Final Examination B. Tech Year 2 for EcE (2006)

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MINISTRY OF SCIENCE AND TECHNOLOGY

DEPARTMENT OF TECHNICAL AND VOCATIONAL EDUCATION


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Final Examination B. Tech Year 2 for EcE (2006)


EcE 04025
Computer Communication Network

Date: 23.10.06

time: 12:30 ~
15:30

Answer Any Five Questions


1(a) Describe the advantages of ISDN.

1(b) Describe different channel types of ISDN. And also explain each channel.


2(a) Discuss about Teleservices.

2(b) Explain Link Access Protocol
-
D Channel (
LAP
-
D).



3(a) What is Token Ring? And explain about Token Ring Network using Figures.

3(b) Write short note the followings:

(i)

The Token Frame (ii) The Data Frame


4(a) What are connectors?

4(b)Briefly explain Network Interface Cards.


5(a) Describe Brid
ge Standards and explain Transparent Bridge.

5(b) Briefly explain Random Access versus Token Passing Protocols.


6(a) Draw IEEE 802.3 frame (1) with an IEEE 802.1q header (2) adds 4 bytes to the
802.3 frame.



6(b) Explain (1) Stop
-
and
-
Wait Flow
-
Control Prot
ocol (2) Sliding Window Flow
Control Protocol.






Solution


1(a)


The

integrated services digital network (ISDN) represent the overhaul
and redesign of our conventional telephone network from an analog system to
an end
-
to
-
end digital network. Th
is comple
tely digital
-
based net
work is
capable of transmitting voice and data communications over a single
telephone line using in expensive and conventional twisted
-
pair
-
cable.



The advantage ISDN offers over other services is that separate connections
are not need for these different transmissions. Thus, instead of having a
telephone line

for voice communications, a second telephone line

fax or
computer dial up connections

and a coaxial cable link for video
communications, a single ISDN connection will
support all of these
transmissions. ISDN also provides a cost
-

effective strategy for internet
working. Instead of paying for dedicated leased lines, remote sites can
interconnect with other sites via dialup links.


1(b)

ISDN defines several different chan
nel types, which provides a logical


separation of user data from signaling and control information.



B channels



The B channel is a 64 kbps clear channel used to transmit computer data,
digitized voi
ce and digitized video. B channel transmissions are either circuit
or packet
-
switched. Data also can be exchanged via frame relay or through a
dedicated leased line arrangement. Since ISDN uses a separate channel (the D
channel) for signaling information 1
00% of the bandwidth allocated for an
ISDN B channel is used for data transmission.





D channels


The D channel is either a 16 kbps or 64 kbps channel depending on the
specific service level provided. It is used to carry signal and control
information
for circuit switched user data. The D channel also can be used to
transmit packet
-
switched user data( provided that no signal or control
information is needed), data from security alarm signals of remote sensing
device that detect fire or instructors, and
low speed information required from
telemetry services such as meter reading.


H channels

The H channel is used for transmitting user data at higher transmission
rates than B channel provides. Four H channels are defined: H 0, H 10, H 11,
and H 12. H 0 co
mprises six B channels for a total capacity of 384 kbps.
Example of applications that might use an H channel include video
conferencing, high speed fax or high speed packet
-
switched data, and high
quality video.


2(a)

Teleservices


ISDN also

supports a se
veral teleservices, including capability for 64
-
kbps
Group IV fax, teletext, and videotex. Teletext and videotex are electronic
information utilities that use computers or standard television sets equipped
with adapters to display information. Teletext is
one
-
way communication
system; videotex is a two
-
way

system. Teletext5 broadcasts data as part of a
televisi
on signal; videotex use cable television or telephone lines to transmit
data. typical applications available from
these services include information
retri
e
v
a
l, electronic transactions
, interperson
al messaging, computing, and
tel
emonitoring.
Clearly, many of these applications are now available via the
Internet's
World Wide Web. However, teletext and videotex services have
been available in countries su
ch
as France, Canada, and
Great Britain since
the early 1980s. Futhermore, trials of these two services were conducted in the
United States during the 1983
-
1984 time periods. Example
s include Videotron
by Knight
-
Ridder Newspapers in southeastern Florida, K
eyfaxcom Electronic


Publishing in Chicago, and Gateway by Times Mirror Videotex Services in
Southern California in 1984.




