Functionality of Auto Negotiation

The function of negotiating between the capability advertised by the physical layer of the OSI reference model is known as Auto Negotiation. This mechanism helps to control the connection of a single MDI(Media Dependent Interface) to a single PHY type.

Ethernet devices supports various speeds and configuration. If a switch is connected to multiple devices with different speeds, it will be difficult to configure each connection manually. Auto negotiation function helps in detecting the mode of the device at the other end and advertise its own capabilities in order to configure the common speed for operation. This function helps in maintaining flexible connectivity among the Ethernet devices.

For negotiation the local device needs to advertise its ability to the remote device and simultaneously receives the abilities of remote device. The advertisement of abilities is done through DME(Differential Manchester Encoding) pages. A page contains all the information required by arbitration state machine for choosing HCD(Highest common denominator), HCD is the highest common technology resolved by the priority resolution function of AN, after which the link is set up. DME pages are not transmitted when Auto Negotiation is complete.

The DME page carries 48 bit with 106 evenly spaces transition positions(a position where transition can occur). It contains a Manchester violation delimiter, the 48 bit page and a single pseudo-random bit. Clock information is represented at odd numbered transition positions, and data information is represented at even numbered transition position. First eight position contains Manchester violation delimiter, it marks the beginning of the page. It contains a transition at position 1 and position 5 only. Each 49 odd-numbered positions will contain a transition, and 49 even-numbered transition position will represent the following information :-

1. If a transition is preset at even-numbered transition position then it represents logical one.

2. If a transition is absent at even-numbered transition position then it represents logical zero.

AN supports two types of pages:-

1. Base Page:- This is the first page to be transmitted by the device, this page advertises the speed available at the local end, it also provides the information of FEC(Forward Error Correction) feature supported. The remaining bits in the base page is used by the arbitration state machine.

2.Next Page:- Next pages are used to transfer null message code, device tag and negotiates the EEE capability of the device. There are two types of Next page encoding supported by AN:-

    a. Message Next Page :- This page has two field i.e. message code field and unformatted code field. Message code field defines how the unformatted code field will be interpreted.

    b. Unformatted Next Page :- This page is used along with the message next page for displaying OUI and device identifier tag values.

Process of Auto Negotiation:

There are three behavioral models defined in IEEE 802.3 Clause 73 for Auto Negotiation.

1. Auto negotiation transmit

2. Auto negotiation receive

3. Auto negotiation arbitration

DME Pages are transmitted by the transmit function of AN, transmitter function makes transition on the MDI lane based upon the encoding mentioned above. On reception the receive function decipher the page based on the transitions on the MDI lane. The arbitration function controls the functioning of transmitter and receiver function. It sends the page to the transmitter and also interprets the information from the received page. Arbiter then uses the priority resolution function to determine the highest common denominator and then connects the MDI to the PHY.