CHAPTER ONE: TELEPHONE NETWORK STRUCTURE

1. Network Structure

1.1 EXISTENCE OF SWITCHING CENTERS

The existence of telephone switching centers is due, among other reasons, to the fact that more material is utilized by not having to cable between each and every subscriber. Nowadays, the latter would be impossible.

To calculate the number of connections between N nodes (subscribers), the following formula can be used:

     ┌─┐
     │N│   N(N-1)
U =  │ │ = ──────
     │2│     2
     └─┘

With switching centers, it’s not necessary to connect all subscribers to each other, but rather to connect all subscribers to a common center that handles their communication.

These types of centers that handle communication for a set of users in a specific area are called LOCAL AREA CENTERS.

The switching equipment is responsible for this function, which is nothing more than routing calls from the calling user to the receiving user. This routing is done based on a number that the calling user indicates to the center and that belongs to the called user, taking into account routing criteria that I will explain in future editions.

The elements responsible for connecting a local center with its subscribers are called the SUBSCRIBER NETWORK or LOCAL NETWORK of the center.

1.2 HIERARCHIZATION OF SWITCHING CENTERS

Since local centers can only cover a local area, it’s necessary to connect them to each other so that users from different local centers can communicate.

Therefore, centers of higher rank are used to connect local areas. These types of centers are called PRIMARY CENTERS.

Each local center depends on a single primary center, and a primary center depends on several local centers.

Primary centers must connect local centers by routing transit calls, that is, calls to subscribers that don’t belong to them.

Although there are types of primary centers that have their own subscribers, who don’t belong to any local center because they are located further away.

The connection between a local center and the primary center is called the primary section, whose composition consists of a set of individual circuits that we’ll call links.

Each link between centers can be the basis of a communication at some point.

Just as subscribers from different areas must be connected, primary centers must also be connected to each other; the number of primary centers must be very high to connect them to each other. Thus, there must be a center of higher category connected to the primary centers, the secondary center.

Each primary center depends only on one secondary center, and several primaries depend on one secondary center.

The function of every secondary center is to connect primary centers to each other. There are no exceptions as in the primary center, since NO type of secondary center has its own subscribers.

The secondary section is the connection between a primary center and the secondary center it depends on (composed of links).

It’s necessary to resort to a center of superior rank so that subscribers from secondary areas can communicate with each other, since the number of secondary centers is high.

For this, a tertiary or NODAL center is used, which constitutes a node of the telephone network.

A NODAL region: coincides with a region (nodal region, of course… LOL).

The secondary center depends on a single tertiary center; several secondary centers will depend on the tertiary center.

The function of a tertiary center is to connect secondary centers to each other. The connection of secondary and tertiary centers is known as the tertiary section and is composed of links.

The connections of subscribers from tertiary areas don’t need to resort to centers of superior rank since there are only 6 in Spain (Madrid, Barcelona, Valencia, Seville, Bilbao, and Leon - 1994 data) and if the connection formula is applied, these are reduced to 15.

The connections between tertiary centers are called quaternary sections or major national routes.

2. HIERARCHICAL NETWORK AND COMPLEMENTARY NETWORK

2.1 HIERARCHICAL NETWORK. FINAL SECTIONS AND FINAL ROUTE

The hierarchical network is defined as the set of subscriber stations and automatic centers connected to each other; thus each depends on an immediately superior category, with those of maximum category connected to each other.

The subscriber line, primary section, secondary section, tertiary section, and quaternary section are the hierarchical network connections between centers (final sections).

Final route, are the final sections to connect two subscribers in the hierarchical network, and there’s only one unique final route in the connection between 2 subscribers.

2.2 COMPLEMENTARY NETWORK. DIRECT SECTIONS. TANDEM CENTERS

Despite the fact that the final route between 2 subscribers is unique, the call can be routed in a shorter way, being more economical and with better service.

This is done through the complementary network, which is superimposed and connected to the hierarchical network.

The complementary network is composed of direct sections, among them are the TANDEM centers.

Direct sections are a set of links that connect 2 centers that, by the logic of the hierarchical network, shouldn’t be connected.

Routing through direct sections is shorter than through final sections.

Direct sections can occur between primary and secondary centers to each other.

