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Ethernet: A Multi-access Network

Ethernet: A Multi-access Network. Rick Graziani Cabrillo College graziani@cabrillo.edu. Ethernet Protocol. Ethernet – Most common LAN technology used today. Multi-access network: Multiple devices on the same medium and able to communicate with each other without the services of a router.

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Ethernet: A Multi-access Network

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  1. Ethernet: A Multi-access Network Rick Graziani Cabrillo College graziani@cabrillo.edu

  2. Ethernet Protocol • Ethernet – Most common LAN technology used today. • Multi-access network: Multiple devices on the same medium and able to communicate with each other without the services of a router. • Supports data bandwidths of 10 Mb/s, 100 Mb/s, 1 Gb/s, 100 Gb/s, and more • Operates in the data link layer and the physical layer. • Defined in the IEEE 802.2 and 802.3 standards.

  3. Multi-access Topology • A logical multi-access topology - Enables a number of nodes to communicate by using the same shared media. • Ethernet LANs – Connected by Ethernet switches (legacy hubs) • “Every node “may” see all the frames that are on the medium. • Data Link Destination Address denote which device the frame is for.

  4. Bus Topology • Original Ethernet used a bus topology. • A bus topologyuses a single backbone segment (length of cable) that all the hosts connect to directly. • Ethernet hubs work the same as a “bus”. • And the reason why we have a minimum Ethernet frame size of 64 bytes and specific cable lengths depending upon bandwidth. (See slot time, 5-4-3 rule)

  5. Today’s Ethernet Networks Use Full Duplex NICs and Switches router switch switch switch switch switch switch switch switch • Multi-access network: Multiple devices on the same medium and able to communicate with each other without the services of a router.

  6. Ethernet is Best Effort Delivery • Ethernet is best-effort delivery, no guarantee. • Nothing in the Ethernet frame to ensure delivery. • Like a trucking service, it doesn’t really know or care about the what it is carrying.

  7. Ethernet: A Multi-access Network Rick Graziani Cabrillo College graziani@cabrillo.edu

  8. Ethernet: Ethernet Frame Rick Graziani Cabrillo College graziani@cabrillo.edu

  9. Ethernet Frame Attributes

  10. Ethernet Frame AttributesEthernet Frame Size • Ethernet II and IEEE 802.3 standards define: • minimum frame size as 64 bytes • maximum as 1518 bytes (1520 with 802.1Q tag) • “Collision fragment" or "runt frame” – Frame less than 64 bytes • If size of a transmitted frame is less than the minimum or greater than the maximum, the receiving device drops the frame (usually)

  11. Ethernet: Ethernet Frame Rick Graziani Cabrillo College graziani@cabrillo.edu

  12. Ethernet: Speed (Bandwidth) and Duplex Rick Graziani Cabrillo College graziani@cabrillo.edu

  13. NIC to NIC • Ethernet protocol is only concerned with how the information gets from one Ethernet NIC to another. • Layer 2, Data Link Layer, device • Connects the device (computer) to the LAN • Responsible for the local Layer 2 address (later) • Default: Full duplex (optional Half duplex) • Common Bandwidth • 10 Mbps, 10/100 Mbps, 10/100/1000 Mbps

  14. Auto negotiation: Speed and Duplex PC-A Port 1 Autonegotiation Duplex Duplex Full Full Half Half Speed 1000 Mb/s Speed 100 Mb/s 100 Mb/s 10 Mb/s 10 Mb/s

  15. What would be the duplex settings? Half-duplex router Full-duplex switch switch switch hub hub switch switch switch switch Full-duplex

  16. Duplex Mismatch I’m half-duplex so I can only send when the link is clear but I am also getting a lot of collisions! I’m full-duplex so I can send when ever I want. S1 S2 Full-duplex Half-duplex S2 will continually experience collisions because S1 keeps sending frames any time it has something to send.

