Wireless & Mobile Networking

Wireless & Mobile Networking CS 752/852 - Spring 2011 Lec #3: Medium Access Control - I Tamer Nadeem Dept. of Computer Science

Data Link Layer (DLL) Main Task of the data link layer: Provide error-free transmission over a link Page 2 Spring 2011 CS 752/852 - Wireless and Mobile Networking

DLL Services Framing The DLL translates the physical layer's raw bit stream into discrete units (messages) called frames. How can the receiver recognize the start and end of a frame? Flow Control Flow control deals with throttling the speed of the sender to match that of the receiver. Usually, this is a dynamic process, as the receiving speed depends on such changing factors as the load, and availability of buffer space. Page 3 Spring 2011 CS 752/852 - Wireless and Mobile Networking

DLL Services Link Management Allocating buffer space, control blocks, agreeing on the maximum message size, etc. Synchronize and initialize send and receive sequence numbers with its peer at the other end of the communications channel Error Control Error control is concerned with insuring that all frames are eventually delivered (possibly in order) to a destination. Three items are required: Acknowledgments, Timers, Sequence Numbers Error Detection and Correction line noise is a fact of life (e.g., signal attenuation, natural phenomenon such as lightning, and the telephone repairman). Error Detecting Codes: Include enough redundancy bits to detect errors and use ACKs and retransmissions to recover from the errors. Error Correcting Codes: Include enough redundancy to detect and correct errors. CRC Checksums Page 4 Spring 2011 CS 752/852 - Wireless and Mobile Networking

DLL = LLC + MAC LANs first began to emerge as potential business tools in the late 1970s IEEE launched Project 802 (1980, February) to define certain LAN standards. Project 802 defined network standards for the physical components of a network (the interface card and the cabling) Define the ways NICs access and transfer data over physical media. These include connecting, maintaining, and disconnecting network devices. The IEEE 802 standards incorporated the specifications in the bottom two OSI layers, the physical layer and the data-link layer. Page 5 Spring 2011 CS 752/852 - Wireless and Mobile Networking

DLL = LLC + MAC More detail was needed at the data-link layer, the 802 standards committee divided the data-link layer into two sublayers. Logical Link Control (LLC) Sublayer: Manages data-link communication: establishing and terminating links, controlling frame traffic, sequencing frames, and acknowledging frames. Media Access Control (MAC) Sublayer: Communicates directly with the NIC to provide shared access to the physical layer: Managing media access, delimiting frames, checking frame errors, and recognizing frame addresses. Page 6 Spring 2011 CS 752/852 - Wireless and Mobile Networking

DLL = LLC + MAC Page 7 Spring 2011 CS 752/852 - Wireless and Mobile Networking

DLL = LLC + MAC Wireless LAN Standards (IEEE 802.11) Page 8 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Medium Access Control (MAC) Page 9 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Introduction Multiple access control channels Each node is attached to a transmitter/receiver which communicates via a channel shared by other nodes Transmission from any node is received by other nodes Node 3 Node 2 Shared Channel Node 4 Node 1 Node N Page 10 Spring 2011 CS 752/852 - Wireless and Mobile Networking

The Channel Access Problem Multiple nodes share a channel A B C Pairwise communication desired Simultaneous communication not possible MAC Protocols Suggests a scheme to schedule communication Maximize number of communications Ensure fairness among all transmitters Page 11 Spring 2011 CS 752/852 - Wireless and Mobile Networking 11

The Trivial Solution A B C collision Transmit and pray Plenty of collisions --> poor throughput at high load Page 12 Spring 2011 CS 752/852 - Wireless and Mobile Networking 12

Classification of MAC Protocols Contention-free MAC TDMA, FDMA, CDMA: Divides channel by time, frequency, or code More applicable to static networks and/or networks with centralized control Contention-based MAC Single Channel vs. Multi-Channels Sender Initiated vs. Receiver Initiated Single Channel and Sender Initiated Protocols Page 13 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Carrier Sense Multiple Access (CSMA) Page 14 Spring 2011 CS 752/852 - Wireless and Mobile Networking

