Ad hoc networks
- 2. Introduction
In computer networking, an ad hoc network refers to a
network connection established for a single session
and does not require a router or a wireless base station.
It is defined as the category of wireless network that
utilize multi-hop radio relaying and capable of
operating without the support of any fixed
infrastructure . hence, is also called infrastructure -less
network.
Routing and Resource management are done in a
distributed manner.
- 3. Types of wireless networks
Wireless
mesh
Cellular network
Hybrid
wireless wireless
network network
Wireless
sensor
network
Single hop wireless network Multi hop wireless network
or
Ad hoc network
- 4. Types of Ad hoc network
1. Wireless mesh network
formed to provide an alternate communication infrastructure
for mobile or fixed nodes/users, without the spectrum reuse
constraints and the requirements of network planning of
cellular networks.
2. Wireless sensor network
used to provide a wireless communication infrastructure
among the sensors deployed in a specific application domain.
- 5. 3. Hybrid wireless network
when two nodes in the same cell want to
communicate with each other, the connection is
routed through multiple wireless hops over the
intermediate nodes. the base station maintains the
information about the topology of the network for
efficient routing.
- 8. Difference
Cellular network Ad hoc network
Fixed infrastructure No infrastructure
Single hop wireless Multi hop wireless
link link
Centralized routing Distributed routing
High cost Low cost
Seamless connectivity Frequent path breaks
due to mobility
- 9. Characteristics
Operating without a central coordinator
Multi-hop radio relaying
Frequent link breakage due to mobile nodes
Constraint resources (bandwidth, computing
power, battery lifetime).
Instant deployment
- 11. General Issue for Ad hoc network
Medium access scheme
Distributed operation
Synchronization
Hidden terminal
Access delay
Fairness
Resource reservation
Capability of power control
- 12. General Issue for Ad hoc network
Routing
Mobility
Bandwidth constraint
Error prone
Minimum rout acquisition
Quick rout reconfiguration
Loop free routing
Security and privacy
- 13. General Issue for Ad hoc network
Multicasting
Robustness
Efficiency
Quality of services
Efficient group management
Scalability
Security
- 14. General Issue for Ad hoc network
Transport layer protocol
UDP
TCP
TORA
SMR
Pricing Scheme
Self organization
- 15. General Issue for Ad hoc network
Security
Denial of services
Resource consumption
Energy depletion
Buffer overflow
Interference
Addressing and Service Discovery
- 17. Infrastructure based versus Ad Hoc Networks
Infrastructure Networks contain special nodes called access
points(APs), which are connected via existing networks.
APs are special in the sense that they can interact with
wireless nodes as well as with the existing wired network.
The other wireless nodes , also known as mobile stations ,
communicate via APs. The APs also act as bridges with
other networks.
Ad hoc LANs do not need any fixed infrastructure. These
networks can be set up on the fly at any place. Nodes
communicate directly with each other or forward messages
through other nodes that are directly accessible.
- 18. The design of infrastructure based networks is simpler
because most of the network functionality lies within
the access point ,whereas the client can remain quite
simple.
In Ad hoc networks, the complexity of each node is
higher because every node has to implement medium
access mechanisms to provide certain quality of
service.
Infrastructure based networks lose some of the
flexibility which wireless networks can offer.
e.g. They cannot be used for disaster relief in cases
where no infrastructure is left, where ad hoc networks
can be used.
- 19. Architecture of Ad Hoc Network
IEEE 802.11 Specifies the most famous family
WLAN in which many products are available
802.11 is a set of IEEE standards that govern
wireless networking transmission methods.
Under 802.11 standards mobile terminals can
operate in two modes
1. Infrastructure Mode
2. Ad Hoc Mode
- 20. Ad Hoc Mode
IEEE 802.11 only covers PHY layer and MAC layer
PHY layer is subdivided into
1. Physical Layer Convergence Protocol (PLCP)
2. Physical Medium Dependent sub layer (PMD)
PHY management include channel tuning and
responsible for higher layer functions
(e.g. control of bridging)
MAC management controls authentication
mechanism and power management to save battery
power.