  17. Ethernet: Speed and Duplex Rick Graziani Cabrillo College graziani@cabrillo.edu

  18. Number Systems: Hexadecimal Rick Graziani Cabrillo College graziani@cabrillo.edu

  19. Rick’s Number System Rules • All digits start with 0 • A Base-n number system has n number of digits: • Decimal: Base-10 has 10 digits • Binary: Base-2 has 2 digits • Hexadecimal: Base-16 has 16 digits • The first column is always the number of 1’s • Each of the following columns is n times the previous column (n = Base-n) • Base 10: 10,000 1,000 100 10 1 • Base 2: 16 8 4 2 1 • Base 16: 65,5364,096256161

  20. Rick Graziani graziani@cabrillo.edu Hexadecimal Digits DecHex 8 8 9 9 10 A 11 B 12 C 13 D 14 E 15 F Hexadecimal: 16 digits DecHex 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7

  21. 0, 1, 2, 3, 4, 5, 6, 7 ,8, 9, A, B, C, D, E, F Hexadecimal Decimal16’s1’s 15 F 16 10

  22. Rick Graziani graziani@cabrillo.edu Why Hexadecimal? • Hexadecimal is perfect for matching 4 bits. • Every combination of 4 bits can be matched with one hex number. • 4 bits can be represented by 1 Hex value • 8 bits can be represented by 2 Hex values

  23. Hexadecimal Digits 4 bits can be represented by 1 Hex value DecHex Binary 8421 8 8 1000 9 9 1001 10 A 1010 11 B 1011 12 C 1100 13 D 1101 14 E 1110 15 F 1111 Hexadecimal: 16 digits DecHex Binary 8421 0 0 0000 1 1 0001 2 2 0010 3 3 0011 4 4 0100 5 5 0101 6 6 0110 7 7 0111

  24. Rick Graziani graziani@cabrillo.edu Hexadecimal Digits 4 bits can be represented by 1 Hex value • Hexadecimal is perfect for matching 4 bits. • Every combination of 4 bits can be matched with one hex number. • 4 bits can be represented by 1 Hex value • 8 bits can be represented by 2 Hex values Dec. Hex. Binary Dec. Hex. Binary 0 0 0000 8 8 1000 1 1 0001 9 9 1001 2 2 0010 10 A 1010 3 3 0011 11 B 1011 4 4 0100 12 C 1100 5 5 0101 13 D 1101 6 6 0110 14 E 1110 7 7 0111 15 F 1111

  25. Using Hex for 8 bits Dec. Hex. Binary Dec. Hex. Binary 0 0 0000 8 8 1000 1 1 0001 9 9 1001 2 2 0010 10 A 1010 3 3 0011 11 B 1011 4 4 0100 12 C 1100 5 5 0101 13 D 1101 6 6 0110 14 E 1110 7 7 0111 15 F 1111

  26. Number Systems: Hexadecimal Rick Graziani Cabrillo College graziani@cabrillo.edu

  27. Ethernet: MAC Addresses Rick Graziani Cabrillo College graziani@cabrillo.edu

  28. MAC Address: Ethernet Identity • Layer 2 Ethernet MAC address is a 48-bit binary value expressed as 12 hexadecimal digits • IEEE requires a vendor to follow two simple rules: • Must use that vendor's assigned OUI as the first 3 bytes • All MAC addresses with the same OUI must be assigned a unique value in the last 3 bytes • aka physical address, bia

  29. MAC Address Format DecBinHexDecBinHex 0 = 0000 = 0 8 = 1000 = 8 1 = 0001 = 1 9 = 1001 = 9 2 = 0010 = 2 10 = 1010 = A 3 = 0011 = 3 11 = 1011 = B 4 = 0100 = 4 12 = 1100 = C 5 = 0101 = 5 13 = 1101 = D 6 = 0110 = 6 14 = 1110 = E 7 = 0111 = 7 15 = 1111 = F OUIunique • An Intel MAC address: 00-21-CC-BA-44-C4 • 0000 0000 - 0010 0001 – 1100 1100 - 1011 1010 – 0100 0100 – 1100 0100 • IEEE OUI FAQs: http://standards.ieee.org/faqs/OUI.html