CSMA Don t transmit A B C Listen before you talk Carrier sense multiple access (CSMA) Can collisions still occur? Defer transmission when signal on channel Advantages Fairly simple to implement Functional scheme that works Disadvantages Can not recover from a collision (inefficient waste of medium time) Page 15 Spring 2011 CS 752/852 - Wireless and Mobile Networking

CSMA reduces chance of collisions reduces the efficiency increases the chance for collisions 1-persistant p-persistant Page 16 Spring 2011 CS 752/852 - Wireless and Mobile Networking

CSMA/CD (Collision Detection) Collisions can still occur: Propagation delay non-zero between transmitters spatial layout of nodes When collision: Entire packet transmission time wasted note: Role of distance & propagation delay in determining collision probability Page 17 Spring 2011 CS 752/852 - Wireless and Mobile Networking

CSMA/CD (Collision Detection) Keep listening to channel While transmitting If (Transmitted_Signal!= Sensed_Signal) Sender knows it s a Collision ABORT Page 18 Spring 2011 CS 752/852 - Wireless and Mobile Networking

2 Observations on CSMA/CD Transmitter can send/listen concurrently If (Sensed - Transmitted = null)? Then success The signal is identical at Tx and Rx Non-dispersive The TRANSMITTER can detect if and when collision occurs Page 19 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Unfortunately Both observations do not hold for wireless Because Page 20 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Wireless Medium Access Control A cannot send and listen in parallel A B C D Signal power Signal not same at different locations CS threshold Distance Page 21 Spring 2011 CS 752/852 - Wireless and Mobile Networking

CSMA with Collision Avoidance (CSMA/CA) CMSA/CA The CSMA/CA algorithm is based on a basic time unit called slot σ. Slot duration (σ) is equal to maximum propagation delay. Time space is slotted at the boundaries of σ. Channel access slotted CSMA can only occur at the boundary of σ Next Frame Slot Time Slotting solved collisions because of propagation delays Page 22 Spring 2011 CS 752/852 - Wireless and Mobile Networking

But Still, More Problems to Solve Page 23 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Hidden Node Collisions Important: D has not heard A, but can interfere at receiver B A B C D Signal power D is the hidden node to A CS threshold Distance Page 24 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Hidden Node Collisions Important: D has not heard A, but can interfere at receiver B A B C D SignalOfInterest( SoI ) SNR Interference( I) Noise( N) SoI I D B A B P P d A transmit AB D transmit d DB SNR A B P A transmit d P N AB D transmit d DB D is the hidden node to A Page 25 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Exposed Node Collisions Y Important: X has heard A, but should not defer transmission to Y X A B C D Signal power X is the exposed terminal to A CS threshold Distance Page 26 Spring 2011 CS 752/852 - Wireless and Mobile Networking

So, how do we cope with Hidden/Exposed Terminals? Page 27 Spring 2011 CS 752/852 - Wireless and Mobile Networking 27

The Emergence of MACA, MACAW, & 802.11 Wireless MAC proved to be non-trivial 1992 - research by Karn (MACA) 1994 - research by Bhargavan (MACAW) Led to IEEE 802.11 committee The standard was ratified in 1999 Page 28 Spring 2011 CS 752/852 - Wireless and Mobile Networking

CA with Control Handshaking - (MACA) Alternative to carrier sensing, i.e. does not use CSMA Multiple access with collision avoidance (MACA) uses a three way handshake to avoid hidden terminal problem (Karn, 90) When node A wants to send a packet to node B, node A first sends a Request-to-Send (RTS) to B On receiving RTS, node B responds by sending Clear-to-Send (CTS) All nodes within one hop of node A hear the RTS and defer their transmissions until corresponding CTS. When a node (such as D) overhears a CTS, it keeps quiet for the duration of the transfer Transfer duration is included in RTS and CTS both Page 29 Spring 2011 CS 752/852 - Wireless and Mobile Networking

