Telecommunications

Telecommunications is the art of moving information from one place to another — impressing it on a carrier, sharing one channel among many users, and managing who talks when. The Associate CET exam covers the core ideas: modulation (AM/FM/SSB), multiplexing, the duplex modes, and how cellular systems are organized. It also covers the two-way radio services, the TIA standards for wireless equipment, satellite links and the telephone line (POTS).

Modulation — putting information on a carrier

Modulation is the process of impressing an information signal (audio, data) onto a high-frequency carrier so it can be transmitted efficiently. You vary one property of the carrier in step with the message. The moment you modulate a carrier, you create sidebands — new frequencies above and below the carrier that actually carry the information.

Bandwidth & the carrier

Every transmission occupies a band of frequencies, not a single point — the bandwidth is the range its sidebands span. More information (or a faster data rate) needs more bandwidth. This is why FM broadcast (wide, high fidelity) uses far more spectrum than an AM or SSB voice channel, and why spectrum is a managed, licensed resource.

Multiplexing — sharing one channel

Multiplexing lets many signals share one transmission medium. The two classic methods divide the resource differently:

Duplex modes — who can talk when

Cellular systems

A cellular network covers a large area by dividing it into many small cells, each served by its own low-power base station. The key advantage is frequency reuse: because each base station only needs to cover a small area, the same channels can be used again in cells far enough apart not to interfere. As a phone moves, the network hands off the call from one cell to the next.

Analog & digital, and the building blocks

Modern telecom is mostly digital: the analog voice is sampled by an ADC into bits, which are far more robust over distance (they can be regenerated cleanly) and easy to multiplex, encrypt, and error-check. A modem (modulator-demodulator) bridges digital data and an analog channel. Repeaters and base stations extend range; protocols govern how the bits are framed and routed.

Circuit switching and packet switching. Competency 20.4 covers wired data and voice networks, and they move traffic in one of two ways. The telephone network was built on circuit switching: a call is set up first, and a path, with its link bandwidth and switch capacity, is reserved for that call alone until it ends. Performance is guaranteed, but the reserved capacity sits idle whenever the call is silent; the links are divided into circuits by the FDM and TDM of section 3. Data networks such as the Internet use packet switching: the data is split into packets that share every link on demand (statistical multiplexing) and move one hop at a time, each switch receiving a whole packet before sending it on (store and forward). There is no call setup and it suits bursty data, but when demand exceeds a link's capacity, packets queue, which adds delay, and can be lost (York University CSE3213 lecture notes). A 1 Mbit/s link divided into 100 kbit/s circuits carries at most 1000 / 100 = 10 calls at once; with packet switching, more than 10 users can share it when each is active only part of the time.

Two-way radio services

Competency 20.1 asks you to describe the major types of two-way radio communications. Each service has its own frequencies, its own modulation and its own part of the FCC rules (47 CFR), so the service tells you what to expect on the bench.

An LMR system is built from portable (handheld) radios, which have lower power and shorter range, mobile (vehicle-mounted) radios, which have more power and longer range, and a dispatch center that can patch users together and relay information (FEMA). A conventional system gives each user group a fixed frequency or set of frequencies. On a trunked system a controller automatically assigns each call an open channel from a shared pool (47 CFR 90.7), so the user keys the microphone, waits for the talk permit tone, then speaks. Analog signals fade gradually into static at the edge of coverage; digital signals stay clear and then drop out sharply.

A repeater receives the field radios on one frequency and retransmits them on another with more power, usually from a high site (47 CFR 90.7 calls it a mobile relay station), so a repeater channel reaches much farther. When there is no repeater in range, radios can talk to each other directly on a simplex channel, also called direct, talkaround or car-to-car, over a shorter distance (FEMA). Amateur radio is covered in the Ham Radio guide.

Wireless standards and the TIA

Competency 20.2 asks you to describe wireless telephone, video and data technology and to name the applicable ANSI/TIA standards. The Telecommunications Industry Association (TIA) is ANSI-accredited. Its committee TR-8 writes the standards for private radio systems and equipment, voice and data (APCO), and committee TR-30 the data interfaces such as RS-485 (TI).

