Notes by Santosh Sir
Notes on DCN
Q. Application of FTP
1. It is very useful to send files securely
2. Its main application is in conjunction with BOOTP and DHCP protocols.
3. It is used along with the TCP in internet applications.
It can be easily configured in ROM or PROM.
Various FTP application software are:










FileZilla – Open source application
CuteFTP Pro- Light and compact
FTP Voyager – good connection and useful user tools
BulletProof FTP- flexible product with good import qualities
FTP Explorer – good drag and drop features
SmartFTP – secure, and fast transfer with good user tools
Wise-FTP – great connection and transfer tools
WS FTP Pro – offers support for an extra fee
CoffeCup Direct FTP – HTML editor and zip archiving tool
AceFTP Pro – exceptional on line and telephone support
Q. SNMP architecture
SNMP is based on the manager/agent model consisting of a manager, an agent, a
database of management information, managed objects and the network protocol.
The manager provides the interface between the human network manager and the
management system.
The agent provides the interface between the manager and the physical device(s) being
managed, such as bridges, hubs, routers or network servers, these managed objects might
be hardware, configuration parameters, performance statistics, and so on… These objects
are arranged in what is known as a virtual information database, called a Management
Information Base, also called MIB.
SNMP allows managers and agents to communicate for the purpose of
accessing these objects.
www.aaryaclasses.com
8976381939
1
Notes by Santosh Sir
Notes on DCN
SNMP uses five basic messages (Get, GetNext, GetResponse, Set, and Trap) to
communicate between the manager and the agent. The Get and GetNext messages allow
the manager to request information for a specific variable.
The agent, upon receiving a Get or GetNext message, will issue a GetResponse message
to the manager with either the information requested or an error indication as to why the
request cannot be processed.
A Set message allows the manager to request a change be made to the value of a specific
variable in the case of an alarm remote that will operate a relay.
The agent will then respond with a GetResponse message indicating the change has been
made or an error indication as to why the change cannot be made.
The Trap message allows the agent to spontaneously inform the manager of an
‘important’ event.
Most of the messages (Get, GetNext, and Set) are only issued by the SNMP manager.
Because the Trap message is the only message capable of being initiated by an agent, it is
the message used by Remote Telemetry Units (RTUs) to report alarms.
This notifies the SNMP manager as soon as an alarm condition occurs, instead of waiting
for the SNMP manager to ask. The small number of commands used is only one of the
reasons SNMP is "simple”.
www.aaryaclasses.com
8976381939
2
Notes by Santosh Sir
Notes on DCN
The other simplifying factor is its reliance on an unsupervised or connectionless
communication link. This simplicity has led directly to its widespread use, specifically in
the Internet Network Management Framework.
Within this framework, it is considered ‘robust’ because of the independence of the
managers from the agents,
e.g. if an agent fails, the manager will continue to function, or vice versa.
Applications of SNMP:





Monitoring device performance
Detecting device faults, or recovery from faults
Collecting long term performance data
Remote configuration of devices
Remote device control
Q. Describe Simplex, half duplex and full duplex.
Data Flow
Communication between two devices can be simplex, half-duplex, or full-duplex as
Shown below:
www.aaryaclasses.com
8976381939
3
Notes by Santosh Sir
Notes on DCN
Simplex
In simplex mode, the communication is unidirectional, as on a one-way street. Only one
of the two devices on a link can transmit; the other can only receive
Keyboards and traditional monitors are examples of simplex devices. The keyboard
can only introduce input; the monitor can only accept output. The simplex mode
can use the entire capacity of the channel to send data in one direction.
Half-Duplex
In half-duplex mode, each station can both transmit and receive, but not at the same time. :
When one device is sending, the other can only receive, and vice versa
The half-duplex mode is like a one-lane road with traffic allowed in both directions.
When cars are traveling in one direction, cars going the other way must wait. In a
half-duplex transmission, the entire capacity of a channel is taken over by whichever of
the two devices is transmitting at the time. Walkie-talkies and CB (citizens band) radios
are both half-duplex systems.
The half-duplex mode is used in cases where there is no need for communication
in both directions at the same time; the entire capacity of the channel can be utilized for
each direction.
Full-Duplex
In full-duplex (also called duplex), both stations can transmit and receive simultaneously
The full-duplex mode is like a 2 way street with traffic flowing in both directions
www.aaryaclasses.com
8976381939
4
Notes by Santosh Sir
Notes on DCN
at the same time. In full-duplex mode, signals going in one direction share the
capacity of the link: with signals going in the other direction. This sharing can occur in
two ways: Either the link must contain two physically separate transmission paths, one
for sending and the other for receiving; or the capacity of the channel is divided
between signals traveling in both directions.
One common example of full-duplex communication is the telephone network.
When two people are communicating by a telephone line, both can talk and listen at the
same time.
The full-duplex mode is used when communication in both directions is required
all the time. The capacity of the channel, however, must be divided between the two
directions.
Q. Difference between analog and digital signal.
Sr.
no.
1
Analog signal
Digital signal
It is a continuous signal
It is a discrete time signal.
2
This signal is easily affected by
noise
This signal is not affected by
noise.
3
Not degrades
4
This signal will degrades as time
progresses
Dgm refer class notes
5
It is used in microphone
It is used in CD burning
6
Difficult to transmit
Easy to transmit
7
More prone to errors
Less prone to errors.
8
Less productivity
Better productivity
9
Less strength signal
More strength signal.
Dgm refer class notes
Q. What are the types of messages of SNMP?
GetRequest
www.aaryaclasses.com
8976381939
5
Notes by Santosh Sir
Notes on DCN
A manager-to-agent request to retrieve the value of a variable or list of variables. Desired
variables are specified in variable bindings (values are not used). Retrieval of the
specified variable values is to be done as an atomic operation by the agent. A Response
with current values is returned.
SetRequest
A manager-to-agent request to change the value of a variable or list of variables. Variable
bindings are specified in the body of the request. Changes to all specified variables are to
be made as an atomic operation by the agent. A Response with (current) new values for
the variables is returned.
GetNextRequest
A manager-to-agent request to discover available variables and their values. Returns a
Response with variable binding for the lexicographically next variable in the MIB. The
entire MIB of an agent can be walked by iterative application of GetNextRequest starting
at OID 0. Rows of a table can be read by specifying column OIDs in the variable
bindings of the request.
GetBulkRequest
Optimized version of GetNextRequest. A manager-to-agent request for multiple iterations
of GetNextRequest. Returns a Response with multiple variable bindings walked from the
variable binding or bindings in the request. PDU specific non-repeaters and maxrepetitions fields are used to control response behavior. GetBulkRequest was introduced
in SNMPv2.
Response
Returns variable bindings and acknowledgement from agent to manager for GetRequest,
SetRequest, GetNextRequest, GetBulkRequest and InformRequest. Error reporting is
provided by error-status and error-index fields. Although it was used as a response to
both gets and sets, this PDU was called GetResponse in SNMPv1.
Trap
Asynchronous notification from agent to manager. Includes current sysUpTime value, an
OID identifying the type of trap and optional variable bindings. Destination addressing
for traps is determined in an application-specific manner typically through trap
configuration variables in the MIB. The format of the trap message was changed in
SNMPv2 and the PDU was renamed SNMPv2-Trap.
InformRequest
www.aaryaclasses.com
8976381939
6
Notes by Santosh Sir
Notes on DCN