MACA examples MACA avoids the problem of hidden terminals A and C want to send to B A sends RTS first C waits after receiving CTS from B RTS CTS CTS A B C MACA avoids the problem of exposed terminals B wants to send to A, C to another terminal now C does not have to wait for it cannot receive CTS from A RTS CTS RTS A B C 30 Page 30 Spring 2011 CS 752/852 - Wireless and Mobile Networking

MACA Limitations MACA does not provide ACK RTS-CTS approach does not always solve the hidden node problem Examples Page 31 Spring 2011 CS 752/852 - Wireless and Mobile Networking

MACAW (MACA for Wireless) RTS-CTS-DS-DATA-ACK RTS from A to B CTS from B to A Data Sending (DS) from A to B Data from A to B ACK from B to A Random wait after any successful/unsuccessful transmission Significantly higher throughput than MACA Does not completely solve hidden & exposed node problems A B C D RTS CTS DS Data Ack Page 32 Spring 2011 CS 752/852 - Wireless and Mobile Networking

IEEE 802.11 Standards Page 33 Spring 2011 CS 752/852 - Wireless and Mobile Networking

IEEE 802.11 MAC Very popular wireless MAC protocol Two Architectures IEEE 802.11 Medium Access Control (PCF+DCF) FHSS DSSS Infrared OFDM MAC PHY Page 34 Spring 2011 CS 752/852 - Wireless and Mobile Networking

802.11 PHY Sublayers Physical layer convergence protocol (PLCP) Provides common interface for MAC Offers carrier sense status & CCA (Clear channel assesment) Performs channel synchronization / training Physical medium dependent sublayer (PMD) Functions based on underlying channel quality and characteristics E.g., Takes care of the wireless encoding Page 35 Spring 2011 CS 752/852 - Wireless and Mobile Networking 35

PLCP PLCP has two structures. All 802.11b systems have to support Long preamble. Short preamble option is provided to improve efficiency when trasnmitting voice, VoIP, streaming video. PLCP Frame format PLCP preamble SFD: start frame delimiter PLCP header 8-bit signal or data rate (DR) indicates how fast data will be transmitted 8-bit service field reserved for future 16-bit length field indicating the length of the ensuing MAC PDU (MAC sublayer s Protocol Data Unit) 16-bit Cyclic Redundancy Code Page 36 Spring 2011 CS 752/852 - Wireless and Mobile Networking 36

PLCP (802.11b) long preamble 192us short preamble 96us (VoIP, video) Page 37 Spring 2011 CS 752/852 - Wireless and Mobile Networking 37

IEEE 802.11 MAC Two modes: DCF (distributed coordination function) PCF (point coordination function) IEEE 802.11 DCF is based on CSMA/CA Physical Carrier Sense Explicit ACK from receiver (for unicast transmission) RTS/CTS reservation frames (Virtual Carrier Sensing) Retry Counters Different Timing Intervals for priorities Page 38 Spring 2011 CS 752/852 - Wireless and Mobile Networking

IEEE 802.11 DCF Basics RTS/CTS & Virtual Carrier Sense RTS-CTS used for frames longer than a Threshold RTS-CTS overhead not efficient for short frames Some environments may not find RTS-CTS useful, e.g. many infrastructure networks Threshold variable can be tuned Virtual carrier sensing Duration field in all frames, including RTS and CTS, monitored by every station Duration field used to construct a network access vector (NAV) Inhibits transmission, even if no carrier detected Page 39 Spring 2011 CS 752/852 - Wireless and Mobile Networking

IEEE 802.11 DCF Basics Retry Counters Counter and timer for each frame Short or long retry counter Lifetime timer Retry counter Incremented for each transmission attempt Use of short versus long retry counter based on Threshold variable Threshold limit ShortRetryLimit for short retry counter LongRetryLimit for long retry counter If threshold exceeded, frame is discarded and upper layer is notified via MAC interface Page 40 Spring 2011 CS 752/852 - Wireless and Mobile Networking

