16-05-2012, 04:36 PM
Simulation of Packet Telephony in Mobile Adhoc Networks Using Network Simulator
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Abstract
Packet Telephony has been regarded as an alternative
to existing circuit switched fixed telephony. To propagate new
idea regarding Packet Telephony researchers need to test their
ideas in real or simulated environment. Most of the research in
mobile ad-hoc networks is based on simulation. Among all
available simulation tools, Network Simulator (ns2) has been
most widely used for simulation of mobile ad-hoc networks.
Network Simulator does not directly support Packet Telephony.
The authors are proposing a technique to simulate packet
telephony over mobile ad-hoc network using network simulator,
ns2.
Keywords-Network Simulator; Mobile Ad-hoc Networks; Packet
Telephony; Simulator; Voice over Internet Protocol.
I. INTRODUCTION
The problem of extending the reach of fixed telephonic
system over an area using mobile ad-hoc network is one of the
research area that has got the attention of Computer Science
research fraternity. One obvious solution to the problem comes
in form of Packet Telephony, used interchangeably with Voice
over Internet Protocol in this work. In packet telephony real
time voice conversations are transmitted from source to
destination using packet switched data networks rather than a
circuit switched telephone network. With the help of Packet
Telephony over mobile ad-hoc networks one can extend the
reach of existing fixed telephony. This whole mechanism of
extending the reach of fixed telephony is also termed as Fixed
to Mobile Convergence (FMC) [1]. When this extension of
fixed telephony is done over a mobile ad-hoc network, the
problem becomes unique due to underlying characteristics of
mobile ad-hoc network. The very nature of mobile ad-hoc
networks makes the extension of telephonic call multi-hop
where each intermediate node acts as potential router. The
solution of extending the reach of wired telephony becomes
highly beneficial with use of license free ISM band for
implementing FMC. To summarize this would help forwarding
telephonic call to a mobile node without any cost.
The effective extension of telephonic call over the mobile
ad-hoc network is constrained by various Quality of Service
requirements as recommended by United Nations Consultative
Committee for International Telephony & Telegraphy
(CCITT). A number of Quality of Service, QoS parameters for
implementation of fixed to mobile convergence in mobile adhoc
networks has been suggested. These parameters include
End to End Delay, Packet Delivery Rate, Packet Drop Rate,
Throughput, Channel Utilization, Jitter etc. Any proposed
system should follow strict QoS requirements to become
practically viable. For example the End to End delay must be
less than 250 ms otherwise the system may appear to be half
duplex and user may complain about distortion and echo. In
other words, QoS plays an important role in implementing
Packet Telephony over Mobile Ad-hoc Networks.
Main deterrents in realizing the QoS based services over
Mobile Ad-hoc Networks are a) Limited bandwidth of Mobile
Ad-hoc Network b) Dynamic Topology of Mobile Ad-hoc
Networks c) Limited Processing & Storing Capabilities of
mobile nodes. Numbers of research works are in progress for
ensuring QoS based Packet Telephony over Mobile Ad-hoc
Networks.
It is not always feasible to develop a real time environment
for conducting research. Then researchers have to resort on
secondary means like simulation. In mobile ad-hoc network
research, simulation techniques have been widely used. A
number of simulation tools for developing mobile ad-hoc
network environment are available. Most notable among these
are Network Simulator (ns2), MATLAB, CSIM, OPNET,
Qualinet, GoMoSlim etc. Out of these ns2 is most widely used
tool for the simulation of mobile ad-hoc networks.
Network Simulator does not support VoIP or Packet
Telephony directly. So a need was felt by the authors to devise
a technique for the simulation of Packet Telephony with
network simulator, ns2. The technique proposed should help
users to test performance of the mobile ad-hoc network under
different permutations and combinations of various network
parameters.
II. RELATED WORK
Kurkowski et al. [2] have conducted a survey on the
techniques employed by various authors for research on mobile
ad-hoc networks. The authors have observed that out of 60%
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authors resorting to simulation based techniques, 44% have
used ns2 for drawing their conclusions.
Paolo Giacomazzi et al. [3] have worked on the issue of
feasibility of fixed to mobile convergence using a mobile adhoc
network. The authors have proposed complete system
architecture for their implementation. The proposed
architecture was then evaluated by the authors in terms of
various quality of service (QoS) parameters like Call Drop
Rate, MOS etc.
III. BACKGROUND
A Mobile Ad-hoc Network, MANET, may be defined as a
collection of autonomous nodes that communicate with each
other by forming multi-hop networks and maintaining
connectivity in decentralized manner. All nodes in the Mobile
Ad-hoc Network are of dynamic nature. This means that the
topology of mobile ad-hoc networks keeps on changing.
