Computer networks short note (version 8)

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Computer Networks IN 2510

Goal of this course

1. To learn how the Internet works

2. To learn the fundamentals of computer networks

Internet

Social impact:

Easy access to knowledge

o Ex: Wikipedia

Electronic commerce

o Ex: PayPal

Personal relationships

o match.com

Discussion without censorship

o The Onion Router

Economic impact:

Advertising-sponsored search

“Long tail” online stores

Online marketplace

Crowdsourcing

Computer Networks

Key problems in computer networking

Reliability despite failures

Network growth and evolution

Allocation of resources like bandwidth

Security against various threats

Upheavals in the past 1-2 decades

Growth/ Tech Driver Upheaval

Emergence of the web Content Distribution Networks

Digital songs/ videos Peer-to-peer file sharing

Falling cost per bit Voice-over-IP calling

Many Internet hosts IPv6

Wireless advances Mobile devices

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Lesson 01 Networking Fundamentals

Outline:

Introduction

Benefits/ Risks of Networking

Types of Networks

Networking Devices

Categorizing Networks

Computer Network:

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 as a

computer network.

Node:

Any active electronic device that connected to a computer network

Can be either a connection point, redistribution point, or a communication endpoint

Capable of creating, receiving, or transmitting information over a communications channel

*note: A passive distribution point such as a distribution frame or a patch panel is consequently not a

node.

Devices (nodes) of a network can be classified as:

1. End user devices

Also called hosts

Provide services to the user directly

Ex: Computers(client/ server), printers, scanners, file server, IBM main frame etc

2. Network devices (see page 17: Network Hardware Components)

Connect end user devices together to allow them to communicate

Ex: repeater, bridge, hub, workgroup switch, router, network cloud

Components of a network (includes nodes plus network hardware plus link component plus apps):

Component Function Example

Application, or app, user Uses the network Skype, iTunes, Amazon

Host, or end-system, edge device, node, source, sink

Supports apps Laptop, mobile, desktop

Router, or switch, node, hub, intermediate system

Relays messages between links Access point, cable/ DSL modem

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Link, or channel Connect nodes Wires, wireless

Big picture of nodes

Figure 1: The demonstration of the word ISP and Network

Figure 2: The generic word cloud can be referred to ISP part of a network

Types of links

1. Full duplex

Bidirectional

Both directions at once

Ex: hand phone

2. Half duplex

Bidirectional

Only for one direction at a time

Ex: walky talky

3. Simplex

Unidirectional

Ex: mass media

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Data Transfer Rate (DTR) A key issue in computer networks, and Measured in bps, Bps, Mbps etc

Data Transfer Rate and bandwidth:

Bandwidth is a measurement of the potential amount of data that can be transferred in a given

time frame, while data transfer rate is the actual amount of data being transferred.

Factors that can impact DTR

Congested routers

o Packet loss is a result jitter in videos, gaps in audio

Improperly configured PCs with inadequate memory and processors

Problems caused by poor DTR

Long wait time for connections and downloads

Inability to complete the download due to endless requests for retransmission of dropped

packets

Poor DTR for live video streams in video conferencing (ex: Skype call get stucked)

Improve DTR on computer Networks

Caching of frequently accessed web pages

Established mirror sites requiring fewer hops

Reduce bandwidth intensive elements such as video

Using compression techniques that minimize traffic

Computer Networks

Advantages of Computer Networks

Accessing databases, transferring, processing and retrieval of data can be done online

Online credit card checking, e-commerce and Electronic Fund Transfer are possible

Easily administered

Provides an efficient means of communication such as e-mail, voice mail, and video

conferencing

Users can be easily added or removed

Tasks of distributed nature can be processed by distributed computer systems by exchanging

data

Provides a way to share data, programs, peripherals, computing power and information

Provides data security (comparing to other communication devices)

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Benefits of a network:

Information sharing

Hardware sharing

Software sharing

Collaborative environment

Challenges

Computer hackers

Malicious software

o Ex: viruses, Worms, Trojan horses

Cost

o Network setup

o Maintenance

Equipment malfunctioning

System failures

Network Types

Scale Type Example

Vicinity PAN (Personal Area Network)

Bluetooth (ex: headset)

Building (limited geographic area)

LAN (Local Area Network)

Wifi, Ethernet

City (medium geographical area)

MAN (Metropolitan Area Network)

Cable, DSL

Country (large geographical area)

WAN (Wide Area Network)

Large ISP

Planet The Internet (network of all networks)

The Internet

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Network Topologies Arrangement of various elements (links, nodes etc) of a computer network. i.e. the topological

structure. It may be depicted physically or logically.

