Announcements: ECE/CS 372 introduction to computer networks. Assign 4 is due this Thursday Lab 4 is due next Tuesday Assignment 5 posted soon

Transcription

1 ECE/CS 372 introduction to computer networks Lecture 14 Announcements: Assign 4 is due this Thursday Lab 4 is due next Tuesday Assignment 5 posted soon Credit for lecture slides to Professor Bechir Hamdaoui Adapted from Jim Kurose & Keith Ross (original copyright) 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 Chapter 6, 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 Chapter 6, 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 Chapter 6, 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 Chapter 6, 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 Chapter 6, 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 Chapter 6, 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 Chapter 6, 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 Chapter 6, 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 Chapter 6, 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 Chapter 6, 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 Chapter 6, 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 (will be discussed next) CSMA/CA: Carrier-Sense Multiple Access (i.e., Wi-Fi) (will be discussed later) Chapter 6, slide 13

14 FDMA and TDMA (review) FDMA Example: 4 users frequency TDMA time frequency time Chapter 6, 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) Chapter 6, 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) Channel output Z i,m = d i. cm We use to mean 0 sender Sent bits: d i Code: c m d 0 = 1 d 1 = nd bit 1 st bit 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 = decoded 2 nd bit d 0 = 1 decoded 1 st bit 2 nd bit 1 st bit Note that c m.c m = 8 Chapter 6, slide 16

17 CDMA: two-senders and interference Chapter 6, slide 17

18 CDMA: two-senders and interference M = length of c m Chapter 6, slide 18

19 CDMA: Example of multiple users See board notes for a CDMA example. Chapter 6, slide 19