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Autonegotiation or auto-sensing refers to a process that allows two Ethernet network ports connected to each other (e.g. the network ports of a computer and those of the router , hub or switch to which it is connected, for example) to independently use the maximum to negotiate and configure the possible transmission speed and the duplex method. The procedure only applies to multi-wire connections ( twisted pair cables ) - but not to WLAN , fiber optic or coaxial cable connections .


Autonegotiation in the Ethernet (also called NWay ) works on layer 1 of the OSI model and is defined in the IEEE standard 802.3u.

Nway or N-way is an older term for auto negotiation in telecommunications. It was developed in 1994 by National Semiconductor in order to eliminate incompatibilities with devices with the 10BASE-T (10MBit / s) that had been common up to that point due to the market launch of Fast Ethernet / 100BASE-TX (100MBit / s).

In contrast to the two slower versions, which also had a fixed configuration of speed and duplex mode, the implementation of autonegotiation is mandatory for Gigabit Ethernet (1000BASE-T). Problems due to incorrect configuration, which are often the cause of poor performance in Ethernet networks, are thus reduced.

Link Pulse

In the case of Ethernet over copper cables ( twisted pair cable ), network nodes that are connected to active network components (such as a hub or switch ) are recorded using voltage pulses, so-called link pulses . In 10 Mbit / s networks, this is implemented by the NLP ( Normal Link Pulse ), a pulse that occurs periodically every 16 ± 8 ms.

In 100/1000 Mbit / s networks (Fast / Gigabit Ethernet), the NLP is replaced by a Fast Ethernet pulse , the Fast Link Pulse ( FLP ). This FLP is also sent out every 16 ± 8 ms, which ensures compatibility with older network cards. These treat the FLP like an NLP and determine the existence of a connection without being able to evaluate the information encoded in the FLP .

Pulse sketch for NLP and FLP (pulse spacing 16 ± 8 ms each)

A series of NLPs as used by 10BASE-T subscribers

Autonegotiation is based on pulses similar to those used by 10BASE-T participants to check the presence of other participants. These pulses are sent every 16 ms (with a tolerance of 8 ms) if there is no data traffic. The pulses are positive unipolar and 100 ns long. They are also called link integrity test ( LIT ) pulses in 10BASE-T terminology, and normal link pulses (NLP) in the autonegotiation specification

A participant detects a link error if there is no data traffic for 50 to 150 ms and a pulse is detected. A recipient acknowledges a valid link with two consecutive LIT pulses.

Three FLPs as used by attendees to indicate their options

Autonegotiation uses similar pulses. They are also positive unipolar and have a duration of 100 ns, but each is replaced with a sequence of 33 pulses. Each sequence is called a fast link pulse (FLP) burst. The time interval between each burst is the same as between NLPs , 16 ± 8 ms.

Link Code Word

The 17 "odd" pulses of an FLP burst represent a clock signal, the 16 even pulses contain data information. An FLC burst is thus composed of a frame of 17 pulses with an interval of 125 µs each. In the middle of two of these frame pulses, there can be another pulse, which corresponds to a logical “1”, or it can be missing for a logical “0”. This creates a 16-bit logical word called a link code word ( LCW ). Bit 0 is the first and bit 15 is the last bit.

Coding of an LCW in an FLP burst

An FLP burst cannot be recognized as an NLP and a 10BASE-T participant will interpret the burst as a link error.

The FLP consists of clock pulses followed by data pulses (33 pulses), with which a 16-bit data word is transmitted. If nothing follows the clock pulse, this corresponds to a value of logic 0; if a subsequent pulse occurs, a cell value of 1.

The 16-bit long data word (LCW) has the following meaning in its basic form as a Base Link Code Word :

16 bits (D0 .. D15) of the link code word with the fields S0..S4, A0..A7, RF, AK, NP

D0…D4 S0….S4 Selector Field (00001 für IEEE 802.3, 00010 für IEEE 802.9)
D5…D12 A0….A7 Technology Ability Field (definiert die möglichen Übertragungsarten des Netzwerkinterfaces)
Folgende Arten sind definiert:
D5 A0 10BASE-T
D6 A1 10BASE-T Full Duplex
D7 A2 100BASE-TX
D8 A3 100BASE-TX Full Duplex
D9 A4 100BASE-T4
D11 A6 asymmetrische PAUSE für Fullduplex-Verbindungen
D12 A7 reserviert
D13 RF: Remote Fault (Fehlerindikator)
D14 AK: Acknowledge (Quittierung eines Datenpaketes)
D15 NP: Next Page (es folgen weitere Datenpakete mit herstellerspezifischen Daten)

Base Link Code Word Definition

Both remote stations for data transmission announce their capabilities in the Technology Ability Field and agree on the best match for both parameters (full duplex before half duplex and high speed before lower). A receiver must receive an LCW three times identically before accepting it and confirms or acknowledges it by setting the ACK bit to "1". If the receiver detects an error or incompatibility, it sets the RF bit to "1". After successful autonegotiation, an LCW with the RF bit set to “0” and the ACK bit set to “1” must be sent at least six times to complete the process.

However, this allows z. B. Do not set 1000BASE-T (a Gigabit Ethernet connection) yet. For this purpose, the transmission of further information in a further "Word" is necessary, since the meaning of the base LCW can no longer be expanded sufficiently to also allow this configuration. For this purpose, the next page bit in the LCW is set to "1", whereupon the next received "Word", as well as one or more subsequent unformatted "pages", are to be interpreted or decoded differently - namely as a so-called message page (MP) according to IEEE Standard 802.3, Annex 28C.

The transfer mode is negotiated on both sides using a priority list in order to determine the maximum possible performance:

1 (highest) 1000BASE-T — Full duplex
2 1000BASE-T — Half duplex
3 100BASE-T2 — Full duplex
4 100BASE-TX — Full duplex
5 100BASE-T2 – Half duplex
6 100BASE-T4
7 100BASE-TX – Half duplex
8 10BASE-T – Full duplex
9 (lowest) 10BASE-T – Half duplex

If a remote station is not in autonegotiation mode (switched off or not supported), the other remote station can determine the transmission speed using parallel detection . It is not possible to determine the duplex mode; thus the half-duplex mode is always selected. In this case, the remote station without autonegotiation must be permanently set to half-duplex, otherwise the result is a duplex mismatch (one side full-duplex, the other side half-duplex). The typical effect of this is a working, but very slow connection.

Fiber Channel

Fiber Channel ports can use autonegotiation to detect the transmission speed.

See also