MODELING AND SIMULATION OF LONG DISTANCE
COMMUNICATION LINKS
Ms. Dhanashree Gadkari, Ms. Preeti Palkar.
Mahindra British Telecom Ltd.
Pune, India.
Email: dhanashree.gadkari@mahindrabt.com, preeti.palkar@mahindrabt.com
distances. The links were subjected to various traffic
intensities and the corresponding delays were compared and
studied.
ABSTRACT:
A Corporation came to Mahindra British Telecom with a need
to maximize the reliability of their long-distance information
transfers. To plan faithful data transfer under virtually any
circumstance, we used OPNET Modeler. This paper describes
how through our studies we modeled and simulated various
long-distance communication links and customized node
models until we got the best results. In doing this, we coded
the process model to eliminate the possibility of
communication failure between the workstations by using
redundancy. The links were subjected to various workloads
and compared. In the final analysis, the various scenarios
created in OPNET Modeler proved to be extremely beneficial
in providing high-reliability solutions for global data transfer.
Long Distance Links:
Choosing a medium:
The information signal is carried by the
bearer, which could be a pair of wires, a radio carrier, a fiber
optic cable, etc. Modern long distance links use either radio as
the medium or fiber optic cable. The decision on which one to
use is driven by economics more than any other factor.
However, capacity, information bandwidth can be well other
deciding factors. Brief summary points on various media are
listed below:
INTRODUCTION:

The importance of Long Distance Communication
Links has touched its zenith because of the sole reason of
Globalization of Business. The situation appears to be such
that modern telecommunication & IT tend to permit the
companies to extend control to distant points. Global networks
mainly operate to provide major paths between continents,
typically spanning oceans. Digital communication networks
via fiber optic cables, microwave links, & satellites are quite
capable of providing connectivity among diverse points in
domestic or global networks. Thus a modern network allows
user information to be routed over a flexible medium.
International satellites, transoceanic cables, transborder
microwave, cable systems, & domestic satellites provide the
long haul links.
Theoretically, any one of these media could be
employed
universally;
however
an
international
telecommunication network must employ an appropriate
mixture. Satellite Communication is a reliable means of
establishing international telecommunication links between
multiple points on the ground, especially to transmit
international video. Transoceanic cables were the first means
of providing international telegraph & telephone links across
water bodies. The fact that cables & satellites are
complementary is borne out by the need for the satellites to
provide temporary replacement capacity to restore service
when a high capacity cable is broken. This gives rise to the
concept of achieving “reliability” by means of “redundancy”.
In this paper, we have used this concept. Using OPNET,
various scenarios were modeled in which radio link is used as
backup for shorter distances while satellite is used for longer

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Copper wire is mature technology, rugged and
inexpensive; maximum transmission speed is limited
Glass fiber:
 Higher speed
 More resistant to electro-magnetic
interference
 Spans longer distances
 Requires only single fiber
 More expensive; less rugged
Radio and microwave don't require physical
connection
Radio and infrared can be used for mobile
connections
Laser also does not need physical connection and
supports higher speeds.
Study of RadioLink:
The design of radio link involves:
1. Setting performance requirements.
2. Site selection and preparation of a path
profile to determine antenna tower
heights.
3. Carrying out a path analysis also called
a link budget.
4. Running a path/site survey.
5. Test of the system prior to cut over of
traffic.
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Radio Link – Fiber Optic Link Comparison:
The advantages of radio link are listed
below:
 Less Expensive
 No requirement of right-of-way
 Less vulnerable to vandalism
 Not susceptible to “accidental” cutting of link
 Often more suited to crossing rough terrain
 Often more practical in heavily urbanized areas
 Convenient back up for fiber optic links.
the no. of satellites required for desired coverage. In addition,
since they travel at high speeds relative to the earth’s surface,
a user may need to be handed off from satellite to satellite as
they pass rapidly overhead. Therefore, steerable antennas are
crucial to maintain continuous service.
