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BCS
-
041


Q1(a)

http://www.ukessays.com/essays/engineering/analog
-
vs
-
digital
-
communication.php


Q1 (b)

The

Open Systems Interconnection (OSI) model

(ISO/IEC 7498
-
1) is a product of the
Open Systems
Interconnection

effort at the

International Organization for Standardization
. It is a prescription of
characterizing and standardizing the functions of a

communications system

in terms of

abstraction layers
.
Similar communication functions are grouped into logical layers. A layer serves

the layer above it and is
served by the layer below it.

For example, a layer that provides error
-
free communications across a network provides the path needed
by applications above it, while it calls the next lower layer to send and receive packets that m
ake up the
contents of that path. Two instances at one layer are connected by a horizontal connection on that layer.

FUNCTIONALITY OF LAYERS

7)


Application Layer :

The application layer provider different services to the application. Example of
services
provided by this layer are file transfer, electronic messaging e
-
mail, virtual terminal access and
network management.

6) Presentation Layer :

The Presentation layer is responsible for protocol conversion, date
encryption/decryption, Expanding graphics com
mand and the date compression. This layer makes the
communications between two host possible.

5) Session Layer :

This layer is responsible for establishing the process
-
to
-
process communication
between the host in the network. This layer is responsible for
establishing and ending the sessions across
the network. The interactive login is an example of services provided by this layer in which the connective
are re
-
connected in care of any interruption.

4) Transport Layer :

This layer is responsible for end
-
to
-
end delivers of messages between the
networked hosts. It first divides the streams of data into chunks or packets before transmission and


then
the receiving computer re
-
assembles the packets. It also guarantee error free data delivery without loss
or dupl
ications.

3) Network Layer :

This layer is responsible for translating the logical network address and names into
their physical address ( MAC address). This layer is also responsible for addressing, determining routes
for sending and managing network prob
lems such as packet switching, data congestion and routines.

2) Data Link Layer :

Data link layer is responsible for controlling the error between adjacent nodes and
transfer the frames to other computer via physical layer. Data link layer is used by hubs
and switches for
their operation.

1) Physical Layer :

Physical


Layer is responsible for transmitting row bit stream over the physical cable.
The physical layer defines the hardware items such as cables, cards, voltages etc.


Q1(C )



Q1(D)

In electronics,

telecommunications and computer networks, multiplexing (short muxing) is a term
used to refer to a process where multiple analog message signals or digital data streams are
combined into one signal over a shared medium. The aim is to share an expensive re
source. For
example, in electronics, multiplexing allows several analog signals to be processed by one analog
-
to
-
digital converter (ADC), and in telecommunications, several phone calls may be transferred using
one wire. In communications, the multiplexed s
ignal is transmitted over a communication channel,
which may be a physical transmission medium. The multiplexing divides the capacity of the low
-
level
communication channel into several higher
-
level logical channels, one for each message signal or
data str
eam to be transferred. A reverse process, known as demultiplexing, can extract the original
channels on the receiver side.

Multiplexing technique is designed to reduce the number of electrical connections or leads in the
display matrix. Whereas driving sig
nals are applied not to each pixel (picture element) individually
but to a group of rows and columns at a time. Besides reducing the number of individually
independent interconnections, multiplexing also simplifies the drive electronics, reduces the cost a
nd
provides direct interface with the microprocessors. There are limitations in multiplexing due to
complex electro
-
optical response of the liquid crystal cell. However, fairly reasonable level of
multiplexing can be achieved by properly choosing the multi
plexing scheme, liquid crystal mixture
and cell designing.



There are two predominant ways to multiplex:

Frequency Division Multiplexing

Time Division Multiplexing

Frequency Division Multiplexing (FDM)

In FDM, multiple channels are combined onto a single aggregate signal for
transmission. The channels are separated in the aggregate by their FREQUENCY.

There are always some unused frequency
spaces between channels, known as "guard
bands". These guard bands reduce the effects of "bleedover" between adjacent
channels, a condition more commonly referred to as "crosstalk".

FDM was the first multiplexing scheme to enjoy widescale network deploymen
t, and
such systems are still in use today. However, Time Division Multiplexing is the
preferred approach today, due to its ability to support native data I/O (Input/Output)
channels.


Time Division Multiplexing

Timeplex is probably the best in the busines
s (IMHO) at Time Division Multiplexing,
as it has 25+ years or experience. When Timeplex was started by a couple of ex
-
Western Union guys in 1969 it was among the first commercial TDM companies in
the United States. In fact, "Timeplex" was derived from TIM
E division
multiPLEXing!

