1. MARCH: Media Access with Reduced Handshake
MARCH does not require any traffic prediction mechanism
The RTS packet is used only for the first packet of the stream
MACA: RTS → CTS → DATA → RTS → CTS →. . .
MARCH: RTS → CTS1 → DATA → CTS2 → DATA →…
The CTS packet: MAC address of the sender, receiver, and the route identification number (RTid)
The RTid is used to avoid misinterpretation of CTS packets and initiation of false CTS-only handshake
For example, when node Y received a CTS with RTid = route 1, it does not respond .
The throughput of MARCH is significantly high when compared to MACA, while the control overhead is much less

4. Contention-Based Protocols with
Reservation Mechanisms

5. D-PRMA: Distributed Packet Reservation Multiple Access Protocol
The channel is divided into fixed and equal sized frames along the time axis.
The RTS/BI and CTS/BI are used for slot reservation and for overcoming the hidden terminal problem
If a terminal wins the contention through mini-slot 1, the extra (m – 1) mini-slots of this slot will be granted to the terminal as the payload
For voice node, the same slot in each subsequent frame can be reserved until the end of its packet transmission

7. CATA: Collision Avoidance Time Allocation Protocol
Support broadcast, unicast, and multicast transmissions simultaneously
Each frame consists of S slots and each slot is further divided into five mini-slots
CMS1: Slot Reservation (SR)
CMS2: RTS
CMS3: CTS
CMS4: not to send (NTS)
DMS: Data transmission

8. CMS3 and CMS4 are used as follows:
CATA (cont.)
Each node receives data during the DMS of current slot transmits an SR in CMS1
Every node that transmits data during the DMS of current slot transmits an RTS in CMS2
CMS3 and CMS4 are used as follows:
The sender of an intend reservation, if it senses the channel is idle in CMS1, transmits an RTS in CMS.
Then the receiver transmits a CTS in CMS3
If the reservation was successful the data can transmit in current slot and the same slot in subsequent frames.
Once the reservation was successfully, in the next slot both the sender and receiver do not transmit anything during CMS3 and during CMS4 the sender transmits a NTS.

9. CATA (cont.)
If a node receives an RTS for broadcast or multicast during CMS2 or it finds the channel to be free during CMS2, it remains idle during CMS3 and CMS4
Otherwise it sends a NTS packet during CMS4
A potential multicast or broadcast source node that receives the NTS packet or detecting noise during CMS4, understands that its reservation is failed
If it find the channel is free in CMS4, which implies its reservation was successful
CATA works well with simple single-channel half-duplex radios

11. FPRP: Five-Phase Reservation Protocol
FPRP is a single-channel TDMA-based broadcast scheduling protocol:
need global time synchronization
fully distributed and scalable
reservation process is localized; it involves only two-hop neighbors
No hidden terminal problem
Time is divided into frames: reservation frame (RF) and information frame (IF)
Each RF has N reservation slots (RS) and each IF has N information slots (IS)
Each RS is composed of M reservation cycles (RCs)
With each RC, a five-phase dialog takes place
Corresponding to IS, each node would be in one of the following three states: transmit (T), receive (R), and blocked (B)

13. Five-phase protocol:
Reservation request: send reservation request (RR) packet to dest.
Collision report: if a collision is detected by any node, that node broadcasts a CR packet
Reservation confirmation: a source node won the contention will send a RC packet to destination node if it does not receive any CR message in the previous phase
Reservation acknowledgment: destination node acknowledge reception of RC by sending back RA message to source
Packing and elimination: use packing packet and elimination packet
An example is shown in Figure 6.22

15. MACA/PR: MACA with Piggy- Backed Reservation
MACA/PR is used to provide real time traffic support
The main components: a MAC protocol (MACAW + non persistent CSMA), a reservation protocol, and a QoS routing protocol
Each node maintains a reservation table (RT) that records all the reserved transmit and receive slots/windows of all nodes
Non-real time packet: wait for a free slot in the RT + random time →RTS → CTS → DATA → ACK

