Cabling lan

From Mycomputer Notes

Layer 2 Technologies

• Ethernet and IEEE 802.3
• Layer 2 Devices
• Data Flow
• Ethernet: 10BASE-T Troubleshooting

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 Ethernet performs three functions: Transmitting and receiving data packets decoding data packets and checking them for valid addresses before passing them to the upper layers of the OSI model detecting errors within data packets or on the network 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  Resources required: – IRQ, I/O address, upper memory addresses, DMA Selection Factors for NICs  Type of network – Ethernet, Token Ring, FDDI  Type of media – Twisted pair, coax, fiber  Type of system bus – PCI, ISA

NIC Operations  Layer 1 & Layer 2 device  Primarily Layer 2 – Communicates with upper layers in the computer  Logical Link Control (LLC) – Has MAC address burned in – Encapsulates data into frames – Provides access to the media  Also Layer 1 – Creates signals and interfaces with the media – On-board transceiver Bridges  Connects two network segments – Can connect different layer 2 protocols  Ethernet, Token Ring, FDDI  Makes intelligent decisions about traffic – Reduces unnecessary traffic – Minimizes collisions – Filters traffic based on MAC address  Maintains address tables  Rarely implemented today – Conceptually important Bridge Operations  Bridging occurs at the data link layer: – Controls data flow – Handles transmission errors – Provides physical addressing – Manages access to the physical medium

Bridge Operations  Transparent to upper layers  Best used in low traffic areas – Can cause bottlenecks  Must examine every packet  Broadcasts – Messages sent to all devices  Destination MAC address unknown  Bridge will always forward  Can cause Broadcast Storm – Network time outs, traffic slowdowns, unacceptable performance Switching Operation Microsegmentation – Each switch port acts as a micro bridge (Layer 2 device) – Multiple traffic paths within the switch – Virtual circuits – Temporarily exist - only when needed – Each data frame has a dedicated path  No collisions  Increases bandwidth availability – Each host gets full bandwidth – Full-duplex capabilities Advantages of Switches  Much faster than bridges – Hardware based, not software  Support new uses – e.g. virtual LANs  Reduce the size of collision domains Advantages of Switches  Allows many users to communicate in parallel – Creates virtual circuits – Creates dedicated segments  Collision free  Maximizes bandwidth  Cost effective – Can simply replace hubs in same cable infrastructure  Minimal disruption  Flexible network management – Software based configuration 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 Bridges  Bridge Drawback: – Bridges increase the latency (delay) in a network by 10-30% – A bridge is considered a store-and-forward device slowing network transmissions, thus causing delay.

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.

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