IT 325 Introduction to Mobile and Wireless Networking

IT 325 Introduction to Mobile and Wireless Networking Week 2: Fundamentals of Wireless Networking & Wireless LANs Elma Avdic Spring 2025  - Trends & Applications Fundamentals of - What is a wireless network? - Wireless vs wired, vs...

IT 325 Introduction to Mobile and Wireless Networking Week 2: Fundamentals of Wireless Networking & Wireless LANs Elma Avdic Spring 2025  - Trends & Applications Fundamentals of - What is a wireless network? - Wireless vs wired, vs Wireless Networks mobile - Types of wireless networks - Link characteristics Lecture 1 - Hidden terminal problem - CDMA Agenda - Understand basics of wireless communication - Differentiate between wireless and mobile networks - Identify key elements of wireless network - Explore types of wireless networks and their architectures Introduction to Wireless Networks - Wireless: transmission of data over el-mag waves w/o physical cables - Wireless networks: networks where devices communicate over wireless links - Key elements: - Wireless hosts (e.g., smartphones, laptops) - Wireless links (e.g., WiFi, cellular) - Base stations (e.g., cell towers, access points) - Network infrastructure (e.g., Internet) - E.g. WiFi, Bluetooth, cellular - Important: mobility, scalability, ease of deployment Wireless dominates connectivity - Health and fitness - Virtual reality - UAVs - Internet of Things - Sensors Demand driver: billions of wireless devices Elements of a wireless network wired network infrastructure Wireless and Mobile Networks: 7- 7 Elements of a wireless network wireless hosts ▪ laptop, smartphone, IoT ▪ run applications ▪ may be stationary (non-mobile) or mobile wired network wireless does not always mean infrastructure mobility! Wireless and Mobile Networks: 7- 8 Elements of a wireless network base station ▪ typically connected to wired network ▪ relay - responsible for sending packets between wired network and wireless wired network host(s) in its “area” infrastructure e.g., cell towers, 802.11 access points Wireless and Mobile Networks: 7- 9 Elements of a wireless network wireless link ▪ typically used to connect mobile(s) to base station, also used as backbone link ▪ multiple access protocol coordinates link access wired network ▪ various transmission rates and distances, infrastructure frequency bands Wireless and Mobile Networks: 7- 10 Characteristics of selected wireless links 14 Gbps 802.11ax 10 Gbps 5G 3.5 Gbps 802.11ac 802.11 af,ah 600 Mbps 802.11n 4G LTE 54 Mbps 802.11g 11 Mbps 802.11b 2 Mbps Bluetooth Indoor Outdoor Midrange Long range outdoor outdoor 10-30m 50-200m 200m-4Km 4Km-15Km Elements of a wireless network infrastructure mode ▪ base station connects mobiles into wired network ▪ handoff: mobile changes base station wired network providing connection into wired infrastructure network Wireless and Mobile Networks: 7- 12 Elements of a wireless network ad hoc mode ▪ no base stations ▪ nodes can only transmit to other nodes within link coverage ▪ nodes organize themselves into a network: route among themselves Wireless and Mobile Networks: 7- 13  Applications Vehicles Emergencies Transmitting news, weather, road condition Early transmission of patient data to the Personal comms using GSM hospital, current status, first diagnosis Position via GPS Replacement of a fixed infrastructure in Local ad-hoc network with vehicles case of earthquakes, hurricanes, etc. close-by to prevent accidents, Crisis, war,... guidance system, redundancy Vehicle data (from buses, high speed Entertainment, education,... trains) can be transmitted in advance Outdoor Internet access for maintenance Intelligent travel guide with up-to-date location dependent information Ad-hoc networks for multi user games Devices Higher percentage of mobile devices connecting to the Internet are embedded devices Standardization: IEEE, 3gpp.org, ietf.org, ITU Challenges Power consumption ○ Limited computing power, low quality displays, small disks due to limited battery capacity ○ CPU power consumption CV2f C: internal capacity, reduced by integration V: supply voltage, can be reduced to a certain limit F: clock frequency, can be reduced temporarily Loss of data ○ Higher probability, has to be included i advance into the design; Limited user interfaces ○ Tradeoff between size of fingers and portability Limited fast memory ○ Limited value of mass memories with moving parts ○ Flash memory or ? as alternative Wireless vs Mobile Networks - Wireless Network: focus on untethered connectivity WiFi, Bluetooth, satellite) - Mobile network: involves device movement and handovers (cellular networks, 4G/5G - Overlap: most mobile networks are also wireless Differences from wired networks… - Mobility: hosts can move - Signal propagation: affected by distance, obstacles - Higher error rates Types of Wireless Networks Wireless Properties - Interference / bit errors - More sources of corruption vs. wired → BER (bit error rate) higher in wireless & other devices using the same frequency (interference) - Path Loss - Signal weakens over distance (attenuates) - Multipath propagation - Signal does not travel in a straight line → takes different paths due to reflections - Often a broadcast medium - All traffic to everyone nearby - Power trade-offs - Important for mobile, battery-powered devices  Wireless Links and Network Characteristics Decreasing signal strength: radio signal attenuates as it propagates through matter (path loss) Interference from other sources: wireless network frequencies (e.g., 2.4 GHz) shared by many devices WiFi, cellular): interference 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 Fading (attenuation) - Wireless radio signal attenuates (loses power) as it propagates (free space “path lossˮ) Multipath Radio signal reflects off objects ground, built environment, arriving at destination at slightly different times Wireless link characteristics (2) SNR: signal-to-noise ratio ○ Larger SNR - easier to extract signal from noise (a good thing) SNR vs BER tradeoffs ○ Given physical layer: increase power → increase SNR → decrease BER ○ Given SNR: choose physical layer that meets BER requirement, giving highest throughput SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate) Summary: Wireless Link Characteristics - Challenges: - Signal attenuation: strength decreases with distance - Interference: from other devices (e.g. microwaves) - Multipath propagation: reflections cause signal overlap - Metrics: - Signal-to-Noise Ratio SNR signal strength vs. noise - Bit Error Rate BER error probability - Relationship: higher SNR → lower BER Wireless Transmission Techniques - Frequency bands: ISM bands 2.4GHz, 5GHz), cellular 800 MHz - 5GHz) - Modulation: BPSK, QAM, OFDM - Access Methods: FDMA, TDMA, CDMA Elements of a Wireless Network Hidden Terminal Problem - Scenario: - Two senders A, C) cannot hear each other - Both transmit to receiver B, causing interference - Example: two devices on opposite sides of a base station - Impact: collisions reduce performance - Solution: protocols like RTS/CTS (next lecture) Wireless link characteristics (3) Multiple wireless senders, receivers create additional problems (beyond multiple access): Code Division Multiple Access (CDMA) - Overview - Channel partitioning protocol - Eahc user has a unique code - All share the same frequency - Benefits: - Multiple users transmit simultaneously - Robust to interference CDMA Encoding and Decoding - Process: - Sender: Data bit (d_i) x Code sequence (c_m) - Receiver: decodes using senderʼs code - Example: - Code: 1,1,1,1,1,1,1,1, d_i = 1 - Encoded: 1,1,1,1,1,1,1,1 - Decode: average recovers d_i  CDMA in practice - Requirements: - Orthogonal codes - Similar signal strengths - Applications: - Cellular networks - Some WiFi standards  - IEEE 802.11 Wireless LANs - WiFi Architecture - Channels and Association Wireless LANs - MAC protocol: CSMA/CA - Frame structure - Mobility in WiFi Lecture 2 - Advanced features in WiFi - PANs: Bluetooth Agenda - Understand the IEEE 802.11 WiFi standard - Learn about CSMA/CA and MAC mechanisms - Explore frame formats and address fields in 802.11 - Discuss mobility management in WiFi networks Intro to IEEE 802.11 - Standardization: defined by IEEE for WLANs - Common variants: 802.11b/g/n/ac/ax WiFi 4, 5, 6 - Evolution: higher data rates 11 Mbps → 14Gbps) - Frequencies used: 2.4GHz & 5GHz - Multiple access: CSMA/CA - Use cases: home, enterprise, campus networks Characteristics of Wireless Links IEEE 802.11 Wireless LAN IEEE 802.11 Year Max data rate Range Frequency standard 802.11b 1999 11 Mbps 30 m 2.4 Ghz 802.11g 2003 54 Mbps 30m 2.4 Ghz 802.11n (WiFi 4) 2009 600 70m 2.4, 5 Ghz 802.11ac (WiFi 5) 2013 3.47Gpbs 70m 5 Ghz 802.11ax (WiFi 6) 2020 (exp.) 14 Gbps 70m 2.4, 5 Ghz 802.11af 2014 35 – 560 Mbps 1 Km unused TV bands (54-790 MHz) Wireless and Mobile 802.11ah 2017 347Mbps 1 KmNetworks: 7-900 37 Mhz ▪ all use CSMA/CA for multiple access, and have base-station