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The Medium Access Control Sublayer

The Medium Access Control Sublayer. Chapter 4. 10/11/2007. EETS 7304. 1. MAC Layer. IP. MAC. PPP. Manch. QAM. IP. IP. MAC. PPP. Manch. QAM. router. LAN. WAN. 10/11/2007. EETS 7304. 2. The Channel Allocation Problem. Static Allocation. Dynamic Allocation. Tc. Ts. Tn.

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The Medium Access Control Sublayer

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  1. The Medium Access ControlSublayer Chapter 4 10/11/2007 EETS 7304 1

  2. MAC Layer IP MAC PPP Manch QAM IP IP MAC PPP Manch QAM router LAN WAN 10/11/2007 EETS 7304 2

  3. The Channel Allocation Problem Static Allocation Dynamic Allocation Tc Ts Tn Ts/N m l N l Nm N l m Tn = (Ts/N)/(1 – r) l m Tc = Ts/(1 – r) Example T1 carrier: N = 24, 6400 bits/frame. Ts = 6400/64000 = 0.1 s. For r = 0.8 Tc = 0.5 s, Tn = 0.5/24 s for the same traffic. 10/11/2007 EETS 7304 3

  4. Dynamic Channel Allocation in LANs Station Model:N independent stations sending messages according to Poisson distribution. Single Channel:A single channel is available to all stations to transmit to and receive from. Carrier Sense:Stations sense if the channel is in use and wait vs. stations cannot tell if the channel is in use. Collision Assumption:If stations transmit at the same time, frames will collide and garbled. All stations can detect collision and retransmit frame later. Frame beginning: Pure Aloha (frame can start transmission at any time) vs. slotted Aloha (frame can start transmission only at a given time instance). 10/11/2007 EETS 7304 4

  5. Multiple Access Protocols ALOHA Carrier Sense Multiple Access Protocols Collision-Free Protocols Wavelength Division Multiple Access Protocols 10/11/2007 EETS 7304 5

  6. Pure ALOHA (1970) In pure ALOHA, frames are transmitted at completely arbitrary times. 10/11/2007 EETS 7304 6

  7. Collision conditions in pure Aloha period is 2t • All frames are of the same size. • Frame transmission can start at any time instant. If another frame starts here we will have collision 10/11/2007 EETS 7304 7

  8. Throughput versus offered traffic for ALOHA systems. (throughput per packet time) Max throughputs: 18% at G = 0.5 for pure 37% at G = 1 for slotted. 10/11/2007 EETS 7304 8

  9. CSMA: Persistent and Non-persistent carr ->check next slot (1-p) carr ->random back off carr -> check after random time no carr -> transmit with prob p. no carr -> transmit p-persistent is slotted nonpersistent 10/11/2007 EETS 7304 9

  10. CSMA with Collision Detection CSMA/CD can be in one of three states: contention, transmission, or idle. Minimum contention slot is 2t where t ( 5 ms/km) is the propagation delay between the two most remote stations. 10/11/2007 EETS 7304 10

  11. Reservation protocols: basic bit-map protocol 10/11/2007 EETS 7304 11

  12. The binary countdown protocol 10/11/2007 EETS 7304 12

  13. Wavelength Division Multiple Access tunable control transmit fixed data transmit S S S S S S Station A Station B tunable data receive fixed control receive m time slots for control station x x x x x x x x x x B x x x x x x x x x x x x A x x x x C n time slots for data plus 1 for status Fiber LAN implementation A wants to send data to B 1. A tunes to listen B status S. S tells B control slots #3 is free. 2. A tunes transmit control and sends to B “data for you in my slot 4” in B control slot #3. 3. B tunes to A output and receives data from slot 4. Channel allocations per station 10/11/2007 EETS 7304 13

  14. 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 10/11/2007 EETS 7304 14

  15. Ethernet 802.3 Base band Segment length in hundredths meters 10 MHz T conn. Vampire taps 10/11/2007 EETS 7304 15

  16. Ethernet Cabling Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T. 10/11/2007 EETS 7304 16

  17. Ethernet coding (+/- 0.85 V) (a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding. 10/11/2007 EETS 7304 17

  18. Ethernet MAC Sublayer Protocol Frame formats. (a) DIX (DEC, Intel Xerox) (b) IEEE 802.3. • Address bit 46 determines local or global address. • Min frame is 64 bytes from dest. address to checksum. 10/11/2007 EETS 7304 18

  19. 7 1 6 6 2 46-1500 4 Pre SOF DA SA Length/type Data FC Ethernet 802.3 • Preamble (PRE). The PRE is an alternating pattern of ones and zeros. • Start-of-frame delimiter (SOF). The SOF is an alternating pattern of ones and zeros, ending with two consecutive 1-bits. • Destination address (DA). Source addresses (SA). • Length/Type indicates either the number of MAC-client data bytes that are contained in the data field of the frame, or the frame type ID if the frame is assembled using an optional format. • Data n bytes (46=< n =<1500) of any value. (The total frame minimum is 64bytes.) • Frame check sequence (FCS) 32-bit cyclic redundancy check (CRC) value, which is created by the sending MAC and is recalculated by the receiving MAC to check for damaged frames.

