Data Communication &Network
DKT224/3
Tutorial 1
1.
Date : 14/1/2010
Describe the wiring dilemma of fully connecting telephones
In the earliest stages of telephone use, calling one telephone from another required the two phones
to be directly connected by a pair of wires carrying electricity. Since every pair must be connected to every
phone to be fully connected, the system is impractical, if not impossible. Consider: 100 telephones takes
4,950 pairs; 1,000 telephones takes nearly half a million wire pairs to be fully connected.
2. Why do COs not guarantee that every call attempt will be successfully connected?
Since it was clear that relatively few interconnecting wire pairs would provide sufficient
connectivity almost all the time, the COs made a business decision to avoid the cost of satisfying every
interconnection request at the risk of being unable to connect every call attempted during peak demand
periods
3. What is distributed access computing?
Distributed access computing, also known as remote access, uses devices called terminals (attached
to a mainframe or a communications link) to allow submission of jobs to the computer from distant
locations
4. How did the decision to limit call bandwidth to the voice band affect data
communication via modems?
The telephone company decision to limit the voice band to 4 kilohertz had the unintended
consequence of limiting the maximum speed achievable by modems over standard telephone lines to
approximately 34 Kbps
5. What is the business case for voice and data network convergence?
Since PDNs were overwhelmingly used for data transmission alone, the average business customer
needed two distinct networks: the telephone system for voice, and PDN for computer connections. Using
and maintaining two separate networks was extremely expensive, resulted in less-than-optimal utilization
of either network, often called for duplicate equipment, and required two sets of technicians. Data itself
also changed, creating a need for a new network to manage video and audio data
6. How do de jure and de facto standards differ?
De jure (“by right”) standards are established together by standards organizations and expert
committees, while de facto (“by fact”) standards are established by popular use.
7. Besides span, what major factor distinguishes LANs from both MANs and
WANs?
A LAN lies entirely within a private domain
8. Why did network development proceed from WANs to LANs instead of the other
way around?
The first computer networks grew from the need to economically utilize mainframe computers. Since
the mainframe was at one location and the computer users could be anywhere in the world, a large
geographical network was necessary to provide access to the mainframe. LANs came about later on, as
PCs led to the need to interconnect in one location
9. Explain the concept of encapsulation
Tutorial 1
Pn. Shahadah Ahmad
Data Communication &Network
DKT224/3
Encapsulation refers to the process by which the data is enclosed into a package for sending. Each
lnetwork layer can operate without knowledge of other layers, or of the package itself. Instead, the layers
use specific headers; each layer only pays attention to its own header’s directions. The layers then pass the
package on to an adjacent layer accordingly.
This process begins with the sending computer. At the topmost layer, a header containing
information for that layer is added, then the package is passed down to the next layer. The package
continues in this way until the second-to-bottom layer, where a trailer may be added before moving
forward into the bottom (always the physical) layer.
On the receiving end, the package travels up the layers, where the corresponding headers tell each
layer what to do before passing the package upward.
10. Why is the noise a problem with analog signaling?
When analog electrical signals are corrupted by noise from another electrical force, it is impossible
to completely remove the noise at the receiving end. This means that signals cannot be restored to their
original state
11a. Describe the similarities and differences in the services provided by (1) a music
program delivered over broadcast radio and (2) music delivered by a dedicated CD
player.
Solution:
Both broadcast radio and a dedicated CD player provide users with similar types of
information and with almost the same performance. However the broadcast radio offers
its service in a real-time fashion with no interaction with users. A CD player, on the other
hand, stores its information on a CD and delivers it on demand.
11b. Describe how these services might be provided and enhanced by providing them
through a communications network.
Solution:
Broadcast radio is typically transmitted in real time using radio waves "over the air."
Broadcast audio programs can also be transmitted over any communication network.
Furthermore, when stored in servers that are attached to a network, "broadcast" audio
programs can be retrieved for listening at a later time. Indeed a server can also store CD
audio material and retrieve it on demand. If the network and server are sufficiently
responsive, it may also be possible to provide the interactivity of a personal CD player
through a network-based service.
12 Briefly differentiate of Metropolitan Area Network (MAN) and Wide Area Network
(WAN) used in the network.
The general factors to differentiate MAN and WAN are size, distances (covered by the
network), structure, and ownership.
SIZE
DISTANCES
OWNERSHIP
STRUCTURE
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MAN
Size between a LAN and a WAN,
May comprise township or city.
More than 10 km ,normally covers
the area inside a town or city
Multinasional company
Protocol used:DQDB &SMDS
WAN
Large geographic area, may comprise a continent,
country or even the whole world
Very large coverage.
Telecommunication Company
Packet switching, ISDN,ATM
Pn. Shahadah Ahmad
Data Communication &Network
DKT224/3
13. Sketch a hybrid topology with a ring backbone and two bus network.
14. In Figure 1, computer A sends a message to computer D via LAN1, router R1 and
LAN2. Draw the contents of the packets and frames at the network and data link layer for
each hop interface.
Tutorial 1
Pn. Shahadah Ahmad
Data Communication &Network
DKT224/3
15. State the differences between TCP and IP. (ANY FOUR ANS)
IP
Placed
at level 3 ( network)
Interconnection
of Packet Switching networks
use the services that the PS networks provide
TCP

