Computer communication network

Computer Communication Networks
Himanshu Sirohi
Department of Computer Applications
Swami Vivekanand Subharti University, Meerut
himanshu.sirohi@ymail.com

Computer Networks:
A computer network is a digital telecommunications network which
allows nodes to share resources. In computer networks, computing
devices exchange data with each other using connections (data links) between
nodes. These data links are established over cable media such as wires or optic
cables, or wireless media such as Wi-Fi.
A computer network is a set of computers connected together for the purpose of
sharing resources. The most common resource shared today is connection to
the Internet. Other shared resources can include a printer or a file server.
The Internet itself can be considered a computer network.
Advantages of Computer Network:
• Sharing devices such as printers saves money.
• Site (software) licenses are likely to be cheaper than buying several standalone
licenses.
• Files can easily be shared between users.
• Network users can communicate by email and instant messenger.
• Security is good - users cannot see other users' files unlike on stand-alone
machines.
• Data is easy to backup as all the data is stored on the file server.

Network Criteria:
• Performance: Performance is the defined as the rate of transferring error free
data. It is measured by the Response Time. Response Time is the elapsed
time between the end of an inquiry and the beginning of a response. Request
a file transfer and start the file transfer. Factors that affect Response Time are:
• Number of Users: More users on a network - slower the network will run
• Transmission Speed: speed that data will be transmitted measured in bits per
second (bps)
• Media Type: Type of physical connection used to connect nodes together
• Hardware Type: Slow computers such as XT or fast such as Pentiums
• Software Program: How well is the network operating system (NOS) written

• Reliability: Reliability is the measure of how often a network is
useable. MTBF (Mean Time Between Failures) is a measure of the
average time a component is expected to operate between failures.
Normally provided by the manufacturer. A network failure can be:
hardware, data carrying medium and Network Operating System.
• Security: Security is the protection of Hardware, Software and Data
from unauthorized access. Restricted physical access to computers,
password protection, limiting user privileges and data encryption are
common security methods. Anti-Virus monitoring programs to
defend against computer viruses are a security measure.

Goals of Computer Networks:
The following are some important goals of computer networks:
• Resource Sharing –
Many organization has a substantial number of computers in operations,
which are located apart. Ex. A group of office workers can share a
common printer, fax, modem, scanner etc.
• High Reliability –
If there are alternate sources of supply, all files could be replicated on
two or, machines. If one of them is not available, due to hardware
failure, the other copies could be used.
• Money Saving- Computer networking is an important financial aspect
for organizations because it saves money. Example- sharing of software
and hardware that is printer.

Types of Networks:
• Local Area Network (LAN):
• It is also called LAN and designed for small physical areas such as an office, group of buildings
or a factory. LANs are used widely as it is easy to design and to troubleshoot. Personal
computers and workstations are connected to each other through LANs. We can use different
types of topologies through LAN, these are Star, Ring, Bus, Tree etc.
• LAN can be a simple network like connecting two computers, to share files and network
among each other while it can also be as complex as interconnecting an entire building.
• LAN networks are also widely used to share resources like printers, shared hard-drive etc.
• Characteristics of LAN
• LAN's are private networks, not subject to tariffs or other regulatory controls.
• LAN's operate at relatively high speed when compared to the typical WAN.
• There are different types of Media Access Control methods in a LAN, the prominent ones are
Ethernet, Token ring.
• It connects computers in a single building, block or campus, i.e. they work in a restricted
geographical area.
• Applications of LAN
• One of the computer in a network can become a server serving all the remaining computers
called clients. Software can be stored on the server and it can be used by the remaining
clients.
• Connecting Locally all the workstations in a building to let them communicate with each
other locally without any internet access.
• Sharing common resources like printers etc are some common applications of LAN.