Different network topologies:

Bus topology

Ring topology

Star topology

Mesh topology

Tree topology

a combination of bus topology and star topology

Hybrid topology

Hybrid networks use a combination of any two or more topologies, in such a way that the resulting network does not exhibit one of the standard topologies (e.g., bus, star, ring, etc.). A hybrid topology is always produced when two different basic network topologies are connected.

Local Area Networks (LAN) Spans a relatively small area

Properties of LAN:

LAN are usually confined to one building or a group of buildings

Usually privately owned

Provides higher DTR

Provide full time connectivity to local services

The most common type of LAN is Ethernet

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Components of LAN:

Router

Bridge

Hub

Ethernet switch

Repeater

Wide Area Networks (WAN) Covers a very large geographical area such as a country, continent or even the whole world

Properties of WAN:

Provide long distance communication of data or information

Operating at low DTRs

Provide full time/ part time connectivity

Connect devices separated over wide, even global areas

Components of WAN:

Router

Communication Server

Modem

Types of WANs:

MAN (Metropolitan Area Network)

PAN (Public Access Network)

VAN (Value Added Network)

VPN (Virtual Private Network)

Metropolitan Area Network (MAN):

A network that interconnects users with computer resources in a geographic area or region

larger than that covered by even a large local area network (LAN)

Interconnection of networks in a city into a single larger network

Interconnection of several LANs by bridging them with backbone lines

Example: subscriber networks, TV service

Public Access Network (PAN):

Could be accessed by public

Examples: image services, web services

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Value Added Network (VAN):

A value-added network (VAN) is a private network provider (sometimes called a turnkey

communications line) that is hired by a company to facilitate electronic data interchanges (EDI)

or provides other network services.

Virtual Private Network (VPN):

A network that uses a public telecommunication infrastructure, such as the Internet, to provide

remote offices or individual users with secure access to their organization’s network.

Example: Research and development work

Became popular as more employees worked in remote locations

Employees can access the network(intranet) from remote locations

The Internet is used as the backbone for VPNs (we are creating this network on top of internet)

Secured networks

o Level of security should be high becoz we have to prevent data from falling into wrong

hands, so we encrypt data before we send

o These systems use encryption and other security mechanisms to ensure that only

authorized users can access the network and that the data cannot be intercepted

o Encryption definition: scrambling the plain text, so that others cannot understand

This kind of networks are also called tunnel networks

o Though these are on internet, these are virtually private.

Figure 3: How a VPN client connect with the VPN sever through tunnel

Figure 4: Big picture of VPN

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Benefits of VPN:

Reduce cost tremendously from reduction of equipment and maintenance costs

o No cables becoz Internet is used as a back bone

Scalability

Secured

o Only authorized people can access

Internetwork:

An internetwork is a collection of individual networks, connected by intermediate networking

devices, that functions as a single large network.

Network bridging technologies are used here

o Network Bridging Definition: Network bridging is the action taken by network

equipment to create an aggregate network from either two or more communication

networks, or two or more network segments.

o 4 types of Network bridging technologies

1. Simple bridging

2. Multiport bridging

3. Learning or transparent bridging

4. Source route bridging

Ex: internet

Figure 5: Internetworking by simple bridging

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Storage Area Network (SAN) High speed special type of network that connects storage devices

Figure 6: SAN is a dedicated Network for attaching servers to storage devices

Characteristics of SAN:

Used to enhance storage with devices, such as disk arrays, tape libraries, and optical jukeboxes

Accessible to servers so that the devices appear to the operating system as locally attached

devices

Has its own network of storage devices that are generally not accessible through the local area

network (LAN) by other devices.

The cost and complexity of SANs dropped in the early 2000s to levels allowing wider adoption

across both enterprise and small to medium-sized business environments.

A SAN does not provide file abstraction, only block-level operations. However, file systems built

on top of SANs do provide file-level access, and are known as shared-disk file systems.

Benefits of SAN:

Simplifies storage administration

Increase performance of the network due to high access speed and efficiency

Adds flexibility and scalability since cables and storage devices do not have to be physically

moved to shift storage from one server to another

Ability to allow servers to boot from the SAN itself

o This allows for a quick and easy replacement of faulty servers since the SAN can be

reconfigured so that a replacement server can use the LUN of the faulty server.

Enables storage replication either implemented by disk array controllers, by server software, or

by specialized SAN devices.

o Replication in computing involves sharing information so as to ensure consistency

between redundant resources, such as software or hardware components, to improve

reliability, fault-tolerance, or accessibility.

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Control Area Network (CAN bus) A serial network of micro controllers, sensors, devices and actuators in a system or subsystem for real

time control applications.

