Multiple Access
Media Access Both are on shared media. Then, what s really the problem? 2
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 3
The Trivial Solution Transmit and pray Plenty of collisions --> poor throughput at high load A B C collision 4
The Simple Fix Transmit and pray Plenty of collisions --> poor throughput at high load Listen before you talk Carrier sense multiple access (CSMA) Defer transmission when signal on channel Don t transmit A B C Can collisions still occur? 5
CSMA collisions 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 6
Collision Detection in Wired Media
CSMA/CD (Collision Detection) Keep listening to channel While transmitting If (Transmitted_Signal!= Sensed_Signal) Sender knows it s a Collision ABORT 8
2 Observations on CSMA/CD Transmitter can send/listen concurrently If (Transmitted - Sensed = null)? Then success The signal is identical at Tx and Rx Non-dispersive The TRANSMITTER can detect if and when collision occurs 9
Unfortunately Both observations do not hold for wireless! Because 10
Wireless Medium Access Control A B C D Signal power Distance 11
Wireless Media Disperse Energy A cannot send and listen in parallel A B C D Signal power Signal not same at different locations Distance 12
Collision Detection Difficult D A B C Signal reception based on SINR Transmitter can only hear itself Cannot determine signal quality at receiver 13
14 Calculating SINR A B C α α CB C transmit C B AB A A B d P I d P SoI N Noise I Interference SoI SignalOfInterest SINR transmit = = + = ) ( ) ( ) ( α α CB C transmit AB A A B d P N d P SINR transmit + = D
Red signal >> Blue signal Red < Blue = collision X A B C D Signal power Distance 15
Important: C has not heard A, but can interfere at receiver B C is the hidden terminal to A X A B C D Signal power Distance 16
Important: X has heard A, but should not defer transmission to Y Y X is the exposed terminal to A X A B C D Signal power Critical fact #1: Interference is receiver driven while CSMA is sender driven Distance 17
So, how do we cope with Hidden/Exposed Terminals? 18
How to prevent C from trasmitting? X A B C D Signal power Distance 19
An Idea! A B C D A node decides to intelligently choose a Carrier sensing threshold (T) The node senses channel If signal > T, then node does not transmit If signal < T, then transmit Possible to guarantee no collisions? 20
An Idea! X A B C D Signal power Distance 21
A Project Idea! Will this solve the wireless MAC problem? Do not transmit in this region X A B C D Signal power T Distance 22
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 23
IEEE 802.11 RTS = Request To Send M Y CTS = Clear To Send S RTS CTS D X K 24
IEEE 802.11 silenced M Y S Data D silenced ACK X silenced K silenced 25
MACA variant: DFWMAC in IEEE802.11 sender receiver idle idle ACK RxBusy time-out NAK; RTS packet ready to send; RTS wait for the right to send CTS; data time-out; RTS data; ACK time-out data; NAK RTS; CTS wait for ACK wait for data ACK: positive acknowledgement NAK: negative acknowledgement RxBusy: receiver busy RTS; RxBusy
802.11 CSMA/CA CSMA: Listen before transmit Collision avoidance when transmitting a packet, choose a backoff interval in the range [0, CW] CW is contention window Count down the backoff interval when medium is idle count-down is suspended if medium becomes busy Transmit when backoff interval reaches 0
Congestion Avoidance: Example busy B1 = 25 wait B1 = 5 data B2 = 20 busy data B2 = 15 wait B2 = 10 B1 and B2 are backoff intervals at nodes 1 and 2
802.11 RTS/CTS + ACK Sender sends RTS with NAV (Network allocation Vector, i.e. reservation parameter that determines amount of time the data packet needs the medium) Receiver acknowledges via CTS (if ready to receive) CTS reserves channel for sender, notifying possibly hidden stations Sender can now send data at once, acknowledgement via ACK Other stations store NAV distributed via RTS and CTS sender DIFS RTS data receiver SIFS CTS SIFS SIFS ACK other stations NAV (RTS) NAV (CTS) defer access DIFS new contention data t
