The OSI 7 layers and the TCP/IP model

1. Same layer on two machines follows a ___; neighbouring layers on one machine meet at an ___.
   Stuck? Peek at 1.1.1 Why layer: protocol, service, interface, peers

   • Layer: one job in the stack; serves the layer above, uses the layer below.
   • Protocol: rules between peers (same layer, different machines): format, meaning, behaviour.
   • Service: what a layer does for the layer above. Interface: how the layer above asks for it.
   • Peer talk is logical; the real path is down → link → up.
   • Layering = modularity + reuse − some overhead.

   Read lesson 1.1.1

2. 1's complement of 10110011?
   Stuck? Peek at 0.2.3 XOR, 1's complement addition and modulo-2 division

   • AND (mask), OR, NOT (flip), XOR (1 where bits differ).
   • a ⊕ a = 0, a ⊕ 0 = a, (a ⊕ k) ⊕ k = a.
   • 1's complement = flip all bits; 1's-complement addition wraps the end carry back in.
   • Modulo-2: + = − = XOR, no carries; bits ↔ polynomial.
   • Mod-2 division: leading 1 → XOR divisor; leading 0 → XOR zeros; remainder = (divisor length − 1) bits.

   Read lesson 0.2.3

3. Hub, switch, router — which one joins different networks?
   Stuck? Peek at 0.1.2 Nodes, links, topologies, LAN/WAN and the boxes in the middle

   • Node = any device; host = node where messages start/end.
   • Point-to-point (2 nodes) vs broadcast (all hear all).
   • Simplex (one way) · half-duplex (two ways, one at a time) · full-duplex (both at once).
   • Links: mesh n(n−1)/2, star n, ring n, tree n−1, bus 1 backbone.
   • LAN (building) ⊂ WAN (cities) ⊂ the Internet (network of networks).
   • Hub repeats to all · switch delivers within a LAN · router picks the next network.

   Read lesson 0.1.2

In the 1970s every company built its own incompatible network stack. The international standards body (ISO) answered with a reference model of seven layers: OSI. Meanwhile the Internet grew on a leaner stack named after its two best-known protocols, TCP/IP. GATE asks about both — which layer does what, and how the two models line up.

Put these jobs in order from the wire upwards: (1) choose a route across many networks; (2) turn bits into signals; (3) deliver to the right program on a host; (4) move data to the next directly connected box.

2 → 4 → 1 → 3: physical (signals), data link (next box), network (route across networks), transport (right program, end to end). The application sits above them.

The seven OSI layers

From the wire up (mnemonic bottom-up: Please Do Not Throw Sausage Pizza Away):

Responsibility of each layer

• Physical: bit-level signalling, voltages, cables, connectors, bit rate.
• Data link: delivery between neighbouring nodes on one link, detecting damaged bits, deciding who uses a shared link.
• Network: delivery from the source host to the destination host across many links, choosing routes.
• Transport: delivery from a process (a running program) on one host to a process on the other; reliability and pacing end to end.
• Session: opening, synchronising and closing a dialogue.
• Presentation: translating formats, compression, encryption.
• Application: the services programs use directly.

Hop-to-hop vs end-to-end

Layers 1–3 run on every node along the path, including routers in the middle: they work hop by hop. Layers 4–7 run only on the two end hosts: they work end to end. This is why a router cares about the network layer but never looks at the transport layer's business.

The TCP/IP (Internet) model

The Internet's practical stack has 5 layers in most textbooks (some count 4 by merging physical and data link into one "link" layer):

Why two models?

OSI is a clean reference for naming and teaching layers; TCP/IP is what the Internet actually runs. The name "TCP/IP" comes from two protocols you meet in Phases 3 and 4; for now it is just the model's name.

Putting it together

• OSI has 7 layers; TCP/IP has 5 (top three OSI layers merged into application).
• Layers 1–3 are hop by hop and run on routers; 4–7 are end to end and run only on hosts.
• Transport = process to process; network = host to host; data link = hop to hop.

• Process-to-process is transport, host-to-host is network — GATE swaps them in options.
• Encryption and compression are presentation in OSI, not application.
• TCP/IP folds session and presentation into the application layer — they do not disappear, they move into programs.
• "Data link = hop to hop", not end to end.

"Please Do Not Throw Sausage Pizza Away" = Physical, Data link, Network, Transport, Session, Presentation, Application (bottom to top).

We know which layer does what. Next (1.1.3): when a message moves down the stack, each layer adds its own bookkeeping to it — what does the message look like on the wire, what is each layer's unit called, and which boxes look at which part?

1. MCQ

   Process-to-process delivery is the responsibility of the:

   1. network layer
   2. transport layer
   3. data link layer
   4. session layer
2. MSQ

   Which OSI layers does a router implement?

   1. Physical
   2. Data link
   3. Network
   4. Transport
3. MCQ

   In the 5-layer TCP/IP model, OSI's session and presentation duties belong to the:

   1. transport layer
   2. application layer
   3. network layer
   4. data link layer
4. MCQ

   Encryption of data in the OSI model belongs to:

   1. Application
   2. Session
   3. Presentation
   4. Transport
5. NAT

   How many OSI layers run only on end hosts (not on routers)?

   Write your answer before opening the key.

• OSI bottom-up: Physical, Data link, Network, Transport, Session, Presentation, Application.
• Scope: link → hop → host-to-host → process-to-process.
• Routers: layers 1–3; hosts: 1–7.
• TCP/IP (5): application (= OSI 5–7), transport, network, data link, physical.
• Encryption/compression = presentation; dialogue control = session.

• OSI model (7 layers)
• physical / data link / network / transport / session / presentation / application layers
• TCP/IP (Internet) model, 5 layers
• process (a running program)
• hop-to-hop vs end-to-end

1. OSI layers bottom-up?
   Reveal answer

   Physical, Data link, Network, Transport, Session, Presentation, Application.
2. Host-to-host vs process-to-process?
   Reveal answer

   Network vs transport.
3. Layers on a router?
   Reveal answer

   1–3.
4. Where does encryption sit in OSI?
   Reveal answer

   Presentation (6).

• Scope ladder: link (physical) → hop (data link) → host-to-host (network) → process-to-process (transport). — deciding which of layers 1–4 a delivery job belongs to.
• Routers = layers 1–3; hosts = all layers. — classic middle-of-path questions (bridges/switches stop at 2, repeaters at 1 — next lesson).