ISDN versus Regular Phone Service


An ISDN telephone can be used to call

someone whose phone service is
provided by the analog
-
based

plain old telephone system. The reverse is also
true. However, we will not be able

to achieve high
-
quality connect sessions
because only one part of the connection is digital; the other is analog. Thus
,
there will not be any improvement in line performanc
e. As for any differences
in telephones, ISDN telephone sets have many more built
-
in functions and
capabilities than conventional analog phones. Aside from that both types of
phoned function similarly, with one excepti
on.


OSI

D Channel

B and H Channels

Layer

Call
-
control

Packet Data

Telem
etry

Circuit
-
switched

Packet
-
switched

3

ITU
-
T
Q.931/I.451

X.25
Protocols

____

N/A

X.25 Protocols

2

ITU
-
T Q.931/(LAP
-
D)I.441

Frame Relay
Protocols

LAP
-
B

1

ISDN I
-
series: I.430 (BRI) and I.431 (PRI)

ISDN I
-
series: I.43
0 (BRI) and I.431 (PRI)


2(b)

Link Access Protocol
-
D Channel (LAP
-
D)


Flap

-

Signals the
beginning or ending of the frame.


Address

-

Provides addressing information. The service access point identifier
identifies where

the layer
-
2 protocol provides servi
ce to layer 3. Specific
address
es identify

specific
services. For example SAPI = 16 is for X.25 packet
data transmissions. The command/response bit specifics whether the frame is
a command or response. The extended address
bit specifies the beginning and
e
nding of the address field. If EA is 0
, then another byte
of address
information follows. An EA of 1 implies that the
current byte is the last byte
or the address. Thus, given a two
-
byte

address, EA = 0 in the higher
-
order
byte and EA = 1 in the lower
-
orde
r byte. The terminal endpoint identifier


represents the spe
cific address or ID assigned to each ISDN terminal
equipment connected to an ISDN network via an S/T interface.


Control

-

Provides layer
-
2 control information

Information
-

Provides layer
-
3 protoc
ol information

and user data. The
protocol discriminator id
entifies the specific layer
-
3 protocol. The length byte
specifics the length

of the CRV field, which is either one or two bytes. The
call reference value is the number assigned to each call. Once a

call is
completed, this number can be reassigned to a new call. Message type
identifies specific message related to circuit
-
switch
ed connections.
For
example, during call setup, the message CONNECT indicates that the
receiving terminal equipment end node
has accepted a call by the initiating
TE. When a call has been completed, the message RELEASE COMPLETE is
sent to indicate that the channel

HAS BEEN TORN DOWN.

CRC

-

Provides for data integrity via CRC checksums.


3(a)

A token ring network is a local ar
ea network technology based on a
token passing for media access control. Data frame on a token ring network
are transmitted from note to node, a clockwise or counterclockwise direction,
over a point to point link. A token ring implemented either as a logic
al ring
using a physical ring topology.


1

2

3

4

5









3(b)

(i
)
The Token Frame

At the data link layer, access to the network is control by a
special,”token”frame, which circulates around the ring when all lobes are idle.

A code vi
olation occurs if there is no transition at the midpoint of a bit
signal. The access control field consist of eight bits: three priority bits (P), a
token bit (T), a monitor bit (M) and three reservation bits (R).




Figure 10.4


(ii
)
The Data Frame



T
he content of a token ring data frame that the first three field of the
data frame are equivalent to the token frame. A lobe that possesses the free
token and has data to transmit changes the access control field’s token bit to 1
and then arguments the tok
en frame by including a frame control field,
destination and sources address, user data, a CRC checksum and a frame
status field.






Figure 10.5


4(a)



Connector attach components together several types of connector are
available, serving various purpo
se. Four example connectors are used to (a)
connect network interface cards, such as an Ethernet card to a cable, (b)
connect cable segment (c) terminate a segment. Connector actually connect
the cable to a terminating resisto
r or an array of resistors and

consequently
known as terminators. The type of connector is usually a function of cable
type. Connectors are classified by their gender, and they do indeed ‘mate’.




