Computer Networks
OSI Model
Section titled “OSI Model”The OSI model is a seven-layer conceptual framework that standardizes and describes the distinct functions of network communication - from physical bit transmission to application-level services - to guide interoperability, design, and troubleshooting.
| No. | OSI Layer | Responsibilities |
|---|---|---|
| 7 | Application | Application-level services, end-user protocols |
| 6 | Presentation | Data representation, encryption, compression |
| 5 | Session | Session and connection management |
| 4 | Transport | End-to-end communication, reliability, flow control |
| 3 | Network | Logical addressing and routing |
| 2 | Data Link | Framing, MAC addressing, error detection/correction |
| 1 | Physical | Physical medium, bit transmission, connectors |
TCP/IP Model
Section titled “TCP/IP Model”As the OSI model is only conceptual, the TCP/IP is the most used for communication on network
| TCP/IP Layer | Name | Responsibilities | Examples |
|---|---|---|---|
| Application | Application data | Application protocols used by programs. The application layer may live on top of a TLS (encryption) layer | HTTP, FTP, SMTP, DNS |
| Transport | Segment | Host-to-host communication, ports, reliability | TCP, UDP |
| Internet | Datagram/Packet | Logical addressing and routing (IP) | IPv4, IPv6, ICMP |
| Link | Frame | Physical addressing and local delivery | Ethernet, Wi‑Fi |
| (part of Link) | Bits | / | Cables, fiber |
Acronyms for Area Network
Section titled “Acronyms for Area Network”- Personal Area Network (PAN)
- Wireless LAN (WLAN)
- Local Area Network (LAN)
- Metropolitan Area Network (MAN)
- Wide Area Network (WAN)
Access to the medium
Section titled “Access to the medium”Access to the medium defines the mechanisms that allow multiple devices to share and coordinate the use of a communication link.
Deterministic Access Methods
Section titled “Deterministic Access Methods”- FDMA (Frequency Division Multiple Access): Each user is assigned a distinct frequency band for communication.
- TDMA (Time Division Multiple Access): Users share the same frequency band but transmit in different time slots.
- CDMA (Code Division Multiple Access): Each user is assigned a unique code, allowing multiple users to transmit simultaneously over the same frequency band.
- OFDMA (Orthogonal Frequency Division Multiple Access): A variation of FDMA in which the subcarrier frequencies are selected to be orthogonal in the Fourier domain, improving spectral efficiency and reducing interference.
Random Multiple Access Methods
Section titled “Random Multiple Access Methods”-
ALOHA: When a collision occurs, the transmitter waits for a randomly chosen delay before retransmitting.
-
Slotted ALOHA: An enhanced version of ALOHA where time is divided into slots, with
T_slot ≥ T_propagation, reducing the probability of collisions. -
CSMA (Carrier Sense Multiple Access): Stations listen to the channel and transmit only if it is free. If a collision occurs, the signal is corrupted for all stations, and the transmitter waits for a random delay before retrying.
-
CSMA/CD (Carrier Sense Multiple Access with Collision Detection): Transmission and collision detection occur simultaneously. The retransmission delay follows a Binary Exponential Backoff (BEB) algorithm, where the contention window starts at size 2 and doubles after each collision.
-
CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance): An extension of CSMA/CD in which the station continues to sense the channel during the BEB delay to reduce the likelihood of collisions.