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Ethernet Fundamentals

Ethernet Fundamentals. Introduction to Ethernet. The success of Ethernet is due to the following factors: Simplicity and ease of maintenance Ability to incorporate new technologies Reliability Low cost of installation and upgrade

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Ethernet Fundamentals

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  1. Ethernet Fundamentals

  2. Introduction to Ethernet • The success of Ethernet is due to the following factors: • Simplicity and ease of maintenance • Ability to incorporate new technologies • Reliability • Low cost of installation and upgrade • Bandwidth can be increased without changing underlying technology Essentially, Ethernet and IEEE 802.3 are the same standards. The original idea for Ethernet grew out of the problem of allowing two or more hosts to use the same medium and prevent the signals from interfering with each other. This problem of multiple user access to a shared medium was studied in the early 1970s at the University of Hawaii

  3. A drawing of the first Ethernet system by Bob Metcalfe of Xerox

  4. IEEE Ethernet naming rules • In BASE band signaling, the data signal is transmitted directly over the transmission medium. • In BROADband signaling, not used by Ethernet, a carrier signal is modulated by the data signal and the modulated carrier signal is transmitted.

  5. Layer 1 Cannot communicate with the upper-layer protocols Cannot name or identify computers Can describe only streams of bits Cannot decide which computer will transmit data from a group in which all are trying to transmit at the same time Layer 2 Communicates with upper-layer protocols using logical link control (LLC) Provides an addressing (or naming) process Uses framing to organize or group the bits Uses a system called Media Access Control (MAC) to control transmissions Layer 1 vs. Layer 2 Layer 2 deals with the limitations of Layer 1

  6. Divided OSI Layer 2 into two sublayers Media Access Control (MAC) – traditional L2 features Transitions down to media Logical link control (LLC) – new L2 features Transitions up to the network layer IEEE Standard

  7. Comparing LAN Standards

  8. OSI Layer 1 and 2 Together Are the Access Protocols • These are the delivery system protocols • Independent of: • Network OS • Upper-level protocols • TCP/IP, IPX/SPX • Sometimes called: • Access methods • Access protocols • Access technologies • Media access • LAN protocols • WAN protocols Ethernet, Fast Ethernet, Gigabit Ethernet, Token Ring, FDDI, Frame Relay, ATM, PPP, and so on

  9. LLC allows part of the data link layer to function independent of LAN access technologies (protocols / methods) Provides services to network layer protocols, while communicating with access technologies below it. LAN access technologies: Ethernet Token Ring FDDI Logical Link Control (LLC)

  10. Participates in the data encapsulation process. LLC PDU between Layer 3 and the MAC sublayer Adds control information to the network layer data to help deliver the packet. It adds two fields: Destination Service Access Point (DSAP) Source Service Access Point (SSAP) Supports both connectionless and connection-oriented upper-layer protocols Allows multiple higher layer protocols to share a single physical data link Logical Link Control (LLC)

  11. Provides MAC Addressing (naming) Depending on access Technology (Ethernet, Token Ring, FDDI), Provides: Data transmission control Collision resolution (retransmission) Layer 2 frame preparation (data framing) Frame Check Sequence (FCS – frame error detection) Media Access Control (MAC)

  12. Media Access Control (MAC) Protocols • Ethernet (IEEE 802.3) • Logical bus topology • Physical star or extended star • Nondeterministic • First-come, first-served • Token Ring (IEEE 802.5) • Logical ring • Physical star topology • Deterministic • Token controls traffic • Older declining technology • FDDI (IEEE 802.5) • Logical ring topology • Physical dual-ring topology • Deterministic • Token controls traffic

  13. Ethernet and the OSI model:

  14. The Media Access Control (MAC) sublayer is concerned with the physical components that will be used to communicate the information. • The Logical Link Control (LLC) sublayer remains relatively independent of the physical equipment that will be used for the communication process to upper layers.

  15. Naming: 48 bits or 12 Hex • The NIC uses the MAC address to assess whether the message should be passed onto the upper layers of the OSI model. • The NIC makes this assessment without using CPU processing time. • The MAC address is burned into ROM on the NIC card

  16. Legacy (Broadcast) Ethernet: Source host builds a Layer 2 data frame. Its own MAC is the source address. The MAC of the target is the destination address. All devices on the segment see the frame. Only the target’s NIC recognizes its MAC address in the Destination Address field. Target host copies and processes the frame. Non-target hosts dispose of or ignore of the frame. How the NIC Uses MAC Addresses

  17. Layer 2 Address Encapsulation and De-encapsulation Source and Destination MAC address fields are part of the frame header.

  18. The following slides address the following topics: Why framing is necessary Frame format diagram Generic frame format Framing

  19. Binary data is a stream of 1s and 0s. Framing breaks the stream into decipherable groupings: Start and stop indicator fields Naming or addressing fields Data fields Quality-control fields (FCS or CRC) Framing is the Layer 2 encapsulation process. A frame is the Layer 2 protocol data unit (PDU). Why Framing Is Necessary

  20. Start Frame Field (multiple bytes in size – preamble + SFD) Address Fields (Source & Destination MAC addresses) Type / Length Field (depending on Ethernet standard) Data Field (46-1500 bytes) FCS (Frame Check Sequence) Field Frame Stop Field (unique value) Generic Frame Format

  21. Media Access Control (MAC): Non-Deterministic (1st come 1st served) Collisions occur and slow transmission Deterministic (taking turns) No Collisions

  22. MAC rules and collision detection/backoff Networking devices detect a collision has occurred when the amplitude of the signal on the networking media increases. (JAM) When a collision occurs, each node that is transmitting will continue to transmit for a short time to ensure that all devices see the collision. (transmit 32-bit jam signal) The devices that were involved in the collision do not have priority to transmit data.

  23. MAC rules and collision detection/backoff: • Host wants to transmit • Is carrier sensed? • Assemble Frame • Start Transmitting • Is a Collision detected? • Keep Transmitting • Is the transmission done? • Transmission complete • Broadcast jam signal • Attempts = Attempts + 1 • Attempts > Too many? • Too many collisions; abort transmission • Algorithm calculates backoff • Wait for t microseconds

  24. This Class: Chapter 5 Labs due! Finish Labs: Chapter 5  no Labs for Chapter 6! Wall Outlet – Layout & cut Cable, Panel installation next week Chapter #6 Test Next Week

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