Computer Networks COE 549 Directional Antennas for Adhoc

Computer Networks COE 549 Directional Antennas for Adhoc Networks Tarek Sheltami KFUPM CCSE COE http: //faculty. kfupm. edu. sa/coe/tarek/coe 549. htm 19 March 2018

Outline § Introduction § IEEE 802. 11 (CSMA/CA) overview § Motivations § Problem statement § Beamforming: Definition, types and advantages. § Basic DMAC § Challenges in Ad-hoc Networks using directional antennas. § Multi-Hop MAC (MMAC) § Beamforming with Power Control § Performance Evaluation 2

Ad Hoc Networks Typically assume Omnidirectional antennas A silenced node C B A 3/19/2018 D 3

Can Directional Antennas Improve Performance? Not possible using Omni C B A 3/19/2018 D 4

A Comparison Issues Omni Directional Spatial Reuse Low High Connectivity Low High Interference Omni Directional Cost & Complexity Low High 3/19/2018 5

Motivation • Are directional antennas beneficial to medium access control in ad hoc networks ? – To what extent ? – Under what conditions ? 3/19/2018 6

IEEE 802. 11 • Sender sends Ready-to-Send (RTS) • Receiver responds with Clear-to-Send (CTS) • RTS and CTS announce the duration of the imminent dialogue • Nodes overhearing RTS/CTS defer transmission for that duration – Network Allocation Vector (NAV) remembers duration 3/19/2018 7

IEEE 802. 11 RTS = Request-to-Send RTS A 3/19/2018 B C D E F 8

IEEE 802. 11 RTS = Request-to-Send RTS A B C D E F NAV = 10 3/19/2018 9

IEEE 802. 11 CTS = Clear-to-Send CTS A 3/19/2018 B C D E F 10

IEEE 802. 11 CTS = Clear-to-Send CTS A B C D E F NAV = 8 3/19/2018 11

IEEE 802. 11 • DATA packet follows CTS. Successful data reception acknowledged using ACK. DATA A 3/19/2018 B C D E F 12

IEEE 802. 11 ACK A 3/19/2018 B C D E F 13

IEEE 802. 11 • Channel contention resolved using backoff – Nodes choose random backoff interval from [0, CW] – Count down for this interval before transmission A Random backoff Data Transmit backoff Wait B Random backoff Wait backoff Data Transmit 3/19/2018 14

Antenna Model 2 Operation Modes: Omni and Directional A node may operate in any one mode at any given time 3/19/2018 15

Antenna Model In Omni Mode: • Let us assume that nodes receive signals with Gain Go In Directional Mode: • Directional Gain Gd (Gd > Go) 3/19/2018 16

Directional Communication Received Power (Tx Gain) * (Rx Gain) • Tx Gain = Transmit gain in the direction of receiver • Rx Gain = Receive gain in the direction of the transmitter B A C Convention: A link shown by overlapping beams along the line joining the transmitter and receiver. Nodes C, A form a link. C, B do not. 3/19/2018 17

Directional Neighborhood Receive Beam B Transmit Beam A C • When C transmits directionally • Node A sufficiently close to receive in omni mode • Node C and A are Directional-Omni (DO) neighbors • Nodes C and B are not DO neighbors 3/19/2018 18

Directional Neighborhood Transmit Beam Receive Beam B A C • When C transmits directionally • Node B receives packets from C only in directional mode • C and B are Directional-Directional (DD) neighbors 3/19/2018 19

Antenna Beamforming • A technique in which the antenna pattern is switched (or steered) to a desired direction. • Two types: switched & steered beam. - Switched beam: can select one from a set of predefined beams/antennas S 3/19/2018 D - Steered beam: can direct the beam to the desired direction. (cost more but better performance) S D 20

Antenna Beamforming 1. Longer range Why? higher antenna gain in the desired direction Benefits: better connectivity and lower end-to-end delay 2. Higher spatial reuse Why? Reduced interference (narrower beamwidth) Benefits: increased capacity and throughput 3/19/2018 21

Research Problem Identify the challenges encountered in MAC when beamforming antennas are used in Ad hoc networks and find the possible solutions of those problems in the literature. 3/19/2018 22

Challenges in Ad-hoc Networks The two most impacted networking mechanisms as a result of using beamforming antennas are 1. Neighbor discovery identifies the one-hop neighbors 2. MAC provides distributed access to the channel 3/19/2018 23

DMAC l l DMAC is MAC with directional (beamforming) Antennas. Two Operation Modes: Omni and Directional A node may operate in any mode at any given time 3/19/2018 24

Basic DMAC • Assumption: Location of neighbors is known. • Sender transmits Directional-RTS (DRTS) • A node listens omni-directionally when idle, – RTS received in Omni mode. • Receiver sends Directional-CTS (DCTS) • DATA, ACK transmitted and received directionally. • Operation is the same as 802. 11 but with directional antennas and , and with the use of DNAV (directional NAV)!! 3/19/2018 25

