Wireless Mobile Communications: Part 2 Mobile Adhoc 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_at_boeing.com
• [phone]

2
Outline

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

3
Wireless LANMAC Protocol
4
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 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 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
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
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
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
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
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
Link State Routing

• At node 5, based on the link state pkts, topology
  table is constructed
• Dijkstras 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 neighbors 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 (100s to 1000's
  up to 10, 000s)
• 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
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
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

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
2
8
3
5
9
1
9
4
6
Hop1
7
10
12
13
Hop2
19
18
21
11
Hopgt2
15
22
36
14
23
17
16
20
29
35
27
25
24
26
28
34
30
32
31
53
Message Reduction in FSR
54
Landmark Routing Protocol

• Logical subnet group of nodes with functional
  affinity with each other (e.g., they move
  together)
• Node logical address ltsubnet, hostgt
• 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

56
References

• References
• 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
• http//www.ietf.org/html.charters/mobileip-charter
  .html