Ex: automatic controlling system of a Toyota car

CAN bus:

A vehicle bus standard designed to allow microcontrollers and devices to communicate with

each other in applications without a host computer. It is a message-based protocol, designed

originally for multiplex electrical wiring within automobiles, but is also used in many other

contexts.

Figure 7: CAN bus example

Communication Media (Transmission Media) Media which network and nodes are connected

Two types of transmission media:

1. Guided (wired) media

Waves are guided along a solid medium

2. Unguided (wireless) media

Provide means for transmitting electromagnetic signals (waves) through air, but without

any guidance to the wave

Figure 8: Big picture of Communication media

Communication media

Wired/ Guided

Twisted pair Coaxial cable Fiber optics

Wireless/ Unguided

Radio waves Microwaves Infrared (IR)

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Twisted Pair cable

Figure 9: Twisted pair cables

Twisted pair:

A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern.

Typically, a number of pairs are bundled together into a cable by wrapping them in a tough

protective sheath as shown in the Figure 9.

Why twisting?

Twisting decreases the crosstalk interference between adjacent pairs in a cable.

Tighter twisting provides much better performance, but also increases the cost.

Usage:

LANs

Two common types of twisted pair cables:

1. UTP – Unshielded Twisted Pair

Subject to external electromagnetic interferences

Ex: Ordinary telephone wire, LANs (Ethernet)

2. STP – Shielded Twisted Pair

Expensive than UTP (therefore not much popular)

Ex: industrial setting where high amounts of electromagnetic interference

Attenuation Characteristic of UTP:

Attenuation increases when diameter (measured in gauge in practice) increases is high.

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Common Applications:

As local loop in telephone lines

Digital subscriber lines (DSL)

LANs (10BaseT, 100BaseT)

o Connector is RJ45 Ethernet

Coaxial cable

Figure 10: Cross section of a coaxial cable

Coaxial cable:

This type of cables consists of a hollow outer cylindrical conductor (also called metallic shield)

that surrounds a single inner wire conductor (also called centre core).

Between the above two, there is a dielectric insulator ring.

Outer conductor (metallic shield) is covered with a plastic jacket (also called outer protective

shield).

Four components of a coaxial cable:

1. Plastic jacket

2. Metallic shield

3. Dielectric insulator

4. Centre core

Benefit due to shielding:

Coaxial cables are much less susceptible to interference or crosstalk than twisted pair.

o Outer conductor can be grounded. Therefore inner conductor is shielded from

interferences and disturbance. i.e. reduced crosstalk.

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Common applications of coaxial cables:

Television distribution (cable TV)

Long distance telephone transmission

LANs

Coaxial cables and twisted pair:

Twisted pair cabling is better suited when cost and installation are an issue and if EMI and

crosstalk are not too much of a problem. However, they do not provide electrostatic shielding

and do not work as well as coaxial cables at higher frequencies.

Fiber Optic cable (FO cable)

Figure 11: Fiber optic cables

Fiber optic cables:

A fiber optic cable consists of a bundle of glass/plastic threads, each of which is capable of

transmitting messages modulated onto pulses of light waves.

Characteristic of FO:

Very high speed

Lack of attenuation (less noise, high purity of signal)

High capacity data transmission (higher bandwidth)

Expensive

Relative safe way to transmit (not easy to trap what is being transmitting

Figure 12: Single mode and Multimode Fiber

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Advantages and disadvantages of twisted pair, coaxial cables, and FO cables

Twisted Pair Coaxial Cables Fiber Optic Cables

Advantages * Cheaper * Less susceptible to electrical interference and crosstalk * Because it is electrically "cleaner", STP wire can carry data at a faster speed

* Support greater cable lengths between network devices than twisted pair. * Extra protective plastic cover that help keep moisture away. * Less susceptible to electrical interference and crosstalk than twisted pair

* One single mode fiber can replace a metal of time larger and heavier. * Multi-mode optical cable has a larger diameter and can be used to carry signal over short distance.

Disadvantages * STP wire is that it is physically larger and more expensive than twisted pair wire. * STP is more difficult to connect to a terminating block.

* Thick coaxial is that it does not bend easily and is difficult to install. * Expensive than twisted pair

* Difficult to make connections to fiber optic cable. * Highly expensive * The optical fiber must be highly polished to allow light to pass with little loss.

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Main Network Hardware Components All networks are made up of basic hardware building blocks to interconnect nodes of a network. i.e end

user nodes devices and network devices (see page 02: Nodes)

Network hardware components include:

Hub

o Electronic device (with a number of ports) used in a LAN to link groups of computers,

hub is a multi-port repeater

Repeaters/amplifiers

o Electronic devices that receive signals and amplify and send them along the network.

