Wireless Mobile Communications: Part 2 – Mobile Ad-hoc Networks (MANET)

1. Wireless Mobile Communications:Part 2 – Mobile Ad-hoc Networks (MANET) Jae H. Kim, Ph.D. Manager/Associate Technical Fellow Boeing Phantom Works jae.h.kim@boeing.com (253) 657-7685

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

3. Wireless LANMAC Protocol

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

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 wirelessLANs (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 802.11 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. DIFS DIFS PIFS SIFS medium busy contention next frame t direct access if medium is free  DIFS 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

18. contention window (randomized back-offmechanism) DIFS DIFS medium busy next frame t direct access if medium is free  DIFS slot time IEEE 802.11 MAC - CSMA/CA • 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. DIFS data sender SIFS ACK receiver DIFS data other stations t waiting time contention 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

20. DIFS RTS data sender SIFS SIFS SIFS CTS ACK receiver DIFS NAV (RTS) data other stations NAV (CTS) t defer access contention 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

21. IEEE 802.1 MAC - DCF • IEEE 802.11 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. Example B1 = 25 B1 = 5 wait data data wait B2 = 10 B2 = 20 B2 = 15 B1 and B2 are backoff intervals at nodes 1 and 2 CW = 31 IEEE 802.11 MAC – DCF (Cont.)

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 802.11 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 0 Routing table at node 5 : 1 3 2 4 Tables grow linearly with # nodes Control overhead grows with network size and mobility 5

33. 1 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 0 {1} {0,2,3} {1,4} 3 2 {1,4,5} 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. 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) • 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. source OLSR Example • 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. RREQ flooded with increasing scope RREP reverses successful flooding path destination destination AODV Example 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