Dear vergita-ga,
Good day!
Q1.)
a.) Define the basic signal theory with the aid of diagrams
In a communications system, information (data, voice, image, video) is
propagated from one point to another by means of electromagnetic
signals. By data we mean entities that convey meaning, or information,
e.g., audio speech, video movie or a text document. Signals are the
electromagnetic representations of data. Transmission refers to data
communication by the propagation and processing of signals.
While the signal is a function of time, it can also be expressed as a
function of frequency, i.e., the signal consists of components of
different frequencies. This frequency-domain concept is more important
in understanding data transmission.
Time-domain concept
With respect to time, a signal can be either continuous or discrete.
In a continuous signal, the signal intensity varies smoothly over a
period of time, i.e., there are no breaks in the signal, e.g., human
speech. The familiar sine wave (y = sin(x)) is an example of a
continuous signal. On the other hand, in a discrete signal, the signal
intensity is maintained at a constant level for some period of time
and then changes to another constant level, e.g., binary 1 and 0.
Frequency-domain concept
Practically speaking, an electromagnetic signal is made up of many
frequencies, for example, the human speech uses a frequency range
between 20 Hz – 200,000 Hz. This frequency range is called as the
spectrum of the signal.
Depending on the transmission media and the communications
environment, either analog or digital signals can be used. An analog
signal is a continuously varying electromagnetic wave that can be
propagated over a variety of media. A digital signal is a sequence of
voltage pulses that may be transmitted over a wire medium. For
example, a constant positive voltage level may represent binary 1, and
a constant negative voltage level may represent binary 0.
Transmission impairments
The transmission of signals faces the following problems:
1. Attenuation – this means that the signal strength decreases with
distance
2. Delay – this problem is peculiar to guided transmission media. This
refers to the phenomenon that the various frequency components of a
signal arrive at the receiver at different times and thereby distort
the signal
3. Noise – refers to any unwanted signals that are inserted somewhere
between the sender and the receiver
b.) How the signal theory affects the choice of transmission methods
and media
Both analog and digital signals may be transmitted over suitable
transmission media. The way these signals are treated is a function of
the transmission system. The choice of transmission media will involve
the various impairments that characterize each medium.
Analog transmission is a means of transmitting analog signals without
regard to their content; the signals may represent analog data (e.g.,
voice) or digital data (e.g., binary data that passes through a
modem). In either case, the analog signal will become weaker
(attenuated) after a certain distance. To achieve longer distances,
the analog transmission system includes amplifiers that boost the
signal.
Digital transmission, in contrast, is concerned with the content of
the signal. A digital signal can be transmitted only a limited
distance before attenuation endangers the integrity of the data. To
achieve greater distances, repeaters are used. A repeater receives the
digital signal, recovers the patterns of 1s and 0s and retransmits a
new signal, thereby overcoming the attenuation.
Transmission media may be classified as under:
1. Guided media – here the waves are guided along a physical path,
e.g., twisted pair, coaxial cable and optical fibre
2. Unguided media – here the waves are not guided, e.g., propagation
through air, vacuum, and sea water
A guided transmission medium is point-to-point if:
1. It provides a direct link between two devices
2. Those are the only two devices sharing the medium
In a multipoint, guided configuration, more than two devices share the
same medium.
Guided transmission media
The twisted pair cable is the most common transmission medium for both
analog and digital data. It is used in the telephone network as well
as in LANs within buildings. For analog signals, amplifiers are
required about every 5 – 6 km. For digital transmission, repeaters are
required every 2 – 3 km. These twisted-pair installations are
generally designed to support voice traffic using analog signaling.
However, by using a modem, they can also handle digital data, though,
at modest speeds.
Coaxial cable can also transmit both analog and digital signals.
However, their frequency characteristics are superior as compared to
twisted pair, and therefore can be used effectively at higher
frequencies and data rates. Coaxial cables are used in a wide variety
of applications:
- Television distribution
- Long-distance telephone transmission
- Short-distance computer system links
- LANs
The first, distributing TV signals to homes, is the most widely used
application of coaxial cables.
Optical fiber has the highest frequency and data rate characteristics
of all the guided media. It is widely used in the following five
applications:
- Long-distance trunks
- Metropolitan trunks
- Rural trunks
- Subscriber loops
- LANs
The following table compares and contrasts these three media:
Frequency Typical Typical Repeater
range attenuation delay spacing
Twisted pair 0 – 3.5 kHz 0.2 dB/km @ 1 kHz 50 µs/km 2 km
(with loading)
Twisted pairs 0 – 1 MHz 3 dB/km @ 1 kHz 5 µs/m 2 km
(multi-pair cables)
Coaxial cable 0 – 500 MHz 7 dB/km @ 10 MHz 4 µs/m 1 – 9 km
Optical fiber 180 – 370 THz0.2-0.5 dB/km 5 µs/m 40 km
[Reference: Digital Communications, by I. Glover and P. Grant, Upper
Saddle River, NJ: Prentice-Hall, (1998)]
Unguided transmission media
The major unguided (or wireless) transmission media are:
- Microwave (terrestrial and satellite)
- Radio
- Infrared
Terrestrial microwave systems are primarily used in long-distance
telecommunications service, as an alternative to coaxial cable or
optical fiber. They are also used for short, point-to-point links
between buildings.