In Time Division Multiplexing, channels "share" the common aggregate based upon
time! There are a variety of TDM schemes, discussed in the following sections:

Conventional Time Division Multiplexing

Statistical Time Division Multip
lexing

Cell
-
Relay/ATM Multiplexing

Conventional Time Division Multiplexing (TDM)

Conventional TDM systems usually employ either Bit
-
Interleaved or Byte
-
Interleaved
multiplexing schemes as discussed in the subsections below.

Clocking (Bit timing) is critica
l in Conventional TDM. All sources of I/O and
aggregate clock frequencies should be derived from a central, "traceable" source for
the greatest efficiency.

Bit
-
Interleaved Multiplexing

In Bit
-
Interleaved TDM, a single data bit from an I/O port is output to

the aggregate
channel. This is followed by a data bit from another I/O port (channel), and so on, and
so on, with the process repeating itself.

A "time slice" is reserved on the aggregate channel for each individual I/O port. Since
these "time slices" for

each I/O port are known to both the transmitter and receiver,
the only requirement is for the transmitter and receiver to be in
-
step; that is to say,
being at the right place (I/O port) at the right time. This is accomplished through the
use of a synchron
ization channel between the two multiplexers. The synchronization
channel transports a fixed pattern that the receiver uses to acquire synchronization.

Total I/O bandwidth (expressed in Bits Per Second
-

BPS) cannot exceed that of the
aggregate (minus the
bandwidth requirements for the synchronization channel).

Bit
-
Interleaved TDM is simple and efficient and requires little or no buffering of I/O
data. A single data

bit

from each I/O channel is sampled, then interleaved and output
in a high speed data strea
m.

Unfortunately, Bit
-
Interleaved TDM does not fit in well with today's microprocessor
-
driven, byte
-
based environment!

Byte
-
Interleaved Multiplexing

In Byte
-
Interleaved multiplexing, complete words (bytes) from the I/O channels are
placed sequentially, one

after another, onto the high speed aggregate channel. Again, a
synchronization channel is used to synchronize the multiplexers at each end of the
communications facility.

For an I/O payload that consists of synchronous channels only, the total I/O
bandwid
th cannot exceed that of the aggregate (minus the synchronization channel
bandwidth). But for asynchronous I/O channels, the aggregate bandwidth CAN BE
EXCEEDED if the aggregate byte size is LESS than the total asynchronous I/O
character size (Start + Data

+ Stop bits). (This has to do with the actual
CHARACTER transmission rate of the asynchronous data being LESS THAN the
synchronous CHARACTER rate serviced by the TDM).

Byte
-
Interleaved TDMs were heavily deployed from the from the late 1970s to around
1985
. These units could support up to 256 KBPS aggregates but were usually found in
4.8 KBPS to 56 KBPS DDS and VF
-
modem environments. In those days, 56 KBPS
DDS pipes were very high speed circuits.



Q2(a)

In

computer communication

theory relating to

packet
-
switched
networks
, a

distance
-
vector routing
protocol

is one of the two major classes of

routing protocols
, the other major class being the

link
-
state
protocol
. Distance
-
vector routing protocols use the

Bellman
-
Ford algorithm
,

Ford

Fulkerson algorithm
,
or

DUAL FSM

(in the case of

Cisco Systems
's protocols) to calculate paths.

A distance
-
vector routing protocol requires that a router informs its neighbours of topology changes
periodically. Compared to

link
-
state protocols
, which require a router to inform all the nodes in a network
of topology changes, distance
-
vector routing protocols have less
computational complexity

and

message
overhead
.
[
citation needed
]

The term

distance vector

refers to the fact that the protocol manipulates

vectors

(
arrays
) of distances to
other nodes in the network.


Method

Routers using distance
-
vector protocol do not have knowledge of the entire path to a destination. Instead
they use two methods:

1.

Direction in which router or
exit interface a packet should be forwarded.

2.

Distance from its destination.

Distance
-
vector protocols are based on calculating the direction and distance to any link in a network.
"Direction" usually means the next hop address and the exit interface. "Dist
ance" is a measure of the cost
to reach a certain node. The least cost route between any two nodes is the route with minimum distance.
Each node maintains a vector (table) of minimum distance to every node. The cost of reaching a
destination is calculated
using various route metrics.

RIP

uses the hop count of the destination
whereas

IGRP

take
s into account other information such as node delay and available bandwidth.

