Tag Archives: auto-negotiation

Knowledge of SFP Auto-Negotiation

FacebookTwitterGoogle+LinkedInRedditTumblrShare

We usually see fiber optic transceiver with descriptions like “10/100/1000 copper SFP” shown in the picture below. Then what does “10/100/1000” mean? In fact, it refers to SFP modules that support 10/100/1000 auto-negotiation. With the function of auto-negotiation, SFP can operate on 10 Mbps, 100 Mbps, and 1000 Mbps. Some knowledge of SFP auto-negotiation will be given in this article.

Copper SFP Auto-Negotiation

What Is Auto-Negotiation?

Today a number of technologies, such as 10Base-T, 100Base-T, and 1000Base-T, use the same RJ-45 connector, creating the potential for connecting electrically incompatible components together and causing network disruption. To eliminate the possibility of dissimilar technologies interfering with each other, the Institute of Electrical and Electronics Engineers (IEEE) developed a protocol known as auto-negotiation. Auto-negotiation allows devices to perform automatic configuration to achieve the best possible mode of operation over a link. Devices with this feature will broadcast their speed (10 Mbps, 100 Mbps, and 1000 Mbps) and duplex (half/full) capabilities to other devices and negotiate the best between two devices.

Types of SFP Auto-Negotiation

There are two types of auto-negotiation that operate simultaneously within the SFP module. One is the 1000Base-T auto-negotiation, the other is 1000BASE-X auto-negotiation. The difference between them is that 1000BASE-T auto-negotiation is conducted over the Cat 5 cable between the two 1000BASE-T devices while 1000BASE-X auto-negotiation is typically conducted between two host systems over fiber. Usually, Gigabit SFP transceivers use auto-negotiation to advertise the following modes of operation: 1000Base-T in full or half duplex, 100Base-TX in full or half duplex, and 10Base-T in full or half duplex.

SFP Auto-Negotiation in Real Applications

A few cases of how SFP auto-negotiation operation works in an actual application are shown below:
Case1: A SFP is inserted into a switch with no copper cable.
Regardless of whether the MAC has 1000Base-X auto-negotiation turned on or off, 1000Base-X auto-negotiation will not complete. Because 1000Base-X auto-negotiation will never complete before 1000Base-T link is established.

Case2: After SFP is inserted into a switch w/ 1000Base-X auto-negotiation, copper cable is inserted.
SFP will store the 1000Base-X abilities advertisements from the MAC. 1000Base-T auto-negotiation will be restarted using abilities advertisements from the MAC. After 1000Base-T link is completed, SFP will send 1000Base-X abilities advertisements and acknowledgement codewords to the MAC. 1000Base-X link will then be established.

Case3: After SFP is inserted into a switch w/ no 1000Base-X auto-negotiation, copper cable is inserted.
SFP will detect that only idles are received from the MAC. 1000Base-T link will be established based on abilities set by hardware strap options on the PHY. After 1000Base-T link is established, the SFP will wait for 200 minutes and go into bypass mode. 1000Base-X link will then be established.

Case4: Both 1000Base-T and 1000Base-X link has been established. Copper cable is then unplugged.
When the copper cable is unplugged, 1000Base-T link will be broken. This will restart auto-negotiation both for 1000Base-X and 1000Base-T.

Case5: Copper cable is first plugged into the SFP, then SFP with cable is inserted into switch.
This case is the same case 2 and 3. If the SFP is powered up with copper cable already plugged in, it will go through the same auto-negotiation process.

After reading this article, you may know more about SFP auto-negotiation. Fiberstore have a lot of 10/100/1000BASE-T auto-negotiation 100m RJ45 copper SFP fiber optic transceivers in stock with high quality and low price. For more information, please visit fs.com.

Related Article: Compatible SFPs for Ubiquiti EdgeSwitch and UniFi switch

Related Article: SFP Module: What’s It and How to Choose It?