Introduction to computer Networks

Computer Network

A computer network is a set o nodes connected by communication links.

Nodes

Can be a computer, printer or any other device capable of send/receiving data generated by other nodes in the network

Example for nodes

• Computer

• Server

• Printer

• Security camera

• Many more (Switches, bridges, routers, etc)

Communication link

Wired link or wireless link

• The link carries the information

Links

Wired; Cable

Wireless: air

End device

Starting point of communication

• Computers

• Network printers

• VoIP Phones

• Telepresence endpoint

• Security cameras

• Mobile handheld devices (Smart phones, tablets, PDAs, Wireless debit/credit card readers, barcode scanner)

Intermediary devices

The ones that connect end devices and allow data transmission on a network: hubs, switches, routers, gateways, firewalls or access points, call tower, modem, internet cloud

Forward data from one node to another

• Switches

• Wireless access point

• Routers

• Security Devices (Firewall)

• Bridges

• Hubs Repeaters

• Cell Tower

Resource sharing

• Computer network mainly used for resource sharing

Basic characteristics of computer network

• Fault tolerance

• Scalability

• Quality of Service (QoS)

• Security

Fault Tolerance

Ability to:

1. Continue working despite failures (takes another route)

2. Ensure no loss of service

Scalability

The ability to:

1. Grow based on the needs

2. Have good performance after growth

• Ex) Network should work after adding 10 computers

• Ex) The Internet

Quality of Service (QoS)

The ability to:

1. Set priorities

2. Manage data traffice to reduce data loss, delay etc

Ex) Delay of one second in email, delay in real time communication matters so router gives priority to VoIP phone than email

Ex) Router should know which data to prioritize

Security

The ability to prevent:

• Unauthorized access

• Misuse

• Forgery

The ability to provide

• Confidentiality

• Integrity

• Availability

Data Communication

• Data communications are the exchange of data between two nodes via some form of link (transmission medium) such as a cable

Data Flow

• Simplex

• Half Duplex

• Full Duplex

Simplex

• Communication is always unidirectional

• One device can transmit and the other device will receive

• Example: Keyboards, Traditional monitors

Half Duplex

• Communication is in both directions but not at the same time

• If one device is sending, the other can only receive and vice versa

• Example: Walkie-talkies

Duplex or Full Duplex

• Communication is in both directions simultaneously

• Device can send a receive at the same time

• Example: Telephone line

Protocols

• All communication schemes will have the following things in common

  • Source or sender

  • Destination or receiver

  • Channel or media

• Rules or protocols govern all methods of communication

Protocol

Protocol = Rule

• It is a set of rules that govern data communication

• protocol determines:

  • What is communicated?

  • How it is communicated?

  • When it is communicated?

Protocols - Human Communicatoin

Protocols are necessary for human communication and include:

• An identified sender and receiver

• common language and grammar

• speed and timing of delivery

• Confirmation or acknowledgement requirements

Protocols - Network Communication

Protocols used in network communications also define:

• Message encoding

• Message formatting and encapsulation

• Message timing

• Message size

• Message delivery options

Message encoding

• The source that is the source computer generates the message, gives it to encoder in order to generate signals, given to the transmitter and transmission medium, receiver, decoder (signal) understanding, message destination

• Converting waves into signals

• 

Message formatting and encapsulation

Agreed format.

Encapsulate the information (Source and information data) to identify the send and the receiver rightly

Message size

Humans break long messages into smaller parts or sentences. Long messages must also be broken into smaller pieces to travel across a network

Message timing

Flow control.

Response timeout.

Message Delivery Options

• Unicast

  • One sender and one receiver

• Multicast

  • Set of receiver but not to all

• Broadcast

  • Sends data to all participants in the network

Peer-to-peer network

• No Centralized administration

• All peers are equal

• Simple sharing applications

• Not scalable

Client Server Network

• Centralized administration

• Request-Response model

• Scalable

• Server may be overloaded

Components of a Computer Network

Nodes, Media and Services

Nodes

• End nodes (end devices)

• Intermediary nodes

Media

• Wired medium (guided medium)

• Wireless medium (unguided medium)

Wired media

• Ethernet straight through cable

• Ethernet crossover cable

• Fiber Optic cable

• Coaxial cable

• USB cableEt

Ethernet cable

- Ethernet straight through cable: connects different devices

• Ethernet cross over cable

  • Two devices of same kind, two switches, two computers, two routers

Fiber Optic cables

• Data carried in form of light waves, fastest mode of wired communication

  

Coaxial cable

- Used for audio or video communication

• Carries data in form of electric signals

USB Cables

Wireless Media

• Infrared (example: short range communication - TV remote control)

• Radio (Example: Bluetooth, Wi-Fi)

• Microwaves (Example: Cellular System)

• Satellite (Example: Long range communication - GPS)