Fragmentation sender receiver DIFS RTS SIFS CTS SIFS frag 1 SIFS ACK SIFS 1 frag 2 SIFS ACK2 other stations NAV (RTS) NAV (CTS) NAV (frag 1 ) NAV (ACK 1 ) DIFS contention data t
802.11 Steps All backlogged nodes choose a random number R = rand (0, CW_min) Each node counts down R Continue carrier sensing while counting down Once carrier busy, freeze countdown Whoever reaches ZERO transmits RTS Neighbors freeze countdown, decode RTS RTS contains (CTS + DATA + ACK) duration = T_comm Neighbors set NAV = T_comm Remains silent for NAV time 31
802.11 Steps Receiver replies with CTS Also contains (DATA + ACK) duration. Neighbors update NAV again Tx sends DATA, Rx acknowledges with ACK After ACK, everyone initiates remaining countdown Tx chooses new R = rand (0, CW_min) If RTS or DATA collides (i.e., no CTS/ACK returns) Indicates collision RTS chooses new random no. R1 = rand (0, 2*CW_min) Note Exponential Backoff Ri = rand (0, 2^i * CW_min) Once successful transmission, reset to rand(0, CW_min) 32
But is that enough? 33
RTS/CTS Does it solve hidden terminals? Assuming carrier sensing zone = communication zone CTS E RTS F A B C CTS D E does not receive CTS successfully Can later initiate transmission to D. Hidden terminal problem remains. 34
Hidden Terminal Problem How about increasing carrier sense range?? E will defer on sensing carrier no collision!!! CTS E RTS F A B C Data D 35
Hidden Terminal Problem But what if barriers/obstructions?? E doesn t hear C Carrier sensing does not help CTS E RTS F A B C Data D 36
Exposed Terminal B should be able to transmit to A RTS prevents this RTS CTS E A B C D 37
Exposed Terminal B should be able to transmit to A Carrier sensing makes the situation worse RTS CTS E A B C D 38
Another Problem Multi-Channel Hidden Terminals Channel 1 Channel 2 RTS A B C A sends RTS Slides Courtesy of So and Vaidya
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
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
Thoughts! 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 The search for the best MAC protocol is still on. However, 802.11 is being optimized too. Thus, wireless MAC research still alive 42
WLAN - IEEE 802.11
Mobile Communication Technology according to IEEE Local wireless networks WLAN 802.11 WiFi 802.11a 802.11b 802.11h 802.11g 802.11i/e/ /n/ /z/aa Personal wireless nw WPAN 802.15 802.15.1 Bluetooth ZigBee 802.15.4 802.15.2 802.15.4a/b/c/d/e/f/g 802.15.5,.6 (WBAN) 802.15.3 802.15.3b/c Wireless distribution networks WMAN 802.16 (Broadband Wireless Access) WiMAX + Mobility [802.20 (Mobile Broadband Wireless Access)] 802.16e (addition to.16 for mobile devices)
IEEE 802.11 Requirements Wi-Fi often used by the public as a synonym for IEEE 802.11-wireless LAN (WLAN). Design for small coverage (e.g. office, home) Low/no mobility High data-rate applications Ability to integrate real time applications and non-real-time applications Use un-licensed spectrum
802.11: Infrastructure STA 1 ESS 802.11 LAN BSS 1 Access Point BSS 2 Portal Distribution System Access Point 802.x LAN STA 2 802.11 LAN STA 3 Architecture similar to cellular networks station (STA) terminal with access mechanisms to the wireless medium and radio contact to the access point access point (AP) station integrated into the wireless LAN and the distribution system basic service set (BSS) group of stations using the same AP portal bridge to other (wired) networks distribution system interconnection network to form one logical network (EES: Extended Service Set) based on several BSS
802.11 - Architecture of An Ad-hoc Network 802.11 LAN Direct communication within a limited range STA 1 IBSS 1 STA 3 Station (STA): terminal with access mechanisms to the wireless medium STA 2 Independent Basic Service Set (IBSS): group of stations using the same radio frequency IBSS 2 STA 5 STA 4 802.11 LAN
IEEE Standard 802.11 mobile terminal fixed terminal application TCP IP access point infrastructure network application TCP IP LLC LLC LLC 802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC 802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY
