Ethernet (802.3), CSMA/CD (Carrier Sense Multiple Access / Collision Detection) | Computer Network

Computer Network | Ethernet (802.3)

Ethernet is the most widely-installed local area network (LAN) technology. Ethernet was originally developed by Xerox from an earlier specification called Aloha net and then developed further by Xerox, DEC, and Intel. An Ethernet LAN typically uses coaxial cable or special grades of twisted pair wires. Ethernet is also used in wireless. Ethernet is standardized as IEEE 802.3 that specifies a CSMA/CD bus network. CSMA/CD (Carrier Sense Multiple Access / Collision Detect) is used to detect the collision in the network. An Ethernet CSMA/CD can be implemented using a Bus or even a Star topology.

Common Ethernet types:

Common Name
Alternative Name
Name Of IEEE Standard
Cable Type, Maximum Length
10 Mbps
IEEE 802.3
Copper, 100 m
Fast Ethernet
100 Mbps
IEEE 802.3u
Copper, 100 m
Gigabit Ethernet
1000 Mbps
IEEE 802.3z
Fiber, 550 m (SX)
5 km (LX)
Gigabit Ethernet
1000 Mbps
IEEE 802.3ab
100 m

802.3 Ethernet frame format:

802.3 Ethernet frame format:

802.3 Ethernet frame format:

Explanation :

Preamble: An 8-byte pattern of binary 1s and 0s used to establish synchronization. The last bit of the preamble is always 0.

Start Frame Delimiter: An 8-bit pattern indicating the formal start of the frame.

Destination Address: An address specifying a specific destination station, a group of stations, or all stations in the LAN. This address can be 16 bits or 48 bits in length, but all stations in the LAN must adhere to one format or the other. 

Source Address: The address of the originating station. This address has the same length requirements as the destination address.

Length: The length measured in bytes, of the actual data, indicating the 802.2 header. This is a 16 bit field. 

Following the header is the 802.2 header and the actual data. At the end of the data is the 802.3 trailer, which includes:

Padding: Extra, non-data bytes can be inserted into the frame to make the overall frame length more palatable to the physical network.

Frame check sequence: At the end of the frame is a 32 bit Cyclic Redundancy Check (CRC) on the data starting with the destination addresses the terminating at the end of the data (not including any padding)

CSMA/CD (Carrier Sense Multiple Access / Collision Detection)

  • CSMA/CD is the protocol used in Ethernet networks to ensure that only one network node is transmitting on the network wire at any one time.
  • Carrier Sense means that every Ethernet device listens to the Ethernet wire before it attempts to transmit. If the Ethernet device senses that another device is transmitting, it will wait to transmit.
  • Multiple Access means that more than one Ethernet device can be sensing (listening and waiting to transmit) at a time.
  • Collision Detection means that when multiple Ethernet devices accidentally transmit at the same time, they are able to detect this error.
CSMA/CD (Carrier Sense Multiple Access / Collision Detection)
CSMA/CD (Carrier Sense Multiple Access / Collision Detection)

Advantages of Ethernet

  • Short access delay at low load.
  • MAC management is relatively simple, distributed.
  • Huge installed base and significant operational experience.

Disadvantages of Ethernet

  • Operation at high traffic load is problematic.
  • Variable/Unbounded delay-not well suited for real-time applications.

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