IEEE 802.11 DCF Basics Timing Intervals Timing intervals are defined that control a station s access to the medium Slot time (SlotTime) Specific value depends on PMD layer Derived from propagation delay, transmitter delay, etc. (20micro-sec for DSSS and 50 for FHSS) Basic unit of time for MAC, e.g. for backoff time is a multiple of slot time Short Inter-Frame Space (SIFS) Shortest interval -- SIF < SlotTime e.g. 10 microsec for FHSS Used for highest priority access to the medium, e.g., for ACK and CTS Allows Data-ACK and RTS-CST to be atomic transactions Page 41 Spring 2011 CS 752/852 - Wireless and Mobile Networking

IEEE 802.11 DCF Basics Timing Intervals Priority (or PCF) Inter-Frame Space (PIFS) PIFS = SIFS + SlotTime Used for Point Coordination Function (PCF) access to the medium Allows priority based access to the medium after ACKs but before contention based access Distributed (or DCF) Inter-Frame Space (DIFS) DIFS = SIFS + 2 SlotTime Used for Distributed Control Function (DCF) access to the medium Results in lower priority access than using SIFS or PIFS Extended Inter-Frame Space (EIFS) EIFS = SIFS + (8 ACK) + PreambleLength + PLCPHeaderLength + DIFS Used in the event that the MAC receives a frame with an error Provides an opportunity for a fast retransmit of the error frame In summary SIFS < SlotTime < PIFS < DIFS << EIFS Page 42 Spring 2011 CS 752/852 - Wireless and Mobile Networking

802.11 DCF Mode Principles When a sender has a data to transmit, it picks a random wait period. The wait period is decremented if the channel is idle When this period expires, the node tries to acquire the channel by sending a RTS packet The Receiving node (destination) responds with a CTS packet indicating that its ready to receive the data The sender then completes the packet transmission If the packet is received without errors, the destination node responds with an ACK If an ACK is not received, the packet is assumed to be lost and the packet is retransmitted Page 43 Spring 2011 CS 752/852 - Wireless and Mobile Networking

802.11 DCF Mode Principles If RTS fails, the node attempts to resolve the collision by doubling the wait period. (This is known as binary exponential back-off (BEB)). Station trying to send an ACK is given preference over a station that is acquiring a channel (Different waiting intervals are specified) A node needs to sense channel for Distributed Inter- Frame Space (DIFS) interval before making an RTS attempt and a Short Inter Frame Space (SIFS) interval before sending an ACK packet Page 44 Spring 2011 CS 752/852 - Wireless and Mobile Networking

802.11 DCF Mode C A B D Deferred CW DIFS C D DIFS Contention Window NAV (RTS) NAV (CTS) RTS A RTS DATA SIFS SIFS SIFS CTS ACK B Page 45 Spring 2011 CS 752/852 - Wireless and Mobile Networking

802.11 DCF Mode Notes Because SIFS is shorter than the DIFS interval, the station sending an ACK attempts transmission before a station sending a data packet In addition to physical channel sensing, virtual carrier sensing is achieved due to NAV (Network allocation vector) field in the packet NAV indicates the duration of current transmission Nodes listening to RTS, or CTS messages back off NAV amount of time before sensing the channel again Several papers describe this protocol and even suggest enhancements. Page 46 Spring 2011 CS 752/852 - Wireless and Mobile Networking

802.11 Contention Window Random number selected from [0,cw] If transmission was successful, set CW = CW min If transmission fails (i.e., no ACK), CW = min{2(cw+1)-1, CW max } Small value for cw Less wasted idle slots time Large number of collisions with multiple senders (two or more stations reach zero at once) Optimal CW for known number of contenders & know packet size Computed by minimizing expected time wastage (by both collisions and empty slots) Tricky to implement because number of contenders is difficult to estimate and can be VERY dynamic Project Idea: Evaluate literature for CW calculation schemes under different scenarios Enhance/New adaptive CW scheme Page 47 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Physical Carrier Sense Mechanisms Energy detection threshold Monitors channel during idle times between packets to measure floor noise Energy levels above this floor noise by a threshold trigger carrier sense DSSS correlation threshold Monitors the channel for Direct Sequence Spread Spectrum (DSSS) coded signal Triggers carrier sense if the correlation peak is above a threshold More sensitive than energy detection (but only works for 802.11 transmissions) High BER disrupts transmission but not detection Carrier can be sensed at lower levels than packets can be received Receive Range Results in larger carrier sense range than transmission range More than double the range in NS2 802.11 simulations Carrier Sense Range Page 48 Spring 2011 CS 752/852 - Wireless and Mobile Networking

