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CS 453 Computer Networks

CS 453 Computer Networks. Lecture 15 Medium Access Control Sublayer. MAC sublayer. Gigabit Ethernet History has shown us that like Peanut butter cookies, you can’t have enough data rate capacity As data rates have grown applications have quickly swelled to fill the capacity

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CS 453 Computer Networks

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  1. CS 453Computer Networks Lecture 15 Medium Access Control Sublayer

  2. MAC sublayer • Gigabit Ethernet • History has shown us that like Peanut butter cookies, you can’t have enough data rate capacity • As data rates have grown applications have quickly swelled to fill the capacity • About the time that Fast Ethernet was hitting the streets, the IEEE 802 committee was working on what became known as Gigabit Ethernet

  3. MAC sublayer • Gigabit Ethernet • IEEE defined Gigabit Ethernet in 1998 and labeled it 802.3z • They apparently thought was the final frontier in terms of ethernet • They were wrong on that point • 802.3z goal to be • 10x faster than 802.3u (Fast Ethernet) • Backward compatible with other Ethernet standards

  4. MAC sublayer • Gigabit Ethernet • Goals • Same Ethernet Frame format • Same min/max frame sizes • Same 48 bit Ethernet addressing scheme • Offer unacknowledged datagram service • Unicast & multicast

  5. MAC sublayer • Gigabit Ethernet • All connections are point-to-point • No multidrop like 803.2 and 802.5 • Any GigE cable – only two devices • One device can be switch or hub • Modes – full-duplex • Normal • Connected to a switch • Send/Receive at same time • What about cable length and collision alarm delay??? • Remember the issue of cable length and the delay time for collision alarm to propagate from the station detecting the collision to the station transmitting?

  6. MAC sublayer • Gigabit Ethernet • In standard Ethernet (802.3) with a multidrop medium… • …the minimum packet size (minus the preamble and SOF flag) is 64 bytes…. • …so that a collision alarm could, worst case, reach from one end of the medium to a transmitting station at the other end of the medium while is it still transmitting… • ..but this can only be true for a maximum length cable… • For 802.3 that is 2500 meters • For full-duplex GigE, this is not an issue • Each cable is “private” for two devices • Collision are not possible • No CSMA/CD • Only cable length issue is signal strength loss

  7. MAC sublayer • Gigabit Ethernet • Half-duplex • All connections are point-to-point, but • Connected to a common hub • … functions like a bus-in-a-box • So, collisions are possible • Uses CSMA/CD • Then, in theory, due to collision alarm propagation delay • Max cable length = [10base2MaxCableLength]/100 = • 2500/100 = 25 meters • That won’t do!

  8. MAC sublayer • Gigabit Ethernet • Half-duplex - Collisions • Carrier Extension • Pad the frame to 512 bytes • Done padded and unpadded by hardware, no changes to software • Poor bandwidth efficiency for small payloads • Frame Bursting • “Bunch up” several frames and transmit at one time • If grouped frame is still less than 512 bytes pad to 512 • Efficient if there a good flow of frames to transmit • Allows cable lengths to 200 meters

  9. MAC sublayer • Gigabit Ethernet • 1000BaseT Encoding • Uses 4 pair of Cat5/Cat6 cable • Five symbols using 5 voltage levels • 00, 01,10,11 and a control symbol • So 2 bits per symbol • Each symbol over one twisted pair • So, 2 bit per symbol * 4 pairs = 8 bits transmitted at same time • 125 Mhz * 8 bits = 1 Gbps

  10. MAC sublayer • Gigabit Ethernet • Flow control • 1 msec delay in processing arriving data = up to 1953 frames lost in 1 msec • GigE uses a flow control frame • If busy host send PAUSE frame == type field = 0x8808 … • First 2 bytes of payload field controls the flow control command • Next bytes contain pause time in 512 nsec increments

  11. 802.11 Wireless LANs • WiFi • Very popular local area networking • Operates in two modes • Using a Base station /Access Point (Infrastructure mode) • Without an Access Point – station to station (adhoc mode)

  12. 802.11 Wireless LANs • 802.11 Physical layer/Data Link Layer • Originally (1997) three transmission media • Infrared • FHSS • DSSS

  13. 802.11 Wireless LANs • 802.11 Physical layer/Data Link Layer • Infrared • Diffused Infrared light at 0.85 or 0.95 microns • 1 or 2 Mbps • Uses Gray code encoding • For 2 bits creates 4 bit codeword with never more than one 1 bit • Cannot penetrate walls – so good cell isolation • Low bandwidth • Inference from sunlight • Not very popular

  14. 802.11 Wireless LANs • 802.11 Physical layer/Data Link Layer • FHSS (remember Hedy Lammar) • 79 1 Mhz channels • At 2.4 GHz ISM band • All stations generate pseudorandom sequence of channels to hop to • If stations use same PRN seed and stay synchronized… • Will hop to the next channel in sequence simultaneously

  15. 802.11 Wireless LANs • 802.11 Physical layer/Data Link Layer • FHSS (remember Hedy Lammar) • Dwell time adjustable • Dwell time must be < 400 msec • Pretty secure from eavesdropping • Sniffer does not know hop sequence or • dwell time • Uses same band as garage door openers, microwave ovens and cordless phones

  16. 802.11 Wireless LANs • 802.11 Physical layer/Data Link Layer • DSSS (Direct Sequence Spread Spectrum) • Data bits combined with higher data rate bit Pseudo noise sequence called “chipping code”… • Then divides data according to spreading ratio • Chipping code is a redundant bit pattern of data bits • Bit errors can be corrected • Difficult to intercept • Difficult to jam