802.11 - Layers and functions MAC access mechanisms, fragmentation, encryption MAC Management synchronization, roaming, MIB, power management PLCP Physical Layer Convergence Protocol clear channel assessment signal (carrier sense) PMD Physical Medium Dependent modulation, coding PHY Management channel selection, MIB Station Management coordination of all management functions PHY DLC LLC MAC PLCP PMD MAC Management PHY Management Station Management
IEEE 802.11 Physical Layer Family of IEEE 802.11 standards: unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz 300 MHz 5.15-5.35 GHz 5.725-5.825 GHz and 802.11b/g 802.11a
The IEEE 802.11 Family Protocol Release Data Freq. Rate (typical) Rate (max) Legacy 1997 2.4 GHz 1 Mbps 2Mbps? Range (indoor) 802.11a 1999 5 GHz 25 Mbps 54 Mbps 802.11b 1999 2.4 GHz 6.5 Mbps 11 Mbps 802.11g 2003 2.4 GHz 25 Mbps 54 Mbps 802.11n 2008 2.4/5 GHz 200 Mbps 540 Mbps ~30 m ~30 m ~30 m ~50 m
802.11a Modulation Use OFDM to divide each physical channel (20 MHz) into 52 subcarriers (312.5 KHz each) 48 data, 4 pilot Adaptive modulation BPSK: 6, 9 Mbps QPSK: 12, 18 Mbps 16-QAM: 24, 36 Mbps 64-QAM: 48, 54 Mbps
802.11 - MAC layer I - DFWMAC Traffic services Asynchronous Data Service (mandatory) exchange of data packets based on best-effort support of broadcast and multicast Time-Bounded Service (optional) implemented using PCF (Point Coordination Function) Access methods DFWMAC-DCF CSMA/CA (mandatory) collision avoidance via randomized back-off mechanism minimum distance between consecutive packets ACK packet for acknowledgements (not for broadcasts) DFWMAC-DCF w/ RTS/CTS (optional) Distributed Foundation Wireless MAC avoids hidden terminal problem DFWMAC- PCF (optional) access point polls terminals according to a list
802.11 - MAC layer II Priorities defined through different inter frame spaces no guaranteed, hard priorities SIFS (Short Inter Frame Spacing) highest priority, for ACK, CTS, polling response PIFS (PCF IFS) medium priority, for time-bounded service using PCF DIFS (DCF, Distributed Coordination Function IFS) lowest priority, for asynchronous data service DIFS DIFS medium busy PIFS SIFS contention next frame direct access if medium is free DIFS t
802.11 Inter Frame Spacing 802.11b 802.11a 802.11g asifstime 10 usec 16 usec 10 usec aslottime 20 usec 9 usec 20 usec (mixed); 9 usec (g only) adiftime (2xSlot+SIFS) 50 usec 34 usec 50 usec; 28 usec 55
802.11 - Frame format Types control frames, management frames, data frames Sequence numbers important against duplicated frames due to lost ACKs Addresses receiver, transmitter (physical), BSS identifier, sender (logical) Miscellaneous sending time, checksum, frame control, data bytes 2 2 6 6 6 6 Duration/ Address Address Address Sequence ID 1 2 3 Control Frame Control Protocol version Type Subtype To DS More Frag Retry Power Mgmt 2 0-2312 4 Address Data 4 bits 2 2 4 1 1 1 1 1 1 1 1 From DS More Data WEP Order CRC
MAC address format scenario to DS from address 1 address 2 address 3 address 4 DS ad-hoc network 0 0 DA SA BSSID - infrastructure 0 1 DA BSSID SA - network, from AP infrastructure 1 0 BSSID SA DA - network, to AP infrastructure network, within DS 1 1 RA TA DA SA DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address
Special Frames: ACK, RTS, CTS Acknowledgement ACK bytes 2 2 6 4 Frame Duration Receiver Control Address CRC Request To Send RTS bytes 2 2 6 6 4 Frame Duration Receiver Transmitter CRC Control Address Address Clear To Send CTS bytes 2 2 6 4 Frame Duration Receiver Control Address CRC
Example: 802.11b Throughout Suppose TCP with 1460 bytes payload 802.11b data frame size (not including preamble): 1536 bytes TCP ACK data frame size (not including preamble): 76 bytes 802.11b ACK frame size 14 bytes Suppose 802.11b at the highest rate 8 bits per symbol 1.375 Msps Q: What is TCP/802.11b throughput? http://www.andrews.edu/~swensen/wifi%20throughput.pdf
Example: 802.11b Throughout Each transaction requires 2,084 µs. At that duration, 479 exchanges can complete per second. With a TCP payload of 1,460 bytes per exchange, the throughput is 5.7 Mbps.