There are high end repeaters and low end repeaters

Routers

o Electronic devices used to ensure messages are sent to their intended destinations

Switches

o Hub can be replaced by a switch; bridge is a primitive version of a switch; therefore

switch can be called a multi-port bridge

Gateway

o Consists of hardware and/ or software that allows communications between dissimilar

networks

Bridges

o Consists of hardware and/ or software that allows communication between two similar

networks

*note

If there are very few nodes, a hub is enough.

If you want segments of end user nodes, use a hub to connect end user nodes to form a segment, and

then use a switch to connect hubs. If you no need segments, you can use a switch to connect all the end

user nodes.

Within LAN- a hub is used, LAN – LAN- a bridge is used, segments of a LAN- a switch is used, LAN – MAN-

a router is used.

Bridges, switches, and hubs are very much alike.

A hub can be replaced by a switch.

Switch can be called a multi-port bridge.

Bridge is a primitive version of a switch.

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Why interconnect?

To separate/ connect one corporate division with another

To connect two LANs with different protocols

To connect a LAN to the internet

To break a LAN into segments to relieve traffic congestion

To provide a security wall between two different types of users

Ti connect WLAN to LAN

Remote access

Refer:

http://www.scorelift.com/resources/computer-networks-basic-hardware-requirements.html

Network interface cards (NIC): A network card, network adapter, or NIC is a piece of computer hardware designed to allow computers

to communicate over a computer network. It provides physical access to a networking medium and

often provides a low-level addressing system through the use of MAC addresses.

Each network interface card has its unique id. This is written on a chip which is mounted on the card.

Figure 13: NIC

Repeaters Repeater is an electronic device that receives a signal, clean it of unnecessary noise, regenerates it and

retransmits it at a higher power level, or to the other side of the obstruction, so that the signal can cover

longer distances without degradation.

Receive signal -> clean signal -> regenerate signal -> retransmit signal

In most twisted pair Ethernet configurations, repeaters are required for cables that runs longer than

100m.

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A repeater with multiple ports is known as a hub.

Repeaters works on the physical layer of the OSI model.

Repeaters require a small amount of time to regenerate the signal. This can cause a propagation delay

which can affect network communication when there are several repeaters in a row. Many network

architectures limit the number of repeaters that can be used in a row.

Figure 14: Repeater and Hub(multi-port repeater)

Hub (Multi-port repeater) A hub interconnects two or more work stations into a LAN. When a workstation transmits to a hub, the

hub immediately resends the data frames to all the connecting links. i.e. if a signal is coming from one

port, it amplifies the signal and repeat (copy) it to other ports.

Hub is used in star topology. A hub works at the physical layer of the OSI model.

Figure 15: 1-Port BNC to 8-Port RJ45 10M Network Hub

10M in the above hub means that 10Mbps data transmission rate. Ports numbered from 1 to 8 are to

connect computers. The uplink is to connect the hub to a switch, router or to another hub.

Uplink also called “Link”, “Out” etc.

Bridge A network bridge connects multiple network segments at the data link layer of the OSI model. Therefore

bridges connect networks and forward frames from one network to another.

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Bridges broadcast to all the ports except the port on which the broadcast was received. However,

bridges do not promiscuously (jumbles) copy traffic to all the points, as hubs do, but learn which MAC

addresses are reachable through specific ports. Once the bridge associates a port and an address, it will

send traffic for that port only.

Bridges learn the association of ports and addresses by examining the source address of frames that it

sees on various ports. Once a frame arrives through a port, its source address is stored and the bridge

assumes that MAC address is associated with that port. The first time a previously unknown destination

address is seen, the bridge will forward the frame to all ports other than the one on which the frame

arrived.

Bridges come in three basic types:

1. Local bridges

2. Remote bridges

3. Wireless bridges

Figure 16: A Bridge

Information only crosses the bridge if they are addressed for a host on the other side (selective

forwarding).

Figure 17: How bridges help to connect two networks.

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Switches (Multi-port Bridges) A network switch is a device that forwards and filters OSI layer 2 (Data link layer) datagrams (chunks of

data communication) between ports (connected cables) based on the MAC address in the packets.

Unlike hubs, switches make decisions of frames on the basis of MAC addresses. A switch has numerous

ports, facilitating a star topology for devices, and cascading additional switches.

Some switches are capable of routing based on Network layer of OSI model addressing additional logical

levels; these are called multi-layer switches.

We connect twisted pair cable with RJ45 connectors to ports on a switch. A switch can have many as 96

ports.