Updates are performed periodically in a distance
-
vector protocol where all or part of a router's routing
table is sent to all its neighbors that are configured to use the same dis
tance
-
vector routing protocol. RIP
supports cross
-
platform distance vector routing whereas IGRP is a

Cisco Systems

proprietary distance
vector routing protocol. Once a router has

this information it is able to amend its own routing table to
reflect the changes and then inform its neighbors of the changes. This process has been described as
‘routing by rumor’ because routers are relying on the information they receive from other ro
uters and
cannot determine if the information is actually valid and true. There are a number of features which can
be used to help with instability and inaccurate routing information.

EGP

and

BGP

are not pure distance
-
vector routing protocols because a distance
-
vector protocol
calculates routes based only on li
nk costs whereas in BGP, for example, the local route preference value
takes priority over the link cost.


Count
-
to
-
infinity problem

The

Bellman
-
Ford algorithm

does not prevent

routing loops

from happening and suffers from the

count
-
to
-
infinity problem
. The core of the count
-
to
-
infinity problem is that if A tells B that it has a path
some
where, there is no way for B to know if the path has B as a part of it. To see the problem clearly,
imagine a subnet connected like as A
-
B
-
C
-
D
-
E
-
F, and let the metric between the routers be "number of
jumps". Now suppose that A is taken offline. In the vec
tor
-
update
-
process B notices that the route to A,
which was distance 1, is down
-

B does not receive the vector update from A. The problem is, B also gets
an update from C, and C is still not aware of the fact that A is down
-

so it tells B that A is only
two jumps
from C (C to B to A), which is false. This slowly propagates through the network until it reaches infinity (in
which case the algorithm corrects itself, due to the relaxation property of Bellman
-
Ford).


EXAMPLE

Write the example from the given li
nk & only Table
-
1,2,3 with theory between them

http://www.cs.bu.edu/fac/byers/courses/791/F99/scribe_notes/cs791
-
notes
-
990923.html


Q2(b)

Write the answer f
rom the given link

http://ninjacraze.hubpages.com/hub/Data
-
Communication


Q2( c)

Write the answer from the given link

http://voip.about.com/od/voipbasics/a/switchingtypes.htm


Q2(d)

Write the answer from the given link (Page No
-
134 to 142)

http://books.google.co.in/books?id=lPiO8J6VC5YC&
pg=PA134&lpg=PA134&dq=process+of+sa
mpling+used+in+digital+communication&source=bl&ots=v6ea3UdSuB&sig=781J4Lkb8YagWB6t
qTLXTW7uM3I&hl=en&sa=X&ei=TdZDUan2DIiKrge8jIGIAg&ved=0CE0Q6AEwAw#v=onepage&q
=process%20of%20sampling%20used%20in%20digital%20communication&
f=false



Q3(a)

http://etd.lib.fsu.edu/theses/available/etd
-
04092004
-
143712/unrestricted/Ch_2MultipleAccess.pdf


Q3 (b)

http://www.universalteacherpublications.com/univ/free
-
asgn/mcs42/page2.htm


Q3 (c)

Network topologies describe the ways in which the elements of a network are connecte
d. They
describe the physical and logical arrangement of network nodes. Let us look at the advantages
different network topologies offer and get to know their shortfalls.


Bus Topology


Advantages

It is easy to handle and implement.


It is best suited for
small networks.


Disadvantages

The cable length is limited. This limits the number of stations that can be connected.

This network topology can perform well only for a limited number of nodes.


Ring Topology


Advantage

The data being transmitted between tw
o nodes passes through all the intermediate nodes. A central
server is not required for the management of this topology.


Disadvantages

The failure of a single node of the network can cause the entire network to fail.

The movement or changes made to networ
k nodes affects the performance of the entire network.


Mesh Topology


Advantage

The arrangement of the network nodes is such that it is possible to transmit data from one node to
many other nodes at the same time.


Disadvantage

The arrangement wherein
every network node is connected to every other node of the network, many
of the connections serve no major purpose. This leads to the redundancy of many of the network
connections.



Q3(d)

Small, fixed
-
length cells can be routed through switches quickly an
d completely in hardware. Small,
fixed
-
size cells also make it easier to build hardware that handles many cells in parallel. Also, they do not
block transmission lines for very long, making it easier to provide quality
-
of service guarantees.

