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Title: Chapter 6 slides, Computer Networking, 3rd edition


1
Chapter 7 Wireless Ad Hoc Networks
2
What is an Ad Hoc Network?
  • Definitions
  • An ad-hoc network is one that comes together as
    needed, not necessarily with any assistance from
    the existing Internet infrastructure
  • Instant infrastructure
  • A MANET is a collection of mobile platforms or
    nodes where each node is free to move about
    arbitrarily
  • A MANET distributed, possibly mobile, wireless,
    multihop network that operates without the
    benefit of any existing infrastructure
    (infrastructure-less), except the nodes themselves

3
Mobile Ad Hoc Networks
  • May need to traverse multiple links to reach a
    destination

4
Mobile Ad Hoc Networks (MANET)
  • Mobility causes route changes

5
Why Ad Hoc Networks ?
  • Ease of deployment
  • Speed of deployment
  • Decreased dependence on infrastructure

6
  • Fundamental Challenges

It is better to know some of the questions than
all of the answers. James
Thurber (1835-1910)
7
1. Energy Efficiency
  • No infrastructure means must rely on batteries
    (or, in general, limited energy resources)
  • Possible solutions
  • Selectively sending nodes into a sleep mode
  • Using transmitters with variable power (the Power
    Control problem)
  • Using energy-efficient paths
  • Using cooperative techniques (still relatively
    new)

8
2. Mobility
  • Mobility-induced route changes
  • Mobility-induced packet losses
  • Mobility patterns may be different
  • Controlled e.g. robots
  • Offers opportunities for improving the network
    functions e.g. connectivity, coverage
  • Uncontrolled e.g. nomadic users
  • Offers challenges to network design
  • But also offers opportunities for improvement,
    e.g.
  • Users carry delay-tolerant data closer to
    destination
  • Delay Tolerant Network (Challenge Networks)

9
3. QoS
  • Providing QoS even in wired networks (e.g. the
    Internet) is a challenging problem
  • Wireless RF channels further complicate the
    problem
  • Unpredictability
  • Medium access broadcast medium with hidden
    terminal problem
  • Possible solutions
  • New MAC design
  • Cross-layer integration allow different layers
    to adapt depending on available information at
    other layers

10
4. Scalability
  • Limited wireless transmission range
  • Whether the network is able to maintain an
    acceptable level of service even as the number of
    nodes is increased
  • How fast the network protocol control overhead
    increases as N increases
  • Possible solutions
  • Introducing hierarchy
  • Utilizing location information
  • Limiting reactions to changes
  • Fixing things (e.g. paths) locally

11
5. Utilizing New Technologies
  • What are the gains that could be achieved by
    using newly available technologies such as
  • Smart directional (beamforming) antennas
  • Increases the spatial reuse in cellular, but how
    about ad-hoc?
  • Can several nodes together act as an antenna
    array? Practical issues?
  • Software Radio
  • The ability to quickly switch the operating
    frequency may provide opportunities, but also
    challenging
  • GPS
  • Location information may help

12
6. Security
  • Ease of snooping on wireless transmissions
  • From crypto point of view, lack of a trusted
    authority is one of the main challenges
  • How to generate/share keys reliably
  • Harder to track or even detect attackers in a
    wireless environment, given that
  • Network relies on in-situ connections to other
    nodes which may be malicious
  • Malicious nodes may be especially harmful by
    injecting bogus control packets
  • DoS attacks that deplete a nodes battery

13
7. Lack of Reference
  • Lack of sufficient experimental data to confirm
    models
  • What does a multi-hop path really mean?
  • What is a link?
  • Simplistic models that do not capture the
    complexities, or complex models that do not lead
    to insights?
  • Are the protocols good enough, have they reached
    closed to the best possible?
  • Good balance between mathematical and
    experimental work

14
Multiple-Layer Problem
  • PHY
  • Adapt to rapid changes in link characteristics
  • MAC
  • Minimize collision, allow fair access, and
    semi-reliably transport under rapid change and
    hidden/exposed terminals
  • Network
  • Determine efficient transmission paths when links
    change often and bandwidth is at a premium
  • Transport
  • Handle delay and packet loss statistics that are
    very different than wired networks
  • Application
  • Handle frequent disconnection and reconnection as
    well as varying delay and packet loss
    characteristics

15
Several Major Issues
  • MAC protocols for ad hoc networks
  • Routing in ad hoc networks
  • Transport protocols for ad hoc networks

