D. PHYSICAL MEDIA

The physical medium is the channel through which information is transmitted. It may be a cable (wired medium) or a wireless transmission (wireless medium). The choice of physical medium determines the maximum data rate, the range, susceptibility to interference and the installation cost.

Related media (guided)	Unbound media (unguided)
Twisted pair (UTP/STP/FTP)	Wi-Fi (802.11a/b/g/n/ac/ax/be)
Coaxial cable (10Base5, CATV)	Laser / infrared (FSO)
Optical fibre (MMF, SMF)	Terrestrial and satellite microwaves
Powerline communication (PLC) cable	Mobile network (4G/5G)

Physical media are evaluated on the basis of: available bandwidth, maximum distance before signal regeneration, resistance to electromagnetic interference (EMI/RFI), ease of installation and cost.

1. Optical fibre

Fibre optics represent one of the greatest technological advances in cabling. They transmit light generated by either a laser or an LED (Light-Emitting Diode, LED). They are unaffected by electromagnetic interference and offer complete protection against passive eavesdropping.

a) Fibre structure

An optical fibre consists of three concentric elements:

· The core: the central part of the fibre through which light rays travel. Made of glass (silica) or high-purity plastic.

· The optical cladding: surrounds the core with a material whose refractive index is lower than that of the core. This difference in refractive index confines the propagation of light rays through total internal reflection.

· The protective coating (coating/buffer): an outer layer that protects the optical fibre from physical damage (impact, moisture, crushing).

Fig. 12 — Structure of an optical fibre (core, cladding, coating)

An optical cable can transmit up to several hundred Gbps over distances of several kilometres — something no copper cable can achieve. Today, it is the standard solution for long distances and high data rates. Optical fibres range from 2 to 864 strands in a single cable (one strand per transmission direction, by convention).

 

b) Types of fibre

Multimode fibre (MMF): light rays follow multiple paths (modes) within the core, which causes modal dispersion that limits the transmission distance and data rate. Used for short distances (LAN, data centres). Transmitter: LED (850 nm) or VCSEL laser (850 nm). Standard dimensions: core 50 µm / cladding 125 µm (or 62.5/125 µm). Performance: from 1 Gbps/km (OM1) to 100 Gbps/100 m (OM5). A distinction is made between step-index fibres and gradient-index fibres (sinusoidal wave, better performance).

Fig. 13 — Propagation in a multimode fibre (index gradient and index jump)

Single-mode fibre (SMF): the light rays follow a single path (fundamental mode) within a very narrow core (5 to 10 µm / 125 µm cladding). The transmitter is a laser (1,310 nm or 1,550 nm) providing a very precise signal. Performance: > 100 Gbps/km, usable over tens to hundreds of kilometres. Chromatic dispersion (different wavelengths propagate at slightly different speeds) is the main limiting factor, corrected by dispersion compensators or DSF (Dispersion Shifted Fibre) fibres.

Fig. 14 — Propagation in a single-mode fiber (single mode, low dispersion)

Feature	Multimode (MMF)	Single-mode (SMF)
Core diameter	50 or 62.5 µm	8–10 µm
Light source	LED / VCSEL (850 nm)	Laser (1 310 / 1 550 nm)
Distance max.	~ 550 m (OM4, 10 Gbps)	Several dozen kilometres
Max. flow rate	100 Gbps / 100 m (OM5)	Tbps with DWDM
Cost of transmitters	Low (VCSEL)	Higher (FP/DFB laser)
Typical use	LAN, data centre	WAN, backbone, FTTH

c) Optical transmitters

Three types of transmitters are used, depending on the application:

LED (Light Emitting Diode): emit infrared light at 850 nm. Used with short-distance multimode fibres. Low cost, long service life, slow modulation.

 

Infrared laser diodes: emit at 1,300 nm. Intermediate performance, used for medium distances.

DFB/FP lasers: wavelengths of 1,310 nm or 1,550 nm. Used with long-haul single-mode fibres and DWDM systems. Very narrow spectral bandwidth, required for dense wavelength division multiplexing.