16. Real time packet:
Transmit real time packets at certain regular intervals
(say CYCLE)
RTS→CTS→DATA (carry reservation info for next data) →ACK→…→DATA (carry reservation info) → ACK
Hear DATA and ACK: update their reservation table
The ACK packet serves to renew the reservation, in addition to recovering from the packet loss
Reservation fail: fail to receive ACK packets for a certain number of DATA packets

18. RTMAC: Real Time Medium Access Control Protocol
The two components: MAC protocol and QoS routing protocol
QoS routing: for end to end reservation + release of bandwidth
MAC: medium access for best effort + reservation for real time
Control packets
Real time : ResvRTS, ResvCTS, and ResvACK, half of DIFS
Best effort: RTS, CTS, and ACK
The duration of each resv-slot is twice the maximum propagation delay
Transmit real time packets first reserves a set of resv-slots
The set of resv-slots for a connection is called a connection-slot
The superframe for each node may not strictly align with the
other nodes (use relative time for all reservation)

20. Contention-Based MAC Protocols
with Scheduling Mechanisms

21. DPS: Distributed Priority Scheduling and MAC in Ad Hoc Networks
Provide differentiated QoS levels to different wireless applications in ad hoc networks
Achievable by QoS-sensitive MAC and network layer scheduling
Distributed scheduling problem with local information
Basic mechanisms:
Piggyback information
Head-of-line (HoL) packet as the packet with the highest priority (lowest index)
RTS, CTS : carry current packet info
DATA, ACK: carry next head-of-line info

23. MAC Protocols with Directional
Antennas

24. Advantages
Reduced signal interference, increase system throughput, and improved channel reuse
Assumptions
Only one radio transceiver can transmit and receive only one
packet at any given time
M directional antennas, each antennas having a conical radiation pattern, spanning an angle of 2π/M radians (See Figure 6.28)
Adjacent antennas never overlap
If a node transmits when all its antennas are active, then the
transmission’s radiation pattern is similar to that of an omnidirectional antennas
Packet transmission--See Figure 6.29

27. Other MAC Protocols

28. MMAC: Multi-Channel MAC Protocol
Assumptions:
No dedicated control channel
N channels for data transmission
Each node maintains a data structure called PreferableChannelList (PCL)
Three types for channels:
High preference channel (HIGH): the channel is selected by the current node and is used by the node
Medium preference channel (MID): a channel which is free and is not being currently used
Low preference channel (LOW): such a channel is already being used in the transmission range of the node by other neighboring nodes

29. MMAC: Multi-Channel MAC Protocol (cont.)
Time is divided into beacon intervals and every node is synchronized by periodic beacon transmissions
Ad hoc traffic indication messages (ATIM) window is used to negotiate for channels for transmission
ATIM, ATIM-ACK, and ATIM-RES (ATIM-reservation) message
ATIM messages takes place on a particular channel
Operation of the MMAC protocol is illustrated in Figure 6.34
The receiver plays a dominant role in channel selection

30. ATIM Window

32. RBAR: Receiver-Based Autorate Protocol
Rate Adaptation Mechanism
run at the receiver instead of being located at the sender
consist of channel quality estimation and rate selection
Rate selection is done during RTS-CTS packet exchange
The RTS and CTS packets carry the chosen modulation rate and the size of the data packet, instead of the duration of the reservation (see Figure 6.37)
The rate chosen by the sender and receiver are different
DRTS calculated by the neighbor nodes of the sender would not be valid if the rates chosen by the sender and receiver are different
Overcome problem: a special MAC header containing a reservation sub header (RSH) which contains control information for determining transmission duration

34. RBAR: Receiver-Based Auto-rate Protocol (cont.)
Channel quality estimation
The receiver uses a sample of the instantaneous received signal strength at the end of RTS reception
Rate selection algorithm
A simple threshold based technique is used e.g. SNR
It is important that the best available channel quality estimates are used for rate selection

35. MAC Protocols for Ad-Hoc Wireless Networks
Introduction
Issues
Design Goals
Classifications
Contention-based Protocols
Contention-based Protocols with reservation mechanisms
Contention-based Protocols without Scheduling mechanisms
MAC Protocols that use directional antennas
Other MAC Protocols

36. Issues
The main issues need to be addressed while designing a MAC protocol for ad hoc wireless networks:
Bandwidth efficiency is defined at the ratio of the bandwidth used for actual data transmission to the total available bandwidth. The MAC protocol for ad-hoc networks should maximize it.
Quality of service support is essential for time-critical applications. The MAC protocol for ad-hoc networks should consider the constraint of ad-hoc networks.
Synchronization can be achieved by exchange of control packets.

