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CS 453 Computer Networks

CS 453 Computer Networks

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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