COSC 85 - Lecture 2

OSI Model or Open Systems Interconnection

Is a conceptual framework used to describe the functions of a networking system.

OSI Model or Open Systems Interconnection

Uses layers to give visual description of what is going on with a particular networking system.

Physical
Data Link
Network
Transport
Session
Presentation
Application

7 Layers of OSI Model:

International Standards Organization (ISO) in 1947

Dedicate to worldwide agreement on international Standards.

Open Systems Interconnection Model (OSI)

An ISO standard that covers all aspects of network communication. It was first introduced in the late 1970s

Application layer

Enables the user to access the network. It provides user interfaces and support for services.

File Transfer – FTP (File Transfer Protocol)
Web Surfing – HTTP/S (Hypertext Transfer Protocol)
Emails – SMTP (Simple Mail Transfer Protocol)
Virtual Terminals – TELNET

Network Services

Presentation Layer

Receives data from applications layer. These data are in the form of numbers or characters.

Presentation layer

Is concerned with the syntax and semantics of the information exchanged between two systems.

Translation

Converts data to binary format for the machine to understand.

Data Compression

Reduces the number of bits that are used to represent the original data.

Encryption

Enhances the security of the data. Secure Sockets Layer (SSL) is used in the encryption and decryption of the data.

Session layer

Helps in setting up and managing connection enabling sending and receiving of data followed by termination of connections or sessions.

Synchronization

The session layer allows a process to add checkpoints, or synchronization points, to a stream of data.

Dialog control

It allows the communication between two processes to take place in either half-duplex or full-duplex mode.

Authentication

A process of verifying the user. A session is established between server and the computer once the user is authenticated.

Authorization

A process used by server to determine if the user have the permission to access a file or a site.

Transport layer

Is responsible for process-to-process delivery of the entire message.

Segmentation

Data received from session layer is divided into small data units called segments.

Source and destination port; sequence number

In segmentation, each segments contains both of this

Flow Control

Transport layer controls the amount of data transmitted to a level that the receiver can process.

Error Control

If some data units never arrive the destination, transport layer uses Automatic Repeat Request schemes to re-transmit the lost or corrupted data.

Transmission Control Protocol (TCP)

Connection-Oriented Transmission. Gives feedback, therefore data that is lost can be re-transmitted. Internet surfing, Emails, FTP, etc.

User Datagram Protocol (UDP)

Connectionless Transmission. No Feedback whether data is really delivered or not. Streaming, Music, Gaming, Voice calls, DNS, etc.

Network layer

Works for the transmission of received data segment from one computer to another located in different networks.

Logical Addressing

IP addressing (IPv4 or IPv6) is done in network layer. Every computer in a network has a unique IP Address.

Routing

A method of moving data packet from source to its destination. It is based on the logical address format of the logical addressing.

Path determination

A method which choosing the best possible path for data delivery from source to destination.

Data Link layer

Receives packet from network layer which contains IP addresses of sender and receiver.

Logical Addressing
Physical Addressing

Two kinds of addressing:

Logical Addressing

Is done at network layer where sender and receiver’s IP Addresses are assigned to each data packet.

Physical Addressing

Is done at data link layer where MAC addresses of both devices are assigned to received the data packet.

Framing

The data link layer divides the stream of bits received from the network layer into manageable data units called frames.

Flow Control

The data link layer imposes a flow control mechanism to avoid overwhelming the receiver.

Error Control

Adding mechanism to detect and retransmit damaged or lost frames.

Access Control

Data link protocols are necessary to determine which device has control over the link at any given time.

Physical Layer

Converts the binaries into signals (either electrical, radio or optical)

Physical Characteristics of interfaces and medium

Physical layer defines the interface between the device and the transmission medium. It also defines the type of transmission medium.

Representation of bits

Physical layer data consists of a stream of bits with no interpretation. To be transmitted, bits must be encoded into signals, electrical or optical.

Data Rate

The physical layer defines the duration of a bit, which is how long it lasts.

Synchronization of bits

The sender and the receiver clocks must be synchronized.

Line configuration

Concerned with the connection of devices to the media.

Physical Topology

It defines how devices are connected to make a network.

Transmission mode

It defines the direction of transmission between two devices.