ETHERNET

ETHERNET

1. ENCAPSULATION

   • SUBLAYER LLC
     • The LLC sublayer takes the protocol data from the network, which is generally an IPv4 packet, and adds control information to help distribute the packet to the destination node. The LLC is used for communication with the upper layers of the application and for the transition of the package to the lower layers for distribution purposes.
   • MAC SUBCAPA
     • The MAC sublayer is the lower sublayer of the data link layer and is implemented by hardware, generally, in the computer's NIC. The specific data is detailed in the IEEE 802.3 standards. In Figure 2, the common Ethernet IEEE standards are detailed.
     • 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.
         • - DELIMITATION OF TRAMS: 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.
         • - ADDRESS: 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.

2. 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.

3. ETHERNET FRAME FIELDS

   • • • • • • • The Preamble (7 bytes) and Frame Start Delimiter (SFD) fields, also called "frame start" (1 byte), are used for synchronization between the sending and receiving devices. These first eight bytes of the frame are used to capture the attention of the receiving nodes. Basically, the first bytes tell the receiver to prepare to receive a new frame.
             • This 6-byte field is the identifier of the intended recipient. Layer 2 uses this address to help the devices determine if the frame is aimed at them. The address of the frame is compared to the MAC address of the device. If they match, the device accepts the frame. It can be a unicast, multicast or broadcast address.
           • this 6-byte field identifies the NIC or the origin interface of the frame. It must be a unicast address.
         • 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.
       • This field (46 to 1500 bytes) contains the encapsulated data of a higher layer, which is a layer 3 PDU or, more commonly, an IPv4 packet. All frames must have at least 64 bytes in length. A small packet is encapsulated, additional bits (called "padding") are used to increase the size of the frame to the minimum size.
     • The frame check sequence (FCS) field (4bytes) is used to detect errors in the frame. It uses a cyclic redundancy check (CRC). The sending device includes the results of a CRC in the field FCS of the frame. The receiving device receives the frame and generates a CRC to look for errors. If the calculations match, it means that no error occurred. The calculations in the data could be a consequence of an interruption of the electrical signals that represent the bits.

4. MAC ADDRESS: ETHERNET IDENTITY

   • In Ethernet, each network device is connected to the same shared media. In the past, Ethernet was, to a greater extent, a medium duplex topology that used a multiple access bus or, more recently, Ethernet hubs. That is, all the nodes received each transmitted frame. To avoid the excessive overhead involved in the processing of each frame, the MAC addresses were created in order to identify the real origin and destination.

5. 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, for example the physical address (MAC) of a computer is 00-18-DE-DD-A7-B2. On a Mac or Linux host, the ipconfig command is used.

6. UNIDIFUSION MAC ADDRESS

   • A unicast MAC address is the unique address used when a frame is sent from a single transmitting device to a single receiving device. The process that a source host uses to determine the destination MAC address is known as the "address resolution protocol (ARP)". The ARP is discussed later in this chapter.

7. MULTI-DISTRIBUTION 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. Multicast addresses can be used in remote games, where many players connect remotely to play the same game. Another use of multicast addresses is distance learning through videoconferencing, where many students are connected to the same class.

8. BASIC NOTIONS OF SWITCHES

   • A layer 2 Ethernet switch uses MAC addresses to make forwarding decisions. Unaware completely what protocol is transmitted in the data portion of the frame, such as an IPv4 packet. The switch makes forwarding decisions only according to layer 2 Ethernet MAC addresses.

9. OBTAINING MAC ADDRESSES

   • The switch arms the MAC address table dynamically after examining the source MAC address of the frames received in a port. The switch forwards the frames after searching for a match between the destination MAC address of the frame and an entry of the MAC address table.

10. METHODS FOR FORWARDING THE TRACES OF THE CISCO SWITCHES

    • STORAGE AND SENDING: A storage and dispatch switch receives the complete frame and calculates the CRC. If the CRC is valid, the switch searches for the recipient address, which determines the exit interface.
    • CUTTING METHOD: the switch that uses the cutting method sends the frame before receiving it in its entirety. At a minimum, the destination address must be read so that the frame can be sent.

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