3.
1 Introduction to Telephone System
A telephone system is used for voice communication over long distances.
It converts sound into electrical signals and transmits them over wires.
Involves key components like microphones, speakers, and switching systems.
Central office switches connect calls between users.
The Public Switched Telephone Network (PSTN) is the backbone of the
system.
3.2 Advantages of Electronic Telephony over Manual Telephony
Electronic switching provides faster call connection.
Reduces human errors and manual labor.
Supports automated dialing and call routing.
Allows multiple services like caller ID and voicemail.
Improves efficiency and reliability of the communication network.
3.3 Block Diagram of Electronic Telephone Exchange
Includes components like subscriber line interface, control unit, and
switching matrix.
Subscriber lines connect to the exchange through interfaces.
Switching matrix routes calls between lines.
Control unit manages call processing and signaling.
Enhances automation and scalability in networks.
3.4 In-band and Out-of-band Signaling System for Telephony
In-band signaling sends control info (like tones) on the same path as voice.
Example: DTMF tones during dialing.
Out-of-band signaling uses a separate channel for signaling.
It’s faster and more secure (e.g., SS7 protocol).
Out-of-band is common in modern systems.
3.5 Pulse Dialing and DTMF
Pulse dialing sends a series of electrical pulses to represent numbers.
Common in rotary phones.
DTMF (Dual-Tone Multi-Frequency) uses tones for each key press.
Used in modern push-button phones.
DTMF is faster and more efficient than pulse dialing.
3.6 Local Loop
It’s the physical link between the subscriber and the local exchange.
Typically a pair of copper wires.
Carries voice and signaling info to/from the exchange.
Vital part of landline telephone systems.
Subject to signal degradation over long distances.
3.7 Various Signals Present on Local Loop
Ringing signals alert the user of an incoming call.
Dial tone indicates readiness to dial.
Busy tone shows the line is in use.
Call progress tones inform about call status.
DC current flows to detect off-hook condition.
3.8 Limitations of Conventional Mobile Phone Systems
Poor signal quality and limited coverage.
Inefficient frequency usage.
No handoff between cells—calls drop during movement.
Manual frequency assignment.
Couldn’t handle increasing subscriber demand.
3.9 Evolution of Cellular Mobile Communication System
Started with 1G (analog voice), evolved to 4G and 5G.
2G introduced digital transmission and SMS.
3G enabled mobile internet and video calling.
4G brought high-speed internet and IP-based services.
5G offers ultra-low latency and IoT support.
3.10 Block Diagram of a Basic Cellular System
Includes Mobile Station (MS), Base Station (BS), and Mobile Switching Center
(MSC).
MS communicates with BS using radio signals.
BS connects to MSC for call processing.
MSC links to PSTN or other mobile networks.
Ensures handoff and mobility management.
3.11 The Terms Mobile Station and Base Station
Mobile Station (MS): The user’s device (e.g., mobile phone).
Base Station (BS): Handles radio communication with MS.
BS includes antennas and transceivers.
Communicates with MSC for call routing.
BS serves one or more cell areas.
3.12 Mobile Switching Centre (MSC)
Central part of mobile network responsible for call control.
Routes calls between base stations and external networks.
Manages mobility, handoff, and subscriber data.
Connects with HLR, VLR, and PSTN.
Ensures smooth communication and billing.
3.13 Forward and Reverse Channels in Mobile Communication
Forward channel: from base station to mobile (downlink).
Reverse channel: from mobile to base station (uplink).
Used for voice, data, and signaling.
Each direction has its own frequency or time slot.
Essential for duplex communication.
3.14 Cellular Channels
Divided into control and traffic channels.
Control channels handle signaling and setup.
Traffic channels carry voice and data.
Frequency reuse allows efficient spectrum use.
Channels are dynamically assigned during communication.