Switches learn which MAC addresses are associated with particular ports by noting source addresses as

frame enter the switch. Broadcast frames are forwarded to all ports of a switch.

Collisions occur only when two separate hosts attempts to communicate with the same third host.

Switches can send and receive information at the same time, so they can send information faster than

hubs.

Figure 18: A switch

Differences between a bridge and a switch:

Switches have many ports; bridges only have two (or some less number of) ports.

Switches perform forwarding in hardware, while bridges perform it in software.

o Bridge may introduce overhead.

o A switch must be able to maintain the full speed of the medium between any two ports.

*note there are confusions

The IEEE standards clearly don’t state that bridges will have only two ports.

There’s nothing in the standard that says bridging must or should be done in software.

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Routers A router is an internetworking device that forwards packets between networks by processing

information found in the datagram or packet. This works at network layer of OSI model.

What is a router made of?

A router has many of the same components as your computer,

CPU

Memory

I/O interfaces (mostly network interfaces)

Operating System

A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP’s

network.

Routers ate located at gateways, the places where two or more networks connect.

Routers are critical devices that keep data flowing between networks and keep the networks connected

to the internet.

A router is an internetworking device that forwards packets between networks by processing

information found in the datagram or packet. In many situations, this information is processed in

conjunction with the routing table (forwarding table). Routers use routing tables to determine what

interface to forward packets. i.e. when data arrives from one of the segments, the router decides, which

segment to forward that data.

Routers often incorporate firewall functions.

Differences between routers and bridges:

Routers Bridges

Each host’s IP address must be configured MAC addresses of hosts are hard wired

If network is reconfigured, IP addresses may need to be reassigned

No network configuration needed

Routing done via RIP or OSPF Routing done by

learning bridge algorithm

spanning tree algorithm

Each router manipulates packet (ex: reduces TTL field)

Bridges do not manipulate frames

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Gateways A network node equipped for interfacing with another network that uses different protocols

Gateways also called protocol converters can operate at any network layer. The activities of a gate are

more complex than that of the router or switch as it communicates using more than one protocol.

Gateway is a term that was once used to refer to a routing device. Today in the TCP/IP world, the term

router is used to describe such a device. The term gateway is now refers to special-purpose devices, that

perform protocol conversions. Gateway implement application layer conversions of information

received from various protocols.

Examples of gateways

VocalTec Gateway

RadVision Gateway

Firewalls Firewalls are the most important aspect of a network with respect to security. A firewall system does

not need every interaction or data transfer monitored by a human, as automated processes can be set

up to assist in rejecting access requests from unsafe sources, and allowing actions from recognized ones.

The vital role firewall play in network security grows in parallel with the constant increase in cyber

attacks for the purpose of stealing/ corrupted data, planting viruses, etc.

Modems Modem is a device or program that enables a computer to transmit data over, for example, telephone

or cable lines. Computer information is stored digitally, whereas information transmitted over telephone

lines is transmitted in the form of analog waves. A modem converts between these two forms.

How networks are categorized

Networks are usually classified using three properties

1. Topology

Bus

Ring

Star

2. Protocol

3. Architecture

P2P

Client/server

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Network Architecture The way a computer network is designed and built.

The two types of network architecture:

1. Peer to peer (P2P/Workgroup) architecture

2. Client/Server architecture

Refer:

http://www.techrepublic.com/article/understanding-the-differences-between-client-server-and-peer-

to-peer-networks/

https://www.youtube.com/watch?v=uliny6fWbvs

Client/Server Architecture In a Client/ Server (Server-based) network, the server is the central location where users share and

access network resources. This dedicated computer controls the level of access that users have for

shared resources. Each computer that connects to the network is called a client computer. In a server

based network, users have one user account and password to log on to the server and to access shared

resources ex: Web authentication at UOM wireless

What can a server do?

Servers can perform multiple roles – some are;

They can run email servers

They van share files (file server)

Control Internet Access (proxy server)

Host Web Sites (web server)

Control printing for multiple computers (print server)

Hardware and software required by to implement Client/Server Architecture

Hardware Software

Server

Enough Backing Storage/RAM

Powerful enough processor (multi-core/multi processor)

Server

Networking Operating System

Client

Desktops

Laptops/tablets

Client

Operating System

Suitable Application software

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Levels of Access:

Servers can also control the levels of access that client machines or users have on the network

o Administrators may have full access whereas normal users (such as pupils) will have

limited access

Rights that may be assigned are:

o Read/ write/ delete…

Figure 19: Client server network

Advantages and disadvantages of Client server network:

Advantages Disadvantages

facilitates resources sharing (centrally administered and control)

facilitate system backup and recovery

enhance security (only administrator can have access to server)

support more users (difficult to achieve with P2P networks

high cost for servers

need expert to configure the network

introduce a single point of failure to the system

congestion In network

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Peer to Peer (P2P/Workgroup) Architecture In a P2P network, a group of computers is connected together so that users can share resources and

information. There is no central location for authenticating users, storing files, or accessing resources.