Acknowledged asynchronous notification from manager to manager. This PDU uses the
same format as the SNMPv2 version of Trap. Manager-to-manager notifications were
already possible in SNMPv1 (using a Trap), but as SNMP commonly runs over UDP
where delivery is not assured and dropped packets are not reported, delivery of a Trap
was not guaranteed. InformRequest fixes this by sending back an acknowledgement on
receipt. Receiver replies with Response parroting all information in the InformRequest.
This PDU was introduced in SNMPv2.
Q. Explain procedure to construct fiber optic cable or
fiber fabrication. (8M)
There are two basic techniques used in the fabrication of optical fibers:
1) Fibers can be drawn directly from melts of silica in crucibles.
2) Vapor phase oxidation.
Double crucible method
The double crucible method can be used to make both silica and halide glass fibers. The
technique is simple and straightforward. One glass rod is made from silica powders for
the core and one for the cladding. The rods are then used as feedstock for each of two
concentric crucibles. The inner contains the molten core, while the outer contains the
cladding. In a continuous process, the fiber is drawn from the molten state. The
disadvantage of this method is the possibility of introducing contaminants during the
melting process. Figure illustrates the double-crucible drawing process.
www.aaryaclasses.com
8976381939
7
Notes by Santosh Sir
Notes on DCN
Double-crucible fiber drawing process
Vapor phase oxidation
With direct drawing, it is difficult to get pure and homogeneous fibers; therefore this
method is not commonly used. The vapor phase oxidation processes have proven to be
more successful. These processes are usually done in two steps:
1) The first being the preparation of the preform.
2) The second being the drawing of the fiber.
Preparation of the perform
The main reason a perform is prepared is to have a "drawable" material that is clean, low
in OH concentration, low in metallic-ion contaminants, and inexpensive. Many
techniques have been developed to prepare these preforms. Some common commercially
used methods are Outside Vapor-Deposition, Modified Chemical Vapor Deposition,
Vapor Phase Axial Deposition, and Plasma Chemical Vapor Deposition. They differ
mainly by the way the soot is deposited.
www.aaryaclasses.com
8976381939
8
Notes by Santosh Sir
Notes on DCN
The perform is made by vapor-phase oxidation, in which two gases, SiCl4 and O2, are
mixed at a high temperature to produce silicon dioxide (SiO2)
Outside Vapor-Deposition
Outside Vapor Deposition (OVD), also called the "soot process", was first developed by
Corning Incorporated. This fiber was the first to have a loss of less then 20dB/km. The
three main steps involved are laydown (Figure4-a), consolidation (Figure4-b), and
drawing. In the laydown process, several materials such as SiCl4, GeCl4, BCl3 and O2 are
allowed to react in a hot flame to produce soot (Figure 5).
Figure 4: Two phases of the OVD process: (a) Laydown; (b) consolidation.
www.aaryaclasses.com
8976381939
9
Notes by Santosh Sir
Notes on DCN
Figure 5: Outside Vapor-Deposition
Fiber Drawing
www.aaryaclasses.com
8976381939
10
Notes by Santosh Sir
Notes on DCN
Figure 9: Schematic of a typical drawing process.
The major steps of a typical drawing process are shown in Figure 9. The preform is put
into a draw furnace, where the bottom tip is heated to melting. This molten piece now
starts to fall, forming a fiber with a 125-µm outer diameter. Diameter-monitoring
equipment controls the actual fiber diameter by changing, if necessary, the rate of
drawing that is executed by a tractor assembly. A coating applicator applies a coating
over the cladding.
www.aaryaclasses.com
8976381939
11
Notes by Santosh Sir
Notes on DCN
Q. Applications of fiber optic
1. Optical fibers are used in transmission of the following signals:
 Telephone signals
 Telex signals
 Internet signals
2. They are used for interconnecting computers in the local area networks.
3. They are used I the defense communication
4. They are used in medical applications.
Q. Explain light propagation through fiber optic.
SIMPLE RAY MODEL
Figure 2 (optical fiber with core, cladding and total internally reflected ray)
For propagation of light inside the core there are two possibilities.
1. A light ray is launched in a plane containing the axis of the fiber. We can then see the
light ray after total internal reflection travels in the same plane i.e., the ray is confined to
the plane in which it was launched and never leave the plane. In this situation the rays
will always cross the axis of the fiber. These are called the Meridional rays. (Fig. 2)
2. The other possibility is that the ray is not launched in a plane containing the axis of
the fiber.
For example if the ray is launched at some angle such that it does not intersect the axis of
the fiber, then after total internal reflection it will go to some other plane. We can see that
in this situation the ray will never intersect the axis of the fiber. The ray essentially will
spiral around the axis of fiber. These rays are called the Skew rays.
So it can be concluded that if the light is to propagate inside an optical fiber it could be
through two types of rays
www.aaryaclasses.com
8976381939
12
Notes by Santosh Sir
Notes on DCN
