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CS412 Introduction to Computer Networking & Telecommunication

Learn about LANs, Ethernet, MAC Sublayer Protocol, Manchester Encoding, Backoff Algorithm, Gigabit Ethernet & Logical Link Control. Explore bridging and data link layer switching techniques.

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CS412 Introduction to Computer Networking & Telecommunication

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  1. CS412 Introduction to Computer Networking & Telecommunication Local Area Networks Chi-Cheng Lin, Winona State University

  2. Topics • LANs - IEEE Project 802 • Ethernet • Data Link Layer Switching

  3. Figure 12-1 LAN Compared with the OSI Model

  4. Figure 12-2 Project 802

  5. Ethernet • Ethernet Cabling • Manchester Encoding • Ethernet MAC Sublayer Protocol • Binary Exponential Backoff Algorithm • Switched Ethernet • Fast Ethernet • Gigabit Ethernet • IEEE 802.2: Logical Link Control • Retrospective on Ethernet

  6. 802.3 and Ethernet • 802.3 • 1-Persistent CSMA/CD LAN, 1 - 10 Mbps • Ethernet • A specific product that almost implements 802.3 • Cabling (baseband) • 10Base5 (thick Ethernet) • 10Base2 (thin Ethernet) • 10Base-T (twisted pair) • 10Base-F (fiber optics) XBaseY Channel capacity Cable type

  7. Ethernet Cabling • The most common kinds of Ethernet cabling.

  8. Ethernet Cabling (2) • Three kinds of Ethernet cabling. • (a) 10Base5, (b) 10Base2, (c) 10Base-T.

  9. Ethernet Cabling (3) • Cable topologies. • (a) Linear (b) Spine (c) Tree (d) Segmented

  10. Manchester Encoding • Why Manchester encoding? • Differentiating 0 bit or idle • Synchronization • Encoding scheme: • Each bit period is divided into 2 equal intervals • Each bit period has a transition in the middle

  11. Manchester Encoding • Manchester encoding • Bit 1: high-low, bit 0: low-high • Differential Manchester encoding • Bit 1: no transition at the start of interval • Bit 0: transition at the start of interval

  12. Ethernet MAC Sublayer Protocol • Frame formats. • (a) DIX Ethernet, (b) IEEE 802.3. • Preamble: 10101010 for synchronization • Start of frame: 10101011

  13. Ethernet MAC Sublayer Protocol • Addresses • Ethernet uses 6 bytes • Support • Unicast: address begins with 0 • Multicasting: 1 + group number • Broadcasting: all 1’s

  14. Figure 12-6 Collision in CSMA/CD

  15. Ethernet MAC Sublayer Protocol (2)

  16. Ethernet MAC Sublayer Protocol • Minimum frame size: 64 bytes • Why? frame_size bits/channel_capacity bps > 2 s In 10-Mbps Ethernet, 2 = 50 s, therefore frame_size > 50 s x 10 Mbps = 500 bits, rounded up to 512 bits = 64 bytes • As the network speed goes up  minimum frame length must go up or maximum cable length must come down

  17. Binary Exponential Backoff Algorithm • Wait time t time slots after a collision • t = a random number between 0 and 2i - 1 after i collisions • t = 1024, for i = 10,...,16 • when i > 16, reset i = 0 • Low delay for light load • Reasonable delay for high load

  18. Switched Ethernet • A simple example of switched Ethernet • If all ports on a card wired together, each card becomes an on-card LAN and forms one collision domain. • If buffer used, one port is a collision domain and no collision will occur.

  19. Figure 12-14 An Ethernet Network Using A Hub One collision domain

  20. Figure 12-15 An Ethernet Network Using a Switch

  21. Fast Ethernet • The original fast Ethernet cabling. 100Base-T: hubs and switches 100Base-F: switches only, with one cable one collision domain

  22. Gigabit Ethernet • Configurations • (a) A two-station Ethernet. • (b) A multistation Ethernet.

  23. Gigabit Ethernet (2) • Gigabit Ethernet cabling.

  24. IEEE 802.2: Logical Link Control • LLC • (a) Position of LLC. (b) Protocol formats.

  25. Logical Link Control • LLC forms the upper half of data link layer (MAC is below LLC) • Purposes • Provides error control and flow control • Hides differences between 802 networks by providing a single format and interface to network layer • Services • Unreliable datagram • Acknowledged datagram • Reliable connection-oriented service

  26. Logical Link Control • Sender • Network layer passes packet to LLC using LLC access primitives • LLC sublayer adds LLC header • Source and destination access points • Control: sequence and acknowledgement numbers • 802.x frame payload field = (LLC header + packet) • Frame is transmitted • Receiver • Reversed process

  27. Restrospective on Ethernet • Has been 20+ years • Simple and flexible • Reliable • Cheap • Easy to maintain • Works easily with TCP/IP • Both IP and Ethernet are connectionless • Evolution – no software change required • Speed: higher and higher • Hubs, switches

  28. Data Link Layer Switching • Bridges from 802.x to 802.y • Local Internetworking • Spanning Tree Bridges • Remote Bridges • Interconnection Devices • Repeaters, hubs, bridges, switches, cut-through switches, routers, gateways

  29. Data Link Layer Switching Multiple LANs connected by a backbone to handle a total load higher than the capacity of a single LAN.

  30. Bridges from 802.x to 802.y • Operation of a LAN bridge from 802.11 to 802.3.

  31. Bridges from 802.x to 802.y (2) • General Problems • Different data formats • Different data rates • Different maximum frame length

  32. Local Internetworking • A configuration with four LANs and two bridges.

  33. Transparent Bridges • Transparency • Plug and play • Operates in Promiscuous Mode • Accepting every frame transmitted on all LANs to which it is attached • Decides • Discard or forward • If forward, to which LAN? • Look up a huge destination address hash table

  34. Transparent Bridges • Hash Table • Initially empty • Flooding algorithm • Backward learning algorithm • Arrival time noted for dynamic topology • Scanned periodically to remove old entries • Routing procedure for an incoming frame • If dest LAN = src LAN then discard • If dest LAN != src LAN then forward • If dest LAN unknown then use flooding

  35. Figure 16.6Learning bridge

  36. Spanning Tree Bridges • To increase reliability • Two or more bridges between 2 LANs • Problem: looping F3 F4 with unknown destination

  37. Spanning Tree Bridges • Solution to looping: Spanning tree bridges • LAN  vertex • Bridge  edge(s)

  38. Remote Bridges • Connects LANs at remote sites • Approach • Putting bridges on each LAN • Connecting bridges point-to-point • Point-to-point link considered as a “hostless” LAN

  39. Interconnection Devices • Repeaters, hubs, bridges, switches, cut-through switches, routers, gateway • Issues: bandwidth and collision domain (a) Which device is in which layer. (b) Frames, packets, and headers.

  40. Interconnection Devices • (a) A hub. (b) A bridge. (c) a switch.

  41. Figure 16.2Repeater

  42. Figure 16.3Function of a repeater A repeater is not an amplifier – an amplifier does not regenerate signals.

  43. Figure 16.4Hubs A hub is a multiport repeater.

  44. Figure 16.5Bridge

  45. Figure 14.16A network with and without a bridge

  46. Figure 14.17Collision domains in a nonbridged and bridged network

  47. Figure 21-16 Switch

  48. Figure 14.18Switched Ethernet

  49. Cut-Through Switch • As soon as the destination header field has been received, the frame can be forwarded. • Faster (shorter delay) • No more store-and-forward? • Bad frames propagation

  50. Figure 21-10 A Router in the OSI Model

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