16
Design Goals for MAC Protocols
  • Allow fair access to the shared radio medium
  • Distributed protocol
  • Available bandwidth must be utilized efficiently
  • Control overhead should be minimized
  • Ensure fair bandwidth allocation to competing
    nodes
  • Reduce the effect of hidden/exposed terminals
  • Effectively manage the power consumption
  • Provide QoS support for real-time traffic
  • Protocol should be scalable

17
Overall Picture
MAC Protocols for Ad Hoc
Contention-based
Contention-based with reservation
Contention-based with scheduling
Other Protocols
  • DPS
  • DWOP
  • DLPS
  • MMAC
  • MCSMA
  • PCM
  • RBAR

Sender initiated
Receiver initiated
  • RI-BTMA
  • MACA-BI
  • MARCH

synchronous
asynchronous
Single channel
Multiple channel
  • D-PRMA
  • CATA
  • HRMA
  • SRMA/PA
  • FPRP
  • MACA/PR
  • RTMAC
  • BTMA
  • DBTMA
  • ICSMA
  • MACAW
  • FAMA

18
Contention-based Protocols with Reservations
  • Use a bandwidth reservation technique
  • Contention occurs only at resource reservation
    phase
  • Node gets an exclusive access to the media once
    bandwidth is reserved
  • D-PRMA
  • Distributed packet reservation multiple access
    protocol
  • SRMA/PA
  • Soft reservation multiple access with priority
    assignment
  • RTMAC
  • Real-time medium access control protocol

19
Contention-based Protocols with Scheduling
  • Focus on packet scheduling at the nodes and
    transmission scheduling of the nodes
  • DPS
  • Distributed priority scheduling
  • DWOP
  • Distributed wireless ordering protocol
  • DLPS
  • Distributed laxity-based priority scheduling

20
Contention-based Protocols w/o Reservation/Schedul
ing
  • MACA
  • Multiple access collision avoidance protocol
  • MACAW
  • Media Access Protocol for Wireless LAN
  • BTMA
  • Busy tone multiple access protocol
  • MARCH
  • Media access with reduced handshake

21
MACA Multiple Access Collision Avoidance
  • Proposed as an alternative to CSMA/CA
  • Handle hidden and exposed terminal issues using
    RTS-CTS
  • RTS and CTS packets carry the expected duration
    of the data transmission
  • A node near the sender that hearing RTS do not
    transmit for a time to receive CTS
  • A node near the receiver after hearing CTS
    differs its transmission
  • If the neighbor hears the RTS only, it is free to
    transmit once the waiting interval is over

neighbor
sender
neighbor
receiver
RTS
RTS
CTS
CTS
Data
Data
22
MACAW Enhancement of MACA
  • Issue 1 potential flow starvation due to BEB
  • Both S1 and S2 have the high volume of traffic,
    S1 seizes the channel first
  • Packets transmitted by S2 get collided and it
    doubles CW
  • The probability that S2 seizes the channel
    decreasing
  • Solution in MACAW
  • Packet header contains the field set to the
    current back-off value of the transmitting node
  • Node receiving this packet copies this value to
    its back-off counter
  • If all the nodes can hear each other, eventually
    they will have the same back-off counter
    (fairness)

BEB BEB copy
S1-AP 48.5 23.82
S2-AP 0 23.82
S1
S2

AP
23
MACAW (Cont.)
  • Issue 2 backoff calculation adjusts too rapidly
  • After every successful transmission, return to
    the case where all stations have a minimal
    backoff counter, and then must repeat a period of
    contention to increase the backoffs
  • Solution in MACAW
  • Gentler adjustment
  • Upon a collision, the backoff interval is
    increased by a multiplicative factor (1.5)
  • Finc(x) MINl.5x, CWmax
  • Upon success it is decreased by 1
  • Fdec(x) MAXx-1, CWmin

24
MACAW (Cont.)
  • Issue 3 Neighbor receivers problem
  • When node A sends an RTS to B, while node C is
    receiving from D, node B cannot reply with a
    CTS, since B knows that D is sending to C
  • When the transfer from C to D is complete, node B
    can send a Request-to-send-RTS (RRTS) to node A
  • Node A may then immediately send RTS to node B

D
C
B
A
25
MACAW (Cont.)
  • This approach, however, does not work in the
    scenario below
  • Node B may not receive the RTS from A at all, due
    to interference with transmission from C