This means that users must remember which computers in the workgroup have the shared resources or

information that they want to access. It also means that users must log on to each computer to access

the shared resources on that computer.

No hierarchy among computers, all are treated the same, No administrator responsible for the network.

Where P2P network is appropriate:

10 or less users

No specialized services required

Security is not an issue

Only limited growth in the foreseeable future (numbers of nodes to be added in future)

Figure 20: P2P network

Advantages and disadvantages of P2P network:

Advantages Disadvantages

easy to install and configure

all the resources and contents are shared by all the peers

more reliable as central dependency is eliminated, failure of one peer doesn’t affect the functioning of other peers

no need of a full time system administrator, user can control the shares resources

cost is comparatively less

since the system is decentralized, difficult to manage by the administrator

difficult to uphold security policy

security is less (malware such as viruses, spy ware, Trojans can easily transmitted over P2P architecture)

data recovery or backup is very difficult (each computer should have its own backup system)

lots of movies, music, and other copyright files are transferred using this type of file transfer. Ex: torrents

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OSI Model The Open Systems Interconnection Model

What is OSI?

A conceptual model that characterizes and standardizes the communication functions of a

telecommunication or computing system without regard to their underlying internal structure

and technology.

Goal of OSI Model:

Interoperability of diverse communication systems with standard protocols

OSI consists of 7 abstract layers:

Figure 21: 7 communication layers of OSI model

How to memorize the sequence?

Programmers Do Not Throw Sausage Pizza Away

*note read the following descriptions from bottom to top, then you will understand easier.

Physical layer (L1):

Provides mechanical, electrical and other functional aids available to activate or deactivate

physical connections, they maintain and transmit bits over. i.e.,s transmission of raw bits across

the transmission media.

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Data link layer (L2):

Ensures a largely error-free transmission, controls access to the transmission medium, providing

checksums and acknowledgment and repeat procedures in case of errors or losses. It breaks the

packets into frames. Error detection and correction applies here.

Network layer (L3):

Ensures the relaying into data packets including the route search (routing), network addresses.

This layer breaks messages into packets and transfers them across the network. Network layer

has the responsibility to send packets to correct destination, this feature is called routing. Nodes

here may be computers, routers, switches etc.

Transport layer (L4):

Ensures the decomposition into data packets and congestion avoidance, this layer receives data

from the session layer and make messages. These messages are passed on to the network layer.

Session layer (L5):

Ensures the process of communication between two systems, treated session terminations i.e.

allows users of different machines to create sessions and communicate, applications create

different transport streams, and session layer can bind all these streams belonging to the same

application ex: if you are doing a video chat session layer combine the audio stream and the

video stream.

Presentation layer (L6):

Converts system specific data representation in an independent form that provides data

compression and encryption, i.e. consider the format of data and appropriate conversions have

to be done

Application layer (L7):

Provide some protocols which applications can communicate with each other ex: FTP,HTTP

This is a model that allows any two different systems to communicate regardless of their underlying

architecture (hardware or software).

The OSI model is not a protocol; it is model for understanding and designing a network architecture that

is flexible, robust and interoperable.

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Benefits of the OSI model:

Reduces complexity

Standardizes interfaces

Facilitates modular engineering

Ensures interoperable technology

Accelerates evolution

Simplifies teaching and learning

Figure 22: How layers are communicating according to OSI model

Every layer communicates with the corresponding layer of the other system using protocols. (Protocols

work with corresponding layers of different machines.)

Every layer communicates with the layer above and below it. We say that it provide some service to

those layers. (Services work with layers of the same machine.) Services take place at interfaces of the

above diagram.

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Types of addresses related to computer networking

Ethernet:

Ethernet is a LAN architecture developed in 1976.

It was commercially introduced in 1980 and standardized in 1985 as IEEE 802.3

Bus and star topologies use Ethernet and Ethernet supports data transfer rates of 10Mbps.

Latest versions of Ethernet support 100Mbps, 1Gbps, etc.

Figure 23: Ethernet evolution through four generations

There are mainly two types of addresses

1. Physical addresses

MAC address

2. Logical addresses

IP address

Media Access Control (MAC) address:

Also called hardware addresses.

