What is network architecture (full)

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CREATED BY SORCIA KRISTI-ROSE D’ARCEUIL
NETWORK ACHITECTURE

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CreatedbySorcia Kristi-RoseD’Arceuil
Table of Contents
Title Page
What is Network Architecture 2
Four basic types of Network Topology 4
Comparison between the OSI Model and TCP/IP model 7
The Seven Layer OSI Model- Physical Layer 8
The Seven Layer OSI Model- Data Link Layer 9
The Seven Layer OSI Model- Network Layer 10
The Seven Layer OSI Model- Transport Layer 10
The Seven Layer OSI Model- Session Layer 11
The Seven Layer OSI Model- Presentation Layer 12
The Seven Layer OSI Model- Application Layer 13
Summary of the Open Systems Interconnection Model 14
References 15

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What is Network Architecture?
Network architecture plays a vital role in the world we live in today without it many of the
technological comforts we enjoy wouldn’t exist. The name implies the function of the phase
“Network Architecture” as it can be defined as the structural and logical layout of the network
which consist of software and communication protocols, transmission equipment, infrastructure
transmission of data both wired and wireless and connectivity between components. There exists
two broad classification of network architectures they include client-server architectures and
peer-to-peer architectures.
In a client-server architectural model, a system is categorized into client and server processors
where the servers provide computational resources which clients consume. Thus servers are
powerful computers with the task of managing printers, network traffic and disk drives. Clients
are defined as workstations (PC’s) on which users run applications. Clients rely on servers for
resources such as such as files, devices, and even processing power. Client-server architectures
are commonly organized into layers called tiers. In a two-tier architecture the interface at the
presentation layer operates on a client while the data layer get stored on a server. In a three-tier
architecture (client-server architecture) the computer data storage, data access, functional process
logic are all developed and maintained as independent modules on separate platforms. While in a
multi-tier architecture the system architecture is a superset of a three-tier architecture and
includes additional layers for data and/or application servers. A Peer-to-peer architecture is the
second type of network architecture system and it is made up of computational nodes with equal
capabilities, for example a system of agents that collaborate to collect, correlate and filter
information.
Nonetheless there are various types of networks and most are classified according to their areas
covered, these areas include PAN (Personal Area Network), LAN (Local Area Network) MAN
(Metropolitan Area Network) and WAN (Wide Area Network). A personal area network (PAN)
is a computer network that can be used for data transmission and communication among personal
devices (intrapersonal) such as personal digital assistants, computers and telephones or for the
connection to the internet (uplink) and a higher level network. A wireless personal area
network (WPAN) is a version of a PAN but it’s carried over a wireless network technologies
such as Wireless UBS, INSTEON, IrDA, Bluetooth, ZigBee, Z-Wave and Body Area Network.
In a local area network (LAN) the computer network interconnects computers using network
media within a limited area such as a school, home, computer laboratory or office building. The
two most commonly used technologies used to build LAN networks are Ethernet and Wi-Fi. As
the name implies a wide area network (WAN) is a network that covers a broad area (national,
international, metropolitan and regional) using leased telecommunication lines. The Internet can
be considered a WAN as many business use WANs to relay data among employees, buyers
clients and suppliers from various geographical locations. The characteristic that defines a LANs,
in contrast to wide area networks (WANs), include a smaller geographic area that has a distinct
non-inclusion of leased telecommunication lines.

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In addition, there are many communication protocols used in networking technology as well as
the layout of a network (network topology) plays a critical role in a networks architecture.
Table1. Below provides a list of the four basic network topologies used in network architecture.

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Table1.Illustratingfourbasictypesof NetworkTopology.
Type of
Network
Topolog
y
Description Image
Point-to-
point
 Simplest topology with a permanent link between
two endpoints and it’s govern by Metcalfe's Law.
 Switched point-to-point topologies are the basic
model of conventional telephony.
 Permanent (dedicated) is a point-to-point
communications channel that appears,to the user,
to be permanently associated with the two
endpoints.
 Eg. Children’s tin can telephone is one example of
a physical dedicated channel.
Bus  Each computer or server is connected to the single
bus cable and a signal from the source travels in
both directions to all machines connected on the
bus cable until it finds the intended recipient.
 Linear bus occurs when all of the nodes of the
network are connected to a common transmission
medium which has exactly two endpoints
 Distributed bus occurs when all of the nodes of
the network are connected to a common
transmission medium which has more than two
endpoints that are created by adding branches to
the main section of the transmission