Basic DMAC Why DNAV (directional Network allocation Vector)? Asnwer: to combat directional exposed terminal problem. increased spatial reuse and throughput D B E A 3/19/2018 C 26

Neighbor discovery New notions of neighbors: C A B Transmit Receive antenna OO Omni Nodes A and B are OO neighbors. OO Nodes C and A are not but DO neighbors. Dir. OO OODO DO Nodes C and B are not but DD neighbors. Dir DODD DD Omni Dir 3/19/2018 27

Neighbor discovery • How to know the direction of the intended node? – CTS, DATA, ACK are much easier than RTS – Two possible ways: • From the AOA (Angle_of_Arrival ) of RTS and CTS. • Or from self location information included in RTS and CTS. – Directing the beam towards the destination for DRTS is challenging. Possible solutions: • Most MAC proposal assumes that this information is available by routing protocol. Each node know its location (by GPS or any location estimation method). • By Ao. A cashing of overheard packets (ex. Takai et al. [2]) • Circular DRTS • ORTS. 3/19/2018 28

Neighbor discovery DMAC by Takai et al. [2] • Goals: send RTS directionally without location knowledge. • Employs DNAV – It is set according to Ao. A of the RTS/CTS dialog • Employs Ao. A cashing – The direction of neighbors is cashed based on the estimation of Ao. A of the overheard packets. • RTS is send directionally if the direction of the intended destination is available in the cash • RTS is sent omnidirectionally if the direction of the destination is not available in the Ao. A cash or CTS is not received after directional RTS transmission. • 3 to 4 times improvement in throughput compared to 802. 11 3/19/2018 29

Neighbor discovery • Extended transmission range – Beamforming enables longer range – Advantages: reduced # of hops, e 2 e delays and better connectivity (sparse networks) – Most of MAC proposals are not able to achieve the maximum possible range • OO, OD link only, – For Maximum range: • DD link – MMAC by Choudhury et al. [3] 3/19/2018 30

Neighbor discovery MMAC by Choudhury et al. [3] - Knowledge of neighbors location is assumed - Goal: improve system performance (e 2 e delay and throughput) by extending the range of transmission (DD link). - Similar to basic DMAC + DD link - DD link can be established by multi-hop RTS (MHRTS) B MHR TS A MHR C TS MHR TS D DRTS 3/19/2018 DO Link DD Link E DCTS DATA 31

Multi Hop RTS – Basic Idea D C A B DO neighbors E DD neighbors F G A source-routes RTS to D through adjacent DO neighbors (i. e. , A-B-C-D) When D receives RTS, it beamforms towards A, forming a DD link 3/19/2018 32

MMAC protocol A transmits RTS towards D D H E F C A 3/19/2018 B G 33

MMAC protocol H updates DNAV D H E F C A 3/19/2018 B G 34

MMAC protocol A transmits M-RTS to DO neighbor B D H E F C A 3/19/2018 B G 35

MMAC protocol B forwards M-RTS to C (also DO) D H E F C A 3/19/2018 B G 36

MMAC protocol A beamforms toward D – waits for CTS D H E F C A 3/19/2018 B G 37

MMAC protocol C forwards M-RTS to D D H E F C A 3/19/2018 B G 38

MMAC protocol D beamforms towards A – sends CTS D H E F C A 3/19/2018 B G 39

MMAC protocol A & D communicate over DD link D H E F C A 3/19/2018 B G 40

MMAC protocol Nodes D and G similarly communicate D H E F C A 3/19/2018 B G 41

Problems in DMAC There are two main problems associated with DMAC: 1. New Hidden Terminals 2. Deafness 3/19/2018 42

Problems in DMAC 1. New Hidden Terminals The node is hidden to the ongoing communication of other node when it didn’t hear the RTS/CTS transmission while it can interfere Case 1. E is out of RTS/CTS range of A/C communication Case 2. Loss in channel state Collision D E D C A The antenna of E is directed twards D RTS/CTS of A/C CANNOT be heard by E A C 3/19/2018 E Collision 43

Problems in DMAC 2. Deafness • A node A is deaf with respect to nodes X, Z, if it cannot receive from nodes X, Z due to beam direction while it can receive if it was in omni mode. • Effects: – Waste the capacity and energy (due unproductive control packets). – Introduce unfairness (increased backoff interval). RTS X A B DATA RTS 3/19/2018 Z X and Z do not know node A is busy. They keep transmitting RTSs to node A 44

Problems in DMAC • Hidden terminals and deafness are the two critical problems in DMAC. • Possible Solution: – Send RTS and/or CTS omnidirectionally while DATA/ACK are sent directionally. Example: DMAC by Ko et al. [5] 3/19/2018 45