The ATM Cell

T
he ATM cell is the fixed
-
length transmission unit defined by the ATM
standard. An ATM cell contains two major types of information: the payload
and the header. The payload is the information to be transferred through an
ATM network. It can include data, vo
ice, image, or video. The header

is the
information used to route the cell through the network and to ensure that the cell
is forwarded to its destination.

Every ATM cell is 53 bytes long. The first 5 bytes contain header information,
and the remaining 48

bytes contain the payload (see

Figure 1
-
8
).


Figure

1
-
8:

ATM Cell



The 5
-
byte header (see

Figure 1
-
9
) contains several different fields

(see
Table 1
-
2
).
The 48 bytes following the header (the payload) contain user data.


Figure

1
-
9:

UNI ATM Cell Header Format



Table


1
-
2:

Fields in ATM Cell Header

Header Field
Name

Location in
Header

Description

GFC (Generic
Flow
Control)1

First four bits of
Byte 1

Controls the flow of traffic across the user network interface (UNI) and
thus into the ATM network.

VPI (Virtual
Path
Identifier)

Second four bits
of Byte 1 and the
first four bits of
Byte 2

Identifies a particular VPC.

A VPC is

a group of virtual connections
carried between two points and may involve several ATM links. VPIs
provide a way to bundle traffic heading to the same destination.

VCI (Virtual
Channel
Identifier)

Second four bits
of Byte 2, Byte 3,
and the first

four
bits of Byte 4

Identifies a particular VCC. A VCC is a connection between two active,
communicating ATM entities. The VCI consists of a concatenation of
several ATM links.

PT (Payload
Type)

The fifth, sixth,
and seventh bits
of Byte 4

Indicates the
type of information in the payload field. ATM cells carry
different types of information that may require different handling by
the network or terminating equipment.

CLP (Cell Loss
Priority)

Eighth bit of Byte
4

Indicates the cell loss priority set by the

user. This bit indicates the
eligibility of the cell for discard by the network under congested
conditions. If the bit is set to 1, the cell may be discarded by the
network if congestion occurs.

HEC (Header
Error
Control)

Byte 5

Contains an error
-
correct
ing code calculated across the previous four
bytes of the header. The HEC detects multiple
-
bit header errors and
can be used to correct single
-
bit errors. The HEC provides protection
against incorrect delivery of messages caused by address errors. The
HEC
does not provide any protection for the payload field itself.

1. For a network
-
to
-
node (NNN) interface, there is no GFC field. These four bits are part of the
VPI field.



Q4(a)

http://www.theparticle.com/cs/bc/net/flowctrl.pdf

Q4(b)



Q4(c )

http://windows.microsoft.com/en
-
in/windows
-
vista/connect
-
two
-
computers
-
using
-
a
-
c
rossover
-
cable

Q4 (d)


A number of disadvantages associated with CDMA; two of the most severe are the
problem of "self
-
interference," and the related problem of the "near
-
far" effect.




Self
-
interference arises from the presence of delayed replicas of signa
l due to
multipath. The delays cause the spreading sequences of the different users to
lose their orthogonality, as by design they are orthogonal only at zero phase
offset. Hence in despreading a given user's waveform, nonzero contributions
to that user's
signal arise from the transmissions of the other users in the
network. This is distinct from both TDMA and FDMA, wherein for reasonable
time or frequency guardbands, respectively, orthogonality of the received
signals can be preserved.



The near
-
far problem

arises from the fact that signals closer to the receiver of
interest are received with smaller attenuation than are signals located further
away. Therefore the strong signal from the nearby transmitter will mask the
weak signal from the remote transmitter
. In TDMA and FDMA, this is not a
problem since mutual interference can be filtered. In CDMA, however, the
near
-
far effect combined with imperfect orthogonality between codes (e.g.
due to different time sifts), leads to substantial interference. Accurate a
nd
fast

power control

appears essential to ensure reliable operation of multi
-
user
DS
-
CDMA systems.



The system is little complicated



The overall performance degrades
with the increase in number of users



Low throughput efficiency



Multi
-
user interference or multiple access interference(MAI)



Multi
-
path fading



Receivers here in CMDA are complex in nature.



It needs more complicated power control for senders.



Self jamming is

a problem in CDMA system.



Due to the proprietary nature, engineering community is not aware of all the CDMA flaws.



CDMA cannot offer international roaming, a large GSM advantage.



CDMA is relatively new, and the network is not as mature as GSM.



Your handse
t can only be used with the provider that you got the phone from.



Difficult to optimize to maximize performance.



Low traffic areas lead to inefficient use of spectrum and equipment process.



Currently, base station equipment is expensive.