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Manager/Associate Technical Fellow

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1 Manager/Associate Technical Fellow
Wireless Mobile Communications: Part 2 – Mobile Ad-hoc Networks (MANET) Jae H. Kim, Ph.D. Manager/Associate Technical Fellow Boeing Phantom Works (253)

2 Outline PART 2: Wireless LAN MAC Protocols
Mobile Ad-hoc Network (MANET) Proactive Reactive Hybrid

3 Wireless LAN MAC Protocol

4 Wireless Protocol Layers
Data Plane Control Plane Application Processing Propagation Model Mobility Frame Processing Radio Status/Setup CS/Radio RTS/CTS Frame Wrapper Ack/Flow Control Clustering Packet Store/Forward VC Handle Flow Control Routing IP Wrapper IP/Mobile IP RSVP Transport Wrapper TCP/UDP Control Channel Radio MAC Layer Network IP Transport Application RTP Wrapper RCTP Link Layer Application Setup

5 Media Access Control (MAC) Layer
MAC protocol: Coordination and scheduling of transmissions among competing neighbors Goals: Low latency, good channel utilization; best effort and real time support MAC layer clustering: Aggregation of nodes in a cluster (= cell) for MAC enhancement Different from network layer clustering, partitioning such as used for routing

6 MAC Protocols CDMA (Code Division Multiple Access)
FDMA (Frequency Division Multiple Access) TDMA (Time Division Multiple Access) ALOHA Slotted ALOHA CSMA (Carrier Sense Multiple Access) DAMA (Demand Assigned Multiple Access) PRMA (Packet Reservation Multiple Access) Reservation TDMA MACA (Multiple Access with Collision Avoidance) Polling SDMA (Space Division Multiple Access)

7 Multiple Access MAC protocol coordinates transmissions from different stations in order to minimize/avoid collisions (1) Random Access: CSMA, MACA (2) Channel Partitioning: TDMA, FDMA, CDMA (3) “Taking turns”: Polling Goal is efficient, fair, simple, decentralized

8 Random Access A node transmits at random (i.e., no a priory coordination among nodes) at full channel data rate If two or more nodes “collide,” they retransmit at random times The random access MAC protocol specifies how to detect collisions and how to recover from them (via delayed retransmissions, for example) Examples of random access MAC protocols: Slotted ALOHA – 36% throughput ALOHA – 18% throughput (c) CSMA and CSMA/CD

9 Slotted ALOHA Time is divided into equal size slots (= full packet size) a newly arriving station transmits a the beginning of the next slot if collision occurs (assume channel feedback, eg the receiver informs the source of a collision), the source retransmits the packet at each slot with probability P, until successful. Success (S), Collision (C), Empty (E) slots S-ALOHA is channel utilization efficient; it is fully decentralized

10 Pure (unslotted) ALOHA
Slotted ALOHA requires slot synchronization A simpler version, pure ALOHA, does not require slots A node transmits without awaiting for the beginning of a slot Collision probability increases (packet can collide with other packets which are transmitted within a window twice as large as in S-Aloha) Throughput is reduced by one half, i.e., S= 1/2e

11 Carrier Sense Multiple Access (CSMA)
CSMA: listen before transmit. If channel is sensed busy, defer transmission Persistent CSMA: retry immediately when channel becomes idle (this may cause instability) Non persistent CSMA: retry after random interval Note: collisions may still exist, since two stations may sense the channel idle at the same time ( or better, within a “vulnerable” window = round trip delay) In case of collision, the entire packet transmission time is wasted

12 Collision Detection CSMA/CD: carrier sensing and deferral like in CSMA. But, collisions are detected within a few bit times. Transmission is then aborted, reducing the channel wastage considerably. Typically, persistent transmission is implemented CSMA/CD can approach channel utilization =1 in LANs (low ratio of propagation over packet transmission time) Collision detection is easy in wired LANs (eg, E-net): can measure signal strength on the line, or code violations, or compare tx and receive signals Collision detection cannot be done in wireless LANs (the receiver is shut off while transmitting, to avoid damaging it with excess power)

13 IEEE 802. 11 MAC - CSMA Protocol
Sense channel idle for DISF (Distributed Inter Frame Space) transmit frame (no Collision Detection) receiver returns ACK after SIFS (Short Inter Frame Space) If channel sensed busy, then binary backoff NAV: Network Allocation Vector (min time of deferral)