37. Issues
The main issues need to be addressed while designing a MAC protocol for ad hoc wireless networks:
Hidden and exposed terminal problems:
Hidden nodes:
Hidden stations: Carrier sensing may fail to detect another station. For example, A and D.
Fading: The strength of radio signals diminished rapidly with the distance from the transmitter. For example, A and C.
Exposed nodes:
Exposed stations: B is sending to A. C can detect it. C might want to send to E but conclude it cannot transmit because C hears B.
Collision masking: The local signal might drown out the remote transmission.
Error-Prone Shared Broadcast Channel
Distributed Nature/Lack of Central Coordination
Mobility of Nodes: Nodes are mobile most of the time.

38. Wireless LAN configuration

39. The 802.11 MAC Sublayer Protocol
(a) The hidden station problem.
(b) The exposed station problem.

40. Design goals of a MAC Protocol
Design goals of a MAC protocol for ad hoc wireless networks
The operation of the protocol should be distributed.
The protocol should provide QoS support for real-time traffic.
The access delay, which refers to the average delay experienced by any packet to get transmitted, must be kept low.
The available bandwidth must be utilized efficiently.
The protocol should ensure fair allocation of bandwidth to nodes.
Control overhead must be kept as low as possible.
The protocol should minimize the effects of hidden and exposed terminal problems.
The protocol must be scalable to large networks.
It should have power control mechanisms.
The protocol should have mechanisms for adaptive data rate control.
It should try to use directional antennas.
The protocol should provide synchronization among nodes.

41. Classifications of MAC protocols
Ad hoc network MAC protocols can be classified into three types:
Contention-based protocols
Contention-based protocols with reservation mechanisms
Contention-based protocols with scheduling mechanisms
Other MAC protocols
MAC Protocols for Ad Hoc
Wireless Networks
Contention-Based
Protocols
Contention-based
protocols with
reservation mechanisms
Contention-based
protocols with
scheduling mechanisms
Other MAC
Protocols
DirectionalAntennas
RI-BTMA
Sender-Initiated
Protocols
MACA-BI
Receiver-Initiated
Protocols
Synchronous
Protocols
Asynchronous
Protocols
MMAC
MARCH
MCSMA
RI-BTMA
D-PRMA
MACA/PR
PCM
MACA-BI
CATA
RTMAC
Single-Channel
Protocols
Multichannel
Protocols
RBAR
MARCH
HRMA
SRMA/PA
MACAW
BTMA
FAMA
DBTMA
FPRP
ICSMA

42. Classifications of MAC Protocols
Contention-based protocols
Sender-initiated protocols: Packet transmissions are initiated by the sender node.
Single-channel sender-initiated protocols: A node that wins the contention to the channel can make use of the entire bandwidth.
Multichannel sender-initiated protocols: The available bandwidth is divided into multiple channels.
Receiver-initiated protocols: The receiver node initiates the contention resolution protocol.
Contention-based protocols with reservation mechanisms
Synchronous protocols: All nodes need to be synchronized. Global time synchronization is difficult to achieve.
Asynchronous protocols: These protocols use relative time information for effecting reservations.

43. Classifications of MAC Protocols
Contention-based protocols with scheduling mechanisms
Node scheduling is done in a manner so that all nodes are treated fairly and no node is starved of bandwidth.
Scheduling-based schemes are also used for enforcing priorities among flows whose packets are queued at nodes.
Some scheduling schemes also consider battery characteristics.
Other protocols are those MAC protocols that do not strictly fall under the above categories.

