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Chapter 4

Ethernet Basic. Chapter 4. Ethernet: A family of network technologies (standards) to build a LAN Logical bus topology Bus physical topology: The “older” Ethernet Using coaxial cable as the bus cable Star physical topology: The “newer” Ethernet (“ Star Bus topology”)

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Chapter 4

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  1. Ethernet Basic Chapter 4

  2. Ethernet: • A family of network technologies (standards) to build a LAN • Logicalbus topology • Busphysical topology: The “older” Ethernet • Using coaxial cable as the bus cable • Starphysical topology: The “newer” Ethernet (“Star Bus topology”) • Using UTP or Fiber Optic cable What is the Ethernet?

  3. Older versus Newer Ethernet Older: BUS Physical Topology Newer: STAR Physical Topology Older: Coaxial Cable as the Bus Cable Newer: UTP Cable as the Patch Cable and the Horizontal Run

  4. Ethernet: • Ethernet frame • CSMA/CD as the network access method/media access control, i.e., a method to determine which computer in the network can access the wire (in the network) at a given moment What is the Ethernet?

  5. Ethernet in the OSI 7 layer model

  6. Ethernet frame • Created, sent, received, read, and destroyed by Ethernet NIC • The advantages of using frame: • Frame defines the maximum data size, and therefore, large data must be broken into smaller pieces. This provides two benefits: • Benefit 1:Preventing one computer from monopolizingthe shared wire in the network (distributing the network use among all computers in the network more evenly) • Benefit 2:Whenever data is corrupted during transmission, the sending computer only has to retransmit one (or few) frame(s) that contains the corrupted data Ethernet Frame

  7. Seven major parts in Ethernet frame: • Part 1: Preamble • 64 bits of alternating 1s and 0s and ends with 11 • Functions: • Enabling the receiving NIC to recognize the beginning of a frame • Giving time to the receiving NIC to realize a coming of a frame and therefore, can make all necessary preparations to receive the frame properly Ethernet Frame

  8. Seven major parts in Ethernet frame: • Part 2 and 3: Receiver/destination MAC address and Sender MAC Address • A program/software called SNIFFER can enable NIC to run in PROMISCUOUS MODE to processALL frames it receives, regardless of their destination MAC addresses • http://www.tamos.com/htmlhelp/monitoring/monitoringusinghubs.htm Ethernet Frame

  9. Hub Computer A With “Packet Sniffer” this computer will accept and process the message For Computer B Computer B Accept the message Drop/destroy the message Drop/destroy the message Drop/destroy the message

  10. Seven major parts in Ethernet frame: • Part 4: Length • Information about the number of BYTES of DATA inside the frame • Part 5: Data • Data that is received from the upper layer of the OSI seven-layer model (Network Layer): PACKET • Minimum number of bytes of data: 64 bytes • Maximum number of bytes of data: 1500 bytes Ethernet Frame

  11. Seven major parts in Ethernet frame: • Part 6: Pad • If the data size is less than 64 bytes, the sender will add more bytes in this part (PAD) to bring the data up to the minimum 64 bytes • Part 7: Frame Check Sequence • Error Detection Information (CRC for the Ethernet) • To detect data transmission error that causes data modification during the transmission Ethernet Frame

  12. Carrier Sense Multiple Access (CSMA)/Collision Detection (CD) • Network access method (media access control) • Carrier Sense: • Every computer in the network must first examine (sense) the BUS/SEGMENT before sending a frame • If the computer detects traffic in the BUS/SEGMENT, it will wait for a moment, and then reexamine (resense) the BUS/SEGMENT • If the computer detects no traffic, it will send the frame to the BUS/SEGMENT CSMA/CD

  13. Multiple Access: • All computers (NICs) in a network perform CSMA/CD, hence, have equal access to the wire (no computer has greater access (priority) to the wire) • Collision Detection: • When two or more computers are sending frames to a network simultaneously, a collision occurs, and all frames will be corrupted/damaged/lost (the transmission is wasted) • During a frame transmission (after sending a frame to a network), the sender’s NIC will always monitor (listen to) the network • If the sender’s NIC detects something else being transmitted in the network, it recognizes that its frame has been corrupted by collision with another frame CSMA/CD

  14. Early ethernet networks 10BaseT = UTP cable (the “T”) based Ethernet 10BaseFL =Fiber Optic cable (the “FL”) based Ethernet Using hub

