Autonegotiation is a signaling mechanism and procedure used by

Ethernet over twisted pair Ethernet over twisted-pair technologies use twisted-pair cables for the physical layer of an Ethernet computer network. They are a subset of all Ethernet physical layers. Early Ethernet used various grades of coaxial cable, but in 1984, Sta ...

by which two connected devices choose common transmission parameters, such as speed, duplex mode, and flow control. In this process, the connected devices first share their capabilities regarding these parameters and then choose the highest performance transmission mode they both support. Autonegotiation is defined in clause 28 of IEEE 802.3. and was originally an optional component in the

Fast Ethernet In computer networking, Fast Ethernet physical layers carry traffic at the nominal rate of 100 Mbit/s. The prior Ethernet speed was 10 Mbit/s. Of the Fast Ethernet physical layers, 100BASE-TX is by far the most common. Fast Ethern ...

standard. It is backwards compatible with the normal link pulses (NLP) used by

10BASE-T 1 (one, unit, unity) is a number representing a single or the only entity. 1 is also a numerical digit and represents a single unit of counting or measurement. For example, a line segment of ''unit length'' is a line segment of length 1 ...

. The protocol was significantly extended in the

Gigabit Ethernet In computer networking, Gigabit Ethernet (GbE or 1 GigE) is the term applied to transmitting Ethernet frames at a rate of a gigabit per second. The most popular variant, 1000BASE-T, is defined by the IEEE 802.3ab standard. It came into use ...

standard, and is mandatory for 1000BASE-T gigabit Ethernet over twisted pair. In the

OSI model The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...

, autonegotiation resides in the

physical layer In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer; The layer most closely associated with the physical connection between devices. This layer may be implemented by a PHY chip. The ...

Standardization and interoperability

In 1995, the

standard was released. Because this introduced a new speed option for the same wires, it included a means for connected network adapters to negotiate the best possible shared mode of operation. The autonegotiation protocol included in

IEEE 802.3 IEEE 802.3 is a working group and a collection standards defining the physical layer and data link layer's media access control (MAC) of wired Ethernet. The standards are produced by the working group of Institute of Electrical and Electronics ...

clause 28 was developed from a patented technology by

National Semiconductor National Semiconductor was an American semiconductor manufacturer which specialized in analog devices and subsystems, formerly with headquarters in Santa Clara, California. The company produced power management integrated circuits, display dr ...

known as NWay. The company gave a letter of assurance for anyone to use their system for a one time license fee. Another company has since bought the rights to that patent. The first version of the autonegotiation specification, in the 1995 IEEE 802.3u

standard, was implemented differently by different manufacturers leading to

interoperability Interoperability is a characteristic of a product or system to work with other products or systems. While the term was initially defined for information technology or systems engineering services to allow for information exchange, a broader defi ...

issues. These problems led many network administrators to manually set the speed and duplex mode of each network interface. However, the use of manually set configuration may also lead to

duplex mismatch On an Ethernet connection, a duplex mismatch is a condition where two connected devices operate in different duplex modes, that is, one operates in half duplex while the other one operates in full duplex. The effect of a duplex mismatch is a l ...

es. Duplex mismatch is difficult to diagnose because the network is nominally working; Simple programs used for network tests such as

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report a valid connection. However, network performance is significantly impacted. The autonegotiation specification was improved in the 1998 release of IEEE 802.3. This was followed by the release of the IEEE 802.3ab

standard in 1999 which specified mandatory autonegotiation for

1000BASE-T In computer networking, Gigabit Ethernet (GbE or 1 GigE) is the term applied to transmitting Ethernet frames at a rate of a gigabit per second. The most popular variant, 1000BASE-T, is defined by the IEEE 802.3ab standard. It came into use ...

. Autonegotiation is also mandatory for 1000BASE-TX and

10GBASE-T 10 Gigabit Ethernet (10GE, 10GbE, or 10 GigE) is a group of computer networking technologies for transmitting Ethernet frames at a rate of 10 gigabits per second. It was first defined by the IEEE 802.3ae-2002 standard. Unlike previous ...

implementations. Currently, most network equipment manufacturers recommend using autonegotiation on all access ports and enable it as a factory default setting.

Function

Autonegotiation can be used by devices that are capable of more than one transmission rate, different duplex modes (half duplex and full duplex), and different transmission standards at the same speed (though in practice only one standard at each speed is widely supported). During autonegotiation, each device declares its ''technology abilities'', that is, its possible modes of operation. The best common mode is chosen, with higher speed preferred over lower, and full duplex preferred over half duplex at the same speed. Parallel detection is used when a device that is capable of autonegotiation is connected to one that is not. This happens if a device does not support autonegotiation or autonegotiation is disabled on a device. In this condition, the device that is capable of autonegotiation can determine and match speed with the other device. This procedure cannot determine duplex capability, so half duplex is always assumed. Other than speed and duplex mode, autonegotiation is used to communicate the port type (single port or multiport) and the master-slave parameters for

gigabit Ethernet In computer networking, Gigabit Ethernet (GbE or 1 GigE) is the term applied to transmitting Ethernet frames at a rate of a gigabit per second. The most popular variant, 1000BASE-T, is defined by the IEEE 802.3ab standard. It came into use ...

