computer networks

1. COMPUTER NETWORKS
BSc IT semester 2
Name: Muhama Mark
BT0072
Roll: 1210000081
Qn 1. Write about different network structures in use.
In any network there exists a collection of machines varying uses or application programs
called hosts. The hosts are connected by a communication subnet or just subnet. There are two
distinct components knows as transmission lines and switching elements. These two components
are most in wide area networks. The transmission lines move bits between machines and
examples are channel, trunks and circuits.
Interface message processor is another name given to switching elements. Here each host is
connected to one or many interface message processors and all traffic from a host goes through
its interface message processor (IMP). There are basically two types of designs for
communication subnets.
Broadcast channels; broadcasting is the transmission of a message from one end or source and
receiving information to the rest of the system most local area networks and a small portion of
wide area networks are of this type (broadcasting). In a local area network, the interface message
processor is reduced to a single chip embedded inside the host or node so that there is always one
host per interface message processor whereas in a wide area network, there might be many hosts
sharing one interface message processor. Broadcasting systems do have a single communication
channel shared by all other machines or hosts on the network. Broadcasting systems also support
transmission to a subnet of machines and this is known as multicasting where a message is sent
to a group of hosts on the network.
Point to point channels; point to point is a connection between two peer machines. The network
contains numerous cables or leased telephone lines; each of the numerous cables or leased
telephone lines connects a pair of interface message processors. It two interface message
processors that do not share a cable with to communicate, they must do this indirectly, via other
IMPs. A subnet using this principle is called point to point, store and forward, or packet switched
subnet
When a point to point subnet is used, an important design issue is what the IMP interconnection
topology should look like. Locale network have a symmetric topology, whereas wide area
networks have asymmetric topology.
Qn 2. Describe the architecture and usage of ISDN.
ISDN refers to integrated service digital network. It’s a protocol that combines digital
telephony and data transport services. This has enabled the transmission of audio, video, and text
over existing telephone lines.
The user access is the most paid area by the standards organization. There’s a common physical
interface defined to be usable for telephone, computer terminal, and video text as well as DTE-
DCE connection. A number of protocols are needed for exchange of control information between
user device and the network.

2. Twisted pair cables are used to provide full duplex digital communication link.
The central office connects numerous ISDN subscriber loop signals to the IDN and provides
subscriber access to dedicated lines, packet switched networks among others. Even multiplexed
access via digital PBX and LAN must also be accounted for.
Usage of integrated service digital network is as follows;
Supplementary service; these services provide additional functionality to the bearer and
teleservices and examples of supplementary services are;
Call waiting; a service whereby someone on the telephone is notified of an incoming call and is
able to place the first call on hold while answering, Reverse charging, Message handling;
Bearer service; these services provide the means to transfer information (voice, data and video)
between users without the network manipulating the content of that information. The network
does not need to process the information and therefore does not change the content. Bearer
services belong to the first three layers of the OSI model and are well defined in the ISDN
standard.
Tele services; here, the network may change or process the contents of the data, these services’
correspond to the layers 4-7 of the OSI model i.e.; transport layer, session layer, presentation
layer and application layer. Teleservices rely on the facilities of the bearer services and are to be
aware of details of the process. Teleservices include telephony which is simply a collective term
for all types of voice equipment that are designed to allow interactive communication between
two points, teletex, videotext, telex and
Conferencing; the world is more connected than ever. With the creation of high-speed Internet,
businesses can conduct meetings over long distances and this has become the norm. This form of
long-distance communication has become known as conferencing, and it is a vital part of today's
global communications strategy.
Qn 3. Explain the concept of framing in Data Link Layer and its importance in
dataCommunication.
Data transmission in the physical layer means moving bits in the form of a signal from the
source to destination. The physical layer provides bit synchronization to ensure that the sender
and receiver use the same bit durations and timing. The data link layer packs bits which have
been moved by the Physical Layer, into frames, so that each frame is distinguishable from
another whiles being transmitted to destination.
Framing in the data link layer separates a message from one source to a destination, or from
other messages to other destinations, by adding a sender address and a destination address. The
destination address defines where the packet is to go; the sender address helps the recipient
acknowledge the receipt.
A message is divided into small frames, if an error exists in one of the frames, only that frame
will be retransmitted but not the whole frame as it is when the message is not broken down.

3. Variable size framing; here frame sizes are not fixed but vary and we need to find a way of
defining the beginning and ending of a frame. Variable size framing is most used in local area
networks than in other networks. We employ two approaches in variable size framing and these
are bit oriented approach and character oriented approach.
Character oriented protocols; Data to be carried are 8 bit characters from a
Coding system (ex: ASCII). The data link layer translates layers raw bit stream into discrete
messages called frames. It’s because the physical layer just accepts a raw bit stream and to
deliver it to destination. This bit stream is not guaranteed to be error free. The number of bits
received may be less than, equal to, or more than the number of bits transmitted. They may also
have different values. It is up to the data link layer to detect and if necessary correct errors. If not
correct at least detect errors and take proper action like ask for retransmission etc. The usual
approach of data link layer is to break the bit stream up into discrete frames and then for the
purpose of error detection or correction, it uses some coding technique that finds some checksum
for each frame. When a frame arrives at the destination, the checksum is recomputed,ifit’s
different from the one contained in the frame, an error has occurred and takes steps to deal with
it
Fixed size framing; in fixed-size framing, there is no need for defining the boundaries of the
frames. The size itself can be used as a delimiter. All this is importance work of data link layer.
Qn 4. Differentiate Noisy and Noiseless channels in Data Communication.
Noise is unwanted signal that is inserted between transmission and reception. What the
source sends is not what the destination receives due to the unwanted signal interfering between
the transmissions of data. Some types of noises are explained below.
Cross talk; Crosstalk is an effect that occurs between neighbor pairs of wires. The electrical
current flowing through one wire will create an electromagnetic field which is captured by the
neighbor wire
Thermal noise; Generated by the thermal agitation of electrons in conductors and it is a function
of temperature. The thermal noise is present in all electronic circuits and transmission media.
Generally, it is mathematically described as a “white noise”.
Impulse;these are irregular pulses or spikes e.g. external electromagnetic interference such as
lightening and it’s an important source of errors for the digital signals.
A channel is a path that data uses in its transmission from source to destination and such channel
include cables.
Therefore a noisy channel is a path that contains other signals other than the original signal sent
by the source and reality we will always have noisy channels as introduced by ClaudeShannon a
formula called the Shannon capacity used for determining he theoretical highest data rate of a
noisy channel
Capacity = bandwidth*log2(1+SNR) where SNR is signal to noise ratio.
We have 3 protocols that we use here
Go back N Automatic Repeat request

