Ethernet - CSMA/CD, collision domain, duplex

Ethernet

Ethernet is a broad family of network technologies. Ethernet previously used a shared communication medium, which could be metallic or optical cable, for data transmission. Ethernet is specified by the IEEE 802.3 standard.

Ethernet is the dominant technology for building local area networks (LANs), but it also has a significant presence in larger networks. Network technology falls into the second (data link) layer of the ISO OSI model, but also into the first (physical) layer. It defines a range of parameters regarding cables (transmission medium) and signals, which is the 1st layer. It also describes the method of network access and the general addressing format, which is the 2nd layer.

Examples of other network technologies that have been largely replaced by Ethernet include FDDI, Token Ring, and ARCNET. Another technology with a significant share in LANs and growing usage is WiFi - a wireless network standardized in IEEE 802.11.

Initially, the transmission medium used was a thick coaxial cable (10Base-5 or thin coaxial cable 10Base-2) in a shared bus configuration. The transmission speed was 10Mb/s. It was gradually replaced by twisted pair cable in a star configuration (using a hub). At a speed of 10Mb/s, it was simply called Ethernet (10Base-T). The latest medium for Ethernet is optical fiber (10Base-F). Over time, speeds increased to 100Mb/s, known as Fast Ethernet (e.g., 100Base-TX), 1Gb/s, known as Gigabit Ethernet (e.g., 1000Base-T), and today we have a standard for 10 Gigabit Ethernet (10GBase-T). Similarly, hubs were replaced by switches, creating point-to-point connections, and the technology is referred to as switched Ethernet, where a shared medium is no longer used.

For certain types of Ethernet, the appropriate cable is always defined. For optics, we can have single-mode or multi-mode fibers with specific parameters. For metallic cables, shielded (STP - Shielded Twisted Pair) or unshielded (UTP - Unshielded Twisted Pair) twisted pair cables are used. Additionally, they are categorized according to their properties and suitability for specific Ethernet types, such as UTP cables of category 3, 4, 5, 5e, 6, 6a, or 7.

Media Access - CSMA/CD

When multiple devices are connected to a shared bus, only one can communicate at a time while others can only listen. The problem arises when multiple devices start communicating simultaneously, causing a collision. For transmission (media access), Carrier Sense Multiple Access With Collision Detection (CSMA/CD) is used as a network control protocol.

CSMA/CD Algorithm

1. The device that wants to transmit listens to see if the line is free
2. If it is free, it starts transmitting the frame
3. The sender listens to detect any collision
4. If a collision is detected, it sends a jam signal
5. After the jam ends, the stations that were transmitting set a random time to wait
6. After the time expires, the process starts again from step 1

Collision Domain

Collision domain is a part of the network where a collision of transmissions from multiple stations can occur. It is a group of devices connected to a shared medium. This is practically an example of old network layouts using coaxial cable or the use of hubs (repeaters), which forward received packets to all interfaces except the one from which they came.

Previously, to divide a network into multiple collision domains (and reduce their size), the network was segmented and connected using routers. However, switches (bridges) appeared, which by their function create separate collision domains on each interface. A switch sends a received packet only to one interface (in a learned state) and performs packet switching.

If an end device or another switch is connected to a switch and the link is full duplex, no collision can occur on this link. Practically, this means that we have two communicating devices on the link, and each device sends data over its own pair of wires.

Duplex

Half duplex means that communication can only be active in one direction at a time.

Full duplex is when we can transmit and receive simultaneously. In Ethernet, two pairs of wires (usually, but sometimes four) are used, with half used only for transmission and half for reception. This eliminates problems and delays caused by the CSMA/CD algorithm, and the theoretical speed is doubled (in fast Ethernet, 100Mb/s is used for transmission and 100Mb/s for reception).

Note: Hubs do not support full duplex communication because collisions can occur. If two stations transmit simultaneously, the hub receives it and sends it to other ports, effectively sending a collision.