OSI Model

Acronym for 7 layers

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Acronym for data in different layers

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What is a network protocol?

a set of rules for exchanging data in a structured format. A network protocol has two principal functions:

• Addressing—Describing where data messages should go. At each OSI model layer, there are different mechanisms for identifying nodes and rules for how they can send and receive messages.

• Encapsulation—Describing how data messages should be packaged for transmission. Encapsulation is like an envelope for a letter, with the distinction that each layer requires its own envelope. At each layer, the protocol adds fields in a header to whatever payload data it receives from an application or other protocol.

BFPSD →

Bits → frames→ packets→ segments →data →data →data

PDNTSPA →

physical → data → network→ transport → session → presentation → application

L1 - Physical: Asynchronous

Uses start and stop bits to indicate when transmissions occur from the sender to the receiver. Like when someone leaves a voicemail.

L1 - Physical: Synchronous

Uses a reference clock to coordinate the transmissions by both sender and receiver

L1 - Physical: Broadband

Divides bandwidth into separate channels. Like when someone watches TV. There is a single coaxial cable that carries hundreds of channels.

L1 - Physical: Baseband

Uses all available frequencies on a medium (cable) to transmit data. Uses a reference clock so it is synchronous communication. Telephone does this because it can only make one phone call at a time and uses all its resources for that one phone call. Another example is if you have a wired network so you use ethernet cords. All the frequency that is available on the ethernet is going to the node, instead of it being wireless then the network shares the connection.

L1 - Physical: Baseband TDM (Time-division multiplexing)

Each session takes a turn, using fixed time slots, to share the medium between all users. It explains how multiple communications share the same physical medium reliably. In practical terms, it’s about getting bits from point A to point B without collisions or confusion.

L1 - Physical: Baseband StatTDM (Statistical Time-division multiplexing)

is a physical-layer technique where multiple devices share a single baseband link, but time slots are assigned dynamically instead of being fixed.

• Devices only get a time slot when they actually have data

• Idle devices don’t waste bandwidth

• Time slots include addressing info so the receiver knows who the data belongs to

L1 - Physical: Baseband FDM (Frequency-division multiplexing)

is a physical-layer technique where a single baseband physical medium is divided into multiple frequency bands, allowing multiple signals to transmit at the same time, each on a different frequency.

• The link’s bandwidth is split into non-overlapping frequency channels

• Each sender uses a different frequency

• Signals transmit simultaneously, not in turns

L1 - Physical: What does multiplexing allow?

Allows multiple people to use a baseband connection at the same time.

L1 - Physical: Types of media

Fiber optic, ethernet, coaxial

L1 - Physical: Types of wireless media

Bluetooth, wifi, NFC

L1 - Physical: Types of infrastructure

hubs, access points, media converters

What are layers 1 devices?

They are essentially repeaters, passing along whatever is received. Whatever they take in, they send right back out. Like if light goes through one end of fiber optic cable, then light comes out the other end.

L2 - Data Link

Packages data into frames and transmits those frames on the network. Call their data frames

L2 - Data Link: MAC

Physical addressing system of a device which operates on a logical topology. A unique 48 bit physical addressing system is assigned to every NIC produced.

L2 - Data Link: Logical Link Control (LLC)

Provides connection services and allows acknowledgment of receipt of messages. Most basic form of flow control. Provides basic error control functions.

L2 - Data Link: Devices

NICs, bridges, switches

L3 - Network

it is about logical addressing and routing — deciding how data gets from one network to another. Has IP addresses. Call their data packets

L3 - Network: Packet Switching

Packet switching is a networking method where data is broken into small packets, sent independently across a network, and reassembled at the destination. Most networks use this.

L3 - Network: Circuit Switching

networking method where a dedicated communication path (circuit) is established before data is sent, and that path remains reserved for the entire session.

L3 - Network: Message Switching

a store-and-forward networking method where an entire message is sent from node to node, stored at each intermediate device, and then forwarded to the next hop only after the full message is received.

L3 - Network: ICMP

Sends error messages and operational information to an IP destination. Has the PING command

L3 - Network: Devices & Stuff

Routers, multi-layer switches, IPv4/6, ICMP

L4 - Transport

Dividing line between the upper layers and the lower layers of the OSI model. Is responsible for end-to-end communication between devices and ensuring that data is delivered reliably, in order, and without errors (depending on the protocol). Call their data segments.

L4 - Transport: TCP

protocol that provides reliable, connection-oriented communication between devices over a network.

• ensures that all data reaches the destination.

• If segments are lost, TCP retransmits them.

• There is a 3 way handshake: syn→syn-ack →Ack

L4 - Transport: UDP

Connectionless protocol that is an unrealiable wat to transport segments across the network because it will drop them and not tell sender but its FASTER THAN TCP

L4 - Transport: Windowing

Allows the clients to adjust the amount of data in each segment

L5 - Session

responsible for establishing, managing, and terminating communication sessions between applications.

L5 - Session: H.323

used to set up, maintain, and tear down voice and video connections. AKA facetime, skype using RTP (real-time transport protocol)

L5 - Session: NetBIOS

Used to share files over a network

L6 - Presentation

Data formatting and encryption. Responsible for how data is formatted, translated, encrypted, and compressed so it can be properly understood by the receiving system. Ex) ASCII

L6 - Presentation: Examples

Scripting languages (HTML), standard text, pictures (gifs, pngs), movie files, encryption algorithms (TLS, SSL)

L7- Application

provides network services directly to end-user applications and is the layer where users interact with the network.

L7- Application: Application Services

Unites communicating components from more than one network application. Ex) POP3, IMAP, SMTP

L7- Application: Service Advertisement

Sending out of announcements to other devices on the network to state the services they offer. Like when you see a printer on “find devices”.