Contains 6 bytes separated with colons or dashes

Ex: 00:A0:CC:23:AF:4A or 00-A0-CC-23-AF-4A

They uniquely identify an adapter on a LAN.

Internet Protocol (IP) address:

IP addresses serve as the location of websites on the internet as well as the workstations that

are connected to the web.

IP addresses are made up of 4 bytes according to IPv4. Each byte is called an Octet.

Ethernet evolution

Standard Ethernet (10Mbps)

Fast Ethernet (100Mbps)

Gigabit Ethernet (1Gbps)

Ten Gigabit Ethernet (10Gbps)

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There are 2 types of IP addresses

1. Static IP addresses

Found on servers and remain the same

A domain name server assigns a human readable web address to each static IP address

to make it more user friendly.

2. Temporary IP addresses

Found only on PC’s are constantly changing each time it is logged on

Temporary IP addresses are assigned by ISP each time it is logged on to the internet

becoz IP addresses are limited. Therefore cannot assign a user a permanent IP address.

Network/ Communication Protocols An agreed se t of rules and procedures for transmitting data between two or more devices

Features determined by the protocol are;

How the sending device indicates it has finished sending the message

How the receiving device indicates it has received the message

What is the type of error checking mechanism to be used

How networks can be classified?

From topology

From protocol

From architecture

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Lesson 02 Virtual LANs (VLANs)

Outline:

Define VLANs

List the benefits of VLANs

Explain how VLANs are used to create broadcast domains

Explain how routers are used for communication between VLANs

List common VLAN types

Define ISL and 802.1Q

Explain the concept of geographic VLANs

Configure static VLANs on switches

Verify and save VLAN configurations

Delete VLANs from a switch configuration

Motivation:

Figure 24: A large flat LAN

Typical problems of large LANs include

Multicast and unknown MAC address traffic

Management issues

Security issues

i.e. everybody shares everybody’s resources

Solution:

Make all departments in its L3 network. i.e. you need so many routers to connect all department

networks. This is highly costly. There comes Virtual LAN concept with L2 switches.

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VLAN introduction VLAN:

Any broadcast domain that is partitioned and isolated in a computer network at the Data Link

Layer (L2)

Logical Network Layer(L3) separation acting in Data Link Layer (L2)

Every VLAN is essentially a Network Layer(L3) IP network

Figure 25: The concept of VLANs

Refer:

https://www.youtube.com/watch?v=Z8s_sxzw3zI

VLANs logically segment switched networks based on the functions, project teams, or applications of the

organization regardless of the physical location or connections to the network.

Figure 26: VLAN segmentation is independent of physical connection or location

Figure 27: There are Data VLANs and Voice VLANs

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*note

VLANs are created to provide segmentation services traditionally provided by physical routers in

LAN configurations.

VLANs provide scalability, security, and network management. Routers in VLAN topologies

provide broadcast filtering, security, and traffic flow management.

Broadcast domain:

A logical division of a computer network, in which all nodes can reach each other other by

broadcast at the data link layer (L2). A broadcast domain can be within the same LAN segment

or it can be bridged to other LAN segments.

*note

A VLAN is a broadcast domain created by one or more switches.

The network design in Figure 26 creates two separate broadcast switches, while the network

design in Figure 27 creates three separate broadcast switches.

Without VLANs Example 01

Figure 28: 3 LANS working separately without configuring it as a VLAN

Each group is on a different IP network and on a different switch. Three switches had used to create the L3 separation

Without VLANs Example 02

2 subnets, but same VLAN

With VLANs Example 01

Figure 29: After configuring as 3 VLANs

Switch is configured with ports on the appropriate VLAN. Still, each group on a different IP network, they are on the same switch.

Without VLANs Example 02

2 subnets, 2 VLANs

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Ports on a switch Refer:

http://www.freeccnastudyguide.com/study-guides/ccna/ch7/7-3-types-switch-ports/

A switch port can be in one of two modes:

1. Access mode

A port in access mode belongs to one specific VLAN and sends and receives regular

Ethernet frames in untagged form.

The switch interfaces connected to devices such as desktops, laptops, printers etc, are

typically configured as access ports.

Used to connect computers etc.

2. Trunk mode

The distinguished feature of trunk ports is that they carry traffic from multiple VLANs at

the same time.

Such interfaces are most commonly configured between two switches, or between a

switch and a router, or even between a server and a switch.

Trunking is a great feature because a single physical link is shared by multiple VLANs still

allowing traffic isolation between VLANs.

Used to coneect two switches.

Figure 30: Trunk port

In absence of such feature we would have required one inter-switch per link or one

Access mode link between switches per VLAN.