Direct sections can occur between centers of different rank, as long as they don’t differ in hierarchy by more than one degree.

Therefore, the following direct sections are allowed:

Same hierarchy:

• Local to local
• Primary to primary
• Secondary to secondary

One degree difference:

• Local to primary
• Primary to secondary
• Secondary to nodal

The fact that they are allowed depends on a previous study.

It’s rare that direct sections exist that don’t meet the conditions, but they can occur (from local to nodal or from primary to nodal).

Tandem centers are transit centers (without subscribers) to which other centers connect (without the tandem centers belonging to the hierarchical network).

They can be of two types: urban and interurban.

The existence of the complementary network makes the path between subscribers no longer unique, but there are several paths among which the centers will have to decide the most appropriate routing.

The complementary network is so widespread that for certain types of traffic, it routes the majority of calls.

3. CATEGORIES OF CENTERS. SINGLE-CENTER AND MULTI-CENTER AREAS

3.1 FUNCTION AND CATEGORY OF CENTERS

According to size, location (rural or urban area), and types of traffic, there are several types of local centers, primary centers… denominated with different names.

3.2 RURAL NETWORK

It’s organized based on primary areas called sectors.

Sectors are rural primary areas whose headquarters is a primary center known as a sector center.

Number of sectors: 356 (about 7 provinces)

The primary center is located in the most important population of the sector, to which local centers are connected by hierarchical network, which handle subscribers located in smaller populations (terminal centers).

Function of the primary center: route transit calls from terminal centers.

The sector center has a dual function:

• As a primary center as the headquarters of its sector
• As a local center for subscribers in its population

Direct sections can exist between Sector Centers or Sector Transit Centers of one province with Sector Centers or Sector Transit Centers of another, including the CAI (Automatic Interurban Center, which we’ll see a bit later) of another province.

Types of Centers:

Sector Center (S.C)

Primary center on which local centers (terminals) located in different
populations depend. It serves as a local center for subscribers in its population.

Sector Transit Center (S.T.C)

Primary center on which local centers (terminals) located in the same or
different populations depend. No subscribers are directly connected to it.

Terminal Center (T.C)

Local center that connects subscribers from one or several populations,
generally small ones. It depends on a primary center (S.C or S.T.C)
located in a different population.

There are other types of rural centers that are less common:

Subsector Center

In principle, it's a T.C capable of performing transits between other T.Cs
in the sector when circuitry is saved with it.

Principal Sector Center

In principle, it's a T.C capable of performing transits between other T.Cs
in the sector when circuitry is saved with them.

Its category is that of a primary center.

Rural Network Architecture:

Subscriber            Subscriber                                    /Subscriber
       \              |                               /TerminalCenter
     TerminalCenter─SectorCenter─TerminalCenterOfSector           /Subscriber
       /                    |                             \TerminalCenter
Subscriber                Subscriber                                \Subscriber

3.3 URBAN NETWORK. SINGLE-CENTER AND MULTI-CENTER AREAS

The single-center area is the service area of a center that, due to the scarce existing demand, is the only one in the population.

These local centers are called urban centers, despite the fact that their category in the hierarchical network is like that of terminal centers.

They are characterized by the fact that the number of subscribers is greater, they depend on a center with primary function located in the same population; their characteristics of Complementary Network connections are different.

As a rule, whenever the number of urban centers in a population is not high, all of them will be interconnected with all the others in their urban area through direct sections.

When the number of direct sections necessary to connect all urban centers to each other is so high, it’s called a Composite Multi-Center Urban Network. This consists of 2 zones: Interior Zone and Exterior Zone.

The structure of the interior zone is analogous to the Simple Multi-Center Urban Network, given that the interior zone corresponds to the oldest urban centers when their number was sufficient to be able to connect them through direct sections.

Urban centers depend on tandem centers, most of them were “rotary” - a very old system that I believe no center currently uses (it was mechanical), although I’ve read that in Barcelona until recently there were some… this system is over 50 years old.

Types of Centers:

Urban Center:

It's a local center with greater capacity than a terminal center. It performs
the connection between subscribers in the same population. It depends on a
center with primary function, although it can be a secondary one, located
in the same urban area.

Urban Tandem Center:

Primary center on which only urban centers from the exterior zone of a
composite multi-center urban area depend.

