MSIT 413: Wireless Technologies Week 8

MSIT 413: Wireless Technologies Week 8 Michael L. Honig Department of EECS Northwestern University November 2017

The Multiple Access Problem How can multiple mobiles access (communicate with) the same base station? Frequency-Division (AMPS) Time-Division (IS-136, GSM) Code-Division (IS-95, 3G) Direct Sequence/Frequency-Hopped Orthogonal Frequency Division (WiMax, 4G) Random Access (802.11, wireless data)

802.11 Random Access Access Point (AP) Station A Station C Station B Terminals send/receive messages (packets) to/from the AP at random times (i.e., when they appear).

Medium Access Control (MAC) Fixed assignment access Each user is assigned a dedicated channel, time slot, or code Appropriate for circuit-switched traffic, transferring long data files Random access: users contend for access to the channel Users may collide, losing packets. Sometimes can negotiate rate (bandwidth, time slots, codes) and power Widely used in wired networks Used in WiFi, and in cellular networks for requesting channel/time slot/code

Cellular Call Setup (Random Access) 1. Call Request 2. Send numbers to switch 3. Page Receiver 4. Request Channel/Time slot/code

ALOHA-Based Random Access Packet arrives transmit Collision? no Base station Sends ack yes Transmitter does not Receive ack Packet is rescheduled With random delay Simple: asynchronous Low throughput under heavy loads (maximum is 18% of incoming packets) Slotted ALOHA Synchronous, maximum throughput increases to 36% Used in GSM to reserve a time slot for voice connection Reservation ALOHA Contention period followed by reserved message slots

ALOHA Protocols

Carrier Sense Multiple Access (CSMA) Packet arrives Sense channel Busy? no Transmit packet yes Delay transmission (non-persistent) Listen before talk (LBT) protocol How do collisions occur?

Carrier Sense Multiple Access (CSMA) Packet arrives Sense channel Busy? no Transmit packet yes Delay transmission (non-persistent) Listen before talk (LBT) protocol Collisions occur if transmitters cannot sense the other transmission (e.g., due to large propagation delay) Lower probability of collision/higher throughput than ALOHA Long propagation times è more collisions ALOHA preferred for wide area applications

CSMA Example

Collision Detection: Worst-Case Delay Station A t=0 Station B Propagation delay T = distance / c A starts transmitting at time 0

Collision Detection: Worst-Case Delay t=0 t < T Station B Station A Propagation delay T = distance / c A starts transmitting at time 0 B starts transmitting just before time T (channel is clear)

Collision Detection: Worst-Case Delay Station A t=0 t < T B senses collision at time T Station B A starts transmitting at time 0 B starts transmitting just before time T (channel is clear) B hears A just after it starts to transmit.

Collision Detection: Worst-Case Delay t=0 t < T Station B Station A B senses collision at time T A starts transmitting at time 0 B starts transmitting just before time T (channel is clear) B hears A just after it starts to transmit. B s initial transmission travels back to A

Collision Detection: Worst-Case Delay Station A t=0 t < T A senses collision at time 2T Station B A starts transmitting at time 0 B starts transmitting just before time T (channel is clear) B hears A just after it starts to transmit. B s initial transmission travels back to A A senses a collision at time 2T

Maximum Separation Station B Station A Propagation delay T = distance / c Worst-case delay before collision is detected is 2T As T increases, probability of collision increases, more bits can be lost during a collision Imposes maximum separation between stations 802.11b: maximum separation between station and router is 100 M T = 200/c = 2/3 microsecond Data rate 11 Mbps è maximum of 16 bits are lost in collision

Carrier Sensing Nonpersistent: After sensing a busy channel, the terminal senses the channel after a random waiting period Persistent: The terminal senses the channel until the channel becomes free. 1-Persistent: After the channel becomes free, the terminal transmits immediately. p-persistent: The terminal transmits with probability p.

