Wireless Networked Systems
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1 Wireless Networked Systems CS 795/895 - Spring 2013 Lec #4: Medium Access Control Power/CarrierSense Control, Multi-Channel, Directional Antenna Tamer Nadeem Dept. of Computer Science
2 Power & Carrier Sense Control Page 2 Spring 2013 CS 795/895 - Wireless Networked Systems
3 Transmission Range The range within which a packet is successfully received if no interference transmitter Dependent on transmission power and attenuation Page 3 Spring 2013 CS 795/895 - Wireless Networked Systems
4 Carrier Sensing Range The range within which a transmitter triggers carrier sense detection transmitter Dependent on antenna sensitivity Page 4 Spring 2013 CS 795/895 - Wireless Networked Systems
5 Interference Range The range within which receivers will be interfered with and suffer a loss receiver Interference range bigger than transmission range? Page 5 Spring 2013 CS 795/895 - Wireless Networked Systems
6 Hidden Nodes Any node within interference range of a receiver can potentially be a hidden node Hidden when not captured by physical carrier sensing (at transmitter) c c in Interference range of b AND out of carrier sense range of a a b Page 6 Spring 2013 CS 795/895 - Wireless Networked Systems
7 Hidden Nodes Any node within interference range of a receiver can potentially be a hidden node Hidden when not captured by virtual carrier sensing (RTS/CTS transmission range) c c in Interference range of b AND out of carrier sense range of a AND out of transmission range of b a b Page 7 Spring 2013 CS 795/895 - Wireless Networked Systems
8 Exposed Nodes Any node within carrier sense range of transmitter and out of interference range of receiver Prevents simultaneous transmissions Reduction in Spatial Reuse d c a b c in carrier sense range of a AND out of interference range of b Page 8 Spring 2013 CS 795/895 - Wireless Networked Systems
9 Network Capacity Carrier sense range affects hidden and exposed node problems Hidden and exposed node problems have opposing effects on system throughput Smaller carrier sense range Larger High number of hidden nodes Low number of exposed nodes Low number of hidden nodes High number of exposed nodes High collision probability High spatial reuse probability Low collision probability Low spatial reuse probability Page 9 Spring 2013 CS 795/895 - Wireless Networked Systems
10 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 I n e f f i c i e n c y : Page 10 Spring 2013 CS 795/895 - Wireless Networked Systems
11 Physical Carrier Sensing - revisited Physical carrier sensing Assume channel idle if carrier less than threshold Current implementations: Fixed threshold à arbitrary point of performance What if we make it dynamic à Optimal operation point to improve the performance of the system Page 11 Spring 2013 CS 795/895 - Wireless Networked Systems
12 Dynamic Physical Carrier Sensing Adaptive threshold as a function of transmitter-receiver distance and receive power Two solutions: [Jing et. al. 04] Unique carrier sensing threshold throughout the network Requires info exchange [Veeravali et. al. 04] Heterogeneous carrier sensing thresholds in the network Local decisions Fairness could be an issue Practical issues: Limits on threshold antenna sensitivity Estimation and measurements Page 12 Spring 2013 CS 795/895 - Wireless Networked Systems
13 Benefits of Power Control Control Factor for Power Control D SINR RX = S I + N > γ i i Tx Rx Control Factor for CST tuning How many power levels are needed to achieve the same control granularity as tuning the carrier sense threshold? Page 13 Spring 2013 CS 795/895 - Wireless Networked Systems
14 Transmission Power Control Choose transmit power levels to maximize physical spatial reuse Tune MAC to ensure nodes transmit simultaneously when possible Spatial reuse = network capacity / link capacity Client 2 AP 1 AP 2 Concurrent transmissions increase spa4al reuse Client 2 AP 2 Client 1 AP 1 Client 1 Spa$al Reuse = 1 Spa$al Reuse = 2 14 Page 14 Spring 2013 CS 795/895 - Wireless Networked Systems