14 Hidden Terminal Effect
CSMA inefficient in presence of hidden terminals Hidden terminals: A and B cannot hear each other because of obstacles or signal attenuation; so, their packets collide at B Solution? CSMA/CA (Collision Avoidance)

15 Collision Avoidance CTS “freezes” stations within range of receiver (but possibly hidden from transmitter); this prevents collisions by hidden station during data RTS and CTS are very short: collisions during data phase are thus very unlikely (similar effect as Collision Detection) Note: IEEE allows CSMA, CSMA/CA and “polling” from AP

16 IEEE 802.11 - MAC Layer Access methods MAC-DCF CSMA/CA (mandatory)
Collision avoidance via randomized “back-off” mechanism Minimum distance between consecutive packets ACK packet for acknowledgements (not for broadcasts) MAC-DCF w/ RTS/CTS (optional) Distributed Foundation Wireless MAC Avoids hidden terminal problem MAC- PCF (optional) Access point polls terminals according to a list

17 802.11 - MAC layer (cont.) 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 t medium busy SIFS PIFS DIFS next frame contention direct access if medium is free  DIFS

18 IEEE MAC - CSMA/CA t medium busy DIFS next frame contention window (randomized back-off mechanism) slot time direct access if medium is free  DIFS station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type) if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time) if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness)

19 IEEE 802.11 MAC - CSMA/CA (cont.)
Sending unicast packets station has to wait for DIFS before sending data receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC) automatic retransmission of data packets in case of transmission errors t SIFS DIFS data ACK waiting time other stations receiver sender contention

20 IEEE 802.11 MAC - RTS/CTS Sending unicast packets
station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium) acknowledgement via CTS after SIFS by receiver (if ready to receive) sender can now send data at once, acknowledgement via ACK other stations store medium reservations distributed via RTS and CTS t SIFS DIFS data ACK defer access other stations receiver sender contention RTS CTS NAV (RTS) NAV (CTS)

21 IEEE MAC - DCF IEEE DCF: Congestion control achieved by dynamically choosing the contention window, CW 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 When backoff interval reaches 0, transmit RTS

22 IEEE 802.11 MAC – DCF (Cont.) Example data wait B1 = 5 B2 = 15 B1 = 25
B1 and B2 are backoff intervals at nodes 1 and 2 CW = 31 data wait B1 = 5 B2 = 15 B1 = 25 B2 = 20 B2 = 10 Example

23 Congestion Avoidance The time spent counting down backoff intervals is a part of MAC overhead Choosing a large CW leads to large backoff intervals and can result in larger overhead Choosing a small CW leads to a larger number of collisions (when two nodes count down to 0 simultaneously)

24 Congestion Control Since the number of nodes attempting to transmit simultaneously may change with time, some mechanism to manage congestion is needed IEEE DCF: Congestion control achieved by dynamically choosing the contention window CW

25 Binary Exponential Backoff in DCF
When a node fails to receive CTS in response to its RTS, it increases the contention window cw is doubled (up to an upper bound) When a node successfully completes a data transfer, it restores CW to CWmin

26 Channel Partitioning (e.g., CDMA)
CDMA (Code Division Multiple Access): exploits spread spectrum (DS or FH) encoding scheme unique “code” assigned to each user; I.e., code set partitioning Used mostly in wireless broadcast channels (cellular, satellite,etc) All users share the same frequency, but each user has own “chipping” sequence (i.e., code) Chipping sequence like a mask: used to encode the signal encoded signal = (original signal) x (chipping sequence) decoding: inner product of encoded signal and chipping sequence (note: the inner product is the sum of the component-by-component products) To make CDMA work, chipping sequences must be chosen orthogonal to each other (i.e., inner product = 0)

27 CDMA Encode/Decode

28 CDMA (cont.) CDMA Properties:
protects users from interference and jamming (in WW II) protects users from radio multipath fading allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”) requires “chip synch” acquisition before demodulation requires careful transmit power control to avoid “capture” by near stations in near-far situations FAA requires use of SS (with limits on tx power) in the Unlicensed Spectrum region (ISM), e.g., 900 MHz and 2.4 GHz (WaveLANs) CDMA used in Qualcomm cellphones (channel efficiency improved by factor of 4 with respect to TDMA)