and ad-hoc network versions 802.11 Network Architecture - Basic Service Set BSS Set of stations with an access point AP - AP connects to wired network - Stations associate with one AP - Extended Service Set ESS Multiple BSSs interconnected - Ad-Hoc Mode: Peer-to-peer communication with AP 802.11 LAN architecture ▪ wireless host communicates with base station Internet base station = access point AP) ▪ Basic Service Set BSS (aka “cellˮ) in infrastructure mode switch or router contains: wireless hosts BSS 1 access point AP base station ad hoc mode: hosts only BSS 2 Channels and Association - Channels: 11 in 2.4 Ghz (e.g., 1, 6, 11 non-overlapping) - Process: - Scan for APs (passive/active) - Select AP (e.g., strongest signal) - Authenticate and associate 802.11: Channels ▪ spectrum divided into channels at different frequencies AP admin chooses frequency for AP interference possible: channel can be same as that chosen by neighboring AP! Example: 2.4 GHz Class 22: 41 802.11: Association ▪ arriving host: must associate with an AP scans channels, listening for beacon frames containing APʼs name SSID) and MAC address selects AP to associate with then may perform authentication Chapter 8 BSS then typically run DHCP to get IP address in APʼs subnet 802.11: passive/active scanning BBS 1 BBS 2 BBS 1 BBS 2 1 1 1 AP 2 2 2 AP 2 AP 1 AP 1 2 3 4 3 H1 H1 passive scanning: active scanning: (1) beacon frames sent from APs (1) Probe Request frame broadcast from H1 (2) association Request frame sent: H1 (2) Probe Response frames sent from APs to selected AP (3) Association Request frame sent: H1 to (3) association Response frame sent selected AP from selected AP to H1 (4) Association Response frame sent from selected AP to H1 IEEE 802.11: multiple access ▪ avoid collisions: 2+ nodes transmitting at same time ▪ 802.11 CSMA - sense before transmitting donʼt collide with detected ongoing transmission by another node ▪ 802.11 no collision detection! difficult to sense collisions: high transmitting signal, weak received signal due to fading canʼt sense all collisions in any case: hidden terminal, fading goal: avoid collisions: CSMA/CollisionAvoidance C A B C A’s C’s signal signal strength B strength A space Medium Access Control (MAC) in 802.11 - Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) - Why avoidance instead of detection? - Wireless nodes cannot listen while transmitting - Hidden and exposed terminal problems - CSMA/CA mechanism: 1. Sense the channel before transmitting (if idle, transmit after DIFS 2. If busy, wait for a random backoff time 3. Transmit if still idle; if collision occurs, retry IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender sender receiver 1 if sense channel idle for DIFS then transmit entire frame (no CD) DIFS 2 if sense channel busy then start random backoff time timer counts down while channel idle data transmit when timer expires if no ACK, increase random backoff interval, repeat 2 SIFS AC K Avoiding collisions (more) idea: sender “reservesˮ channel use for data frames using small reservation packets ▪ sender first transmits small request-to-send RTS) packet to BS using CSMA RTSs may still collide with each other (but theyʼre short) ▪ BS broadcasts clear-to-send CTS in response to RTS ▪ CTS heard by all nodes sender transmits data frame other stations defer transmissions RTS/CTS Mechanism for Collision Avoidance - Purpose: reduce hidden terminal problem - RTS/CTS: reserves channel → reduces hidden terminal issues - Steps: 1. Sender sends a Request-to-Sent RTS) frame 2. Receiver responds with a Clear-to-Send CTS) frame 3. Sender transmits data; receiver ACKs (confirms frame receipt) - Tradeoff: useful for long frames but adds overhead Collision Avoidance: RTS-CTS exchange A B AP RTS(A) RTS(B ) reservation collision RTS(A) CTS(A) CTS(A) time DATA (A) Wireless and Mobile Networks: 7- 49 defer ACK(A) ACK(A) 802.11 Frame Format - Frame control: type, subtype, control flags - Duration: time reservation for transmission - Address fields: 4 MAC address fields for complex routing - Sequence control: helps with ordering and retransmissions - Frame body: contains data payload - FCs: ensures integrity with error checking 802.11 frame: addressing 2 2 6 6 6 2 6 0 - 2312 4 frame durati address address address seq address control on 1 2 3 control 4 payload CRC Address 1: MAC address Address 4: used only in of wireless host or AP to ad hoc mode receive this frame Address 2: MAC address Address 3: MAC address