D
C
B
A
26
BTMA Busy Tone Multiple Access
  • One of the earliest solutions for hidden terminal
    problem
  • Multi-channel protocol
  • Control channel used for busy tone transmission
  • Data channel used for data transmission
  • Three variants
  • BTMA (Busy Tone Multiple Access)
  • DBTMA (Dual Busy Tone Multiple Access)
  • RI-BTMA (Receiver-Initiated BTMA)

27
BTMA (Cont.)
  • Basic idea
  • Node senses the control channel to check whether
    the busy tone is active
  • If not, turns on busy tone signal and starts data
    transmission
  • If yes, waits for a random period of time and
    repeats
  • Any node that senses the carrier on the incoming
    data channel also transmits a busy tone
  • Pros and Cons
  • Simple with extremely low collision probability
  • Bandwidth utilization is low (blocked in two-hop
    neighbor)
  • Multiple channels

28
Several Major Issues
  • MAC protocols for ad hoc networks
  • Routing in ad hoc networks
  • Transport protocols for ad hoc networks

29
Why is Routing in MANET Different?
  • No specific nodes dedicated for control
  • Host mobility
  • Link failure/repair due to mobility may have
    different characteristics than those due to other
    causes
  • Rate of link failure/repair may be high when
    nodes move fast
  • Different node characteristics
  • E.g. power constraints, multiple access issues
  • New performance criteria may be used
  • Route stability despite mobility
  • Energy consumption

30
Unicast Routing Protocols
  • Many protocols have been proposed
  • Some have been invented specifically for MANET
  • Others are adapted from previously proposed
    protocols for wired networks
  • No single protocol works well in all environments
  • Some attempts made to develop adaptive protocols

31
MANET Protocol Zoo
  • Topology based routing
  • Proactive approach, e.g., DSDV.
  • Reactive approach, e.g., DSR, AODV, TORA.
  • Hybrid approach, e.g., Cluster, ZRP.
  • Position based routing
  • Location Services
  • DREAM, Quorum-based, GLS, Home zone etc.
  • Forwarding Strategy
  • Greedy, GPSR, RDF, Hierarchical, etc.

32
Routing Protocols
  • Proactive protocols
  • Determine routes independent of traffic pattern
  • Traditional link-state and distance-vector
    routing protocols are proactive
  • Reactive (on-demand) protocols
  • Discover/maintain routes only when needed
  • Source-initiated route discovery
  • Hybrid protocols

33
Trade-Off
  • Latency of route discovery
  • Proactive protocols may have lower latency since
    routes are maintained at all times
  • Reactive protocols may have higher latency
    because a route from X to Y will be found only
    when X attempts to send to Y
  • Overhead of route discovery/maintenance
  • Reactive protocols may have lower overhead since
    routes are determined only if needed
  • Proactive protocols can (but not necessarily)
    result in higher overhead due to continuous route
    updating

34
Tradeoff (Cont.)
  • Which approach achieves a better trade-off
    depends on the traffic and mobility patterns
  • Reactive protocols may yield lower routing
    overhead than proactive protocols when
    communication density is low
  • Reactive protocols tend to loose more packets
    (assuming that network layer drops packets if a
    route is not known)
  • Proactive protocols perform better with high
    mobility and dense communication graph

35
Single Path vs. Multipath
  • Single path
  • Use one path from source to destination
  • Similar to wired routes
  • Advantages
  • Simple to implement
  • Disadvantages
  • Source must find a new route to destination if
    old one fails
  • Multipath
  • Use more than one path from source to destination
  • Advantages
  • Load balancing can occur
  • Higher tolerance to link failures
  • Disadvantages
  • Adds complexity to receiver and sender

36
Short Hops vs. Long Hops
  • Research to date suggests short-hop
  • Provides lower energy consumption
  • Lower transmission power needed due to shorter
    distance between nodes
  • Provides higher link capacity
  • Higher received signal strength due to shorter
    distance between nodes
  • Long-hop intuitively should have less total delay
    due to
  • Less total hops
  • Smaller total processing delay

37
Some Existing Wireless Routing Protocols
  • DSDV
  • WRP
  • CGSR
  • STAR
  • OLSR
  • FSR
  • HSR
  • GSR
  • DSR
  • AODV
  • ABR
  • SSA
  • FORP
  • PLBR
  • CEDAR
  • ZRP
  • ZHLS
  • RABR
  • LBR
  • COSR
  • PAR
  • LAR
  • OLSB