Various frequency bands commonly used are:
C band (4-8 GHz)
Ku band (10-18 GHz)
Ka band (18-31 GHz)
With a higher frequency band and a corresponding shorter
wavelength, smaller antennas can be used to receive the
signal. These bands are preferred as they solve the problem of
orbital crowding and are not shared by terrestrial services.
Study Of Satellite:
A satellite communication system, distinguished by
its global coverage, inherent broadcast capability, bandwidthon-demand flexibility, and the ability to support mobility, is an
excellent candidate to provide broadband integrated Internet
services to globally scattered users. Satellite networks can
serve as broadband access networks, high-speed backbone
networks connecting heterogeneous networks or simply as
communication links between users with fixed or mobile
terminals. It is an extension of LOS microwave technology.
The satellite must be within LOS of the participating earth
terminal. A satellite system consists of a space segment and a
ground segment. The ground system consists of gateway
stations, a network control center, and operation control
centers. The space segment consists of satellites, which may
be classified into geostationary orbit (GSO), and
nongeostationary orbit (NGSO) satellite, including medium
earth orbit (MEO) and low earth orbit (LEO) satellite,
according to the orbit altitude above the earth’s surface.
A geostationary orbit is 35,786 km above equator. Its
revolution around the earth is synchronized with the earth’s
rotation. Hence it appears fixed to an observer on the earth’s
surface. Its high altitude allows each GSO satellite to cover
approximately one third of the earth’s surface, excluding the
high latitude areas.
The cost of launching these satellites is high. Due to
its high altitude and inherent signal degradation with distance,
large antennas and transmission power are required for both
the GSO satellite and ground terminals. The most significant
problem is the large propagation delay for GSO satellite links.
The MEO orbit is approximately 3000 km up to the GSO orbit
from the earth with a typical round trip propagation delay less
than that of GSO.
The LEO orbit is located 200-3000 km above the
earth’s surface with round trip propagation delay, which is
comparable to that of a terrestrial link. The duration of LEO
periods is 95 to 120 minutes. LEO systems try to ensure a high
elevation for every spot on earth to provide a high quality of
communication link. Each LEO satellite will only be visible
from the earth for around 10 minutes. Since LEO/MEO
satellites are closer to the earth, the necessary antenna size and
transmission power level are much smaller; but their footprints
are also much smaller. The lower the orbit altitude, the greater
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
Satellite – Fiber Optic Link Comparison:
Drawbacks of satellite:
Limited information bandwidth
Excessive delay when popular bandwidth satellite systems
are utilized.
OUR WORK:
This paper comprises of the study of the performance
of various long distance communication links. There are two
scenarios, one, which compares physical link with radio
medium and other which compares physical link with satellite
medium. The basic parameter of concern in the study is the
end to end delay incurred by the various mediums on basis of
which they are compared.
Scenario 1:
The network model consists of two similar nodes,
which are connected via a duplex physical link as well as
duplex radio link. Basically, the radio link acts as a back up
for the physical link.
Figure 1. Network model of the radio-link scenario.
The node model is identical for both the nodes. It consists of:
Generator,
queue,
processor,
point-to-point
transmitter and receiver, radio transmitter and receiver. The
statistic wire is connected from queue to processor. The
statistic of the queue, which is used for decision making, is
“queue size (bits)”. The threshold value set for queue size is
1900000 bits. The current value being below it, the data is sent
on the physical link, else on the radio.
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Fig. 2 Node model of the node.
Fig. 5 Node model of the ground node.
We have used LEO for the satellite. The process model of the
two nodes are identical to those in scenario 1. The node model
of the ground node consists of a control and data signal
transmitter and receiver and a point-to-point transmitter
receiver pair. The node model of the satellite comprises of pair
of antenna, radio transmitter and radio receiver for data and
control signals. The threshold value set for queue size is
1000000 bits. The control signal is sent throughout the
simulation duration time which is of 10 hours. Initially the
queue size condition is validated and then if the threshold has
crossed then the data is transmitted only if the satellite is in
LOS of both the nodes, else stored in the processor itself.