of of wireless host or AP router interface to which AP transmitting this frame is attached 802.11 frame: addressing Internet H1 R 1 802.3 Ethernet frame R1 MAC addr H2 MAC addr MAC dest addr MAC source addr AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address 3 802.11 WiFi frame 802.11 frame: addressing duration of reserved frame sequence # (for reliable data transmission time (RTS/CTS) transfer) 2 2 6 6 6 2 6 0 - 2312 4 frame durati address address address seq address control on 1 2 3 control 4 payload CRC 2 2 4 1 1 1 1 1 1 1 1 protocol to from more Wireless powerand more Mobile type subtype retry Networks: mgt 7- 53 WEP rsvd version AP AP frag data frame type (RTS, CTS, ACK, data) Mobility in 802.11 Networks - Handoff process: when a device moves between APs - How? Within same subnet: - Move between APs - Keep IP address - Switch learns new AP via self-learning - Handoff types: - Hard handoff: connection breaks before switching - Soft handoff: overlapping connections for smoother transition - Challenges: latency, packet loss, security considerations 802.11: mobility within same subnet ▪ H1 remains in same IP subnet: IP address can remain same ▪ switch: which AP is associated with H1? self-learning (Ch. 6): switch will see frame from H1 and “remember” which switch port can be used to reach H1 Wireless and Mobile Networks: 7- 55 H1 BBS 2 BBS 1 Advanced features in 802.11 - Rate adaptation: - Adjusts rate based on SNR - Power management: - sleep/wake cycles save energy 802.11: advanced capabilities Rate adaptation ▪ base station, mobile dynamically 10-1 change transmission rate (physical 10-2 layer modulation technique) as 10-3 BER mobile moves, SNR varies 10-4 10-5 10-6 1. SNR decreases, BER increase as node 10-7 10 20 30 40 moves away from base station SNR(dB) 2. When BER becomes too high, switch to QAM256 (8 Mbps) QAM16 (4 Mbps) lower transmission rate but with lower BER BPSK (1 Mbps) operating point 802.11: advanced capabilities power management ▪ node-to-AP “I am going to sleep until next beacon frameˮ AP knows not to transmit frames to this node node wakes up before next beacon frame ▪ beacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sent node will stay awake if AP-to-mobile frames to be sent; otherwise sleep again until next beacon frame Personal Area Networks: Bluetooth - Overview: - Short-range 10m, low-power - Ad hoc, no infrastructure - Piconet: - Master + up to 7 clients - Frequency-hopping spread-spectrum FHSS Personal area networks: Bluetooth ▪ less than 10 m diameter ▪ replacement for cables (mouse, C P keyboard, headphones) P M radius of coverage ▪ ad hoc: no infrastructure C P C ▪ 2.4-2.5 GHz ISM radio band, up to 3 P Mbps ▪ master controller / client devices: M master controller client device master polls clients, grants requests for C client transmissions P parked device (inactive) Personal area networks: Bluetooth ▪ TDM, 625 μsec sec. slot ▪ FDM: sender uses 79 frequency C P channels in known, pseudo-random P radius of M order slot-to-slot (spread spectrum) coverage other devices/equipment not in piconet only C C P P interfere in some slots ▪ parked mode: clients can “go to sleep” (park) and later wakeup (to preserve M master controller battery) client device C ▪ bootstrapping: nodes self-assemble P parked device (plug and play) into piconet (inactive)  Pandemic + Bluetooth Class 23: 62 Source: Research Areas Wireless communication ○ Transmission quality (bandwidth, error rate, delay) ○ Modulation, coding, interference ○ Medium access, regulations Mobility ○ Location dependent services ○ Location transparency ○ Quality of service support (delay, jitter, security) Portability ○ Power consumption ○ Limited computing power, sizes of display ○ Usability Security ○ Privacy ○ Data integrity ○ Tacking ○ Encryption ○ Law enforcement Mobile communications through the Layers: Application Layer Service Location Simple Reference Model New/adaptive applications Multimedia Transport Layer Congestion/glow control Quality of Service Network Layer Addressing, routing Device Location Hand-over Data Link Layer Authentication Media access/control Multiplexing Encryption Physical Layer Modulation Interference Attenuation Frequency Handovers Glossary - Hosts - DIFS - Links - SIFS - Base station - RTS - SNR - CTS - BER - ACK - CDMA - AP - Backoff - BSS - MAC addr - SSID - Frame - Channels - FCS - Beacon frames - Rate adaptation - CSMA/CA - Power mgmt - Chipping sequence - Orthogonal codes - Piconet - FHSS