a) Meridional rays: The rays which always pass through the axis of fiber giving high
optical intensity at the center of the core of the fiber.
b) Skew Rays: The rays which never intersect the axis of the fiber, giving low optical
intensity at the center and high intensity towards the rim of the fiber.
Q. What are the management components?
Management Components
To do management tasks, SNMP uses two other protocols: Structure of Management
Information (SMI) and Management Information Base (MIB). In other words,
management
on the Internet is done through the cooperation of the three protocols SNMP,
SMI, and MIB
Structure of Management Information
The Structure of Management Information, version 2 (SMIv2) is a component for
network
management. Its functions are
1. To name objects
2. To define the type of data that can be stored in an object
3. To show how to encode data for transmission over the network
SMI is a guideline for SNMP. It emphasizes three attributes to handle an object: name,
data type, and encoding method
Management Information Base (MIB)
The Management Information Base, version 2 (MIB2) is the second component used in
network management. Each agent has its own MIB2, which is a collection of all the
objects that the manager can manage. The objects in MIB2 are categorized under 10
different groups: system, interface, address translation, ip, icmp, tcp, udp, egp, transmission,
and snmp. These groups are under the mib-2 object in the object identifier tree. Each group
has defined variables and/or tables.
e.g.
www.aaryaclasses.com
8976381939
13
Notes by Santosh Sir
Notes on DCN
Q. What are the losses in fiber optic cable?
www.aaryaclasses.com
8976381939
14
Notes by Santosh Sir
Notes on DCN
www.aaryaclasses.com
8976381939
15
Notes by Santosh Sir
Notes on DCN
www.aaryaclasses.com
8976381939
16
Notes by Santosh Sir
Notes on DCN
www.aaryaclasses.com
8976381939
17
Notes by Santosh Sir
Notes on DCN
www.aaryaclasses.com
8976381939
18
Notes by Santosh Sir
Notes on DCN
www.aaryaclasses.com
8976381939
19
Notes by Santosh Sir
Notes on DCN
Q. Explain hidden station and Exposed station problem.
Hidden station problem:
www.aaryaclasses.com
8976381939
20
Notes by Santosh Sir
Notes on DCN
Hidden nodes in a wireless network refer to nodes that are out of range of other nodes or a
collection of nodes. Take a physical star topology with an access pointwith many nodes
surrounding it in a circular fashion: Each node is within communication range of the AP, but
the nodes cannot communicate with each other, as they do not have a physical connection
to each other. In a wireless network, it is likely that the node at the far edge of the access
point's range, which is known as A, can see the access point, but it is unlikely that the same
node can see a node on the opposite end of the access point's range, C. These nodes are
known as hidden. The problem is when nodes A and C start to send packets simultaneously
to the access point B. Because the nodes A and C are out of range of each other and so
cannot detect a collision while transmitting, Carrier sense multiple access with collision
detection (CSMA/CD) does not work, and collisions occur, which then corrupt the data
received by the access point. To overcome the hidden node problem, RTS/CTS handshaking
is implemented in conjunction with the Carrier sense multiple access with collision
avoidance (CSMA/CA) scheme. The same problem exists in a MANET.
The hidden node problem can be observed easily in widespread (>50m
radius) WLAN setups with many nodes that use directional antennas and have high upload.
This is why IEEE 802.11 is suited for bridging the last mile for broadband access only to a
very limited extent. Newer standards such as WiMAX assign time slots to individual stations,
thus preventing multiple nodes from sending simultaneously and ensuring fairness even in
over-subscription scenarios.
IEEE 802.11 uses 802.11 RTS/CTS acknowledgment and handshake packets to partly
overcome the hidden node problem. RTS/CTS is not a complete solutionand may
decrease throughput even further, but adaptive acknowledgments from the base station can
help too.
Exposed station problem:
In wireless networks, the exposed node problem occurs when a node is prevented
fromsending packets to other nodes due to a neighboring transmitter. Consider an example
of 4 nodes labeled R1, S1, S2, and R2, where the two receivers are out of range of each
other, yet the two transmitters in the middle are in range of each other. Here, if a
transmission between S1 and R1 is taking place, node S2 is prevented from transmitting to
R2 as it concludes after carrier sense that it will interfere with the transmission by its
neighbor S1. However note that R2 could still receive the transmission of S2 without
interference because it is out of range of S1.
IEEE 802.11 RTS/CTS mechanism helps to solve this problem only if the nodes are
synchronized and packet sizes and data rates are the same for both the transmitting nodes.
When a node hears an RTS from a neighboring node, but not the corresponding CTS, that
www.aaryaclasses.com
8976381939
21
Notes by Santosh Sir
Notes on DCN
node can deduce that it is an exposed node and is permitted to transmit to other neighboring
nodes.
Q. What are the types of physical links in Bluetooth?
Physical Links
Between master and slave(s), different types of links can be established:


