Day-16: Ethernet.

By | May 22, 2022

When Ethernet was developed, if you talk about 1970’s , It was developed by Xerox but later on they added another few partners like DEC and Intel into this project and the name they kept as DIX .

Ethernet Features.

  • Ethernet – Most common LAN technology used today.
  • Supports data bandwidths of 10, 100, 1000, 10,000, 40,000, and 100,000 Mbps (100 Gbps)
  • Operates in the data link layer and the physical layer.
  • Defined in the IEEE 802.2 and 802.3 standards.
  • Ethernet relies on the two separate sublayers of the data link layer to operate:
    • Logical Link Control (LLC)
    • MAC

Network Interface Card (NIC)

  • Layer 2, Data Link Layer, device
  • Connects the device (computer) to the LAN
  • Responsible for the local Layer 2 address.
  • Common Layer 2 NICs:
    • Ethernet
    • Token Ring
  • Common Bandwidth
    • 10 Mbps, 10/100 Mbps, 10/100/1000 Mbps, 10/100/1000/10000 Mbps.


Ethernet provides a method for controlling how the nodes share access through the use a Carrier Sense Multiple Access (CSMA) technology . If multiple devices on a single medium attempt to forward data simultaneously, the data will collide resulting in corrupted, unusable data

Carrier Sense Multiple Access (CSMA) process 

Carrier sense is basically developed for preventing the collision on the wire , It means when you are sending the frame on Ethernet you have to listen that wire first whether the media is free or busy if it is busy you have to wait for certain amount of time before you send .normally 9.25 micro second you have to wait . This is basically useful in Half duplex network But in full duplex it is not necessary .

  • Used to first detect if the media is carrying a signal
  • If no carrier signal is detected, the device transmits its data
  • If two devices transmit at the same time – data collision

CSMA/CD and Collisions

  • CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
  • Listens to the network’s shared media to see if any other users on “on the line” by trying to sense a neutral electrical signal or carrier.
  • If no transmission is sensed, then multiple access allows anyone onto the media without any further permission required.
  • If two PCs detect a neutral signal and access the shared media at the exact same time, a collision occurs and is detected.
  • The PCs sense the collision by being unable to deliver the entire frame (coming soon) onto the network. (This is why there are minimum frame lengths along with cable distance and speed limitations.)
  • When a collision occurs, a jamming signal is sent out by the first PC to detect the collision.
  • Using either a priority or random backoff scheme, the PCs wait certain amount of time before retransmitting.
  • If collisions continue to occur, the PCs random interval is doubled, lessening the chances of a collision.

CSMA/Collision Detection

  • With today’s intermediate devices (full-duplex switches), collisions do not occur.
  • Processes utilized by CSMA/CD are really unnecessary.
  • Wireless connections in a LAN environment still have to take collisions into account.

CSMA/Collision Avoidance (CSMA/CA) media access method

  • Device examines the media for the presence of data signal – if the media is free, the device sends a notification across the media of its intent to use it
  • The device then sends the data.
  • Used by 802.11 wireless networking technologies

Evolution of the Ethernet .

  • 1979 Bob Metcalfe developed Ethernet at XEROX PARC
  • 1980 DEC-Intel-Xerox (DIX) publish first original 10 Mbps Ethernet Standard over thick coaxial cable
  • 1985 IEEE 802.3 used DIX standard and published standard with the title IEEE 802.3 Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications
  • Supplements
    • 1985 10BASE2 Thin Ethernet
    • 1990 10BASE-T Twisted-pair
    • 1995 100BASE-T Fast Ethernet and Autonegotiation
    • 1997 Full Duplex Standard
    • 1998 1000BASE-X Gigabit Ethernet

MAC Address

  • Layer 2 Ethernet MAC address is a 48-bit binary value expressed as 12 hexadecimal digits.
  • IEEE requires a vendor to follow two simple rules:
    • Must use that vendor’s assigned OUI as the first 3 bytes.
    • All MAC addresses with the same OUI must be assigned a unique value in the last 3 bytes.

