Ethernet FRAMES

When a PHY communicates with another PHY, the signal is converted inside the PHY.
A digital signal is sent from an OSI reference model layer 2 device such as SWITCH to PHY using an interface called MII (Media Independent Interface).Data transmission is possible because the digital signal transmission format is standardized.
This article explains how PHY receives digital signals using MII.

Data Transfer Method

OSI Reference Model

The OSI reference model classifies the mechanisms of communication functions in computer networks into seven categories. Each layer has a different role, and layer 2 determines the data transfer method within the same network.
Layer 1 has physical functions such as signal conversion.

Encapsulation

With data alone, it is unclear how the data will be processed or where it will be sent, but by adding a header to the data, the processing method and destination become clear, making it possible to transmit information appropriately. This is called "encapsulation." The headers at each layer add different information. Processing proceeds from the upper layer to the lower layer, and the data is sent as an electrical signal to the opposing device.

The L2 header adds destination and source information, while the L1 header adds a signal to start sending data.

Decapsulation

The data received as an electrical signal has its headers removed from the lower layer to the higher layer, until only the data is extracted.
This is called "de-encapsulation."

Packets sent and received

A MAC frame is FRAMES that adds information (described below) to data processed at Layer 3. This article explains the general FRAMES specified in IEEE802.3.

The part from the Destination address field to the Frame Check Sequence field is called FRAMES, and the part from the Preamble field to the ETD is called a packet. The SFD field, Address field, Length/Type field, and Frame Check Sequence field are fixed length.
Packets are transmitted in order starting with the Preamble field, and the bits of each octet are transmitted least significant bit first.

FRAME

Address field

This field indicates the destination MAC address and source MAC address. This information allows you to determine the destination of the packet and identify the source station. Each address field is 6 octets.

Length/Type field

It has one of two meanings depending on the numerical value of the 2 octets.

Length: 1500 or less (0x05DC or less)

Indicates the number of octets contained in the data FRAMES.

Type: 1536 or higher (0x0600 or higher)

Indicates the format of the data FRAMES.

Data field

This is data from the upper layer. This data is encapsulated and passed to the lower layer. A typical FRAMES is 46 to 1500 octets. If the minimum FRAMES length requirement (64 octets) is not met, additional bits are added per octet.

Frame Check Sequence field

FRAMES is used to detect errors in transmitted FRAMES. A 4-octet CRC (Cyclic Redundancy Check) value is added to all fields of the frame except the FCS field.

PACKET

Preamble

This starts packet transmission and sends a signal for receiver synchronization. It is sent at the beginning of each packet and is specified to be at least 56 bits long. The signal consists of repeated "1" and "0", with the final bit specified to be "0".
Example: "10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101010 "

SFD (Start Frame Delimiter)

It follows the preamble and indicates the start of the data frame. It is specified as "10101011".

DATA(FRAME)

This is the FRAME data. After this data is encapsulated, signal conversion is performed. The total amount of data is a multiple of 8 bits.

EFD (End Frame Delimiter)

It indicates the end of transmission and serves to turn off the transmitter.

MII data stream

Sending method using sending PIN

In the case of MII, the bits of each octet are transmitted and received as two nibbles (4 bits).
Bits 0 to 3 of the octet correspond to bits 0 to 3 of the first nibble, bits 4 to 7 of the octet correspond to bits 0 to 3 of the second nibble, and so on.

Initial bit sent on the send pin

As shown in the diagram above, the octet signal is processed, and data is sent after the preamble and SFD. After a packet has been sent, the shortest waiting time to send the next packet is 96 bits.