Advantages of Local Area Networks:
• Resource Sharing: Computer resources like printers, modems, DVD-ROM drives and hard disks can be shared
with the help of local area networks. This reduces cost and hardware purchases.
• Software Applications Sharing: It is cheaper to use same software over network instead of purchasing separate
licensed software for each client a network.
• Easy and Cheap Communication: Data and messages can easily be transferred over networked computers.
• Centralized Data: The data of all network users can be saved on hard disk of the server computer. This will help
users to use any workstation in a network to access their data. Because data is not stored on workstations locally.
• Data Security: Since, data is stored on server computer centrally, it will be easy to manage data at only one place
and the data will be more secure too.
• Internet Sharing: Local Area Network provides the facility to share a single internet connection among all the
LAN users. In Net Cafes, single internet connection sharing system keeps the internet expenses cheaper.
• Disadvantages of LAN
• High Setup Cost: Although the LAN will save cost over time due to shared computer resources, but the initial
setup costs of installing Local Area Networks is high.
• Privacy Violations: The LAN administrator has the rights to check personal data files of each and every LAN
user. Moreover he can check the internet history and computer use history of the LAN user.
• Data Security Threat: Unauthorized users can access important data of an organization if centralized data
repository is not secured properly by the LAN administrator.
• LAN Maintenance Job: Local Area Network requires a LAN Administrator because, there are problems of
software installations or hardware failures or cable disturbances in Local Area Network. A LAN Administrator is
needed at this full time job.
• Covers Limited Area: Local Area Network covers a small area like one office, one building or a group of nearby
buildings.

Metropolitan Area Network (MAN):
• It was developed in 1980s.It is basically a bigger version of LAN. It is also called MAN and
uses the similar technology as LAN. It is designed to extend over the entire city. It can be
means to connecting a number of LANs into a larger network or it can be a single cable.
It is mainly hold and operated by single private company or a public company.
• Characteristics of MAN
• It generally covers towns and cities (50 km)
• Communication medium used for MAN are optical fibers, cables etc.
• Data rates adequate for distributed computing applications.
• Advantages of MAN
• Extremely efficient and provide fast communication via high-speed carriers, such as fiber
optic cables.
• It provides a good back bone for large network and provides greater access to WANs.
• The dual bus used in MAN helps the transmission of data in both directions
simultaneously.
• A MAN usually encompasses several blocks of a city or an entire city.
• Disadvantages of MAN
• More cable required for a MAN connection from one place to another.
• It is difficult to make the system secure from hackers and industrial espionage(spying)
graphical regions.

Wide-Area Networks:
• A wide area network (also known as WAN), is a large network of information that is not
tied to a single location. WANs can facilitate communication, the sharing of information
and much more between devices from around the world through a WAN provider.
• Characteristics of WAN:
• Multiple computers are connected together.
• It connects devices that are separated by a broader geographical area than a LAN.
• A WAN usually interconnects multiple LANs.
• Communication links between computers are provided by telephone networks, public
data networks, satellites, etc.
• Advantages of WAN:
• WAN covers larger geographical area. Hence business offices situated at longer distances
can easily communicate.
• Like LAN, it allows sharing of resources and application software's among distributed
workstations or users.
• The software files are shared among all the users. Hence all will have access to latest files.
This avoids use of previous versions by them.
• Organizations can form their global integrated network through WAN. Moreover it
supports global markets and global businesses.

Transmission Modes in Computer Networks:
• Transmission mode refers to the mechanism of transferring of data between two
devices connected over a network. It is also called Communication Mode. These
modes direct the direction of flow of information. There are three types of
transmission modes. They are:
• Simplex Mode
• Half duplex Mode
• Full duplex Mode
•Simplex Mode- In this type of transmission mode, data can be sent only in
one direction i.e. communication is unidirectional. We cannot send a message
back to the sender. Unidirectional communication is done in Simplex Systems
where we just need to send a command/signal, and do not expect any response
back.
• Examples of simplex Mode are loudspeakers, television broadcasting, television
and remote, keyboard and monitor etc.

HALF DUPLEX Mode:
• Half-duplex data transmission means that data can be transmitted in both
directions on a signal carrier, but not at the same time.
• For example, on a local area network using a technology that has half-duplex
transmission, one workstation can send data on the line and then immediately
receive data on the line from the same direction in which data was just
transmitted. Hence half-duplex transmission implies a bidirectional line (one that
can carry data in both directions) but data can be sent in only one direction at a
time.
• Example of half duplex is a walkie- talkie in which message is sent one at a time
but messages are sent in both the directions.
FULL DUPLEX Mode: In full duplex system we can send data in both the
directions as it is bidirectional at the same time in other words, data can be sent in
both directions simultaneously.
• Example of Full Duplex is a Telephone Network in which there is communication
between two persons by a telephone line, using which both can talk and listen at
the same time.