Fig. 3 Process model of the processor.
This scenario is simulated thrice by varying the intensity of the
workload. The data is generated at inter arrival arguments of 0.6
seconds (fastest data generation rate), 0.8 seconds and 1.0 seconds
(slowest data generation rate).
Scenario 2:
The network model consists of two similar nodes,
which are connected via a duplex physical link as well as
duplex satellite link. Basically, the satellite link acts as a back
up for the physical link.
Fig. 6 Node model of the satellite.
This scenario is simulated thrice by varying the intensity of
the workload. The data is generated at inter arrival arguments
of 0.6 seconds (fastest data generation rate), 0.8 seconds, 1.0
and 1.2 seconds (slowest data generation rate).
Fig. 4 Network model of satellite-link scenario.
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Graphs and Conclusion:
Scenario 1:The following graph depicts the ETE delay for
various workloads.
Run 1: Inter arrival args = 0.6 seconds.
Run 2: Inter arrival args = 0.8 seconds.
Run 3: Inter arrival args = 1.0 seconds.
The queue size and throughput graphs for one of the nodes are
as follows:
Fig. 9 Queue size & Throughput of physical link & radio
receiver.
Fig. 7 ETE delay of the physical link.
Fig. 10 Throughput of physical link & radio transmitter.
Thus, we can conclude that whenever the queue size goes
above the threshold value, data is transferred on the radio link.
Also, the maximum value of ETE delay for radio, 15000 sec.
is almost eight times that for the physical link which is 1750
sec. If ETE radio delay needs to be kept below a certain value,
then we need to compromise on other parameters of the
network, such as the distance between the nodes or the
intensity of workload.
Fig. 8 ETE delay of the radio transmission.
Data rate for physical link : 1000000000 bits/sec.
Data rate for radio transmitter/receiver : 9600 bits/sec.
Service rate of queue : 1000 bits/sec.
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Scenario 2:
The following graph depicts the
ETE delay for various workloads.
Run 1:
Inter arrival args = 0.6 seconds.
Run 2:
Inter arrival args = 0.8 seconds.
Run 3:
Inter arrival args = 1.0 seconds.
Run 4:
Inter arrival args = 1.2 seconds.
Fig. 11 End to end delay of physical link.
Fig. 13 Queue size and throughput of link and satellite node
transmitter.
Fig. 12 End to end delay of satellite.
Data rate for physical link : 1000000000 bits/sec.
Data rate for radio transmitter/receiver : 9600 bits/sec.
Service rate of queue : 1000 bits/sec.
The delay for the satellite medium is comparatively high. The
reason for this being that transmission on this medium occurs
only when the satellite is in LOS of both the ground nodes,
else the data gets stored in the queue. Also for the slowest data
generation rate entire data is transferred on the physical link
medium.
Fig. 14 Throughput of radio and physical link receivers.
Hence we can conclude that whenever the queue size goes
above the threshold value and also the satellite is in the LOS
then successful data transmission takes place.
The queue size has been used as a deciding factor for
deflection of traffic in this paper. Similarly, any other
parameter could be used such as the kind of data being
generated, congestion of link, etc.
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FUTURE ENHANCEMENTS:
 The network level model could be refined to involve two
subnets, each comprising of multiple stationary/mobile
workstations and a single basestation.
 Consideration of encapsulation/decapsulation of data
being transmitted and received to achieve more reliability.
REFERENCES:
[1] Schiller J.H. “MOBILE COMMUNICATIONS”
[2] OPNET, "OPNET Manuals”, Vol. 1 - 8, OPNET
Technologies, Inc, Washington, USA, 1989-2000.
[3]
Freeman,
ENGINEER”.
“TELECOMMUNICATION
SYSTEM
[4] Yurong Hu and Victor O.K. Li, “Satellite-Based Internet:
A Tutorial” IEEE Communications Magazine, March 2001
Vol. 39 No. 3.
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