  15. 10BaseT: Cabling CAT3 or higher, two pairs, UTP cable RJ-45 connectors Also known as CRIMP Connecting UTP cable to RJ-45 connector: CRIMPING Crimping standard (see Figure 4.12 on Page 67) TIA/EIA 568 A standard TIA/EIA 568 B standard

  16. 10BaseT: Specifications 10 Mbps Baseband Maximum distance between the hub and the node (computer/NIC): 100 meters Maximum number of nodes per hub: 1024 Network topology: Star Bus ‏ Cable: UTP, CAT3 or higher, two pairs, RJ-45 connectors

  17. 10BaseFL/10BaseF: Specifications 10 Mbps Baseband Maximum distance between the hub and the node (computer/NIC): 2000meters (2 kilometers)‏ Network topology: Star Bus Cable: Multimode Fiber Optic

  18. 10BaseFL/10BaseF: Specifications Advantages: Longer cable without repeater Immune to electromagnetic interference (EMI) More difficult to tap More scalable Scalability = ability to be adapt to (to be used with) a new, faster technology Disadvantages: More expensive More difficult to install More fragile

  19. Multiple Ethernet Hubs Why using multiple Ethernet Hubs (multiple segments)? If one hub is down, the other hub(s) still works → NO SINGLE POINT OF FAILURE IN THE HUB

  20. Multiple Ethernet Hubs How to connect multiple Ethernet Hubs (multiple Ethernet switches)? Crossover cable (UTP, two pairs, CAT3 or higher): reversing the sending and receiving pairs on the end of the cable (see page 70)‏ One end of a cable is connected to the regular port, the other end is connected to the regular port at the other hub Crossover Port (also called Crossover, Uplink, In-port, Out-port): Use regular cable, NOT crossover cable One end of the regular cable (NOT crossover cable) is connected to the Crossover Port, the other end is connected to the regular port at the other hub Autosensing Port (a regular port that can turn to be Crossover Port automatically whenever necessary)‏ One end of a regular cable is connected to the Autosensing Port, the other end is connected to the regular port at the other hub

  21. Connecting Ethernet Segments

  22. Multiple Ethernet Hubs Hubs must be connected in a single line (daisy chain) Why? This type of connection will simply extend the “logical bus cable inside the hubs,” hence, maintain the “logical bus topology” Hubs cannot be connected in “hierarchical configuration” (see page 70 figure 4.17) Why? This type of connection will connect the “logical bus cables inside the hubs” in a configuration other than a “logical bus topology”

  23. Collision Domain (all computers are connected to a hub(s)): • A network in which there will be collision if two or more computerstransmit frames simultaneously • Collisions can significantly slow down the network. Why? Because every time there is a collision, the sender will automatically retransmit the frame (the original transmission is wasted) • A SWITCH or BRIDGE (i.e., a two-port switch) can divide a network into multiple collision domains Collision Domain

  24. Switch Computer A For Computer B Switch Computer B Accept the message

  25. Switch does the switching by using its lookup table (i.e., an electronic database storing a list of ports and their corresponding MAC addresses) When a switch is just installed, as the lookup table is still empty, it acts like a hub. However, every time it receives a frame in a port, it will store the port number and the sender MAC address of the frame in the lookup table (see this animation: http://www.cisco.com/image/gif/paws/10607/lan-switch-transparent.swf) Switch

  26. Connecting multiple switches: • Use the procedures to connect multiple hubs • Spanning Tree Protocol • Enabling a switch to detect bridge loop (i.e., three or more switches that are connected to one another forming a loop) and to automatically shut down one port to break the loop • The problem with bridge loop • Broadcast storm Switch

  27. FAQ: • Can I break a network into multiple “collision domains” by using a Router? • Yes, you can • So, why do I use Switch instead of a Router? • Switch is cheaper (although the price of Router continues going down) • Switch is easier to install (e.g., no (not much) configuration is needed) Collision Domain

  28. When do we use hub (repeater), switch (bridge), and router? • Hub • Switch • Router • Router can do what a switch can do, but it is more expensive and more difficult to install (e.g., requires more configuration). • Only router can connect two different networks. • Switch: To connect computers within a LAN • Router: To connect one LAN to another LAN • If you want to install packet sniffer to monitor incoming traffics to your network, you can get hub Hub (repeater) versus Switch (bridge) versus router More sophisticated, more expensive, more difficult to install

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