Ethernet

Ethernet

1. Ethernet is the predominant LAN technology in the world. Ethernet works on the data link layer and the physical layer. Ethernet protocol standards define many aspects of network communication, including format, size, timing, and frame coding.

   • Ethernet encapsulation
     • Ethernet works on the data link layer and the physical layer. It is a family of network technologies that are defined in the IEEE 802.2 and 802.3 standards.

2. MAC Sublayer

   • Data encapsulation
     • The process of data encapsulation includes the arming of frames before the transmission and the disarming of frames at the moment of reception. To arm the frame, the MAC layer adds a header and a trailer to the PDU of the network layer.
       • Data encapsulation provides three main functions:
         • Frame delimitation
           • The framework process provides important delimiters that are used to identify a group of bits that make up a frame. These delimiting bits provide synchronization between the transmit and receive nodes.
         • Addressing
           • The encapsulation process contains the layer 3 PDU and also provides addressing of the data link layer.
         • Error detection
           • Each frame contains a trailer used to detect transmission errors.
   • Media access control
     • The second task of the MAC sub-layer is the access control to the medium. The control of access to the medium is responsible for placing the frames in the media and removing them from them. As the name implies, it controls access to the media. This sublayer communicates directly with the physical layer.

3. Evolution of Ethernet

   • Since the creation of Ethernet in 1973, standards evolved to specify faster and more flexible versions of the technology. This ability of Ethernet to improve over time is one of the main reasons why its use is so widespread. The first versions of Ethernet were relatively slow, with a speed of 10 Mbps, while the latest versions work at 10 Gbps and even faster. Scroll through the timeline in Figure 1 to see the different versions of Ethernet.

4. Ethernet raster fields

   • The minimum Ethernet frame size is 64 bytes, and the maximum is 1518 bytes. This includes all bytes of the "Destination MAC Address" field up to the "Frame Check Sequence (FCS)" field inclusive. The "Preamble" field is not included when describing the size of a frame.
   • Any frame less than 64 bytes in length is considered a collision fragment or a short frame, and is automatically discarded by the receiving stations. The frames of more than 1500 bytes of data are considered "jumbos" or giant baby frames.

5. Frame processing

   • Often, the MAC address is known as the "physical address (BIA)" because, historically, this address is physically recorded in the read-only memory (ROM) of the NIC. In other words, the address is permanently encoded in the ROM chip.
   • When the computer boots, the first thing the NIC does is copy the MAC address of the ROM to the RAM. When a device forwards a message to an Ethernet network, it attaches the header information to the frame. The header information contains the source and destination MAC addresses.

6. Representations of the MAC address

   • On a Windows host, you can use the ipconfig / all command to identify the MAC address of an Ethernet adapter. In Figure 1, note that it is indicated on the screen that the physical address (MAC) of the computer is 00-18-DE-DD-A7-B2. If you have access, we suggest you try this on your own computer. On a Mac or Linux host, the ipconfig command is used.
   • Depending on the device and the operating system, you can see several representations of MAC addresses, as shown in Figure 2. Cisco routers and switches use the format XXXX.XXXX.XXXX, where X is a hexadecimal character.
   • Unicast MAC address
     • In Ethernet, different MAC addresses are used for unicast, broadcast and layer 2 multicast communications.
     • A unicast MAC address is the unique address used when a frame is sent from a single transmitting device to a single receiving device.
   • Broadcast MAC address
     • Broadcast packets have a destination IPv4 address that contains only one (1) numbers in the host portion. This numbering in the address means that all hosts in that local network (broadcast domain) will receive and process the packet. Many network protocols, such as DHCP and ARP, use broadcasting.
   • Multicast MAC address
     • Multicast addresses allow a source device to send a packet to a group of devices. A multicast group IP address is assigned to the devices that belong to a multicast group.