Line Configuration:
• Line configuration refers to the way two or more communication devices
attached to a link. Line configuration is also referred to as connection. A Link is
the physical communication pathway that transfers data from one device to
another. For communication to occur, two devices must be connected in same
way to the same link at the same time.
There are two possible line configurations.
1. Point-to-Point
2. Multipoint
• Point-to-Point- A Point to Point Line Configuration Provide dedicated link
between two devices use actual length of wire or cable to connect the two end
including microwave & satellite link. Infrared remote control & tv’s remote
control.
• The entire capacity of the channel is reserved for transmission between those
two devices. Most point-to-point line configurations use an actual length of wire
or cable to connect the two ends, but other options, such as microwave or
satellite links, are also possible.
• Point to point network topology is considered to be one of the easiest and most
conventional network topologies. It is also the simplest to establish and
understand. To visualize, one can consider point to point network topology as
two phones connected end to end for a two way communication.

Multipoint Configuration-
• Multipoint Configuration also known as Multidrop line configuration one or
more than two specific devices share a single link capacity of the channel is
shared.
More than two devices share the Link that is the capacity of the channel is
shared now. With shared capacity, there can be two possibilities in a Multipoint
Line Configuration:
• Spatial Sharing: If several devices can share the link simultaneously, its called
Spatially shared line configuration
• Temporal (Time) Sharing: If users must take turns using the link , then its called
Temporally shared or Time Shared Line Configuration

Network Topology:
Network topology is the arrangement of the elements of a communication
network. Network topology can be used to define or describe the arrangement of
various types of telecommunication networks, including command and control
radio networks, industrial fieldbuses and computer networks.
Network topology is the way a network is arranged, including the physical or
logical description of how links and nodes are set up to relate to each other.
OR
Network Topology is the schematic description of a network arrangement,
connecting various nodes(sender and receiver) through lines of connection.
Types of Network Topologies:
1. Bus Topology
2. Ring Topology
3. Star Topology
4. Mesh Topology
5. Star Topology

Bus Topology:
• Bus topology is a network type in which every computer and network device is
connected to single cable. When it has exactly two endpoints, then it is
called Linear Bus topology.

Ring Topology:
• It is called ring topology because it forms a ring as each computer is connected
to another computer, with the last one connected to the first. Exactly two
neighbors for each device.

Star Topology:
• In this type of topology all the computers are connected to a single hub through
a cable. This hub is the central node and all others nodes are connected to the
central node.

Mesh Topology:
• A network setup where each computer and network device is interconnected
with one another, allowing for most transmissions to be distributed even if one
of the connections go down. It is a topology commonly used for wireless
networks. Below is a visual example of a simple computer setup on a network
using a mesh topology.

Tree Topology:
• It has a root node and all other nodes are connected to it forming a hierarchy. It
is also called hierarchical topology. It should at least have three levels to the
hierarchy.

Open Systems Interconnection model (OSI model):
• The model was developed by the International Organization for Standardization
(ISO) in 1984. It is now considered the primary Architectural model for inter-
computer communications.
•
• The Open Systems Interconnection (OSI) reference model is a descriptive
network scheme. It ensures greater compatibility and interoperability between
various types of network technologies.
• The OSI model describes how information or data makes its way from application
programs (such as spreadsheets) through a network medium (such as wire) to
another application program located on another network.
•
• The OSI reference model divides the problem of moving information between
computers over a network medium into SEVEN smaller and more manageable
problems.
• This separation into smaller more manageable functions is known as layering.

A LAYERED NETWORK MODEL:
• The OSI Reference Model is composed of seven layers, each specifying particular
network functions.
• The process of breaking up the functions or tasks of networking into layers
reduces complexity.
• Each layer provides a service to the layer above it in the protocol specification.
• Each layer communicates with the same layer’s software or hardware on other
computers.
• The lower 4 layers (transport, network, data link and physical —Layers 4, 3, 2,
and 1) are concerned with the flow of data from end to end through the
network.
• The upper four layers of the OSI model (application, presentation and session—
Layers 7, 6 and 5) are orientated more toward services to the applications.
• Data is encapsulated with the necessary protocol information as it moves down
the layers before network transit.

THE SEVEN OSI REFERENCE MODEL LAYERS:

LAYER 7: APPLICATION LAYER:
• The application layer is the OSI layer that is closest to the user.
• It provides network services to the user’s applications.
• It differs from the other layers in that it does not provide services to any other
OSI layer, but rather, only to applications outside the OSI model.
• Examples of such applications are spreadsheet programs, word processing
programs, and bank terminal programs.
• The application layer establishes the availability of intended communication
partners, synchronizes and establishes agreement on procedures for error
recovery and control of data integrity.
• LAYER 6: PRESENTATION LAYER:
• The presentation layer ensures that the information that the application layer of
one system sends out is readable by the application layer of another system.
• If necessary, the presentation layer translates between multiple data formats by
using a common format.
• Provides encryption and compression of data.
• Examples:- JPEG, MPEG, ASCII, EBCDIC, HTML.

