STP

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.