LAN Design

LAN Design:

Ethernet

When we talk about a LAN, Ethernet is the most popular physical layer LAN technology today. Its standard is defined by the Institute for Electrical and Electronic Engineers as IEEE Standard 802.3, but was originally created by Digital Intel Xerox (DIX). According to IEEE, information for configuring an Ethernet as well as specifying how elements in an Ethernet network interact with one another is clearly defined in 802.3. 

For half-duplex Ethernet 10BaseT topologies, data transmissions occur in one direction at a time, leading to frequent collisions and data retransmission. In contrast, full-duplex devices use separate circuits for transmitting and receiving data and as a result, collisions are largely avoided. A collision is when two nodes are trying to send data at the same time. On an Ethernet network, the node will stop sending when it detects a collision, and will wait for a random amount of time before attempting to resend, known as a jam signal. Also, with full-duplex transmissions the available bandwidth is effectively doubled, as we are using both directions simultaneously. You MUST remember: to enjoy full-duplex transmission, we need a switch port, not a hub, and NICs that are capable of handling full duplex. Ethernet’s media access control method is called Carrier sense multiple access with collision dectection (CSMA/CD). Because of Ethernets collision habits it is also known as the “best effort delivery system.” Ethernet cannot carry data over 1518 bytes, anything over that is broken down into “travel size packets.” 

 

Fast Ethernet

For networks that need higher transmission speeds, there is the Fast Ethernet standard called IEEE 802.3u that raises the Ethernet speed limit to 100 Mbps! Of course, we need new cabling to support this high speed. In 10BaseT network we use Cat3 cable, but in 100BaseT network we need Cat 5 cables. The three types of Fast Ethernet standards are 100BASE-TX for use with level 5 UTP cable, 100BASE-FX for use with fiber-optic cable, and 100BASE-T4 which utilizes an extra two wires for use with level 3 UTP cable. 

Gigabit Ethernet

Gigabit Ethernet is an emerging technology that will provide transmission speeds of 1000mbps. It is defined by the IEEE standard The 1000BASE-X (IEEE 802.3z). Just like all other 802.3 transmission types, it uses Ethernet frame format, full-duplex and media access control technology. 

Token Ring

Token Ring is an older standard that isn't very widely used anymore as most have migrated to some form of Ethernet or other advanced technology. Ring topologies can have transmission rates of either 4 or 16mbps. Token passing is the access method used by token ring networks, whereby, a 3bit packet called a token is passed around the network. A computer that wishes to transmit must wait until it can take control of the token, allowing only one computer to transmit at a time. This method of communication aims to prevent collisions. Token Ring networks use multistation access units (MSAUs) instead of hubs on an Ethernet network. For extensive information on Token Ring, read Cisco's Token Ring/IEEE 802.5 tutorial.