Understanding Telecommunication Subsystems and Network Functions
Subsystems
Network Subsystem Digital Connection
This subsystem’s role is to establish connections for:
- Payment processing
- Connections between trunks
- Connections between trunked subscribers through the switching stage
Central Processor Subsystem
This subsystem controls network connections and contains the necessary programs and data to initiate and terminate connections. It also records data and transmits it to the Input/Output subsystem.
This subsystem is often duplicated for redundancy in many systems.
Input/Output Subsystem
This subsystem acts as the interface, allowing the central system to interact with the outside world, including:
- Payment gateways
- Other central systems
- Operation and maintenance terminals
Here are some devices that assist the control system in performing its functions:
- Recorders
- Transmitters
- Receiver units
- Pricing mechanisms
- Synchronization devices
- Timing devices
Chapter 6: The Basic Functions I
The Basic Functions
The basic functions are:
- Interconnection
- Control
- Signaling with subscriber terminals
- Signaling with other plants
- Operational functions
- Synchronization
- Timing
- Packet switching
Interconnection
The switching system must facilitate communication channels between all subscribers within a central system and between subscribers and links connecting to other plants. This is achieved through the network connection.
Control
This is arguably the most complex function, integrating numerous secondary functions to control the system. These secondary functions can vary between systems and are not considered basic functions individually.
General Functions of Digital Switching Systems
Switching refers to the interconnection of terminals arranged in groups, ranging from a large population to an individual or small group.
Signaling Function with Terminals
The switching system must exchange signals with terminals to establish communication, enabling actions such as:
- Detecting a subscriber’s call attempt
- Notifying a subscriber of an incoming call
- Receiving information for call establishment
- Informing the terminal of congestion and connection failure
- Advising the terminal to begin sending signals (digits)
This information is closely related to the interface with subscriber lines.
Signaling Function with Other Plants
Since telecommunication networks consist of multiple switching centers, these centers must cooperate to establish communication between terminals connected to different centers.
This function involves exchanging signals between plants and is related to the interface with central links, common channel signaling, and control mechanisms.
The Basic Functions II
Operating Functions
Systems must support operational, maintenance, administration, and reconciliation functions for efficient and cost-effective network operation.
Synchronization
In interconnected digital switching systems using digital transmission media, synchronization is crucial for a robust ISDN foundation. Without synchronization, service quality can be negatively impacted.
Synchronization involves ensuring all digital stations within the network operate with an identical or near-identical clock signal in terms of frequency and phase.
To achieve this, central systems utilize internal and external reference clocks and procedures to select the appropriate clock based on network conditions.
Timing
The timing function utilizes a clock signal to generate various time reference signals, derived from the basic clock, ensuring harmonious operation of the entire switching system.
Packet Switching
For integrated services digital networks (ISDN), the PBX must support data terminal connections. Certain services require the digital system to support packet switching functions, not just circuit switching. Therefore, digital switching centers must incorporate components capable of handling protocols and information characteristics of packet switching networks.
Telecommunications and Telephony Concepts
Telecommunication refers to communication over a distance. This can involve communication between individuals, individuals and machines, or machines themselves. The distance can range from a few meters to millions of kilometers.
The general model involves a source terminal where information is generated and a presentation terminal where it is received and presented in an understandable format. The process of sending information between these terminals is called transmission.
Transmission typically involves electrical energy propagating through a suitable medium. Since information is not always in electrical form, transducers are used to convert it into electrical signals and back. For example, a TV camera converts images into electrical signals.
Telephony
Telephony is the science of transmitting sound over a distance. It involves the study and utilization of methods and procedures for sound transportation.
In telephony, both terminals are identical and are known as subscriber terminals or telephones.
Types of Telephony
Based on the transmission medium, telephony can be categorized as:
- Wired Telephony: Uses wires as the transmission medium.
- Wireless Telephony: Uses air as the transmission medium.
In telephony, microphones convert sound into electrical current variations, and telephone handsets convert these variations back into sound.
Point-to-Point Connections and Networks
Point-to-point connections involve connecting two terminals directly. As the number of terminals increases, the complexity and cost of connecting them directly also increases. This leads to the concept of networks.
Telecommunications Networks
A network consists of terminals connected to a central point called a node. The node establishes a single path between any two terminals wishing to communicate. This reduces complexity and cost compared to direct connections.
In a large geographical area, multiple nodes can be interconnected to facilitate communication between different groups of terminals.
Networks provide shared resources for communication and only establish connections when required. They operate based on protocols, which are rules governing signal access, routing, and transmission. Signaling refers to the special signals used for network operation.
Transmission, Switching, and Signaling in the Telephone Network
The telephone network is a switched network where the node is called a central office. It connects groups of telephone terminals through central offices, which are interconnected.
The network provides communication channels and performs switching functions to connect terminals. This process is supervised by signaling, which involves special signals exchanged between terminals and central offices.
When a subscriber makes a call, the network performs various functions, including detecting the call attempt, providing a dial tone, transmitting dialed digits, connecting the calling and called parties, providing call status signals (e.g., ringing, busy), and disconnecting the call after the conversation ends.
These functions involve transmission, switching, and signaling, highlighting the intelligent nature of the Public Switched Telephone Network (PSTN).