Lecture 2 network

OSI MODEL (Open Systems Interconnection)

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

OSI MODEL

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

1. Physical

2. Data Link

3. Network

4. Transport

5. Session

6. Presentation

7. Application

OSI Model 7 layers

International Standards Organization (ISO)

was established in 1947, dedicated 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 late 1970s.

7) Application Layer

• used by network applications.

• enables user to access network. It provides user interfaces and support for services.

Network Services

1. File Transfer - FTP (File Transfer Protocol)

2. Web Surfing - HTTP/S (Hypertext Transfer Protocol)

3. Emails - SMTP (Simple Mail Transfer Protocol)

4. Virtual Terminals - TELNET

6) Presentation Layer

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

6) Presentation Layer

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

1. Translation

2. Data Compression

3. Encryption

Functions of Presentation Layer

Translation

Converts data to binary format for machine to understand

Data Compression

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

Encryption

Enhances security of data.

Secure Sockets Layer (SSL)

is used in encryption and decryption of data.

Session Layer 5

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

1. Synchronization

2. Dialog Control

3. Authentication

4. Authorization

Functions of the Session Layer

Synchronization

Allows a process to add checkpoints or synchronization points to a stream of data.

Dialog Control

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

Authentication

process of verifying user.

• A session is established between server and computer once user is authenticated.

Authorization

Process used by server to determine if user has permission to access a file or a site.

4) Transport Layer

- Responsible for process-to-process delivery of entire message.

- provides enhancements to services of network layer.

- main task: ensure that data sent from one computer arrives reliably, in correct sequence, and without errors at receiving computer.

1. Segmentation

2. Flow Control

3. Error Control

Functions of the Transport Layer

Segmentation

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

• Each segment contains a source and destination port and sequence number.

Flow Control

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

• Example: A mobile phone connected to a server.

• The server can process data up to 100 Mbps, while mobile phone can only process 10 Mbps.

Error Control

If some data units never arrive at destination, transport layer uses

Automatic Repeat Request (ARQ)

schemes to retransmit lost or corrupted data.

Transmission Control Protocol (TCP)

• Connection-oriented transmission

• Gives feedback; data that is lost can be retransmitted

• Used for Internet surfing, Emails, FTP, etc.

User Datagram Protocol (UDP)

• Connectionless transmission

• No feedback whether data is really delivered or not

• Used for streaming, music, gaming, voice calls, DNS, etc.

3) Network Layer

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

1. Logical Addressing

2. Routing

3. Path Determination

Functions of the Network Layer

Logical Addressing

IP addressing (IPv4 or IPv6) is done at network layer.

• Every computer in a network has a unique IP address.

• It assigns sender and receiver IP addresses to each segment so that each data packet can reach correct destination.

Routing

Method of moving data packets from source to destination based on logical addressing.

Path Determination

Choosing best possible path for data delivery from source to destination.

• OSPF (Open Shortest Path First)

• BGP (Border Gateway Protocol)

• IS-IS (Intermediate System to Intermediate System)

2) Data Link Layer

Receives packets from network layer which contain IP addresses of sender and receiver.

1. Logical Addressing

2. Physical Addressing

Two kind of addressing:

Logical Addressing

Done at network layer where sender and receiver IP addresses are assigned to each data packet.

Physical Addressing

Done at data link layer where MAC addresses of both devices are assigned to receive data packet.

1. Framing

2. Flow Control

3. Error Control

4. Access Control

Functions of Data Link Layer

Framing

Divides stream of bits received from network layer into manageable data units called frames.

Flow Control

Imposes a flow control mechanism to avoid overwhelming receiver.

Error Control

Adds mechanisms to detect and retransmit damaged or lost frames.

Access Control

Determines which device has control over link at any given time.

1) Physical Layer

Converts binaries into signals (electrical, radio, or optical).

Functions of the Physical Layer

1. Physical Characteristics of Interfaces and Medium

2. Representation of Bits

3. Data Rate

4. Synchronization of Bits

5. Line Configuration

6. Physical Topology

7. Transmission Mode

Physical Characteristics of Interfaces and Medium

Defines interface between device and transmission medium, including type of transmission medium.

Representation of Bits

Defines how bits (binary data) are encoded into signals (electrical or optical).

Data Rate

Defines duration of a bit (how long it lasts).

Synchronization of Bits

Ensures sender and receiver clocks are synchronized.

Line Configuration

Concerned with how devices connect to media.

Physical Topology

Defines how devices are physically connected to make a network.

Transmission Mode

Defines direction of transmission between two devices