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Star  Each network host is connected to a centralhub/
switch with a point-to-point connection. The
switch is the server and the peripherals are the
clients.
 Extended star occurs when a network that is
based upon the physical star topology has one or
more repeaters between the centralnode (the 'hub'
of the star) and the peripheral or 'spoke' nodes.
 Distributed Star occurs when individual
networks that are based upon the physical star
topology connected in a linear fashion with no
central or top level connection point
 Advantage- the simplicity of adding additional
nodes. Disadvantage- is that the hub represents a
single point of failure.
Ring  A topology that is set up in a circular fashion in
which data travels around the ring in one direction
and each device on the ring acts as a repeater to
keep the signal strong as it travels.
 When a device sends data, it must travel through
each device on the ring until it reaches its
destination. Every node is a critical link.
 There is no server computer present; all nodes
work as a server and repeat the signal. The
disadvantage of this topology is that if one node
stops working, the entire network is affected or
stops working.
Picture Source: http://homepages.uel.ac.uk/u0330814/ring.html

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In the same way humans need communication to survive and thrive, technology requires the
same communication abilities to function and operate. At the core of any network is a highly
specialized highway of interconnected components and protocols that allow it to operate as it
should. In 1977 the International Organization for Standardization (ISO) made an effort to
regulate a computer networking by creating the Open Systems Interconnection’s Model. (OSI
Model.) This OSI model attempts to standardize the internal functions of a communication
system with distinct abstraction layers. The OSI model isn't a protocol but rather it is a model
created to ensure the architecture of a network is flexible, robust and interoperable with the
primary responsibility to facilitate communication between the different systems without
requiring any changes to the logic of the underlying software and hardware. In the model
communication functions are divided into seven logical layers each layer serves the layer above
it and the layer below it. The seven OSI layers begin with the 7th layer the Application layer that
provides different services to the application, the 6th layer the Presentation layer converts the
information while the 5th Session layer handles problems which are not communication issues.
Transport is the 4th layer and it provides end to end communication control while the 3rd layer,
the Network layer routes the information in the network. Data Link, (2nd layer) provides error
control and lastly the 1st layer, the physical layer connects the entity to the transmission media.
For example, a layer that provides error-free communications across a network provides the path
needed by applications above it, while it calls the next lower layer to send and receive packets
that make up the contents of that path. Two instances at one layer are connected by a horizontal
connection on that layer.
As mentioned previously, the underlying protocols that allow a network to be successful are just
as important the network topology. Thus the first computer networking model with standardized
protocols was created and funded by DARPA, initially known as the DoD model but later
became known as the Transmission Control Protocol (TCP) and the Internet Protocol (IP)
(TCP/IP) for its most important protocols. TCP/IP protocols are maintained by the Internet
Engineering Task Force (IETF) which provides an end-to-end connectivity specifying how data
should be transmitted, addressed, routed and received at the destination. The TCP/IP is organized
into four abstraction layers from lowest to highest; the link layer, the internet layer, transport
layer and the application layer. All layers are used to sort related protocols according to the
scope of networking involved. The first layer, the link layer contains communication
technologies for a single network segment (link), the second layer (internet layer) connects hosts
across independent networks and thus establishes internetworking. The transport layer is the
third layer and it handles host-to-host communication while the last layer, the application layer
provides process-to-process application data exchange. Table2. Below gives a detailed
comparison between the OSI and the TCP/IP models.

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Table2. The Comparison between the OSI Model and TCP/IP model.
OSI (Open System Interconnection) TCP/IP(Transmission Control Protocol
/ Internet Protocol)
OSI provides layer functioning and also defines
functions of all the layers.
In OSI model the transport layer guarantees the
delivery of packets.
Follows horizontal approach
OSI model has a separate presentation layer.
OSI is a general model.
TCP/IP model is more based on protocols
and protocols are not flexible with other
layers.
In TCP/IP model the transport layer does
not guarantees delivery of packets.
Follows vertical approach.
TCP/IP does not have a separate
presentation layer.
TCP/IP model cannot be used in any other
application
Network layer of OSI model provide both
connection oriented and connectionless service.
OSI model has a problem of fitting the protocols
in the model
Protocols are hidden in OSI model and are easily
replaced as the technology changes.
OSI model defines services, interfaces and
protocols very clearly and makes clear distinction
between them.
It has 7 layers
The Network layer in TCP/IP model
provides connectionless service.
TCP/IP model does not fit any protocol
In TCP/IP replacing protocol is not easy.
In TCP/IP it is not clearly separated its
services, interfaces and protocols.
It has 4 layers
Source: Fictitious data, for illustration purposes only

