Wireless media

Overview#

Radio is the medium most people meet first — cellular networks, Bluetooth, smart home devices, and Wi-Fi. It is convenient because nothing has to be cabled to the client, and difficult for exactly the same reason. The medium is shared with every other transmitter in range, including ones on networks nobody in the building controls.

The organizing question for any shared medium is how systems take turns without destroying each other’s transmissions. Wireless answers it differently from wired media, and the reason is physical: a station cannot hear the channel over its own transmission, so it cannot detect a collision while causing one.

Taking turns on a shared channel#

A Wi-Fi station cannot listen while it transmits, so it cannot detect a collision the way a wired station can. It tries to avoid one instead. Carrier-sense multiple access with collision avoidance (CSMA/CA) works like this:

A station listens for activity on the channel it intends to use.
If the channel is busy, it waits and checks again.
If the channel is clear, it transmits.
The receiver checks the frame for errors and returns an acknowledgment (ACK).
If no ACK arrives, the sender treats the frame as lost and retries.

Two optional control frames protect longer transmissions. A request to send (RTS) announces how long the sender expects to occupy the channel. The receiver answers with a clear to send (CTS), which other stations in range hear as an instruction to wait.

A typical Wi-Fi network is hub-and-spoke: several devices associate with one wireless router or access point. In 802.11 terminology the client devices are stations.

802.11 standards#

The Institute of Electrical and Electronics Engineers (IEEE) publishes the standards for both wireless and wired LAN technology. The 802.11 family covers wireless LANs.

Standard	Max PHY rate	Band	Nominal range	Notes
802.11a	54 Mbps	5 GHz	35 m	Early OFDM standard, less crowded band
802.11b	11 Mbps	2.4 GHz	40 m	Slower, longer reach, noisy band
802.11g	54 Mbps	2.4 GHz	40 m	802.11b-compatible, faster
802.11n (Wi-Fi 4)	600 Mbps	2.4 and 5 GHz	70 m	Dual-band, MIMO antennas, channel bonding
802.11ac (Wi-Fi 5)	6.9 Gbps	5 GHz	35 m	Wider channels, more simultaneous devices
802.11ax (Wi-Fi 6)	9.6 Gbps	2.4 and 5 GHz	70 m	OFDMA; holds up better in crowded environments

Wi-Fi 6E extends 802.11ax into the 6 GHz band where regulators allow it. A separate amendment, 802.11h, added dynamic frequency selection and transmit power control so 5 GHz networks can share spectrum with radar systems; it is an amendment to 5 GHz operation, not a band variant of 802.11g.

Cellular networks#

Cellular networks are built and operated by telecom carriers. A mobile device associates with a nearby tower, which connects onward to the carrier’s core network.

2G and 3G are deprecated and largely decommissioned.
4G and 5G carry modern mobile traffic.
6G is still in research and standardization.

Long Term Evolution (LTE) drove the transition from voice-centric cellular networks to data networks capable of ordinary Internet access. 5G improves on it with higher throughput and lower latency, but at shorter range per site, which means denser tower deployments.

Narrowband-IoT (NB-IoT) is a low-power profile that occupies a narrow slice of the carrier’s spectrum. It transmits slowly — roughly 20–100 kbps — but penetrates walls and reaches underground locations well, which suits metering and sensor devices that send small readings infrequently.

The Global System for Mobile Communications (GSM) family identifies subscribers with a SIM card, which is what makes a device portable between networks and carriers internationally. Code division multiple access (CDMA) was a competing approach used in the United States by carriers such as Verizon and Sprint; CDMA networks identified devices in the network rather than with a removable card. Those networks have been retired in favor of LTE and 5G.

Satellite#

Satellite links carry traffic where terrestrial coverage does not reach. The distance a signal travels to orbit and back introduces latency, signal strength falls off with the square of the distance, and atmospheric conditions degrade the link further.

Geostationary (GEO) satellites orbit at roughly 35,000 km (22,000 miles) and hold a fixed position over one point on the ground. A single GEO satellite covers a large area consistently and the ground dish can stay pointed in one direction, but the round trip adds substantial latency.

Low-earth orbit (LEO) satellites orbit much closer, which cuts latency considerably. They do not hold a fixed position, so the ground station has to track them — mechanically, or with an electronically steered phased array — and coverage depends on a constellation of many satellites handing off to each other. Providers such as Starlink offer consumer Internet access this way.

Global Positioning System (GPS) and its counterpart constellations are a different use of the same infrastructure. A receiver measures how long signals took to arrive from several satellites and solves for its own position.

Study-note shortcuts worth correcting#

Advertised rates are shared, aggregate PHY rates. The 9.6 Gbps figure for Wi-Fi 6 is the theoretical total across the channel under ideal conditions, not throughput available to one device.
Published ranges are nominal. Walls, interference, antenna design, and transmit power move the real number substantially in either direction.
CSMA/CA avoids collisions; CSMA/CD detects them. The difference is physical: a wired station can hear the cable while transmitting, and a wireless station cannot hear itself over its own transmission.
802.11g has no 5 GHz variant. Radar coexistence in 5 GHz came from 802.11h.
Cellular did not abandon the SIM card. CDMA was the network type that identified devices without one. LTE and 5G use SIM and eSIM.
GPS position fixing is trilateration, not triangulation. The receiver solves from measured distances, not measured angles.

Suggested practice: watch a wireless link negotiate#

On a network you own:

On a wireless client, use iw dev or nmcli dev wifi list to see the band, channel, and channel width in use, and match them to the 802.11 table above.
Move the client further from the access point and repeat. Watch the negotiated rate change while the standard stays the same — the generation is a ceiling, not a promise.
Run iw dev <interface> scan | grep -E "SSID|freq|signal" and count how many networks share your channel. That contention is invisible and is usually the real explanation for a slow link.
Compare the rate your client negotiated against the headline number for its Wi-Fi generation. The gap is the difference between an aggregate PHY rate and one device’s share of it.
Check ip -s link on the wireless interface for error and drop counters. A link can associate successfully and still be marginal.

This shows what your equipment negotiated. It does not show why — signal quality and interference need dedicated tools to measure rather than infer.

Transmission media — the overview, and how radio compares with copper and glass.
Wired media — copper and fiber, and the collision handling that a cable makes possible.
The OSI model — where physical signalling and framing sit relative to each other.
Network appliances — the access points terminating these links.

Sources and further reading#

This page was edited from my own study notes, taken from Ian Neil’s CompTIA Network+ certification guide, and checked against the primary sources:

IEEE 802.11 Wireless LAN Working Group — the wireless LAN standards and their amendments.
GPS.gov — official reference for GPS system operation and positioning.
FCC Part 15 rules — the unlicensed-band power limits that bound real-world range in the US.

Regulatory domains differ by country, so channel availability and permitted transmit power vary with where the equipment is operating, not just what it supports.