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Computer Networks

Computer Networks. Bhushan Trivedi, Director, MCA Programme, at the GLS Institute of Computer Technology, Ahmadabad. Chapter 6. Medium access sub layer (MAC). The need for arbitration. Introduction and need for MAC layer Shared Channel Concept Collision detection and avoidance

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Computer Networks

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  1. Computer Networks Bhushan Trivedi, Director, MCA Programme, at the GLS Institute of Computer Technology, Ahmadabad

  2. Chapter 6 Medium access sub layer (MAC)

  3. The need for arbitration Introduction and need for MAC layer Shared Channel Concept Collision detection and avoidance Channel acquisition issues Ad-hoc and Infrastructure modes

  4. Channel acquisition solutions Sender transmit whenever wishes to First version of Ethernet and 802.11 or WiFi Using token as a prerogative to transmit Reservation before sending The sender sends only if receiver confirms in wireless 802.11 case, ad-hoc mode Central authority to decide 802.11 using access points

  5. Infrastructure or PCF mode

  6. Ad hoc Mode

  7. Both modes together in same network

  8. Wired MAC Layer ALOHA and Slotted ALOHA Ethernet Fast Ethernet Gigabit Ethernet The 10 Gb Ethernet Future versions Dual speed cards, auto-negotiation and upgrades 802.2, the LLC layer

  9. ALOHA connecting islands at Hawaii

  10. The collision Problem inALOHA

  11. Slotted ALOHA solution

  12. Ethernet First Version with Bus Topology

  13. Ethernet and CSMA/CD Refrain from transmitting if channel is busy Realize collision almost immediately Wait for a random period before retransmitting.. collision only if the first sender’s signal has not reached the second sender inherently poor design for heavy traffics.

  14. Classic Ethernet 802.3 Two standards, DIX and 802.3 (IEEE) Initially Bus and Hubs were used Later switches took over

  15. Classsic Ethernet Connection Types

  16. Star Topology

  17. Actual layout of the network

  18. Cascading the network

  19. Switch improved the performance by reducing collision domain

  20. Ethernet Frame Structure DIX

  21. Ethernet Frame Structure IEEE

  22. Multiplexing at Ethernet

  23. CSMA/CD Carrier Sense Multiple Access Collision Detection Interframe gap Maximum distance limitation Minimum frame size limitation More bandwidth deteriorates performance

  24. Binary Exponential Backoff Sender sends immediately with idle channel Continues to listen while transmitting In case of a collision, the sender waits for a random period (maximum of two time slots) In case they collide again, the interval is just doubled every time it experiences a collision, When doubling is repeated to the slot size to 0–1023 it will not increase further

  25. A Fast Ethernet 100 Mb version of Ethernet (802.3u) The frame structure is same permits only star topology. Hubs or switches are a must. The bus topology and coaxial cables are not used

  26. Fast Ethernet Connections

  27. The DC Component of a signal

  28. A signal central around a positive value is equal to a signal centered at zero plus a positive continuous voltage signal (DC Component)

  29. The Paradigm Shift: PtoP from shared Coaxial cable and multi-drop is discouraged Point to Point and full duplex transmissions Reducing collision domain Distance limit is lifted, Ethernet is used for first mile now Ethernet remain unfazed after 40 years

  30. Gigabit Ethernet Even faster Ethernet 1000 MHz (802.3z) Full and Half Duplex operations

  31. Gigabit Ethernet Connections

  32. Gigabit Ethernet Frame bursting and hardware extensions

  33. 8B/10B Coding More than six 0s or 1s are not allowed in a 10-bit word Four identical bits are not permitted in a sequence The same coding scheme is also used in fiber channel All fiber-based mechanisms use 8B/10B Cat cables use 4B/5B which is different

  34. The Pause Frame for Flow Control

  35. 10Gb Much faster Ethernet 802.ae, ak, ap, an etc Standard and non standard variants Backplane Ethernet Explicit support for WAN

  36. 10Gb connections

  37. 40-100 Gb Super fast Ethernet 802.3ba 100 Gb over 40 Km Under standardization when this is being written Link aggregation Dense Wavelength Division Multiplexing

  38. 40-100G Ethernet

  39. Wireless MAC Layer The Wireless LAN protocol (802.11) The DCF Mode Fragmentation in DCF mode The PCF mode Managing PCF and DCF modes together The 802.11 frame

  40. Wireless transmission in 802.11 Signal fading (free space path loss) Interference Multipath fading Restriction on power LOS vs NLOS Placement of physical obstacle Frequency range Modulation scheme

  41. Placement of obstacles

  42. DCF (Ad Hoc) mode Computers communicate directly Compulsory mode With or without CSMA/CA Collision invites Binary exponential backoff Unlike Ethernet, stop and wait is used Fragments are sent when noise is higher

  43. Ad Hoc mode with CSMA/CA

  44. AdHoc mode without CSMA/CA

  45. Fragments in 802.11

  46. Continue

  47. The PCF mode Access point determines who will send when beacon frames for polling each station Default and protected modes and SSID Default mode Must know the SSID Protected Mode MAC address IP address

  48. Access point primitives

  49. Continue…

  50. Different Inter-frame spacing

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