44. Contention-based protocols
MACAW: A Media Access Protocol for Wireless LANs is based on MACA (Multiple Access Collision Avoidance) Protocol
MACA
When a node wants to transmit a data packet, it first transmit a RTS (Request To Send) frame.
The receiver node, on receiving the RTS packet, if it is ready to receive the data packet, transmits a CTS (Clear to Send) packet.
Once the sender receives the CTS packet without any error, it starts transmitting the data packet.
If a packet transmitted by a node is lost, the node uses the binary exponential back-off (BEB) algorithm to back off a random interval of time before retrying.
The binary exponential back-off mechanism used in MACA might starves flows sometimes. The problem is solved by MACAW.

45. MACA Protocol The MACA protocol. (a) A sending an RTS to B.
(b) B responding with a CTS to A.

46. MACA examples MACA avoids the problem of hidden terminals
A and C want to send to B
A sends RTS first
C waits after receiving CTS from B
MACA avoids the problem of exposed terminals
B wants to send to A, C to another terminal
now C does not have to wait for it cannot receive CTS from A A C
RTS
CTS
CTS B A C
RTS
RTS
CTS B

47. MACAW
Variants of this method can be found in IEEE as DFWMAC (Distributed Foundation Wireless MAC),
MACAW (MACA for Wireless) is a revision of MACA.
The sender senses the carrier to see and transmits a RTS (Request To Send) frame if no nearby station transmits a RTS.
The receiver replies with a CTS (Clear To Send) frame.
Neighbors
see CTS, then keep quiet.
see RTS but not CTS, then keep quiet until the CTS is back to the sender.
The receiver sends an ACK when receiving an frame.
Neighbors keep silent until see ACK.
Collisions
There is no collision detection.
The senders know collision when they don’t receive CTS.
They each wait for the exponential backoff time.

48. MACA variant: DFWMAC in IEEE802.11
sender
receiver
idle
idle
packet ready to send; RTS
data;
ACK
RxBusy
time-out;
RTS
wait for the
right to send
RTS;
CTS
time-out 
data;
NAK
ACK
time-out 
NAK;
RTS
CTS; data
wait for
data
wait for ACK
RTS; RxBusy
ACK: positive acknowledgement
NAK: negative acknowledgement
RxBusy: receiver busy

49. Contention-based protocols
Floor acquisition Multiple Access Protocols (FAMA)
Based on a channel access discipline which consists of a carrier-sensing operation and a collision-avoidance dialog between the sender and the intended receiver of a packet.
Floor acquisition refers to the process of gaining control of the channel. At any time only one node is assigned to use the channel.
Carrier-sensing by the sender, followed by the RTS-CTS control packet exchange, enables the protocol to perform as efficiently as MACA.
Two variations of FAMA
RTS-CTS exchange with no carrier-sensing uses the ALOHA protocol for transmitting RTS packets.
RTS-CTS exchange with non-persistent carrier-sensing uses non-persistent CSMA for the same purpose.

50. Contention-based protocols
Busy Tone Multiple Access Protocols (BTMA)
The transmission channel is split into two:
a data channel for data packet transmissions
a control channel used to transmit the busy tone signal
When a node is ready for transmission, it senses the channel to check whether the busy tone is active.
If not, it turns on the busy tone signal and starts data transmissions
Otherwise, it reschedules the packet for transmission after some random rescheduling delay.
Any other node which senses the carrier on the incoming data channel also transmits the busy tone signal on the control channel, thus, prevent two neighboring nodes from transmitting at the same time.
Dual Busy Tone Multiple Access Protocol (DBTMAP) is an extension of the BTMA scheme.
a control channel used for control packet transmissions (RTS and CTS packets) and also for transmitting the busy tones.

51. Contention-based protocols
Receiver-Initiated Busy Tone Multiple Access Protocol (RI-BTMA)
The transmission channel is split into two:
a data channel for data packet transmissions
a control channel used for transmitting the busy tone signal
A node can transmit on the data channel only if it finds the busy tone to be absent on the control channel.
The data packet is divided into two portions: a preamble and the actual data packet.
MACA-By Invitation (MACA-BI) is a receiver-initiated MAC protocol.
By eliminating the need for the RTS packet it reduces the number of control packets used in the MACA protocol which uses the three-way handshake mechanism.
Media Access with Reduced Handshake (MARCH) is a receiver-initiated protocol.