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The Seven Layer OSI Model
Physical Layer
It can be said that all the network hardware belongs to the physical layer however this statement
isn’t necessary accurate because the physical layer defines a number of network functions, not
just hardware cables and cards. Hardware devices generally implement multiple layers because
all hardware must have some relation to the physical layer in order to send data over the network.
eg. The physical layer and the data link layer are both utilized by an Ethernet network interface
card. The physical layer is perhaps the most complex layer in the OSI architecture due to the
plethora of available hardware technologies because it is the fundamental layer underlying the
logical data structures of the higher level functions. Within the semantics of the OSI network
architecture, the physical layer provides a mechanical, electrical and procedural interface to the
transmission medium as properties of the frequencies to broadcast on, electrical connectors,
modulation scheme to use and similar low-level parameters are specified here. The three major
responsibilities of the physical layer include;
 Definition of Hardware Specifications: wireless radio, operation of cables, connectors,
transceivers, network interface cards and other hardware devices.
 Encoding and Signaling: transform data from bits that reside within a device into signals
that can be sent over the network.
 Data Transmission and Reception: correct encoding, transmission of data and receiving
of data.
 Topology and Physical Network Design: hardware-related design issues, eg, WAN and
LAN.
All in all, the physical layer technologies generally deal with the actual ones and zeroes that are
sent over the network.
For example, when considering network interconnection devices, the simplest ones operate at the
physical layer: repeaters, conventional hubs and transceivers. These devices have absolutely no
knowledge of the contents of a message. They just take input bits and send them as output.
Devices like switches and routers operate at higher layers and look at the data they receive as
being more than voltage or light pulses that represent one or zero.

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Data Link Layer
The data link network consist of the wired and wireless local area networking technologies
within a network and it is divided into the logical link control
(LLC) and media access control (MAC). Data itself can be classified into “Data types” based on
their values which can fall in one of four classifications; Real, Integer and Boolean. The LLC
sublayer acts as an interface between the media access control (MAC) sublayer and the network
layer but functionally the logical link control itself provides multiplexing mechanisms that make
it possible for several network protocols (Decnet, Appletalk, IP and IPX) to coexist within a
multipoint network and transported over the same network medium. The LLC is usually
considered a DLL sublayer as it provides services to the layer above it (network layer) and hides
the rest of the details within the data link layer which allows different technologies to work
seamlessly with the higher layers. The IEEE 802.2 LLC protocol is used by local area
networking technologies while on the other hand, the Media Access Control (MAC) is as
important as it avoids conflicts within the network medium because it regulates the access to the
network. Eg. Ethernet uses the CSMA/CD method of media access control, while Token Ring
uses token passing. In addition to the LLC and MAC, data framing, addressing, error detection
and handling are additional activities performed by the data linking layer. Data framing can be
defined as the final encapsulation of higher-level messages into frames that are sent over the
network at the physical layer. When it comes to addressing the names implies its function,
labeling information with a particular destination location. A hardware address or Mac address is
the unique number on each device that is used by the data link layer protocol to ensure that data
intended for a specific machine gets to it properly. Lastly at the lower levels of the network
stack, the data link layer handles any errors that occur here, this is known as error detection and
handling.

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Network Layer
In contrast to the previous layer which only deals with devices that are local to each other, the
network layer is concerned with getting data, packaging output with the correct network address,
selecting routes and maintaining the quality of service when a data sequences is transmitted from
one computer to another destination host on a different network. Transition really begins at this
layer from the more abstract functions (higher layers) into the tasks required to get data to its
destination. This layer also recognizes and forwards to the transport layer incoming messages for
local host domains. The internet is possible at the network layer because routers are able to
perform at this level by sending data throughout the extended network. The network layer might
also perform fragmentation and reassembly and report delivery errors. The transport layer, in
relation to the transport layer continues this abstraction transition as you go up the OSI protocol
stack.
Transport Layer
As with any act of communication there must be a channel through which the communication
must move through. When humans verbally communicate, they do so by speaking, this is a
channel. Writing a letter is another channel. The Transport layer acts as the channel by which
networking can travel to from one device to another. Thus the overall job of the transport layer is
to provide the necessary functions to enable communication between software application
processes on different computers as it acts as a “liaison” between the abstract world of
applications at the higher layers and the concrete functions of layers one to three. Not only can
the line of communication at the transport layer be horizontal but also data can be moved to
upper layers as well. Thus, despite being associated the lower layers the transport of data results
in the layer having a fair bit in common with the layers 5 through 7. Flow control, segmentation/
de-segmentation and error control are the means by which this layer controls the reliability of
any given link and it can retransmit any link that fails.

