1. CS 372 – introduction to computer networks* Announcements: r Final exam on Friday  The materials after chapters 1,2  Emphasis on the material covered after Midterm  50 minutes exam in this class r Verify your grades Acknowledgement: slides drawn heavily from Kurose & Ross * Based in part on slides by Bechir Hamdaoui and Paul D. Paulson.

2. Chapter 6 outline  Introduction  Wireless links, characteristics  CDMA r IEEE 802.11 wireless LANs (“wi-fi”) r Mobility Chapter 6, slide 2

3. Wireless and Mobile Networks Background: r computer nets: laptops, palmtops, PDAs, Internet-enabled phone  promise anytime-anywhere mobile Internet access r two important (but different) challenges  wireless: communication over wireless link  mobility: handling the mobile user who changes point of attachment to network Chapter 6, slide 3

4. Elements of a wireless network network infrastructure wireless hosts r laptop, PDA, IP phone r run applications r may be stationary (non-mobile) or mobile  wireless does not always mean mobility Chapter 6, slide 4

5. 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”  e.g., cell towers, 802.11 access points Chapter 6, slide 5

6. Elements of a wireless network network infrastructure wireless link r typically used to connect mobile(s) to base station r may also be used as backbone link r link access coordinated by multiple access protocol r various data rates, transmission distance, etc. Chapter 6, slide 6

7. 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 Chapter 6, slide 7

8. 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 r e.g. ZigBee Chapter 6, slide 8

9. 9 Wireless network taxonomy Infrastructure Infrastructure-less Single Hop Multiple hops 1. hosts connect to base station 2. base station connects to larger Internet E.g.: WiFi/Cellular 1. no base station 2. no connection to larger Internet E.g.: Bluetooth 1. hosts may have to relay via multiple nodes (multi-hop) 2. connects to larger Internet E.g.: mesh network 1. no base station 2. no connection to larger Internet 3. may have to relay via others to reach a given node E.g.: MANET large Internet WiFi Network Mesh Network large Internet Bluetooth Network Mobile Ad-Hoc Network (MANET)

10. Wireless Link Characteristics Differences from wired link ….  decreased signal strength: radio signal attenuates as it propagates through matter (path loss)  interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well  multipath propagation: radio signal reflects off objects, arriving at destination at slightly different times …. make communication across wireless link (even a point to point) much more “difficult” Chapter 6, slide 10

11. Chapter 6, slide 11 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 attenuation: r B, A hear each other r B, C hear each other r A, C can not hear each other interfering at B

12. IEEE 802.11: multiple access r avoid collisions: 2 + nodes transmitting at same time r 802.11: CSMA - sense before transmitting  don’t collide with ongoing transmission by other node r 802.11: no collision detection!  difficult to sense collisions when transmitting due to weak received signals (fading)  can’t sense all collisions in any case: hidden terminal, fading  goal: avoid collisions: CSMA/CA (Collision Avoidance) A B C A B C A’s signal strength space C’s signal strength Two scenarios where collision cannot be detected Chapter 6, slide 12

13. Wireless traffic control r Wireless uses collision avoidance (CA) rather than collision detection (CD)  Transmitting computer sends very short "reservation" message to receiver  Destination responds with short message reserving slot for sender r Response from destination is broadcast so all potential senders are notified of the reservation Chapter 6, slide 13

14. Avoiding collisions 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 base station using CSMA  RTSs may still collide with each other (but they’re short) r Base station broadcasts clear-to-send CTS in response to RTS r CTS heard by all nodes  sender transmits data frame  other stations defer transmissions Avoid data frame collisions completely using small reservation packets Chapter 6, slide 14

15. Wireless CSMA/CA r Any station may receive simultaneous requests  Results in collision at receiver  Both requests are lost  Neither sender gets a reservation  Both senders use random backoff and retry r Any station may receive closely spaced requests  Selects one  Sends message to selected sender  Sender that is not selected uses backoff/retry Chapter 6, slide 15

16. Collision Avoidance: RTS-CTS exchange AP A B time RTS(A) RTS(B) RTS(A) CTS(A) DATA (A) ACK(A) reservation collision defer Chapter 6, slide 16

