1. 10/11/2007 EETS 73041 The Medium Access Control Sublayer Chapter 4

2. 10/11/2007 EETS 73042 MAC Layer LAN WAN router Manch MAC PPP QAM IP Manch MAC IP QAM PPP IP

3. 10/11/2007 EETS 73043 The Channel Allocation Problem Static AllocationDynamic Allocation Tc = Ts/(1 –  )      Tn = (Ts/N)/(1 –  ) Tn Ts/N N Tc Ts Example T1 carrier: N = 24, 6400 bits/frame. Ts = 6400/64000 = 0.1 s. For  = 0.8 Tc = 0.5 s, Tn = 0.5/24 s for the same traffic.

4. 10/11/2007 EETS 73044 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).

5. 10/11/2007 EETS 73045 Multiple Access Protocols ALOHA Carrier Sense Multiple Access Protocols Collision-Free Protocols Wavelength Division Multiple Access Protocols

6. 10/11/2007 EETS 73046 Pure ALOHA (1970) In pure ALOHA, frames are transmitted at completely arbitrary times.

7. 10/11/2007 EETS 73047 Collision conditions in pure Aloha period is 2t If another frame starts here we will have collision All frames are of the same size. Frame transmission can start at any time instant.

8. 10/11/2007 EETS 73048 Throughput versus offered traffic for ALOHA systems. Max throughputs: 18% at G = 0.5 for pure 37% at G = 1 for slotted. (throughput per packet time)

9. 10/11/2007 EETS 73049 (1-p) carr ->random back off CSMA: Persistent and Non-persistent no carr -> transmit carr -> check after random time nonpersistent no carr -> transmit with prob p. p-persistent is slotted carr ->check next slot

10. 10/11/2007 EETS 730410 CSMA with Collision Detection CSMA/CD can be in one of three states: contention, transmission, or idle. Minimum contention slot is 2  where  ( 5  s/km) is the propagation delay between the two most remote stations.

11. 10/11/2007 EETS 730411 Reservation protocols: basic bit-map protocol

12. 10/11/2007 EETS 730412 The binary countdown protocol

13. Wavelength Division Multiple Access 10/11/2007 EETS 730413 Station A Station B tunable control transmit fixed data transmit tunable data receive fixed control receive 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. Fiber LAN implementation Channel allocations per station

14. 10/11/2007 EETS 730414 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

15. 10/11/2007 EETS 730415 Ethernet 802.3 10 MHz Segment length in hundredths meters Base band Vampire taps T conn.

16. 10/11/2007 EETS 730416 Ethernet Cabling Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T.

17. 10/11/2007 EETS 730417 Ethernet coding (+/- 0.85 V) (a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding.

18. 10/11/2007 EETS 730418 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.

19. 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. FCDataLength/typeSADASOFPre 446-150026617

20. 10/11/2007 EETS 730420 Collision detection can take as long as 2  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 2  = 50 mksec = 500 bits = 64 bytes.

21. 10/11/2007 EETS 730421 Binary exponential backoff 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/k gives A max -> 1/e for k -> inf. Average number of contention slots =    jA(1 – A) j-1 = 1/A. Therefore, channel efficiency = P/(P + 2  /A) Frame first collision second collision for i-th collision station randomly chooses between 0 and 2 i - 1 contention slots until 1023. P sec

22. 10/11/2007 EETS 730422 Efficiency of Ethernet at 10 Mbps with 512- bit slot times

23. 10/11/2007 EETS 730423 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.

24. 10/11/2007 EETS 730424 Gigabit Ethernet (a) A two-station Ethernet. (b) A multistation Ethernet.

25. 10/11/2007 EETS 730425 Gigabit Ethernet cabling

26. 10/11/2007 EETS 730426 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.).

27. 10/11/2007 EETS 730427 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

28. 10/11/2007 EETS 730428 Part of the 802.11 protocol stack FHSS - Frequency Hopping Spread Spectrum DSSS - Direct Sequence Spread Spectrum OFDM - Orthogonal Frequency Division Multiplexing HR-DSSS - High Rate DSSS MAC: PCF - Point Coordin. Funct.: Base Station Polls mobiles within its cell. DCF - Distributed Coordination Function further discussed.

29. 10/11/2007 EETS 730429 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.

30. 10/11/2007 EETS 730430 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.

31. 10/11/2007 EETS 730431 The use of virtual channel sensing using CSMA/CA NAV - Network Allocation Vector

32. 10/11/2007 EETS 730432 A fragment burst Due to the noisy wireless channels data packets are split into short fragments.

33. 10/11/2007 EETS 730433 Interframe spacing in 802.11

34. 10/11/2007 EETS 730434 The 802.11 Data Frame Structure Frames must process in order Encrypted using WEP More frames to follow Toggle bit to put receiver to sleep Retransmitted frame Data, Control, Mngmnt. RTS, CTS Frame out of Cell More fragments to follow For NAV For Base Station 12 bit frame, 4 bits fragment

35. 10/11/2007 EETS 730435 802.11 Distribution Services Association (Attach) Disassociation (Detach) Reassociation (Handoff) Distribution (Frame routing) Integration (Protocol conversion)

36. 10/11/2007 EETS 730436 802.11 Intracell Services Authentication (of mobile) Deauthentication (on leave) Privacy (encryption) Data Delivery (data exchange)

37. 10/11/2007 EETS 730437 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

38. 802.11 vs. 802.16 802.11802.16 environment indooroutdoor mobility mobilefixed access point-to-multipointpoint-to-point distance meterskm spectrum ISM (900 MHz-5.7 GHz)10 - 66 GHz (mm waves) QoS internetmultimedia (con orient) errors detection correction (Hamming) securityoptionalmandatory bit rate10 Mbps100 Mbps

39. 10/11/2007 EETS 730439 The 802.16 Protocol Stack

40. 10/11/2007 EETS 730440 The 802.16 Physical Layer

41. 10/11/2007 EETS 730441 The 802.16 Physical Layer (2) Frames and time slots for time division duplexing.

42. 10/11/2007 EETS 730442 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)

43. 10/11/2007 EETS 730443 The 802.16 Frame Structure (a) A generic frame. (b) A bandwidth request frame.