Fig. 15-16 — Optical emitters: LED (850 nm) and laser (1,310/1,550 nm)

 

d) Connecteurs fibre optique

There are many different types of fibre-optic connectors. Each connector features a ferrule (a precision ceramic or metal cylinder) that holds the fibre in place and aligns it.

The most commonly used connectors:

           

Fig. 17a-17b — ST (bayonet, left) and SC (push-pull, right) connectors

Fig. 17c — Dual MIC connector (used in FDDI networks))

Connector	Fastening	Typical use
ST (Straight Tip)	Bayonet (twist-lock)	Multimode LAN networks
SC (Subscriber Connector)	Push-pull (clip)	FTTH, LAN, WAN — the most common
LC (Lucent Connector)	Push-pull petit format	SFP transceiver, datacenters
FC (Ferrule Connector)	Back	Measuring instruments, telecoms
MIC (Media Interface Connector)	Double, clip	FDDI (obsolete)
SMA	Back	Military applications, instruments
FCPC	Screw-mounted with PC polishing	Single-mode fibres

e) Types of polishing (ferrule)

The type of polish on the optical connector (ferrule) determines the quality of the connection and the insertion loss:

· PC (Physical Contact): the end face is polished to produce a slightly domed convex surface. Reflection attenuation: −40 dB.

· UPC (Ultra Physical Contact): more pronounced curvature than PC, higher quality. Reflection attenuation: −50 dB. Blue connector.

· APC (Angled Physical Contact): convex-shaped and polished at an 8-degree angle, reducing unwanted reflections. Reflection attenuation: −60 dB. Green connector. Used in multi-subscriber FTTH (GPON) and CATV systems. This is the most recommended type of polish for long-distance links.

f) Connection techniques (coupling)

· Mechanical coupling: two connectors joined end-to-end by means of a precision adapter. Detachable connection.

· Mechanical splice: a permanent connection made by mechanical clamping. Used for repairs following a break. Insertion loss: ~0.5 dB.

· Fusion splicing: the process of joining two fibres using an electric arc (fusion splicer). The most reliable and least disruptive solution. Insertion loss: < 0.1 dB. Used for FTTH deployments and backbones.

2. Twisted-pair cables (UTP/STP/FTP)

In the simplest case, a pair of wires is used to carry the signal. The electric current generates an induced current in neighbouring lines (crosstalk), which interferes with the signal and reduces its effectiveness. Twisting the pairs cancels out this interference: the induced currents in each half-turn cancel each other out.

Fig. 18 — 4-pair twisted-pair UTP cable (structure and twisting of the pairs)

Twisted-pair cable comes in three main types:

UTP (Unshielded Twisted Pair): four pairs of wires enclosed in a PVC sheath. The most common, cheapest and easiest to install. Susceptible to external electromagnetic interference.

FTP (Foiled Twisted Pair): an overall aluminium shield surrounds the four pairs. Protection against electromagnetic interference (EMI) without individual shielding of the pairs.

STP (Shielded Twisted Pair): each pair is individually shielded, and the whole cable is then shielded. Maximum protection against EMI and crosstalk. A stiffer and heavier cable. Used in industrial or highly disrupted environments.

a) Cabling categories

Category	Bandwidth	Max. flow rate	Typical use
Cat 3	sixteen megahertz	Ten megabits per second	Telephony, 10BaseT Ethernet
Cat 5	100 MHz	One hundred megabits per second	Fast Ethernet 100BaseTX
Cat 5e	100 MHz	1 Gbps	Gigabit Ethernet 1000BaseT
Cat 6	250 MHz	1 Gbps (10 Gbps  55 m)	Gigabit, structured cabling
Cat 6A	500 MHz	10 Gbps / 100 m	10GBaseT, modern offices
Cat 7	600 MHz	10 Gbps	Data centres
Cat 8	2,000 MHz	25–40 Gbps / 30 m	Servers, Edge Data Centre