• LAYER 5: SESSION LAYER:
• The session layer defines how to start, control and end conversations (called
sessions) between applications.
• This includes the control and management of multiple bi-directional messages
using dialogue control.
• It also synchronizes dialogue between two hosts' presentation layers and
manages their data exchange.
• The session layer offers provisions for efficient data transfer.
• Examples- SQL, ASP(AppleTalk Session Protocol).
• LAYER 4: TRANSPORT LAYER:
• The transport layer regulates information flow to ensure end-to-end connectivity
between host applications reliably and accurately.
• The transport layer segments data from the sending host's system and
reassembles the data into a data stream on the receiving host's system.
• The boundary between the transport layer and the session layer can be thought
of as the boundary between application protocols and data-flow protocols.
Whereas the application, presentation, and session layers are concerned with
application issues, the lower four layers are concerned with data transport
issues.
• Layer 4 protocols include TCP (Transmission Control Protocol) and UDP (User
Datagram Protocol).

LAYER 3: NETWORK LAYER
• Defines end-to-end delivery of packets.
• Defines logical addressing so that any endpoint can be identified.
• Defines how routing works and how routes are learned so that the packets can be
delivered.
• The network layer also defines how to fragment a packet into smaller packets to
accommodate different media.
• Routers operate at Layer 3.
• Examples:- IP, IPX, AppleTalk.
• LAYER 2: DATA LINK LAYER
• The data link layer provides access to the networking media and physical transmission
across the media and this enables the data to locate its intended destination on a
network.
• The data link layer provides reliable transit of data across a physical link by using the
Media Access Control (MAC) addresses.
• The data link layer uses the MAC address to define a hardware or data link address in
order for multiple stations to share the same medium and still uniquely identify each
other.
• Concerned with network topology, network access, error notification, ordered delivery of
frames, and flow control.
• Examples:- Ethernet, Frame Relay, FDDI.

LAYER 1: PHYSICAL LAYER:
• The physical layer deals with the physical characteristics of the transmission
medium.
• It defines the electrical, mechanical, procedural, and functional specifications for
activating, maintaining, and deactivating the physical link between end systems.
• Such characteristics as voltage levels, timing of voltage changes, physical data
rates, maximum transmission distances, physical connectors, and other similar
attributes are defined by physical layer specifications.
• Examples:- EIA/TIA-232, RJ45, NRZ.

TCP/IP Model:
• The OSI Model we just looked at is just a reference/logical model. It was
designed to describe the functions of the communication system by dividing the
communication procedure into smaller and simpler components. But when we
talk about the TCP/IP model, it was designed and developed by Department of
Defense (DOD) in 1960s and is based on standard protocols. It stands for
Transmission Control Protocol/Internet Protocol. The TCP/IP model is a concise
version of the OSI model. It contains four layers, unlike seven layers in the OSI
model. The layers are:

1. Network Access Layer:
• This layer corresponds to the combination of Data Link Layer and Physical Layer of the OSI
model. It looks out for hardware addressing and the protocols present in this layer allows for
the physical transmission of data.
We just talked about ARP being a protocol of Internet layer, but there is a conflict about
declaring it as a protocol of Internet Layer or Network access layer. It is described as residing in
layer 3, being encapsulated by layer 2 protocols.
2. Internet Layer:
• This layer parallels the functions of OSI’s Network layer. It defines the protocols which
are responsible for logical transmission of data over the entire network. The main
protocols residing at this layer are :
• IP – stands for Internet Protocol and it is responsible for delivering packets from the
source host to the destination host by looking at the IP addresses in the packet
headers. IP has 2 versions:
IPv4 and IPv6. IPv4 is the one that most of the websites are using currently. But IPv6 is
growing as the number of IPv4 addresses are limited in number when compared to
the number of users.
• ICMP – stands for Internet Control Message Protocol. It is encapsulated within IP
datagrams and is responsible for providing hosts with information about network
problems.
• ARP – stands for Address Resolution Protocol. Its job is to find the hardware address
of a host from a known IP address. ARP has several types: Reverse ARP, Proxy ARP,
Gratuitous ARP and Inverse ARP.

