ECE/CS 372 introduction to computer networks. Lecture 13

Transcription

1 ECE/CS 372 introduction to computer networks Lecture 13 Announcements: HW #4 hard copy due today Lab #5 posted is due Tuesday June 4 th HW #5 posted is due Thursday June 6 th Pickup midterms Acknowledgement: slides drawn heavily from Kurose & Ross Chapter 6, slide 1

2 Wireless and Mobile Networks Background: success of wireless: # wireless (mobile) phone subscribers now exceeds # wired phone subscribers! Internet: anytime & anywhere: laptops, PDAs, tablets, iphones, MagicJack, IP-enabled devices two important (but different) challenges wireless: communication over wireless link mobility: handling the mobile user who changes point of attachment to network Wireless and Mobile Networks, slide 2

3 Elements of a wireless network wireless hosts network infrastructure e.g.: laptop, iphone run: applications stationary or mobile wireless does not always mean mobility Wireless and Mobile Networks, slide 3

4 Elements of a wireless network network infrastructure base station bridge: typically connected to wired network relay: responsible for sending pkts between backbone network and wireless host(s) e.g., cell towers, access points Wireless and Mobile Networks, slide 4

5 Elements of a wireless network network infrastructure wireless link connects: mobiles to base station multiple access protocol: coordinates link access various data rates, transmission distance Wireless and Mobile Networks, slide 5

6 Elements of a wireless network network infrastructure infrastructure mode bridge: base station connects mobiles into wired network handoff: mobile changes base station providing connection into wired network Wireless and Mobile Networks, slide 6

7 Elements of a wireless network ad hoc mode no base stations limited range: nodes can only transmit to other nodes within link coverage multi-hop: nodes organize themselves into a network: route among themselves Wireless and Mobile Networks, slide 7

8 Wireless network taxonomy Infrastructure Infrastructure-less Single Hop 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 large large Internet Internet Multiple hops 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 Mobile Ad-Hoc Bluetooth Network WiFi Network (MANET) Network Mesh Network Wireless and Mobile Networks, slide 8

9 Outline Wireless wireless characteristics multiple access schemes TDMA/FDMA CDMA Wi-Fi wireless LANs CSMA/CA IEEE Mobility principles: addressing routing to mobile users mobile IP Wireless and Mobile Networks, slide 9

10 Wireless Link Characteristics Differences from wired link. decayed 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) multipath propagation: radio signal reflects off objects ground, arriving at destination at slightly different times. make communication across (even a point to point) wireless link much more difficult Wireless and Mobile Networks, slide 10

11 Wireless Network Characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): C A B C A B A s signal strength C s signal strength Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each other, meaning that A, C are unaware of their interference at B space Signal attenuation: B, A hear each other B, C hear each other A, C can not hear each other can interfere at B Wireless and Mobile Networks, slide 11

12 Outline Wireless wireless characteristics multiple access schemes TDMA/FDMA CDMA Wi-Fi wireless LANs CSMA/CA IEEE Mobility principles: addressing routing to mobile users mobile IP Wireless and Mobile Networks, slide 12

13 Multiple Access Schemes Q: How can multiple users share the medium? FDMA: Frequency Division Multiple Access (seen before) TDMA: Time Division Multiple Access (seen before) CDMA: Code Division Multiple Access (i.e., Cellular) (will be discussed next) CSMA/CA: Carrier-Sense Multiple Access (i.e., Wi-Fi) (will be discussed later) Wireless and Mobile Networks, slide 13

14 FDMA and TDMA (review) FDMA Example: 4 users frequency TDMA time frequency time Wireless and Mobile Networks, slide 14

15 CDMA all users use all frequency (like TDMA) all users send at all time (like FDMA) allows multiple users to coexist and transmit simultaneously a unique code assigned to each user encoding at sender: (original data) x (code) decoding at receiver: (encoded signal) x (code) Wireless and Mobile Networks, slide 15

