📡 A Level · Chapter 14
Communication & Internet Technologies
Protocols · TCP/IP · Circuit vs Packet Switching
14.1 Protocols
What is a protocol?
A protocol is an agreed set of rules that governs how data is formatted, transmitted, received and acknowledged between devices. Without protocols, two computers from different manufacturers could never communicate — it’s like agreeing on a language before holding a conversation.
Why are protocols essential?
- Devices are made by different manufacturers
- Ensures data is sent/received in a compatible format
- Defines error checking, flow control, addressing
- Allows interoperability across the entire internet
TCP/IP Protocol Suite — The Four-Layer Stack
Protocol implementations are modelled as layers. Each layer has specific responsibilities and only communicates with the layers directly above and below it.
4
Application Layer
Closest to the user. Provides the interface for network-aware apps. Handles data formatting and presentation.
HTTP/HTTPS
FTP
SMTP
POP3
IMAP
BitTorrent
3
Transport Layer
Breaks data into segments. Manages end-to-end communication, error checking, flow control. TCP provides reliability; UDP is faster but unreliable.
TCP
UDP
2
Internet / Network Layer
Handles logical addressing (IP addresses) and routing of packets across multiple networks. Determines the best path.
IP (IPv4/IPv6)
ICMP
1
Link Layer (Network Access)
Physical transmission of data on the local network. Converts data to signals for the medium (copper, fibre, wireless).
Ethernet
Wi-Fi (802.11)
MAC
How layers work together
When you send an email: Application (SMTP formats email) → Transport (TCP segments it) → Internet (IP adds source/dest addresses) → Link (Ethernet converts to electrical signals). On the other end, each layer unwraps its header in reverse.
Key Application Layer Protocols
HTTP (HyperText Transfer Protocol) — transfers web pages from server to browser. Stateless: each request is independent.
HTTPS adds TLS/SSL encryption — all data is encrypted in transit. Used wherever security matters (banking, login pages).
Client → Server: GET /index.html HTTP/1.1
Server → Client: 200 OK (sends HTML page)
FTP (File Transfer Protocol) — transfers files between a client and server. Uses two separate connections: one for control commands, one for data. Supports authentication (username/password).
Note
FTP sends credentials in plain text — SFTP or FTPS adds encryption for secure file transfer.
| Protocol | Purpose | Direction |
|---|---|---|
| SMTP | Simple Mail Transfer Protocol — sends email from client to server, or between servers | Outgoing |
| POP3 | Post Office Protocol 3 — downloads email to device and (by default) deletes from server | Incoming |
| IMAP | Internet Message Access Protocol — accesses email on server; stays synced across multiple devices | Incoming |
POP3 vs IMAP
POP3: emails live on your device. IMAP: emails live on the server, accessible from anywhere. Most modern apps (Gmail, Outlook) use IMAP.
BitTorrent is a peer-to-peer (P2P) protocol — files are distributed across many users rather than downloaded from one central server.
- File is split into many small chunks/pieces
- Each downloader simultaneously uploads pieces they already have
- A tracker coordinates who has which pieces
- As more people download, the network gets faster (not slower)
Analogy
Instead of one person handing you a full textbook, 50 classmates each give you a different chapter simultaneously — and you share pages you already have while receiving others.
14.2 Circuit Switching vs Packet Switching
There are two fundamental approaches to routing data across a network:
🔌 Circuit Switching
- Dedicated physical path established before transmission
- Path reserved for entire duration
- Guaranteed bandwidth
- No routing delays mid-call
- Inefficient — line idle during silence
- If path fails → connection drops
Telephone PSTN ISDN
VS
📦 Packet Switching
- Data split into independent packets
- Each packet routed independently
- Packets may take different routes
- Reassembled at destination
- Efficient — bandwidth shared dynamically
- Resilient — packets reroute around failures
Internet TCP/IP
Anatomy of a Packet
Every packet contains a header with routing info, the data payload, and error-checking bits:
Source
IP
IP
Dest.
IP
IP
Seq.
No.
No.
DATA PAYLOAD
Check-
sum
sum
| Field | Purpose |
|---|---|
| Source IP | Where the packet came from |
| Destination IP | Where the packet is going |
| Sequence number | Allows reassembly in correct order |
| Data payload | The actual content (fragment of the original message) |
| Checksum | Error detection — receiver checks if data was corrupted in transit |
The Role of a Router in Packet Switching
Router’s job
A router reads the destination IP address in each packet’s header, consults its routing table, and forwards the packet to the next best hop toward the destination. Routers don’t care about the full path — they just decide the next step, and the packet finds its own way.
- Segmentation: Sending computer splits message into numbered packets
- Header added: Each packet gets source IP, dest IP, sequence number, checksum
- Routing: Each router reads destination IP and forwards to next router (packets may take different routes)
- Arrival: Packets arrive at destination, possibly out of order
- Reassembly: TCP uses sequence numbers to reassemble packets in correct order
- Error checking: If checksum fails or packet lost → TCP requests retransmission
Analogy
Imagine sending a 5-part letter in separate envelopes via different postal routes. Some arrive out of order, one gets delayed. The recipient reassembles them using the numbered pages. That’s packet switching.
⚡ Exam Essentials
- Know all 4 TCP/IP layers and their protocols by name
- Be able to describe the journey of a message from one host to another
- Compare circuit vs packet switching — benefits, drawbacks, use cases
- Explain the role of a router: reading headers and using routing tables
- Know what each field in a packet header is for
- BitTorrent = P2P, not client-server — know why it scales well