Interurban Tandem Center:

Transit center that performs functions:
• As an urban tandem center for various terminal centers in its same
  metropolitan area
• As a sector center for terminal centers located outside the metropolitan
  area and/or automatic center outside the metropolitan area

Its category is that of a primary center.

3.4 INTERPROVINCIAL TRAFFIC. THE CAI AND THE NODAL

The CAI (Automatic Interurban Center) connects by hierarchical network the headquarters of the sectors and by hierarchical route to the urban centers of its urban area, with this the telephone traffic within the secondary area (of the province) is resolved.

Another mission of the CAI is to route interprovincial traffic.

The functions of the CAI can be done with two different centers, one specific to provincial transit, the CAP (Provincial Automatic Center), and another dedicated to national or interprovincial traffic called CAN (National Automatic Center).

Types of Centers:

Automatic Interurban Center (CAI):

Secondary center responsible for transit traffic destined or proceeding to
the primaries or locals that depend on it, whether the traffic is provincial
or interprovincial (national). It has no subscribers directly connected.

National Automatic Center (CAN):

Secondary center that routes national transit traffic (between centers
dependent on it), located in the same province and centers located in
different provinces. It doesn't route transit traffic between centers
that depend on it.

Provincial Automatic Center (CAP):

Secondary center that only routes the traffic that depends on it (provincial).

Nodal Center:

Tertiary center through which several secondary centers of a region connect
and traffic is directed to other nodal regions.

---

2. TELEPHONE SWITCHING

It’s the connection of some users with others, performed by the centers, more specifically by the switching equipment.

The subscriber through their terminal indicates the phone number they want to contact, the request is received by their local center which will enable a link between itself and the destination center or center that interconnects with the destination center.

Link Types:

• Outgoing - Calls leaving the center
• Incoming - Calls arriving at the center
• In transit - Calls passing through the center
• Local - Calls within the same center

If a bidirectional link is used, the incoming or outgoing call service cannot be performed at the same instant.

CONNECTION NETWORK

It’s the set of electromechanical or electronic circuits that form the switching unit, constituting the basic support of the connection.

It’s formed by a determined number of cross points necessary for communication.

Classification:

By Signal Type:

• Analog - Continuous signal switching
• Digital - Discrete signal switching

By Switching Type:

• Spatial network - Physical path switching
• Temporal network - Time-based switching
• Space-time network - Combined approach

What’s currently used is the space-time network. The European system consists of Pulse Code Modulation (PCM) along with time multiplexing.

Using space-time networks, it’s achieved to provide service to more subscribers with fewer equipment.

For multiplexing, a frame of 125 microseconds duration is created with 32 channels (3.9 ms each). The speed is 2 Mb/s.

MIC Technique (Pulse Code Modulation):

            ┌─ MIC TECHNIQUE ─┐
            │                 │
            │ • Sampling:     │
            │   Takes a sample│
            │   every 125 μs  │
            │                 │
            │ • Quantization: │
            │   Through a     │
            │   logarithmic   │
            │   quantizer     │
            │                 │
            │ • Encoding:     │
            │   8 bits        │
            └─────────────────┘

A channel is taken and during 3.9 microseconds it’s carried to the output, then the next channel is taken…

Note: Channels 1 and 16 are not for conversation, but for frame alignment and signaling respectively.

CONTROL UNIT

These are the circuits responsible for selecting cross points so that the call can be made.

It’s responsible for routing calls through free cross points to reach the destination (dialed phone number).

Operation Stages:

1. Concentration Stage

The number of incoming circuits to the control unit is reduced according
to traffic (or call volume).

The relationship between the number of inputs and outputs is known as
the concentration index.

2. Distribution Stage

The circuits from the control unit are received to perform connections
without eliminating the circuits.

3. Expansion Stage

The circuits from the central unit are related to the required outputs.

Control System Types:

Analog Systems:

A) Progressive Control

Performs routing without considering the next switching stage.
Main drawback: saturation

B) Common Control

Selects the entire communication path taking into account the next
switching stage.

Digital Systems:

There are 3 methods to perform the communication path, call billing, intensity control, saturation (traffic volume), maintenance help…

A) Wired Logic

Electromechanical elements are replaced by electronic elements, reducing
maintenance and consumption. The entire circuitry had to be changed,
which made it expensive and was never carried out.