15 Transmission Power Control in Practice For simple scenario à easy to compute optimal transmit power May or may not enable simultaneous transmit AP 1 d 12 AP 2 Protocol builds on iterative pair-wise optimization d 11 d 22 d 21 Client 2 Adjusting transmit power à requires adjusting carrier sense thresholds Client 1 15 Page 15 Spring 2013 CS 795/895 - Wireless Networked Systems
16 Multi-Channel Networking Page 16 Spring 2013 CS 795/895 - Wireless Networked Systems
17 802.11b Channels Splits 2.4 GHz band into 11 channels of 22 MHz each Channels 1, 6 and 11 don t overlap IEEE b also operates in the highly populated 2.4 GHz ISM band (2.40 to GHz), which provides only 83 MHz of spectrum to accommodate a variety of other products, including cordless phones, microwave ovens, other WLANs, and WPANs such as Bluetooth Maximum allowable radiated emission limited to 100 mw Page 17 Spring 2013 CS 795/895 - Wireless Networked Systems
18 802.11a Channels Page 18 Spring 2013 CS 795/895 - Wireless Networked Systems
19 802.11g/n Channels Page 19 Spring 2013 CS 795/895 - Wireless Networked Systems
20 Summary Table of IEEE (Wi-Fi) Family Page 20 Spring 2013 CS 795/895 - Wireless Networked Systems
21 Proper Usage of Channels Page 21 Spring 2013 CS 795/895 - Wireless Networked Systems
22 Channel Assignment Page 22 Spring 2013 CS 795/895 - Wireless Networked Systems
23 Channel Assignment Page 23 Spring 2013 CS 795/895 - Wireless Networked Systems
24 Issues - Fairness Page 24 Spring 2013 CS 795/895 - Wireless Networked Systems
25 Issues - Utilization Transmitter-receiver pairs use independent channels that don t overlap to avoid interference. Realistically, transmitter power output is NOT uniform at all frequencies of the channel. Channel A Channel B Channel C Channel D Fixed Block of Radio Frequency Spectrum PROBLEM: Transmitted power of some freqs. < max. permissible limit Results in lower channel capacity and inefficient usage of the spectrum Power Channel A Channel B Channel C Channel D Real Usage Wastage of spectrum Page 25 Spring 2013 CS 795/895 - Wireless Networked Systems
26 Multi-Channel Communication Multiple Channels available in IEEE channels in b 12 channels in a Utilizing multiple channels can improve throughput Allow simultaneous transmissions 1 1 defer Single channel 2 Multiple Channels Page 26 Spring 2013 CS 795/895 - Wireless Networked Systems
27 Problem Statement Using k channels does not translate into throughput improvement by a factor of k Nodes listening on different channels cannot talk to each other 1 2 Constraint: Each node has only a single transceiver Capable of listening to one channel at a time Goal: Design a MAC protocol that utilizes multiple channels to improve overall performance Modify DCF to work in multi-channel environment Page 27 Spring 2013 CS 795/895 - Wireless Networked Systems
28 Multi-Channel Hidden Terminals Consider the following naïve protocol Static channel assignment (based on node ID) Communication takes place on receiver s channel Sender switches its channel to receiver s channel before transmitting Page 28 Spring 2013 CS 795/895 - Wireless Networked Systems
29 Multi-Channel Hidden Terminals Channel 1 Channel 2 RTS A B C A sends RTS Page 29 Spring 2013 CS 795/895 - Wireless Networked Systems
30 Multi-Channel Hidden Terminals Channel 1 Channel 2 CTS A B C B sends CTS C does not hear CTS because C is listening on channel 2 Page 30 Spring 2013 CS 795/895 - Wireless Networked Systems
31 Multi-Channel Hidden Terminals Channel 1 Channel 2 DATA RTS A B C C switches to channel 1 and transmits RTS Collision occurs at B Page 31 Spring 2013 CS 795/895 - Wireless Networked Systems
32 Nasipuri s Protocol Assumes N transceivers per host Capable of listening to all channels simultaneously Sender searches for an idle channel and transmits on the channel [Nasipuri99WCNC] Extensions: channel selection based on channel condition on the receiver side [Nasipuri00VTC] Disadvantage: High hardware cost Page 32 Spring 2013 CS 795/895 - Wireless Networked Systems