Priority

Upon receipt of the technology abilities of the other device, both devices decide the best possible mode of operation supported by both devices. Among the modes that are supported by both devices, each device chooses the one that is highest priority. The priority among modes is as follows:

Electrical signals

Autonegotiation is based on pulses similar to those used by

devices to detect the presence of a connection to another device. These ''link integrity test'' (LIT) pulses are sent by Ethernet devices when they are not sending or receiving any frames. They are unipolar positive-only electrical pulses of a nominal duration of , with a maximum pulse width of , generated at a time interval with a timing variation tolerance of . A device detects the failure of a link if neither a frame nor two of the LIT pulses is received for 50-150 ms. For this scheme to work, devices must send LIT pulses regardless of receiving any. In the autonegotiation specification these pulses are called normal link pulses (NLP). NLPs used by autonegotiation are still unipolar, positive-only, and with a nominal duration of ; but each LIT is replaced by a pulse burst consisting of 17 to 33 pulses sent apart. Each pulse burst is called a fast link pulse (FLP) burst. The time interval between the start of each FLP burst is the same as between NLPs. The FLP burst consists of 17 NLP at a time interval with a tolerance of . Between each pair of two consecutive NLPs (i.e. at after first NLP of the pulse pair) an additional positive pulse may be present. The presence of this additional pulse indicates a logical 1, its absence a logical 0. As a result, every FLP contains a 16-bit data word. This data word is called a ''link code word'' (LCW). The bits of the LCW are numbered from 0 to 15, where bit 0 corresponds to the first possible pulse in time and bit 15 to the last.

The base link code word

Every fast link pulse burst transmits 16 bits of data known as a link code word. The first such word is known as a ''base link code word'', and its bits are used as follows: * 0–4: selector fieldindicates which standard is used between IEEE 802.3 and IEEE 802.9 * 5–12: technology ability fielda sequence of bits that encode the possible modes of operations among the 100BASE-T and 10BASE-T modes (see below) * 13: remote faultset to one when the device is detecting a link failure * 14: acknowledgementthe device sets this to one to indicate the correct reception of the base link code word from the other party; this is detected by the reception of at least three identical base code words. Upon receiving these three identical copies, the device sends a link code word with the acknowledge bit set to one from six times to eight times. * 15: next pageused to indicate the intention of sending other link code words after the base link code word The technology ability field is composed of eight bits. For IEEE 802.3, these are as follows: * bit 0: device supports 10BASE-T * bit 1: device supports 10BASE-T in full duplex * bit 2: device supports 100BASE-TX * bit 3: device supports 100BASE-TX in full duplex * bit 4: device supports 100BASE-T4 * bit 5: device supports

pause frame Ethernet flow control is a mechanism for temporarily stopping the transmission of data on Ethernet family computer networks. The goal of this mechanism is to avoid packet loss in the presence of network congestion. The first flow control mech ...

* bit 6: device supports asymmetric pause for full duplex * bit 7: reserved The link code words are also called ''pages''. The base link code word is therefore called a base page. The next page bit of the base page is 1 when the device intends to send other pages, which can be used to communicate other abilities. These additional pages are sent only if both devices have sent base pages with a next page bit set to 1. The additional pages are still encoded as link code words (using 17 clock pulses and up to 16 bit pulses).

Message and unformatted next page

The base page is sufficient for devices to advertise which ones among the 10BASE-T, 100BASE-TX and 100BASE-T4 modes they support. For gigabit Ethernet, two other pages are required. These pages are sent if both devices have sent base pages with a next page bit set to one. The additional pages are of two kinds: ''message pages'' and ''unformatted pages''. These pages are still 16-bit words encoded as pulses in the same way as the base page. Their first eleven bits are data, while their second-to-last bit indicates whether the page is a message page or an unformatted page. The last bit of each page indicates the presence of an additional page. The 1000BASE-T supported modes and master-slave data (which is used to decide which of the two devices acts as the master, and which one acts as the slave) are sent using a single message page, followed by a single unformatted page. The message page contains: * half duplex capability * whether the device is single port or multiport * whether master/slave is manually configured or not * whether the device is manually configured as master or slave The unformatted page contains a 10-bit word, called a master-slave seed value.

Duplex mismatch

A duplex mismatch occurs when two connected devices are configured in different duplex modes. This may happen, for example, if one is configured for autonegotiation while the other one has a fixed mode of operation that is full duplex (no autonegotiation). In such conditions, the autonegotiation device correctly detects the speed of operation, but is unable to correctly detect the duplex mode. As a result, it sets the correct speed but assumes half-duplex mode. When a device is operating in full duplex while the other one operates in half duplex, the connection works reliably only at a very low throughput. A full-duplex device may transmit data while it is receiving. However, if the half-duplex device receives data while it is sending, it senses a

collision In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great fo ...

and aborts transmission and then attempts to resend the frame. The full-duplex device will report

frame check sequence A frame check sequence (FCS) is an error-detecting code added to a frame in a communication protocol. Frames are used to send payload data from a source to a destination. Purpose All frames and the bits, bytes, and fields contained within ...

(FCS) errors on the aborted transmissions. Depending on timing, the half-duplex device may sense a late collision, which it will interpret as a hard error rather than a normal consequence of CSMA/CD and may not attempt to resend the frame. The full-duplex device does not detect any collision and assumes the frame was received without error. This combination of (late) collisions reported at the half-duplex end and FCS errors reported by the full-duplex end are indicators that a duplex mismatch is present.

Patents

Autonegotiation is covered by the US patents , , , , , , ; European Patent Applications SN 93308568.0 (DE, FR, GB, IT, NL); Korean Patent No. 286791; Taiwanese Patent No. 098359; Japanese Patent No. 3705610; Japanese Patent 4234. Applications SN H5-274147; Korean Patent Applications SN 22995/93; Taiwanese Patent Applications SN 83104531.

References

External links

Ethernet Autonegotiation Best Practices

Gigabit Ethernet Auto-Negotiation

Clause 28 & 40 AutoNegotiation An introduction to the Auto-Negotiation process

Auto-Negotiation, how does it work?

10/100/1000 Link Timing
{{Ethernet Ethernet Network protocols he:Ethernet#Autonegotiation