Ethernet (802.3) | Computer Networks - Computer Science Engineering (CSE) PDF Download

Introduction to Ethernet

Ethernet was developed in the mid-1970s by researchers at the Xerox Palo Alto Research Center (PARC). It serves as a practical example of a broader technology called carrier sense, multiple access with collision detection (CSMA/CD), which is used in local area networks.

In CSMA/CD, "carrier sense" refers to the ability of all nodes to determine whether a network link is idle or busy. "Collision detect" means that while nodes are sending data, they listen for any interference, or collision, with frames from other nodes.

Key Features

• Standardized by: IEEE 802.3

• Media: Twisted pair cables, fiber optics, and coaxial cables.

• Data Rates: Ranges from 10 Mbps to 400 Gbps and beyond (like 800 Gbps being developed)

• Topology: Primarily star (modern) or bus (legacy)

• Access Method: CSMA/CD (Carrier Sense Multiple Access with Collision Detection) — though mostly obsolete in switched Ethernet.

Physical Properties of Ethernet

Coaxial Cable: An Ethernet segment uses a coaxial cable similar to that used for cable TV, but with some differences. The Ethernet cable can be up to 500 meters long and typically has an impedance of 50 ohms, whereas cable TV cables usually have an impedance of 75 ohms. To set up the segment, taps (connection points) need to be spaced at least 2.5 meters apart.

Transceiver: A transceiver is a small device attached to the tap on the coaxial cable. Its functions include:

• Detecting when the Ethernet line is not in use.
• Sending signals when a host is transmitting data.
• Receiving incoming signals.

The transceiver connects to an Ethernet adapter, which is then plugged into the host computer or device.

Repeaters: Multiple Ethernet segments can be connected using a repeater. A repeater’s role is to forward digital signals, similar to how an amplifier boosts analog signals. However, there are limitations: no more than four repeaters can be placed between any two hosts, which restricts the total range of an Ethernet network to 2,500 meters.

Maximum Hosts: Under specific standards, an Ethernet network can support a maximum of 1,024 hosts. At the end of each segment, terminators are used to absorb the signal. This absorption prevents the signal from bouncing back and causing interference with subsequent signals, ensuring smoother data transmission.

Standards of Ethernet

There are several standards of Ethernet, each with its own specifications:Ethernet Cable Standards

• 10Base5: This is the first physical standard defined in the IEEE 802.3 model. It is also known as thick net or thick Ethernet. A segment of the original 10Base5 cable can be up to 500 meters long.
• 10Base2: The second implementation defined by the IEEE 802 series is called 10Base2. It is also known as thin-net, cheapnet, or thin Ethernet. In this standard, '10' indicates a network operation speed of 10 Mbps, 'Base' signifies the use of a baseband system, and '2' denotes a segment length limit of 200 meters.
• 10Base-T: This is the most popular standard defined in the IEEE 802.3 series, known as 10Base-T or twisted pair Ethernet. The "T" refers to twisted pair cabling. A 10Base-T segment is typically limited to a length of less than 100 meters.