3201 Computer Networks – 2014/2015 Handout: Questions Bank

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Computer Network Questions Bank with Answers

Question1:

There is considerable confusion in the literature between a computer network and a

distributed system. Explain this statement.

Computer networks vs. distributed systems:

Distributed System Computer Networks

- A collection of independent

computers appears to its users as a

single coherent system

- The coherent model and software

are absent

- Layer of software on top of OS

called “middleware” is responsible

for implementing this model.

- Users are exposed to the actual

machines

- The distinction between a network and a distributed system lies with the

software, rather than the hardware.

Question2:

Define the following terms: ANSI, UDP, ISO, FTP, IP, NAT, PAT, SNAT, and ICS.

ANSI American National Standards Institute

Is the primary organization for fostering the development of technology standards in the

United States,

A private nonprofit organization that oversees the development of voluntary consensus

standards

UDP User Datagram Protocol

Offers only a minimal transport service -- non-guaranteed datagram delivery -- and gives

applications direct access to the datagram service of the IP layer

ISO International Organization for Standardization

Is an international standard-setting body composed of representatives from various

national standards organizations

FTP file transfer protocol

Is a standard network protocol used to transfer files from one host to another host over

a TCP-based network, such as the Internet.

IP internet protocol

IP is the primary protocol in the Internet Layer of the Internet Protocol Suite and has the

task of delivering datagrams from the source host to the destination host solely based on

the addresses.

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NAT Network Address Translation

An Internet standard that enables a local-area network (LAN) to use one set of IP

addressesfor internal traffic and a second set of addresses for external traffic.

PAT port address translation

A type of network address translation. During PAT, each computer on LAN is translated

to the same IP address, but with a different port number assignment.

SNAT Source Network Address Translation (source-nat or SNAT)

Allows traffic from a private network to go out to the internet.

ICS Internet connection sharing

A method for connecting multiple computers in a LAN to the Internet through a single

connection and a single IP address.

Question3:

Which of the OSI layers handles each of the following?

Transmitting raw bits over a communication channel.

Physical Layer

How to keep a fast transmitter from drowning a slow receiver in data.

Data Link Layer

Controls the operation of the subnet

Network Layer

Concerned with the syntax and semantics of the information transmitted.

Presentation Layer

Dividing the transmitted bit stream into frames.

Data Link Layer

Determining which route through the subnet to use.

Network Layer

Question4:

Explain the use of Reliable and Unreliable connection.

Reliable means data never lost ,its done through acknowledge sent from the receiver

to the sender.

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- Acknowledge process introduces overheads and delayes

- Some times acknowledge not accepted such as digitized voice traffic

Reliable services in the sense that we are assured of the receiver receiving the data ,

and in

Unreliable : means ( no acknowledge) connectionless services are often called

datagram service.

Unreliable the receiver might not actually get the data. This is monitored by

usually setting a feedback required or not mechanism.

When to use them:

Reliable In cases like FTP, we might require that the sender acknowledge the

receipt. For this we use reliable connection oriented services.

Unreliable In other cases such as database queries, it is not really required that the

receiver sends an acknowledgement. In these cases, we use unreliable connectionless

services.

Question5:

Discuss the advantages and disadvantages of a wireless topology.

Advantages of a Wireless Topology:

• The nice thing about wireless networks is the lack of cabling. The wireless

network requires only base backbone segments to connect the wireless cells to the

wired network if there is one.

• Troubleshooting failed devices and cells is very easy and makes failed

components easy to find and replace.

• Easy Management

Disadvantages of a Wireless Topology:

• Signal interference. Other devices and machinery that emit radio frequencies or

“noise” can cause interference and static, which can disrupt the bubble of

communication around the cell. Another source of noise is lightning during

storms. This noise is the same static you hear when lightning strikes while you are

speaking on a phone.

• Blockage can occur in structures that are made of thick stone or metal, which do

not allow radio frequencies to pass through easily. This drawback usually can be

overcome somewhat by changing the frequency used by the devices to a higher

frequency.

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• Signal interception. means unwanted third parties could intercept wireless

communications without physically being on the premises; they would simply

have to be within the signal range.

Question6:

a) Given a channel of bandwidth F=3 KHz and bit rate b=300, 600, 1200, 2400,

38000 bps and we want to transmit n=8 bits (01100010) through that channel.

