4.9.2b Network Protocols — AQA 7517

The OSI Model — 7 Layers

The OSI (Open Systems Interconnection) model is a conceptual framework that describes how data travels from one device to another across a network. Each layer handles a specific task and communicates with the layers above and below it.

Layer	Name	Protocols/Examples	Function
7	Application	HTTP, HTTPS, FTP, SMTP, DNS, POP3	Interface for network services used by applications
6	Presentation	SSL/TLS, JPEG, MPEG, ASCII	Data formatting, encryption, compression
5	Session	NetBIOS, PPTP	Establishes, maintains, terminates communication sessions
4	Transport	TCP, UDP	End-to-end communication, flow control, error correction
3	Network	IP, ICMP, routers	Logical addressing (IP), routing packets between networks
2	Data Link	Ethernet, Wi-Fi MAC, switches	Physical addressing (MAC), framing, error detection
1	Physical	Cables, NICs, hubs, repeaters	Bit transmission over physical medium (electrical/optical/radio)

Mnemonic (bottom to top): Please Do Not Throw Sausage Pizza Away

TCP vs UDP

Feature	TCP (Transmission Control Protocol)	UDP (User Datagram Protocol)
Connection	Connection-oriented (3-way handshake)	Connectionless
Reliability	Guaranteed delivery — lost packets retransmitted	No guarantee — packets may be dropped
Order	Packets reordered at destination	No ordering
Speed	Slower (overhead)	Faster (low overhead)
Use cases	Web browsing, email, file transfer	Video streaming, online gaming, VoIP, DNS

TCP 3-way handshake: SYN → SYN-ACK → ACK. Used to establish a reliable connection before data transfer begins.

Key Application Layer Protocols

Protocol	Port	Purpose
HTTP	80	HyperText Transfer Protocol — requesting/serving web pages (unencrypted)
HTTPS	443	HTTP Secure — HTTP encrypted via TLS/SSL
FTP	20/21	File Transfer Protocol — transferring files between client and server
SMTP	25/587	Simple Mail Transfer Protocol — sending email
POP3	110	Post Office Protocol — downloading email from server (deletes from server)
IMAP	143	Internet Message Access Protocol — email stored on server, synced across devices
DNS	53	Domain Name System — resolves domain names to IP addresses
SSH	22	Secure Shell — encrypted remote access to command line

Encapsulation and Decapsulation

As data passes down the OSI layers, each layer adds its own header (and sometimes trailer) — this is called encapsulation. At the destination, each layer strips its header in reverse — this is decapsulation.

Application data → Transport adds TCP/UDP header → Network adds IP header → Data Link adds MAC frame header → Physical sends as bits
At destination: each layer processes and strips its header and passes data up

Exam tip: AQA 7517 expects you to name OSI layers in order (bottom to top or top to bottom), give the function of each layer, and identify which protocol operates at which layer. Learn the port numbers for common protocols — questions like "A firewall blocks port 443, what does this affect?" require this knowledge. TCP vs UDP: know when each is appropriate and why.

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Worksheet — 4.9.2b The OSI Model & Protocols

8 questions · instantly marked · AQA 7517 standard

Q1Name the seven layers of the OSI model, from layer 1 to layer 7.[4]

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1 mark each: Layer 1 Physical [1]; Layer 2 Data Link [1]; Layer 3 Network [1]; Layer 4 Transport [1]; Layer 5 Session [1]; Layer 6 Presentation [1]; Layer 7 Application [1].

Q2Explain the difference between TCP and UDP. Give one use case for each.[4]

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TCP: connection-oriented [1]; guarantees delivery by retransmitting lost packets [1]; packets are reordered at destination [1]; use case: web browsing, email, file download [1]. UDP: connectionless [1]; no guaranteed delivery — packets may be lost [1]; faster due to low overhead [1]; use case: video streaming, online gaming, VoIP [1].

Q3State the function of the Network layer (Layer 3) of the OSI model. Name the main protocol at this layer.[2]

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Network layer: responsible for logical addressing (IP addresses) [1] and routing data packets between networks [1]; the main protocol is IP (Internet Protocol) [1].

Q4Explain what is meant by encapsulation in the context of the OSI model.[2]

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As data passes down the OSI layers, each layer adds its own header (and sometimes trailer) containing control information [1]; this wraps the data from the layer above [1]; so by the time data reaches the Physical layer, it has headers from all layers added to it [1]. At the destination, each layer strips its header (decapsulation).

Q5State the port numbers for HTTP, HTTPS, and FTP.[2]

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HTTP: port 80 [1]; HTTPS: port 443 [1]; FTP: port 20 (data) and 21 (control) [1].

Q6Explain why IMAP is generally preferred over POP3 for users who access their email from multiple devices.[2]

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IMAP stores email on the server and syncs across all devices [1]; so if you read an email on your phone, it is marked as read on your laptop too [1]. POP3 downloads email to a single device and typically deletes it from the server [1], so other devices cannot access the same emails.

Q7Explain the TCP three-way handshake. Why is it needed?[2]

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Three-way handshake: (1) Client sends SYN packet to server [1]; (2) Server responds with SYN-ACK [1]; (3) Client sends ACK to confirm [1]. Needed: to establish a reliable connection and synchronise sequence numbers before data transfer begins, ensuring both sides are ready [1].

Q8A school network administrator blocks port 443 on the firewall. What effect will this have? Justify your answer.[2]

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Port 443 is used by HTTPS [1]; blocking it will prevent users from accessing secure (encrypted) websites [1]; most modern websites use HTTPS so this would block the majority of web browsing [1]; users may still access unencrypted HTTP (port 80) but this is unsecure and most sites redirect HTTP to HTTPS [1].

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The OSI Model — 7 Layers

TCP vs UDP

Key Application Layer Protocols

Encapsulation and Decapsulation

Worksheet — 4.9.2b The OSI Model & Protocols

Mini Test — OSI Model & Protocols