3. Host-to-Host Layer:
• This layer is analogous to the transport layer of the OSI model. It is responsible
for end-to-end communication and error-free delivery of data. It shields the
upper-layer applications from the complexities of data. The two main protocols
present in this layer are :
• Transmission Control Protocol (TCP) – It is known to provide reliable and error-
free communication between end systems. It performs sequencing and
segmentation of data. It also has acknowledgment feature and controls the flow
of the data through flow control mechanism. It is a very effective protocol but
has a lot of overhead due to such features. Increased overhead leads to
increased cost.
• User Datagram Protocol (UDP) – On the other hand does not provide any such
features. It is the go-to protocol if your application does not require reliable
transport as it is very cost-effective. Unlike TCP, which is connection-oriented
protocol, UDP is connectionless.
4. Application Layer:
• This layer performs the functions of top three layers of the OSI model:
Application, Presentation and Session Layer. It is responsible for node-to-node
communication and controls user-interface specifications. Some of the protocols
present in this layer are: HTTP, HTTPS, FTP, TFTP, Telnet, SSH, SMTP, SNMP, NTP,
DNS, DHCP, NFS, X Window, LPD. Have a look at Protocols in Application Layer for
some information about these protocols.

Transmission Media:
• Transmission media is a communication channel that carries the information from the
sender to the receiver. Data is transmitted through the electromagnetic signals.
• The main functionality of the transmission media is to carry the information in the form
of bits through LAN(Local Area Network).
• It is a physical path between transmitter and receiver in data communication.
• In a copper-based network, the bits in the form of electrical signals.
• In a fiber based network, the bits in the form of light pulses.
• In OSI(Open System Interconnection) phase, transmission media supports the Layer 1.
Therefore, it is considered to be as a Layer 1 component.
• The electrical signals can be sent through the copper wire, fiber optics, atmosphere,
water, and vacuum.
• The characteristics and quality of data transmission are determined by the characteristics
of medium and signal.
• Transmission media is of two types are wired media and wireless media. In wired media,
medium characteristics are more important whereas, in wireless media, signal
characteristics are more important.
• Different transmission media have different properties such as bandwidth, delay, cost and
ease of installation and maintenance.
• The transmission media is available in the lowest layer of the OSI reference model,
i.e., Physical layer.

Guided Media:
• It is defined as the physical medium through which the signals are transmitted.
• It is also known as Bounded media.
UnGuided Transmission:
• An unguided transmission transmits the electromagnetic waves without using
any physical medium. Therefore it is also known as wireless transmission.
• In unguided media, air is the media through which the electromagnetic energy
can flow easily.

Integrated Service for Digital Network:
These are a set of communication standards for simultaneous digital transmission
of voice, video, data, and other network services over the traditional circuits of the
public switched telephone network. Before Integrated Services Digital Network
(ISDN), the telephone system was seen as a way to transmit voice, with some
special services available for data. The main feature of ISDN is that it can integrate
speech and data on the same lines, which were not available in the classic
telephone system.
Services of ISDN:
The following are the interfaces of ISDN:
• Basic Rate Interface (BRI) –
There are two data-bearing channels (‘B’ channels) and one signaling channel
(‘D’ channel) in BRI to initiate connections. The B channels operate at a
maximum of 64 Kbps while the D channel operates at a maximum of 16 Kbps.
The two channels are independent of each other. For example, one channel is
used as a TCP/IP connection to a location while the other channel is used to send
a fax to a remote location. In iSeries ISDN supports basic rate interface (BRl).

The basic rate interface (BRl) specifies a digital pipe consisting two B channels of
64 Kbps each and one D channel of 16 Kbps. This equals a speed of 144 Kbps. In
addition, the BRl service itself requires an operating overhead of 48 Kbps.
Therefore a digital pipe of 192 Kbps is required.
2. Primary Rate Interface (PRI) –
Primary Rate Interface service consists of a D channel and either 23 or 30 B
channels depending on the country you are in. PRI is not supported on the iSeries.
A digital pipe with 23 B channels and one 64 Kbps D channel is present in the usual
Primary Rate Interface (PRI). Twenty-three B channels of 64 Kbps each and one D
channel of 64 Kbps equals 1.536 Mbps. The PRI service uses 8 Kbps of overhead
also. Therefore PRI requires a digital pipe of 1.544 Mbps.
3. Broadband-ISDN (B-ISDN) –
Narrowband ISDN has been designed to operate over the current communications
infrastructure, which is heavily dependent on the copper cable however B-ISDN
relies mainly on the evolution of fiber optics. According to CCITT B-ISDN is best
described as ‘a service requiring transmission channels capable of supporting rates
greater than the primary rate.

Computer communication network

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