The most important applications of satellite microwave systems are in:
1. Television distribution
2. Long-distance telephone transmission
3. Private business networks (using VSAT)
Perhaps the most important application of radio is mobile telephony.
Infrared signals are typically used to connect various peripheral
devices to the computer, e.g., a wireless keyboard connected to the
computer using infrared waves.
Q2.) Compare and contrast the following communication channels:
Multiplexing means sending multiple signals (each with a given
transmission capacity requirement) on a carrier (with large
transmission capacity) at the same time as a single, complex signal
and then recovering the separate signals at the receiving end.
Multiplexing is done because:
1. Higher data transmission capacity means more efficiency of the
facility that, in turn, reduces the cost per individual signal
(similar to economies of scale)
2. Capacity (of the carrier) required for a single signal is
relatively modest (similar to car pooling logic)
a.) Time Division Multiplexing (TDM)
Time-division multiplexing (TDM) involves dividing the carrier into
two (or more) channels based on time slices, i.e., the common channel
is allotted to several different signals, one at a time, in
alternating time slots. Each individual data stream is reassembled at
the receiving end based on the timing.
There are two variations of TDM: synchronous TDM and statistical TDM.
Generally, TDM refers to synchronous TDM. Digital signals are commonly
multiplexed using TDM. In synchronous TDM, the time slots are
preassigned to sources and are fixed. This may lead to unwanted
wastage. Statistical (or asynchronous or intelligent) TDM resolves
this issue by dynamically allotting time slots on demand.
A multiplexer (a type of circuit), working at the source, accepts the
input from each individual end user, breaks each signal into segments,
and assigns the segments to the composite signal in a rotating,
repeating sequence. The composite signal thus contains data from
multiple senders. At the destination, a demultiplexer separates the
individual signals and routes them to the proper end users. Thus, a
two-way communications circuit needs to have a
multiplexer/demultiplexer at each end of the carrier.
The salient feature of TDM is its flexibility – it allows for
variation in the number of signals being sent along the line, and
constantly adjusts the time intervals to make optimum use of the
available bandwidth. A familiar example of the use of TDM is on the
Internet where the traffic can change drastically from hour to hour.
b.) Frequency Division Multiplexing (FDM)
Frequency-division multiplexing (FDM) is a familiar and widely used
form of multiplexing wherein the transmission facility is divided into
channels by splitting the total frequency band (of the carrier) into
narrow bands, each allotted to an individual signal (sub-channeling).
Cable TV systems, carrying multiple video channels on a single cable,
are acommon example of FDM. Analog signals are commonly multiplexed
using FDM.
Here also, a multiplexer accepts the input from each individual end
user, and generates a signal on a different frequency for each of the
inputs. At the destination, the individual signals are separated out
by a demultiplexer and routed to the proper end users. Therefore, FDM
also requires a multiplexer/demultiplexer at each end of the carrier
to support two-way communication.
The biggest advantage of FDM is its speed, as each input signal is
sent and received at maximum speed at all times.
c.) Wave Division Multiplexing (WDM)
WDM is an optical transmission technique in which multiple streams of
data are transmitted over a single optical fiber as light rays of
different wavelengths. It exploits the fact that light of different
wavelengths does not interfere.
Dense wave division multiplexing (DWDM) is describes systems that
support (normally) 16 (or more) channels. In contrast, “coarse” WDM
implies the use of two or four (and up to eight or sixteen) channels
on a fiber.
WDM allows to simultaneous transmission of different data formats
(e.g., IP, SONET, ATM) at different rates as each channel is
demultiplexed at the end of the transmission back into the original
source.
Q3.) Explain the selection criteria for LAN interconnection devices
such as:
I have described the devices in detail in my previous answer. Here, I
focus on the selection criteria only.
The broad criteria for selection of any network component are
explained below:
1. Strategic criteria:
1.1 Organization Fit – define and match the features, benefits, and
risks inherent in network technology to the needs and capacities of
the organization and people it serves
1.2 Appropriate Technology – choose and use the type of network
technology that meets a given application in the most efficient and
effective manner
1.3 Usable Design – select, modify, and use technology that meets the
need of the users and the organization
2. Tactical criteria:
2.1 Manageable and Supportable Technology – this means that the
networks or systems can be managed and supported by the people using
the technology.
2.2 Interoperable – it should be possible for one type of network or
system to communicate or work with another regardless of vendor
2.3 Scalable – minimal change should be effected in current
configurations to accommodate growth
2.4 Flexible Management – the network resources should be monitored
and managed strategies that leverage software-based remote management
tools with other resources
2.5 Portable – the products should support several hardware platforms
with different versions or should have built-in capabilities for
switching between them
2.6 Secure – the security of the LAN should not be compromised by flaw
in the interconnection device
The generic guidelines for selecting any of the interconnection
devices are as follows:
1. Switch or router? A router supports only one communication at a
time; a switch, on the other hand, enables simultaneous multiple
communications by allowing messages to be routed from one port to
another
2. Bridge or router? Routers are more intelligent than bridges in that
they can constantly adjust to the changing network conditions.
