From Mycomputer Notes

Revision as of 16:51, 12 September 2006

We know that each LAN is unique but many designs aspects are common to all of them. Most of them fallow the same communication standanrd and shared the same components. This chapter presents information on elements of Ethernet LANs and common LAN devices. Ethernet was first implemented by the Digital, Intel, and Xerox group (DIX). DIX created and implemented the first Ethernet LAN specification, which was used as the basis for the Institute of Electrical and Electronics Engineers (IEEE) 802.3 specification, released in 1980 Together Ethernet and IEEE 802.3 currently maintain the greatest market share of any LAN protocol. Today, the term Ethernet is often used to refer to all carrier sense multiple access/collision detection (CSMA/CD) LAN’s that generally conform to Ethernet specifications, including IEEE 802.3. IEEE 802.3 and Ethernet v2.0

Ethernet performs three functions

1. Transmitting and receiving data packets
2. Decoding data packets and checking them for valid addresses before passing them to the upper layers of the OSI model
3. Detecting errors within data packets or on the network

Physical Layer Implementation

Ethernet Media & Connector

The cable and connector specifications used to support Ethernet implementations are derived from the EIA/TIA standards. The categories of cabling defined for the Ethernet are derived from the EIA/TIA-568 (SP2840) Commercial Building Telecommunications Wiring Standards. The EIA/TIA specefies an RJ-45 connector to UTP cable.

The file compares the cable and connector specifications for the most popular Ethernet implementations

LAN Devices

Many types of devices, other then a workstation, make-up the LAN. This divices include repeaters, hubs, bridges and switches.

Repeater 

The purpose of the Repeater is to regenerate and retime network signals at the bit level to allow them to travel longer distance on the media. The 4 repeater rule for 10Mbps bus based Ethernet known as "5-4-3" is used as standard when extending the LAN.

Hubs 

Are multiport repeaters and it generally has 4 to 24 ports and it is most commonly used in Ethernet 10BASET & 100BASET. Using a Hub changes the network topology from linear, each device plugs directly to the wire, to a Start Topology. The hub electronically repeats data arriving from the cable out of all ports connected to the same network signal. Hubs comes in 3 flaveours:

• Active --> must be plug to an electrical outlet.
• Intelligent --> same functionality as an acitve hub but they include a microprocessor chip and some diagnostic capabilities.
• Passive --> They only function is to be a physical connetion point. They do not boost or clean the signal.

All the devices connected to the hub can hear all the trafic therefore Hubs maintained a single Collision Domain.

Bridges 

When times comes and we need to break up a LAN into smaller easily managed segments it is time to introduce a Bridge. Segmenting a single LAN into smaller segments can extend the capabilities of the LAN because it decreases the trafic through the entire LAN. Other devices used to connect netwokr segments together are (bridges, swithes, routers and gateways). A bridge makes intelligent decisionsabout whether or not to pass signals onto the next segment of a newtork, thus reducing traffic. It cab be used to connect dissimilar Layer 2 Protocols (i.e Ethernet, Token Ring, FDDI)

Switching 

A switch is a sort of a multiport bridge, but much faster, and the number of port depends on how many networks segments are to be linked. The switch chooses the port to which the destination device or workstaion is connected. It alleviates congestion in Ethernet LAN by reducing trafic and increasing bandwith.

Broadcast Domains

All hosts connected to the same switch are still in the same broadcast domain. A broadcast from one node will be seen by all other nodes connected through the LAN switch

Segmentation 

Two primary reasons for segmenting a LAN:

• Isolate traffic between segments
• Achieve more bandwidth per user by creating smaller collision domains

Know when to segment

It is important to note that even though 100% of the bandwidth may be available, Ethernet networks perform best when kept under 30-40% of full capacity. Bandwidth usage that exceeds the recommended limitation results in increased collisions.

Routers 

The Router is a layer 3 (Network) device, but operates at layers 1-3. Routers create the highest level of segmentation because of their ability to make exact determinations of where to send the data packet.

Because routers perform more functions than bridges, they operate with a higher rate of latency.

NIC

In the CSMA/CD access method, networking devices with data to transmit over the networking media work in a listen-before-transmit mode. NICs

• Provides ports for network connection
• Communicate with network via serial connection
• Communication with computer through parallel connection

NIC 

Controls the host's access to the medium. NIC only looks at the bits and no any address information.

• Layer 1 & Layer 2 device
• Communicates with upper layers in the computer
• Logical Link Control (LLC)
• Has MAC address burned in
• Encapsulates data into frames
• Consider a Layer 2 device because each individual NIC carries an unique MAC address
• Creates signals and interfaces with the media On-board transceiver

Selection Factors for NICs

• Type of network (Ethernet, Token Ring, FDDI)
• Type of media (Twisted pair, coax, fiber)
• Type of system bus (PCI, ISA)