B) Stored Program Control (SPC)

A program was developed that stored in memory and controlled the cross
points. The software was easily updatable, gaining flexibility.

Centralized SPC:

The processor has access to all system resources (the center) and is
responsible for performing all switching functions.

Generally, two micros are available in case one fails.

Distributed SPC:

Only access to certain resources is available, only the functions of a
part of the system are performed. To manage the entire center, several
micros are needed, each handling certain functions.

This is the most normal, as the center's operation doesn't depend on
one (or two) micros; if one of them goes down, it doesn't imply that
the center stops functioning, only some aspect of it.

Another advantage is that a modular system is available to which it's
easy to add new functions, characteristics...

CLASSIFICATION OF AUTOMATIC SWITCHING SYSTEMS

Classification based on the control unit and connection network:

[MECHANICAL]

Rotary Systems:

Based on rows and columns, the Rotary belongs to this type.

[ELECTROMECHANICAL]

Crossbar System:

Consists of:
• Emitter multiselector
• Grids
• Receiver
• Translator

Examples:
• Pentaconta 1000 (crossbar)
• ARD

[SEMI-ELECTRONIC]

They have an electromechanical connection network and an electronic
control unit.

Example: Pentaconta 2000

[ELECTRONIC]

Connection network and electronic control unit, the entire system is digital.

Examples:
• AXE (Ericsson)
• 5ESS (AT&T - yes, it has UNIX inside :D)
• 1240 (Alcatel)

1240 ALCATEL SYSTEM

Digital and space-time connection network controlled by distributed stored program.

1240 Modules:

Subscriber Interface Module (Remote Subscriber Unit)

Connects distant subscribers and those who don't have physical medium
with the center (wireless).

Data Interface Module

Switch data in packet mode and circuit mode.

Digital Link Modules

Provides service to digital links (120 subscribers). Uses MIC technique
with 16 bits per interval communicating at 4Mb/s.

Digital Subscriber Module

Provides service to 128 subscribers (not all centers provide service
to this type of subscriber).

Analog Link Module

Those necessary to provide service to the center's links.

Analog Subscriber Module

Provides service to 128 subscribers, basic line functions.

Clock and Tones Module

Marks the working speed (8.192 MHz), also indicates the tones of:
• Call alert
• Line busy
• Ring

They usually have two of these modules.

Peripherals and Maintenance Module

Alarm alert for failures. Screens, tapes, disks... for function
maintenance. Two modules are usually available.

Common Channel Module

Common channel signaling system (SSCC#7) used for signaling between centers.

Service Circuits Module

Capacity of 32 service circuits (emitters/receivers).

Operator Interface Module

Up to 15 operator positions.

Auxiliary Circuitry:

It's the circuitry necessary for the modules to function. Control
functions of the 8086 micro, terminal interface unit, memory, and clock.

Circuits:

• Link circuits
• Synchronization circuits
• Clock circuits

Programming: To program this center, the CHILEN language is used.

BASIC FUNCTIONS OF A COMMUNICATION CHANNEL

Interconnection: Interconnected with the nodes.

Control: Along with the previous one, it’s the most basic function. Establishes the maintenance and liberation of the path.

Supervision: Responsible for supervising:

• Subscriber lines and links
• Conversation path

SIGNALING SYSTEMS

Subscriber Signaling:

Tone	Frequency	Purpose
Dial tone	-	Invitation to dial
Call tone	400 Hz	Path free
Busy tone	400 Hz	Line occupied
Error tone	400 Hz	Communication not completed
Call alert	25 Hz, up to 75V	Ringing signal
Call waiting	100 Hz	Call waiting indication
Billing tone	12 KHz	Billing signal

Subscriber → Network Signaling:

• Decadic keyboard (uses pulses)
• MF keyboard (uses tones)

Inter-center Signaling:

+ Storage and analysis of received information (depends on the system)
+ Assign outgoing link of the distant center
+ Receive information (when pressing certain numbers)
+ Storage and analysis of received information (depends on the switching system)
+ Selection and connection: search for a free outgoing link and activation
+ Exploitation: billing and consumption
+ Synchronization: time measures for each step of synchronization
+ Timing: achieve that all centers act at the same time (clock frequency)
+ Packet switching

COMMUNICATION PROTOCOLS

The paths that the modules use to communicate are 4 Mb/s, generated because each channel consists of 16 bits.