  17. 802.11 Wireless LANs • 802.11 Physical layer/Data Link Layer • 802.11a Orthogonal Frequency Division Multiplexing (OFDM) • 54 Mbps • 5 GHz ISM band • 52 frequency channels – 48 data, synchronization • Phase shift modulation up to 18 Mbps • QAM from 18 Mbps to 54 Mbps

  18. 802.11 Wireless LANs • 802.11 Physical layer/Data Link Layer • 802.11b - High Rate Direct Sequence Spread Spectrum • 11 million chipping codes per second ~ 11 Mbps in 2.4 GHz band • Actual preceded 802.11a • 1,2, 5.5 and 11 Mbps • Slower than 802.11a but range much greater

  19. 802.11 Wireless LANs • 802.11 Physical layer/Data Link Layer • 802.11g OFDM • Enhancement to 802.11g • Approved in 2001 • Operates in the 2.4 GHz ISM band • Up to 54 Mbps

  20. 802.11 Wireless LANs • Station B tries to communicate with A… • C cannot hear B’s communication and tries to communicate B A C

  21. 802.11 Wireless LANs • D transmits to C • A wants to transmit to B, but • Hears noise, delays transmission, • unnecessarily B A C D

  22. 802.11 Wireless LANs • 802.11 can’t use CSMA/CD • Can’t use “dead air” to indicate that it is ok to transmit • Need protocol to coordinate medium access • DCF – Distributed Coordination Function • PCF – Point Coordination Function

  23. 802.11 Wireless LANs • DCF – Distributed Coordination Function • CSMA/CA - Collision Avoidance • Physical Channel Sensing • Virtual Channel Sensing

  24. 802.11 Wireless LANs • Virtual Channel Sensing • A transmits a Request To Send (RTS) to B • B responds with Clear To Send (CTS) to A • C can hear RTS so self imposes Network Allocation Vector (NAV), can’t transmit until hears ACK from B • D does not hear RTS but hears CTS from B, self imposes NAV until is hears ACK Diagram from Tanenbaum (2003), pg. 297

  25. 802.11 Wireless LANs • DCF – Distributed Coordination Function • Another problem – Wireless medium is inherently noisy and unreliable • Probability of large frame getting through without error is relatively small… • And will need retransmitted • Solution: Frame fragmenting

  26. 802.11 Wireless LANs • DCF – Distributed Coordination Function • Fragmented frames • Break large frames up into small frames • After sending RTS and receiving CTS, … • Sender sends a burst of frame fragments Diagram from Tanenbaum (2003), pg. 297

  27. 802.11 Wireless LANs • PCF – Point Coordination Function • Media Access is control from a Point (Access Point) • AP polls stations and asks if they have a frame to send • Transmission order is determined by AP

  28. 802.11 Wireless LANs • PCF – Point Coordination Function • AP broadcasts beacon frame every 0.01 to 0.1 seconds • Beacon frame contains hopping sequences, dwell times, synchronization clock, etc. • Invites stations to “log on” to polling service • Once on polling service AP guarantees designated fraction of bandwidth • Therefore, can make QOS guarantee

  29. 802.11 Wireless LANs • Can use DCF and PCF at the same time • Requires wait periods or Interframe Spacing • SIFS – Short Interframe Spacing • PIFS – PCF Interframe Spacing • DIFS – DCF Interframe Spacing • EIFS – Extended Interframe Spacing

  30. 802.11 Wireless LANs • 802.11 Interframe Spacing • After • SIFS – control frames or next fragments • PIFS – PCF frames may be sent • DIFS – DCF frames may be sent • EIFS – bad frame recovery can be started Diagram from Tanenbaum (2003), pg. 297

  31. 802.11 Wireless LANs • 802.11 Frame Structure Frame Control From: ANSI/IEEE Std. 802.11 , 1999 edition (R2003)

  32. 802.11 Wireless LANs • 802.11 Frame Structure • Frame Control • Protocol version – allow multiple versions of protocol • Type – Data, Management, Control • Subtype – RTS, CTS • To DS/From DS – going to/coming from distribution system (i.e. ethernet) • MF – more fragments coming • Retry – frame is a retry of a previous frame • Pwr – controls power of receiving station • More – more frames to come • W – Frame encrypted with WEP algorithm • O – frames must be processed in order From: ANSI/IEEE Std. 802.11 , 1999 edition (R2003)

  33. 802.11 Wireless LANs • 802.11 Frame Structure • Data frame • Duration – how long the frame and ACK will use channel • Address 1/Address 2 – Source Address/Destination Address • Address 3/Address 4 – Base station (source/destination) addresses for intercell traffic • Sequence – fragment sequence number • Data – payload max length 2312 bytes • Checksum - From: ANSI/IEEE Std. 802.11 , 1999 edition (R2003)

  34. 802.11 Wireless LANs • 802.11 Services • Distributions Services • Assocation – allows stations to connect to access point • Disassociate – breaks relationship between station and access point (leave network) • Reassociation – handoff station to another access point • Distribution – routing local air or wired network • Integration – bridging/conversion to other addressing/framing formats

  35. 802.11 Wireless LANs • 802.11 Services • Station Services • Authentication – authenicates station to access point • Deauthentication - - logs station out of network cell • Privacy – encryption/decryption • Data delivery – move data from station to station

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