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CREATED BY SORCIA KRISTI-ROSE D’ARCEUIL

NETWORK ACHITECTURE

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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. Illustrating four basic types of Network Topology.

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 central hub/

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 central node (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 the OIS protocol layer

• 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 & Termination Session

• Data Translation Compression & EncryptionPresentation

• User Application Service Application

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References

1. The TCP/IP Guide, Version 3.0 - Version Date: September 20, 2005.

http://www.tcpipguide.com/free/t_PhysicalLayerLayer1.htm

2. Introduction: Classification of Network Architecture

http://www.cs.toronto.edu/~marbach/COURSES/CSC358_S14/classification.pdf

3. Stephen McQuerry, Cisco Systems, CCNA Self-Study: Interconnecting Cisco Network Devices,

2nd Edition. Published Nov 19, 2003.

4. Matthew Gast. O'Reilly Media, Inc. 2005, 802.11 Wireless Networks

5. Stephen McQuerry, Cisco Systems, CCNA Self-Study: Introduction to Cisco Networking Technologies (INTRO). Published Mar 9, 2004