Synchronous Connection-Oriented (SCO) link
Asynchronous Connection-Less (ACL) link
SCO LINK
The type of radio link used for voice data. An SCO link is a set of reserved
timeslots on an existing ACL link. Each device transmits encoded voice data in
the reserved timeslot. There are no retransmissions, but forward error
correction can be optionally applied. SCO packets may be sent every 1, 2 or 3
timeslots.
Enhanced SCO (eSCO) links allow greater flexibility in setting up links: they
may use retransmissions to achieve reliability, allow a wider variety of packet
types, and greater intervals between packets than SCO, thus increasing radio
availability for other links.
ACL LINK
In the slots not reserved for SCO links, the master can exchange packets with
any slave on a per-slot basis. The ACL link provides a packet-switched
connection between the master and all active slaves participating in the piconet.
Both asynchronous and isochronous services are supported. Between a master
www.aaryaclasses.com
8976381939
22
Notes by Santosh Sir
Notes on DCN
and a slave only a single ACL link can exist. For most ACL packets, packet
retransmission is applied to assure data integrity.
A slave is permitted to return an ACL packet in the slave-to-master slot if and
only if it has been addressed in the preceding master-to-slave slot. If the slave
fails to decode the slave address in the packet header, it is not allowed to
transmit.
ACL packets not addressed to a specific slave are considered as broadcast
packets and are read by every slave. If there is no data to be sent on the ACL
link and no polling is required, no transmission shall take place.
Q. Difference between FTP and TFTP.
Sr.
no.
1
FTP
TFTP
Special-purpose; much simpler
7
General-purpose and hence, a more complex
protocol
Needs more memory due to its complexity
Interactive
Greater overhead & reliable control needed.
Due to above, it depends on TCP transport
layerprotocol
Uses TCP port #20 as data port & port #21 as
the connection port
Used over the WWW
8
9
10
Control and data are separated
Authentication is present
Reliable data transfer
Control and data are not separated
Authentication is not present
Unreliable data transfer
2
3
4
5
6
Needs very small memory
Unidirectional
Less overhead and no control needed.
Due to above, it depends on UDP transport
layer protocol
Uses UDP port #69
Used only on local networks due to lack of
security
Q. Explain fiber optic cable connectors.
ST
ST (an AT&T Trademark) is probably still the most popular connector
for multimode networks (ca. 2005), like most buildings and campuses. It
has a bayonet mount and a long cylindrical 2.5 mm ceramic (usually) or
polymer ferrule to hold the fiber. Most ferrules are ceramic, but some
are metal or plastic
www.aaryaclasses.com
8976381939
23
Notes by Santosh Sir
Notes on DCN
SC
SC is a snap-in connector also with a 2.5 mm ferrule that is widely used
for it's excellent performance. It was the connector standardized in TIA568-A, but was not widely used at first because it was twice as
expensive as a ST. Now it's only a bit more expensive and much more
common It's a snap-in connector that latches with a simple push-pull
motion. It is also available in a duplex configuration.
FC
FC was one of the most popular singlemode connectors for many years.
It also uses a 2.5 mm ferrule, but some of the early ones use ceramic
inside stainless steel ferrules. It screws on firmly, but you must make
sure you have the key aligned in the slot properly before tightening. It's
been mostly replaced by SCs and LCs.
LC
LC is a small form factor connector that uses a 1.25 mm ferrule, half the
size of the SC. Otherwise, it's a standard ceramic ferrule connector,
easily terminated with any adhesive. Good performance, highly favored
for single mode.
FDDI - ESCON
Besides the SC Duplex, you may occasionally see the FDDI and
ESCON* duplex connectors which mate to their specific networks. They
are generally used to connect to the equipment from a wall outlet, but
the rest of the network will have ST or SC connectors. Since they both
use 2.5 mm ferrules, they can be mated to SC or ST connectors with
adapters.
MT-RJ
MT-RJ is a duplex connector with both fibers in a single polymer ferrule.
It uses pins for alignment and has male and female versions. Multimode
only, field terminated only by prepolished/splice method.
Opti-Jack
www.aaryaclasses.com
8976381939
24
Notes by Santosh Sir
Notes on DCN
The Panduit Opti-Jack is a neat, rugged duplex connector cleverly
designed aournd two ST-type ferrules in a package the size of a RJ-45.
It has male and female (plug and jack) versions.
Volition
3M's Volition is a slick, inexpensive duplex connector that uses no
ferrule at all. It aligns fibers in a V-groove like a splice. Plug and jack
versions, but field terminate jacks only.
LX-5
LX-5 is like a LC but with a shutter over the end of the fiber.
MT
MT is a 12 fiber connector for ribbon cable. It's main use is for
preterminated cable assemblies and cabling systems. Here is a 12 fiber
MT broken out into 12 STs.
This connector is sometimes called a MTP or MPO which are
commercial names.
MU
MU looks a miniature SC with a 1.25 mm ferrule. It's more popular in
Japan.
Q. What are the applications of LASER?
Scientific