- 21. MAC Layer
Provides “ Asynchronous Data Service “ and “ Time
Bounded Service “
802.11 only offers the “ Asynchronous Data Service “ in
Ad-hoc Mode , but both services can be offered by
Infrastructure Mode.
Basic access mechanism defined for IEEE 802.11 are
1. Distributed Coordination Function (DCF)
2. Point Coordination Function (PCF)
DCF only offers asynchronous service , while PCF
offers both asynchronous and time bounded service
MAC mechanism also called distributed foundation
wireless medium access control (DFWMAC)
- 22. Basic DFWMAC using CSMA/CA
Mandatory access mechanism for IEE 802.11 is based
on CSMA/CA
CSMA/CA mechanism shown in figure.
- 23. If medium is busy , nodes have to wait for the duration of
DIFS , entering a contention phase.
Each node now choose a random back off time within a
contention window and delays medium access for this
random amount of time.
As soon as a node senses the channel is busy , it has lost this
cycle and has to wait for the next chance.
The basic CSMA/CA mechanism is not fair
To provide fairness , IEEE 802.11 adds back off timer
Each node selects random amount of waiting time
If certain station does not get access to the medium in first
cycle it stops its back off timer , waits for the channel to be
idle again for DIFS and starts the counter again.
- 24. DFWMAC with RTS/CTS
Hidden terminal problem may occur in 802.11 ,if one
station can receive two others, but those cannot
receive each other.
To deal with this problem, mechanism using two
control packets , RTS and CTS .
After waiting for DIFS , the sender can issue a request
to send (RTS) control packet.
Every node receiving this RTS now has to set its net
allocation vector (NAV) in accordance with the
duration field
The NAV than specifies earliest point at which the
station can try to access the medium.
- 25. If the receiver receives the RTS , it answers with the
clear to send (CTS) waiting for SIFS
This CTS packet contains duration field and receivers
have to adjust their NAV.
Now all nodes within receiving distance around sender
and receiver are informed that they have to wait more
time before accessing the medium.
This mechanism reserves the medium for one sender
exclusively.
It is also called a Virtual Reservation Scheme.
- 26. ROUTING ALGORITHM
TABLE DRIVEN
Maintain the route table
The Wireless Routing Protocol localizes the updates to
immediate neighbors.
The Cluster Gateway Switch Routing
protocol reduces the size of the tables and amount of
information propagation by having each cluster of
nodes select a cluster head.
- 27. Source initiated on demand
On-demand routing protocols are characterized by a
path discovery mechanism which is initiated when a
source needs to communicate with a destination that it
does not know how to reach.
- 28. Classification of protocols
Based on the routing information update
mechanism
Proactive or table-driven routing protocols
Reactive or on demand routing protocols
Hybrid routing protocols
Based on the use of Temporal information for
Routing
Routing protocols using past temporal information
Routing protocols that use future temporal information
- 29. Based on Topology Information Organization
Flat topology routing protocols
Hierarchical topology routing protocols
Based on the Utilization of specific Resources
Power aware routing
Geographical information assisted routing
- 30. Table Driven Routing Protocol
Destination sequenced distance-vector protocol
(DSDV)
According to this protocol every node maintains
a table that contains the shortest distance and
the first node on the shortest path to every
other node in the network.
It incorporate table update with increasing
sequence number tag to prevent loops, to
counter to the count-to-infinity problem, and
for faster convergences.
- 31. DSDV (continue ..)
Destination Next Hop Distance Sequence Number
A’s routing table :
A A 0 S205_A
B B 1 S334_B
C C 1 S198_C
D D 1 S567_D
E D 2 S767_E
F D 2 S45_F
- 32. Advantage of DSDV
The availability of the routers to all destination at all
times implies that much less delay is involved in the
route setup process.
The mechanism of incremental updates with sequence
number tags makes the existing wired network
protocol adaptable to ad hoc networks.