Binary Exponential Backoff Packet arrives transmit Collision? no Base station Sends ack yes Transmitter does not Receive ack Packet is rescheduled After R time slots R is random Maximum of 16 retries Time slot 2 x (maximum round trip delay) After 1 st collision: R=0 or 1 with equal probability After 2 nd collision: R=0,1,2, or 3 with equal probability After i th collision (i=1,,10): R is selected between 0 and 2 i -1

Performance Throughput (S): Average number of successful packet transmissions per unit time. Normalized throughput: Percentage of successful packet transmissions (per time slot or time unit) Average Delay (D): Average waiting time before successful transmission Offered Traffic (G): Number of packet transmission attempts per packet time slot includes both new arrivals and retransmissions. Performance depends on the propagation delay across the network relative to the packet duration.

Throughput vs. Offered Load Slotted non-persistent CSMA non-persistent CSMA Throughput Slotted Aloha p-persistent CSMA Aloha Offered load

Throughput vs. Offered Load Slotted non-persistent CSMA non-persistent CSMA Throughput 18% Aloha Slotted Aloha p-persistent CSMA 0.5 Offered load

Delay vs. Throughput Delay Throughput

CSMA with Collision Detection (CSMA/CD) Nodes detect a collision in progress, and stop transmitting before the entire packet is transmitted. Assumes nodes can hear each other when they are transmitting. Appropriate for wired channels. Problems with wireless channels: Nodes cannot transmit and receive at the same frequency at the same time. Not all nodes may be in range of each other.

Hidden Terminal Problem Station A Station B Station D Station C A is transmitting to B. C wants to transmit to D.

Hidden Terminal Problem Coverage area for station C. Station A Station B Station D Station C A is transmitting to B. C wants to transmit to D. C may not sense A s transmission, causing a collision at B.

Example: Cognitive Radio Primary users of spectrum share with secondary users Secondary users cannot disrupt primary service Proposed for TV white space unused TV channels available for secondary use TV transmitter TV receiver

Exposed Terminal Problem Station A Station B Station D Station C B is transmitting to A. C wants to transmit to D

Exposed Terminal Problem Coverage area for station C. Station A Station B Station D Station C B is transmitting to A. C wants to transmit to D. C senses B s transmission, and does not transmit even though it would not cause interference at A.

Basic Problem Carrier sensing determines whether or not there are interfering sources near the transmitter, whereas we want to know if there are active receivers nearby.

Solutions Busy-tone multiple access (BTMA) Separate control channel used to indicate that the channel is idle or busy. An active station transmits a busy tone on the control channel. Each receiver that senses a busy tone turns on its own busy tone. Used in ad hoc networks. Digital or Data Sense Multiple Access (DSMA) Used in FDD cellular mobile data networks Forward channel periodically broadcasts a busy/idle bit for the reverse link. Mobile transmits if bit is in idle state; base station sets bit to busy. Not carrier sensing: sensing is performed after demodulation. Multiple Access with Collision Avoidance (MACA)

Hidden Terminal Problem Station A Station B Station D Station C A is transmitting to B. C wants to transmit to D.

Revealing the Hidden Terminal Station A RTS Station B Coverage area for station C. Station D Station C A first sends a Request to Send (RTS) packet to B.

Revealing the Hidden Terminal CTS Coverage area for station C. Station A Station B Station D Station C A sends a Request to Send (RTS) packet to B. B sends a Clear to Send (CTS) packet to A; heard by C!

Revealing the Hidden Terminal data Coverage area for station C. Station A Station B Station D Station C A sends a Request to Send (RTS) packet to B. B sends a Clear to Send (CTS) packet to A; heard by C! C is silent for duration of A s transmission (specified in CTS)

Revealing the Hidden Terminal Coverage area for station C. Station A Station B Station D Station C What if C hears RTS, but not CTS?

Exposed Terminal Station A RTS Station B Coverage area for station C. Station D Station C C will not hear the CTS from A.