52. Contention-based Protocols with Reservation Mechanisms
Contention occurs during the resource (bandwidth) reservation phase.
Once the bandwidth is reserved, the node gets exclusive access to the reserved bandwidth.
QoS support can be provided for real-time traffic.
Distributed packet reservation multiple access protocol (D-PRMA)
It extends the centralized packet reservation multiple access (PRMA) scheme into a distributed scheme that can be used in ad hoc wireless networks.
PRMA was designed in a wireless LAN with a base station.
D-PRMA extends PRMA protocol in a wireless LAN.
D-PRMA is a TDMA-based scheme. The channel is divided into fixed- and equal-sized frames along the time axis.

53. Access method DAMA: Reservation-TDMA
Reservation Time Division Multiple Access
every frame consists of N mini-slots and x data-slots
every station has its own mini-slot and can reserve up to k data-slots using this mini-slot (i.e. x = N * k).
other stations can send data in unused data-slots according to a round-robin sending scheme (best-effort traffic)
e.g. N=6, k=2
N * k data-slots
N mini-slots
reservations for data-slots
other stations can use free data-slots
based on a round-robin scheme 26

54. Distributed Packet Reservation Multiple Access Protocol (D-PRMA)
Implicit reservation (PRMA - Packet Reservation Multiple Access):
a certain number of slots form a frame, frames are repeated
stations compete for empty slots according to the slotted aloha principle
once a station reserves a slot successfully, this slot is automatically assigned to this station in all following frames as long as the station has data to send
competition for this slots starts again as soon as the slot was empty in the last frame 1 2 3 4 5 6 7 8
time-slot
reservation
ACDABA-F
frame1 A C D A B A F
ACDABA-F
frame2 A C A B A
collision at reservation attempts
AC-ABAF-
frame3 A B A F
A---BAFD
frame4 A B A F D t
ACEEBAFD
frame5 A C E E B A F D 25

55. Contention-based protocols with Reservation Mechanisms
Collision avoidance time allocation protocol (CATA)
based on dynamic topology-dependent transmission scheduling
Nodes contend for and reserve time slots by means of a distributed reservation and handshake mechanism.
Support broadcast, unicast, and multicast transmissions.
The operation is based on two basic principles:
The receiver(s) of a flow must inform the potential source nodes about the reserved slot on which it is currently receiving packets. The source node must inform the potential destination node(s) about interferences in the slot.
Usage of negative acknowledgements for reservation requests, and control packet transmissions at the beginning of each slot, for distributing slot reservation information to senders of broadcast or multicast sessions.

56. Contention-based protocols with Reservation Mechanisms
Hop reservation multiple access protocol (HRMA)
a multichannel MAC protocol which is based on half-duplex, very slow frequency-hopping spread spectrum (FHSS) radios
uses a reservation and handshake mechanism to enable a pair of communicating nodes to reserve a frequency hop, thereby guaranteeing collision-free data transmission.
can be viewed as a time slot reservation protocol where each time slot is assigned a separate frequency channel.
Soft reservation multiple access with priority assignment (SRMA/PA)
Developed with the main objective of supporting integrated services of real-time and non-real-time application in ad hoc networks, at the same time maximizing the statistical multiplexing gain.
Nodes use a collision-avoidance handshake mechanism and a soft reservation mechanism.

