Encapsulation, headers, PDU names, addresses and devices per layer

1. Which OSI layer is host-to-host? Which is process-to-process?
   Stuck? Peek at 1.1.2 The OSI 7 layers and the TCP/IP model

   • 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.

   Read lesson 1.1.2

2. Service vs protocol in one line each?
   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

3. Bits in 1500 bytes?
   Stuck? Peek at 0.2.1 Bits, bytes and prefixes: powers of 10 for rates, powers of 2 for sizes

   • 1 B = 8 b. Lower-case b = bit, upper-case B = byte.
   • Rates: k = 10³, M = 10⁶, G = 10⁹ (bps).
   • Sizes: K = 2¹⁰, M = 2²⁰, G = 2³⁰ (bytes) — unless the question says 1000.
   • Time to push out = (size in bits) ÷ (rate in bit/s).
   • ms = 10⁻³ s, µs = 10⁻⁶ s, ns = 10⁻⁹ s.

   Read lesson 0.2.1

Put a letter in an envelope, put that envelope in a courier pouch, put the pouch in a mail bag on a truck. Each wrapper carries the address its handler needs — the truck driver reads only the bag label, never your letter. A message moving down a network stack is wrapped the same way, once per layer. GATE expects you to name each wrapping, size it, and say which box reads which label.

A 500-byte message passes down through 5 layers and each adds 20 bytes of its own information. How big is it on the wire, and what fraction is your actual message?

500 + 5 × 20 = 600 bytes; your message is 500/600 ≈ 83.3 % of what is sent. The rest is per-layer bookkeeping called headers.

Encapsulation and decapsulation

A layer never looks inside its payload: to it, the upper layer's header and data are just bytes to carry.

The PDU names

The unit a layer exchanges with its peer is its PDU (protocol data unit). The names are fixed exam vocabulary:

What a header carries

A header holds what the peer needs to do its job: addresses, lengths, sequence or type information, and check values. Each layer's header carries its own kind of address:

Which device works at which layer

A box "works at" the highest layer whose header it reads to decide what to do:

A frame on the wire

Putting it together, a frame crossing one link looks like this (sizes illustrative):

Putting it together

• Each layer wraps the unit from above with its own header (data link adds a trailer too).
• PDUs: message, segment, packet/datagram, frame, bits.
• A device works at the highest layer whose header it reads: hub 1, switch 2, router 3.

• The data link layer adds a trailer too; the other layers add only headers.
• "Packet" and "datagram" both name the network-layer PDU; "segment" is transport; "frame" is data link.
• Link-layer addresses change at every hop; network-layer addresses stay the same from source to destination.
• A switch reads only the link header — it never decides routes between networks.

"My Sister Pours Fizzy Bubbles": Message, Segment, Packet, Frame, Bits — top to bottom.

We can now describe our message on the wire as a frame wrapping a packet wrapping a segment wrapping a message. Next (Ch 1.2): between our laptop and the server there are many routers — does the network reserve a whole path for us in advance like a phone call, or do our packets find their own way?

1. MCQ

   The PDU of the network layer is called a:

   1. frame
   2. segment
   3. packet (datagram)
   4. message
2. NAT

   A 900-byte message passes down 4 layers, each adding a 25-byte header. Total bytes sent?

   Write your answer before opening the key.

3. MSQ

   Which statements are correct?

   1. A switch reads the link-layer header.
   2. A router reads the network-layer header.
   3. A hub reads the network-layer header.
   4. The link layer usually adds a trailer as well as a header.
4. MCQ

   As a packet crosses three routers from source to destination, which address normally stays the same?

   1. The link-layer address
   2. The network-layer address
   3. Both
   4. Neither
5. NAT

   Data 1460 B, headers 20 B + 20 B + 14 B, trailer 4 B. Fraction of the frame that is data (3 decimals)?

   Write your answer before opening the key.

• Encapsulation: payload + header (+ trailer at data link); decapsulation in reverse.
• PDUs: message (app) → segment (transport) → packet/datagram (network) → frame (data link) → bits.
• Addresses: link (per hop, changes) · network (end to end, fixed) · which-program id · names.
• Devices: hub/repeater 1 · bridge/switch 2 · router 3 · gateway/host up to 7.
• Efficiency = data / (data + all headers + trailer).

• encapsulation / decapsulation
• header, trailer, payload
• PDU
• message (application PDU)
• segment (transport PDU)
• packet / datagram (network PDU)
• frame (data link PDU)
• link-layer address (changes per hop)
• network-layer address (end to end)
• repeater, bridge, gateway (device layers)

1. PDU names top to bottom?
   Reveal answer

   Message, segment, packet/datagram, frame, bits.
2. Which layer adds a trailer?
   Reveal answer

   Data link.
3. Highest layer of a switch? a router?
   Reveal answer

   Switch 2, router 3.
4. Which address changes at every hop?
   Reveal answer

   The link-layer address.

• Efficiency after encapsulation = data / (data + Σ headers + trailers). — one message carried in one unit at each layer (no splitting into smaller pieces).
• Device layer = highest header it must read: hub 1, switch 2, router 3. — classic device-to-layer matching questions.