Nitin Vaidya University of Illinois at Urbana-Champaign Wireless Networking Primer (few topics that may help in understanding other lectures) Nitin Vaidya University of Illinois at Urbana-Champaign

What Makes Wireless Interesting? Absence of wires facilitate mobility Signal attenuation Spatial reuse Diversity Multi-user diversity Antenna diversity Time diversity Frequency diversity Wireless devices often battery-powered Broadcast medium makes it easier to snoop on, or tamper with, wireless transmissions

Transmission “Range” Whether a transmission is received reliably or not depends on Transmit power level Channel conditions (time-varying) Interference (time-varying) Noise (not deterministic) Packet size Modulation scheme (bit rate) Error control coding Transmission rate Transmission not received by all “neighbors” reliably Not all nodes can “hear” each other Time-varying outcome of transmissions

Medium Access Protocol (MAC) Wireless channel is a shared medium, requiring suitable MAC protocol. Performance of the MAC protocol depends on Channel properties Physical capabilities Single interface? One packet at a time? One channel at a time? Antenna diversity? Assume single interface, single channel, single antenna, one packet at a time, small propagation delay

“Basic” Protocol Simple rule (a distributed protocol): Transmit packet immediately (if not transmitting already) Shortcomings No provision for reliability No detection of “collisions”

Reliability: A Retransmission Protocol Stop-and-wait

A Mechanism to Reduce Collision Cost Packet loss may occur due to collisions. To reduce cost: “Reserve” the wireless channel before transmitting data Send short control packets for reservation Collision may occur for control packets, but they are short  lower collision cost Once channel reserved, data transmission (hopefully) reliable

RTS-CTS Exchange Node A sends RTS to B B responds with CTS Duration of proposed transmission specified in RTS B responds with CTS Host A sends data Other hosts overhearing RTS keep quiet for duration of proposed transmission Works alright if all nodes within “range” of each other

RTS-CTS RTS-CTS reduce collision cost If data packets too small, sending RTS-CTS not beneficial A possible implementation: Send RTS-CTS only for data packets with size > RTS-threshold

Carrier Sense Multiple Access (CSMA) (to reduce collisions) Listen-before-you-talk A host may transmit only if the channel is sensed as idle

Carrier Sensing (Approximation) Implementation using Carrier Sense (CS) threshold Pcs If received signal power < CS threshold  Channel idle Else channel busy In reality, efficacy of carrier sensing affected by noise & interference.

Carrier Sense Multiple Access (CSMA) D perceives idle channel although A is transmitting A B C D distance power D’s CS Threshold

Carrier Sense Multiple Access (CSMA) D perceives busy channel when A transmits D B C A distance power D’s CS Threshold

Trade-Off Large carrier sense threshold  More transmitters  Greater spatial reuse & more interference Trade-off between spatial reuse and interference

Impact of CS Threshold on Interference Suppose C transmits even though A is already transmitting A B C D Path gain g = received power / transmit power

Hidden Terminals

Hidden & Exposed Terminals Collisions may occur despite carrier sensing Smaller carrier sensing threshold can help But increases the incidence of exposed terminals ?

Hidden & Exposed Terminals Cannot eliminate all collisions using carrier sensing Trade-off between hidden and exposed terminals Optimal carrier sense threshold function of network “topology” and traffic characteristics

Collision Detection Ethernet uses carrier sensing & collision detection (CSMA/CD) Transmitter also listens to the channel Mismatch between transmitted & received signal indicates mismatch Stop transmitting immediately once collision is detected  Reduces time lost on a collision

Collision Detection in Wireless Networks Receiving while transmitting: Received signal dominated by transmitted signal Collision occurs at receiver, not the transmitter  Collision detection difficult at the transmitter without feedback from the receiver

Solutions for Hidden Terminals Busy-tone Virtual carrier sensing Carrier sensing mechanism discussed earlier will be referred to as physical carrier sensing, to differentiate with virtual carrier sensing

Virtual Carrier Sensing RTS specifies duration of transmission CTS also includes the duration Any host hearing RTS or CTS stay silent as shown RTS CTS

Virtual Carrier Sensing Host C may not receive RTS and still cause collision at host B SINR = Signal-to-interference-and-noise ratio = S / (I + N) Assume “SINR-threshold model”  assume that SINR  necessary/sufficient for reliable delivery (approximation of reality)

SINR for RTS reception at C is upper bounded as If C transmits while A is receiving an Ack from B, SINR for Ack reception at A is upper bounded as RTS CTS

It is possible to find path gains for which we have and 

Virtual Carrier Sensing C’s silent interval below is not adequate to ensure reliable Ack reception at A Similarly, D’s silent interval not adequate to ensure reliable data reception at B RTS CTS

Virtual Carrier Sensing - Modification Greater protection from interference Reduce book-keeping with multiple nearby transmitters

“Space Reserved” by Virtual CS RTS RTS Reminder: “Range” not necessarily circular in practice

Physical & Virtual CS Physical carrier sensing (PCS) & virtual carrier sensing (VCS) may be used simultaneously Channel assumed idle only if both PCS and VCS indicate that the channel is idle

Backoff Intervals Channel sensing not enough to prevent multiple nodes to start transmitting “at nearly the same time” Reduce such collisions by controlling access probability Implementation using backoff intervals: Choose backoff interval uniformly in range [0, cw-1] Initialize a counter by this value Decrement counter after each slot if channel detected idle Transmit when counter reaches 0

Responding to Packet Loss To reduce collisions due to excessive load on the channel, access probability should be reduced May be achieved by increasing the window over which backoff interval is chosen Exponential backoff : [0,c-1]  [0,2c-1]

