Layered Models and Physical Media

Why layers

Networking is described in layers so that each can be changed independently: a web application does not care whether it runs over fibre, copper or wireless. Two models are in common use.

The OSI model

1. Physical — cables, connectors, radio, voltages and signalling.
2. Data link — frames on a local link; MAC addresses; Ethernet and wireless.
3. Network — logical addressing and routing between networks; IP.
4. Transport — end-to-end delivery; TCP and UDP; ports.
5. Session — establishing and managing conversations.
6. Presentation — encoding, encryption, compression.
7. Application — the protocols applications use: HTTP, SMTP, DNS.

The practical TCP/IP model collapses these into four: link, internet, transport and application. The OSI numbering survives because engineers use it as shorthand — “a layer 2 problem” means the local link, “layer 3” means routing and IP.

Encapsulation

Data is wrapped as it descends the stack: application data gets a transport header (a TCP segment), then an IP header (a packet), then a frame header and trailer for the local link. Each device strips only the layers it needs. A switch reads the frame; a router reads the packet; only the destination host reads the application data.

Copper and fibre

• Twisted pair — the standard structured cabling. Category 5e and 6 support gigabit; category 6a supports 10 gigabit. The practical limit for a run is one hundred metres including patch leads, and exceeding it produces intermittent faults that are very hard to diagnose.
• Fibre — immune to electromagnetic interference, far greater distance, and electrically isolating, which matters in industrial environments and between buildings. Single-mode for long distance, multi-mode for shorter runs.
• Termination quality — most physical faults are bad terminations, damaged cable, or dirty fibre connectors. Certify structured cabling when it is installed and keep the results.

Wireless

• Wi-Fi is a shared medium: all devices on a channel take turns, so a busy access point slows everyone regardless of the internet connection behind it.
• The 2.4 GHz band travels further and penetrates walls better but has few non-overlapping channels and heavy interference. The 5 GHz and 6 GHz bands offer more capacity over shorter range.
• Coverage design means planning access point placement, channel allocation and power — not adding another access point wherever someone complains, which usually increases interference.
• Use WPA3 where supported, WPA2 with a strong passphrase otherwise, and enterprise authentication with individual credentials rather than a shared key for organisational networks.
• Guest access should be isolated from internal networks entirely.