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Question Bank

Advanced Computer Networks

Q.1 What do you mean by Subnet Masking?

The subnet mask is used to determine which portion of the IP address is the network address and

which is the host address. This means that the portions of network to host in an IP address can

change. The most common subnet mask is 255.255.255.0. The simple explanation is that

wherever there is a 255, this indicates that it is the network portion. Wherever there is a 0, this

indicates the host portion.

Q.2 Define Beacons?

Beacon are used to determine their own absolute locations based on GPS or manual

configuration. The majority of nodes can then derive their absolute location by combining an

estimate of their position relative to the beacons with the absolute location information provided

by the beacons.

Q.3 Differentiate between TCP and UDP.

Transmission Control Protocol (TCP) :

1) Transmission Control Protocol (TCP) is a connection oriented protocol, which means the

devices should open a connection before transmitting data and should close the connection

gracefully after transmitting the data.

2) Transmission Control Protocol (TCP) assures reliable delivery of data to the destination.

3) Transmission Control Protocol (TCP) protocol provides extensive error checking mechanisms

such as flow control and acknowledgment of data.

4) Sequencing of data is a feature of Transmission Control Protocol (TCP).

5) Delivery of data is guaranteed if you are using Transmission Control Protocol (TCP).

6) Transmission Control Protocol (TCP) is comparatively slow because of these extensive errorchecking mechanisms

7) Multiplexing and De multiplexing is possible in Transmission Control Protocol (TCP) using

TCP port numbers.

8) Retransmission of lost packets is possible in Transmission Control Protocol (TCP).


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User Datagram Protocol (UDP) :

1) User Datagram Protocol (UDP) is Datagram oriented protocol with no overhead for opening,

maintaining, and closing a connection.

2) User Datagram Protocol (UDP) is efficient for broadcast/multicast transmission.

3) User Datagram protocol (UDP) has only the basic error checking mechanism using

checksums.

4) There is no sequencing of data in User Datagram protocol (UDP) .

5) The delivery of data cannot be guaranteed in User Datagram protocol (UDP) .

6) User Datagram protocol (UDP) is faster, simpler and more efficient than TCP. However, User

Datagram protocol (UDP) it is less robust then TCP

7) Multiplexing and De multiplexing is possible in User Datagram Protcol (UDP) using UDPport numbers.

8) There is no retransmission of lost packets in User Datagram Protcol (UDP).

Q.4 Explain the main features of OSPF ?

Authentication of routing messages.

Additional hierarchy.

Load balancing.

Q. 5 What do you mean by Subnetting ?

Subnetting gives us a simple way to reduce the total number of network numbers that are

assigned. The idea is to take a single IP network number and allocate the IP address with that

network to several physical networks, which are now referred to as subnets.

Q.6 Define quality of service? What are the two categories of QoS attributes?

The quality of service defines a set of attributes related to the performance of the connection. For

each connection, the user can request a particular attribute each service class is associated with a

set of attributes.

The two main categories are :

User Oriented

Network Oriented


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Q.7 Why is SSL important?

Secure Socket Layers are critical for protecting private information. Every email sent by us,

every website visited by us and every piece of data leaving our computer can be seen by more

than just the intended recipient UNLESS IT IS SECURELY ENCRYPTED. SSL is the protocol

or standard for internet security, and working without it is akin to leaving our car unlocked in atheft prone area.

Q.8 Comment on the security of Public Key Schemes?

Security of Public Key Schemes:

a. Llike private key schemes, brute force exhaustive search attack is always theoretically possible

but keys used are too large (>512bits) .

b. Security relies on a large enough difference in difficulty between easy (en/decrypt) and hard

(cryptanalyse) problems

Q.9 Mention some of the properties of Digital Signatures ?

Digital Signature Properties :

must depend on the message signed

must use information unique to sender to prevent both forgery and denial

must be relatively easy to produce

must be relatively easy to recognize & verify

be computationally infeasible to forge

Q. 10 What are the key benefits of public key cryptography ?

The primary benefit of public key cryptography is that it allows people who have no preexisting

security arrangement to exchange messages securely. The need for sender and receiver to share

secret keys via some secure channel is eliminated; all communications involve only public keys,

and no private key is ever transmitted or shared. Some examples of public-key cryptosystems are

Elgamal, RSA & Diffie-Hellman.

Q. 11 Define Cryptanalysis, Cryptanalyst, Cryptology and Cryptosystem?

a. Cryptanalysis is the study of mathematical techniques for attempting to defeat cryptographic

techniques, and, more generally, information security services.

b. Cryptanalyst is someone who engages in cryptanalysis.


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c. Cryptology is the study of cryptography and cryptanalysis.

d. Cryptosystem is a general term referring to a set of cryptographic primitives used to provide

information security services. Most often the term is used in conjunction with primitives

providing confidentiality, i.e., encryption.