RTS Collision Station A RTS Station B Station E RTS Station C RTS messages from E and B collide à exponential backoff

Corrupted CTS Station A CTS Station B Station E Data, RTS, or CTS Station C CTS message from A is corrupted due to interference from E à exponential backoff by B

MACA Protocol (RTS/CTS) Transmitter Receiver Request to Send (RTS), packet length Clear to Send (CTS), packet length Data Ack Terminals receiving either an RTS or CTS must not transmit for the duration of the packet. (What if the terminal hears RTS but not CTS?) Collision occurs if multiple nodes transmit an RTS, or the CTS is not heard due to other interference. Collision è binary exponential back-off

Wireless Local Area Networks (WLANs) Low mobility, high data rates within confined region (building or campus) Competitive with other wireless data systems (4G, fixed wireless access) Unlicensed bands Industrial, Scientific, Medical (ISM): 2.4 GHz National Information Infrastructure (UNII): 5 GHz Must accept interference, therefore uses spread spectrum signaling, or random access with collision avoidance. 40

Range-Rate Tradeoff WWAN IEEE 802.22 Range WMAN WLAN WPAN ZigBee 802.15.4 Bluetooth 802.15.1 IEEE 802.20 WiMax IEEE 802.16 WiFi 802.11 802.15.3 802.15.3a 802.15.3c 0.01 0.1 1 10 100 1000 Data Rate (Mbps) 41

Overview of 802.11 Standard IEEE 802.11 2.4 GHz 850 to 950 nm FHSS DS-SS Diffuse IR 2 Mbps 4GFSK 1 Mbps 2GFSK 2 Mbps DQPSK 1 Mbps DBPSK IEEE 802.11b Extension 5.5 Mbps DQPSK-CCK 11 Mbps DQPSK-CCK 42

Overview of 802.11 Standard IEEE 802.11 discontinued 2.4 GHz 850 to 950 nm FHSS DS-SS Diffuse IR 2 Mbps 4GFSK 1 Mbps 2GFSK 2 Mbps DQPSK 1 Mbps DBPSK IEEE 802.11b Extension discontinued 5.5 Mbps DQPSK-CCK 11 Mbps DQPSK-CCK 43

802.11b vs. 802.11g 802.11b 802.11g 2.4 2.485 Ghz Interference from Microwave, Bluetooth 3 non-overlapping channels Max Speed 11 Mbps Direct Sequence Spread Spectrum (DSSS) CSMA/CA same same channels 6 to 54 Mbps via OFDM 5.5 and 11 Mbps via DSSS CSMA/CA 44

802.11g vs. 802.11a 802.11g 802.11a 2.4 2.485 Ghz Interference from Microwave, Bluetooth 3 non-overlapping channels 6 to 54 Mbps via OFDM fallback to DSSS CSMA/CA Indoor/outdoor range: 38m/140m 5 Ghz range 5 channels Max Speed 54 Mbps Orthogonal Frequency Division Multiplexing (OFDM) CSMA/CA Shorter range 45

802.11g vs. 802.11n 802.11g 802.11n 2.4 2.485 Ghz Interference from Microwave, Bluetooth 3 non-overlapping channels (20 MHz) 6 to 54 Mbps via OFDM fallback to DSSS CSMA/CA Indoor/outdoor range: 38m/140m 2.4 or 5 Ghz MIMO enhancement to 11g/11a Rates (20 MHz/40MHz): 72.2 / 150 Mbps per MIMO stream Up to 4 MIMO streams Indoor/outdoor range: 70m / 250m 46

Peer-to-Peer (Ad Hoc) Configuration Single Cell Station A Station C Station B Mobile devices are referred to as Stations. Each Station can communicate directly with another Station. System referred to as Independent Basic Service Set (IBSS) 47

Infrastructure Configuration Access Point Single Cell Station A Station C Station B Access Point is analogous to a cellular Base Station System referred to as Infrastructure Basic Service Set (BSS) 48

Extended BSS (EBSS) Configuration Distribution System (DS) BSS Connect APs via a wired network System referred to as Extended BSS APs have a BSSID System has a SSID Basic Service Set 49

Association/Disassociation Active scanning: laptop scans different frequencies Passive scanning: laptop waits for beacon on a particular frequency 50

Association/Disassociation Probe Request 51

Association/Disassociation Probe Request Probe Response 52

Association/Disassociation Probe Request Probe Response Authenticate 53

Association/Disassociation Probe Request Probe Response Authenticate Associate Request 54

Association/Disassociation Probe Request Probe Response Authenticate Associate Request Associate Response 55

Association/Disassociation Probe Request Probe Response Authenticate Associate Request Associate Response Disassociate (No ack) 56

802.11b: Physical Layer CCK (Complementary Code Keying): Each 8-bit symbol is mapped to 8 four-phase (QPSK) symbols. 57 The signatures (codes) are orthogonal.

