Ethernet Frame
- (The Golden Gate Bridge, San Francisco, California - Jeff M. Wang)
- Ethernet Frame
An Ethernet frame is a data link layer protocol data unit and uses the underlying Ethernet physical layer transport mechanisms. In other words, a data unit on an Ethernet link transports an Ethernet frame as its payload. An Ethernet frame contains three parts; an Ethernet header (Preamble, SFD, Destination, Source, and Type), Encapsulated data (Data and Pad), and an Ethernet trailer (FCS).
An Ethernet frame is preceded by a preamble and start frame delimiter (SFD), which are both part of the Ethernet packet at the physical layer. Each Ethernet frame starts with an Ethernet header, which contains destination and source MAC addresses as its first two fields. The middle section of the frame is payload data including any headers for other protocols (for example, Internet Protocol) carried in the frame. The frame ends with a frame check sequence (FCS), which is a 32-bit cyclic redundancy check used to detect any in-transit corruption of data.
The following image shows an example of an Ethernet frame.
- (Ethernet Frame - CompterNetworknotes)
* The preamble field
An Ethernet packet starts with a seven-octet preamble and one-octet start frame delimiter (SFD). The preamble consists of a 56-bit (seven-byte) pattern of alternating 1 and 0 bits to make the pattern '10101010', allowing devices on the network to easily synchronize their receiver clocks, providing bit-level synchronization.
Preamble bytes help the receiving device to identify the beginning of an Ethernet frame. When a device receives 7 continuous bytes of the same pattern (10101010), it assumes that the incoming data is an Ethernet frame and it locks the incoming bit-stream.
* The SFD field
The SFD, Start Frame Delimiter, is the eight-bit (one-byte) value that marks the end of the preamble, which is the first field of an Ethernet packet, and indicates the beginning of the Ethernet frame. This byte also stores the same pattern, except the last bit. In the last bit, it stores 1 instead of the 0.
For Ethernet variants transmitting serial bits instead of larger symbols, the (uncoded) on-the-wire bit pattern for the preamble together with the SFD portion of the frame is 10101010 10101010 10101010 10101010 10101010 10101010 10101010 10101011; The bits are transmitted in order, from left to right.
The SFD byte indicates the receiving device that the next byte is the destination MAC address of the Ethernet frame.
* Destination MAC address
This field is 6 bytes long. It contains the MAC address of the destination device. MAC address is 6 bytes or 48 bits (1 byte = 8 bits, 6x8 = 48bits) long. For convenience, usually, it is written as 12-digit hexadecimal numbers (such as 0000.0A12.1234).
The destination MAC address allows the receiving device to determine whether an incoming frame is intended for it or not. If a frame is not intended for the receiving device, the receiving device discards that frame.
* Source MAC address
This field is also 6 bytes long. It contains the MAC address of the source device. It helps the receiving device in identifying the source device. The following image shows an example of both types of address in the frame.
* The Type field
The type field is 2 bytes long. This field stores information about the protocol of the upper layer (network layer). It is used to indicate which protocol is encapsulated in the payload of the frame and is used at the receiving end by the data link layer to determine how the payload is processed. The same field is also used to indicate the size of some Ethernet frames.
The Data Link layer of the source computer prepares, packs and loads the Ethernet frame in the media. The Data link layer of the destination computer picks the Ethernet frame from the media. After picking the Ethernet frame, the Data link layer of the destination computer unpacks, processes, and hands over that Ethernet frame to the upper layer for further processing.
If multiple protocols are running in the upper (network) layer of the destination computer, the data link layer will fail to hand over the received frame to the upper layer as it does not know to which protocol it should give the received frame.
The type field solves this issue. This field allows the sender computer to insert the information of the upper layer protocol. Through this information, the data link layer of the destination computer can easily determine the upper layer protocol to which it should hand over the received frame.
Modern LAN implementations mostly use the IP protocol in the network layer. There are two variants of the IP protocol; IPv4 and IPv6. If the type field has value IP or ox800, the frame is carrying the data of the IPv4 protocol. If the type field has value IPv6 or 0x86dd, the frame is carrying the data of the IPv6 protocol.
* Data and Pad field
This field stores the encapsulated data of the upper layer. This field has a size limit of 46 bytes (minimum) to 1500 bytes (maximum). Due to this limit, the network (upper) layer can't pack more or less data in a single packet (encapsulated data of the upper layer). If data is less than the minimum requirement, padding is added. If data is more than the maximum limit, extra data is packed in the next packet.
Each frame contains up to 1500 bytes of data. Minimally, a frame must contain at least 46 bytes of data, even if this means the host has to pad the frame before transmitting it. The reason for this minimum frame size is that the frame must be long enough to detect a collision.
* FCS (Frame Check Sequence)
All frames and the bits, bytes, and fields contained within them, are susceptible to errors from a variety of sources. The FCS field contains a number that is calculated by the source node based on the data in the frame. This number is added to the end of a frame that is sent. When the destination node receives the frame the FCS number is recalculated and compared with the FCS number included in the frame. If the two numbers are different, an error is assumed and the frame is discarded.
The FCS field is 4 bytes long. This field stores a 4 bytes value that is used to check whether the received frame is intact or not. The sender device takes all fields of the frame except the FCS field, and runs them through an algorithm, known as the CRC (Cyclic Redundancy Check). The CRC algorithm generates a 4-byte result, which is placed in this FCS field.
When the destination device receives a frame, it takes the same fields and runs them through the same algorithm. If the result matches with the value stored in the FCS field, the frame is considered good and is processed further. If both values do not match, the frame is considered bad and is dropped.
[More to come ...]