Problems in DMAC by Ko et al. [5] - Knowledge of neighbors location is assumed - Multiple directional antennas for each nodes (switched beam) - Goal: increase spatial reuse while reducing control packet collisions. - DATA/ACK is directional - CTS is omnidirectional = OCTS - Two schemes for RTS: - Scheme 1 : DRTS (Directional RTS) only - Scheme 2 : ORTS/DRTS X A S D B S can send to D but not to X Both schemes send DRTS 3/19/2018 A D S B Scheme 2 sends RTS in all directions (ORTS) if no antenna is blocked 46

Problems in DMAC by Ko et al. (Cont. ) Performance • Offers about 50% better throughput compared to IEEE 802. 11, depends on Topology • Scheme 1 vs. Scheme 2: – Scheme 2 tries to reduce collision of control packets at the source while scheme 1 tries maximize spatial reuse in the vicinity of the source. – No significant performance difference 3/19/2018 47

Problems with DMAC Possible Solution to unfairness caused by Deafness: Tone. DMAC by Choudury et al. [6] • • • Goal: to reduce the effect of unfairness caused by Deafness by identify Deafness from congestion RTS/CTS/DATA/ACK are sent directionally After RTS/CTS/DATA/ACK exchange, A and B send their tones omnidirectinally. neighboring nodes that overhear the tones will know that node A or B was engaged in communication. Throughput is 2 times better than DMAC. C will know that B was deaf. It will reset the backoff window – Fairness is improved. to the minimum value. A_TONE B_TONE A B DATA 3/19/2018 A_TONE RTS C B_TONE 48

DMAC Tradeoffs • Benefits – Better Network Connectivity • Disadvantages – Hidden terminals – Deafness – Spatial Reuse – No DD Links 3/19/2018 49

Impact of Beamforming on Ad-hoc Networking: MAC , Neighbor discovery, Route discovery Our Goal is to study the impact of Antenna beamforming on MAC. Examples: (Assume CSMA/CA ) Without beamforming A C B D Exposed terminal problem C A B C D No problem C A A B D E 3/19/2018 With beamforming No problem E Deafness Problem 50

51 r/2 r beamforming Area = A Beamforming only Area = A/6 A rough comparison of relative interferen directional beamwidth, and r 4 pr degrees 3/19/2018

Performance • Simulation – – – – Qualnet simulator 2. 6. 1 Constant Bit Rate (CBR) traffic Packet Size – 512 Bytes 802. 11 transmission range = 250 meters DD transmission range = 900 m approx Beamwidth = 60 degrees Channel bandwidth 2 Mbps Mobility - none 3/19/2018 52

MMAC Hop Count • Max MMAC hop count = 3 – Too many DO hops increases probability of failure of RTS delivery – Too many DO hops typically not necessary to establish DD link C B A 3/19/2018 D DO neighbors E DD neighbors F G 53

MMAC - Concerns • High traffic – lower probability of RTS delivery • Multi-hop RTS may not reach DD neighbor due to deafness or collision • No more than 3 DO links is used for each DD link • Neighbor discovery overheads may offset the advantages of MMAC 3/19/2018 54

Aligned Routes in Grid 3/19/2018 55

Unaligned Routes in Grid 3/19/2018 56

“Random” Topology 3/19/2018 57

“Random” Topology: delay 3/19/2018 58

Mobility • Nodes moving out of beam coverage in order of packet-transmission-time – Low probability • Antenna handoff required – – MAC layer can cache active antenna beam On disconnection, scan over adjacent beams Cache updates possible using promiscuous mode Evaluated in [Roy. Choudhury 02_Tech. Report] 3/19/2018 59

Broadcast • Several definitions of “broadcast” – Broadcast region may be a sector, multiple sectors Broadcast Region A – Omni broadcast may be performed through sweeping antenna over all directions [Roy. Choudhury 02_Tech. Report] 3/19/2018 60

References 1. 2. 3. 4. 5. 6. Basagni, M. Conti, S. Giordano, I. Stojmenovic, eds, Mobile Ad Hoc Networking, IEEE Press/Wiley, August 2004. M. Takai, et al. , “Directional virtual carrier sensing for directional antennas in mobile ad hoc networks”, ACM Mobi. Hoc 2002, pp 39 -46, June 2002 R. R. Choudhury, X. Yang, N. H. Vaidya, and R. Ramanathan, “Using directional antennas for medium access control in ad hoc networks”, MOBICOM 2002, pp 59 -70, September 2002 N. S. Fahmy, T. D. Todd and V. Kezys, “Ad hoc networks with smart antennas using IEEE 802. 11 -based protocols”, IEEE ICC 2002, pp 3144 -3148, May 2002 Y-B Ko, V. Shankarkumar and N. H. Vaidya, “Medium access control protocols using directional antennas in ad hoc networks”, IEEE INFOCOM 2000, pp 13 -21 Choudhury, R. R. and Vaidya, N. H. , “Deafness: a MAC problem in ad hoc networks when using directional antennas” ICNP 2004, Proceedings of the 12 th IEEE International Conference on Network Protocols, pp: 283 - 292 , 2004 61