A wide variety of interferometric techniques

Raman spectroscopy

Laser induced breakdown spectroscopy

Atmospheric remote sensing

Investigating nonlinear optics phenomena
www.aaryaclasses.com
8976381939
25
Notes by Santosh Sir
Notes on DCN

Holographic techniques employing lasers also contribute to a number of measurement techniques.

Laser based LIght Detection And Ranging (LIDAR) technology has application
in geology, seismology, remote sensing and atmospheric physics.

Lasers have been used aboard spacecraft such as in the Cassini-Huygens mission.

In astronomy, lasers have been used to create artificial laser guide stars, used as reference objects
for adaptive optics telescopes.
Military
Directly as an energy weapon
Directed energy weapons are being developed, such as Boeing's Airborne Laser which was constructed
inside a Boeing 747. Designated the YAL-1, it is intended to kill short- and intermediate-range ballistic
missiles in their boost phase.
Defensive countermeasures
Defensive countermeasure applications can range from compact, low power infrared countermeasures to
high power, airborne laser systems. IR countermeasure systems use lasers to confuse the seeker heads on
heat-seeking anti-aircraft missiles.
Disorientation
Some weapons simply use a laser to disorient a person.
Medical

Cosmetic surgery (removing tattoos, scars, stretch marks, sunspots, wrinkles, birthmarks, and hairs):
see laser hair removal. Laser types used in dermatologyinclude ruby (694 nm), alexandrite (755 nm),
pulsed diode array (810 nm), Nd:YAG (1064 nm), Ho:YAG (2090 nm), and Er:YAG (2940 nm).

Eye surgery and refractive surgery

Soft tissue surgery: CO2, Er:YAG laser

Laser scalpel (General surgery, gynecological, urology, laparoscopic)

Photobiomodulation (i.e. laser therapy)

"No-Touch" removal of tumors, especially of the brain and spinal cord.

In dentistry for caries removal, endodontic/periodontic procedures, tooth whitening, and oral surgery
www.aaryaclasses.com
8976381939
26
Notes by Santosh Sir
Notes on DCN
Q. Compare SVC and PVC.
Sr. No. SVC
1
A source and a destination
Connect when data are being
Transferred.
2
SVC creates a temporary, short
Connection.
3
An SVC require establishing
And terminating phases.
4
SVC is cheap cost as compare
With PVC.
5
SVC is on-demand circuit
Established by user signal.
6
A SVC is established by UNI
Signaling methods.
7
8
PVC
A source and a destination
May choose to have a PVC.
PVC create a permanent and
Continuous connection.
PVC connection setup is simple.
PVC is costly.
This virtual circuit values are
Manual.
A PVC in the usual meaning is a
VC that is not signaled by the
End points.
A switch in the path fails, the SVC Failure of a link causes a soft PVC
Is broken and would have to be
To route around the outage and
Reconnected.
Remain available.
E.g. ATM, X.25 etc.
E.g. Frame Relay, ATM, X.25, etc.
www.aaryaclasses.com
8976381939
27