1 / 70

The Medium Access Control Sublayer

The Medium Access Control Sublayer. Chapter 4. The Channel Allocation Problem. Static Channel Allocation in LANs and MANs Dynamic Channel Allocation in LANs and MANs. Dynamic Channel Allocation in LANs and MANs. Station Model. Single Channel Assumption. Collision Assumption.

charlie
Télécharger la présentation

The Medium Access Control Sublayer

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. The Medium Access ControlSublayer Chapter 4 CS522 Tanenbaum

  2. The Channel Allocation Problem • Static Channel Allocation in LANs and MANs • Dynamic Channel Allocation in LANs and MANs CS522 Tanenbaum

  3. Dynamic Channel Allocation in LANs and MANs • Station Model. • Single Channel Assumption. • Collision Assumption. • (a) Continuous Time.(b) Slotted Time. • (a) Carrier Sense.(b) No Carrier Sense. CS522 Tanenbaum

  4. Multiple Access Protocols • ALOHA • Carrier Sense Multiple Access Protocols • Collision-Free Protocols • Limited-Contention Protocols • Wavelength Division Multiple Access Protocols • Wireless LAN Protocols CS522 Tanenbaum

  5. Pure ALOHA In pure ALOHA, frames are transmitted at completely arbitrary times. CS522 Tanenbaum

  6. Pure ALOHA (2) Vulnerable period for the shaded frame. CS522 Tanenbaum

  7. Pure ALOHA (3) A station transmits a frame whenever it has data to send. When the frame collides with other frame(s), retransmit it by waiting a random amount of time. What is the throughput (or efficiency) of an ALOHA channel? Let N (no. of stations using an ALOHA channel) = ∞ , assume infinite population, new arrival is a Poisson process, λn = mean new frame arrival rate (frame/sec), µ = channel capacity (frame/sec), T=1/ µ=frame time (sec/frame) S = λn/µ= λn (frame/sec). 1/µ (sec/frame) = mean new frames per frame time, S is traffic density or channel utilization, λr = mean retransmitted frame rate. The combined traffic of new and old (retransmission) traffic is still a Poisson process with G= mean departure frames per frame time=(λn+λr)/µ=λT. G is actual traffic density. G can be controlled by adjusting retransmission time. Can S be controlled? What is the system throughput? S or G? Analysis: • If S>1, almost every frame will collide. • For reasonable throughput, 0<S<1. G > S. • At low load, S ≈ 0 → G ≈ S. • At high load, G>S. CS522 Tanenbaum

  8. Pure ALOHA (4) • What is the relationship between G and S? S = GP0P0 = the probability that a frame does not suffer a collision. • How to find P0?Note that the combined frame traffic is a Poisson process, therefore,Pr[k] = the probability that k frames are generated during a frame time T = (λ=λn+λr, T=1/µ → λT=G substitute λT with G ) = • But observe Figure 4-2, the vulnerable period of a frame is 2 frame time. • Pr’[k] = (T’=2T, λT’=2G) • P0 = Pr’[0] = . • Observation:S depends on incoming traffic.G depends on S. For each S, there are two G values in Figure 4-3. CS522 Tanenbaum

  9. What is the max. throughput of Pure ALOHA? How to control G to get maximum S? Let  , Smax = The best we can hope for the channel utilization is 18.4%. Is 18.4% channel utilization for the ALOHA system too low? For a terminal user sends 60 character/msg every 2 min., input rate is 0.5 char/sec = 5 bits/sec (assume 10 bit async. transmission) For 4800 bps channel and 10% utilization, it can support 96 interactive users. For bursty, interactive traffic, pure ALOHA is sufficient and simple. CS522 Tanenbaum

  10. Slotted ALOHA Transmission time is divided into slots. Stations with data to transmit will wait until the starting time of a slot. Time of vulnerable period is reduced to 1 frame (slot) time. (Require synchronization devices, e.g., a central station broadcasts the clock signal to all stations for synchronizing the slots.) Pr[k] = P0 = Pr[0] = e-G S=GP0=Ge–G. Let  , Smax = CS522 Tanenbaum

  11. PURE ALOHA vs. Slotted ALOHA Throughput versus offered traffic for ALOHA systems. CS522 Tanenbaum

  12. Carrier Sense MA protocols Protocols in which stations listen for a carrier (i.e. a transmission medium) and act accordingly, e.g. MA protocols used by LANs. 1-Persistent CSMA When a station has data to send, it listens to the channel. If the channel is busy, it waits until channel idle. When the channel is idle, it transmits a frame (with probability = 1). Propagation delay effect: At t0, station 1 detects idle and sends a frame. At t0+τ−ε, station 2 detects idle and sends a frame.2 At t0+τ, station 2 detects collision. CS522 Tanenbaum

  13. Carrier Sense MA Protocols(2) At t0+τ−ε+τ, station 1 detects collision. In worst case, only about 2τ time later can station 1 detect the collision. The longer the cable, the longer stations have to wait to be sure that there is no collision. Non-persistent CSMA • If the channel is busy, it waits for random period then sense the channel again. • Better channel utilization but longer delays than 1-persistent CSMA. p-persistent CSMA(applies to slotted channel) • If the channel is idle, it transmits with prob = p (with 1-p, it defers until next slot). • If the next slot is idle again, do the same thing. CS522 Tanenbaum