Connectors are also frequently
labeled

by their type. Three
common types are DB type, ce
ntronics and DIN. DB connectors serve as an
interface between a computer and a peripheral device such as a printer or


external modern. Several types of DB connector exist and are distinguished by
the number of pins they contain. DIN(Deutsche
Industries

Nor
m, a G
erman
industrial standard) and ele
ctronics

connectors ar
e
similar

to DB type
connectors excepts DIN connectors are circular instead of rectangular and
centronics connector contain ‘teeth’ instead of pins. Other connector type
include coax, V
-
type, fi
ber, video, SCSI
and modular.







4(b)


Network interfacing cards (NIC) operates at the data link layer.
Many
people think of network interface cards as Ethernet cards, but this not entirely
correct. An Ethernet card is a network interface card used in
Ethernet/802.3
networks
.

Not all network
interface cards are Ethernet cards, through. A
network interface card is known b many names. Some of the more common
ones are LAN adapter, network adapter, networ
k

card, and
network board.
Generally, we prefer to ca
ll them NIC.



Figure 6.13


5(a)

Bridge standard


Two bridge standards have been defined by IEEE. The first is
transparent bridge and is used in IEEE 802.3 and 802.5 networks. The second
is a sourcing routing bridge which was introduced by IBM and is used
exclusively in token ring networks.
Bridges pass

frames between LANs and
provide filtering. They allow frame from a node on one network to be
forwarded to a node on another network but discard any frames destined for
the same network from which the frames
originated. Thus bridges keeps local
traffic local but forward traffic destined for a remote network. Since bridge
operates at the data link layer, they check the hardware address of a particular
network interface card to determine whether to forward or di
scount a frame.






Transparent Bridge



A transparent bridge is a “plug and play” unit. Operating in”
promiscuous mode” a transparent bridge captures every frame that is
transmitted on all the networks to which the bridge is connected. The brid
ges
examine every frame it receive and extracts each frame’s source address,
which is then added to a “learned address” table maintained by the bridge.
Eventually, this table contains an entry for each unique source address and the
port on which the frame
was received.


5(b)


Random access protocol defines how a node can access a
communication channel. These protocols employ the philosophy that a node
can transmit whenever it has data to transmit. Random access protocols imply
contention; is a phenomenon in

which more than one entity competes to do
something at the same time.







Token passing protocol LAN is token bus, which is defined in
IEEE 802.4. a token bus network is characterized as a logical ring on a
physical bus. Physically, the network resemble
s a bus topology, but logically
it is arranged as a ring with respect to passing the token from lobe to lobe.


6(b)



(i)

Stop and Wait Flow Control Protocol


An example of a very simple flow control protocol is the stop
-
and
-
wait
proto
col. As inferred by its name, stop and
-
wait requires the sender to transmit
one frame and then wait for the receiver to acknowledge receipt of this frame.
The acknowledgement sent by the receiver is a basic frame that simply in
form the sender that the rec
eiver is now ready to accept another data frame.
Asender must wait until it receivers an acknowledgement from the receiver
for the frame it transmitted before it is permitted to transmit another frame. If
a receiver withholds an acknowledgement, then the f
low of data between
sender and receiver stops. Although stop and wait is very effective it is also
very impractical for modern networking environments. First as described, stop
and wait uses a simplex transmission; data frames flow in only one direction.
I
n most data communication environments, data transmission is full duplex.
Second the protocol is ideal when transmitting larger frames. Unfortunately,
larger frames are generally partitioned into smaller data unit to a commended
a receiver’s limited buffer

size. Small frames
size

also facilitates faster error
detection and reduces the amount of data that require retransmission in event
that an error is detected.

z






Figure 5.7

6(b)

(ii)

Sliding Window Flow
-
control Protocol


An enhancement to the stop an wai
t protocol is the sliding window
concept, which improves data flow by having the receiver inform the sender
of its available buffer space. The sliding window concept is implemented by
requiring the sender to sequentially number each data frame it sends and

by
having the sender and receiver maintain information about the number of
frames they can respectively send or receive. Flow control based on this
concept is called sliding window protocol.

A general example of how the sliding window concept is used
for
flow control is provided in which show a simplex data transmission from
host A (the sender) to host B (the receiver). We assume a three
-
bit window
field. An explanation of how this concept is implemented follows.









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