57. Contention-based protocols with Reservation Mechanisms
Five-Phase Reservation Protocol (FPRP)
a single-channel time division multiple access (TDMA)-based broadcast scheduling protocol.
Nodes uses a contention mechanism in order to acquire time slots.
The protocol assumes the availability of global time at all nodes.
The reservation takes five phases: reservation, collision report, reservation confirmation, reservation acknowledgement, and packing and elimination phase.
MACA with Piggy-Backed Reservation (MACA/PR)
Provide real-time traffic support in multi-hop wireless networks
Based on the MACAW protocol with non-persistent CSMA
The main components of MACA/PR are:
A MAC protocol
A reservation protocol
A QoS routing protocol

58. Contention-based protocols with Reservation Mechanisms
Real-Time Medium Access Control Protocol (RTMAC)
Provides a bandwidth reservation mechanism for supporting real-time traffic in ad hoc wireless networks
RTMAC has two components
A MAC layer protocol is a real-time extension of the IEEE DCF.
A medium-access protocol for best-effort traffic
A reservation protocol for real-time traffic
A QoS routing protocol is responsible for end-to-end reservation and release of bandwidth resources.

59. Contention-based protocols with Scheduling Mechanisms
Protocols in this category focus on packet scheduling at the nodes and transmission scheduling of the nodes.
The factors that affects scheduling decisions
Delay targets of packets
Traffic load at nodes
Battery power
Distributed priority scheduling and medium access in Ad Hoc Networks present two mechanisms for providing quality of service (QoS)
Distributed priority scheduling (DPS) – piggy-backs the priority tag of a node’s current and head-of-line packets o the control and data packets
Multi-hop coordination – extends the DPS scheme to carry out scheduling over multi-hop paths.

60. Contention-based protocols with Scheduling Mechanisms
Distributed Wireless Ordering Protocol (DWOP)
A media access scheme along with a scheduling mechanism
Based on the distributed priority scheduling scheme
Distributed Laxity-based Priority Scheduling (DLPS) Scheme
Scheduling decisions are made based on
The states of neighboring nodes and feed back from destination nodes regarding packet losses
Packets are recorded based on their uniform laxity budgets (ULBs) and the packet delivery ratios of the flows. The laxity of a packet is the time remaining before its deadline.

61. MAC Protocols that use directional Antennas
MAC protocols that use directional antennas have several advantages:
Reduce signal interference
Increase in the system throughput
Improved channel reuse
MAC protocol using directional antennas
Make use of an RTS/CTS exchange mechanism
Use directional antennas for transmitting and receiving data packets
Directional Busy Tone-based MAC Protocol (DBTMA)
It uses directional antennas for transmitting the RTS, CTS, data frames, and the busy tones.
Directional MAC Protocols for Ad Hoc Wireless Networks
DMAC-1, a directional antenna is used for transmitting RTS packets and omni-directional antenna for CTS packets.
DMAC-1, both directional RTS and omni-directional RTS transmission are used.

62. Other MAC Protocols Multi-channel MAC Protocol (MMAC)
Multiple channels for data transmission
There is no dedicated control channel.
Based on channel usage channels can be classified into three types: high preference channel (HIGH), medium preference channel (MID), low preference channel (LOW)
Multi-channel CSMA MAC Protocol (MCSMA)
The available bandwidth is divided into several channels
Power Control MAC Protocol (PCM) for Ad Hoc Networks
Allows nodes to vary their transmission power levels on a per-packet basis
Receiver-based Autorate Protocol (RBAR)
Use a rate adaptation approach
Interleaved Carrier-Sense Multiple Access Protocol (ICSMA)
The available bandwidth is split into tow equal channels
The handshaking process is interleaved between the two channels.

63. ROUTING PROTOCOLS

64. ROUTING PROTOCOLS Reactive protocols (on-demand)
Determine route if and when needed
Source initiates route discovery
Example: DSR (dynamic source routing)
Proactive protocols (table-driven)
Traditional distributed shortest-path protocols
Maintain routes between every host pair at all times
Based on periodic updates; High routing overhead
Example: DSDV (destination sequenced distance vector)
Hybrid protocols
Adaptive; Combination of proactive and reactive
Example : ZRP (zone routing protocol)

65. Protocol Trade-offs Reactive protocols Proactive protocols
Lower overhead since routes are determined on demand
Significant delay in route determination
Employ flooding (global search)
Control traffic may be bursty
Proactive protocols
Always maintain routes
Little or no delay for route determination
Consume bandwidth to keep routes up-to-date
Maintain routes which may never be used
Which approach achieves a better trade-off depends on the traffic and mobility patterns