Calculate for each above bit rates the following:

Number of harmonics passed, Time of transmission T[msec.], Frequency

of first harmonic

Write your conclusion and draw the binary signal (original signal) and the

successive approximation to the original signal when passing one

harmonic, 2 harmonics, 4 harmonics and 8 harmonics.

B (bps) T [msec] 1st harmonic #harmonics passed

300 26.67 37.5 80.01

600 13.33 75.02 39.99

1200 6.67 149.93 20.01

2400 3.33 300.3 9.99

38000 0.21 4761.9 0.63

n = number of bits

Time =(𝑛𝑜. 𝑜𝑓 𝑏𝑖𝑡𝑠

𝑏𝑖𝑡𝑟𝑎𝑡𝑒) then mul by 1000 to convert from sec to msec

1st harmonic =( 𝑏𝑖𝑡𝑟𝑎𝑡𝑒

𝑛𝑜. 𝑜𝑓 𝑏𝑖𝑡𝑠)

# harmonics =f * (𝑛𝑜. 𝑜𝑓 𝑏𝑖𝑡𝑠

𝑏𝑖𝑡𝑟𝑎𝑡𝑒) then mu by 1000 to transfer frequency from khz to mhz

Maximum bit rate with 3 KHz bandwidth will be 2400 bit/sec

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Question7:

F=32 KHz

Bit rate=300, 600, 1200, 2400, 38000 bps

N=16 bits

Bit

rate

b

bit/sec

Time of

transmission

T=n/b * 1000 msec

Frequency

of 1st

harmonic

F0=1/T Hz

No. of

harmonic

passed=

F * T= F * n/b

300 53.33 18.75 853.28

600 26.66 37.5 426.56

1200 13.33 75 213.28

2400 6.66 150 106.56

38000 0.42 2380 6.72

Conclusion:

Maximum bit rate with 16 KHz bandwidth will be 2400 bit/sec

Question8:

Given a channel of bandwidth f = 8 khz and bitrate b= 300, 600, 1200, 2400, & 38000

bps and we want to transmit n = 2 bytes through that channel.

Calculate for each above bitrates the following

- Number of harmonics passed

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- Time of transmission T [msec]

- Frequency of the first harmonic

B (bps) T [msec] 1st harmonic #harmonics passed

300 53.33 18.75 426

600 26.67 37.5 213

1200 13.33 75 106

2400 6.67 150 53

38000 0.42 2375 3

n = number of bits n= 2*8 =16 bit

Time =(𝑛𝑜. 𝑜𝑓 𝑏𝑖𝑡𝑠

𝑏𝑖𝑡𝑟𝑎𝑡𝑒) then mul by 1000 to convert from sec to msec

1st harmonic =( 𝑏𝑖𝑡𝑟𝑎𝑡𝑒

𝑛𝑜. 𝑜𝑓 𝑏𝑖𝑡𝑠)

# harmonics =f * (𝑛𝑜. 𝑜𝑓 𝑏𝑖𝑡𝑠

𝑏𝑖𝑡𝑟𝑎𝑡𝑒) then mu by 1000 to transfer frequency from khz to mhz

Question9:

What is meant by each of the following terms: ( ARP , HTTP , HTTPS, NTP,SMTP,

FTP, SSH ).

Address Resolution Protocol The Address Resolution Protocol (ARP)

HTTP and HTTPS: The Hypertext Transfer .

Network Time Protocol (NTP): The Network Time Protocol (NTP)

Simple Mail Transfer Protocol (SMTP): The Simple Mail Transfer Protocol

File Transfer Protocol (FTP): The File Transfer Protocol .

Secure Shell (SSH): The Secure Shell

Question10:

Compare between the ISO-OSI Reference model and the TCP/IP model.

Criteria OSI TCP/IP

Number of Layers 7 layer’s:

1. Application Layer

2. Presentation Layer

3. Session Layer

4. Transport Layer

5. Network Layer

6. Data Link Layer

7. Physical Layer

4 layer’s:

1. Application Layer

2. Transport Layer

3. Internet Layer

4. Host-to-network

Protocols/Layers 1. Application Layer (HTTP, FTP,

NFS, NTP …).

1. Application Layer (TELNET,

FTP, SMTP, DNS).

2. Transport Layer (TCP, UDP).

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2. Presentation Layer (MIME, TLS,

SSL, XDR).

3. Session Layer (SAP, PPTP,

SOCKS…).

4. Transport Layer (TCP, UDP,

SPX, DCCP…).

