--- title: "Communication protocol and OSI architecture" subject: "Telecommunications" chapter: 8 key: "semester_8/telecommunications:protocol-osi-architecture" source: https://www.bctengineer.com/q/semester_8/telecommunications/protocol-osi-architecture site: BCT Engineering --- # Communication protocol and OSI architecture Telecommunications · Chapter 8: Data Communication - Asked in: 2068 Bhadra (old course), 2+14 marks - Video: [What is OSI Model? full Explanation | Networking](https://www.youtube.com/watch?v=1msEo8PIcbw) — Learn Coding - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/protocol-osi-architecture - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-8 ## Question What is communication protocol? Explain the OSI architecture used in data communication. ## Short answer A protocol is a set of rules governing data exchange, specifying syntax, semantics, and timing. The OSI model has seven layers: Physical (bit transmission), Data Link (framing, MAC), Network (addressing/routing), Transport (end-to-end reliability), Session, Presentation, and Application. ## Answer #### Communication protocol A **communication protocol** is a set of rules and conventions that govern the exchange of data between two communicating entities. Both ends must follow the same protocol to understand each other. A protocol defines three key elements: - **Syntax:** format of data and control information (field order and sizes, e.g. a header of 20 bytes). - **Semantics:** meaning of each field and the action to take (e.g. what an ACK or error code means). - **Timing:** when and how fast data is sent, sequencing and speed matching. Examples: HDLC (data link), IP (network), TCP (transport), HTTP and SMTP (application), Q.931 in ISDN. #### OSI reference model The **Open Systems Interconnection (OSI)** model was developed by **ISO** (ISO 7498, 1984) so that systems from different vendors can communicate. It divides the communication task into **seven layers**. Each layer performs a defined set of functions, uses the services of the layer below and gives services to the layer above. Layers on two machines talk to each other by **peer protocols**, while actual data moves down one stack, across the medium and up the other. Layering principles: a layer for each different level of abstraction, well-defined functions, small interface flow across boundaries, and enough layers to separate functions but not too many. ```text Host A Host B +----------------+ +----------------+ | 7 Application |<---------------->| 7 Application | | 6 Presentation |<---------------->| 6 Presentation | | 5 Session |<---------------->| 5 Session | | 4 Transport |<---------------->| 4 Transport | | 3 Network |<->[3]<---->[3]<->| 3 Network | | 2 Data link |<->[2]<---->[2]<->| 2 Data link | | 1 Physical |<->[1]<---->[1]<->| 1 Physical | +-------+--------+ router router +-------+--------+ +========= physical medium =========+ ``` Layers 1–3 are **network-dependent** (also run in intermediate nodes); layers 4–7 are **end-to-end** (only in hosts). #### 1. Physical layer - Sends the raw **bit stream** over the medium. - Defines mechanical (connectors, pins), electrical (voltage levels, bit duration), functional (meaning of pins) and procedural specifications. - Line coding, bit synchronisation, data rate, simplex/half/full duplex, topology. - Examples: RS-232/V.24, X.21, 10Base-T, ISDN I.430. Data unit: **bit**. #### 2. Data link layer - Turns the raw line into a reliable link between **adjacent nodes**. - **Framing** (start/end of frame, bit stuffing), physical (MAC) addressing. - **Error control** with CRC and ARQ (retransmission), **flow control** (sliding window). - In LANs it is split into **LLC** and **MAC** (medium access, e.g. CSMA/CD) sublayers. - Examples: HDLC, LAPB, LAPD, PPP, IEEE 802.3. Data unit: **frame**. #### 3. Network layer - Delivers packets from **source host to destination host** across many networks. - **Logical addressing** (IP address), **routing** (choosing paths), switching, congestion control, fragmentation, internetworking. - Connection-oriented (virtual circuit) or connectionless (datagram) service; accounting. - Examples: IP, X.25 packet layer, ICMP. Data unit: **packet**. #### 4. Transport layer - **End-to-end (process-to-process)** reliable delivery of the complete message. - Segmentation and reassembly, **port addressing**, end-to-end error and flow control, connection management, multiplexing several sessions on one network connection. - Hides network details from upper layers; gives the quality of service required. - Examples: TCP, UDP, ISO TP0–TP4. Data unit: **segment**. #### 5. Session layer - Establishes, manages and terminates **sessions** (dialogues) between applications. - **Dialogue control:** who talks when (half/full duplex, token management). - **Synchronisation:** inserts checkpoints so that after a failure transfer restarts from the last checkpoint, not from the start. - Examples: RPC, NetBIOS, ISO 8327. #### 6. Presentation layer - Concerned with **syntax and semantics** of the information exchanged. - **Translation** between different codes (ASCII, EBCDIC) and data formats (ASN.1). - **Encryption/decryption** for security and **compression** to reduce bits. - Examples: JPEG, MPEG, SSL/TLS encryption, ASN.1. #### 7. Application layer - Gives the user and application programs access to network services. - Services: file transfer and access (FTAM, FTP), e-mail (X.400, SMTP), directory (X.500), virtual terminal, web (HTTP), network management. #### Data flow and encapsulation Each layer adds its own **header** (and the data link layer also a trailer) to the data from the layer above; the receiver removes them in reverse order. ```text Data Layer 7 AH|Data 7 (headers PH|AH|Data 6 added at SH|PH|AH|Data 5 each layer) TH|SH|PH|AH|Data = segment 4 NH|TH|SH|PH|AH|Data = packet 3 DH|NH|....