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LANs - Ethernet Style

LANs - Ethernet Style. Review what LAN devices do? What media types are used? How does Ethernet really work? Why are MAC addresses so important if we have IP addresses?. Devices. What layer device?. What does it do? Connects LAN segments; Filters traffic based on MAC addresses; and

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LANs - Ethernet Style

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  1. LANs - Ethernet Style • Review what LAN devices do? • What media types are used? • How does Ethernet really work? • Why are MAC addresses so important if we have IP addresses?

  2. Devices What layer device? • What does it do? • Connects LAN segments; • Filters traffic based on MAC addresses; and • Separates collision domains based upon MAC addresses.

  3. Devices What layer device? • What does it do? • Since it is a multi-port bridge, it can also • Connect LAN segments; • Filter traffic based on MAC addresses; and • Separate collision domains • However, switches also offer full-duplex, dedicated bandwidth to segments or desktops.

  4. Devices What layer device? • What does it do? • Concentrates LAN connections from multiple devices into one location • Repeats the signal (a hub is a multi-port repeater)

  5. Devices What layer device? • What does it do? • Interconnects networks and provides broadcast control • Determines the path using a routing protocol • Re-encapsulates the packet in the appropriate frame format and switches it out the interface • Uses logical addressing (i.e. IP addresses) to determine the path

  6. Media Types

  7. LAN Technologies Three Most Common Used Today in Networking

  8. Ethernet/802.3 • Cable Specifications: • 10Base2 • Called Thinnet; uses smaller coax • Max. distance = 185 meters (almost 200) • 10Base5 • Called Thicknet; uses thick coax • Max. distance = 500 meters • 10BaseT • Uses Twisted-pair • Max. distance = 100 meters 10 means 10 Mbps

  9. Ethernet/802.3 • Ethernet is broadcast topology. • What does that mean? • Every devices on the Ethernet segment sees every frame. • Frames are addressed with source and destination MAC addresses.

  10. Ethernet/802.3 • Ethernet is broadcast topology. • What does that mean? • When a device sends out a frame, every device must look at it to determine if the MAC address matches. If it does, it opens the frame. • If it does not match the host’s MAC, then it discards the frame with no further processing.

  11. Ethernet/802.3 • Ethernet is broadcast topology. • When a source does not know the destination or wants to communicate with every device, it encapsulates the frame with a broadcast MAC address: FFFF.FFFF.FFFF • What is the main network traffic problem caused by Ethernet broadcast topologies? Collisions ! ! !

  12. Ethernet/802.3 • Ethernet topologies use shared media. • That means media access is controlled on a “first come, first serve” basis. • This results occasionally in collisions between the data of two simultaneously transmitting devices.

  13. Ethernet/802.3 • Since the collision is between two electric signals, you end up with an unknown signal, which cannot be understood by any host except as a wreck. Neither signal can be recovered. The signal is lost.

  14. Ethernet/802.3 • Collisions cannot be avoided due to the nature of Ethernet’s shared medium. They can be reduced by increasing the number of collision domains. • Once a collision happens, the mess must be cleared up and the pathway reopened for business. • Collisions are resolved using what method?

  15. Ethernet/802.3 • CSMA/CD (Carrier Sense Multiple Access with Collision Detection) • CSMA/CD works in this way: • A node needing to transmit listens for activity on the media. If there is none, it transmits. • The node continue to listen. A collision is detected by a spike in voltage (a bit can only be a 0 or a 1--it cannot be a 2) • The node generates a jam signal to tell all devices to stop transmitting for a random amount of time (back-off algorithm). • When media is clear of any transmissions, the node can attempt to retransmit.

  16. Address Resolution Protocol • In broadcast topologies, we need a way to resolve unknown destination MAC addresses. • ARP is protocol where the sending device sends out a broadcast ARP request which says, “What’s you MAC address?” FFFF.FFFF.FFFF

  17. Address Resolution Protocol • If the destination device exists on the same LAN segment as the source, then the destination device replies with its MAC address.

  18. Address Resolution Protocol • However, if the destination and source are separated by a router, the router will not forward the broadcast (an important function of routers). Instead the router replies with its own MAC address. When the message reaches the router, it reprocesses and sends the message out another port towards the destination IP address using its own MAC address. • Source and destination IP addresses do not change, but the MAC changes with each router that handles the message.

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