Project PDA

A widely-installed type of local area network (LAN) technology. An Ethernet LAN typically uses coaxial cable or special grades of twisted pair wires. The most commonly installed Ethernet systems are called 10BASE-T and provide transmission speeds up to 10 Mbps. "Fast Ethernet" refers to 100 Mbps Ethernet networks.

Ethernet evolved into the complex networking technology that today powers the vast majority of local computer networks. The coaxial cable was later replaced with point-to-point links connected together by hubs and/or switches in order to reduce installation costs, increase reliability, and enable point-to-point management and troubleshooting. StarLAN was the first step in the evolution of Ethernet from a coaxial cable bus to a hub-managed, twisted pair network. The advent of twisted-pair wiring enabled Ethernet to become a commercial success.

Ethernet has been standardized as IEEE 802.3. Its star-topology, twisted pair wiring form became the most widespread LAN technology in use from the 1990s to the present, largely replacing competing LAN standards such as coaxial cable Ethernet, token ring, FDDI, and ARCNET. In recent years, Wi-Fi, the wireless LAN standardized by IEEE 802.11, has been used in addition to or instead of Ethernet in many installations.

 Fast Ethernet provides 10 times the bandwidth of traditional 10 Mbps networks. There are two main schools in fast Ethernet territory: 100Base-X and 100VG-AnyLAN. 100VG-AnyLAN uses all four pairs. 25MHz signals are transmitted on each of the pairs. Contrast this with 20Mhz signals that are divided on the two wire pairs for 10Base-T.

Some managed switches offer a variety of tools to combat these issues including:
Spanning-tree protocol to maintain the active links of the network as a tree while allowing physical loops for redundancy.

Various port protection features, as it is far more likely an attacker will be on an end system port than on a switch-switch link.

• VLANs to keep different classes of users separate while using the same physical infrastructure.
• fast routing at higher levels to route between those VLANs.
• Link aggregation to add bandwidth to overloaded links and to provide some measure of redundancy, although the links won't protect against switch failure because they connect the same pair of switches.

CSMA/CD shared medium Ethernet

Ethernet originally used a shared coaxial cable (the shared medium) winding around a building or campus to every attached machine. A scheme known as carrier sense multiple access with collision detection (CSMA/CD) governed the way the computers share the channel. The scheme was relatively simple compared to competing technologies token ring or token bus.

Ethernet repeaters and hubs

For signal degradation and timing reasons, coaxial Ethernet segments had a restricted size which depended on the medium used. For example, 10BASE5 coax cables had a maximum length of 500 metres (1,640 feet). Also, as was the case with most other high-speed buses, Ethernet segments had to be terminated with a resistor at both ends. For coaxial cable based Ethernet, each end of the cable had a 50-ohm resistor and heat sink attached. Typically this was built into a male BNC or N connector and attached to the last device on the bus (or if vampire taps were in use to a socket mounted on the end of the cable just past the last device). If this was not done or if there was a break in the cable the AC signal on the bus was reflected, rather than dissipated, when it reached the end. This reflected signal was indistinguishable from a collision, and so no communication could take place.

Bridging and switching

While repeaters could isolate some aspects of Ethernet segments, such as cable breakages, they still forwarded all traffic to all Ethernet devices. This created practical limits on how many machines could communicate on an Ethernet network. To alleviate this, bridging was created to communicate at the data link layer while isolating the physical layer. With bridging, only well-formed packets are forwarded from one Ethernet segment to another; collisions and packet errors are isolated. Bridges learn where devices are, by watching MAC addresses, and do not forward packets across segments when they know the destination address is not located in that direction.

Dual speed hubs (Fast Ethernet)

In the early days of Fast Ethernet, Ethernet switches were relatively expensive devices. However, hubs suffered from the problem that if there were any 10BASE-T devices connected then the whole system would have to run at 10 Mbit. Therefore a compromise between a hub and a switch appeared known as a dual speed hub. These devices consisted of an internal two-port switch, dividing the 10BASE-T (10 Mbit) and 100BASE-T (100 Mbit) segments. The device would typically consist of more than two physical ports. When a network device becomes active on any of the physical ports, the device attaches it to either the 10BASE-T segment or the 100BASE-T segment, as appropriate. This prevented the need for an all-or-nothing migration from 10BASE-T to 100BASE-T networks. These devices are often known as dual-speed hubs, since the traffic between devices on the same segment is not switched.

Simple switched Ethernet networks still suffer from a number of issues:

• They suffer from single points of failure; e.g., if one link or switch goes down in the wrong place the network ends up partitioned.
• It is possible to trick switches or hosts into sending data to your machine even if it's not intended for it, as indicated above.
• It is possible for any host to flood the network with broadcast traffic forming a denial of service attack against any hosts that run at the same or lower speed as the attacking device.
• They suffer from bandwidth choke points where a lot of traffic is forced down a single link.