5. Network Layer (IP, ICMP, Apple

Talk….).

6. Data Link Layer (IEEE 802.3,

PPP, SDLC….).

7. Physical Layer (IEEE 802.16,

USB, Bluetooth, Hubs, DSL….).

3. Internet Layer (IP).

4. Host-to-network Layer

(ARPANET, SATNET, Packet

radio, LAN).

Role/Layer Application Layer: Contains a variety

of protocols that are commonly needed

by users.

Presentation Layer: Manages theses abstract data structures

& allows higher-level data structures to

be defined & exchanged, as computers

with different data representations to

communicate.

Session Layer: The user negotiate with the session layer

to establish a connection with a process

on another machine to establish a

session, the user must provide the remote

address he wants to connect to the

operation of setting up a session between

2 processes is often called “binding”.

Transport Layer: We mean by “end-to-end” or from

“source-to-destination” a program or a

source machine carries only a

conversation with the similar program on

a destination machine using the message

headers & control messages.

Network Layer: Controls the operation of the subnet.

Data Link Layer:

Data link layer takes “raw

transmission bits” & break it up

into “Data frames”.

Creates & recognizes frames

boundaries, this can be done by

attaching special bit patterns to

Application Layer: Contains all the higher level protocols.

Transport Layer: Allow peer entities on the source &

destination hosts to carry on a

conversation.

Internet Layer:

Uses packet switching

techniques, based on a

connectionless internetworking

Defines packet format & protocol

called “IP”.

Host-to-network Layer: The host has to connect to the network

using some protocol, so it can send IP

packets to it.

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Question11:

Classify the different switching techniques used in computer networks and which one is

used.

Different switching techniques:

- Circuit Switching:

The switching equipment within the telephone system seeks out a physical path all the

way from your telephone to the receiver's telephone

- Message Switching:

No physical path is established in advance between sender and receiver. Instead, when

the sender has a block of data to be sent, it is stored in the first switching office (i.e.,

router) and then forwarded later, one hop at a time. Each block is received in its entirety,

inspected for errors, and then retransmitted.

- Packet Switching:

There is no limit at all on block size, which means that routers (in a modern system) must

have disks to buffer long blocks. It also means that a single block can tie up a router-

router line for minutes, rendering message switching useless for interactive traffic.

Packet switching Circuit switching

Doesn’t setup a physical connection

between sender and receiver inadvance

Setup physical connection inadvance

Acquire the bandwidth only on

transmission and then releases it

Holds the bandwidth until the end of

transmission

No bandwidth waste Bandwidth is wasted

Circuits are never dedicated Circuits are dedicated

IMPs provide speed, code conversion as

well as some correction

No

IMPs provide buffering in the main

memory install and forward routing

Buffering is to be in the secondary

memory for sender and receiver only

the beginning & the end of the

frame “01111110”.

Problem of flooding slow

receivers from fast senders must

be taken into consideration.

Problem of transmission on both

directions must be solved.

Physical Layer: Concerned with transmitting raw bits

over a communication channels.

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Cost doesn’t depend on the distance

between the two ends

Charge = f ( traffic, time )

Cost doesn’t depend on the traffic (no. of

bits) sent

Charge = f ( distance, time )

Used for coputer networks Rarely used

Packets may arrive to the rciever in order

different than transmission order

Message arrive in order

Question12:

Compare between the 5 different IP classes in computer networks (ranges, Subnet masks

and number of clients).

IP

Class

Ranges Subnet masks Number of

Clients

A 1 - 126 255.0.0.0 16,777,214

B 128 - 191 255.255.0.0 65,534

C 192 - 223 255.255.255.0 254

D 224 - 239 Multicast address N/A

E 240 - 255 Reserved for future

use

N/A

Question13:

What is meant by loopback test on 127.0.0.1?

Messages sent to 127.0.0.1 do not appear in the network. The network adapter

loopback(send back) all messages to the sending application. That is it is used

when a program access a network service running on the same computer itself.

That is loopback address is used to test the self-connectivity.

Question14:

Define the following terms: SAP, UTP, STP, TCP, FTP, IP, Computer Network,

Protocol, NOS

SAP service access point

A Service Access Point (SAP) is an identifying label for network endpoints used

in Open Systems Interconnection (OSI) networking.

STP shielded twisted pair

Twisted pair cabling is a type of wiring in which two conductors (the forward and return

conductors of a single circuit) are twisted together for the purposes of canceling

out electromagnetic interference (EMI) from external sources

UTP unshielded twisted pair

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UTP cable is also the most common cable used in computer networking.

