§1.2.2Physical Media

The brief life of a bit

To define what a physical medium is, follow one bit.

A bit starts in a host, passes through a series of links and routers, and ends in another host. Along the way it is transmitted many times — once for each hop. For every one of those hops, the bit is sent by propagating electromagnetic waves or optical pulses across a physical medium physical medium Simple The material or space that actually carries the bits: wire, fibre or air. Precise The medium across which a bit propagates as electromagnetic waves or optical pulses between one transmitter-receiver pair. Examples: twisted-pair copper wire, coaxial cable, multimode fiber-optic cable, terrestrial radio spectrum, satellite radio spectrum. introduced in ch. 1 — open in glossary .

The medium does not have to be the same for each hop.

Media fall into two groups.

• With guided media guided media Simple Media where the signal travels inside a solid cable. Precise Physical media in which the waves are guided along a solid medium, such as fiber-optic cable, twisted-pair copper wire, or coaxial cable. introduced in ch. 1 — open in glossary , the waves are guided along a solid: a fibre-optic cable, a twisted-pair copper wire, a coaxial cable.
• With unguided media unguided media Simple Media where the signal travels through air or space, with no cable. Precise Physical media in which the waves propagate in the atmosphere and in outer space, such as in a wireless LAN or a digital satellite channel. introduced in ch. 1 — open in glossary , the waves propagate in the atmosphere or in outer space: a wireless LAN (Local Area Network) Local Area Network A network covering one building, campus or home. introduced in ch. 1 , a satellite channel.

What the media actually cost

Before the details, one economic fact that shapes every decision that follows.

The cost of the physical link — the copper, the glass — is often minor compared with the other costs of networking. In particular, the labour of installing it can be orders of magnitude higher than the cost of the material itself.

This is why many builders install twisted pair, optical fibre and coaxial cable into every room of a building at once. Even if only one is used at first, another may well be wanted soon, and laying it later means paying the labour twice.

Twisted-pair copper wire

Twisted-pair copper wire twisted-pair copper wire Simple Two insulated copper wires twisted around each other, the cheapest and most common cable. Precise Two insulated copper wires, each about 1 mm thick, arranged in a regular spiral pattern. The twist reduces electrical interference from similar pairs nearby. A wire pair constitutes a single communication link; unshielded twisted pair (UTP) is common for LANs at 10 Mbps to 10 Gbps. introduced in ch. 1 — open in glossary is the least expensive and most commonly used guided medium. Telephone networks have used it for more than a hundred years. More than 99 % of the wired connections from a telephone handset to the local telephone switch still use it.

It is two insulated copper wires, each about 1 mm thick, arranged in a regular spiral. The wires are twisted together to reduce electrical interference from similar pairs lying close by. Several pairs are usually bundled into one cable inside a protective shield, and one wire pair makes one communication link.

UTP (Unshielded Twisted Pair) Unshielded Twisted Pair The copper cable commonly used for LANs inside a building; category 6a reaches 10 Gbps over 100 m. introduced in ch. 1 is what LANs inside a building normally use. Data rates run from 10 Mbps to 10 Gbps, depending on the thickness of the wire and the distance between transmitter and receiver.

Coaxial cable

Like twisted pair, coaxial cable coaxial cable Simple A copper cable whose two conductors sit one inside the other, shielded so it can carry more. Precise A cable of two copper conductors that are concentric rather than parallel. With its insulation and shielding it achieves high data rates, and it can be used as a guided shared medium: several hosts connect directly to the cable and each receives whatever the others send. introduced in ch. 1 — open in glossary has two copper conductors — but they are concentric rather than parallel, one inside the other. With that construction, plus special insulation and shielding, coaxial cable achieves high data rates.

It is common in cable television systems, and as §1.2.1 described, those systems have been coupled with cable modems to give homes Internet access at hundreds of Mbps. The transmitter shifts the digital signal to a specific frequency band, and the resulting analog signal travels to one or more receivers.

That last phrase matters. Coaxial cable can be used as a guided shared medium: a number of hosts can be connected directly to the cable, and each of them receives whatever the others send.

Optical fibre

An optical fibre optical fiber Simple A thin glass thread that carries data as pulses of light, one pulse per bit. Precise A thin, flexible medium that conducts pulses of light, each pulse representing a bit. A single fiber supports tens or hundreds of gigabits per second, is immune to electromagnetic interference, has very low signal attenuation up to 100 km, and is very hard to tap. introduced in ch. 1 — open in glossary is a thin, flexible medium that conducts pulses of light. Each pulse is one bit.

A single fibre supports enormous bit rates — tens or even hundreds of gigabits per second. It has three further advantages:

1. It is immune to electromagnetic interference.
2. It has very low signal attenuation attenuation Simple How much a signal weakens as it travels along a link. Precise The loss of signal strength with distance along a physical medium, measured in decibels. It is the property that makes optical fiber the preferred long-haul medium: fiber has very low attenuation up to 100 km, while copper loses strength quickly, and faster as the signalling frequency rises. introduced in ch. 1 — open in glossary up to 100 kilometres.
3. It is very hard to tap, which matters for security.

