M06:Wireless and Mobile Networks. Corinna Schmitt

Transcription

1 M06:Wireless and Mobile Networks Corinna Schmitt

2 Acknowledgement 2016 M06 2

3 Background q # wireless (mobile) phone subscribers now exceeds # wired phone subscribers (5-to-1)! q # wireless Internet-connected devices equals # wireline Internetconnected devices Laptops, Internet-enabled phones promise anytime untethered Internet access q Two important (but different) challenges Wireless: Communication over wireless link Mobility: Handling the mobile user, who changes point of attachment to network 2016 M06 3

4 Introduction 2016 M06 4

5 Elements of a Wireless Network (1) 2016 M06 5

6 Elements of a Wireless Network (2) Wireless hosts q laptop, smartphone q run applications q may be stationary (nonmobile) or mobile - wireless does not always mean mobility 2016 M06 6

7 Elements of a Wireless Network (3) Base station q typically connected to wired network q relay - responsible for sending packets between wired network and wireless host(s) in its area - e.g., cell towers, access points 2016 M06 7

8 Antenna Examples 2016 M06 8

9 Elements of a Wireless Network (4) Wireless Link q Typically used to connect mobile(s) to base station q Also used as backbone link q Multiple access protocol coordinates link access q Various data rates, transmission distance 2016 M06 9

10 Characteristics of Selected Wireless Links 2016 M06 10

11 Elements of a Wireless Network (5) Infrastructure mode q Base station connects mobiles into wired network q Handoff: mobile changes base station providing connection into wired network 2016 M06 11

12 Ad Hoc Mode (1) Ad hoc mode q No base station q Nodes can only transmit to other nodes within link coverage q Nodes organize themselves into a network: route among themselves 2016 M06 12

13 Ad Hoc Mode (2) q Example SecureWSN new 2016 M06 13

14 Wireless Network Taxonomy 2016 M06 14

15 Examples VANET MANET MESH Bluetooth M06 15

16 Excursion Frequencies and Generations Jochen Schiller: Mobile Communications 2016 M06 16

17 Frequencies for Communication (1) 2016 M06 17

18 Frequencies for Communication (1) q q q VHF-/UHF-ranges for mobile radio Simple, small antenna for cars, deterministic propagation characteristics, reliable SHF and higher for dedicated radio links, satellite communication Small antenna, focusing, large bandwidth available Wireless LANs use frequencies in UHF to SHF spectrum Some systems planned up to EHF, limitations due to absorption by water or oxygen molecules (resonance frequencies) 2016 M06 18

19 Frequencies and Regulation - Cellular q ITU-R holds auctions for new frequencies and manages frequency bands worldwide (WRC, WorldRadio Conferences) 2016 M06 19

20 BAKOM Switzerland LTE Licence Auction (1) q Under offer (2010) 61 blocks in 11 categories of 5 frequency bands Half of those are utilized currently by Swisscom, Sunrise, Orange/Salt Minimal offers between 2.7 and 21.3 Million CHF per block Frequency bands at 800 MHz and 900 MHZ are more expensive Covering largest distances 2016 M06 20

21 BAKOM Switzerland LTE Licence Auction (2) q Complexity: Swiss telecom providers hired external auctioning expertise Redundant, high-performance PCs for auction software Redundant, high-speed, low-delay, fixed Internet access links Redundant and complete decision power at company q Auction run until Feb 22, 2012 afternoon Minimal income for federal government in Bern (expected) 630 million CHF Final income generated 997 million CHF 2016 M06 21

22 q Switzerland Assignment of LTE Frequencies q Austria 2016 M06 22

23 Frequencies (and Regulation) - WLAN 2016 M06 23

24 Projections with/on Public Mobile q Relative penetration levels of mobile technology HSPA & LTE Advancements, GSMA, 2012 Technology q Theoretical peak network rates 2016 M06 24

25 LTE Addendum (1) q LTE Advanced Pro 3GPP approved Release 13 plan for early 2016 MTC enhancements Public safety features, such as D2D and ProSe Small cell dual-connectivity and architecture Carrier aggregation enhancements Interworking with Wi-Fi Licensed assisted access (at 5 GHz) 3D/FD-MIMO Indoor positioning Single cell-point to multi-point Work on latency reduction M06 25

26 LTE Addendum (2) 2016 M06 26

27 Wireless, Mobile, Public Network Generation (1) 2016 M06 27

28 Wireless, Mobile, Public Network Generation (2) 2016 M06 28

29 Development of Mobile Telecom Systems 2016 M06 29

30 Back to Lecture 2016 M06 30

31 Wireless Links, Characteristic, CDMA 2016 M06 31

32 Wireless Link Characteristic (1) q Important 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 ground, arriving ad destination at slightly different times q... make communication across (even a point to point) wireless link much more difficult 2016 M06 32

33 Wireless Link Characteristic (2) Multipath Propagation M06 33

34 Wireless Link Characteristic (3) q Signal-to-Noise Ratio (SNR) Larger SNR easier to extract signal from noise (a good thing ) q SNR versus Bit-Error-Ratio (BER) tradeoffs Given physical layer: increase power à increase SNR à decrease BER Given SNR: choose physical layer that meets BER requirement, giving highest thruput SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate) 2016 M06 34

35 Wireless Link Characteristic (4) q Multiple wireless senders and receivers create additional problems (beyond multiple access) q Hidden terminal problem B, A hear each other B, C hear each other A, C cannot hear each other means A, C unaware of their interference at B q Signal attenuation B, A hear each other B, C hear each other A, C cannot hear each other interfering at B 2016 M06 35