66. Dynamic Source Routing (DSR)
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

67. 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

68. Broadcast transmission
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

69. 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

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

71. 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

72. 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

73. 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

74. 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
Represents RREP control message

75. 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

76. Data Delivery in DSR Y Z DATA [S,E,F,J,D] S E F B C M L J A G H D K I
Packet header size grows with route length

77. Dynamic Source Routing: Advantages
Routes maintained only between nodes who need to communicate
reduces overhead of route maintenance
Route caching can further reduce route discovery overhead
A single route discovery may yield many routes to the destination, due to intermediate nodes replying from local caches

78. DSR: Disadvantages
Packet header size grows with route length due to source routing
Flood of route requests may potentially reach all nodes in the network
Potential collisions between route requests propagated by neighboring nodes
insertion of random delays before forwarding RREQ
Increased contention if too many route replies come back due to nodes replying using their local cache
Route Reply Storm problem
Stale caches will lead to increased overhead

79. Ad Hoc On-Demand Distance Vector Routing (AODV)
DSR includes source routes in packet headers
Resulting large headers can sometimes degrade performance
particularly when data contents of a packet are small
AODV attempts to improve on DSR by maintaining routing tables at the nodes, so that data packets do not have to contain routes
AODV retains the desirable feature of DSR that routes are maintained only between nodes which need to communicate

80. AODV Route Requests (RREQ) are forwarded in a manner similar to DSR
When a node re-broadcasts a Route Request, it sets up a reverse path pointing towards the source
AODV assumes symmetric (bi-directional) links
When the intended destination receives a Route Request, it replies by sending a Route Reply (RREP)
Route Reply travels along the reverse path set-up when Route Request is forwarded

81. Route Requests in AODV 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

82. Broadcast transmission
Route Requests in AODV Y
Broadcast transmission Z S E F B C M L J A G H D K I N
Represents transmission of RREQ

83. Route Requests in AODV Y Z S E F B C M L J A G H D K I N
Represents links on Reverse Path

84. Reverse Path Setup in AODVY Z S E F B C M L J A G H D K I N
Node C receives RREQ from G and H, but does not forward
it again, because node C has already forwarded RREQ once

85. Reverse Path Setup in AODVY Z S E F B C M L J A G H D K I N

86. Reverse Path Setup in AODVY Z S E 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 RREQ

87. Forward Path Setup in AODVY Z S E F B C M L J A G H D K I N
Forward links are setup when RREP travels along
the reverse path
Represents a link on the forward path

88. Route Request and Route Reply
Route Request (RREQ) includes the last known sequence number for the destination
An intermediate node may also send a Route Reply (RREP) provided that it knows a more recent path than the one previously known to sender
Intermediate nodes that forward the RREP, also record the next hop to destination
A routing table entry maintaining a reverse path is purged after a timeout interval
A routing table entry maintaining a forward path is purged if not used for a active_route_timeout interval

89. Link Failure
A neighbor of node X is considered active for a routing table entry if the neighbor sent a packet within active_route_timeout interval which was forwarded using that entry
Neighboring nodes periodically exchange hello message
When the next hop link in a routing table entry breaks, all active neighbors are informed
Link failures are propagated by means of Route Error (RERR) messages, which also update destination sequence numbers

90. Route Error
When node X is unable to forward packet P (from node S to node D) on link (X,Y), it generates a RERR message
Node X increments the destination sequence number for D cached at node X
The incremented sequence number N is included in the RERR
When node S receives the RERR, it initiates a new route discovery for D using destination sequence number at least as large as N
When node D receives the route request with destination sequence number N, node D will set its sequence number to N, unless it is already larger than N

91. AODV: Summary Routes need not be included in packet headers
Nodes maintain routing tables containing entries only for routes that are in active use
At most one next-hop per destination maintained at each node
DSR may maintain several routes for a single destination
Sequence numbers are used to avoid old/broken routes
Sequence numbers prevent formation of routing loops
Unused routes expire even if topology does not change