Figure 31: Separate Access mode link between switches per each VLAN

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There are two ways a switch port can settle down into one of the above modes. Those two ways are:

1. Static method

You can manually configure a switch port to be in the access or trunk mode in the static

method.

The network administrator is responsible for keying in the mapping between the ports

and VLANs.

2. Dynamic method

You can let Dynamic Trunking Protocol (DTP) run on an interface to negotiate trunking in

the dynamic method.

The ports are able to dynamically work out their VLAN configuration.

This uses a software database of MAC address to VLAN mapping which the network

administrator must set op first.

Trunking protocol examples are ISL 0r 802.1Q

*note

Each switch port can be assigned to a different VLAN.

Ports assigned to the same VLAN share broadcasts.

Ports that do not belong to that VLAN do not share these broadcasts.

Static membership:

Static membership VLANs are called port-based and port-centric membership VLANs.

As device enters the network, it automatically assumes the VLAN membership of the port to

which it is attached.

The default VLAN for every port in the switch is the management VLAN, i.e. VLAN1 and may not

be deleted.

All other ports on the switch may be reassigned to alternate VLANs.

*important note

Figure 32: 2 VLANs 2 Subnets

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VLANs are assigned on the switch port. There is no VLAN assignment done on the host usually.

In order for a host to be a part of that VLAN, it must be assigned an IP address that belongs to the

proper subnet. VLAN = Subnet

Assigning a host to the correct VLAN is a 2 step process:

1. Connect the host to the correct port on the switch.

2. Assign to the host the correct IP address depending on the VLAN membership.

Dynamic membership:

Dynamic membership VLANs are created through Network management software.

CiscoWorks 2000 or CiscoWorks for Switched Internetworks is used to create Dynamic VLANs.

Dynamic VLANs allow for membership based on the MAC address of the device connected to

the switch port.

As a device centers the network, it queries a database within the switch for a VLAN

membership.

Benefits of VLANs:

Key benefit of VLANs is that they permit the Network Administrator to organize the LAN logically instead

of physically.

The administrator is able to all of the following:

Easily move workstations on the LAN

Easily add workstations to the LAN

Easily change the LAN configuration

Easily control network traffic

Improve security

*note

For the communication of two computers that are belonging to two different VLANS we need a router.

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VLAN Tagging There are two types of protocols:

Tagging Method Media Description

Inter Switch Link (ISL) *also called CISCO Proprietary

Fast Ethernet ISL header encapsulates the LAN frames and there is a VLAN ID field in the ISL header

Frame is Lengthened

802.1Q Fast Ethernet IEEE defined Ethernet VLAN protocol

header is modified

Cisco recommends using 802.1Q

802.1Q Trunking Protocol A special tag is added to frame depending on the VLAN number.

If we did not tag the information, the switch will finds it difficult to distinguish between VLANs.

VLAN tags are added only when transmitting through trunk links.

Every 802.1Q trunk port must have one Native VLAN. It is the default VLAN. The traffic for the Native

VLAN does not get tagged.

Native VLAN Native VLAN is an 802.1Q concept: frames belonging to Native VLAN are sent untagged through trunk

ports.

Native VLAN concept has been introduced as a way to provide backward compatibility to a device that

doesn’t support VLAN tagging: if a switch port is configured to be a trunk unconditionally without Native

VLAN concept only NIC that support VLAN tagging could be connected to the port.

Not all PC Network adapters support VLAN tags so the authors of 802.1Q standard introduced 802.1Q to

provide backward compatibility to allow a dumb device to connect to the network on single VLAN =

Native VLAN.

To be noted that Cisco ISL has no Native VLAN concept. 802.1Q is most used nowadays.

*note

Do practical with Cisco packet tracer.

Creating a VLAN

View the list of VLANs

Assigning a Port to a VLAN

Delete the VLAN membership of a port

Assigning mode of a port access/ trunk etc

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Lesson 03 Network Layer The network layer is responsible for the source-to-destination delivery of a packet possible across

multiple networks.

If two systems are connected to the same link, there is usually no need fpr a network layer. However, if

the two systems are attached to different networks, there is often a need for the network layer to

accomplish source-to-destination delivery.

Important network device: Router

Device that does routing part in a VLAN is the router. Inside a LAN, if we don’t have different VLANs, we

don’t need a router, a switch is enough. However to connect a LAN to a WAM or internet we need a

router.

Inside a subnet only layer 2 is enough. i.e., physical address or the MAC address is enough.

To send beyond LAN, we need an additional address. i.e., a logical address or an IP address is needed.

Functions of router limited up to network layer.

If the congestion at router is very high, packets may get lost.

Figure 33: Functions of router limited up to network layer.

IP addresses