Ethernet

Ethernet Operations

Ethernet operates on a shared medium with multiple devices connected. To prevent signal jamming from different devices, Ethernet uses a protocol called CSMA/CD, which stands for Carrier Sense Multiple Access with Collision Detection. The CSMA/CD process occurs before and during the transmission of signals over the Ethernet cable and can be broken down into the following steps:

CSMA/CD Functions:

1) Check the sender's readiness to send packets.

2) Verify that the Ethernet cable is not in use by sending dummy data onto the medium. If the dummy data transmits without collision, the medium is considered idle. This is the carrier sense portion, where it senses for signals from other connected devices. If a collision occurs, it refrains and retries after a random time.

3) Transmit data and listen for collisions - At this point, data is transmitted, and the device listens for collision signals. If a collision occurs, it halts the transmission and emits a jam signal, instructing all devices using the medium to pause and retry after a random interval. Then, it restarts the process of sending a dummy signal.

4) If no signal collision is detected, the sender completes the frame transmission and prepares for the next transmission.

This protocol allows multiple devices to share a single communication medium effectively.

LAYER 2 Devices

HUBS

Ethernet hubs, also known as network hubs, are devices that simply forward the data they receive. They do not modify the data or discriminate between ports when sending out signals. Often referred to as "dumb" devices, their main advantage lies in allowing multiple connections to a single signal source. It is important to note that Ethernet hubs function within a single, large broadcast domain. What a hub receives, it broadcasts through all open and available ports. I will explain collision and broadcast domains in subsequent writing.

Let's take a detour to discuss collision domains and broadcast domains to enhance our understanding of Ethernet switches.

Broadcast Domain and Collision Domain.

The first thing to understand about collision and broadcast domains is that they are logical constructs. A broadcast domain is a logical division of a network in which any signal received from any source is retransmitted to every device within that domain. A hub groups all connected devices into a single broadcast domain, which means any signal sent by one device is broadcasted to all devices connected to the hub, resulting in significant inefficiency. In contrast, switches connect devices into a single network but with a key advantage: they can learn MAC addresses. If a message is addressed to a specific MAC, the switch, upon recognizing the destination MAC, will not broadcast the message but will instead send it directly to the corresponding collision domain. Each port on a switch creates its own collision domain.

A collision domain, on the other hand, is a segment of a network where data packets can collide if multiple devices transmit messages simultaneously. Collisions become even more apparent when the switch is set to half-duplex, which prevents devices from communicating at the same time.

Here is a summary to help you recognize what each device group does.

Hubs: Hubs constitute a single large broadcast domain and a single large collision domain.
Switches: A single large broadcast domain exists unless VLANs are implemented, which segment the large broadcast into smaller sections. Each connected device has a separate collision domain.
Router: Separate broadcast domains mean that each interface is an individual network. This is why routers do not allow interfaces with overlapping networks. Additionally, they separate collision domains.

some more take home points.

Devices within the same broadcast domain do not require routing to communicate; they can connect directly. For example, devices in the same VLAN and network can interact with each other without routing.
Devices within the same collision domain may interfere with each other's signals.

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Switches.

Switches are devices capable of assigning individual ports, and thus individual devices, to separate collision domains. They place every device into a single large broadcast domain, meaning all broadcasts by default will reach all connected devices. To address this issue, switches can segment the broadcast domain into separate zones using VLANs. Each device within a specific VLAN will only receive broadcasts on that VLAN. Switches broadcast messages for an unknown MAC address, but require only one broadcast to learn a valid MAC address. Once the MAC address location is known, the switch builds a forwarding table, mapping ports to MAC addresses, eliminating the need to broadcast to a known MAC address.