  20. Collision detection can take as long as 2t This is to sense a collision before end of the frame reach far end. In 10 Mbps LAN 1 bit is 100 nsec, and max segment 2500 m round trip delay is 2t = 50 mksec = 500 bits = 64 bytes. 10/11/2007 EETS 7304 20

  21. Binary exponential backoff Frame second collision for i-th collision station randomly chooses between 0 and 2i - 1 contention slots until 1023. Frame P sec first collision Simplified version with persistance p: If k stations contend for a channel the probability that any of k gets a channel is: A = kp(1 – p)k-1 p = 1/kgives Amax -> 1/e for k -> inf. Average number of contention slots = S000 jA(1 – A)j-1 = 1/A. Therefore, channel efficiency = P/(P + 2t/A) 10/11/2007 EETS 7304 21

  22. Efficiency of Ethernet at 10 Mbps with 512-bit slot times 10/11/2007 EETS 7304 22

  23. Switched Ethernet Plug-in card handles collision domain either usual way or w/out collision. Frames destined outside plug-in domain are switched over backplane to the destined plug-in card. 10/11/2007 EETS 7304 23

  24. Gigabit Ethernet (a) A two-station Ethernet. (b) A multistation Ethernet. 10/11/2007 EETS 7304 24

  25. Gigabit Ethernet cabling 10/11/2007 EETS 7304 25

  26. IEEE 802.2: Logical Link Control (a) Position of LLC. (b) Protocol formats. When reliable service is required the LLC (Logical Link Control) layer is added on the top of MAC layer. LLC is HDLC based (frame sequencing etc.). 10/11/2007 EETS 7304 26

  27. Wireless LANs (WiFi) The 802.11 Protocol Stack The 802.11 Physical Layer The 802.11 MAC Sublayer Protocol The 802.11 Frame Structure Services 10/11/2007 EETS 7304 27

  28. Part of the 802.11 protocol stack MAC: PCF - Point Coordin. Funct.: Base Station Polls mobiles within its cell. DCF - Distributed Coordination Function further discussed. FHSS - Frequency Hopping Spread Spectrum DSSS - Direct Sequence Spread Spectrum OFDM - Orthogonal Frequency Division Multiplexing HR-DSSS - High Rate DSSS 10/11/2007 EETS 7304 28

  29. Wireless LAN (bandwidth is 11 to 54 MHz) (a) A transmitting: C is out of range and cannot hear A. (b) B transmitting: C hears B and falsely concludes that it cannot transmit to D. Before transmitting the transmitting station wants to know whether is any activity around receiver. CSMA tells only activity around transmitter. Solution is that sender stimulate receiver to transmit short frame. 10/11/2007 EETS 7304 29

  30. DCF: wireless MACA (Multiple Access with Collision Avoidance) C hears RTS A to B (30 bytes) frame with the length of the frame to follow. D hears B responding to A with a CTS (copying the length of the next frame). A starts transmitting. 10/11/2007 EETS 7304 30

  31. The use of virtual channel sensing using CSMA/CA NAV - Network Allocation Vector 10/11/2007 EETS 7304 31

  32. A fragment burst Due to the noisy wireless channels data packets are split into short fragments. 10/11/2007 EETS 7304 32

  33. Interframe spacing in 802.11 10/11/2007 EETS 7304 33

  34. The 802.11 Data Frame Structure For Base Station For NAV 12 bit frame, 4 bits fragment Frames must process in order Data, Control, Mngmnt. RTS, CTS Encrypted using WEP Frame out of Cell More frames to follow More fragments to follow Toggle bit to put receiver to sleep Retransmitted frame 10/11/2007 EETS 7304 34

  35. 802.11 Distribution Services Association (Attach) Disassociation (Detach) Reassociation (Handoff) Distribution (Frame routing) Integration (Protocol conversion) 10/11/2007 EETS 7304 35

  36. 802.11 Intracell Services Authentication (of mobile) Deauthentication (on leave) Privacy (encryption) Data Delivery (data exchange) 10/11/2007 EETS 7304 36

  37. Broadband Wireless (WiMax) 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 10/11/2007 EETS 7304 37

  38. 802.11 802.16 environment indoor outdoor mobility mobile fixed access point-to-multipoint point-to-point distance meters km spectrum ISM (900 MHz-5.7 GHz) 10 - 66 GHz (mm waves) QoS internet multimedia (con orient) errors detection correction (Hamming) security optional mandatory bit rate 10 Mbps 100 Mbps 802.11 vs. 802.16

  39. The 802.16 Protocol Stack 10/11/2007 EETS 7304 39

  40. The 802.16 Physical Layer 10/11/2007 EETS 7304 40

  41. The 802.16 Physical Layer (2) Frames and time slots for time division duplexing. 10/11/2007 EETS 7304 41

  42. The 802.16 MAC Sublayer Protocol Service Classes Constant bit rate service (for T1) Real-time variable bit rate service (for multimedia) Non-real-time variable bit rate service (file transfer) Best efforts service (for contended upstream traffic) 10/11/2007 EETS 7304 42

  43. The 802.16 Frame Structure (a) A generic frame. (b) A bandwidth request frame. 10/11/2007 EETS 7304 43

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