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Chabot College

Chabot College. ELEC 99.09 VLAN. Data Link Sublayers. LLC (Logical Link Control). MAC (Media Access Control). Ethernet II (DIX Ethernet). Ethernet II. 1972 Xerox PARC begins work 1982 Ethernet v.2 spec released. 16 bits. variable. 64 bits. 48 bits. 48 bits. 32 bits. Preamble. DA.

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Chabot College

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  1. Chabot College ELEC 99.09 VLAN

  2. Data Link Sublayers LLC (Logical Link Control) MAC (Media Access Control)

  3. Ethernet II (DIX Ethernet) Ethernet II • 1972 Xerox PARC begins work • 1982 Ethernet v.2 spec released 16 bits variable 64 bits 48 bits 48 bits 32 bits Preamble DA SA Type Data FCS

  4. 802.3_Raw (Novell) Ethernet_802.3 16 bits variable 64 bits 48 bits 48 bits 32 bits Preamble DA SA Length Data FCS Ethernet II 16 bits variable 64 bits 48 bits 48 bits 32 bits Preamble DA SA Type Data FCS

  5. IEEE 802.3 (with 802.2 LLC) Ethernet_802.2 (an 802.3 frame with LLC) 16 bits 8 bits 64 bits 48 bits 48 bits 8 bits Preamble DA SA Length DSAP SSAP variable 32 bits Data FCS The 802.2 Protocol (LLC): Service Access Points serve the same purpose as a Protocol Type field.

  6. Ethernet Frame Types • Ethernet II • Ethernet 802.3 (with 802.2 LLC) • Ethernet 802.3 RAW (Novell only) • Ethernet SNAP These can coexist on the same LAN, but in order to directly communicate, nodes must use the same frame type.

  7. Ethernet Characteristics • Media Access Method: CSMA/CD • Broadcast • One station transmits at a time • Best-effort delivery

  8. Ethernet Drawbacks

  9. Ethernet Drawbacks Performance negatively affected by: • The data frame broadcast delivery nature of Ethernet/802.3 LANs • CSMA/CD access methods allow only one station to transmit at a time. • Network congestion due to increased bandwidth demands from multimedia applications such as video and the Internet. • Normal latency (propagation delay) of frames as they travel across the LAN layer 1 media and pass through layer 1, 2 and 3 networking devices. • Extending the distances of the Ethernet/802.3 LANs using Layer 1 repeaters.

  10. Standard Ethernet • Can either send OR receive.

  11. The Need for Speed

  12. The Need for Speed

  13. Balance is Key

  14. Propagation Delay • Propagation delay (latency) is the time a frame or packet of data takes to travel from the source station or node to its final destination on the network.

  15. What causes latency? • Since Ethernet LANs use CSMA/CD to provide best effort delivery there must be a certain amount of latency in the system to detect collisions and negotiate transmission rights on the network. • The greater the number of devices the greater the latency or propagation delay.

  16. Ethernet Transmission Times • The time it takes for the Data Link layer to “hand off” to the cable (Physical Layer).

  17. Extending the Media

  18. Improving LAN performance Performance can be improved by: • Segmenting the network using Bridges, Routers and LAN Switches • Moving to full duplex transmitting • Upgrading to the Fast Ethernet Standard 802.3u • Upgrading to the Gig Ethernet Standard 802.3z and 802.3ab

  19. Why Segment LANs?

  20. Segmenting and Collisions

  21. Segmenting with Bridges

  22. Bridges Add Latency Increases propagation delay by up to 30%

  23. Segmentation With Routers 20-40% loss of throughput

  24. Segmentation with Switches

  25. How Switches Work • A switch can create a network that behaves like it only has two nodes - the sender and the receiver. • These two nodes share the 10 Mbps bandwidth between them, available bandwidth can reach closer to 100%.

  26. How Switches Work • Switches are high speed multi-port bridges with one port for each node or segment of the LAN. • A switch segments a LAN into microsegments creating collision free domains from one larger collision domain.

  27. Microsegmentation

  28. Switch Latency • Switches add latency, but they can overcome this by forwarding frames before they are completely received.

  29. Full-Duplex Ethernet • Allows the transmission of a packet and the reception of a different packet at the same time. • Requires two pairs of wires and a switched connection between each node. • Point-to-point connection, nearly collision free. • No negotiations for bandwidth.

  30. Full-Duplex Ethernet Offers 100% bandwidth in both directions (potential 20 Mbps, 200 Mbps, etc).

  31. Switches and Broadcasts

  32. Switches Live at Layer 2

  33. Switches Create Virtual Circuits

  34. Switches Learn the Network

  35. Content Addressable Memory • An Ethernet switch can learn the address of each device on the network by • reading the source address of each packet transmitted and • noting the port where the frame was heard • The switch then adds this information to its forwarding database. Addresses are learned dynamically. This means that as new addresses are read they are learned and stored in content addressable memory (CAM).When a source is read that is not found in the CAM it is learned/stored for future use.

  36. Aging Out Each time an address is stored it is time stamped. This allows for addresses to be stored for a set period of time. Each time an address is referenced or found in the CAM, it receives a new time stamp. Addresses that are not referenced during set period of time are removed from the list. By removing aged or old addresses the CAM maintains an accurate and functional forwarding database.

  37. Benefits of Switching • Improved manageability. • Easy to migrate from shared media.

  38. Symmetric Switching

  39. Asymmtric Switching

  40. Memory Buffering Two ways to buffer incoming and outgoing frames: • port-based memory buffering • shared memory buffering

  41. Port-Based Memory Buffering • Packets are stored in queues that are linked to specific incoming ports. A packet is transmitted to the outgoing port only when all the packets ahead of it in the queue have been successfully transmitted.

  42. Port-Based Drawbacks • It is possible for a single packet to delay the transmission of all the packets in memory because of a busy destination port. This delay occurs even if the other packets can be transmitted to open destination ports.

  43. Shared Memory Buffering • Deposits all packets into a common memory buffer that is shared by all the ports on the switch. The amount of memory allocated to a port is determined by how much is required by each port. This is called dynamic allocation of buffer memory.

  44. Two Switching Methods

  45. Cut-through v. Store & Forward

  46. Spanning-Tree Protocol • allows redundant switched/bridged paths without suffering the effects of loops in the network.

  47. STP States

  48. Virtual Local-Area Networks

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