However, this also makes them more processing-intensive and also
slower than bridges
3. Hub or switch? The bandwidth available to network users is the
major differentiating factor between hubs and switches. For small
networks, a hub may be the best choice. If the users of the network
require more bandwidth, using a switch will increase the efficiency of
the network.
The specific criteria in choosing LAN interconnection devices are as
follows:
a.) Hub
Hubs are used to extend the reach of your network. They should satisfy
the following evaluation criteria:
Multiple topology/media support – a hub should be able to support
mix-and-match of various LAN technologies, e.g., Ethernet, ATM, etc.
Fault tolerance – the system and the productivity of the network
should not be negatively affected if any hub component fails
Slot independence – the hub should not impose any limitations as far
as installation of a module in a slot is concerned, i.e., we should be
able to install any module in any slot
High-speed backplanes – their capacity determine the overall capacity
of the hub as they can carry a large amount of data, address, etc.
Routing and bridging – the ability to integrate bridging and routing
plays an important role in smaller networks, as it eliminates a need
for external components
Latency – by latency, we mean the time taken by a hub to forward a
data packet that it has received. Common sense suggests that a lower
latency rate will lead to a faster performance
Management – these advanced management facilities (generally found in
modular hubs) allow the networks to managed with more control and
greater flexibility. The features to be compared include utilities,
monitoring functionality, and training costs
Scalability – it refers to the capability to expand a network with
minimal changes. We should be able to integrate products from previous
with new model lines; it should also be easy for us to migrate to new
platforms with minimal disruption to existing operations
b.) Repeater
Repeaters are nowadays embedded in the other LAN interconnection
devices and therefore their selection criteria are not covered here.
c.) Switch
The following criteria should be kept in mind while selecting a
switch:
1. Ensure that it doesn't drop frames
2. Check the switch latency
3. Use cut-through switches for time-sensitive applications; otherwise
store-and-forward switches will suffice
4. Use dedicated switched ports for multimedia stations
5. Try to keep a 1:1 ratio between demand and resource. Or, provide
multiple lines into one server
6. Determine, in advance, the percentage of bad frames in the network
(baselining) before installing switch
7. Get switches with remote monitoring capability embedded in ports to
save time and money in the long run
8. Be careful while placing switches having “back pressure” control
mechanism into the network
d.) Bridge
Bridges transmit data between dissimilar networks.
e.) Gateway
f.) Router
Routers increase the speed and the efficiency of data transmission.
The following are the evaluation criteria for choosing routers:
1. WAN Ports – a WAN port is required for each physical connection to
a service provider. The number of WAN ports on the router should be
adequate for satisfying the present needs keeping in mind scalability.
A single port is adequate to connect to the Internet, but a second
port is required to establish a backup link for reliability. For
connecting multiple sites with leased lines, you will need a port for
each link
2. Bandwidth (Line Speed) – the router must be able to support the
expected traffic, with some room for growth or expansion. The
aggregation of traffic at regional or central sites resulting from
connections to multiple remote locations should be taken into
consideration. Also take into account the compression capabilities of
the router
3. Supported Protocols – choose a router that supports all the
protocols needed for the protocols and the services chosen for the
network
4. Maximum Number of Links
Additional links:
An exhaustive treatment of the subject can be found in this book
Data & Computer Communications, by William Stallings, Pearson
Education, Inc. (2000)
An excellent listing of sites on this topic
http://directory.google.com/Top/Computers/Data_Communications/Reference/Tutorials/
A good collection of tutorials on telecommunications
http://www.iec.org/online/tutorials/
An online chapter on fundamentals of telecommunications
http://www.privateline.com/manual/one.html
The web site of MRV Communications, Inc. has a good tutorial on WDM
http://www.mrv.com/technology/wdm.php
An introduction to WDM by Cisco
http://www.cisco.com/warp/public/779/servpro/solutions/optical/docs/whatiswdm.html
An Anixter Technology white paper on Ethernet switching
http://www.itmweb.com/essay522.htm
Community Access Program web site of Government of Canada offers
advice on networks
http://cap.ic.gc.ca/english/8643.shtml
An excellent collection of Information Resource and Network Management
topics maintained by Professor Max Maw of the Philadelphia University
http://faculty.philau.edu/mawm/Intro.htm
The Cisco Product Documentation section provides lots of information
and resources
http://www.cisco.com/univercd/home/home.htm
Search Strategy:
Telecommunications fundamentals
://www.google.com/search?q=telecommunications+fundamentals&hl=en&lr=&ie=ISO-8859-1
wave division multiplexing (WDM)
://www.google.com/search?hl=en&ie=ISO-8859-1&q=wave+division+multiplexing+%28WDM%29
criteria for selecting hubs
://www.google.com/search?q=criteria+for+selecting+hubs&hl=en&lr=&ie=ISO-8859-1
network+management+choosing+hubs
://www.google.com/search?hl=en&ie=ISO-8859-1&q=network%2Bmanagement%2Bchoosing%2Bhubs
Hope this helps!
Regards,
reeteshv-ga |
