Ethernet is the most widely used local area network (LAN) technology. Ethernet was designed to fill the middle ground between long-distance, low-speed networks and specialized, computer-room networks carrying data at high speeds for very limited distances. Ethernet is well suited to applications where a local communication medium must carry sporadic, occasionally heavy traffic at high peak data rates.

Ethernet network architecture has its origins in the 1960s at the University of Hawaii, where the access method that is used by Ethernet, carrier sense multiple access/collision detection (CSMA/CD), was developed. Xerox Corporation’s Palo Alto Research Center (PARC) developed the first experimental Ethernet system in the early 1970s. This was used as the basis for the Institute of Electrical and Electronic Engineers (IEEE) 802.3 specification released in 1980.

Shortly after the 1980 IEEE 802.3 specification, Digital Equipment Corporation, Intel Corporation, and Xerox Corporation jointly developed and released an Ethernet specification, Version 2.0, that was substantially compatible with IEEE 802.3. Together, Ethernet and IEEE 802.3 currently maintain the greatest market share of any LAN protocol. Today, the term Ethernet is often used to refer to all carrier sense multiple access/collision detection (CSMA/CD) LAN’s that generally conform to Ethernet specifications, including IEEE 802.3.

Ethernet and IEEE 802.3 specify similar technologies; both are CSMA/CD LANs. Stations on a CSMA/CD LAN can access the network at any time. Before sending data, CSMA/CD stations listen to the network to determine if it is already in use. If it is, then they wait. If the network is not in use, the stations transmit. A collision occurs when two stations listen for network traffic, hear none, and transmit simultaneously. In this case, both transmissions are damaged, and the stations must retransmit at some later time. Backoff algorithms determine when the colliding stations can retransmit. CSMA/CD stations can detect collisions, so they know when they must retransmit.

Both Ethernet and IEEE 802.3 LANs are broadcast networks. This means every station can see all of the frames, regardless of whether they are the intended destination of that data. Each station must examine the received frames to determine if they are the destination. If so, the frame is passed to a higher layer protocol within the station for appropriate processing.

Differences between Ethernet and IEEE 802.3 LANs are subtle. Ethernet provides services corresponding to Layer 1 and Layer 2 of the OSI reference model. IEEE 802.3 specifies the physical layer, Layer 1, and the channel-access portion of the data link layer, Layer 2, but does not define a Logical Link Control protocol. Both Ethernet and IEEE 802.3 are implemented through hardware. Typically, the physical part of these protocols is either an interface card in a host computer or circuitry on a primary circuit board within a host computer.

There are at least 18 varieties of Ethernet, which have been specified, or are in the specification process. -  

The table in Figure highlights some of the most common and important Ethernet technologies.

The Ethernet and IEEE 802.3 frame fields are described in the following summaries:

• preamble - The alternating pattern of 1's and 0's tells receiving stations that a frame is Ethernet or IEEE 802.3. The Ethernet frame includes an additional byte that is the equivalent of the Start of Frame (SOF) field specified in the IEEE 802.3 frame.

• start-of-frame (SOF) - The IEEE 802.3 delimiter byte ends with two consecutive 1 bits, which serve to synchronize the frame-reception portions of all stations on the LAN. SOF is explicitly specified in Ethernet.

• destination and source addresses - The first 3 bytes of the addresses are specified by the IEEE on a vendor-dependent basis. The last 3 bytes are specified by the Ethernet or IEEE 802.3 vendor. The source address is always a unicast (single-node) address. The destination address can be unicast, multicast (group), or broadcast (all nodes).

• type (Ethernet) - The type specifies the upper-layer protocol to receive the data after Ethernet processing is completed.

• length (IEEE 802.3) - The length indicates the number of bytes of data that follows this field.

• data (Ethernet) - After physical-layer and link-layer processing is complete, the data contained in the frame is sent to an upper-layer protocol, which is identified in the type field. Although Ethernet version 2 does not specify any padding, in contrast to IEEE 802.3, Ethernet expects at least 46 bytes of data.

• data (IEEE 802.3) - After physical-layer and link-layer processing is complete, the data is sent to an upper-layer protocol, which must be defined within the data portion of the frame. If data in the frame is insufficient to fill the frame to its minimum 64-byte size, padding bytes are inserted to ensure at least a 64-byte frame.

• frame check sequence (FCS) - This sequence contains a 4 byte CRC value that is created by the sending device and is recalculated by the receiving device to check for damaged frames.

Ethernet is a shared-media broadcast technology – summarized in the Figure - . The access method CSMA/CD used in Ethernet performs three functions:

1. transmitting and receiving data packets
2. decoding data packets and checking them for valid addresses before passing them to the upper layers of the OSI model
3. detecting errors within data packets or on the network

In the CSMA/CD access method, networking devices with data to transmit over the networking media work in a listen-before-transmit mode. This means when a device wants to send data, it must first check to see whether the networking media is busy. The device must check if there are any signals on the networking media. After the device determines the networking media is not busy, the device will begin to transmit its data. While transmitting its data in the form of signals, the device also listens. It does this to ensure no other stations are transmitting data to the networking media at the same time. After it completes transmitting its data, the device will return to listening mode. -

Networking devices are able to tell when a collision has occurred because the amplitude of the signal on the networking media will increase. When a collision occurs, each device that is transmitting will continue to transmit data for a short time. This is done to ensure that all devices see the collision. Once all devices on the network have seen that a collision has occurred, each device invokes an algorithm. After all devices on the network have backed off for a certain period of time (different for each device), any device can attempt to gain access to the networking media once again. When data transmission resumes on the network, the devices that were involved in the collision do not have priority to transmit data. The Figure summarizes the CSMA/CD process. 