33 Wu s Protocol [Wu00ISPAN] Assumes 2 transceivers per host One transceiver always listens on control channel Negotiate channels using RTS/CTS/RES RTS/CTS/RES packets sent on control channel Sender includes preferred channels in RTS Receiver decides a channel and includes in CTS Sender transmits RES (Reservation) Sender sends DATA on the selected data channel Page 33 Spring 2013 CS 795/895 - Wireless Networked Systems
34 Wu s Protocol (cont.) Advantage No synchronization required Disadvantage Each host must have 2 transceivers Per-packet channel switching can be expensive Control channel bandwidth is an issue Too small: control channel becomes a bottleneck Too large: waste of bandwidth Optimal control channel bandwidth depends on traffic load, but difficult to dynamically adapt Page 34 Spring 2013 CS 795/895 - Wireless Networked Systems
35 Multi-Channel MAC (MMAC) Protocol Assumptions Each node is equipped with a single transceiver The transceiver is capable of switching channels Channel switching delay is approximately 250us Per-packet switching not recommended Occasional channel switching not to expensive Multi-hop synchronization is achieved by other means Page 35 Spring 2013 CS 795/895 - Wireless Networked Systems
36 Multi-Channel MAC (MMAC) Divide time into beacon intervals At the beginning of each beacon interval, all nodes must listen to a predefined common channel for a fixed duration of time (ATIM window) Nodes negotiate channels using ATIM messages Nodes switch to selected channels after ATIM window for the rest of the beacon interval Page 36 Spring 2013 CS 795/895 - Wireless Networked Systems
37 Preferred Channel List (PCL) Each node maintains PCL Records usage of channels inside the transmission range High preference (HIGH) Already selected for the current beacon interval Medium preference (MID) No other vicinity node has selected this channel Low preference (LOW) This channel has been chosen by vicinity nodes Count number of nodes that selected this channel to break ties Page 37 Spring 2013 CS 795/895 - Wireless Networked Systems
38 Channel Negotiation In ATIM window, sender transmits ATIM to the receiver Sender includes its PCL in the ATIM packet Receiver selects a channel based on sender s PCL and its own PCL Order of preference: HIGH > MID > LOW Tie breaker: Receiver s PCL has higher priority For LOW channels: channels with smaller count have higher priority Receiver sends ATIM-ACK to sender including the selected channel Sender sends ATIM-RES to notify its neighbors of the selected channel Page 38 Spring 2013 CS 795/895 - Wireless Networked Systems
39 Channel Negotiation Common Channel Selected Channel A Beacon B C D Time ATIM Window Beacon Interval Page 39 Spring 2013 CS 795/895 - Wireless Networked Systems
40 Channel Negotiation Common Channel Selected Channel A ATIM ATIM- RES(1) Beacon B ATIM- ACK(1) C D Time ATIM Window Beacon Interval Page 40 Spring 2013 CS 795/895 - Wireless Networked Systems
41 Channel Negotiation Common Channel Selected Channel A ATIM ATIM- RES(1) Beacon B ATIM- ACK(1) C ATIM- ACK(2) D ATIM ATIM- RES(2) Time ATIM Window Beacon Interval Page 41 Spring 2013 CS 795/895 - Wireless Networked Systems
42 Channel Negotiation Common Channel Selected Channel A ATIM ATIM- RES(1) RTS DATA Channel 1 Beacon B ATIM- ACK(1) CTS ACK Channel 1 C ATIM- ACK(2) CTS ACK Channel 2 D ATIM ATIM- RES(2) RTS DATA Channel 2 Time ATIM Window Beacon Interval Page 42 Spring 2013 CS 795/895 - Wireless Networked Systems
43 Performance Evaluation ns-2 simulator Transmission rate: 2Mbps Transmission range: 250m Traffic type: Constant Bit Rate (CBR) Beacon interval: 100ms Packet size: 512 bytes ATIM window size: 20ms Default number of channels: 3 channels Compared protocols : IEEE single channel protocol DCA: Wu s protocol MMAC: Proposed protocol Page 43 Spring 2013 CS 795/895 - Wireless Networked Systems