  14. Persistent and Nonpersistent CSMA Comparison of the channel utilization versus load for various random access protocols. Smaller p has better throughput but at what cost? CS522 Tanenbaum

  15. CSMA with Collision Detection CSMA/CD can be in one of three states: contention, transmission, or idle. CS522 Tanenbaum

  16. Collision-Free Protocols The basic bit-map protocol. CS522 Tanenbaum

  17. Collision-Free Protocols (2) The binary countdown protocol. A dash indicates silence. CS522 Tanenbaum

  18. Limited-Contention Protocols Acquisition probability for a symmetric contention channel. CS522 Tanenbaum

  19. Adaptive Tree Walk Protocol The tree for eight stations. CS522 Tanenbaum

  20. Wavelength Division Multiple Access Protocols Wavelength division multiple access. CS522 Tanenbaum

  21. Wireless LAN Protocols A wireless LAN. (a) A transmitting. (b) B transmitting. CS522 Tanenbaum

  22. Wireless LAN Protocols (2) The MACA protocol. (a) A sending an RTS to B. (b) B responding with a CTS to A. CS522 Tanenbaum

  23. Ethernet • Ethernet Cabling • Manchester Encoding • The Ethernet MAC Sublayer Protocol • The Binary Exponential Backoff Algorithm • Ethernet Performance • Switched Ethernet • Fast Ethernet • Gigabit Ethernet • IEEE 802.2: Logical Link Control • Retrospective on Ethernet CS522 Tanenbaum

  24. Ethernet Cabling The most common kinds of Ethernet cabling. CS522 Tanenbaum

  25. Ethernet Cabling (2) Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T. CS522 Tanenbaum

  26. Ethernet Cabling (3) Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented. CS522 Tanenbaum

  27. Ethernet Cabling (4) (a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding. CS522 Tanenbaum

  28. Ethernet MAC Sublayer Protocol Frame formats. (a) DIX Ethernet, (b) IEEE 802.3. CS522 Tanenbaum

  29. Ethernet MAC Sublayer Protocol (2) CS522 Tanenbaum

  30. Ethernet Performance Efficiency of Ethernet at 10 Mbps with 512-bit slot times. CS522 Tanenbaum

  31. Switched Ethernet A simple example of switched Ethernet. CS522 Tanenbaum

  32. Fast Ethernet The original fast Ethernet cabling. CS522 Tanenbaum

  33. Gigabit Ethernet (a) A two-station Ethernet. (b) A multistation Ethernet. CS522 Tanenbaum

  34. Gigabit Ethernet (2) Gigabit Ethernet cabling. CS522 Tanenbaum

  35. IEEE 802.2: Logical Link Control (a) Position of LLC. (b) Protocol formats. CS522 Tanenbaum

  36. Wireless LANs • The 802.11 Protocol Stack • The 802.11 Physical Layer • The 802.11 MAC Sublayer Protocol • The 802.11 Frame Structure • Services CS522 Tanenbaum

  37. The 802.11 Protocol Stack Part of the 802.11 protocol stack. CS522 Tanenbaum

  38. The 802.11 MAC Sublayer Protocol (a) The hidden station problem. (b) The exposed station problem. CS522 Tanenbaum

  39. The 802.11 MAC Sublayer Protocol (2) The use of virtual channel sensing using CSMA/CA. CS522 Tanenbaum

  40. The 802.11 MAC Sublayer Protocol (3) A fragment burst. CS522 Tanenbaum

  41. The 802.11 MAC Sublayer Protocol (4) Interframe spacing in 802.11. CS522 Tanenbaum

  42. The 802.11 Frame Structure The 802.11 data frame. CS522 Tanenbaum

  43. 802.11 Services Distribution Services • Association • Disassociation • Reassociation • Distribution • Integration CS522 Tanenbaum

  44. 802.11 Services Intracell Services • Authentication • Deauthentication • Privacy • Data Delivery CS522 Tanenbaum

  45. Broadband Wireless • Comparison of 802.11 and 802.16 • The 802.16 Protocol Stack • The 802.16 Physical Layer • The 802.16 MAC Sublayer Protocol • The 802.16 Frame Structure CS522 Tanenbaum

  46. The 802.16 Protocol Stack The 802.16 Protocol Stack. CS522 Tanenbaum

  47. The 802.16 Physical Layer The 802.16 transmission environment. CS522 Tanenbaum

  48. The 802.16 Physical Layer (2) Frames and time slots for time division duplexing. CS522 Tanenbaum

  49. The 802.16 MAC Sublayer Protocol Service Classes • Constant bit rate service • Real-time variable bit rate service • Non-real-time variable bit rate service • Best efforts service CS522 Tanenbaum

  50. The 802.16 Frame Structure (a) A generic frame. (b) A bandwidth request frame. CS522 Tanenbaum

More Related