Data Transmission in a Network
1. From Message to Bits — the Journey and the Wrapping
A message does not travel as one solid block. It is broken into pieces and each piece is wrapped in headers as it descends the protocol stack:
application data (HTTP request: "GET /index.html")
│ + TCP header
▼
segment ┌────────┬──────────────────────┐
│ TCP │ data │
└────────┴──────────────────────┘
│ + IP header
▼
packet ┌────────┬────────┬─────────────┐
│ IP │ TCP │ data │
└────────┴────────┴─────────────┘
│ + Ethernet header/trailer
▼
frame ┌───────┬────────┬────────┬─────┬─────┐
│ Frame │ IP │ TCP │data │ FCS │
└───────┴────────┴────────┴─────┴─────┘
│
▼
BITS → signal over the medium (copper / fibre / radio)
This process is called encapsulation; the reverse process at the receiver is decapsulation. Each layer only reads the header meant for it — which is exactly what makes a network of completely different machines work.
2. Why Layering?
A layered model divides a complex job into smaller, independent tasks: each layer offers a service to the layer above and uses the services of the layer below, and a change in one layer does not break the others. This gives modularity, easier troubleshooting and standard interfaces (so any vendor's hardware and software can interoperate).
3. The OSI Model (7 layers) and TCP/IP Stack
| # | OSI layer | Function | PDU | Devices/protocols |
|---|---|---|---|---|
| 7 | Application | User services: web, mail, file transfer | Data | HTTP, FTP, SMTP, DNS |
| 6 | Presentation | Translation, encryption, compression | Data | SSL/TLS, JPEG, MPEG |
| 5 | Session | Establish, manage, synchronise dialogues | Data | RPC, NetBIOS, sockets |
| 4 | Transport | End-to-end delivery, segmentation, flow/error control | Segment | TCP, UDP |
| 3 | Network | Logical addressing and routing between networks | Packet | IP, ICMP, ARP, routers |
| 2 | Data link | Framing, MAC addressing, error detection on a link | Frame | Ethernet, Wi-Fi, switches |
| 1 | Physical | Bits on the medium: voltages, pins, cables, radio | Bit | UTP, fibre, hubs, RJ-45 |
TCP/IP mapping: Application (OSI 5-7) · Transport (4) · Internet/Network (3) · Network access/Link (1-2). The Internet uses this four/five-layer practical model, and the OSI model remains the teaching/reference model. Mnemonic for OSI top-down: All People Seem To Need Data Processing.
4. Addressing at Each Layer
| Layer | Address | Size | Identifies | Example |
|---|---|---|---|---|
| Application | URL / domain name | variable | A resource | https://pyqdeck.com/paper |
| Transport | Port number | 16 bits | A process/service on a host | 80 (HTTP), 443 (HTTPS), 53 (DNS), 25 (SMTP) |
| Network | IP address | 32 bits (IPv4) / 128 bits (IPv6) | A host on the internet | 142.250.192.46 |
| Data link | MAC address | 48 bits | A network interface on the LAN | 00:1A:2B:3C:4D:5E |
| Physical | — | — | — | Signals/pins |
A frame on a LAN carries all three: MAC to reach the next hop, IP to reach the final host, port to reach the right process on that host.
5. Switching Techniques
| Basis | Circuit switching | Message switching | Packet switching (datagram) |
|---|---|---|---|
| Path | Dedicated physical path set up first | No path; whole message stored and forwarded | No path; message split into packets, each routed independently |
| Bandwidth use | Wasted while silent | Efficient, but long messages block links | Very efficient — links shared statistically |
| Delay | Fixed setup delay, then low delay | High (store-and-forward per hop) | Variable (jitter), but low average |
| Failure handling | Call drops | Retransmit message | Route around failure, packet by packet |
| Example | Traditional telephone (PSTN) | Old telegraph/telex networks | The Internet (IP), most modern data |
Virtual circuit packet switching (Frame Relay, ATM, MPLS) is a middle ground: packets follow a pre-established logical path with labels, giving predictable ordering (used by ISPs and enterprise WANs).
6. The Journey of a Web Request (worked end-to-end example)
1. User types https://pyqdeck.com in the browser
2. DNS resolves the name → IP address (say 142.250.192.46) [DNS query]
3. TCP three-way handshake with the server: SYN → SYN-ACK → ACK [Transport]
4. TLS handshake establishes encryption [Presentation]
5. HTTP GET /index.html sent; the packet is built [Application]
6. The packet is placed in an Ethernet/Wi-Fi frame with the
default-gateway (router) MAC address as the destination [Data link]
7. The router forwards it hop by hop: each router looks at the IP
address, consults its routing table, decrements TTL, and
rewraps the packet in a NEW frame for the next link [Network]
8. The server's reply travels back along the chosen route
9. Frames are unwrapped bottom-up; the browser renders the page
10. Lost segments are retransmitted; the connection closes (FIN/ACK)
7. Control Mechanisms That Make It Work
- Framing — packet boundaries marked so the receiver knows where one ends and the next begins.
- Error detection — checksums and CRC in the frame trailer; corrupted frames are dropped (Ethernet) or retransmitted (TCP).
- Flow control — the receiver advertises a window so a fast sender cannot swamp a slow receiver.
- Error/retransmission control — sequence numbers plus acknowledgements; timeouts trigger retransmission.
- Congestion control — TCP slow start, congestion avoidance and detection (packet loss) slow the sender when the network is overloaded.
- Ordering — sequence numbers let the receiver reassemble packets that arrive out of order.
- Routing — routers choose paths using static routes or dynamic protocols (RIP, OSPF, BGP).
- Multiplexing — many applications/connections share one link (many ports over one IP, many IPs over one cable).
- TTL (Time To Live) — a hop counter that prevents packets from looping forever; TTL 0 → packet discarded and ICMP "time exceeded" returned.
8. LAN vs WAN Transmission Path
In a LAN, delivery is one or two hops: source → switch → destination, using MAC addresses within one broadcast domain. In a WAN/internet, the same data crosses a dozen or more routers, each making an independent forwarding decision, with higher latency (milliseconds, not microseconds), possible reordering, and paths that may change mid-transfer.
9. Exam Points
- Why is layering used? Explain encapsulation with a diagram — 5-7 marks, very frequent.
- Draw the OSI reference model and state the function of each layer — 7-10 marks, almost certain.
- Map the OSI layers onto the TCP/IP model — 5 marks.
- Explain the addressing used at each layer (port, IP, MAC) with sizes — 5 marks.
- Differentiate circuit switching, message switching and packet switching — 5 marks, very frequent.
- Trace the steps of loading a web page through the layers — 5-7 marks.
- Explain flow control, error control and congestion control — 5 marks.
- What is TTL and why is it needed? — 3 marks.