On 802.11 Issues RTS/CTS & Carrier Sense When RTS/CTS is useful? Should Carrier Sensing replace RTS/CTS? Interference Range vs. Carrier Sense Range How effective CSMA carrier sense? BER & Date rate and Transmission Range (data rate affect the SNR threshold and hence the transmission range but not the physical CS) Contention Window Size Is ACK necessary? MACA said no ACKs. Let TCP recover from losses The search for the best MAC protocol is still on. However, 802.11 is being optimized too. Thus, wireless MAC research still alive Page 49 Spring 2011 CS 752/852 - Wireless and Mobile Networking

On RTS/CTS & Carrier Sense Does RTS/CTS (Virtual CS) solve hidden terminals? Assuming carrier sensing zone = communication zone A B C D CTS E RTS F E does not receive CTS successfully Can later initiate transmission to D. Hidden terminal problem remains. Page 50 Spring 2011 CS 752/852 - Wireless and Mobile Networking 50

On RTS/CTS & Carrier Sense Hidden Terminal: How about increasing Physical Carrier Sense range?? E will defer on sensing carrier no collision!!! CTS E RTS F A B C Data D Page 51 Spring 2011 CS 752/852 - Wireless and Mobile Networking

On RTS/CTS & Carrier Sense Exposed Terminal: B should be able to transmit to A Carrier sensing makes the situation worse RTS CTS E A B C D Page 52 Spring 2011 CS 752/852 - Wireless and Mobile Networking

On RTS/CTS & Carrier Sense 802.11 does not solve HT/ET completely Only alleviates the problem through RTS/CTS and recommends larger CS zone Large CS zone aggravates exposed terminals Spatial reuse reduces A tradeoff RTS/CTS packets also consume bandwidth Moreover, backing off mechanism is also wasteful Carrier sense relies on channel measurements at the sender to infer the probability of reception at the receiver! Project Idea: Evaluation of the benefits and drawbacks of carrier sense Scheme to intelligently choose a Carrier sensing threshold Evaluate tracking correlation between channel conditions at the sender and at the receiver. Page 53 Spring 2011 CS 752/852 - Wireless and Mobile Networking

On Contention Window Size Optimal CW for known number of contenders & know packet size Computed by minimizing expected time wastage (by both collisions and empty slots) Tricky to implement because number of contenders is difficult to estimate and can be VERY dynamic 802.11 adaptive scheme is unfair Under contention, unlucky nodes will use larger cw than lucky nodes (due to straight reset after a success) Lucky nodes may be able to transmit several packets while unlucky nodes are counting down for access Fair schemes should use same cw for all contending nodes Project Idea: Evaluate literature for CW calculation schemes under different scenarios Enhance/New adaptive CW scheme Page 54 Spring 2011 CS 752/852 - Wireless and Mobile Networking

On Interference Range vs. Carrier Sense Range 802.11 physical layer (e.g., Direct Sequence Spread Spectrum (DSSS) used in 802.11b) Capture effect: two transmissions received by the same receiver, the signals of the stronger transmission will capture the receiver radio, and signals of the weaker transmission will be rejected as noise. Simple and widely accepted model: Capturing stronger signal Capturing stronger frame Received Frame Received Frame Frame 1 Frame 2 Frame 1 Frame 2 Page 55 Spring 2011 CS 752/852 - Wireless and Mobile Networking