There are two bits in the first octet of a MAC address that are used to define certain aspects of the MAC address. The least significant bit of the first octet of the MAC address is the Individual/Group(I/G) address bit and the next to least significant bit of the first octet of the MAC address is the Universally or Locally(U/L) administered address bit.

The Individual/Group address bit is used to signify if the destination MAC address is a unicast or a multicast/broadcast Layer 2 address. If the bit is set to 0 then it is an Individual MAC address or a unicast MAC address. If the bit is set to 1 then it is a Group address and is a multicast/broadcast address.

The Universally Administered or Locally Administered address bit is used to tell if the MAC address is the burned-in-address(BIA) or a MAC address that has been changed locally. Maybe this bit served a purpose in the past but with modern applications on Windows or Unix systems you can change the MAC address to almost anything and this bit does not have to be set to tell the system that you are no longer using the manufacturer’s BIA. If the bit is set to 0 then the MAC address is recognized as a BIA MAC address. When the bit is set to 1 then the MAC address is recognized as being changed from the BIA to a unique MAC address that is locally setup.

Ethernet Charecterstics.

  • The Ethernet protocol uses MAC addresses to identify the source of the Ethernet frame and the destination of the Ethernet frame.
  • Whenever any computer sends an Ethernet frame, it includes the MAC address on its NIC as the Source “MAC” Address.
  • Every device with an Ethernet NIC has a MAC addresses assigned:
    • Examples, workstations, servers, printers, switches, and routers
  • MAC addresses are sometimes referred to as burned-in addresses (BIAs)
  • Examples: 00-05-9A-AC-78-00, 00:05:7A:3C:78:00, or 0A05.9A3C.7800.
  • Ethernet header contains the source and destination MAC address.
  • Each NIC views information to see if the destination MAC address in the frame matches the device’s physical MAC address stored in RAM.
  • No match, the device discards the frame.
  • Matches the destination MAC of the frame, the NIC passes the frame up the OSI layers, where the decapsulation process takes place.

Ethernet Frames.


The very first field is preamble. Preamble means it’s a wake up call . Like it informs the wire that hey everyone here is a ethernet frame going .

DA/SA– DA is destination address and SA is source address . But these are the MAC addresses .

Type:-As we know each layer encapsulate something , So the type code is basically something to represent the frame types like ARP, IPv4 and IPv6 etc etc .All the protocols are having number so that number will be seated in the type code filed .

Data & Padding :- These are basically our data.FCS:-FCS runs a formula and calculate some numbers and put into the packet and sent . when other end will receive that frame they will calculate it and that should be comeout with that same FCS value what the sender had . if the value went different then it means there is somewhere the few bits of the packet has been changed or modified or corrupted.

IEEE 802.3

When the DIX handed it to IEEE which is our 802.3 for standisation they added few extra field into the frame header .

SD:-Starting  delimiter , 

Type Filed has been replaced with the length field but it works differently, the length field says how big the data size is . The type code is same as the 802.2 header , this is the field which do upper and lower layer encapsulation  .

Ethernet Cabling Details

In todays network we are not using the coaxial cable , instead we uses twisted cable .

When you connect two devices of different types together, you use a straight through cable. When you connect two devices of the same type together, you use a crossover cable.


 There is one more complication to take into account when you are buying network cables.  If your device implements MDI and MDI-X, you don’t need to worry about crossover cables. MDI is the Medium Dependent Interface standard and it operates inside the socket of a NIC. It can detect which wire transmissions arrive on rather than doggedly listening on pins 3 and 6. MDI-X stands for Medium Dependent Interface – Crossover. So, if your computer, printer, switch, router or modem has MDI printed on the box, you only need to buy a straight through cable.

Normally if you are connecting the two devices which both are in same layer device you can use the cross over cables . if the devices are in different layer then you have to use the Straight over cable .

Connections between specific devices, such as switch-to-switch, switch-to-router, switch-to-host, and router-to-host device, once required the use of a specific cable types (crossover or straight-through).

Modern Cisco switches support the mdix auto interface configuration command to enable the automatic medium-dependent interface crossover (auto-MDIX) feature.

The auto-MDIX feature is enabled by default, therefore a cable change is not needed

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