Telecommunications Fundamentals: Network Infrastructure & Transmission

Early Telephone Technology & Specifications

Alexander Graham Bell invented the telephone in 1876. Early telephony involved transistor technology. Key specifications included a central voltage of -48V, impedance ranging from 400-1750 ohms, and a 440 Hz tone. The busy tone marking was typically 350-current, with frequencies between 480-620 Hz. DTMF (Dual-Tone Multi-Frequency) technology became standard. Common cable specifications included a diameter of 0.4 mm, an impedance of 280 ohms/km, and a transmitting frequency range of 300-3400 Hz.

Subscriber & Urban Cabling

The cable inside the rosette connects old and new subscriber equipment to the Network Termination Point (NTP/PTR). Urban distribution cables, such as those used by Laredo for telephone service, often feature a 0.5 mm diameter, polyvinyl insulation, and a steel core with 4 conductors.

Main Distribution Frame & Network Components

The Main Distribution Frame (MDF) is a metal frame where power strips are mounted. It connects the subscriber network to circuits originating from switching equipment. Key components of the telecommunications network include:

• Internal network
• Dispersion
• Distribution
• Feeder sections

PABX Systems Explained

PABX (Private Automatic Branch Exchange) is telephone equipment that acts as a private central switch. It uses software to provide switching functions to users within an organization, allowing internal calls and access to the public network without each user needing a separate public line.

ISDN & Digital Network Access Types

Radio transmission speeds can reach 64 kbps. ISDN (Integrated Services Digital Network) offers different types:

• Narrowband ISDN: Up to 2 Mbps
• Broadband ISDN: Over 2 Mbps

ISDN access types include:

• Basic Rate Interface (BRI): 2 B-channels (64 kbps each for voice or data) + 1 D-channel (16 kbps for signaling).
• Primary Rate Interface (PRI): 30 B-channels (64 kbps each) + 1 D-channel (16 kbps).

Analog & Digital Bus Architectures

Analog buses in installations are often designed to be identical from the Network Termination Point (NTP/PTR), establishing a baseline. Digital buses represent a different set of infrastructure considerations.

ADSL & Symmetric DSL Differences

ADSL (Asymmetric Digital Subscriber Line) features asymmetric transmission speeds, meaning the download speed is different (usually higher) than the upload speed. In contrast, Symmetric DSL offers equal upload and download speeds.

Understanding Multiplexing Principles

Multiplexing is the process of combining multiple data streams into a single shared communications channel, maximizing its capacity.

Multiplexing Techniques: FDM & TDM

Frequency Division Multiplexing (FDM) is a transfer method where each subchannel utilizes a distinct portion of the transmission bandwidth. For example, a band from 0.3 to 3.4 kHz with a carrier frequency of 3 to 3.4 kHz, where carriers are spaced, and the maximum frequency to transmit is 4 kHz. Time Division Multiplexing (TDM), on the other hand, allows each subchannel to appear in the channel during a specific, high-frequency time slot for transmission.

Modulation & Switching Fundamentals

Modulation is the process by which information is encoded onto a carrier signal for transmission over a medium. Commutation (Switching) is the process of interconnecting transmission channels or telecommunication circuits for the duration of a signal transmission. This typically involves a sequence:

• Circuit request
• Circuit confirmation
• Information transmission
• Circuit disconnection

Types of Modulation: AM, FM, & SSB

Amplitude Modulation (AM) involves combining a signal channel with a carrier wave, where the amplitude of the carrier is varied in proportion to the message signal. For example, a 3 kHz carrier wave modulated to 3500 kHz. Frequency Modulation (FM) varies the frequency of the carrier wave in accordance with the amplitude of the modulating signal; higher amplitude results in higher frequency deviation. Single-Sideband Modulation (SSB/BLU) transmits only one sideband of the modulated signal, suppressing the carrier and the other sideband. For instance, with a carrier frequency of 1000 kHz, the upper sideband might be between 1005 and 1010 kHz, and the lower between 990 and 995 kHz.

Digital Multiplexing Concepts: MDT & MDF

• MDT (Multiplexing Digital Transmitters): The total bandwidth of the transmission medium is assigned to each channel during a specific time interval, typically utilizing digital transmitters.
• MDF (Multiplexing Digital Frequencies): The total bandwidth of the transmission medium is divided into fractions, with each fraction assigned to a specific channel.