|Data|DT = frame 2 0110100101110... = bits 1 ``` #### Summary table | Layer | Main function | Data unit | Device/example | |---|---|---|---| | 7 Application | User services | Message | HTTP, FTP, X.400 | | 6 Presentation | Translate, encrypt, compress | Message | ASCII, JPEG, TLS | | 5 Session | Dialogue, checkpoints | Message | RPC, NetBIOS | | 4 Transport | End-to-end reliability | Segment | TCP, UDP | | 3 Network | Routing, logical address | Packet | IP, router | | 2 Data link | Framing, error/flow control | Frame | HDLC, bridge | | 1 Physical | Bits on medium | Bit | RS-232, repeater | #### Advantages of the layered OSI model - Divides a complex task into small, manageable parts. - Changes in one layer do not affect others (modularity). - Equipment from different vendors can work together (open standards). - Clear framework for teaching, design and troubleshooting. The OSI model is mainly a reference model; practical networks mostly use the TCP/IP suite, whose layers map onto OSI (application ≈ 5–7, transport = 4, internet = 3, network access = 1–2). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which set correctly lists the three key elements a communication protocol defines? A. Syntax, semantics, timing B. Syntax, routing, addressing C. Framing, timing, encryption D. Semantics, topology, timing Answer: A. A protocol defines syntax (format), semantics (meaning and action) and timing (when and how fast data is sent). (Key point: Three elements of a protocol) 2. Which protocol element decides what an ACK or an error code means and what action follows? A. Syntax B. Timing C. Semantics D. Encapsulation Answer: C. Semantics gives meaning to each field and the action to take; syntax is only the format. (Key point: Semantics) 3. In the OSI model, which layers also run in intermediate nodes such as routers? A. Layers 4 to 7 B. Layers 1 to 3 C. Layers 5 to 7 only D. Layer 4 only Answer: B. Layers 1-3 are network-dependent and run in intermediate nodes; layers 4-7 are end-to-end and run only in hosts. (Key point: Network-dependent vs end-to-end layers) 4. What is the purpose of synchronisation checkpoints in the session layer? A. To encrypt the data B. To route packets around congestion C. To restart a transfer from the last checkpoint after a failure D. To convert ASCII to EBCDIC Answer: C. Checkpoints let a transfer resume from the last one instead of from the start after a failure. (Key point: Session layer checkpoints) 5. Which is the correct pairing of layer and data unit? A. Network - frame; Data link - packet B. Transport - segment; Network - packet; Data link - frame C. Transport - packet; Network - segment D. Physical - frame; Data link - bit Answer: B. Transport gives segments, network packets, data link frames and physical bits; each lower layer adds its header (data link also a trailer). (Key point: Encapsulation and data units) ## Related in Telecommunications - [Short note: OSI architecture](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ashort-note-osi-architecture) — Telecommunications, chapter 8 - [Communication protocol and its need](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Acommunication-protocol-need) — Telecommunications, chapter 8, asked 3× - [Switching techniques in data communication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aswitching-techniques-data-comm) — Telecommunications, chapter 8, asked 3× - [Flow control mechanisms in data communication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aflow-control-mechanisms) — Telecommunications, chapter 8 - [SS7 vs OSI; MTP-L2 signal units](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-vs-osi-mtp2-units) — Telecommunications, chapter 5 - [Short note: Routing algorithm in data communication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Arouting-algorithm-data-comm) — Telecommunications, chapter 8, asked 2× ## Related in other subjects - [I/O functions used with data files](https://www.bctengineer.com/question.md?key=semester_1%2Fcomputer_programming%3Afile-io-functions) — Computer Programming, chapter 10, asked 2× - [Data warehouse architecture and components](https://www.bctengineer.com/question.md?key=semester_7%2Fdata_mining%3Adw-architecture-components) — Data Mining, chapter 1 - [National data warehouse process architecture](https://www.bctengineer.com/question.md?key=semester_7%2Fdata_mining%3Anepal-national-data-warehouse) — Data Mining, chapter 1 - [Architecture of data mining system](https://www.bctengineer.com/question.md?key=semester_7%2Fdata_mining%3Adm-system-architecture) — Data Mining, chapter 1