On Interference Range vs. Carrier Sense Range Power path loss model: Capture model: Given: R=250m, R C=550, l =2, α=5 d Interference Range: I 1 2 I C Inefficiency: Page 56 Spring 2011 CS 752/852 - Wireless and Mobile Networking

On Interference Range vs. Carrier Sense Range Project Idea: How to estimate interference range (distance) Propagation Delay? Interference Aware MAC Scheme Page 57 Spring 2011 CS 752/852 - Wireless and Mobile Networking

On Transmission Date rate Bit error (p) for BPSK and QPSK : Received Power Channel Bandwidth SNR Floor Noise Data Rate Page 58 Spring 2011 CS 752/852 - Wireless and Mobile Networking

On Transmission Date rate Page 59 Spring 2011 CS 752/852 - Wireless and Mobile Networking

On ACKnowledgment APs typically backlogged with traffic Persistent traffic possibility of optimization Use implicit ACK optimization Piggyback the CTS with ACK for previous dialog 802.11 Gain Implicit ACK Page 60 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Backoff Backoff Backoff Backoff Backoff Backoff On ACKnowledgment The optimization timeline 802.11 Implicit ACK Hybrid Channel Access T R T R T R RTS RTS RTS CTS CTS CTS Data Data Data ACK RTS Poll +ACK CTS RTS CTS +ACK Data Data Data Poll +ACK ACK RTS Data CTS +ACK Page 61 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Performance Analysis of the IEEE 802.11 Distributed Coordination Function (Giuseppe Bianchi) Page 62 Spring 2011 CS 752/852 - Wireless and Mobile Networking

802.11 DCF Throughput Analysis (Bianchi) Objective: Analytical Evaluation of Saturation Throughput Assumptions: Fixed number of stations having packet for transmission Each packet collide with constant and independent probability Model bi-dimensional process {s(t), b(t)} with discrete-time Markov chain Analysis divided into two parts: Study the behavior of single station with a Markov model Study the events that occur within a generic slot time & expressed throughput for both Basic & RTS/CTS access method Page 63 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Markov Chain Model Page 64 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Markov Chain Model Closed form solution for Markov chain Stationary Probability Page 65 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Markov Chain Model Probability τ that a station transmits in randomly chosen slot time When m =0 no exponential backoff is considered probability τ results independent of p In general τ depends on conditional collision probability p Page 66 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Throughput Analysis Normalized system throughput S Probability of transmission P tr Probability of successful transmission P s Page 67 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Throughput Analysis Normalized system throughput Specify T s and T c to compute throughput for DCF access mechanism Page 68 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Throughput Analysis Considering System via Basic Access mechanism Packet header H = PHY hrd +MAC hrd Propagation delay δ Page 69 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Throughput Analysis Packet transmission via RTS/CTS Access mechanism Page 70 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Model Validation Compared analytical results with that obtained by means of simulation Analytical model extremely accurate Analytical results (lines) coincide with simulation results (symbols) in both Basic Access & RTS/CTS cases Page 71 Spring 2011 CS 752/852 - Wireless and Mobile Networking Saturation throughput analysis vs. simulation

Maximum Saturation Throughput τ depends on n, W, and m Analytical model determines maximum achievable saturation throughput Page 72 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Performance Evaluation Greater the network size lower is the throughput for basic access Saturation throughput vs. initial window size for Basic Access mechanism Page 73 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Performance Evaluation Throughput of Basic Access mechanism depends on W W depends on number of terminals High value of W gives excellent throughput performance Saturation throughput vs. initial window size for Basic Access mechanism Page 74 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Performance Evaluation Throughput obtained with RTS/CTS mechanism Independent of value of W Saturation throughput vs. initial window size for RTS/CTS mechanism Page 75 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Performance Evaluation Number of transmissions per packet increases as W reduces & network size n increases. Average number of transmissions per packet Page 76 Spring 2011 CS 752/852 - Wireless and Mobile Networking

Questions Page 77 Spring 2011 CS 752/852 - Wireless and Mobile Networking