4. Several frames are sent before receiving an acknowledgment from the sender. A copy is kept at
the source until the acknowledgement arrives. Each frame sent is given a sequence number.
Sliding window protocol;this is an abstract concept.it deals with a range of sequence that is a
concern of the sender and receiver. Send sliding window is the range concerned with the sender
and receive sliding window is the range concerned with the receiver.
Stop and wait automatic repeat request
Error control mechanisms are added to the stop and wait protocol. To detect and correct
corrupted frames, we add redundancy bits to the data frame.
Noiseless channel is a channel or path that data uses for its safe transmission from source to
destination without any noise or interference, basically the data reaches the destination as clear
as it was sent without any interference.
The formula for a noiseless channel is known as nyquist bit rate formula which defines the
theoretical maximum bit rate
Bitrate = 2*bandwidth*log2L.
In noiseless channel we have two protocols that we use
Simplest protocol; this is has no error control or flow control and it is unidirectional, data
frames travel in only one direction. Here the receiver can never be overwhelmed with incoming
frames and can handle any frame it receives no matter what size.
Stop and wait protocol. This protocol has flow control. This protocol works in a way that the
source sends a frame and waits for an auxiliary ACK which is a confirmation from the recipient
that it has received the frame. It’s still unidirectional protocol but the auxiliary ACK travels the
opposite side
Qn 5. Explain Connection states and POP3 commands and responses.
POP3 refers for post office protocol version 3. This is an electronic mail protocol with both
client (sender/receiver) and server functions. POP3 supports basic functions for electronic mail
retrieval.
POP3 uses a well-known port 110 and establishes a TCP connection to the server. And the
communication between the client and server passes through three distinct states.
The server first sends a greeting massage to the client. The next state is the authenticate state
where the client authenticates itself to the server. It can use both user id and password, AUTH or
APOP.
After successful authentication, transaction state is next where the client can accesses the
mailbox. With POP3, mail is downloaded from the server to the client and then deleted on the
server. Since email messages are downloaded to the client and removed from the server, this
means that there is not a centralized location where email messages are kept. This makes the
POP3 protocol undesirable in a centralized backup solution for a small business.
Commands and responses of POP3 protocol.

5. Commands are sent and in turn responses are expected and such responses start with a status
indicator +OK for success and –ERR for failure. A response can be up to 512 characters long.
Some of the commands are listed below.
During authorization, we have the following commands
USER name uses username for authentication,
PASS password uses password for authentication,
APOP name digest; the name and message digest 5 (MD5) to be used for authentication
QUIT; this is used for terminating the process.
AUTH mechanism; this states the authentication / encryption mechanism to be used
During transaction, we have commands like
STAT used to retrieve the number of messages and the total size of the message,
DELEmsg to delete message and number message,
RSET to cancel any previous delete commands among others,
NOOP means do nothing and the server return a positive response,
RETR msg to retrieve message number message.
Qn 6. Explain the two modes of IP Security.
Internet protocol security is a collection of protocols that provide security for a packet at the
network level or 3 layer of the OSI model. It has two modes namely tunnel and transport modes.
Tunnel mode. IPsec protects the entire packet. It takes the entire packet and applies IPsec
security methods then adds a new IP header. This new has different information than the original
IP header. Tunneling is used to carry traffic of one protocol over the network of a different
protocol. Tunneling is useful when computer workers at home are connecting to a central
computer location.The packets are protected by AH, ESP, or both.
AH is a short form of authentication header and tunnel mode is used when both side of the
security association are gateways like between routers, firewalls. A merit of the tunnel mode is
that the encapsulated IP datagram is totally protected and another possibility of using private
addresses because data is always given another IP header other than the original in a sense that
the encapsulated that can include a private IP address but extra processor power is needed to
operate the tunnel mode of IPsec.
ESP is an acronym of encapsulating security pay load. This is used as a mechanism for integrity
check, authentication and encryption to the IP datagrams.Its working is like the way
Authentication header works in the tunnel mode.
Transport mode. In this mode of security protection is given to what is delivered from the
transport layer to the network layer. The network payload is protected by the transport mode but
not the IP header. The IPsec header and trailer are added to the packet coming from the transport
layer and the IP header is added later to the information. This method is commonly used when
there’s need for host to host or site to site protection of data. Both the sending host and receiving
host use IPsec for encrypting and decrypting the information.

6. Authentication header also can be applied in transport mode. Here the authentication header is
inserted immediately after the IP header. As known transport mode is used by hosts and not
gateways. This mode reduces the processing power but it changeable fields are not authenticated.
Using ESP in transport mode, the ESP is inserted immediately after the IP header. Neither
authentication nor encryption is provided for the IP header. False packets might be delivered for
processing making it a de merit.