Ethernet network becomes LAN standards today. Compared to it’s competitor 20 years ago – Token ring network, Ethernet network win the battle due to it’s superior characteristic – easy and cheaper. From the original commercial specifications that transferred data 10 megabits per second (Mbps) to the 10 gigabits per second (Gbps) rates today, Ethernet network has evolved and become the most prolific LAN protocol ever.
Ethernet network defines both Layer 1 and Layer 2 functions of the OSI reference model. The OSI physical and data link layers work together to provide the function of delivery of data across a wide variety of types of physical networks. Some obvious physical details must be agreed upon before communication can happen, such as the cabling, the types of connectors used on the ends of the cables, and voltage and current levels used to encode a binary 0 or 1.
Data link layer defines the rules (the protocols) to determine when a computer is allowed to use the physical network when the computer should not use the network, and how to recognize errors that occurred during transmission of data.
The term Ethernet refers to a family of protocols and standards that together define the physical and data link layers of the world’s most popular type of LAN. Many variations of Ethernet exist including:
1. 10BASE-T
2. Fast Ethernet
3. Gigabit Ethernet.
10Base-T Ethernet network
10BASE-T Ethernet network allowed the use of telephone cabling that was already installed, or simply allowed the use of cheaper, easier-to-install cabling when new cabling was required. 10BASE-T networks make use of devices called hubs. The physical 10BASE-T Ethernet network uses Ethernet cards in the computers, cabling, and a hub.
The hubs used to create a 10BASE-T Ethernet network are essentially multiport repeaters. That means that the hub simply regenerates the electrical signal that comes in one port and sends the same signal out every other port, therefore collisions can still occurs.
Ethernet network 10Base-T cabling
Ethernet network 10Base-T typically uses Category 5 UTP network cables with RJ-45 connectors.
- The cable used to connect the PCs to the hub is called a straight-through cable.
- The cable used to connect between switches and hubs is crossover cable.
Ethernet Networking
Performance issues: collisions
10BASE2, 10BASE5, and 10BASE-T Ethernet would not work without CSMA/CD. However, because of the CSMA/CD algorithm, Ethernet becomes more inefficient under higher loads; Ethernet began to degrade when the load began to exceed 30 percent utilization.
CSMA/CD logic helps prevent collisions and also defines how to act when a collision does occur. The CSMA/CD algorithm works like this:
1. A device with a frame to send listens until the Ethernet network is not busy.
2. When the Ethernet network is not busy, the sender begins sending the frame.
3. The sender listens to make sure that no collision occurred.
4. Once the senders hear the collision, they each send a jamming signal, to ensure that all stations recognize the collision.
5. After the jamming is complete, each sender randomizes a timer and waits that long.
6. When each timer expires, the process starts over with Step 1.
Reducing collisions – LAN switches
The term collision domain defines the set of devices for which their frames could collide. All devices on a 10BASE2, 10BASE5, or 10BASE-T network using a hub risk collisions between the frames that they send, so all devices on one of these types of Ethernet networks are in the same collision domain.
LAN switches overcome the problems created by collisions and the CSMA/CD algorithm by removing the possibility of a collision. Switches do not create a single shared bus, like a hub; they treat each individual physical port as a separate bus. Switches use memory buffers to hold incoming frames as well, so when two attached devices send a frame at the same time, the switch can forward one frame while holding the other frame in a memory buffer, waiting to forward one frame until after the first one has been forwarded.
Full-Duplex – eliminates collisions
The original Ethernet specifications used a shared bus, over which only one frame could be sent at any point in time, it is called half-duplex. LAN switches with only one device cabled to each port of the switch allow the use of full-duplex operation. Full duplex means that an Ethernet card can send and receive concurrently.
Ethernet network summary
Physical layer protocols define how to deliver data across a physical medium. Data-link protocols make that physical network useful by defining how and when the physical network is used. Ethernet network defines the OSI Layer 1 functions for Ethernet network, including cabling, connectors, voltage levels, and cabling distance limitations, as well as many important OSI Layer 2 functions.
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