802.11 Protocol Architecture Logical link control Contention-free service Contention service MAC Layer Point Coordination Function (PCF) Distributed Coordination Function (DCF) Air interface (802.11/11a/11b/11g) PCF: Point Coordination Control Function (infrastructure) 59

Point Coordination Function (PCF) Access Point Single Cell Station A Station C Station B Access Point Becomes a Scheduler Contention Free Mode (TDMA) Optional MAC feature not widely available in products. 60

Inter Frame Spacing (IFS) Station has to detect a minimum idle time before transmit Time depends on type of Frame, type of MAC SIFS: Short IFS (used by ACKs, CTS) PIFS: PCF IFS used by PCF Frames DIFS: DCF IFS used by DCF Frames SIFS < PIFS < DIFS Relative values of IFS used to prioritize Medium Access 62

802.11 CSMA/CA 63

Basic Access Method DIFS Contention window PIFS DIFS Busy medium SIFS Backoff window Next frame Slot time Time Defer access Select slot using binary exponential backoff 64

IEEE 802.11 Medium Access Control Logic 65

Distributed Control Function (DCF) Network Allocation Vector: Prevents transmissions by other terminals. Virtual carrier sensing: RTS/CTS packets state the duration of the packet. All stations within range must remain silent during that period. 66

802.11 Frame Structure 2 2 6 6 6 2 6 0-2312 4 Frame Duration Addr 1 Addr 2 Addr 3 Sequence Addr 4 Data CRC Control Control Total Header Size 34 bytes Variable packet size 67

802.11 Frame Structure 2 2 6 6 6 2 6 0-2312 4 Frame Duration Addr 1 Addr 2 Addr 3 Sequence Addr 4 Data CRC Control Control Total Header Size 34 bytes Variable packet size Addr 1 to Addr 4: (Receiver/Transmitter Addresses; Source/Destination AP addresses) Frame Control: indicates if frame is data, RTS, CTS, or other type of control. Duration: indicates time in microseconds that the channel will be allocated for transmission (virtual carrier sensing) 70

Power Management Incoming data is buffered. To conserve power, mobile device is typically idle (does not listen for AP). 72

Power Management Beacon announces packet arrival. Incoming data is buffered. To conserve power, mobile device is typically idle (does not listen for AP). Wakes up at designated times to receive a burst. 73

Power Management Transmits packet(s). Incoming data is buffered. To conserve power, mobile device is typically idle (does not listen for AP). Wakes up at designated times to receive a burst. 74

Core Networking Functions Call setup Mobility management Handoff 75

Wireline Call Setup Switch wire pair Helen s phone off hook PSTN Information Flow network address Bob s phone 76

Wireline Call Setup Switch wire pair Helen s phone off hook dial tone PSTN Information Flow network address Bob s phone 77

Wireline Call Setup Switch wire pair Helen s phone off hook dial tone keystrokes PSTN Information Flow network address alert signal Bob s phone 78

Wireline Call Setup Switch wire pair Helen s phone off hook dial tone keystrokes ring indication PSTN Information Flow network address alert signal Bob s phone 79

Wireline Call Setup Switch wire pair Helen s phone off hook dial tone keystrokes ring indication remove ring indication PSTN Information Flow network address alert signal off hook Bob s phone 80

Wireline Call Setup Switch wire pair Helen s phone off hook dial tone keystrokes ring indication remove ring indication PSTN Information Flow conversation network address alert signal off hook Bob s phone 81

Cellular Call Setup 1. Call Request 82

Cellular Call Setup 1. Call Request 2. Send numbers to switch 3. Page Receiver 4. Request Channel 83

Cellular Call Setup (cont.) 5. Switch assigns channels 6. Cellular conversation is set up 84

Information Flow for Cellular Call 85