Ethernet is a broadcast transmission medium. This means that all devices on a network can see all data that passes along the networking media. However, not all the devices on the network will process the data. Only the device whose MAC address and IP address matches the destination MAC address and destination IP address carried by the data will copy the data.

Once a device has verified the destination MAC and IP addresses carried by the data, it then checks the data packet for errors. If the device detects errors, the data packet is discarded. The destination device will not notify the source device regardless of whether the packet arrived successfully or not. Ethernet is a connectionless network architecture and is referred to as a best-effort delivery system.

Signal encoding is a way of combining both clock and data information into a stream of signals over a medium.  The rules of Manchester encoding define a 0 as a signal that is high for the first half of the period and low for the second half. It defines a 1 as a signal that is low for the first half of the period and high for the second half. The rules define a 1 as a signal that is low for the first half of the period and high for the second half.  

10BASE-T transceivers are designed to send and receive signals over a segment that consists of 4 wires - 1 pair of wires for transmitting data, and 1 pair of wires for receiving data. 

Note: Manchester encoding results in 0 being encoded as a high-to-low transition and 1 being encoded as a low-to-high transition. Because both 0's and 1's result in a transition to the signal, the clock can be effectively recovered at the receiver.

In a LAN, where the star topology is used, the networking media is run from a central hub out to each device attached to the network. The physical layout of the star topology resembles spokes radiating from the hub of a wheel. As the graphic shows, a central point of control is used in a star topology. When a star topology is used, communication between devices attached to the local area network is via point-to-point wiring to the central link or hub. All network traffic in a star topology passes through the hub.

The hub receives frames on a port, then copies and transmits (repeats) the frame to all of the other ports. The hub can be either active or passive. An active hub connects the networking media as well as regenerates the signal. In Ethernet where hubs act as multiport repeaters, they are sometimes referred to as concentrators. By regenerating the signal, active hubs enable data to travel over greater distances. A passive hub is a device used to connect networking media and does not regenerate a signal.

One of the star topology’s advantages is that it is considered the easiest to design and install. This is due to the networking media being run directly out from a central hub to each workstation area. Another advantage is its ease of maintenance since the only area of concentration is located at the hub. In a star topology, the layout used for the networking media is easy to modify and troubleshoot. Workstations can be easily added to a network employing a star topology. If one run of networking media is broken or shorted, then only the device attached at that point is out of commission, the rest of the LAN will remain functional. In short, a star topology means greater reliability.

In some ways a star topology's advantages can also be considered disadvantages. For example, while limiting one device per run of networking media can make diagnosis of problems easier, it also increases the amount of networking media required, which adds to the setup costs. And, while the hub can make maintenance easier, it represents a single point of failure (if the hub breaks, everyone's network connection is lost).

TIA/EIA-568-A specifies that the physical layout, or topology that is to be used for horizontal cabling, must be a star topology. This means the mechanical termination for each telecommunications outlet/connector is located at the patch panel in the wiring closet. Every outlet is independently and directly wired to the patch panel. 

The TIA/EIA-568-A specification, for the maximum length of horizontal cabling for unshielded twisted pair cable, is 90 m. The maximum length for patch cords at the telecommunications outlet/connector is 3 m, and the maximum length for patch cords/jumpers at the horizontal cross-connect is 6 m.

The maximum distance for a run of horizontal cabling, that extends from the hub to any workstation, is 100 m. (actually 99 m. but it is commonly rounded up to 100 m.) This figure includes the 90 meters for the horizontal cabling, the 3 meters for the patch cords, and the 6 meters for the jumpers at the horizontal cross-connect. Horizontal cabling runs in a star topology radiate out from the hub, much like the spokes of a wheel. This means that a LAN that uses a star topology could cover the area of a circle with a radius of 100 m.

There will be times when the area to be covered by a network will exceed the TIA/EIA-568-A specified maximum length that a simple star topology can accommodate. For example, envision a building where the dimensions are 200 m x 200 m. A simple star topology that adhered to the horizontal cabling standard specified by TIA/EIA-568-A could not provide complete coverage for that building. 

As indicated in the Figure , workstations E, F, and C are located outside the area that can be covered by a star topology that adheres to TIA/EIA-568-A specifications. As shown, they are not part of the local area network. So users at these workstations wanting to send, share, and receive files, would have to use sneakernet. Because no one wants to return to the days of sneakernet, some cable installers are tempted to solve the problem of a star topology's inadequate coverage by extending the length of the networking media beyond the TIA/EIA-568-A specified maximum length.  

When signals first leave a transmitting station, they are clean and easily recognizable. However, the longer the cable length, the weaker and more deteriorated the signals become as they pass along the networking media. If a signal travels beyond the specified maximum distance, there is no guarantee that when it reaches a NIC card, the NIC card will be able to read it.

If a star topology cannot provide enough coverage for an area to be networked, the network can be extended through the use of internetworking devices that do not result in attenuation of the signal. This resulting topology is designated as an extended star topology. By using repeaters, the distance over which a network can operate is extended. Repeaters take in weakened signals, regenerate and retime them, and send them back out onto the network.