class="current-li label label--small">Understanding Ethernet Encapsulation In Networking
Ethernet and wireless LANs (WLANs) are the two most commonly deployed LAN technologies. Unlike wireless, Ethernet uses wired communications, including twisted pair, fibre-optic links, and coaxial cables. In this article, we are going to talk about ethernet encapsulation in networking.
Ethernet operates in the data link layer and the physical layer. It is a family of networking technologies defined in the IEEE 802.2 and 802.3 standards. Ethernet supports the following data bandwidths:
- 10 Mbps
- 100 Mbps
- 1000 Mbps (1 Gbps)
- 10,000 Mbps (10 Gbps)
- 40,000 Mbps (40 Gbps)
- 100,000 Mbps (100 Gbps)
As shown in the figure, Ethernet standards define both the Layer 2 protocols and the Layer 1 technologies.
Ethernet and the OSI Modes
Ethernet Frame Fields
The minimum Ethernet frame size is 64 bytes and the maximum is 1518 bytes. This includes all bytes from the destination MAC address field through the frame check sequence (FCS) field. The preamble field is not included when describing the size of the frame.
Any frame less than 64 bytes in length is considered a “collision fragment” or “runt frame” and is automatically discarded by receiving stations. Frames with more than 1500 bytes of data are considered “jumbo” or “baby giant frames”.
If the size of a transmitted frame is less than the minimum, or greater than the maximum, the receiving device drops the frame. Dropped frames are likely to be the result of collisions or other unwanted signals. They are considered invalid. However, the Fast Ethernet and Gigabit Ethernet interfaces of some Cisco Catalyst switches can be configured to support larger jumbo frames..
The diagram shows the fields of an Ethernet frame. From left to right the fields and their length are Preamble and SFD, 8 bytes; destination MAC address, 6 bytes; source MAC address, 6 bytes; type/length, 2 bytes; data, 46 – 1500 bytes; and F C S, 4 bytes. Excluding the first field, the total number of bytes in the remaining fields is between 64 – 1518 bytes.
Ethernet Frame Fields
|Preamble and Start Frame Delimiter Fields||The Preamble (7 bytes) and Start Frame Delimiter (SFD), also called the Start of Frame (1 byte), fields are used for synchronization between the sending and receiving devices. These first eight bytes of the frame are used to get the attention of the receiving nodes. Essentially, the first few bytes tell the receivers to get ready to receive a new frame.|
|Destination MAC Address Field||This 6-byte field is the identifier for the intended recipient. As you will recall, this address is used by Layer 2 to assist devices in determining if a frame is addressed to them. The address in the frame is compared to the MAC address in the device. If there is a match, the device accepts the frame. Can be a unicast, multicast or broadcast address.|
|Source MAC Address Field||This 6-byte field identifies the originating NIC or interface of the frame. A source MAC address can only be a unicast address.|
|Type / Length||This 2-byte field identifies the upper layer protocol encapsulated in the Ethernet frame. Common values are, in hexadecimal, 0x800 for IPv4, 0x86DD for IPv6 and 0x806 for ARP.
Note: You may also see this field referred to as EtherType, Type, or Length.
|Data Field||This field (46 – 1500 bytes) contains the encapsulated data from a higher layer, which is a generic Layer 3 PDU, or more commonly, an IPv4 packet. All frames must be at least 64 bytes long. If a small packet is encapsulated, additional bits called a pad are used to increase the size of the frame to this minimum size.|
|Frame Check Sequence Field||The Frame Check Sequence (FCS) field (4 bytes) is used to detect errors in a frame. It uses a cyclic redundancy check (CRC). The sending device includes the results of a CRC in the FCS field of the frame. The receiving device receives the frame and generates a CRC to look for errors. If the calculations match, no error occurred. Calculations that do not match are an indication that the data has changed; therefore, the frame is dropped. A change in the data could be the result of a disruption of the electrical signals that represent the bits.|
MAC Address Format
The figure is three columns showing the decimal and hexadecimal equivalents of select 4-bit binary numbers. From left to right, the column headings are decimal, binary, and hexadecimal. Each column has 16 rows below the header.
Decimal and Binary Equivalents of 0 to F Hexadecimal
An Ethernet MAC address is a 48-bit binary value expressed as 12 hexadecimal digits (4 bits per hexadecimal digit). Hexadecimal digits uses the numbers 0 to 9 and the letters A to F. The figure shows the equivalent decimal and hexadecimal values for binary 0000 to 1111. Hexadecimal is commonly used to represent binary data. IPv6 addresses are another example of hexadecimal addressing.
Figure 1 shows a table with Decimal values between 0 and 15 with the Binary and Hexadecimal equivalents. This table demonstrates why Hexadecimal has letters A through F along with numbers 0 through 9. Figure 2 shows that a MAC address can be represented with dashes, colons or periods.
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