Modern Ethernet, the most common data networking standard, utilizes UTP cables.

TCP transmission control protocol

TCP provides reliable, ordered delivery of a stream of bytes from a program on one

computer to another program on another computer. TCP is the protocol that major

Internet applications such as the World Wide Web, email, remote administration and file

transfer rely on

FTP file transfer protocol

File Transfer Protocol (FTP) is a standard network protocol used to transfer files from

one host to another host over a TCP-based network, such as the Internet.

IP internet protocol

IP is the primary protocol in the Internet Layer of the Internet Protocol Suite and has the

task of delivering datagrams from the source host to the destination host solely based on

the addresses.

Computer Network

Group of computers connected to each other in order to exchange information such as

- torage or printers.

Protocol

Is an agreement between the communicating parties on how communication is to

proceed.

NOS network operating system

A networking operating system (NOS), is the software that runs on a server and enables

the server to manage data, users, groups, security, applications, and other networking

functions. The network operating system is designed to allow shared file and printer

access among multiple computers in a network

Question15:

Explain the different Modulation techniques used to transmit data throw the computer

network channels.

Modulation:

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(a) A binary signal

(b) Amplitude modulation

(c) Frequency modulation

(d) Phase modulation

Question16:

What is meant by 10Base2 and 1000BaseCX?

10base2 --> 10 means transmission rate is 10 bps ,, base means baseband ,, 2

means thinnet coaxial cable as the maximum size of the cable is 200 m or 185 m

1000baseCX --> 1000 means transmission rate is 1000 bps ,, base means

baseband transmission ,, CX means coaxial cable

Question17:

Describe the concept of satellite communication and compare it with the terrestial links.

Satellite vs terrestial links:

Terristal links Satellite

Propagation delay 6 Msec/Km = 𝟏𝟎𝟑

𝟑∗𝟏𝟎𝟖 Typical propagation delay 270 Msec

To send x kbits over a 9600 bps is (𝒙∗𝟏𝟔𝟎𝟎

𝟗𝟔𝟎𝟎) To send x kbits over a 5-Mbps is 270 MS

+ 𝑥

𝟓∗𝟏𝟎6ms

The satellite is faster and the propagation delay is independent of distance between

sender and receiver

Question18:

Draw the wire configuration to connect PC-to-PC and to connect PC-to-Hub.

Using a Straight Through Cable to Connect Two Hosts:

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Using a Crossover Cable to Connect Two Hosts:

Question19:

List two ways in which the OSI reference model and TCP/IP reference model are the

same. Now list two ways in which they differ.

Similarities between TCP/IP and OSI reference Model:

· Both models are based on layered protocols

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· Both have a network, transport and application layer

· In both models, the transport service can provide a reliable end-to-end byte

stream

On the other hand, they differ in several ways:

· The numbers of layers are different

· TCP/IP does not have session or presentation layers

· OSI does not support internetworking and OSI has both connection oriented

and connectionless service in the network layer

Question20:

The following Character encoding is used in a data link protocol:

A: 01000111; B: 11100011; FLAG: 01111110; ESC: 11100000

Show the bit sequence transmitted (In Binary) for the four –character frame: A B ESC

FLAG when each of the following framing methods are used:

1. Character Count

2. Flag bytes with byte stuffing.

3. Starting and ending flag bytes, with bit stuffing.

o Character count

00000100 01000111 11100011 11100000

01111110 o Flag bytes with byte stuffing

01111110 01000111 11100011 11100000 11100000 11100000 01111110 01111110

Flag A B Esc (byte

stuffed) Esc

Esc (byte

stuffed) flag Flag

o Starting and ending flag bytes, with bit stuffing

01111110 01000111 110100011 111000000 011111010 01111110

flag A B Esc flag Flag

Question21:

What is meant by TDMA and FDMA? Explain each of them and write their delay equations. TDMA: Time Division with Multiple Access Time is divisible into slots of time. Each user is assigned a fixed number of predefined

channel time slots. The user has access to entire channel bandwidth but only during its allocated slots.

FDMA: Frequency Division with Multiple Access

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Bandwidth is divided by the number of users. Each user is assigned fraction where by

the assigned buffer it can be seen that the system can be modeled as sub channel, each

of them viewed as an queuing system with a constant service time for the packet

transmission time for each sub channel.