The three ways to share a wireless channel among many users are FDMA (each user gets a frequency), TDMA (each user gets a time slot) and CDMA (each user gets a code and everyone shares the band): the multiple-access versions of the multiplexing in section 3. The cellular layout in section 5 and the digital building blocks in section 6 apply to all of them. The RF guide covers the analog modulation (AM, FM, PM, SSB and CW) in section 3, and its section 10 compares the signaling of GSM, IS-95 CDMA, LTE, 5G NR, P25 Phase 1 and 2 and digital TV, channel by channel.

Satellite communications

Competency 20.3 asks you to describe satellite communications principles. NEETS describes two kinds of communications satellite: a passive satellite only reflects signals back to Earth, while an active satellite works as a repeater. An earth station sends to the satellite on the uplink frequency; the satellite amplifies the signal, converts it to the downlink frequency and sends it back down. The equipment that does this on board is the transponder, and its amplifier can only deliver so much power, so the uplink level is managed to keep it out of saturation (University of Toronto course notes). An earth station has a large, highly directional antenna, a highly sensitive receiver with its preamplifier mounted right behind the antenna to make up for downlink losses, and a powerful transmitter (NEETS: 10 W to 20 kW).

NASA gives the geosynchronous height as 42164 km from Earth's centre, about 36000 km above the surface; NEETS rounds it to 19300 nautical miles. A higher satellite moves more slowly, and only at this height does one orbit take a whole day (NASA: 23 h 56 min 4 s). NEETS adds two operating effects: a handover, when one earth terminal yields control to another as the satellite leaves its coverage, and sun conjunction, when the satellite lines up with the sun and the sun's noise disrupts the link.

The telephone line (POTS)

Competency 20.5 asks you to describe a basic telephone (POTS) circuit with its common wiring and splicing conventions. The two wires of the line are the tip and the ring, named after the parts of the old switchboard plug. The central office (CO) powers the line from a −48 V battery: the tip sits at 0 V and the ring at −48 V relative to ground (UBC lecture notes; other sources give −48 to −52 V).

Wiring. A single-line RJ11 jack uses the center two of its six positions: pin 4 is the tip and pin 3 the ring. In old-style four-wire (quad) station cable the tip is green and the ring red; in twisted-pair cable they are white/blue and blue. A second line (RJ14) uses pins 2 and 5: tip black and ring yellow in quad wire, white/orange and orange in twisted pair. RJ11, RJ14 and RJ25 use the same six-position plug and differ only in how many contacts are wired: one, two or three lines.

Splicing. Multipair telephone cable follows the 25-pair color code. Each pair has a major color from white, red, black, yellow and violet and a minor color from blue, orange, green, brown and slate, which gives 5 × 5 = 25 pairs. The tip conductor carries the major color with the minor as its stripe and the ring the reverse, so pair 1 is white/blue (tip) and blue/white (ring), and pair 25 is violet/slate. Splices join color to color, tip to tip and ring to ring. Larger cables are bundled in 25-pair binder groups marked with ribbons in the same colors. The cable itself is covered in the Cabling guide.

On the job: a portable that cannot reach the repeater

Job practice, not exam content. A typical sequence when a user reports that a portable radio hears dispatch but cannot be heard back. The system's documentation and your agency's procedures come first.

1. Confirm the zone and channel. Radios hold both repeater and simplex channels, grouped in zones (FEMA). A user on the wrong zone, or on a simplex channel, will not reach the repeater at all.
2. Think about coverage first. Range depends on transmit power, the antenna, the radio's condition, terrain and buildings; a portable has less power than a mobile, and the repeater retransmits with more power than the portable has (FEMA), so hearing the repeater does not prove the repeater can hear the portable.
3. Check the radio's own path. Look at the antenna, its connector and the battery, then compare with a known-good radio at the same spot. If the second radio is heard and the first is not, the fault is in the first radio.
4. Keep users talking. While the fault is found, users close to each other can switch to the simplex (talkaround) channel and talk radio-to-radio (FEMA).
5. Record what you found. Note the location, channel, radio and result so the site or the radio can be checked against it later (Work Procedures).