Mobile Ad-hoc Networks do not have fixed routers. All
nodes in these networks can act as routers. Apart from this
mobile ad-hoc networks are characterizes by a number of other
salient features like range limitation, unreliable media,
interference from other sources, dependency on the willingness
of intermediate nodes, scarcity of power and vulnerability to
security threats etc.
Mobile Ad-hoc Networks have been found to be very
useful in emergency search and rescue operations. The reason
behind this is their small deployment time. Moreover their
deployment cost is also small.
Voice over Internet Protocol represents a set of rules and
techniques to transport telephonic conversation over Internet
Protocol. VoIP has proved to be one of the most admired and
utilized application of internet these days. VoIP can prove to be
a very beneficiary application. VoIP can help in achieving
Fixed to Mobile Convergence (FMC) over mobile ad-hoc
networks. The process behind this idea of achieving FMC over
mobile ad-hoc network is illustrated in figure 2. In this figure
various nodes are encircled representing their range. Various
nodes with coinciding ranges may be termed as neighbors. In
this figure node B is neighbor to nodes A and C. Different
neighbors can exchange data through the channel. The
extension of call from the fixed telephone to the node E can be
explained as:
Initially analog voice conversations are digitized and then
compressed using some suitable codec. Afterwards these
compressed conversations are packetized in form of IP packets
and then transported to E using underlined routing protocol. At
E the packet are converted back to analog telephonic
conversations. The main hurdle in implementing FMC over
MANETs comes from the dynamic nature (see figure 1) and
limited node range in these networks.
Figure 1. Dynamic Topology of MANETs
Figure 2. Fixed to Mobile Convergence over Mobile ad-hoc
network
IV. SYSTEM ARCHITECTURE
System architecture represents the protocol layer used for
the implementation of a network. During this work we have
used a system architecture[4]-[5] composed of five network
layers (see figure 3). Various responsibilities are distributed
between layers as below:
A. Application Layer
The functions provided by this layer consist of digitizing &
compressing the telephonic conversations in accordance with
the available bandwidth. As already mentioned major
constraint in implementing Packet Telephony [6] and hence
FMC over the mobile ad-hoc networks comes from the limited
bandwidth these networks posses. Some effective compression
technique is required to overcome this limitation. A number of
compression algorithms have been suggested by the
International Telecommunication Union, ITU. Out of these G
.729 codec [7] working at 8 kbps has been found to be most
useful in scenarios where available bandwidth is small
compared to the overall traffic load.
B. Transport Layer
One needs to choose between TCP and UDP for
implementing transport layer. TCP is connection oriented
protocol whereas UDP is comparatively unreliable
connectionless protocol. The implementation of TCP would
require higher bandwidth as compared to implementation of
A B
C
A
B
C
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UDP. In case of wireless mobile ad-hoc networks with limited
available bandwidth UDP is the obvious choice. To overcome
the limitations of UDP in relatively unreliable mobile ad-hoc
network RTP (Real Time Transport Protocol) is run on the top
of UDP. RTP provides services like payload identification,
sequence numbering etc to the UDP. Almost every device uses
a standard RTP to transmit audio and video.
C. Network Layer
Network layer in case of the mobile ad-hoc networks plays
a central and pivotal role owing to the decentralized nature of
mobile ad-hoc networks. All nodes participating in a mobile
ad-hoc network acts as a potent router and forward the packets
received from neighbors. A number of routing algorithms for
mobile ad-hoc networks have been proposed. The routing
algorithms for mobile ad-hoc networks have been classified
[9]-[13] into two categories viz. topology based routing
algorithms and position based routing algorithms. Due to
various limitations most practical mobile ad-hoc networks
employ topology based routing algorithms. Some major
algorithms belonging to this category are DSR [14], DSDV
[15], AODV [16], TORA.
D. MAC Layer
MAC layer plays a critical role in the successful
implementation of mobile ad-hoc networks. Mobile ad-hoc
networks have scarcity of available channel bandwidth.
Moreover MAC layer not only has the responsibility of channel
sharing but also hides the complexity of wireless network from
upper layers.
APPLICATION LAYER
(G.729 Codec)
TRANSPORT LAYER
(RTP over UDP)
NETWORK LAYER
DSR/DSDV/TORA/AODV)
DATA LINK LAYER
(LLC & IEEE 802.11 MAC)
PHYSICAL LAYER
(IEEE 802.11 PHY)
Figure 3. The Network Architecutre
So, intelligent selection of MAC layer is very important. A
number of MAC solutions are available these days. A good
survey on these can be found in [17]-[21]. IEEE based MAC
solutions have been most widely used in practical mobile adhoc
networks.
E. Physical Layer
The responsibility with physical layer is to take data from
source to destination. A number of physical layer solutions like
IEEE 802.11 PHY [22]-[24], UTLA-TDD[25] are available.
During this work we have implemented legacy IEEE 802.11
based physical layer.