29 Frequency Hopping (FH)
Frequency spectrum sliced into frequency subbands (e.g., 125 subbands in a 25 MHz range) Time is subdivided into slots; each slot can carry several bits (slow FH) A typical packet covers several time slots A transmitter changes frequency slot by slot (frequency hopping) according to unique, predefined sequence; all users are clock and slot synchronized Ideally, hopping sequences are “orthogonal” (i.e., non overlapped); in practice, some conflicts may occur

30 Mobile Ad-Hoc Networks (MANET) Routing Protocols

31 Proactive, Table Driven Routing
Distance Vector: Destination-sequenced distance vector (DSDV), Bellman-Ford Routing control overhead linearly increasing with network size Convergence problems (count to infinity); potential loops Link State: Open Shortest Path First (OSPF) Link update flooding overhead caused by frequent topology changes Not scalable to network size and mobility …

32 Distance Vector Routing table at node 5 : 1 3 2 4
Routing table at node 5 : Destination Next Hop Distance 2 3 1 1 3 2 4 Tables grow linearly with # nodes Control overhead grows with network size and mobility 5

33 Link State Routing At node 5, based on the link state pkts, topology table is constructed: Dijkstra’s Algorithm can then be used for the shortest path {1} 1 2 3 4 5 {0,2,3} 1 3 {1,4} {1,4,5} 2 4 {2,3,5} 5 {2,4}

34 Reactive, On-Demand Routing
Routes are established “on demand” as requested by the source Only the active routes are maintained by each node Channel/Memory overhead is minimized Two leading methods for route discovery: source routing and backward learning (similar to LAN interconnection routing)

35 Routing Protocol Choices?
OSPFv2 is one of the most heavily used IGPs in the Internet today Commercially, we have alternatives: Cisco EIGRP (proprietary, in Distance Vector class of protocols) RIP (legacy Distance Vector, considered inferior for large networks) IS-IS (OSI link-state protocol, many of same issues as with OSPFv2)

36 What is OSPFv2? A “link state” routing protocol for unicast traffic
Simple concept: Assign costs to links Give every router a complete map of the network Execute a shortest path calculation for every destination Build a routing table with next-hop information for all destinations

37 OSPFv2 Area Hierarchy OSPFv2 uses an “area hierarchy” to summarize groups of nodes The backbone is called “Area 0” Every additional area must attach to the backbone Routes to different areas are summarized (aggregated) before re-distribution Cost of area hierarchy is loss of precision in the routes, complexity, and topology restrictions OSPFv2 also uses route summarization between “autonomous systems” This is method of scaling in ADNS

38 Basic OSPFv2 Operation Routers transmit “Hello” messages every 10 seconds to each neighbor Hello messages also contain a list of neighbors from whom Hellos have been received If you see yourself in your neighbor’s Hello message, you know you have a 2-way link Peer routers then synchronize their databases Routers use a reliable flooding algorithm to disseminate link and network information

39 OSPFv2 Problems Flooding of “link state advertisements” causes overhead to grow with a) number of nodes, b) mobility Hello message traffic over slow links Convergence time (operationally it is a lot larger than one would expect) and route “flapping” Difficult for different areas to peer with one another

40 OSPFv2 over MANET? No interface type defined for wireless, broadcast-based networks “Ethernet”-like interface (broadcast) does not function correctly in wireless network Point-to-multipoint interface creates too much overhead (does not capitalize on broadcast capability) No support for Quality-of-Service-based link metrics (for load balancing) Used to be in the protocol specification, but was removed

41 Other Alternatives The Internet Engineering Task Force (IETF) is considering the standardization of new “Mobile Ad-hoc Network (MANET)” routing protocols Optimized for wireless operation Various strategies for scaling to large networks Designed for most severe of NBN conditions (when regular infrastructure breaks down or is non-existent)

42 What is MANET ?

43 Wireless Multihop Routing
Mobility Need to scale to large numbers (100’s to 1000's up to 10, 000’s) Unreliable radio channel (e.g., fading, external interference) Limited bandwidth Limited power Need multimedia applications (QoS)