IEEE 802.11 Distributed Coordination Function (DCF) Physical & virtual carrier sensing (RTS-CTS) Contention window (cw) : Backoff chosen uniformly in [0,cw] Exponential backoff after a packet loss Contention window reset to CWmin on a success

Infrastructure-Based Networks

Hybrid Networks Ad Hoc Networks

Routing Protocols for Mobile Ad Hoc Networks (MANET) Proactive protocols Determine routes independent of traffic pattern Traditional link-state and distance-vector routing protocols are proactive (and could be extended for MANET) Reactive protocols Maintain routes only if needed Hybrid protocols Similar solutions may be used in “mesh” networks

Example of Reactive Routing: Dynamic Source Routing (DSR) [Johnson96] When node S wants to send a packet to node D, but does not know a route to D, node S initiates a route discovery Source node S floods Route Request (RREQ) Each node appends own identifier when forwarding RREQ

Route Discovery in DSR Y Z S E F B C M L J A G H D K I N Represents a node that has received RREQ for D from S

Broadcast transmission Route Discovery in DSR Y Broadcast transmission Z [S] S E F B C M L J A G H D K I N Represents transmission of RREQ [X,Y] Represents list of identifiers appended to RREQ

Route Discovery in DSR Y Z S [S,E] E F B C M L J A G [S,C] H D K I N Node H receives packet RREQ from two neighbors: potential for collision

Route Discovery in DSR Y Z S E F B [S,E,F] C M L J A G H D K [S,C,G] I Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once

Route Discovery in DSR Y Z S E F [S,E,F,J] B C M L J A G H D K I N [S,C,G,K] Nodes J and K both broadcast RREQ to node D Since nodes J and K are hidden from each other, their transmissions may collide

Route Discovery in DSR Y Z S E [S,E,F,J,M] F B C M L J A G H D K I N Node D does not forward RREQ, because node D is the intended target of the route discovery

Route Discovery in DSR Destination D on receiving the first RREQ, sends a Route Reply (RREP) RREP is sent on a route obtained by reversing the route appended to received RREQ RREP includes the route from S to D on which RREQ was received by node D

Route Reply in DSR Y Z S RREP [S,E,F,J,D] E F B C M L J A G H D K I N Represents RREP control message

Dynamic Source Routing (DSR) Node S on receiving RREP, caches the route included in the RREP When node S sends a data packet to D, the entire route is included in the packet header hence the name source routing Intermediate nodes use the source route included in a packet to determine to whom a packet should be forwarded

Data Delivery in DSR Y Z DATA [S,E,F,J,D] S E F B C M L J A G H D K I Packet header size grows with route length

DSR Optimization: Route Caching Each node caches a new route it learns by any means When node S finds route [S,E,F,J,D] to node D, node S also learns route [S,E,F] to node F When node K receives Route Request [S,C,G] destined for node, node K learns route [K,G,C,S] to node S When node F forwards Route Reply RREP [S,E,F,J,D], node F learns route [F,J,D] to node D When node E forwards Data [S,E,F,J,D] it learns route [E,F,J,D] to node D A node may also learn a route when it overhears Data packets

Use of Route Caching When node S learns that a route to node D is broken, it uses another route from its local cache, if such a route to D exists in its cache. Otherwise, node S initiates route discovery by sending a route request Node X on receiving a Route Request for some node D can send a Route Reply if node X knows a route to node D Use of route cache can speed up route discovery can reduce propagation of route requests

Use of Route Caching S E F B C M L J A G H D K I N Z [S,E,F,J,D] [E,F,J,D] S E [F,J,D],[F,E,S] F B [J,F,E,S] C M L J A G [C,S] H D [G,C,S] K I N Z [P,Q,R] Represents cached route at a node (DSR maintains the cached routes in a tree format)

Use of Route Caching: Can Speed up Route Discovery [S,E,F,J,D] [E,F,J,D] S E [F,J,D],[F,E,S] F B [J,F,E,S] C M L [G,C,S] J A G [C,S] H D K [K,G,C,S] I N RREP RREQ Z When node Z sends a route request for node C, node K sends back a route reply [Z,K,G,C] to node Z using a locally cached route

Route Caching: Beware! Stale caches can adversely affect performance With passage of time and host mobility, cached routes may become invalid A sender host may try several stale routes (obtained from local cache, or replied from cache by other nodes), before finding a good route Can affect higher layer performance adversely (e.g., TCP) [Holland99]

Rate Region Rate region characterizes rates that can be supported simultaneously on various links Useful in determining a transmission “schedule” l 1 Feasible Rate vector 1

Rate Region Rate region = all feasible rate vectors Determined by Channel state Power constraints Physical capabilities & constraints: Examples: Use multiple channels simultaneously? Number of interfaces

Rate Region Simple example scenarios Downlink scenario (common transmitter) Uplink scenario (common receiver) B 2 1 B 2 1

Downlink Scenario Treating interference as noise B 2 1

Downlink Scenario: Treating Interference as Noise W = 10 MHz P = 1 mW

Downlink Scenario: Treating Interference as Noise Power-sharing

Downlink Scenario Power-sharing & Time-sharing

Downlink Scenario: Power-sharing & Bandwidth sharing

Downlink Scenario: Successive Interference Cancellation B 2 1 At node 1, treat other Signal as interference

Downlink Scenario: Successive Interference Cancellation B 2 1 At node 2, “cancel” the interference 

Downlink Scenario: Successive Interference Cancellation B 2 1 Decode signal for 1, and “cancel” it Decode signal for 2

Downlink Scenario: Successive Interference Cancellation

For more information … See tutorials at http://www.crhc.illinois.edu/wireless/tutorials.html UIUC course ECE/CS 439 Wireless Networks slides at http://users.crhc.illinois.edu/nhv/09spring.439/