Q.12 Briefly explain SNMP ?

Simple Network Management Protocol (SNMP) is an "Internet-standard protocol for managing

devices on IP networks. Devices that typically support SNMP include routers, switches, servers,

workstations, printers, modem racks, and more." It is used mostly in network management

systems to monitor network-attached devices for conditions that warrant administrative attention.

SNMP is a component of the Internet Protocol Suite as defined by the Internet Engineering Task

Force (IETF). It consists of a set of standards for network management, including an application

layer protocol, a database schema, and a set of data objects.

Q.13 What is the function of SMTP?

The protocol that supports electronic mail on the Internet is called Simple Mail Transfer

(SMTP). It is a system for sending messages to other computer users based on e-mail addresses.

SMTP provides mail exchange between users on the same or different computers.

Q.14 Differentiate between BRI and PRI services offered in ISDN.

Basic Rate Interface (BRI) consists of 2B + D channels which stands for 2 Bearer channels

having 64 kbps capacity for data and one D channel having 16 kbps capacity for control

signalling. Having a separate channel for handshaking and control is called "out of band"

signalling. The 2B channels can be bonded together for a single data channel with a 128 kbps

transfer rate.

Primary Rate Interface (PRI) consists of 23B + D channels which stands for 23 Bearer

channels of 64 kbps capacity for data and one D channel of 64 kbps capacity for control

signalling. The Bearer channels can be bonded in any combination as required.

Q.15 Briefly explain ARP Protocol.

Address Resolution Protocol (ARP) belongs to the Network layer. It can be considered as a

mechanism for mapping addresses between the Network logical addresses and MAC (MediaAccess Control) layer physical addresses. For example: the Network layer protocol IP is not

aware of 48 bit MAC addresses such as Ethernet. Similarly the MAC layer protocol such as

Ethernet does not know anything about 32 bit IP addresses. ARP provides the mechanism to

map MAC addresses to IP addresses in a temporary memory space known as the ARP cache.

Q.16 How does ARP works ?


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The basic operation of ARP is as follows :

a.) When the IP layer wants to communicate with another device on the network, it checks the

ARP cache to see if there is a match with an Ethernet address.

b.) If there is no matching entry in the ARP cache, an ARP broadcast datagram is sent out thatbasically says "Does anybody know whose Ethernet address belongs to this IP address?".

c.) The receiving station that has the IP address, responds with an ARP datagram that says "This

is my IP address and here is my Ethernet address".

d.) The ARP cache is updated and the original IP layer information is then passed on to the MAC

layer for processing.

Q.17 Briefly explain RARP Protocol.

Reverse Address Resolution Protocol (RARP) is the reverse of ARP. It is a mechanism to map

MAC addresses to IP addresses. It is used mainly by diskless workstations upon boot-up to find

out their IP addresses from a BOOTP server. The BOOTP server contains all of the boot-up

configuration files that the workstation needs to boot-up.

Q.18 How does RARP works?

On NICs (network interface cards) there is an empty DIP socket that is used for holding a Boot

PROM. The Boot PROM holds a special software program that tells the workstation that upon

powering up, to go and find a BOOTP server. One of the first tasks of the workstation is to find

out its IP address. The MAC layer address is burnt into the NIC and is already known. A RARP

broadcast datagram is sent out that asks "Does any BOOTP server know what my IP addressis?". The BOOTP server will reply with "Here's the IP address that belongs to your MAC

address". Once the IP address is known, then the rest of the configuration files can be

downloaded and the diskless workstation booted up.

Q.19 Explain RMON.

RMON is a MIB definition that provides for remote network monitoring and manager to

manager communications. This standard defines the information that a device must collect to

provide an accurate picture of a network's health. The information that the RMON MIB provides

is divided into the following 9 groups:

1. Segment Statistics: Statistics on the attached segment: packets, octets, collisions...

2. History: Data collected by the statistics group for future retrieval and analysis

3. Alarms: Thresholds can be set for certain alarm condition. (Collisions/sec)

4. Events: Thresholds can be set for certain event conditions to trigger certain activities.


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5. Host: Statistics can be gathered on individual nodes or hosts on the LAN.

6. Host Top N: Statistics can be gathered on individual nodes over a user-specified time.

7. Traffic Matrix: Shows the amount of traffic and number of errors between any host pair.

8. Packet Capture: Allows capturing of entire packets or individual slices indicating protocols

used.

9. Filters: The filter group specifies the criteria that the packet is going to be captured by.

Q.20 Define ADSL.

Asymmetrical Digital Subscriber Line (ADSL) is a method to use the existing analog local loop

lines for digital data transfer to and from the home. It is asymmetrical in that the upstream

transfer rate is slower than the downstream data rate. This means that the data transfer from the

premise (home) to the CO is a different rate than the data transfer from the CO to the home.

References :

Introduction to Data Communications

By Eugene Blanchard

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