Ethernet overview

Ethernet began as a common media protocol whereby all hosts interacted over a single 10Mbps cable or channel. If a organize wanted to connect on the channel, the would first listen come make sure no other communications were taking place. It would certainly then begin transmitting and likewise listen for any collisions with various other hosts the may have actually started transmitting in ~ the very same time. If a collision to be detected, each hold would back off because that a random time period before attempting one more transmission. This protocol ended up being known together Carrier sense Multiple access with Collision Detection (CSMA/CD). As Ethernet speeds advanced from 10Mbps come 100Mbps to 1000Mbps (GbE), a shared channel was no much longer practical. Today, Ethernet does no share a channel, however instead, each endpoint has actually a committed full duplex connection to a switch the forwards the data come the correct destination endpoint.

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Ethernet is a class 2 protocol contrasted to TCP/IP i beg your pardon is a layer 3 protocol. Let’s use a rail analogy to describe this. A shipping agency has a container through a bar code identifier the it needs to move from the west coastline to the east coastline using two different railway providers (call them western Rail and Eastern Rail). Western Rail picks up the container, reads the bar code, tons it ~ above a flatcar and also sends that halfway throughout the country through numerous switching yards. The flat vehicle has its very own bar code, i m sorry is used at the switching garden to reroute the flat car to the destination. Half way across the country, east Rail currently reads the bar password on the container, tons it onto one more flatcar, and also sends it the remainder of the means across the nation through several an ext switching yards.


In this analogy, the bar code on the container is favor the TCP/IP header. As the frame (container) start the an initial Ethernet network (Western Rail), the TCP/IP header is read and also an Ethernet header (flatcar bar code) is attached i beg your pardon is provided to forward the packet through numerous Ethernet switches (railroad convert yards). The packet may then it is in stripped that the Ethernet header within a class 3 TCP/IP router and also forwarded to a final Ethernet network (Eastern Rail), where an additional Ethernet header is appended based on the TCP/IP header information and the packet is sent to its final destination. The rail is favor a great 2 network and is just responsible for moving the container across its domain. The shipping firm is prefer the class 3 network and is responsible for the destination attend to (container bar code) and for making sure the container arrives at the destination. Let’s look in ~ the Ethernet frame style in figure 2.13.



The following is a summary of the header fields presented in the figure. An interframe gap of at least 12 bytes is used between frames. The minimum structure size consisting of the header and also cyclic redundancy inspect (CRC) is 64 bytes. Jumbo frames can take the maximum structure size up to roughly 16K bytes.

Preamble and also start-of-frame (SoF): The preamble is supplied to get the receiving serializer/deserializer up to speed and also locked onto the little bit timing the the obtained frame. In most instances today, this have the right to be done with simply one byte leaving one more six bytes available to deliver user proprietary information between switches. A SoF byte is provided to signal the start of the frame.

Destination Media accessibility Control (MAC) address: every endpoint in the Ethernet network has an address called a MAC address. The destination MAC address is provided by the Ethernet switches come determine exactly how to forward packets v the network.

Source MAC address: The resource MAC resolve is additionally sent in each frame header i beg your pardon is provided to support address learning in the switch. For example, once a brand-new endpoint joins the network, it can inject a frame with an unknown designation MAC. Every switch will then transfer this frame out every ports. By looking at the MAC resource address, and the port number the the frame came in on, the switch can learn whereby to send future frames destined come this new MAC address.

Virtual regional area network tags (optional): VLANs to be initially occurred to enable companies to create multiple virtual networks in ~ one physics network in stimulate to address issues such as security, network scalability, and network management. For example, the accounting department might want to have a different VLAN 보다 the engineering department therefore packets will remain in their own VLAN domain in ~ the larger physical network. The VLAN sign is 12-bits, giving up come 4096 various virtual LANs. It likewise contains structure priority information. We will certainly provide more information ~ above the VLAN tag in thing 5.

Ethertype: This ar can be supplied to either carry out the dimension of the payload or the form of the payload.

Payload: The payload is the data being transported from resource to destination. In plenty of cases, the payload is a great 3 frame such as a TCP/IP frame.

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CRC (frame inspect sequence): Each framework can be checked for corrupted data making use of a CRC.