38
Dynamic Source Routing (DSR)
  • Reactive, source-based
  • When node S wants to send a packet to node D, but
    does not know a route to D, node S initiates a
    route discovery
  • Source node S floods Route Request (RREQ)
  • Each node appends own identifier when forwarding
    RREQ

39
Route Discovery in DSR
Y
Z
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Represents a node that has received RREQ for D
from S
40
Route Discovery in DSR
Y
Broadcast transmission
Z
S
S
E
F
B
C
M
L
J
A
G
H
D
K
I
N
Represents transmission of RREQ
X,Y Represents list of identifiers appended
to RREQ
41
Route Discovery in DSR
Y
Z
S
S,E
E
F
B
C
M
L
J
A
G
S,C
H
D
K
I
N
  • Node H receives packet RREQ from two neighbors
  • potential for collision

42
Route Discovery in DSR
Y
Z
S
E
F
S,E,F
B
C
M
L
J
A
G
H
D
K
S,C,G
I
N
  • Node C receives RREQ from G and H, but does not
    forward
  • it again, because node C has already forwarded
    RREQ once

43
Route Discovery in DSR
Y
Z
S
E
F
S,E,F,J
B
C
M
L
J
A
G
H
D
K
I
N
S,C,G,K
  • Nodes J and K both broadcast RREQ to node D
  • Since nodes J and K are hidden from each other,
    their
  • transmissions may collide

44
Route Discovery in DSR
Y
Z
S
E
S,E,F,J,M
F
B
C
M
L
J
A
G
H
D
K
I
N
  • Node D does not forward RREQ, because node D is
    the intended target of the route discovery

45
Route Discovery in DSR
  • Destination D on receiving the first RREQ, sends
    a Route Reply (RREP)
  • RREP is sent on a route obtained by reversing the
    route appended to received RREQ
  • RREP includes the route from S to D on which RREQ
    was received by node D

46
Route Reply in DSR
Y
Z
S
RREP S,E,F,J,D
E
F
B
C
M
L
J
A
G
H
D
K
I
N
RREP S,C,G,K,D
Represents RREP control message
47
Dynamic Source Routing (DSR)
  • Node S on receiving RREP, caches the route
    included in the RREP
  • When node S sends a data packet to D, the entire
    route is included in the packet header
  • Hence the name source routing
  • Intermediate nodes use the source route included
    in a packet to determine to whom a packet should
    be forwarded

48
DSR Optimization Route Caching
  • Each node caches a new route it learns by any
    means
  • When node S learns that a route to node D is
    broken
  • Uses another route from its local cache, if such
    a route to D exists in its cache
  • Otherwise, node S initiates route discovery by
    sending a route request
  • Intermediate node X on receiving a Route Request
    for some node D can send a Route Reply
  • If node X knows a route to node D
  • Use of route cache
  • Can speed up route discovery
  • Can reduce propagation of route requests

49
DSR Pros and Cons
  • Advantages
  • Less memory storage needed at each node since
    full routing table is not needed
  • Lower overhead needed because no periodic update
    message are necessary
  • Nodes do not need to continually inform neighbors
    they are still operational
  • Disadvantages
  • Possible transmission latency due to reactive
    approach
  • Stale routes can occur if links change frequently
  • Message size increases as path length increases
  • Collisions between route requests propagated by
    neighboring nodes
  • Route Reply Storm due to nodes replying using
    their local cache

50
Several Major Issues
  • MAC protocols for ad hoc networks
  • Routing in ad hoc networks
  • Transport protocols for ad hoc networks

51
Transmission Control Protocol (TCP)
  • Reliable ordered delivery
  • Implements congestion avoidance and control
  • Reliability achieved by means of retransmissions
    if necessary
  • End-to-end semantics
  • Acknowledgements sent to TCP sender confirm
    delivery of data received by TCP receiver
  • Ack for data sent only after data has reached
    receiver

52
Challenges
  • Throughput unfairness
  • Unfairness at MAC layer
  • Transport layer should take this into account
  • Resource constraints
  • Power and bandwidth constraints
  • Separation of congestion control and reliability
    control
  • Completely decoupled transport layer
  • Wired network transport protocol completely
    separated from underlying layer
  • Ad hoc network interaction with network and MAC
    layer is expected for adaptability

53
Challenges (Cont.)
  • Misinterpretation of congestion
  • Traditional mechanism packet loss, timeout
  • Ad hoc loss/delay due to
  • High bit error rate due to varying link condition
  • Packet collisions due to contention and hidden
    terminal
  • Path breaks due to node mobility
  • Dynamically changing topology
  • Frequent path breaks
  • Partitioning and merging of networks
  • High delay in reestablishment of path