16 CDMA Encode/Decode: one sender only Each user is assigned a unique code: c m = [ ] (length of c m is M = 8 in this example) d = (1,0) = (1,-1) sender Sent bits: d i Code: c m d 1 = d 0 = nd bit 1 st bit We use -1 to mean 0 Channel output Z i,m = d i. cm Z 1,m = d 1. c m nd bit channel output Z 0,m = d 0. c m st bit channel output Z 1,m = d 1. c m Z 0,m = d 0. c m Decoding at receiver: d i = (Z i,m. cm )/8 = (d i. cm. cm )/8 = d i receiver Received bits Code: c m d 1 = -1 decoded 2 nd bit d 0 = 1 decoded 1 st bit 2 nd bit 1 st bit Note that c m.c m = 8 Wireless and Mobile Networks, slide 16

17 CDMA: two-senders and interference Wireless and Mobile Networks, slide 17

18 CDMA: two-senders and interference M = length of c m Wireless and Mobile Networks, slide 18

19 CDMA: Example of multiple users Codeword v = (v0, v1), M=2 Data to send i.e Transmitted word = (v, -v, v, v) Two users User 1 code = (1,-1), data = (1,0,1,1) User 2 code = (1,1), data = (0,0,1,1) User 1 transmits (1,-1, -1, 1, 1, -1, 1, -1) User 2 transmits (-1, -1, -1, -1, 1, 1, 1, 1) Signal = (0, -2, -2, 0, 2, 0, 2, 0) Receiver = (1, -1) (0, -2, -2, 0, 2, 0, 2, 0) => (2, -2, 2, 2) = (1, 0, 1, 1) Receiver = (1,1) (0, -2, -2, 0, 2, 0, 2, 0) => (-2, -2, 2, 2) = (0, 0, 1, 1) Wireless and Mobile Networks, slide 19

20 Wireless networks CSMA/CD Ethernet, interference not as serious as WiFi, what to do when collisions are detected CDMA Cellular, central entity (TDMA and FDMA like) CSMA/CA WiFi 2.4ghz spectrum hard to assign codes/ frequencies to manage allocations for all WiFi devices, don t know who will be transmitting, can t detect collisions as with Ethernet so try to avoid them Generally you can expect to see CSMA like protocols anytime you don t have control over who can access the medium FDMA, TDMA and CDMA type protocols will be in place when you have central control where each device will register and request segment of medium Wireless and Mobile Networks, slide 20

21 Outline Wireless wireless characteristics multiple access schemes TDMA/FDMA CDMA Wi-Fi wireless LANs CSMA/CA IEEE Mobility principles: addressing routing to mobile users mobile IP Wireless and Mobile Networks, slide 21

22 IEEE : multiple access There are two access operating modes Infrastructure based mode Ad hoc based mode Wireless and Mobile Networks, slide 22

23 IEEE : multiple access There are two multiple access functions: Point Coordination Function (PCF) TDMA-like access Point Coordinator (PC) polls users in a roundrobin fashion No contention Synchronous Infrastructure mode Distributed Coordination function (DCF) CSMA-like access Random access: listenbefore-talk Contention-like medium Asynchronous Both infrastructure and adhoc modes Wireless and Mobile Networks, slide 23

24 IEEE multiple access Contention and contention free periods: CFP CP CFP CP Contention-Free Period (CFP) Synchronous traffic Point Coord. Fct (PCF) is the access method (optional, very few APs or adapters implement this) Contention Period (CP) Asynchronous traffic Distr. Coord. Fct (DCF) is the access method Access point alternates between CFP and CP modes Wireless and Mobile Networks, slide 24

25 IEEE DCF MAC one at a time: 2 + nodes send at same time => collision CSMA - sense before transmitting don t collide with ongoing transmission by other node no collision detection difficult to sense collision when transmitting due to weak sigl goal: avoid collisions: CSMA/C(ollision)A(voidance) use Acknowledgment mechanism to recover from collision C A B C A B A s signal strength C s signal strength space Two scenarios where collision cannot be detected Wireless and Mobile Networks, slide 25