OSI_Functions

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OSI Functions

Transmission of information always starts from a sending device (SOURCE) and ends at the receiving device (DESTINATION). Data from the SOURCE travels down the OSI model layers, is then transmitted, and travels back up the model to the DESTINATION. Each OSI layer performs a different function on the transmitted data.

SOURCE

OSI Model Functions

DESTINATION

01101010000100101010011010

Data Link Layer

Session Layer

Physical Layer

Transport Layer

Application Layer

Network Layer

Presentation Layer

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Reliable transmission of dataConnection-oriented (establishes a reliable connection between the sender and receiver using a three-way handshake)Guaranteed deliveryUses flow control (slower)Uses error detection and retransmission mechanismsUses a checksum mechanism to detect errorsCritical applications

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Unreliable transmission of dataConnectionless (does not use the thrree-way handshake to establish a connection before transmitting data)No guaranteed delivery of dataFaster protocol since there is no flow controlAny lost or corrupted packets are ignoredNo error detectionReal-time applications

Transport Layer-Protocols

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Registered

23 Telnet

49152 - 65565

67 DHCP

Network Port Types

80 HTTP

0 - 1023

20 FTP

123 NTP

Dynamic

22 SSH

Port Numbers

69 TFTP

Service ports identify specific services or applications running on a device.

Well-known

25 SMTP

1024 - 49151

53 DNS

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110 POP3

21 FTP

Transport Layer-Ports

Receives SYN (seq = m)

A three-way handshake creates a connection between a local host/client and server. It is a three-step method designed to allow both communicating ends to initiate and negotiate the parameters of the network TCP socket connection at the same time before data such as HTTP or FTP transmits. Multiple TCP socket connections can be transmitted in both directions simultaneously. A three-way handshake is also known as a TCP handshake or SYN-SYN-ACK, and requires both the client and server to exchange SYN (synchronization) and ACK (acknowledgement) packets before actual data communication begins.

Sends SYN (seq = m)

Receives SYN (seq = n)Receives ACK(ack = m + 1)

Transport Layer-Handshake

Receives ACK (ack = n + 1)

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Sends SYN (seq = n) Sends ACK (ack = m + 1)

Sends ACK (ack = n + 1)

Packet 2

Packet Transmission

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Packet 5

Packet 1

Packet Re-assembly

Packet 3

Packet Assembly

Transport Layer-Assembly

Packet 4

TCP breaks down data into smaller packets for transmission, and then reassembles those packets into the original data at the receiver end. Each packet contains a header with the source and destination addresses and the sequence number of the packet. TCP uses sequence numbers to ensure that packets are transmitted and received in the correct order. At the receiver end, TCP reassembles the packets into the original data using the sequence number of each packet to ensure that the packets are reassembled in the correct order. If a packet is lost or corrupted during transmission, TCP can use the sequence number to request that the packet be retransmitted. By breaking data into smaller packets, TCP can ensure that data is transmitted reliably and efficiently over a network.

Presentation Layer

Computers create and use hundreds of different codes to create text files, graphics, sound, animation, and video. The presentation layer uses encoders and decoders to identify different file types and the codes used to create them. This information enables the receiving station to decode and use the data in the proper application.

Use application for .doc

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The Presentation Layer is responsible for formatting and presenting data to be transmitted over a network. The data must be presented in a standard format that can be understood by both the SOURCE and the DESTINATION.

Use application for .mp3

Directory services

The Application Layer acts as a bridge between the network and the software application running on a computer.

The Application Layer is the topmost layer and is responsible for providing services and interfaces for end user applications to access network resources and defines protocols and standards that applications use to communicate and exchange data with other applications or services like HTTP, FTP, SMTP, DNS, and SNMP.

Remote login

Network management

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File transfer

Encryption

It is a common practice to encrypt data before transmitting it across the Internet. Encrypting data requires an encryption algorithm, configuration settings, and a key. The Presentation Layer communicates this information to the destination system. Examples include AES, 256-bit, and key=ABC#2050

The Presentation Layer communicates the type of algorithm used to compress data before being transmitted. It must also inform the receiving station which algorithm was used so that the data can be de-compressed. Examples include zip, gzip, bzip2, or tar.

Compression

IP + Port + Est#

Session Layer

Socket

Session established

The Session Layer establishes, maintains, synchronizes, and terminates sessions between end-user applications. A session is an end-to-end connection between the user and the Internet connections established. The Session Layer manages a session by initiating sockets. Sockets consist of the destination IP address, tcp/udp port number, and establish and acknowledge numbers used by tcp to open new sessions. This layer enables users to maintain multiple, simultaneous Internet connections.

click user to start sessions

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Assembly

Handshake

Transport Layer

The Transport Layer ensures that data transmits reliably between devices on a network. It is the layer that provides end-to-end communication services for applications and services running on different devices.Flow control ensures that data is transmitted at a rate that the DESTINATION can handle, avoiding data loss or congestion on the network. The SOURCE adjusts the rate of transmission based on feedback received from the DESTINATION.

Protocols

Ports

The Network layer assigns logical addresses such as IP addresses to devices on the network.

The Network layer selects the best path for data packets to travel from SOURCE to DESTINATION by using routing tables.

Routing Table

10.16.205.20

The Network Layer’s primary function is to facilitate the transfer of data between different networks by providing logical addressing, routing, and internet working capabilities. It uses QoS mechanisms to prioritize specific types of traffic based on predefined parameters so that time-sensitive data such as real-time video or voice data receives preferential treatment in terms of bandwidth, latency, and reliability.

The Network layer performs network address translation (NAT) to allow devices with private IPs to communicate outside their local netowk using a public IP address.

10.16.205.254

The Network layer can fragment large data packets into smaller pieces that can be transmitted across different networks. It also reassembles the fragments.

192.168.1.15

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Network Layer

The Network layer monitors network traffic and implements congestion control mechanisms to ensure efficient data transmission.

192.168.1.254

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Data Link Layer

00-B0-D3-4E-29-A2

The Data Link Layer provides reliable and error-free transmission of data frames between adjacent network nodes over a physical link. The Data Link Layer consists of two sublayers: the Media Access Control (MAC) sublayer and the Logical Link Control (LLC) sublayer.

The Media Access Control sublayer enables devices to share a physical media and be able to communicate. For example, CSMA/CD is a collision detection method that provides equal access for all devices. Each deice listens to determine if another device is transmitting. A collision occurs when two devices do hear transmission of data and they both transmit. This results in a data collision, and the devices then resend.

The Data Link Layer assigns physical addresses such as Media Access Control (MAC) addresses to devices connected to the network. MAC addresses are unique identifiers that distinguish one device from another on a local network segment.

The Data Link Layer divides the incoming data stream from the Netwok Layer into frames. A frame includes data, control information, and error detection bits.

00-B0-D3-85-2F-1E

00-FC-32-8F-C2-26

00-B0-D3-C4-35-19

The Physical Layer converts the digital data generated by the higher layers into a physical signal for transmission over the network medium.

011010100

The Physical Layer receives the incoming signals from the transmission medium and decodes them back into digital data. It performs signal detection, amplification, and equalization to ensure accurate reception.

Physical Layer

The Physical Layer deals with the electrical, mechanical, and procedural aspects of transmitting data. It defines the physical medium, such as copper wires, optical fibers, or wireless frequencies, through which data is transmitted. It also specifies the characteristics of the physical interface, including voltage levels, signaling rates, and transmission distances.

The Physical Layer also defines the characteristics of the physical medium including cables, connectors, and transmission rates.