54
Performance of TCP
  • Several factors affect TCP performance in MANET
  • Wireless transmission errors
  • Multi-hop routes on shared wireless medium
  • For instance, adjacent hops typically cannot
    transmit simultaneously
  • Route failures/changes due to mobility

55
Throughput over Multi-Hop Wireless Paths
  • Connections over multiple hops are at a
    disadvantage compared to shorter connections,
    because they have to contend for wireless access
    at each hop

TCP Throughput using 2 Mbps 802.11 MAC
56
Impact of Caching
  • Route caching has been suggested as a mechanism
    to reduce route discovery overhead
  • Each node may cache one or more routes to a given
    destination
  • When a route from S to D is detected as broken,
    node S may
  • Use another cached route from local cache, or
  • Obtain a new route using cached route at another
    node

57
Why Performance Degrades With Caching
  • When a route is broken, route discovery returns a
    cached route from local cache or from a nearby
    node
  • After a time-out, TCP sender transmits a packet
    on the new route
  • However, the cached route has also broken after
    it was cached
  • Another route discovery, and TCP time-out
    interval
  • Process repeats until a good route is found

58
To Cache or Not to Cache
  • Caching can result in faster route repair
  • Faster does not necessarily mean correct
  • If incorrect repairs occur often enough, caching
    performs poorly
  • Need mechanisms for determining when cached
    routes are stale

59
Caching and TCP performance
  • Caching can reduce overhead of route discovery
    even if cache accuracy is not very high
  • But if cache accuracy is not high enough, gains
    in routing overhead may be offset by loss of TCP
    performance due to multiple time-outs

60
How to Improve Throughput (Bring Closer to Ideal)
  • Network feedback
  • Inform TCP of route failure by explicit message
  • Let TCP know when route is repaired
  • Probing
  • Explicit notification
  • Reduces repeated TCP timeouts and backoff

61
TCP with ELFN
  • Explicit Link Failure Notification
  • Not totally new, e.g., ECN bits in TCP
  • To provide the TCP sender with information about
    link and route failures, so that it can avoid
    responding to the failures as if congestion has
    occurred
  • How does it work?
  • When a TCP sender receives an ELFN disables its
    retransmission timers and enters a stand-by
    mode
  • While on standby A packet is sent at periodic
    intervals to probe the network to see if a route
    has been established
  • If an acknowledgment is received leaves stand-by
    mode and restores the retransmission timers

62
Performance with Explicit Notification
63
Issues Network Feedback
  • Network knows best (why packets are lost)
  • ? Network feedback beneficial
  • ? Need to modify transport network layer to
    receive/send feedback
  • ? Need mechanisms for information exchange
    between layers
  • Holland99 discusses alternatives for providing
    feedback (when routes break and repair)
  • Chandran98 also presents a feedback scheme

64
TCP Performance
  • Two factors result in degraded throughput in
    presence of mobility
  • Loss of throughput that occurs while waiting for
    TCP sender to timeout
  • This factor can be mitigated by using explicit
    notifications and better route caching mechanisms
  • Poor choice of congestion window and RTO values
    after a new route has been found
  • How to choose cwnd and RTO after a route change?

65
Issues Window Size After Route Repair
  • Same as before route break may be too optimistic
  • Same as startup may be too conservative
  • Better be conservative than overly optimistic
  • Reset window to small value after route repair
  • Let TCP figure out the suitable window size
  • Impact low on paths with small delay-bw product

66
Issues RTO After Route Repair
  • Same as before route break
  • If new route is long, this RTO may be too small,
    leading to timeouts
  • Same as TCP start-up (6 second)
  • May be too large
  • May result in slow response to next packet loss
  • Another plausible approach
  • RTOnew f(RTOold, route-lengthold,
    route-lengthnew)
  • E.g. RTOnew RTOold route-lengthnew/route-leng
    thold
  • Not evaluated yet
  • Pitfall RTT is not just a function of route
    length

67
Summary
  • Still many remained topics related to wireless ad
    hoc networks
  • More research opportunities in
  • Wireless mesh networks
  • With fixed infrastructure as wireless
    infrastructure
  • Multi-radio multi-channel architecture
  • Wireless sensor networks
  • Energy consumption is one of the key challenges
  • Application specific demands, including
    localization, coverage, event detection/collection
    , etc.
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