44 mobile ad-hoc networking Applications
Placeholder for mobile ad-hoc networking Applications (animation)

45 Mobile Ad-hoc Routing Protocols
Proactive: Conventional table-driven routing Optimized Link State Routing (OLSR) Destination Sequenced Distance Vector (DSDV) Fisheye State Routing (FSR) Source Tree Adaptive Routing (STAR) Hierarchical State Routing (HSR) Reactive: On-Demand routing: Ad-hoc On-Demand Distance Vector (AODV) Dynamic Source Routing (DSR) Temporarily Ordered Routing Algorithm (TORA) Location Assisted Routing (LAR) Hybrid Routing: Zone routing

46 OLSR Protocol Optimized Link State Routing (OLSR) Most like OSPF
developed by INRIA, France categorized as “proactive” protocol Most like OSPF Shortest Path First (SPF)-based algorithm Unreliable flooding algorithm Sets up distribution tree to disseminate routing information (nodes are called Multipoint Relays)

47 OLSR Example 20 nodes 100 bi-directional links (not shown)
source 20 nodes 100 bi-directional links (not shown) 19 tree links (shown) 16 leaves (not filled) 4 non-leaf nodes Only 4 non-leaf nodes forward updates generated by the source In flooding, all 20 nodes would forward updates.

48 AODV Routing Protocol Ad-hoc on-demand distance vector
Developed by Perkins, Royer, Das categorized as “reactive” MANET protocol Unknown routes are queried for by a flooding algorithm Recently used routes are cached for future use Several implementations exist, and extensions for QoS and IPv6 defined

49 AODV Example RREQ flooded with increasing scope destination
RREP reverses successful flooding path source

50 Hierarchical Routing Use hierarchical routing to reduce table size and table update overhead Proposed hierarchical schemes include: Fisheye (implicit hierarchy induced by "scope") Zone routing (hybrid scheme) Landmark Routing

51 Fisheye State Routing Topology data base at each node
Similar to link state (e.g., OSPF) Routing information is periodically exchanged with neighbors only ( “Global” State Routing) Similar to distance vector, but exchange entire topology matrix Routing update frequency decreases with distance to destination Higher frequency updates within a close zone and lower frequency updates to a remote zone Highly accurate routing information about the immediate neighborhood of a node; progressively less detail for areas further away from the node

52 Scope of Fisheye Hop=1 Hop=2 Hop>2 2 8 5 3 9 1 9 6 4 7 10 13 12 18
19 11 21 Hop>2 15 36 22 14 23 16 17 20 29 35 25 27 24 26 28 34 30 32 31

53 Message Reduction in FSR
5 1 2 4 3 0:{1} 1:{0,2,3} 2:{5,1,4} 3:{1,4} 4:{5,2,3} 5:{2,4} LST HOP

54 Landmark Routing Protocol
Logical Subnet Landmark Logical subnet: group of nodes with functional affinity with each other (e.g., they move together) Node logical address = <subnet, host> A Landmark is elected in each subnet Every node keeps Fisheye Link State table/routes to neighbors up to hop distance N Every node maintains routes to all Landmarks

55 Landmark Routing (cont.)
A packet to local destination is routed directly using Fisheye tables A packet to remote destination is routed to corresponding Landmark based on logical addr Once the packet gets within Landmark scope, the direct route is found in Fisheye tables Benefits: dramatic reduction of both routing overhead and table size; scalable to large networks Landmark Logical Subnet

56 References References Books IETF Working Group URL
UCLA Course note, “Ad-hoc Wireless Routing,” CS 218- Fall 2002. UCLA Course note, “Ad-hoc Nets – MAC Layer: Part 1,” CS 218- Fall 2002. UCLA Course note, “Ad-hoc Nets – MAC Layer: Part 2,” CS 218- Fall 2002. Books James D. Solomon, Mobile IP The Internet Unplugged, Prentice Hall, 1998. Charles E. Perkins, Mobile IP Design Principles and Practices, Addison-Wesley, 1998. Christian Huitema, Routing in the Internet, Prentice Hall, 2000. Jochen Schiller, Mobile Communications, Addison-Wesley, 2000. Charles E. Perkins, Ad-Hoc Networking, Addison-Wesley, 2001. IETF Working Group URL

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