Ethernet

Ethernet was developed in 1972 as a way to connect newly invented computers to newly invented laser printers. Although recognized even at that time as a remarkable technology breakthrough, few people would have wagered that the ability to connect computers and devices would change communication on the same scale as the invention of the telephone and change business on the scale of the Industrial Revolution. Several competing protocols have emerged since 1972, but Ethernet remains the dominant standard for connecting computers into LANs.

Ethernet is a shared resource where all end stations (computers, servers, etc.) all have access to the transmission medium at the same time. The result is that only one device can send information at a time. Given this limitation, there are two viable solutions:

• Use a sharing mechanism - If all end stations are forced to share a common wire, then rules must ensure that each end station waits its turn before transmitting, or, in the event of simultaneous transmissions, follows rules for retransmitting.
• Divide the shared segments and insulate them - Another solution is to use devices that reduce the number of end stations sharing a resource at any given time.

What They Gave Away
In the 1970s, Xerox Corporation assembled a group of talented researchers to investigate new technologies. The new group was located in the newly opened Palo Alto Research Center (PARC), well away from the corporate headquarters in Connecticut.

In addition to developing Ethernet, the brilliant folks at the PARC invented the technology for what eventually became the personal computer, the graphical user interface (GUI), laser printing, and very large scale integration (VLSI).

Inexplicably, Xerox failed to recognize the brilliance (and commercial viability) of many of these homegrown innovations and let other companies (such as Intel, Microsoft, and Apple) use and market these technologies.

Ethernet Collisions
In a traditional LAN, several users all share the same port on a network device and compete for resources (bandwidth). The main limitation of such a setup is that only one device can transmit at a time. Segments that share resources in this manner are called collision domains because if two or more devices transmit at the same time, the information collides and both endpoints must resend their information. Typically, the devices both begin a random countdown before attempting to retransmit.

This method works well for a small number of users on a segment, each having relatively low bandwidth requirements. As the number of users increase, the efficiency of collision domains decreases sharply, to the point where overhead traffic (management and control) clogs the network.

Smaller Segments
You can divide segments to reduce the number of users and increase the bandwidth available to each user in the segment. Each new segment created results in a new collision domain. Traffic from one segment or collision domain does not interfere with other segment, thereby increasing the available bandwidth of each segment. In the following example, each segment has greater bandwidth, but all segments are still on a common backbone and must share the available bandwidth.

The basic tools for segmenting an Ethernet LAN into more collision domains follow:

• Bridges
• Routers
• Switches

This At-A-Glance sheet discusses segmenting using bridges and routers.

Increasing Bandwidth
In addition to creating segments to increase available bandwidth, you can use a faster medium such as optical fiber or gigabit Ethernet. Although these technologies are faster, they are still shared media; collision domains still exist and eventually experience the same problems as slower media.

Ethernet Segment
A segment is the simplest form of network, where all devices are directly connected. In this type of arrangement, disconnecting or adding a computer disables the segment.

Hubs
Hubs enable you to add and remove computers without disabling the network but do not create additional collision domains.

Repeaters
Repeaters simply extend the transmission distance of an Ethernet segment.

Bridges
Bridges are simple Layer 2 devices that create new segments, resulting in fewer collisions. Bridges must learn the addresses of the computers on each segment to avoid forwarding traffic to the wrong port.

Switched Ethernet
You can think of a LAN switch as a high-speed, multiport bridge with a brain. Switches not only give each end station a dedicated port (meaning there are no collisions), but they also allow end stations to transmit and receive at the same time, greatly increasing the efficiency of the LAN.

LAN Routers
LAN-based routers greatly extend the speed, distance, and intelligence of Ethernet LANs. Routers also allow traffic to travel along multiple paths.

Routers, however, do require a common protocol between the router and end stations.