Ethernet overview

Ethernet began as a common media protocol whereby all hosts communicated over a single 10Mbps cable or channel. If a hold wanted to connect on the channel, it would very first listen come make sure no other communications were taking place. It would then begin transmitting and also listen for any kind of collisions with other hosts the may have started transmitting at the exact same time. If a collision to be detected, each host would earlier off because that a random time period before attempting another transmission. This protocol ended up being known together Carrier feeling Multiple accessibility with Collision Detection (CSMA/CD). As Ethernet speeds progressed from 10Mbps come 100Mbps to 1000Mbps (GbE), a common channel was no longer practical. Today, Ethernet does no share a channel, yet instead, every endpoint has actually a specialized full duplex connection to a switch the forwards the data to the correct location endpoint.

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Ethernet is a layer 2 protocol contrasted to TCP/IP i beg your pardon is a great 3 protocol. Let’s use a rail analogy to describe this. A shipping company has a container with a bar password identifier that it requirements to move from the west coast to the east shore using two separate railway service providers (call them western Rail and also Eastern Rail). Western Rail choose up the container, reads the bar code, lots it top top a flatcar and sends the halfway throughout the country through several switching yards. The flat vehicle has its own bar code, i beg your pardon is supplied at the switching garden to reroute the flat vehicle to the destination. Fifty percent way across the country, east Rail currently reads the bar password on the container, tons it onto an additional flatcar, and sends that the remainder of the method across the nation through several an ext switching yards.

In this analogy, the bar password on the container is choose the TCP/IP header. Together the structure (container) enters the very first 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 supplied to forward the packet through number of Ethernet switches (railroad convert yards). The packet may then be stripped that the Ethernet header in ~ a layer 3 TCP/IP router and also forwarded to a last Ethernet network (Eastern Rail), where one more Ethernet header is appended based on the TCP/IP header information and also the packet is sent out to its final destination. The railroad is choose a class 2 network and also is just responsible for moving the container throughout its domain. The shipping firm is favor the layer 3 network and is responsible because that the destination deal with (container bar code) and for making sure the container come at the destination. Stop look at the Ethernet frame style in figure 2.13.

The following is a summary of the header fields shown in the figure. One interframe void of at least 12 bytes is used in between frames. The minimum framework size consisting of the header and cyclic redundancy check (CRC) is 64 bytes. Jumbo frames can take the maximum frame size increase to approximately 16K bytes.

Preamble and start-of-frame (SoF): The preamble is used to acquire the receiving serializer/deserializer up to speed and locked onto the bit timing that the obtained frame. In most cases today, this have the right to be done with just one byte leaving an additional six bytes obtainable to carry user proprietary information between switches. A SoF byte is offered to signal the begin of the frame.

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

Source MAC address: The resource MAC attend to is likewise sent in each frame header i beg your pardon is supplied to support resolve learning in the switch. For example, once a new endpoint joins the network, it deserve to inject a framework with an unknown designation MAC. Each switch will certainly then transfer this frame out every ports. By looking at the MAC resource address, and the port number the the structure came in on, the switch have the right to learn wherein to send future frames destined come this brand-new MAC address.

Virtual regional area network tags (optional): VLANs to be initially arisen to permit companies to produce multiple virtual networks in ~ one physics network in order to address issues such as security, network scalability, and also network management. Because that example, the accountancy department might want to have a various VLAN 보다 the design department for this reason packets will remain in their very own VLAN domain in ~ the bigger physical network. The VLAN sign is 12-bits, providing up come 4096 different virtual LANs. It additionally contains framework priority information. We will provide more information on the VLAN sign in chapter 5.

Ethertype: This field can be provided to either provide the size of the payload or the form of the payload.

Payload: The payload is the data gift transported from source to destination. In numerous cases, the payload is a class 3 framework such as a TCP/IP frame.

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