Ethernet Protocol Basics

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During the 90s of last century many protocols existed in various networks and architectures. Engineers had developed diverse types of protocols; each for a specific data communication usage such as data transfer, voice, TV broadcast, data centers, and so on. Among which you can find Token Ring, FDDI, ATM and more.

However, due to the simplicity and to the low cost of Ethernet components, it has prevailed eventually, and is now the most widely used protocol in Local Area Networks (LAN). Faster Ethernet protocol were defined that can fit many of the above mentioned applications and network.

Actually Ethernet is a family of protocols, each of a different speed and all of them are based of packet transfer, as defined by the IEEE 802.3 standard group. Today there are Ethernet protocols and components that span from 10Mbit/sec (10 millions bits per second), to 100Mbit/sec (a.k.a. Fast Ethernet), 1Gbit/sec (1 billion bits per second), 10Gbit/sec, 40Gbit/sec and lately also 100Gbit/sec.

Ethernet is a physical layer protocol (layer1 and 2 of the 7 layers of the OSI model). It defines the way an Ethernet frame (packet) is transferred on the cable/fiber, and how the Ethernet switches along the way can direct the frame to its destination.

Ethernet can run on twisted pair copper cables and on fiber cables. The copper cables usually can transfer Ethernet packet up to 100 meters, and are used mainly in inner building networks. Fiber cables can reach tens of kilometers and are used to interconnect between far campuses and cities.

Being a physical layer protocol, Ethernet is a carrier of data for higher layer protocols such as IP (Internet protocol). Actually, most of the cases in which people discuss IP protocol, it is assumed they discuss Ethernet protocol as the carrier. The IP protocol enables to communicate between two logically separated networks. I.e. Ethernet enables direct communication within a logical network, called VLAN (Virtual Lan), while you must use IP protocol to perform proper routing of Ethernet packet between the VLANs.

Most of the electronics devices we use, such as personal computers and laptops, communicate by sending each other Ethernet packets. They are notated as Ethernet stations. Each such station has a unique address (a globally unique number comprises of 48 bits). This is called a MAC address. Each station has a network interface card (NIC) which is aware of this number and would not let an Ethernet packet with another MAC address to enter the station itself. Also, this MAC address is added to any packet transmitted from this NIC. Eventually, any packet carries the unique source address along its way.

The main component in an Ethernet network is the Ethernet switch (For clarity we discuss here Switched Ethernet, and ignore an older Shared Ethernet scheme, which is almost obsolete). A switch is connected to the stations either by a copper cable or a fiber cable, and can connect to various speed stations. Since all the stations, regardless of the speed itself use the same packet format, the switch enables all stations, regardless of their speed to interconnect.

As defined by the IEEE 802.3 group, the Ethernet switch is designed to detect the Ethernet packet origin (Ethernet Source Address) and the packet destination (Ethernet Destination Address). The destination and the source addresses are the first parameters in an Ethernet packet, and are read by the switch. A table of addresses within the switch constantly learns and keeps records of the addresses that are included in the packets entering the switch over the time, and organizes the address table such that each address is related to the port from which certain source address entered the switch. This table is served to direct each incoming packet to its relevant output port, so that the packet should reach its destination eventually.

The detection of the addresses, learning the addresses, and the decision where to switch the packet to, are all done by hardware means, as fast as possible. This is why Ethernet can reach 100Gbits/second.

The IEEE 802.3 definition of Ethernet includes, of course, some more notions such as Full-Duplex, Flow Control, Cut-Through, Store-and-Forward, MAC, Broadcast and Unicast packets, Auto-negotiation, Spanning Tree (STP), and more. We will discuss these topics in further articles.

More related information can be found at Communication Books, and Electronics Books, and at My Laptop.

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