1. Final Exam Wednesday 3/18/2015 Tech LR4 3- 5 PM 1

2. Help out Marcel! (because he's been so helpful to all of you) 1) Go to http://networks.cs.northwestern.edu/softid/ http://networks.cs.northwestern.edu/softid/ 3) Install the SoftID add-on to your browser (Chrome and Firefox supported) for FREE 4) Feel good about yourself. NOTE: Chrome version release may be delayed by a day (dealing with the Chrome marketplace) NOTE: This add-on is to support research on user recommendations. Details are available on link (above)

3. 3 Link Layer r 5.4 Link-Layer Addressing r 5.5 Ethernet r 5.6 Hubs and switches

4. 4 Hubs … physical-layer (“dumb”) repeaters: m bits coming in one link go out all other links at same rate m all nodes connected to hub can collide with one another m no frame buffering m no CSMA/CD at hub: host NICs detect collisions twisted pair hub

5. 5 Switch r link-layer device: smarter than hubs, take active role m store, forward Ethernet frames m examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment r transparent m hosts are unaware of presence of switches r plug-and-play, self-learning m switches do not need to be configured

6. 6 Switch: allows multiple simultaneous transmissions r hosts have dedicated, direct connection to switch r switches buffer packets r Full duplex r switching: A-to-A’ and B- to-B’ simultaneously, without collisions m not possible with dumb hub A A’ B B’ C C’ switch with six interfaces (1,2,3,4,5,6) 1 2 3 4 5 6

7. 7 Switch Table r Q: how does switch know that A’ reachable via interface 4, B’ reachable via interface 5? r A: each switch has a switch table, each entry: m (MAC address of host, interface to reach host, time stamp) r looks like a routing table! r Q: how are entries created, maintained in switch table? m something like a routing protocol? A A’ B B’ C C’ switch with six interfaces (1,2,3,4,5,6) 1 2 3 4 5 6

8. 8 Switch: self-learning r switch learns which hosts can be reached through which interfaces m when frame received, switch “learns” location of sender: incoming LAN segment m records sender/location pair in switch table A A’ B B’ C C’ 1 2 3 4 5 6 A A’ Source: A Dest: A’ MAC addr interface TTL Switch table (initially empty) A 1 60

9. 9 Switch: frame filtering/forwarding When frame received: 1. record link associated with sending host 2. index switch table using MAC dest address 3. if entry found for destination then { if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived

10. 10 Self-learning, forwarding: example A A’ B B’ C C’ 1 2 3 4 5 6 A A’ Source: A Dest: A’ MAC addr interface TTL Switch table (initially empty) A 1 60 A A’ r frame destination unknown: flood A’ A  destination A location known: A’ 4 60 selective send

11. 11 Interconnecting switches r switches can be connected together A B  Q: sending from A to G - how does S 1 know to forward frame destined to G via S 4 and S 3 ?  A: self learning! (works exactly the same as in single-switch case!) S1S1 C D E F S2S2 S4S4 S3S3 H I G

12. 12 Self-learning multi-switch example Suppose C sends frame to I, I responds to C  Q: show switch tables and packet forwarding in S 1, S 2, S 3, S 4 A B S1S1 C D E F S2S2 S4S4 S3S3 H I G 1 2

13. 13 Institutional network to external network router IP subnet mail server web server

14. 14 Switches vs. Routers r both store-and- forward devices m routers: network-layer devices (examine network-layer headers) m switches are link-layer devices (examine link- layer headers) r routers maintain routing tables, implement routing algorithms r switches maintain switch tables, implement filtering, learning algorithms application transport network link physical network link physical link physical switch datagram application transport network link physical frame datagram

15. 15 Summary comparison

16. 16 Chapter 6: Wireless and Mobile Networks Background: r # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! r computer nets: laptops, smartphones, Internet- enabled phone promise anytime untethered Internet access r two important (but different) challenges m communication over wireless link m handling mobile user who changes point of attachment to network

17. 17 Chapter 6 outline 6.1 Introduction Wireless r 6.2 Wireless links, characteristics m CDMA r 6.3 IEEE 802.11 wireless LANs (“wi-fi”)

18. 18 Elements of a wireless network network infrastructure wireless hosts r laptop, smart-phone r run applications r may be stationary (non- mobile) or mobile m wireless does not always mean mobility

19. 19 Elements of a wireless network network infrastructure base station r typically connected to wired network r relay - responsible for sending packets between wired network and wireless host(s) in its “area” m e.g., cell towers or 802.11 access points

20. 20 Elements of a wireless network network infrastructure wireless link r typically used to connect mobile(s) to base station r multiple access protocol coordinates link access r various data rates, transmission distance

21. 21 Elements of a wireless network network infrastructure infrastructure mode r base station connects mobiles into wired network r handoff: mobile changes base station providing connection into wired network

