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Auto-negotiation of Ethernet optical ports and electrical ports

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Auto-negotiation of Ethernet optical ports and electrical ports

1. Background and Principles

1.1 Introduction to Auto-Negotiation

The auto-negotiation mode is that the port automatically adjusts its speed to the highest common level according to the connection speed and duplex mode of the other end device, that is, the fastest speed and duplex mode that both ends of the line can have. Generally, auto-negotiation refers to rate, duplex, and flow control auto-negotiation. When a network port that supports auto-negotiation is connected, a standard negotiation mechanism can be used to advertise the rate and duplex mode supported by the peer. After the negotiation is complete, the rate of both parties (including duplex mode) is automatically set to the highest rate supported by both parties (including duplex mode).

1.2 Principles of Auto-Negotiation

The auto-negotiation function allows a network device to transmit the supported working mode information to the peer end on the network and accept the corresponding information that may be transmitted by the peer end, so as to solve the auto-negotiation problem of the duplex mode and the 10M/100M/1000M rate. The auto-negotiation function is implemented by the chip design at the physical layer. Therefore, no dedicated data packet is used or any upper-layer protocol overhead is brought.

The basic mechanism of auto-negotiation is as follows: Each network device sends an FLP (fast connection pulse) during power-on, management command sending, or user intervention, and the negotiation information is encapsulated in these FLP sequences. By extracting the data, you can obtain the working mode supported by the peer device and other information used to negotiate the handshake mechanism. When a device cannot respond effectively to FLP and returns only one NLP (common connection pulse), it is used as an 10BASE-T compatible device. The FLP and NLP are used only on unshielded twisted pairs, and cannot be applied to the optical media.

The auto-negotiation mechanism of the electrical port is different from that of the optical port. When the optical port is in auto-negotiation mode, the port in auto-negotiation mode sends the C code to perform auto-negotiation. The electrical port uses the FLP for negotiation. For forced ports, I streams are used. In addition, for an electrical port, negotiation occurs before link signal transmission. For optical ports, link synchronization must be established before negotiation.

2. Electrical port auto-negotiation

2.1 Electrical Port Auto-Negotiation Process

2.1.1 Electrical Port Auto-Negotiation Process

If both ends support auto-negotiation, they receive the FLP of each other and decode the information in the FLP. Obtain the connection capability of the other party. The auto-negotiation capability value of the peer end is recorded in the auto-negotiation peer capability register. At the same time, set bit (bit5) of the status register (PHY standard register address 1) to 1. When auto-negotiation is not complete, this bit is always 0.

Table 1 Port rate priority


Working Mode


100BASE-TX Full-duplex






10BASE-T Full-duplex



Then, the system selects the connection mode with the highest priority according to the maximum connection capability of each other. Table 1 lists the priority of the connection mode. For example, if both parties support 10M and 100M, the rate is connected according to 100M. Both full-duplex and half-duplex modes are supported. In this case, the full-duplex connection is used. After a connection is established, the FLP stops sending, as shown in Figure 1. The FLP message is sent again only when the link is interrupted or the auto-negotiation restart command is obtained.


Figure 1 Auto-negotiation mode at both ends

2.1.2 One end forced one end auto-negotiation

In order to ensure that the peer can also connect if the peer cannot support auto-negotiation, a mechanism called Parallel Detection is introduced. In the case where auto-negotiation is turned on at one end and auto-negotiation is turned off at the other end, the establishment of the connection is implemented by the parallel detection function. The parallel detection mechanism is such that on the device port with auto-negotiation capability, if the FLP is not received, it is detected whether there is a characteristic signal of the 10M link or a characteristic signal of the 100M link. If the device is a 10M device and does not support auto-negotiation, a normal link pulse (NLP) is sent on the link, as shown in Figure 2. NLP simply means that the device is in place and does not contain additional information. If it is a 100M device and does not support auto-negotiation, the 4B/5B encoded idle symbol is always sent on the link without data, as shown in Figure 3.

If the parallel detection mechanism detects NLP, it knows that the other party supports the 10M rate; if the 4B/5B encoded idle symbol is detected, it knows that the other party supports the 100M rate. However, whether the other party supports full-duplex and whether or not the flow control frame is supported is not available. Therefore, in this case, the other party is considered to support only half-duplex, does not support full-duplex, and does not support flow control frames.

Based on the above principle, when the peer does not open auto-negotiation, the party that opens the auto-negotiation can only negotiate the half-duplex mode.

Figure 2 Schematic diagram of the forced (10M) mode process on one end of the auto-negotiation end


Figure 3 Schematic diagram of the forced (100M) mode process on one end of the auto-negotiation end


2.1.3 Summary of Electrical Port Auto-Negotiation Results

According to the preceding negotiation principles, the auto-negotiation result of the electrical port is summarized in Table 2.

Table 2 Electrical port auto-negotiation result table

Working      Mode

Auto-Negotiation      Result

Local      end

Peer      end

Local      end

Peer      end



10M/100M/1000M Full

10M/100M/1000M Full

10M Half

10M Half

10M Half

10M Full

10M Half

10M Full

100M Half

100M Half

100M Half

100M Full

100M Half

100M Full

1000M Full

1000M Full

1000M Full

2.2 Ge optical port auto-negotiation process

2.2.1 Introduction to Optical Port Auto-Negotiation

A GE optical port can work in forced or auto-negotiation mode. In the 802.3 specification, the GE optical port supports only the 1000M rate, and supports the full duplex (Full) and half duplex (Half) two duplex modes.

The basic difference between auto-negotiation and force is that the code streams sent when the physical link is established are different. The /C/ code is sent in auto-negotiation mode, that is, the Configuration code stream, and the /I/ code is sent in forced mode, that is, the idle code stream.

2.2.2 Optical port auto-negotiation

Both ends are configured to work in auto-negotiation mode.

The two parties send /C/ code streams to each other. If three identical /C/ codes are received consecutively and the received code stream matches the local working mode, a /C/ code with an ACK response is returned to the peer end. After receiving the Ack message, the peer end considers that the two /C/ codes can communicate with each other and sets the port to the UP state, as shown in the following figure.


Figure 4 Auto-negotiation between the two ends of an optical port


One end is set to the auto-negotiation mode, and the other end is set to the forcible mode.

The auto-negotiation end sends the /C/ code stream, and the forced end sends the /I/ code stream. The forced end cannot provide the local negotiation information to the peer end, and cannot return the Ack response to the peer end. Therefore, the auto-negotiation end is Down. However, the forcible end itself can identify the /C/ code, and considers that the peer end is the port that matches itself. Therefore, the local port is directly set to the UP state, as shown in Figure 5.


Figure 5 Forced negotiation at one end of an optical port


Both ends are set to the forced mode.

The two parties send /I/ streams to each other. After receiving the /I/ code stream, one end considers that the peer end is the port that matches itself and sets the local port to the UP state, as shown in Figure 6.


In addition, for the double-ended forced connection mode, if the distal end of the fiber is pulled out, the far end will be down, but the far end can still be sent, and a one-way on-off condition will be formed. If the remote line is configured with a local private line or private network service, a one-way switch will occur. However, if the tunnel service is configured, the tunnel layer will be disconnected in both directions.


This article is reproduced from Xu Ziwen

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