  30. Ethernet MACMAC Address Representations

  31. The MAC Address MAC Address MAC Address • The Ethernet protocol uses MAC addresses to identify the source of the Ethernet frame and the destination of the Ethernet frame. • Whenever is computer sends an Ethernet frame, it includes the MAC address on its NIC as the Source “MAC” Address. • We will learn later how it learns the Destination “MAC” Address…. (ARP)

  32. Ethernet MACUnicast MAC Address

  33. Ethernet MACBroadcast MAC Address

  34. Ethernet MACMulticast MAC Address Multicast MAC address is a special value that begins with 01-00-5E in hexadecimal Range of IPV4 multicast addresses is 224.0.0.0 to 239.255.255.255

  35. Ethernet: MAC Addresses Rick Graziani Cabrillo College graziani@cabrillo.edu

  36. Ethernet: Switches and Broadcast Domains Rick Graziani Cabrillo College graziani@cabrillo.edu

  37. Full-duplex Switches • Full-duplex is allows simultaneous communication between a pair of stations or devices. • 100% bandwidth utilization

  38. Broadcast Domain

  39. Ethernet: Switches and Broadcast Domains Rick Graziani Cabrillo College graziani@cabrillo.edu

  40. Ethernet: CSMA/CD, Hubs and Collision Domains Rick Graziani Cabrillo College graziani@cabrillo.edu

  41. Original Ethernet – Shared Bus Not for me  It's for me!  Not for me  • When an Ethernet frame is sent all devices on the “bus” receive it. • What do they do with it? • All devices check the destination MAC address to see if it matches their MAC address

  42. Collisions Abbreviated MAC Addresses 1111 2222 3333 nnnn X • When two devices transmit at the same time we have a collision Collision!

  43. Media Access Control Carrier Sense Multiple Access with Collision Detection (CSMA/CD) process • NICs operating in half duplex • Used to first detect if the media is carrying a signal • If no carrier signal is detected, the device transmits its data • If two devices transmit at the same time - data collision • Devices sense collision and stop transmitting – algorithm to determine when to send

  44. (Hub)

  45. Collision Domain 2222 1111 1111 2222 X 4444 3333 • NICs and hub ports operate in half duplex • Insufficient use of bandwidth • About 50% bandwidth utilization (one direction only) • Similar to 802.11 (CSMA/CA) 5555 Shared Collision Domain 3333 4444

  46. Switches (and routers) segment collision domains

  47. Ethernet: CSMA/CD, Hubs and Collision Domains Rick Graziani Cabrillo College graziani@cabrillo.edu

  48. Ethernet: Switch Forwarding Process Rick Graziani Cabrillo College graziani@cabrillo.edu

  49. Forwarding decisions Router (multilayer switch): Layer 3 device • Connects different subnets • Makes forwarding decision based on layer 3 – Destination IP Address • Maintains a routing table Switch: Layer 2 device • Connects devices on same link layer • Makes forwarding decision based on layer 2 – Destination MAC Address • Maintains a MAC address table Hub: Layer 1 device • Multiport repeater • Makes forwarding decision based on layer 1 – forwards bits • Everything that comes in one port, is sent out all other ports (regenerated)

  50. Switch: Learning and Forwarding 1. Learn - Examine source MAC address and incoming port # • In MAC address table? • No: Add source MAC and port # to table (start 5 minute timer) • Yes: Reset 5 minute timer 2. Forward – Examine destination MAC address • Unicast • In MAC address table? • No: Forward out all ports except incoming port • Yes: Forward out port for that MAC address (learned previously) • Broadcast/Multicast (unless using IGMP) • Forward out all ports except incoming port

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