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Session layer
Likewise to the previous layers, the name of the session layer implies its function. A session is a
link between two software applications which allows them to exchange data over a prolonged
period of time. The session layer is also the 5th layer and it is the first layer of the higher levels
for the OSI model to be focused mainly on software application issues rather than all practical
matters related to addressing, packing and delivery of data. The primary job of session layer is to
provide the means necessary to manage, set up and end sessions although the session layer
software products are more a set of tools than specific protocols. These session-layer tools are
normally provided to higher layer protocols through command sets often called application
program interfaces or APIs. Common APIs include NetBIOS, TCP/IP Sockets and Remote
Procedure Calls (RPCs). Most programmers are interested in the tools (API’s) of the session
layer as they are used to develop application software that is able to communicate using TCP/IP
without having to know the implementation details of how TCP/IP works.

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Presentation Layer
The personation layer is it in self a very unique layer. It is second from the top and has a limited
responsibility in the reference model protocol stack. The name of this layer suggests its main
function as well: it deals with the presentation of data, more specifically it is in charge of taking
care of any issues that might arise when data sent from a system needs to be viewed in a different
way by the other system. Additionally this layer takes care of any special processing that must be
done to data from the time an application tries to send it until the time it is sent over the network.
Syntax and Semantics are used mainly by the higher level entities (application layer) to transfer
data. The presentation layer then provides the mapping for the transfer to occur and it does so by
the encapsulation of the presentation service data units into session protocol data units which are
then passed down the stack. Thus providing independence from data representation by
translating between application and network formats. One of the three (Translation, Compression
and encryption) most important tasks undertaken by the presentation layer is translation.
Different types of computer systems (PC’s, Macintoshes and UNIX systems) have distinct
characteristics and represent data in different ways, it is the job of the presentation layer to hide
these differences between machines. The compression function improves the throughput of data
while the encryption function ensures the security of the data as it travels down the protocol
stack.

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Application Layer
This is the final layer of the protocol stack and it is the only layer that is closest to program user.
The main responsibilities at this layer are simply to implement the functions that are needed by
users of the network and to issue the appropriate commands to make use of the services provided
by the lower layers. However, the application layer is not limited to this, in the OSI model the
application layer provides services for user applications to employ thus they implement the
functions performed by uses to accomplish various task of the network. For example, when using
a web browser, the browser is actually a software application on your PC. It doesn’t reside at the
application layer rather, it makes use of the services offered by a protocol that operates at the
application layer, which is called the Hypertext Transfer Protocol (HTTP). The distinction
between the browser and HTTP is subtle, but important. Nonetheless there exist many
application layer protocols that enable various functions at this layer some more include; FTP,
SMTP, DHCP, NFS, Telnet, SNMP, POP3, NNTP. Nonetheless not all application users use the
application layer of a network in the same way, in addition to, not all uses of the application
layer are by applications the operating system itself can and does use services directly at the
application layer.

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Summary of theOISprotocollayer
• encoding
• Signaling
• Physical Data Trasmissions
• Hardware Specifications
Phyiscal
• LLC
• Meida Access Conrol
• Dat framing
• Adressing
• Error Detection & Handling
Data Link
• Local Addressing
• Datagram Encapsulation
• Fragmentation and Reassembly
• Error Handling & Diagnostics
Network
• Process-Level Addresing
• Multiplexing/ Demultiplexing
Connections
• Sedmentations & Reassembly
• Acknowledgments &
Retransmissions
Trasport
• Session Establishment
• Management & TerminationSession
• Data Translation Compression
& EncryptionPresentation
• User Application ServiceApplication

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References
1. The TCP/IP Guide,Version3.0 - VersionDate: September20, 2005.
http://www.tcpipguide.com/free/t_PhysicalLayerLayer1.htm
2. Introduction: ClassificationofNetworkArchitecture
http://www.cs.toronto.edu/~marbach/COURSES/CSC358_S14/classification.pdf
3. StephenMcQuerry, CiscoSystems, CCNA Self-Study:InterconnectingCiscoNetworkDevices,
2nd Edition.PublishedNov19, 2003.
4. Matthew Gast. O'ReillyMedia,Inc.2005, 802.11 WirelessNetworks
5. StephenMcQuerry, CiscoSystems,CCNA Self-Study:Introductionto CiscoNetworking
Technologies(INTRO).PublishedMar9, 2004

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