Services

• e-Mail

• Storage services

• File sharing

• Instant messaging

• Online game

• Voice over IP

• Video telephony

• World Wide Web

Classification of Computer Networks

1. Local Area Network (LAN)

2. Metropolitan Area Network (MAN)

3. Wide Area Network (WAN)

Local Area Network (LAN)

A Local Area Network (LAN) is a computer network that interconnects computers within a limited are such as a residence, school, laboratory, university campus or office building

LAN Devices

• Wired LAN (Ex: Ethernet - Hub, Switch)

• Wireless LAN (Ex: Wi-FI)

Metropolitan Area Network (MAN)

• A metropolitan area network (MAN) is a computer network that interconnects users with computer resources in a geographic region of the size of a metropolitan area (City)

MAN Devices

• Switches/Hubs

• Routers/Bridges

Wide Area Network (WAN)

A wide area network (WAN) is a telecommunications network that extends over a large geographical area for the primary purpose of computer networking

• Communication at a large distance

WAN Devices

• End devices and intermediary devices

The Internet

New trends

• BYOD (Bring Your Own Device)

• Online collaboration

• Cloud computing

Cloud Computing

It is the on-demand availability of computer systems resources, especially data storage and computing power, without direct active management by the user

Network Topology

Arrangement of nodes of a computer network

Topology = layout

Physical Topology and Logical Topology

Physical topology - Placement of various nodes

Logical topology - Deals with the data flow in the network

Bus Topology

• All data transmitted between nodes in the network is transmitted over the common transmission medium and is able to be received by all nodes in the network simultaneously

• A signal containing the address of the intended receiving machine travels from a source machine in both directions to all machines connected to the bus until it finds the intended recipientPros

• Only one wire - less expensive

• Suited for temporary network

• Node failures does not affect others

Cons

• Not fault tolerant (no redundancy) if transmission medium fails

• Limited cable length

• No security

Ring Topology

• A ring topology is a bus topology in a closed loop

• Peer to peer LAN topology

• Two connections: one to each of its nearest neighbors

• Unidirectional

• Sending and receiving data takes place with the help of a TOKEN (TOKEN means nodes turn to send data, TOKEN circulated throughout circle)

Pros:

• Performance better than Bus topology because it is in a closed loop

• Can cause bottleneck due to weak links (a point of congestion in a production system that prevents it from functioning at full speed)

• All nodes with equal access

Cons:

• Unidirectional. Single point of failure will affect the whole network

• high In load - low in performance

• No security

Star Topology

• Every node is connected to a central node called a hub or switch

• Centralized management

• All traffic must pass through the hub or switch

Pro:

• Easy to desing and implement

• Centralized administration

• Scalable

Con:

• Single point of failure affects the whole network

• Bottlenecks due to overloaded switch/hub

• Increased cost due to switch/hub

Mesh Topology

• Each node is directly connected to every other nodes in the network

• Fault tolerant and reliable

Pros:

• Fault tolerant

• Reliable

Cons:

• Issues with broadcasting messages

• Expensive and impractical for large networks

Hybrid Topology

IP Address

IP stands for Internet Protocol

• Every node in the computer network is identified with the help of IP address

IP Address (IPV4)

• Every node in the computer network is identified with the help of IP address

• Logical address

• Can change based on the location of the device

• Assigned by manually or dynamically

• Represented in decimal and it has 4 octets (x.x.x.x)

• 0.0.0.0 to 255.255.255.255 (32 bits)

MAC Address

MAC stands for Media Access Control

• Every node in the LAN is identified with the help of MAC address

• IP address = location of a person

• MAC address = name of the person

• routers need ip address

• Switches need mac address

• Physical address or hardware address

• Unique

• Cannot be changed

• Assigned by the manufacturer

• Represented in hexadecimal

• Example: 70-20-84-00-ED-FC (48 bits)

• Separator: hyphen ( -), period(.), and colon (:)

IP Address vvs MAC address

IP Address

• Needed for communication

• 32 bit

• Reprecented in decimal

• Router needs IP address to forward data

• Example: 10.10.23.56

MAC Address

• Needed for comunication

• 48 bits

• Represented in hexadecimal

• Switch needs MAC address to forward data

• Example: 70-20-84-00-ED-FC

Port addressing

Reaching our city = Reaching our network (IP adress)

Reaching our apartment = Reaching the host ( MAC address)

Reaching the right person = Reaching the right process (Port address)

Port Address or Port Number

• In a node, many process will be running

• Data which are sent/received must reach the right process

• Every process in a node is uniquely identified using port numbers

• Port = communication endpoint

• Fixed port numbers and dynamic port numbers ( 0 - 65535)

Example:

Fixed port numbers: 25,80 etc,

OS assigned dynamic port numbers: 62414

Before sending the data, any node must

• Attach source IP address and destination IP address

• Attach source MAC address and destination MAC address

• Attach source port number and destination port number

Switching

Switching in computer network helps in deciding the best route for data transmission if there are multiple paths in a larger network

