LESSON 2 : OSI and TCP Network Layers

OSI

Open Systems Interconnection

OSI Model

It's a 7-layered framework for designing and implementing computer networks

OSI Model

provides a standardized way to understand and communicate how data is transmitted over a network

1. Physical (Layer 1)

2. Data Link (Layer 2)

3. Network (Layer 3)

4. Transport (Layer 4)

5. Session (Layer 5)

6. Presentation (Layer 6)

7. Application (Layer 7)

The 7 layers of the OSI model are:

Layer 1 – Physical Layer

The lowest layer of the OSI reference model

Layer 1 – Physical Layer

It is responsible for the actual physical connection between the devices

Layer 1 – Physical Layer

contains information in the form of bits

Layer 1 – Physical Layer

responsible for transmitting individual bits from one node to the next

Layer 1 – Physical Layer

When receiving data, this layer will get the signal received and convert it into 0s and 1s and send them to the Data Link layer, which will put the frame back together

Layer 1 – Physical Layer

Hub, Repeater, Modem, and Cables

1. Bit Synchronization

2. Bit Rate Control

3. Physical Control

4. Physical Topologies

5. Transmission Mode

Functions of the Physical Layer (5)

Bit Synchronization

The physical layer provides the synchronization of the bits by providing a clock. This clock controls both sender and receiver thus providing synchronization at the bit level

Bit Rate Control

The Physical layer also defines the transmission rate i.e. the number of bits sent per second

Physical Topologies

Physical layer specifies how the different, devices/nodes are arranged in a network i.e. bus topology, star topology, or mesh topology

Transmission Mode

Physical layer also defines how the data flows between the two connected devices. The various transmission modes possible are Simplex, half-duplex and full duplex

Layer 2 – Data Link Layer (DLL)

responsible for the node-to-node delivery of the message

Layer 2 – Data Link Layer (DLL)

main function of this layer is to make sure data transfer is error-free from one node to another, over the physical layer

Layer 2 – Data Link Layer (DLL)

When a packet arrives in a network, it is the responsibility of the____ to transmit it to the Host using its MAC address

Frame

Packet in the Data Link layer is referred to as ___

Layer 2 – Data Link Layer (DLL)

Switches and Bridges

• Logical Link Control (LLC)

• Media Access Control (MAC)

The Data Link Layer is divided into two sublayers:

Layer 2 – Data Link Layer (DLL)

The packet received from the Network layer is further divided into frames depending on the frame size of the NIC (Network Interface Card)

Layer 2 – Data Link Layer (DLL)

encapsulates Sender and Receiver’s MAC address in the header.

Layer 2 – Data Link Layer (DLL)

The Receiver’s MAC address is obtained by placing an ARP (Address Resolution Protocol) request onto the wire asking, “Who has that IP address?” and the destination host will reply with its MAC address.

1. Framing

2. Physical Addressing

3. Error Control

4. Flow Control

5. Access Control

Functions of the Data Link Layer (5)

Framing

a function of the data link layer. It provides a way for a sender to transmit a set of bits that are meaningful to the receiver. This can be accomplished by attaching special bit patterns to the beginning and end of the frame.

Physical Addressing

After creating frames, the Data link layer adds physical addresses (MAC addresses) of the sender and/or receiver in the header of each frame.

Error Control

The data link layer provides the mechanism of error control in which it detects and retransmits damaged or lost frames.

Flow Control

The data rate must be constant on both sides else the data may get corrupted thus, flow control coordinates the amount of data that can be sent before receiving an acknowledgment

Access Control

When a single communication channel is shared by multiple devices, the MAC sub-layer of the data link layer helps to determine which device has control over the channel at a given time

Layer 3 – Network Layer

works for the transmission of data from one host to the other located in different networks

Layer 3 – Network Layer

takes care of packet routing i.e. selection of the shortest path to transmit the packet, from the number of routes available

Layer 3 – Network Layer

The sender and receiver’s IP address are placed in the header by the _________

Packet

Segment in the Network layer is referred to as ______

Layer 3 – Network Layer

implemented by networking devices such as routers and switches

1. Routing

2. Logical Addressing

Functions of the Network Layer (2)

Routing

The network layer protocols determine which route is suitable from source to destination. This function of the network layer is known as routing.