36 Code Division Multiple Access (CDMA) q Unique code assigned to each user; i.e., code set partitioning All users share same frequency, but each user has own chipping sequence (i.e., code) to encode data Allows multiple users to coexist and transmit simultaneously with minimal interference (if codes are orthogonal ) q Encoded signal = (original data) X (chipping sequence) q Decoding Inner-product of encoded signal and chipping sequence 2016 M06 36

37 CDMA Encode/Decode (1) sender data bits code d 1 = d 0 = slot 1 slot 0 Z i,m = d i. c m channel output Z i,m slot 1 channel output slot 0 channel output received input code receiver slot 1 slot 0 D i = Σ Z i,m. c m m=1 M M d 1 = -1 slot 1 channel output d 0 = 1 slot 0 channel output 2016 M06 37

38 CDMA: Two-sender Interference Sender 1 channel sums together transmissions by sender 1 and 2 Sender 2 using same code as sender 1, receiver recovers sender 1 s original data from summed channel data! 2016 M06 38

39 IEEE Wireless LANs ( WiFi ) 2016 M06 39

40 IEEE Wireless LAN q b GHz unlicensed spectrum Up to 11 Mbps Direct sequence spread spectrum (DSSS) in physical layer All hosts use same chipping code q a: 5-6 GHz range, up to 54 Mbps q g: GHz range, up to 54 Mbps q n: Multiple antennae, GHz range, up to 200 Mbps q All use CSMA/CA for multiple access q All have base station and ad hoc networl version 2016 M06 40

41 LAN Architecture q Wireless host communicates with base station Base station = access point (AP) q Basic Service Set (BSS) ( cell ) in infrastructure mode contains: Wireless hosts Access point (AP): Base station Ad hoc mode: hosts only 2016 M06 41

42 802.11: Channels, Association q b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies AP admin chooses frequency for AP Interference possible: channel can be same as that chosen by neighboring AP! q Host: 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 [Chapter 8] Will typically run DHCP to get IP address in AP s subnet 2016 M06 42

43 802.11: Passive/active Scanning q Passive scanning: (1) Beacon frames sent from Aps (2) Association Request frame sent: H1 to selected AP (3) Association Response frame sent from selected AP to H1 q Active scanning: (1) Probe Request frame broadcast from H1 (2) Probe Response frames sent from APs (3) Association Request frame sent: H1 to selected AP (4) Association Response frame sent from selected AP to H M06 43

44 IEEE : Multiple Access q Avoid collisions: 2+ nodes transmitting at same time q : CSMA - sense before transmitting Do not collide with ongoing transmission by other node q : no collision detection! Difficult to receive (sense collisions) when transmitting due to weak received signals (fading) Cannot sense all collisions in any case: hidden terminal, fading Goal: Avoid collisions à CSMA/C(ollision)A(voidance) 2016 M06 44

45 IEEE MAC Protocol: CSMA/CA q 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 q receiver If frame received OK Return ACK after SIFS (ACK needed due to hidden terminal problem) DIFS sender data ACK receiver SIFS 2016 M06 45

46 Avoiding Collisions q Idea: Allow sender to reserve channel rather than random access of data frames: avoid collisions of long data frames q Sender first transmits small request-to-send (RTS) packets to BS using CSMA RTSs may still collide with each other (but they re short) q BS broadcasts clear-to-send (CTS) in response to RTS q CTS heard by all nodes Sender transmits data frame Other stations defer transmissions q Avoid data frame collisions completely using small reservation packets! 2016 M06 46

47 Collision Avoidance: RTS-CTS exchange A RTS(A) RTS(A) AP reservation collision B RTS(B) CTS(A) CTS(A) DATA (A) defer time ACK(A) ACK(A) 2016 M06 47

48 Frame: Addressing (2) frame control duration address 1 address 2 address 3 seq control address 4 payload CRC 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 Address 2: MAC address of wireless host or AP transmitting this frame 2016 M06 48

49 Frame: Addressing (2) H1 R1 router Internet R1 MAC addr H1 MAC addr dest. address source address frame AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address M frame

50 IEEE MAC Management q Synchronization: Try to find a LAN, try to stay within a LAN Timer q Power management: Sleep-mode without missing a message Periodic sleep, frame buffering, traffic measurements q Association/Re-association (Roaming): Integration into a LAN Roaming, i.e. change networks by changing access points Scanning, i.e. active search for a network q MIB - Management Information Base: Managing, read, write 2016 M06 50

51 IEEE Roaming q No or bad connection? Then perform: Scanning: Scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer Re-association Request: Station sends a request to one or several AP(s) Re-association Response: Success: AP has answered, station can now participate Failure: continue scanning AP accepts Re-association Request Signal the new station to the distribution system The distribution system updates its data base (i.e., location information) Typically, the distribution system now informs the old AP so it can release resources 2016 M06 51

52 Further Reading Introduction Mobility Wireless 6.5 Principles: addressing and routing to mobile users 6.6 Mobile IP 6.7 Handling mobility in cellular networks 6.8 Mobility and higher-layer protocols 6.2 Wireless links, characteristics CDMA 6.3 IEEE wireless LANs ( Wi-Fi ) 6.4 Cellular Internet Access architecture standards (e.g., GSM, LTE) 2016 Not addressed in lecture! See book chapter 6 M06 52