorithm can detect speed and set that appropriately. The duplex

setting of the remote device is indeterminable by the auto-negotiating device. Following the

IEEE standard, the auto-negotiating device falls back to half-duplex. This presents an issue if

the remote device is set to full-duplex. Typically in such a scenario, users complain of slow

network connectivity and application timeouts. These symptoms will be explained in detail in

the section discussing Duplex.

Lastly, it should be noted that, according to the IEEE specification the use of gigabit Ethernet

requires the use of auto-negotiation therefore 1000Mb/s is not a valid hard-coded option in a

true IEEE compliant networking device.

Speed

IEEE 802.3u introduced 100Mb/s to what was previously only a 10 Mb/s Ethernet networking

world. Now that computers had a choice of what speed to communicate a procedure needed to

be introduced to govern this decision. With the introduction of a third speed, 1000 Mb/s or

Gigabit Ethernet, this procedure became even more important. Thus the auto-negotiation

protocol was created and the NWay algorithm adapted to provide a plug and play solution to this

decision making process while still maintaining complete backwards compatibility with the 10

Mb/s protocol.

The 10 Mb/s standard detects an active link with another network device through the

transmission and reception of Link Integrity Test (LIT) pulses whenever the device is not actively

sending or receiving data. These LIT pulses or Normal Link Pulses (NLP), as the name was

later changed to, consist of a single uni-polar positive-only pulse for the duration of 100ns at an

interval of 16ms with a +/-8ms window.

The auto-negotiation protocol introduced with the 100 Mb/s standard transmits a Fast Link Pulse

(FLP) instead of an NLP. A single FLP burst consists of a series of 33 pulses. Each burst of 33

pulses is 2ms long in total and fall into the same transmission interval of 16ms +/- 8ms. The

individual pulses are 125 µs with 62.5µs +/- 7µs between pulses. Diagram 1 illustrates this

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timing. The individual pulses alternate between clock pulses and data pulses with the first and

all successive odd numbered pulses being clock pulses. Each of the 16 data pulses (with each

pulse or lack of pulse representing a 1 or a 0, respectively) consist of a single bit of data and

collectively add up to 16 bits of 2 bytes of data. These 2 bytes make up the link code word

(LCW) which contains the information needed for auto-negotiation.

16 ms

802.3u Standard

Fast Link Pulse (FLP) Burst

16 Data Pulses (two Bytes)

alternated with 17 Clock Pulses

33 Pulses per Burst

Data Pulse

Clock Pulse

16 ms

802.3 Standard

Normal Link Pulses (NLP) or

Link Integrity Test Pulses (LIT)

2 ms

Diagram 1. NLP and FLP Timing.

There are multiple LCW formats but the most important LCW is the base page. This base page

is the transmission stating the capabilities of that device. The first 5 bits only have two valid

values. They state either to use IEEE 802.3 (Ethernet) or IEEE 802.9 (IsoEthernet over Cat3

twisted pair). The next 5 bits state what speed and duplex combinations that a device can

communicate. Bits A5 and A6 are used for Flow Control and D14 is used to acknowledge a

negotiation. The last bit, D15 is used to denote the need to use Next Page, a more advanced

LCW used to negotiate Gigabit speeds and controls. Diagram 2 illustrates the Base Page.

D10

D11

D12

D13

D14

Ack

D15

Selector Field

S0 thru S4

802.3 = 00001

802.9 = 00010

Technology Ability Field

A0 thru A7

10BaseT

10BaseT-FD

100BaseTX

100BaseTX-FD

100BaseT4

PAUSE

Asymmetric Pause

operation for full

duplex links

A7 Reserved

Other Fields

RF Remote Fault

Ack Acknowledge

NP Next Page

Diagram 2. LCW Base Page.

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