Those properties have made fibre the preferred long-haul guided medium, particularly for links across oceans. Many long-distance telephone networks now use fibre exclusively, and it is prevalent in the Internet backbone.

So why is anything else still used? Because the optical devices — the transmitters, receivers and switches at each end — are expensive. That cost has kept fibre out of short-haul uses such as a LAN inside a building, or the last stretch into a home.

The standard link speeds are called OC (Optical Carrier) Optical Carrier The standard for fibre link speeds; OC-n runs at n × 51.8 Mbps. introduced in ch. 1 , running from 51.8 Mbps to 39.8 Gbps. The rule is simple: OC-n runs at n × 51.8 Mbps. Standards in use include OC-1, OC-3, OC-12, OC-24, OC-48, OC-96, OC-192 and OC-768.

Why frequency, not just distance, decides how far copper reaches

This is the effect that separates the media, and the book states its consequences without drawing them. So here is the picture.

Set the frequency to 1 MHz and copper looks usable for a kilometre. Drag it to 200 MHz — the rate a fast LAN needs — and twisted pair crosses the recoverable limit within about a hundred metres. Fibre stays flat on the floor of the plot the whole time.

Terrestrial radio channels

Radio channels terrestrial radio channel Simple A link that carries signals through the air between antennas on the ground. Precise A channel carrying signals in the electromagnetic spectrum, requiring no physical wire, able to penetrate walls and reach mobile users. Its characteristics depend on the propagation environment and distance, and it suffers path loss, shadow fading, multipath fading and interference. introduced in ch. 1 — open in glossary carry signals in the electromagnetic spectrum. They are attractive for four reasons:

• no physical wire has to be installed;
• they can penetrate walls;
• they give connectivity to a user who is moving;
• they can carry a signal a long way.

Their behaviour depends heavily on the propagation environment and the distance. Four effects work against them:

• Path loss path loss Simple A radio signal gets weaker the further it travels. Precise The decrease in radio signal strength as the signal travels over a distance. Together with shadow fading it is one of the environmental effects that determine the characteristics of a radio channel. introduced in ch. 1 — open in glossary — the signal weakens with distance.

• Shadow fading shadow fading Simple A radio signal gets weaker when it has to pass around or through an obstacle. Precise The decrease in radio signal strength caused by obstructing objects that the signal must travel around or through. introduced in ch. 1 — open in glossary — the signal weakens going around or through obstacles.

• Multipath fading multipath fading Simple Copies of the same radio signal arrive by different routes and interfere with each other. Precise The distortion of a radio signal caused by its reflection off interfering objects, so that several copies arrive at the receiver by paths of different lengths. introduced in ch. 1 — open in glossary — the signal reflects off objects, so copies arrive by different routes and interfere.

• Interference — from other transmissions and other electromagnetic signals.

The book sorts terrestrial radio into exactly three groups by distance.

Personal devices such as wireless headsets, keyboards and medical devices operate over the shortest distances. Wireless LAN technologies use the local-area channels. Cellular access uses the wide-area ones. Chapter 7 covers radio channels in detail.

Satellite radio channels

A communication satellite links two or more Earth-based microwave transmitter/receivers, known as ground stations ground station Simple The dish on the ground that talks to a satellite. Precise An Earth-based microwave transmitter/receiver linked by a communication satellite. The satellite receives on one frequency band, regenerates the signal with a repeater, and transmits on another. introduced in ch. 1 — open in glossary . The satellite receives on one frequency band, regenerates the signal using a repeater, and transmits it on another.

Two types are used.

A geostationary satellite geostationary satellite Simple A satellite that stays above the same spot on Earth, very high up. Precise A communication satellite placed in orbit 36,000 kilometres above the Earth's surface, so that it permanently remains above the same spot. The round trip through it introduces a propagation delay of 280 milliseconds; such links can operate at hundreds of Mbps. introduced in ch. 1 — open in glossary permanently remains above the same spot on Earth. That is achieved by placing it in orbit 36,000 kilometres above the surface. The huge distance from ground station up through the satellite and back down introduces a propagation delay of 280 milliseconds. Even so, satellite links can operate at hundreds of Mbps, and they are often used in areas with no DSL or cable access at all.

LEO satellites leo satellite Simple A satellite in a low orbit that moves across the sky, so many are needed for full coverage. Precise A low-earth orbiting satellite, placed much closer to Earth than a geostationary satellite and not remaining above one spot. LEO satellites rotate around the Earth and may communicate with each other as well as with ground stations; many are needed to give an area continuous coverage. introduced in ch. 1 — open in glossary are placed much closer to Earth and do not stay above one spot. They rotate around the Earth, as the Moon does, and may communicate with each other as well as with ground stations. Because each one is only briefly overhead, many satellites must be placed in orbit to give an area continuous coverage. Many low-altitude systems are in development, and LEO technology may be used for Internet access in future.

The six media together

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