ISO-OSI 7-Layer Network Architecture

Physical Layer

Functionality:
1. Hardware Specification:
  - Details of physical cables, network interface cards, wireless radios.
  - Examples: Coaxial Cable, Hybrid Cable, Wireless Card, Network Card.
2. Encoding and Signalling:
  - How bits are encoded in the medium.
  - Modulation techniques, e.g., Binary Phase Shift Keying.
3. Data Transmission and Reception:
  - Responsible for the transfer of each bit of data.
  - Transmission is not completely reliable, and there is no error correction.
4. Topology and Network Design:
  - Integral part of the physical layer.
  - Involves network design, placement of routers, switches, hubs, server locations, and choosing network topologies (ring, bus, star).

Functionality:
1. Framing:
  - Breaks input data into frames, cares about frame boundaries and size.
2. Acknowledgment:
  - Sent by the receiver to inform the source about error-free frame reception.
3. Sequence Numbering:
  - Acknowledges which frame was received.
4. Error Detection:
  - Deals with damaged, lost, or duplicated frames.
5. Retransmission:
  - Retransmits the packet if the source doesn't receive acknowledgment.
6. Flow Control:
  - Regulates the flow of information between fast transmitters and slow receivers.

Functionality:
1. Routing:
  - Determines how packets are routed from source to destination.
  - Types: Static (based on static tables), Dynamic (follows different routes), Semi-Dynamic (chosen at the start of each conversation).
1. Services:
  - Connectionless Service: Treats each packet independently.
  - Connection-Oriented Service: Establishes a connection before all packets follow the same route.
2. Congestion Control:
  - Manages congestion, minimizes packet dropping, and informs the source of dropped packets.
3. Internetworking:
  - Connects multiple networks, acts as one large network.
  - Uses routers, switches, and bridges for connectivity.

Functionality:
1. Multiplexing / Demultiplexing:
  - Creates distinct network connections for each transport connection.
  - May multiplex several transport connections onto the same network connection.
2. Fragmentation and Re-assembly:
  - Splits data into smaller units for transmission and re-assembles at the receiving end.
3. Types of Service:
  - Decides the type of service provided to the session layer.
  - May be perfectly reliable, reliable within tolerances, or unreliable.
4. Error Control:
  - Performs error detection and recovery for reliable services.
5. Flow Control:
  - Regulates the flow of information between fast and slow hosts.
6. Connection Establishment / Release:
  - Establishes and releases connections across the network.
  - Requires a naming mechanism for communication.

Concept of Sessions:
- Authentication required when a user logs in to a remote server.
- Establishes and maintains sessions for data transfer.
Session Management:
- Re-establishes connection if a session breaks down.
- Ensures transparent data transfer restarts from the point of interruption.
Synchronization:
- Introduces checkpoints in case of broken connections.
- Ensures transactions on a database are not lost even if the user has not committed.
Dialogue Control:
- Determines whose turn it is to speak in a session.
- Useful in video conferencing.

Syntax and Semantics:
- Concerned with the syntax and semantics of transmitted information.
- Defines data structures in an abstract way with standard encoding.
Data Exchange:
- Manages abstract data structures and allows the exchange of higher-level structures.
- Encodes data in a standard agreed-upon network format.
Compatibility:
- Converts data transmitted by one machine to be compatible with another.
- Addresses issues arising from different data representations (e.g., 'Big Endian' vs. 'Little Endian').
Additional Functions:
- Handles compression, encryption, and other tasks related to data representation.

User Network Access:
- Contains application protocols that provide user access to the network.
- Choice of protocols and functions is up to the individual user.