Ethernet
From ProPHOTO WIKI
Contents |
Introduction
The foundation for most modern networks, Ethernet provides the basic mechanisms used by devices to communicate. Formally standardized as IEEE 802.3, it specifies the basic communication protocols, electrical signaling mechanisms and physical media used to build these networks. In and of itself, Ethernet only provides basic facilities - however when combined with higher level protocols such as TCP/IP it provides the fundamental resources necessary to build complex networks.
The basic Ethernet standards have been expanded over the years in order to provide higher transfer rates and expanded functionality. Later standards such as Fast Ethernet, Gigabit Ethernet and WiFi all built on the basis of these standards. This has taken it from simple 10mbps networks based on coaxial cabling to sophisticated 10gbps networks running over optical fibre.
Physical Requirements
Most conventional Ethernet networks operate with point-to-point links using 8-conductor unshielded twisted pair cabling with RJ45 jacks. Each device on the network is connected to a switch or hub, which, in turn, distribute the data to the other nodes on the network. This allows networks to be built from a central location and, when switches are used, can provide direct communication between devices.
Cable Grades
This cabling is provided in a number of different 'categories' in order to suit a number of different variants of the standard. While conventional 10mbps Ethernet networks only require Category 3 wiring, Fast Ethernet and Gigabit Ethernet require at least Category 5 wiring. As Category 5e wiring has now become mainstream, its cost has come to the point where it is rarely necessary to use lower grade cabling.
Cable Length
Individual strings of copper wiring used for Ethernet networks should not exceed 100 meters (328ft) for proper operation. If longer runs are required, optical fibre can be used for cable lengths as long as 2km (1.25 miles). These requirements are based on timing issues, so higher quality cabling cannot work around these limitations.
Optional Functionality
Power over Ethernet (IEEE 802.3af)
A newer addition to the Ethernet standards, 802.3af allows power to be delivered alongside data on a single cable. Using otherwise unused conductors within conventional Category 5 cables, this system can provide up to 11 watts of power. This allows devices in remote locations to be serviced by a single cable run, reducing the complexity of the underlying infrastructure.
Note that there are a number of proprietary PoE implementations that don't comply to the 802.3af standard. These systems perform the same function, however they are only compatible with other devices using the same implementation.
Full-Duplex/Half-Duplex
Ethernet networks can either work in full-duplex or half-duplex modes. In the later configuration, the same channel is used for sending and receiving data - as such, only one task can be handled at any given time. When operating in this configuration the provided bandwidth must be shared between the two directions.
In the former configuration, each Ethernet link has two seperate channels - one for sending data and one for receiving data. This allows data to be sent and received in parallel, and provides the rated throughput in each direction. Almost all modern networking equipment uses this configuration, so the distinction is typically only necessary for older devices.
Auto-Negotiation
When two devices are connected, this technology allows them to detect the capabilities of one another and select the fastest communication mode available. For instance, if a Gigabit Ethernet network adapter is connected to a Fast Ethernet switch the adapter will switch to the slower mode in order to provide backwards compatibility with the switch. As above, nearly all modern networking equipment supports this technology.
Auto-MDI/MDI-X
When connecting two hubs or switches, one typically must use a special 'crossover cable' that exchanges the send and receive lines on each end of the cable. Some newer hubs and switches have logic that automatically detects this issue and corrects for it within the device itself, so no special cables are needed. Note that while this is an optional feature on Ethernet and Fast Ethernet devices, it is a mandatory feature for all Gigabit Ethernet devices.
See Also
- Fast Ethernet - 100mbps variant of the Ethernet standard. Almost all modern networking equipment supports this standard.
- Gigabit Ethernet - 1000mbps variant of the Ethernet standard. Has widespread support in networking adapters, however other networking devices (eg switches and routers) are still lagging behind.
- WiFi - A set of popular extensions to this standard that provide wireless networking capabilities.
- Switch - An Ethernet device that allows a number of computers to communicate it an efficient manner.
