03 OSI Model

OSI Model

• Open System Interconnection Model
• 7 layers reference model that defines and separates networking hardware and software into distinct layers and functions

Please	Do	Not	Throw	Sausage	Pizza	Away
Physical	Data Link	Network	Transport	Session	Presentation	Application

Physical Layer

• **Protocol Data Unit: **Bits
• **Carry bit stream over physical medium **
• Bit Transmission Rate is defined by physical layer
• Synchronisation of bits: Sender and receiver use the same bit rate and be synchronised to prevent data loss or corruption

Representation of Bits

• Bits encoded into electrical/optical signals
• Digital signals use discrete voltages
• Analog signals are continuous waveforms that represent data as continuously varying voltage, current or EM radiation

Line Configuration

• Connection of devices to the media (point to point or multipoint)
• Point to Point: Dedicated link between 2 devices; entire capacity of the link is reserved for transmission between the 2 devices
• Multipoint: More than two devices share a single link

Physical Topology

• Defines how devices are connected to make a network

**Mesh **Topology - Each device/node has a dedicated point-to-point link to every other node - Peer-to-Peer connectivity
**Star **Topology - Each device/node has a dedicated point-to-point link to the central controller - Client-Server connectivity
**Bus Topology - Multipoint connection - Long cable acts as backbone - Devices are connected to bus cable by drop lines **and taps
**Ring **Topology - Each device has dedicated point-to-point link with 2 devices on either side - Signal passed along ring in one direction from device to device until it reaches destination

Transmission

Transmission Media

• Wired transmission
  • Twisted pair cable: Ethernet LANS
  • Coaxial cable: Cable TV, older networks
  • Fiber Optic cable: transmit data at high speed over long distance
• Wireless transmission
  • Radio waves: WiFi and mobile networks
  • Microwaves: Satellite and long distance comms
  • Infrared waves: short range comms (TV remote)

Transmission Mode

Simplex - Monodirectional Communication - E.g. Keyboards, monitors
Half-Duplex - Each device can both transmit and receive, but not at the same time - E.g. Walkie-talkies
Full-Duplex - Both devices can transmit and receive simultaneously - E.g. Telephone

Data Link Layer

• **Protocol Data Unit: **Frame
• Allows direct communication between two devices on the same local network
• Flow control: Prevents overwhelming receiver if rate of data absorbed is less than rate of data produced
• **Error control: **Detecting and retransmitting damaged or lost frames
  • Parity Bits: Detect single-bit errors
  • Checksums: Hashing data to detect corruption or changes
  • Cyclic Redundancy Check (CRC): Detect errors from noise or interference
• **Access control: **Data link protocols determine which device has control over the link
  • Media Access Control (MAC) to avoid collisions
  • Carrier Sense Multiple Access (CSMA) with Collision Detection / Avoidance
  • CSMA/CD: Ethernet: devices **detect **collisions and mitigate them
  • **CSMA/CA: **Wireless networks: **avoid **collisions by conducting handshake
• Physical addressing
  • Adds a header to the frame to define the sender and/or receiver of the frame
  • If frame is intended for system outside the sender’s network, receiver address is of the connecting device that connects the network to the next one

Components of Frame

• **Header **with MAC address of **source **and destination
• **Payload **or Data
• **Trailer **with error-checking info

Sublayers

• LLC (Logical Link Control)
  • Interface with network layer, handles error checking and flow control
• MAC (Media Access Control)
  • Handles channel access and physical addressing
• MAC Address: 6 byte hardware address
  • **First 3 bytes: **OUI

Organisationally Unique Identifier - Identifies Manufacturer

• **Last 3 bytes: **

Device-Specific identifier

Network Layer

• Responsible for source-to-destination (machine-to-machine) delivery of a packet
• Logical addressing
  • If a packet passes network boundary, network layer adds a header that includes the logical addresses of sender and receiver
• Routing
  • When independent networks or linked are connected together, connecting devices (routers or switches) route or switch the packets to final destination

Transport layer

• Protocol Data Unit: Segment / Datagram
• Process-to-process (application-to-application) delivery of the message
• Ensures whole message arrives intact and in order: error and flow control at the source-to-destination level
• **Service-point addressing **
  • Computers often run several programs at the same time
  • Source-to-destination delivery must be from a specific process on one computer to specific process on another
  • Transport layer header contains Service-point address (or port address)
• Segmentation and Reassembly
  • Message is divided into transmittable segments with a sequence number
  • These segments are reassembled at the transport layer of the recipient
• Connection Control
  • Connectionless transport **layer **treats each segment as independent packets → Delivers segment to the transport layer at destination machine
  • Connection-oriented transport **layer **makes connection with transport layer at destination machine → Delivers datagram and terminating at end
• Flow control is performed end to end
• Error control is process to process

Session Layer

• Provides ability for presentation entities to organise communication
• This is done for multiple communication sessions taking place at the same time
• Session initiation and teardown: Starting and ending sessions between communicating entities, allowing data transfer to happen between
• Token management: Manages communication mode (simplex, half or full duplex)
• Session-connection to Transport-connection (connection-oriented transfer): Map between transport layer connections and the sessions taking place

Presentation Layer

• Responsible for the way data is presented to the application
• Negotiates the form of data to be transferred (through Transfer Syntax)
• Presentation layer can provide compression, encryption, translation

Application Layer

• Defining services at user-end
• Provides identification of services provided
• Defining Quality of Service (QoS) parameters for the application
• Defining security mechanisms (access control, authentication)
• Synchronisation of communication applications (connection-oriented)

Encapsulation

Flow of data from application to physical layer

End-to-End Data Flow

At Host A

1. Application Layer: Decides it needs to communicate with Host B
2. Presentation Layer: Conducts required **transformations **(compression, encryption, translation)
3. Session Layer:** Initiates communication session** → Data is segmented, **Header **(containing sequence and acknowledgment number) is added to each segment
4. Transport Layer: **Puts data **into **segments **and decides on connection control
5. Network Layer:** Puts data** into **packets **and **headers **are added to packets
6. Data Link Layer: **Puts data **into **frames **and adds header and trailer to each frame
7. Physical Layer: Dealt with as raw bits → transferred through physical channel to the **router **

At Router

1. Router reads up to network layer **header **to route data (partial decapsulation)

At Host B

1. Physical Layer: Raw bits are **elevated **as **frames **into data link layer
2. Data Link Layer: **Header **and **trailer **are read by data link layer and removed
3. Network Layer: Header of packet is **read **to determine if it is correct destination → **Header **is removed
4. Transport Layer: **Header **of each segment read to determine sequence number to arrange segments → **Header **is removed
5. Session Layer determines if this is the end of this session → Requests to end session if so → Else session layer waits for more data
6. Presentation Layer: Decompression, **decryption **and translation
7. Application Layer: Received by user