17. IEEE 802.11 Wireless LAN r 802.11b  2.4-5 GHz  Rate: up to 11 Mbps r 802.11a  5-6 GHz range  up to 54 Mbps r 802.11g  2.4-5 GHz range  up to 54 Mbps r 802.11n: multiple antennae  2.4-5 GHz range  up to 200 Mbps r all use CSMA/CA for multiple access r all have base-station and ad-hoc network versions Chapter 6, slide 17

18. 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 Chapter 6, slide 18

19. Chapter 6, slide 19 802.11: Channels, association r 802.11b/g LAN: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies  AP admin chooses frequency/channel for AP  interference possible: same channel can be chosen by two neighboring APs r host/laptop: must associate with an AP  scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address  selects AP to associate with  may perform authentication  typically runs DHCP to get IP address in AP’s subnet

20. 802.11: passive/active scanning AP 2 AP 1 H1 BBS 2 BBS 1 1 2 2 3 4 Active Scanning: (1) Probe Request frame broadcast from H1 (2) Probes response frame sent from APs (3) Association Request frame sent: H1 to selected AP (4) Association Response frame sent: H1 to selected AP AP 2 AP 1 H1 BBS 2 BBS 1 1 2 3 1 Passive Scanning: (1) beacon frames sent from APs (2) association Request frame sent: H1 to selected AP (3) association Response frame sent: H1 to selected AP Chapter 6, slide 20

21. ZigBee r wireless (ad-hoc) mesh networking standard  mesh networking provides high reliability and larger range. r based on the IEEE 802.15.4 standard for wireless personal area networks  low cost allows the technology to be widely deployed in wireless control and monitoring applications  low power-usage allows longer life with smaller batteries Chapter 6, slide 21

22. What is mobility? r spectrum of mobility, from the network perspective: no mobility high mobility mobile wireless user, using same access point mobile user, passing through multiple access points while maintaining ongoing connections ( like cell phone) mobile user, connecting/ disconnecting from network using DHCP. Chapter 6, slide 22

23. 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?  self-learning (Ch. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H1 Chapter 6, slide 23

24. 24 Mobility terminology home network: permanent “home” of mobile (e.g., 128.119.40/24) Permanent address: address in home network, can always be used to reach mobile e.g., 128.119.40.186 home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote wide area network correspondent correspondent: wants to communicate with mobile

25. 25 Mobility terminology Care-of-address: address in visited network. (e.g., 79,129.13.2) wide area network visited (or foreign) network: network in which mobile currently resides (e.g., 79.129.13/24) Permanent address: remains constant (e.g., 128.119.40.186) foreign agent: entity in visited network that performs mobility functions on behalf of mobile.

26. 26 Mobile Addressing r search all phone books? r call her parents? r expect her to let you know where he/she is? I wonder where Alice moved to? Consider friend frequently changing addresses, how do you find her?

27. 27 Routing approaches r Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange.  routing tables indicate where each mobile located  no changes to end-systems

28. 28 Routing approaches r Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange.  routing tables indicate where each mobile located  no changes to end-systems r Let end-systems handle it:  indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote  direct routing: correspondent gets foreign address of mobile, sends directly to mobile not scalable to millions of mobiles

29. Mobility via Indirect Routing wide area network home network visited network 3 2 4 1 correspondent addresses packets using home address of mobile home agent intercepts packets, forwards to foreign agent foreign agent receives packets, forwards to mobile mobile replies directly to correspondent Chapter 6, slide 29

30. Mobility via Direct Routing wide area network home network visited network 4 2 4 1 correspondent requests, receives foreign address of mobile correspondent forwards to foreign agent foreign agent receives packets, forwards to mobile mobile replies directly to correspondent 3 Chapter 6, slide 30

31. Wireless, mobility: impact on higher layer protocols r logically, impact should be minimal …  best effort service model remains unchanged  TCP and UDP currently run over wireless, mobile r … but performance-wise:  packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoff  TCP interprets loss as congestion, will decrease congestion window un-necessarily  limited bandwidth of wireless links r Many unsolved problems  Research areas ! Chapter 6, slide 31