Wireless Ad-Hoc Networks
Wireless Ad-hoc Networks Network without infrastructure Use components of participants for networking Examples Single-hop: All partners max. one hop apart Bluetooth piconet, PDAs in a room, gaming devices Multi-hop: Cover larger distances, circumvent obstacles Bluetooth scatternet, TETRA police network, car-to-car networks Internet: MANET (Mobile Ad-hoc Networking) group
Manet: Mobile Ad-hoc Networking Mobile Router Manet Mobile Devices Mobile IP, DHCP Fixed Network Router End system
Routing Goal: determine good path (sequence of routers) thru network from source to dest Global information: all routers have complete Topology, link cost info link state algorithm Decentralized: router knows physically-connected neighbors, link costs to Neighbors routers exchange of info with neighbors Distance vector routing: the routing table is constructed from a distance vector at each node routing table (at each host): the next hop for each destination in the network
Distance Vector Routing Distance vector at node E D (E,D,C) c(e,d) + shortest(d,c) ==2+2 = 4 D (E,D,A) c(e,d) + shortest(d,a) ==2+3 = 5 D (E,B,A) c(e,b) + shortest(b,a) ==8+6 = 14
Routing Problem Highly dynamic network topology Device mobility plus varying channel quality Separation and merging of networks possible Asymmetric connections possible N 7 N 6 N 6 N 7 N 1 N 1 N 2 N 3 N 3 N 2 N 4 N 5 good link weak link N 4 N 5 time = t 1 time = t 2
Traditional routing algorithms Distance Vector periodic exchange of messages with all physical neighbors that contain information about who can be reached at what distance selection of the shortest path if several paths available Link State periodic notification of all routers about the current state of all physical links router get a complete picture of the network Example ARPA packet radio network (1973), DV-Routing every 7.5s exchange of routing tables including link quality updating of tables also by reception of packets routing problems solved with limited flooding
Routing in Ad-hoc Networks THE big topic in many research projects Far more than 50 different proposals exist The most simplest one: Flooding! Reasons Classical approaches from fixed networks fail Very slow convergence, large overhead High dynamicity, low bandwidth, low computing power Metrics for routing Minimal Number of nodes, loss rate, delay, congestion, interference Maximal Stability of the logical network, battery run-time, time of connectivity
Problems of Traditional Routing Algorithms Dynamic of the topology frequent changes of connections, connection quality, participants Limited performance of mobile systems periodic updates of routing tables need energy without contributing to the transmission of user data, sleep modes difficult to realize limited bandwidth of the system is reduced even more due to the exchange of routing information links can be asymmetric, i.e., they can have a direction dependent transmission quality
Distance Vector Routing Early work on demand version: AODV Expansion of distance vector routing Sequence numbers for all routing updates assures in-order execution of all updates avoids loops and inconsistencies Decrease of update frequency store time between first and best announcement of a path inhibit update if it seems to be unstable (based on the stored time values)
Distance Vector Routing B A Link 1 Link 2 C Link 3 Link 5 Link 4 Destination Link Hop count A 4 2 B 4 2 C 4 1 D local 0 E 6 1 E D Link 6 Consider D Initially nothing in routing table. When it receives an update from C and E, it notes that these nodes are one hop away. Subsequent route updates allow D to form its routing table.