Logical Addressing

To identify each device inter-network uniquely, the network layer defines an addressing scheme. The sender and receiver’s IP addresses are placed in the header by the network layer. Such an address distinguishes each device uniquely and universally

Layer 4 – Transport Layer

provides services to the application layer and takes services from the network layer

Segments

The data in the transport layer is referred to as ______

Layer 4 – Transport Layer

It is responsible for the end-to-end delivery of the complete message.

Layer 4 – Transport Layer

provides the acknowledgment of the successful data transmission and re-transmits the data if an error is found

Layer 4 – Transport Layer

Protocols used in ______ are TCP, UDP NetBIOS, PPTP.

Layer 4 – Transport Layer

At the Receiver’s side, ________ reads the port number from its header and forwards the Data which it has received to the respective application. It also performs sequencing and reassembling of the segmented data.

1. Segmentation and Reassembly

2. Service Point Addressing

Functions of the Transport Layer

Segmentation and Reassembly

This layer accepts the message from the (session) layer and breaks the message into smaller units. Each of the segments produced has a header associated with it. The transport layer at the destination station reassembles the message

Service Point Addressing

To deliver the message to the correct process, the transport layer header includes a type of address called service point address or port address.

• Services Provided by Transport Layer

• Connection-Oriented Service

• Connectionless Service

Thus, by specifying this address, the transport layer makes sure that the message is delivered to the correct process. (3)

Layer 5 – Session Layer

responsible for the establishment of connections, management of connections, terminations of sessions between two devices

Layer 5 – Session Layer

It also provides authentication and security. Protocols used in the Session Layer are NetBIOS, PPTP

1. Session Establishment

2. Synchronization

3. Dialog Controller

Functions of the Session Layer (3)

Session Establishment, Maintenance, and Termination

The layer allows the two processes to establish, use, and terminate a connection

Synchronization

this layer allows a process to add checkpoints that are considered ______ points in the data. These ________ points help to identify the error so that the data is re-synchronized properly, and ends of the messages are not cut prematurely, and data loss is avoided

Dialog Controller

The session layer allows two systems to start communication with each other in half-duplex or full duplex

Layer 6 – Presentation Layer

also called the Translation layer

Layer 6 – Presentation Layer

The data from the application layer is extracted here and manipulated as per the required format to transmit over the network

Layer 6 – Presentation Layer

Protocols used in the _______ are JPEG, MPEG, GIF, TLS/SSL, etc.

1. Translation

2. Encryption/ Decryption

Functions of the Presentation Layer (2)

Translation

For example, ASCII to EBCDIC

Encryption/ Decryption

Data encryption translates the data into another form or code. The encrypted data is known as the ciphertext, and the decrypted data is known as plain text. A key value is used for encrypting as well as decrypting data.

Data encryption

translates the data into another form or code

ciphertext

encrypted data is known as

plain text

decrypted data is known as

Compression

Reduces the number of bits that need to be transmitted on the network

Layer 7 – Application Layer

At the very top of the OSI Reference Model stack of layers, we find the ______ which is implemented by the network applications. These applications produce the data to be transferred over the network.

Layer 7 – Application Layer

This layer also serves as a window for the application services to access the network and for displaying the received information to the user

Layer 7 – Application Layer

Protocols used in the _______ are SMTP, FTP, DNS, etc.

1. Network Virtual Terminal (NVT)

2. File Transfer Access and Management (FTAM)

3. Mail Services

4. Directory Services

Functions of the Application Layer

Network Virtual Terminal (NVT)

It allows a user to log on to a remote host

File Transfer Access and Management (FTAM)

This application allows a user to access files in a remote host, retrieve files in a remote host, and manage or control files from a remote computer

Mail Services

Provide email service

Directory Services

This application provides distributed database sources and access for global information about various objects and services