The first two bits are from the communication protocol, used to distinguish the type of information traveling, and there are 4:

Code	Type	Description
00	Clear	Link released
01	Selection	The switching to perform
10	Data	Contains the message
11	Information	Control information

THE SWITCHER

It’s constituted by integrated circuits, called switching ports and connected by a time-multiplexed BUS.

Each of the circuits is formed by two independent ports. The function is to receive a signal (MIC), analyze it, memorize it, and perform the switching to output through another port. All ports synchronize at the same frequency of 8.192 Hz.

A port can perform up to 16 switchings, as there are 16 MIC lines arriving.

EXTERIOR PLANT

It’s composed of all the telephone plant located outside the buildings that house the switching equipment buildings…

Link Network:

Are the connections between centers that allow the connection of different
subscribers from different centers. They can be:

• Urban: don't leave the primary area
• Interurban: cross the secondary area

Subscriber Network:

Set of elements that allow the electrical connection between subscriber
equipment (yes, the phones..) and the local center.

Subscriber Distributor:

Element that limits the exterior plant of the switching systems.
It's distinguished:

┌─ Subscriber Distributor ─┐
│                          │
│ • Frame:                 │
│   Metal structure that   │
│   supports all elements  │
│                          │
│ • Vertical side:         │
│   Has strips where pair  │
│   termination is done    │
│                          │
│ • Horizontal side:       │
│   Subscriber line        │
│   termination            │
│                          │
│ • Bridge wires:          │
│   Communicate external   │
│   network with center    │
│                          │
│ • Interconnection point: │
│   Network flexibility    │
│   element, connects with │
│   any output             │
│                          │
│ • Distribution point:    │
│   Last point of network  │
│   from which pairs are   │
│   distributed to subs    │
│                          │
│ • Terminal cable:        │
│   Remains in center      │
│   (from distributor to   │
│   switching equipment)   │
│                          │
│ • Supply cable:          │
│   Connects terminal      │
│   cable with intercon.   │
│   points                 │
│                          │
│ • Service cable:         │
│   Between distribution   │
│   point and building     │
│   registry box           │
│                          │
│ • Dispersion network:    │
│   Set of service cables  │
└──────────────────────────┘

NETWORK INTELLIGENCE

Rediris is a digital network superimposed on the basic telephone network. It handles many things, among them the management of special numbers.

Special Numbers:

Number	Service	Description
900	Automatic reverse charge
901	Shared call
902	Calling subscriber pays	Universal number for SAT
904	Personal diversion	Before 082
905	Passive calls	Surveys, contests…
9051	Special billing
9055	Quantity of steps independent of time
906	Interest line
083	Reverse charge call	Credit call

CIR (Network Intelligence Center)

It's responsible for centralizing intelligence functions. It's formed by
a processor that manages a database (huge database!) in real time, which
allows it to manage network services.

The operation in a special call is as follows:
We call the special phone number, which by the prefix will route the call
to a special center, the CIR picks up the request that asks the AIR
(which we'll see in a moment) to indicate the treatment that should be
given to each call, at the same time it communicates with the CAS which
gives us the subscriber plan information.

AIR (Network Intelligence Agency):

Digital switching centers that perform the transit and interconnection
function between the PSTN and REDIRIS.

It performs basic functions at the local level, but most services must
be consulted to the CIR, retaining the call.

These centers are formed by three types of modules:

• Switching module
• Communication module
• Administration module

FE (Special Functions module):

Handles basic functions to not overload the AIR and free it from work.

Examples: locution, voice recognition, tone reception...

The modules are formed by databases.

CAS:

Is unique in the entire network; it's the service administration center.
Through this service, access to the network is allowed through the CIRs,
for parameter modification (billing, services..). It connects to AIR cards.

SERI (Network Operation System):

Allows performing operation and maintenance functions, makes global
checks of the entire system (measurements, alarms..).

SGR (Network Management System):

Is unique, manages traffic, supervises AIRs and links. It connects to AIR cards.