Placed at level 4 (transport level)

Connection-oriented
protocol
adapt packet lengths to supported lengths
• Provides a reliable unicast end-to-
(fragmentation)
end byte stream over an unreliable
“Best
Effort” delivery
internetwork.
• Provide more facilities than UDP

no prevention of packet loss

independent treatment of
Error recovery, flow control and
packets (no “flow” concept


in normal IPv4)
TCP connection and concept
sequence integrity not
• Stream data transfer – segments
guaranteed
• Reliability – sequence number, ACK,
Routers use destination network, not host address


retransmission
need only to know how to
• Flow control – to avoid overflow,
reach a network
window-mechanism
no influence of source host
on route (normal Ipv4)

reliability
no knowledge of full path to destination
• Multiplexing – based on port numbers
• Logical connection – each
connection is identified by socket
necessary

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send to “next hop”
Pn. Shahadah Ahmad
Data Communication &Network
DKT224/3
16. Describe the differences between connectionless-oriented and connectionoriented transmission.
Connection-oriented must established the connection before transfer/transmit data and
terminated the connection when the session is finished.(eg.TCP) On the other hand
connectionless did not require any establishement as transfer of data can be done
immediately.(eg. UDP).
17. Computer A sends multicast packet to computer B, C and D situated 1km away
through a router. Assuming that the transmission rate of the link is 10 Mbps and the
IP packet header is 20 bytes and the network access header is 28 bytes. Assume the
propagation speed is 200 m/µsec.
i)
Determine the total frame size in byte if each Ethernet frame carries 1032
bytes of IP “payload” data.
ii) Determine the mean time to send an Ethernet frame from computer A,
measured from the beginning of transmission to the end of reception.
iii) Determine the mean time to send an Ethernet frame from computer A,
measured from the beginning of transmission to the end of reception if the
transmission rate of the link is increased to 10 Gbps.
From the results obtained in (ii) and (iii) above, state the conclusion.
i)
ii)
1032 + 20 + 28 =1080 bytes
1µsec  200 metre
X sec  1000 metre, therefore X = 1000/200 = 5 µsec = Tp
Since using multicast, the meantime encountered is actually the trans mission
time for packet transfer . all transmission are done simultaneously.
Tf = Frame size / Link capacity = (1080 x 8 bps) / 10 x 106 = 864 µsec
Total time = Tf + Tp =( 864 + 5 ) µsec = 869 µsec
iii) Tf = Frame size / Link capacity = (1080 x 8 bps) / 10 x 109 = 864 nsec
Total time = Tf + Tp =( 0.864 + 5 ) µsec = 5.864 µsec
Tutorial 1
Pn. Shahadah Ahmad
Data Communication &Network
DKT224/3
The higher the link rate, the faster will be the time taken to transmit.
18. Explain the usage of the following commands : Ping, traceroute, Ipconfig and
Netstat.
•
•
•
•
Ping- to Determines reachability & round-trip delay
Traceroute – to find route from local host to a remote host
Ipconfig - Information about each IP interface of a host
DNS hostname, IP addresses of DNS servers, physical address of network card,
IP address, …
Netstart - Queries a host about TCP/IP network status
Status of network drivers & their interface cards
#packets in, #packets out, errored packets, …
Tutorial 1
Pn. Shahadah Ahmad