IP/Network

 

 

 

 

Ethernet : Auto Negotiation

 

Currently (as of Oct 2014), if you buy a PC most of them would come out with a network card which support Gigabit ethernet. But you cannot gurantee that all the other network components (e.g, Router, Switch and other PC) which directly talking to your PC support the same level of capability. They may support only older technology like 10/100 Base. If they trying to talk in different technology (specification), it is highly likely that the communication does not go through since electrically characteristics (PHY specification) is different.

 

How to resolve this issue ?

To resolve this issue, each network components go through special PHY/Link layer procedure called "AutoNegotiation" to figure out the physical capability of the other party. This negotiation procedure is specified in 28. Physical layer link signaling for Auto-Negotiation on twisted pair of 802.3.

 

< Basic Mechanism of the Negotiation >

 

The basic mechanism is to pass the informaion about its capability in a special "Link Test Pulse" called Fast Link Pulse (FLP). Each device capable of Autonegotiation issue FLP in follows situations :

  • Power Up
  • On command from Management Layer
  • Due to user interaction

FLP burst is made up of a series of link integrity test pulses and each Link Integrity Test pulse is made up of alternating clock/data sequence. The receving network component extract the data part (link codewords) from this pulses and figure out Operation Mode or handshake mode of Auto negotiation.

 

Some old network component (e.g, 10-BASE T) does not support FLP, it only support NLP (Normal Link Pulse). In this case, the network fails to respond FLP and it would send NLP. In this case, the other party would think that its counter part is capable of only 10-BASE T and communicate in 10-BASE T.

 

< Structure of FLP Burst >