Hence wired network protocol can be applied to the ad
hoc network by less modification.
- 33. Disadvantage of DSDV
A small network with high mobility or a large network
with low mobility can completely choke the available
bandwidth. Hence this protocol suffers from excessive
control overhead that is proportional to the number of
nodes in the network.
In order to obtain information about a particular
destination node, a node has to wait for a table update
message initiated by the same destination node.
- 34. Wireless Routing protocol
It is also known as WRP protocols.
It is same as DSDV protocol ,but contain some
additional flag in table like status of the path which is
simple path(correct) or a loop(error), or the
destination node not marked(null).
- 36. Disadvantage of WRP
Complexity of maintenance of multiple tables
demands a larger memory and greater processing
power from the nodes.
At the high mobility, the control overhead involved in
updating table entries is almost the same as DSDV. So
it is not suitable for highly dynamic and also for very
large ad hoc networks.
- 37. Cluster head Gateway Switch Routing
protocol
It is also called CGSR protocols
CGSR organizes nodes into clusters, with coordination
among the member of each cluster instructed nodes
named “cluster head”.
CGSR protocol creates a fixed region in the network.
Each node in the cluster region is known as cluster
member and they all are connected with pivot node
which is called cluster-head .Two cluster region are
connected via node which are place in intersection
region of two cluster region and called cluster-gateway.
- 39. Advantage
CGSR is a hierarchical routing scheme which enables
partial coordination between nodes by electing
cluster-heads. Hence , better bandwidth utilization is
possible.
It is easy to implement priority scheduling scheme
with token scheduling and gateway code scheduling.
- 40. Disadvantage
It increases in path length and instability in the system
at high mobility when the rate of change of cluster-
heads is high.
To avoid gateway conflicts, more resources are
required.
The power consumption at the cluster-head node is
also a matter of concern because the battery-draining
rate at the cluster-head is higher than that at a normal
node.
- 41. On-Demand routing protocols
Dynamic Source Routing Protocol (DSR)
In this routing protocol does not require periodic
‘hello’ packet transmission, which are used by a node
to inform its neighbors of its presence. The basic
approach of this protocol during the routing
construction phase is to establish a route by flooding
Route-Request packets in the network. The
destination node,
On receiving a Route-Request packet, respond by
sending Route-Reply packets back to the resource.
- 43. Advantage
Do not exchange routing update periodically, so
overhead transmission is greatly reduced
Can refer to cache for the new route when link fails.
- 44. Disadvantages
Scalability problem: High route discovery latency for
large network.
High mobility problem: although the packet dropped
may not be substantional, the overhead traffic will
increase a lot.
- 45. Ad Hoc On-Demand Distance
Vector
In AODV, a field of the number of hops is used in the
route record, instead of a list of intermediate router
addresses.
Each intermediate router sets up a temporary reverse
link in the process of a route discovery.
This link points to the router that forwarded the
request.
- 46. Advantage
AODV is loop-free due to the destination sequence
numbers associated with routes. Therefore, it offers
quick convergence when the ad hoc network topology
changes which, typically, occurs when a node moves in
the network
Disadvantage
Poor scalability is a disadvantage of AODV.
- 47. Hybrid routing protocols
Zone Routing Protocol (ZRP)
ZRP is formed by two
sub-protocols, the Intrazone Routing Protocol (IARP)
and the Interzone Routing Protocol (IERP).
- 48. IARP is “a limited scope proactive routing protocol
used to improve the performance of existing globally
reactive routing protocols”. It relies on the service of a
certain neighbor discovery protocol (NDP) to provide
neighbor information. IARP may use a scheme based
on the time-to-live (TTL) field in IP packets to
control the zone range.
IERP is the reactive routing component of ZRP. This
scheme is responsible for finding a global path. It
avoids global queries for destinations that would be
sent to surrounding hop neighbors. When global
queries are required, “the routing zone based
broadcast service can be used to efficiently guide
route queries outward, rather than blindly relaying
queries from neighbor to neighbor” .