22. 22 Elements of a wireless network Ad hoc mode r no base stations r nodes can only transmit to other nodes within link coverage r nodes organize themselves into a network: route among themselves

23. 23 Wireless Link Characteristics Differences from wired link …. m decreased signal strength: radio signal attenuates as it propagates through matter (path loss) m interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); m multipath propagation: radio signal reflects off objects ground, arriving ad destination at slightly different times …. make communication across (even a point to point) wireless link much more “difficult”

24. 24 Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): A B C Hidden terminal problem r B, A hear each other r B, C hear each other r A, C can not hear each other means A, C unaware of their interference at B A B C A’s signal strength space C’s signal strength Signal fading: r B, A hear each other r B, C hear each other r A, C can not hear each other interferring at B

25. 25 Code Division Multiple Access (CDMA) r used in several wireless broadcast channels (cellular, satellite, etc) standards r unique “code” assigned to each user; i.e., code set partitioning r all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data r encoded signal = (original data) X (chipping sequence) r decoding: inner-product of encoded signal and chipping sequence r allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

26. 26 CDMA Encode/Decode slot 1 slot 0 d 1 = -1 111 1 1 - 1 - 1 -1 - Z i,m = d i. c m d 0 = 1 111 1 1 - 1 - 1 - 1 - 111 1 1 - 1 - 1 -1 - 111 1 1 - 1 - 1 -1 - slot 0 channel output slot 1 channel output channel output Z i,m sender code data bits slot 1 slot 0 d 1 = -1 d 0 = 1 111 1 1 - 1 - 1 -1 - 111 1 1 - 1 - 1 - 1 - 111 1 1 - 1 - 1 -1 - 111 1 1 - 1 - 1 -1 - slot 0 channel output slot 1 channel output receiver code received input D i =  Z i,m. c m m=1 M M

27. 27 CDMA: two-sender interference

28. 28 Chapter 6 outline 6.1 Introduction Wireless r 6.2 Wireless links, characteristics m CDMA r 6.3 IEEE 802.11 wireless LANs (“wi-fi”)

29. 29 IEEE 802.11 Wireless LAN r 802.11b m 2.4-5 GHz unlicensed radio spectrum m up to 11 Mbps m widely deployed, using base stations r 802.11a m 5-6 GHz range m up to 54 Mbps r 802.11g m 2.4-5 GHz range m up to 54 Mbps r All use CSMA/CA for multiple access r All have base-station and ad-hoc network versions

30. 30 802.11 LAN architecture r wireless host communicates with base station m base station = access point (AP) r Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains: m wireless hosts m access point (AP): base station m ad hoc mode: hosts only BSS 1 BSS 2 Internet hub, switch or router AP

31. 31 802.11: Channels, association r 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies m AP admin chooses frequency for AP m interference possible: channel can be same as that chosen by neighboring AP! r host: must associate with an AP m scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address m selects AP to associate with m may perform authentication [Chapter 8] m will typically run DHCP to get IP address in AP’s subnet

32. 32 IEEE 802.11: multiple access r avoid collisions: 2 + nodes transmitting at same time r 802.11: CSMA - sense before transmitting m don’t collide with ongoing transmission by other node r 802.11: no collision detection! m difficult to receive (sense collisions) when transmitting due to weak received signals (fading) m can’t sense all collisions in any case: hidden terminal, fading m goal: avoid collisions: CSMA/C(ollision)A(voidance) A B C A B C A’s signal strength space C’s signal strength

33. 33 IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if sense channel idle for DIFS then transmit entire frame (no CD) 2 if sense channel busy then start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, repeat 2 802.11 receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) sender receiver DIFS data SIFS ACK

34. 34 Avoiding collisions (more) idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames r sender first transmits small request-to-send (RTS) packets to BS using CSMA m RTSs may still collide with each other (but they’re short) r BS broadcasts clear-to-send CTS in response to RTS r RTS heard by all nodes m sender transmits data frame m other stations defer transmissions Avoid data frame collisions completely using small reservation packets!

35. 35 Collision Avoidance: RTS-CTS exchange AP A B time RTS(A) RTS(B) RTS(A) CTS(A) DATA (A) ACK(A) reservation collision defer

36. 36 frame control duration address 1 address 2 address 4 address 3 payloadCRC 226662 6 0 - 2312 4 seq control 802.11 frame: addressing Address 2: MAC address of wireless host or AP transmitting this frame Address 1: MAC address of wireless host or AP to receive this frame Address 3: MAC address of router interface to which AP is attached Address 4: used only in ad hoc mode

37. 37 Internet router AP H1 R1 AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3 802.11 frame R1 MAC addr H1 MAC addr dest. address source address 802.3 frame 802.11 frame: addressing

38. 38 hub or switch AP 2 AP 1 H1 BBS 2 BBS 1 802.11: mobility within same subnet router r H1 remains in same IP subnet: IP address can remain same r switch: which AP is associated with H1? m self-learning (Ch. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H1