1. CS 453 Computer 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 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 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 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 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 802.3z goal to be 10x faster than 802.3u (Fast Ethernet) Backward compatible with other Ethernet standards

4. MAC sublayer Gigabit Ethernet Goals 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 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 Modes – full-duplexNormal 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… 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 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 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 = Max cable length = [10base2MaxCableLength]/100 = 2500/100 = 25 meters 2500/100 = 25 meters That won’t do!

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

9. MAC sublayer Gigabit Ethernet 1000BaseT Encoding 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 Flow control 1 msec delay in processing arriving data = up to 1953 frames lost in 1 msec 1 msec delay in processing arriving data = up to 1953 frames lost in 1 msec GigE uses a flow control frame 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 AB C

21. 802.11 Wireless LANs D transmits to C A wants to transmit to B, but Hears noise, delays transmission, unnecessarily AB 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 From: ANSI/IEEE Std. 802.11, 1999 edition (R2003) Frame Control

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