`ACtE0501` Introduction to computer networks and physical layer¶

A computer network is a set of interconnected devices that exchange data using agreed rules called protocols.
A protocol specifies syntax, timing, sequencing, and error handling.
A standard is a formally documented specification so equipment from different vendors can interoperate.
A layer hides internal details from the layer above and offers services upward.
Encapsulation means each lower layer adds its own header, and sometimes trailer, around upper-layer data.

OSI layer	Main role	Typical examples	Closest TCP/IP layer
Application	User-facing network services	HTTP, SMTP, DNS	Application
Presentation	Data format, compression, encryption	TLS record formatting, character encoding	Usually folded into Application
Session	Dialog control, synchronization	Session setup, checkpoints	Usually folded into Application
Transport	End-to-end delivery	TCP, UDP	Transport
Network	Logical addressing, routing	IP, ICMP	Internet
Data link	Framing, MAC addressing, local delivery	Ethernet MAC, PPP, 802.11 MAC	Link / Network access
Physical	Bits on medium	Copper, fiber, radio	Link / Network access

Decisive cues:

OSI has 7 layers; TCP/IP is usually taught as 4 or 5 layers.
TCP/IP is the deployed Internet architecture; OSI is the conceptual reference model.
Do not treat OSI and TCP/IP as one-to-one exact protocol stacks.
FTP and Telnet are application-layer protocols carried over TCP, so Application is the highest OSI layer named by their use.

Body or family	Common association
ISO	OSI reference model
IEEE 802	LAN and MAN standards such as 802.3 and 802.11
IETF	Internet RFCs: IP, TCP, UDP, BGP, TLS-related specs
ITU-T	Telecommunication recommendations

Device	Typical OSI layer	Forwarding basis	Collision domain	Broadcast domain
Hub	Physical	Repeats all bits	One shared domain	One shared domain
Bridge	Data link	MAC address	Separates collisions	Usually same broadcast domain
Switch	Data link	MAC address table	One per port	Same VLAN is one broadcast domain
Router	Network	IP routing table	Separates collisions	Separates broadcast domains

Recognition traps:

A hub does not inspect addresses.
A switch is not a router; a switch forwards frames within a LAN, while a router forwards packets between networks.
A bridge and a Layer-2 switch are conceptually similar, but the switch has many ports and hardware forwarding.

Physical layer defines signaling, voltage or optical levels, modulation at bit level, connectors, and bit timing.
Units commonly tested: bit rate in b/s, symbol rate in baud, bandwidth in Hz.
Bit rate equals symbol rate only when one symbol carries one bit.

Medium	Type	Main strengths	Main limitations
Twisted pair	Guided copper	Cheap, easy, LAN standard	More attenuation and EMI than fiber
Coaxial cable	Guided copper	Better shielding than twisted pair	Bulkier
Optical fiber	Guided optical	Very high bandwidth, low attenuation, EMI immune	Higher installation precision
Radio / microwave	Unguided	Mobility, wireless coverage	Interference, fading, regulation
Infrared	Unguided	Short-range line-of-sight links	Blocked easily, short range

Baseband means the original signal occupies the medium directly.
Broadband means multiple channels occupy different frequency bands.
Fiber uses light, not electric current, so it is immune to electromagnetic interference.
Copper media are more vulnerable to attenuation, crosstalk, and external noise.

If a question asks which device breaks broadcast domains, choose router, not switch.
If a question asks which model has seven layers, choose OSI.
If a question asks which medium is immune to EMI, optical fiber is the usual answer.

OSI is the conceptual 7-layer model; TCP/IP is the deployed Internet stack.
Hub repeats bits, switch uses MAC, router uses IP.
Physical layer handles signaling on the medium, not routing.
Fiber: high bandwidth, low attenuation, EMI immunity.
Protocol defines communication rules; standard defines the agreed public specification.

ACtE0501 Introduction to computer networks and physical layer¶