Destination Sequenced Distance Vector (DSDV) A Link 2 Broken Link 4 D Link 1 C Link 6 Broken B Link 3 Link 5 Network partitions into two isolated islands Disadvantage of Distance Vector Routing is formation of loops. Let Link 2 break, and after some time let link 3 break. E
Destination Sequenced Distance Vector (DSDV) After Link 2 is broken, Node A routes packets to C, D, and E through Node B. Node B detects that Link 3 is broken. It sets the distance to nodes C, D and E to be infinity. Let Node A in the meantime transmit a update saying that it can reach nodes C, D, and E with the appropriate costs that were existing before i.e., via Node B. Node B thinks it can route packets to C, D, and E via Node A. Node A thinks it can route packets to C, D, and E, via Node B. A routing loop is formed Counting to Infinity problem. Methods that were proposed to overcome this
Destination Sequenced Distance Vector (DSDV) A D Lin k 1 BrokenLin k 2 Broken C Lin k 4 Lin k 6 B Lin k 3 Lin k 5 Network partitions into two isolated islands Node A s update is stale!!! Sequence number indicated for nodes C,D, and E is lower than the sequence number maintained at B. Looping avoided! E Each routing table entry is tagged with a sequence number that is originated by the corresponding destination node in that entry.
Dynamic Source Routing I Split routing into discovering a path and maintaining a path Discover a path only if a path for sending packets to a certain destination is needed and no path is currently available Maintaining a path only while the path is in use one has to make sure that it can be used continuously No periodic updates needed!
Dynamic Source Routing II Path discovery broadcast a packet with destination address and unique ID if a station receives a broadcast packet if the station is the receiver (i.e., has the correct destination address) then return the packet to the sender (path was collected in the packet) if the packet has already been received earlier (identified via ID) then discard the packet otherwise, append own address and broadcast packet sender receives packet with the current path (address list) Optimizations limit broadcasting if maximum diameter of the network is known caching of address lists (i.e. paths) with help of passing packets stations can use the cached information for path discovery (own paths or paths for other hosts)
DSR: Route Discovery Sending from C to O P R C G Q A B E H I K M O D F J L N
DSR: Route Discovery Broadcast P R [O,C,4711] C [O,C,4711] G Q A B E H I K M O D F J L N
DSR: Route Discovery P R [O,C/B,4711] C [O,C/G,4711] G [O,C/G,4711] Q A B E [O,C/E,4711] H I K M O D F J L N
DSR: Route Discovery P R C G Q A [O,C/B/A,4711] B D E F H I [O,C/G/I,4711] K [O,C/E/H,4711] L J M N O [O,C/B/D,4711] (alternatively: [O,C/E/D,4711])
DSR: Route Discovery P R C G Q A B E H I [O,C/G/I/K,4711] K M O D F J L N [O,C/B/D/F,4711] [O,C/E/H/J,4711]
DSR: Route Discovery P R C G Q A B E H I K [O,C/G/I/K/M,4711] M O D F J L N [O,C/E/H/J/L,4711] (alternatively: [O,C/G/I/K/L,4711])
DSR: Route Discovery P R C G Q A B E H I K M O D F J L N [O,C/E/H/J/L/N,4711]
DSR: Route Discovery P R C G Q A B E H I K Path: M, K, I, G M O D F J L N
Dynamic Source Routing III Maintaining paths after sending a packet wait for an acknowledgement (if applicable) listen into the medium to detect if other stations forward the packet (if possible) request an explicit acknowledgement if a station encounters problems it can inform the sender of a packet or look-up a new path locally
Interference-based routing Routing based on assumptions about interference between signals N 1 N 2 R 1 S 1 N 3 N 4 N 5 N 6 R 2 S 2 neighbors (i.e. within radio range) N 7 N 8 N 9 Transmissions along the red and blue routes will mutually interfere
Examples for Interference based Routing Least Interference Routing (LIR) calculate the cost of a path based on the number of stations that can receive a transmission Max-Min Residual Capacity Routing (MMRCR) calculate the cost of a path based on a probability function of successful transmissions and interference Least Resistance Routing (LRR) calculate the cost of a path based on interference, jamming and other transmissions LIR is very simple to implement, only information from direct neighbors is necessary