44 Wireless LAN - Throughput Aggregate Throughput (Kbps) MMAC DCA nodes 64 nodes MMAC DCA Packet arrival rate per flow (packets/sec) Packet arrival rate per flow (packets/sec) MMAC shows higher throughput than DCA and Page 44 Spring 2013 CS 795/895 - Wireless Networked Systems
45 Throughput of DCA and MMAC (Wireless LAN) Aggregate Throughput (Kbps) channels 3 channels Packet arrival rate per flow (packets/sec) channels 3 channels Packet arrival rate per flow (packets/sec) DCA MMAC MMAC shows higher throughput compared to DCA Page 45 Spring 2013 CS 795/895 - Wireless Networked Systems
46 Directional Antenna Page 46 Spring 2013 CS 795/895 - Wireless Networked Systems
47 Omni-directional Antennas Silenced Node S D B A C Page 47 Spring 2013 CS 795/895 - Wireless Networked Systems
48 Directional Antennas S D B Not possible using Omni A C Page 48 Spring 2013 CS 795/895 - Wireless Networked Systems
49 Comparison Issues Omni Directional Spatial Reuse Low High Connectivity Low High Interference Omni Directional Cost & Complexity Low High Page 49 Spring 2013 CS 795/895 - Wireless Networked Systems
50 Issues MAC Proposals differ based on How RTS/CTS transmitted (omni, directional) Transmission range of directional antennas Channel access schemes Omni or directional NAVs Gain of directional antennas is equal to the gain of omnidirectional antennas? Page 50 Spring 2013 CS 795/895 - Wireless Networked Systems
51 Antenna Model Two Operation modes : Omni & Directional A node may operate in any one mode at any given time Omni Mode: Omni Gain = G o Idle node stays in Omni mode. Directional Mode: Capable of beamforming in specified direction Directional Gain = G d (G d > G o ) Page 51 Spring 2013 CS 795/895 - Wireless Networked Systems
52 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 Receive Beam Transmit Beam B 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 Page 52 Spring 2013 CS 795/895 - Wireless Networked Systems
53 Directional Communication Receive Beam Transmit 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 Page 53 Spring 2013 CS 795/895 - Wireless Networked Systems
54 Advantage of Directional Antenna Spatial reuse Possible to carry out multiple simultaneous transmissions in the same neighborhood Higher gain Greater transmission range than omni-directional Two distant nodes can communicate with a single hop Routes with fewer hops Page 54 Spring 2013 CS 795/895 - Wireless Networked Systems
55 DCF? Physical Physical Carrier Carrier Sense Sensing Virtual Carrier Sensing Page 55 Spring 2013 CS 795/895 - Wireless Networked Systems
56 Basic DMAC Protocol Channel Reservation A node listens omni-directionally when idle Sender transmits Directional-RTS (DRTS) using specified transceiver profile Physical carrier sense Virtual carrier sense with Directional NAV RTS received in Omni mode (only DO links used) Receiver sends Directional-CTS (DCTS) DATA,ACK transmitted and received directionally Page 56 Spring 2013 CS 795/895 - Wireless Networked Systems
57 Basic DMAC Protocol Directional NAV (DNAV) Table Nodes overhearing RTS or CTS set up directional NAV (DNAV) for that Direction of Arrival (DoA) Tables that keeps track of the directions towards which node must not initiate a transmission New transmission initiated only if direction of transmission does not overlap with DNAV, i.e. if (θ > 0) RTS H ε = 2ß + Θ E 2*ß ε θ DNAV If Θ> 0, New transmission can be initiated C CTS B Page 57 Spring 2013 CS 795/895 - Wireless Networked Systems
58 Problems with Basic DMAC (1/4) Hidden Terminal Problems due to asymmetry in gain A does not get RTS/CTS from C/B A RTS B Data C Page 58 Spring 2013 CS 795/895 - Wireless Networked Systems
59 Problems with Basic DMAC (2/4) Hidden Terminal Problems due to unheard RTS/CTS D A B C Page 59 Spring 2013 CS 795/895 - Wireless Networked Systems
