Ethernet

Introduction

Ethernet History

Broadcast Network Operation

Ethernet Protocol

Ethernet at 10Mbps

Ethernet at 100Mbps

Ethernet at 1000Mbps and above

Ethernet at 10 Mbps

The first Ethernet standard (Ethernet II) worked at a speed of 10 Mbps. This page will describe the technology used at this point in time.

10Base5 and 10Base2

The initial Ethernet implementations used coaxial cable to connect the stations to each other. Two forms of coaxial cable were used, 10Base5 cable known as thick ethernet and a thinner coaxial cable, 10Base2 also known as thin ethernet.

10Base5 uses 10mm wide coaxial cable which allows up to 100 nodes over a maximum distance of 500m. The hardware used in Thick Ethernet is divided in two major parts – one of them is the network interface card (NIC), which handles the digital aspects of communication. The second part is an analog electronic device, called a transceiver, which handles analog signals. The transceiver is attached directly to the Ethernet cable, and a separate cable, known as Attachment Unit Interface (AUI) cable, connects the transceiver to the NIC in the computer.

10Base2 uses 5mm wide and allows up to 30 nodes over a maximum distance of 185m. Thin Ethernet generally costs less than Thick Ethernet. The hardware that performs the transceiver function is built into the NIC, so no external transceivers are needed. Also Thin Ethernet doesn’t use an AUI cable to attach the NIC to the communication medium, instead it attaches directly to the back of each computer using a BNC connector. The medium is a flexible cable than connects from the NIC on one computer directly to the NIC on another computer.

10BaseT with hubs

A newer form of Ethernet has become much more popular. Called 10BaseT, it uses twisted pair wiring instead of coaxial cable. 10BaseT doesn’t have a shared physical medium like the other wiring schemes. Instead, it has an electronic device, called an Ethernet hub, which serves as the centre of the network. Each computer is connected to the hub by twisted pair wiring using RJ-45 connectors. Electronic components in the hub simulate a physical cable, making the entire system operate like a conventional Ethernet. The hub repeats any signal from an input to all of its outputs, thus replacing the broadcast feature of the cable bus.

Hubs have several advantages over a single cable bus system. Firstly, the hubs allow for a centralised monitoring and maintenance infrastructure over the network. Secondly, 10BaseT could use Category 3 twisted pair wire, which is commonly already present in modern office buildings for the telephone system.

Switches

A hub works in much the same way as a single bus (as it is intended to) but this means that if two stations transmit at the same time there is going to be a collision, even if the messages are going to different destinations. If, instead of connecting all of the stations on a hub to a single backplane (i.e. effectively a single shared medium), we could have multiple paths. This allows messages from different sources to different destinations to occur in parallel. This is the principle behind an Ethernet switch, which breaks from the basic CSMA/CD concept of Ethernet and effectively produces a switched network. These switches look just like a 10BaseT hub to the NIC, but there will be less collisions in the network (although collisions will still occur, for example if two messages have the same destination).

It is also possible to connected a switch to a Network Interface Card using two twisted pairs to create a full duplex 10Mbps connection, i.e. a connection which can both send and receive at the same moment of time at full speed.

Bridges

A Bridge is an electronic device that connects two Ethernet segments. It handles complete frames and has the same network interface as a computer. When the bridge receives a frame from one segment, it verifies that the frame is valid and if so forwards the copy of the frame to the other segment if necessary. Therefore, two Ethernet segments connected by a bridge behave like a single Ethernet – any pair of computers on the extended Ethernet can communicate.

A Bridge performs frame filtering. This means that the Bridge doesn't forward the frame if it knows that the destination address is at the same side as the source of the frame. It does this by building up a list of computer addresses on a segment from the source address of messages sent by that segment.

Broadcast and collision domains

The Broadcast Domain represents the domain of nodes which can be addressed in a LAN. Separate LANs will have different broadcast domains. The Collision Domain of a network is the domain in which a collision will occur if two nodes attempt to transmit data at the same time. A LAN comprising a single segment, or multiple segments connected by repeaters will function as a single collision domain. A switch, router or bridge will not forward a collision signal, and will therefore separate the collision domain.

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