A Plethora of Ad Hoc Routing Protocols Flat proactive FSLS Fuzzy Sighted Link State FSR Fisheye State Routing OLSR Optimized Link State Routing Protocol (RFC 3626) TBRPF Topology Broadcast Based on Reverse Path Forwarding reactive AODV Ad hoc On demand Distance Vector (RFC 3561) DSR Dynamic Source Routing (RFC 4728) DYMO Dynamic MANET On-demand Hierarchical CGSR Clusterhead-Gateway Switch Routing HSR Hierarchical State Routing LANMAR Landmark Ad Hoc Routing ZRP Zone Routing Protocol Geographic position assisted DREAM Distance Routing Effect Algorithm for Mobility GeoCast Geographic Addressing and Routing GPSR Greedy Perimeter Stateless Routing LAR Location-Aided Routing
Further Difficulties and Research Areas Auto-Configuration Assignment of addresses, function, profile, program, Service discovery Discovery of services and service providers Multicast Transmission to a selected group of receivers Quality-of-Service Maintenance of a certain transmission quality Power control Minimizing interference, energy conservation mechanisms Security Data integrity, protection from attacks (e.g. Denial of Service) Scalability 10 nodes? 100 nodes? 1000 nodes? 10000 nodes? Integration with fixed networks
Clustering of Ad-hoc Networks Base station Internet Cluster head Cluster Super cluster
Challenges in WiFi Again, explosion of users, devices Interference, interference, interference Heavy interference /contention when accessing the AP, no QoS support Inter-AP interference Interference from other devices (microwave, cordless phones) in the same frequency band Mobility support Seamless roaming when users move between APs Normally low speed (3-10mph)
Challenges in Ad-hoc Networks A flexible network infrastructure Peer-to-peer communications No backbone infrastructure Routing can be multi-hop Topology is dynamic Challenges Devices need to self-manage to survive Manage interference (similar to WiFi but without AP, much harder) Manage connectivity and routing (node mobility and unreliable links) Transmission, access, and routing strategies for ad-hoc networks are generally ad-hoc User collaboration is a good direction but there are always selfish / malicious users
How does Wireless affect Networking? Wireless access is different from Ethernet access Wireless routing is different from IP routing Wireless security is different from wired security
Wireless Access vs. Ethernet Access Ethernet: fixed connection, always on, stable, fixed rate Wireless: unreliable connection, competition based, fading/unreliable, dynamic rate, limited bandwidth Critical: how to coordinate among devices to avoid interference Mobility Cellular: centralized, base station tells each device when and how to send/receive data WLAN + Ad hoc: distributed, CSMA, compete and backoff neighbor discovery + topology control Rate adaptations
Wireless Routing vs. Wired Routing Aside from traditional multi-hop routing Mobility: route discovery and maintenance Interference, interference, interference Multi-hop interference mitigation Spectrum assignment, multi-channel networks
Why is Security more of a Concern in Wireless? No inherent physical protection Physical connections between devices are replaced by logical associations Sending and receiving messages do not need physical access to the network infrastructure (cables, hubs, routers, etc.) Broadcast communications Wireless usually has a broadcast nature Transmissions can be overheard by anyone in range Anyone can generate transmissions, which will be received by other devices in range which will interfere with other nearby transmissions and may prevent their correct reception (jamming)
Wireless Attacks Eavesdropping is easy Injecting bogus messages into the network is easy Replaying previously recorded messages is easy Illegitimate access to the network and its services is easy Denial of service is easily achieved by jamming More