• One-to-one connection

Circuit switching

• A dedicated path is established between the sender and receiver

• Before data transfer, connection will be established ifrst

• Ex: Telephone network

3 Phases in circuit switching

1. connection establishment

2. data transfer

3. connection disconnection

Message switching

• Store and forward mechanism

• message is transferred as a complete unit and forwarded using store and forward mechanism at the intermediary node

• Broken into pieces, intermediary receives small pieces and forwards data

• Not suited for streaming media and real-time applications

Packet switching

• The internet is.a packet switched network

• message is broken into individual chunks called as packets

• Each packet is sent individually

• Each packet will have source and destination IP address with sequence number

• Sequence numbers will help the receiver to

  • Reorder the packets

  • Detect missing packets and

  • Send acknowledgments

Two approaches to packet swtiching

1. Datagram Approach

2. Virtual Circuit Approach

Packet switching - Datagram approach

• Datagram packet switching is also known as connectionless switching

• each indepdnent entity is called as datagram

• Datagrams contain destination infroamtion and the intermediary devices uses this information to forward datagrams to right destination

• In a datagram packet switching approach, the path is not fixed

• Intermediate nodes take the routing decisions to forward the packets

• Reorder done by the receiver

Packet switching - Virtual Circuit Approach

• Virtual Circuit Switching is also known as connection-oriented switching

• In the case of Virtual circuit switching, a preplanned route is established before the messages are sent

• Call request and call accept packets are used to establisht he econnection between sender and receiver

• In this approach, the path is fixed for the duration of a logical connection

Layering in Computer Networks

Layering means decomposing the problem into more manageable components (layers)

Pros;

• It provides more modular design

• Easy to troubleshoot

Protocols

• It is a set of rules that governs data communication

• The protocols in each layer governs the activities of the data communication

Layered Architectures

The OSI Reference Model

The TCP/IP Model

The OSI Model

• OSI stands for Open System Interconnection

• It is a model for understanding and designing a network architecture that is flexible, robust and interoperable

• Developed by the international standards for organizations (ISO)

• The OSI model is not a protocl

• It is only a guideline and hence it is referred as OSI reference model

The OSI Model

• Purpose of the OSI model is to show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software

• The OSI model was never fully implemented

The TCP/IP Model

• TCP/IP = Transmission Control Prtocol/Internet Protocol

• The TCP/IP protocol suite was developed prior to the OSI model

• Therefore, the layers in the TCP/IP protocol suite do not exactly match those in the OIS model

• TCP/IP is a hierarchial protocol made up of interactive modules, each of which provides a specific functionality

• One layer takes care of port addressing, mac addressing, ip addressing

Layers in OSI Reference Model

Application layer - Away

Presentation layer - Pizza

Session layer - Sausage

Transport layer - Throw

Network layer - Not

Data link layer - Do

Physical layer - Please

(Don’t want any intermediate node to access application data; focus on network data link and physical)

Application Layer

Enables the user to access the network resources

• Application needed to send data to device

Services provided by application layer

• File Transfer and Access Management (FTAM)

• Mail Services

• Directory services

Presentation Layer

• It is concerned with the syntax and semantics (meaning of each section) of the information exchange between two systems

Services provided

• Translation

  • If computer will send data will convert it into a common format which is accepted by all devices

• Encryption

  • Plain text into unreadable text so hackers cannot understand the message

• Compression

  • Multimedia messages (audio,video,text)

Session layer

It establishes, maintains and synchronizes the interaction among communicating devices

Services

• Dialog control

  • Two processes are communicating (full duplex, half duplex)

• Synchronization

  • session layer allows checkpoints, checkpoint ensure

Transport Layer

It is responsible for process to process delivery of the entire message

Services:

• Port addressing

• Segmentation and reassembly

• Connection control

  • Connection or connectionless

• End-to-end flow control

• Error control

Network Layer

It is responsible for delivery of data from the original source to the destination network

Services:

• Logical addressing

  • IP addresses

• Routing

  • Finding the best rule for the packet to be transported

Data Link Layer

• It is responsible for moving data (frames) from one node to another node

Services

• Framing

  • Groups bits of 0s and 1s

• ‘Physical addressing

  • MAC addresses

• Flow control

• Error control

• Access control

Physical Layer

Responsible for transmitting bits over a medium. It also provides electrical and mechanical specifications

Services

• Physical characteristic of the media

• Representation of bits

  • Encoding, how 0s and 1s turned into signals

• Data rate

• Synchronization of bits

• Line configuration

• Physical topology

SNMP (Simple Network Management Protocol)

• Monitor networking equipment

• Remotely monitor information

OID (Object Identifier)

• Anything that can be monitored

• Identification 1.3.6.1.2.1.2.2.1.8

• Similar to an IP address

  • sysuptime.0

MIB (Management information Base)

• Numberical OID turned to words based OID

• MIBs easier

Network Monitoring System (NMS)