60 Problems with Basic DMAC (3/4) Shape of Silence Regions Region of interference for omnidirectional transmission Region of interference for directional transmission Page 60 Spring 2013 CS 795/895 - Wireless Networked Systems
61 Problems with Basic DMAC (4/4) Deafness Z RTS A B DATA RTS X X does not know node A is busy. X keeps transmitting RTSs to node A Page 61 Spring 2013 CS 795/895 - Wireless Networked Systems
62 Multi-Hop MAC Protocol (MMAC) Attempts to exploit the extended transmission range Make Use of DD Links Direction-Direction (DD) Neighbor A C A and C can communication each other directly Page 62 Spring 2013 CS 795/895 - Wireless Networked Systems
63 Multi-Hop MAC Protocol (MMAC) Protocol Description : Multi-Hop RTS Based on Basic DMAC protocol DO neighbors B RTS C DD neighbors A D G F T DATA R S 63/26 Page 63 Spring 2013 CS 795/895 - Wireless Networked Systems
64 Multi-Hop MAC Protocol (MMAC) Channel Reservation Send Forwarding RTS with Profile of node F Fowarding RTS B C A D G F T DATA R S 64/26 Page 64 Spring 2013 CS 795/895 - Wireless Networked Systems
65 Spatial Fairness of Different nodes have different neighbors à experience different contention environments. Nodes at the overlapping coverage area of the WLANs suffer from lower throughput Page 65 Spring 2013 CS 795/895 - Wireless Networked Systems
66 Use of Directional Antenna Directional antenna is a well known method to reduce the interference and to increase the range and the capacity for wireless networks. Fairness relieved through interference reduction Page 66 Spring 2013 CS 795/895 - Wireless Networked Systems
67 S-MAC: Sectorized Antenna Dedicated Rx per sector/antenna #3 #2 Tx can switch to different antennas Self-interference cancellation between Tx and Rx in different sectors #4 r N #1 s Consistent channel information at different nodes #5 R #8 No hidden nodes or deafness problem #6 #7 I Addresses the hidden node problem and the deafness problem by continuously monitoring the channel in all directions (sectors) at all time Page 67 Spring 2013 CS 795/895 - Wireless Networked Systems
68 Operation of S-MAC (example I) D H DMAC Hidden Node due to asymmetric gain A RTS CTS RTS E B F G C Collision Page 68 Spring 2013 CS 795/895 - Wireless Networked Systems
69 Operation of S-MAC (example I) D H SMAC: Hidden Node due to asymmetric gain avoidance A CTS from F rcvd RTS not sent by A CTS RTS E B F G C Page 69 Spring 2013 CS 795/895 - Wireless Networked Systems
70 Operation of S-MAC (example II) D H Hidden Node due to unheard RTS/CTS avoidance A CTS RTS E B F G E waits for B-F to finish C Page 70 Spring 2013 CS 795/895 - Wireless Networked Systems
71 Operation of S-MAC (example II) Deafness Prevention D H A E B F G E is aware C is Transmitting C Page 71 Spring 2013 CS 795/895 - Wireless Networked Systems
72 Performance Evaluation NS-2 simulator is used b with transmission rate 11 Mbps. Transmission range of 250m and carrier sensing range is 550m. All nodes are stationary. UDP traffics packets with average packet size 1000 bytes. Four way handshake (RTS/CTS/DATA/ACK) is used. Simulated duration of 50 seconds and each point is averaged from 5 independent runs. Page 72 Spring 2013 CS 795/895 - Wireless Networked Systems
73 Simulation Scenarios Infrastructure mode is used. APs are upgraded with S-MAC of 4 sectors (1 Tx & 4 Rx). All STAs still use omni directional antenna (regular MAC). Network of 2x2 grid of overlapping Each AP has 40 clients that are distributed uniformly in its coverage area. Page 73 Spring 2013 CS 795/895 - Wireless Networked Systems
74 Simulation Results Improvement arises from reduced interference with sector antennas and reduced collision from the S-MAC protocol. Total throughput does not change significantly as the number of sectors increases from 2 to 4. No significant change was found with different antenna orientations. Page 74 Spring 2013 CS 795/895 - Wireless Networked Systems
75 Questions Page 75 Spring 2013 CS 795/895 - Wireless Networked Systems
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