Telecommunication

                                    Telecommunication

                                                The telecommunications network can transmit a variety of information, in two basic forms, analog and digital. In this lesson we will examine both. This information may be transmitted over a circuit/channel or over a carrier system.

Where: A circuit/channel is a transmission path for a single type of transmission service (voice or data) and is generally referred to as the smallest subdivision of the network. A carrier, on the other hand, is a transmission path in which one or more channels of information are processed, converted to a suitable format and transported to the proper destination.

The two types of carrier systems we will be discussing in this lesson are:

1. FDM (Frequency Division Multiplexing) - analog

2. TDM (Time Division Multiplexing) - digital

There are various ways to look at signaling.

Analog versus Digital

Electrical versus Optical

Signals can also be distinguished by their function:

Voice Transmission - By far the largest number of signals found on the network are voice frequency conversations.

Programming - These signals are from radio or television. They may be voice or music or video. The laer two requiring special circuits and facilities because or their wide frequency bandwidth requirements.

Network Control - These are the signals used for network housekeeping and control. There are four basic types:

Alerting

Addressing

Information

Supervision

Supervise the Call

The supervision functions of the switch tend to be overlooked because they are transparent to the customer. They are, however, extremely important because they directly impact the efficient functioning of the switch itself. Supervision functions include :

Relatively soon after the introduction of the telephone it became obvious that the direct connection network had some very distinct trade-offs. Advantages were:

• Speed of call processing

• Privacy Control over the call

• Disadvantages, however, included:

• Each destination required separate phones and cable pairs

• Limited connectivity

• Expense Lack of standards

• Switchboards and the Centralized Network

When Theodore Vail hired the out-of-work railroad engineers to become the earliest AT&T employees, the concept of a centralized point of switch ing for telephone calls became real. As stated in Lesson 1, it was the railroad engineers who had created a hierarchical process for the routing of local and long distance railroad trains (Figure 4.5). The long distance trains were routed from switch house to switch house along "express rail" lines which were known as trunks. Once the train reached the furthermost switch house or hub, the passengers would be required to change trains and board a local train which would travel the distribution network to the end destination. The trains that traveled across the distribution network were slower speed trains with many more stops than the express trunks that connected the switch houses. The same concept was employed with the newly developing telephone network.

Manual Switchboards and Centralized Networks

To realize the vision of a Centralized Network it was necessary to create a central switching point, thus the development and deployment of the manual switchboard. At this location, later known as the central office, each customer had a single connection and the ability to be connected to any other customer.

To complete a call, a patch cord which terminated in two jack locations was used. One jack represented the originator. The other jack represented the called party, or outgoing trunk. The implementation of centralized switching dramatically increased the interconnectivity in the network and along with it the value of the telephone to the customer. The concept of long distance was a natural evolution of the switchboard. Long distance calls were processed initially by the operator physically daisy chaining switchboards together with patch cord on outgoing/incoming trunks. This became an automatic function of the electromechanical switch.

While the centralized network did have its advantages, its early trade-offs were: Lack of control over the call by the customer (all processing was done by the operator). Dependency on the operator (for call set up and billing). Lack of privacy (the operator supervised the call for disconnection and billing). As the switches became more automatic, these issues diminished greatly.

Introduction of an Automated Message Accounting (AMA) (perforated paper tape).

Similarities with the Step-by-Step switch:

Approximately the same footprint (floor space required)

Noisy Electromechanical Manual maintenance

Power Principal

Components of the crossbar switch are sets of contacts called cross points  mounted one above the other along the vertical bars. These cross points are essential to setting up the call path.

To close a set of cross points and complete a connection through the switch, a horizontal bar moves and then a vertical bar. Up to ten simultaneous electrical paths can be established when connections are made in a crossbar switch. It is over these electrical paths customers talk to each other.

Network Overview:

A system of interconnected elements linked by facilities (i.e., physical connections) over which traffic will flow. The traffic may be conversations, information, or complex video or audio services. The telecommunications network must also be able to control the interconnected elements

Two Distinct Types of Networks:

1. Direct Connect Network

2. Centralized Network

Direct Connect Network

It results in a congested and costly network configuration.

Alexander Graham Bell was responsible for the direct connect network. The direct connection formula is:

C = U(U-1)/2

Where: U = Number of Users; C = Number of Connections Advantages:

Private

Customer has complete control over the call (point to point)

The major disadvantages of a direct connect network are:

• Expensive

• Complex

• Railroad Network

Thus, the invention of the telephone and the capability to communicate via the telephone presented a new challenge:

How to allow connection between telephones in different locations without directly wiring the telephones to each other

Theodore Vail was responsible for the telephone network as we know it today. Because of Vail, the early railroads were used as the model for development of a new kind of telephone network - the centralized telephone network. Basic Components of the Railroad Network.

1. Hub (switch)

2. Trunk

3. Local

4. Distribution

5. Centralized Network

Various network components are connected to a centralized point (such as a switch in a central office) which handles switching and routing functions.

In the centralized network structure, the functions of control and interconnection (i.e., switching) are primary. Centralized systems are virtually unlimited in how large they can become. Their major advantage is that customers can be interconnected through switching centers for worldwide communications. Additional considerations of the centralized network are:

• Some loss of control over the call by the customer

• The switch controls routing and connection (i.e., if there is excessive congestion a call may be blocked)

• Possible loss of privacy (although there are many safeguards against this)

• Location of centralized switching system

• Capacity

• Network Components and Architecture

Physical components required for telecommunication networks are:

• Transmission Facilities

• Local Loop

• IOF - Interoffice facilities

• Switching Systems

• Customer Premise Equipment (CPE)

• Transmission Facilities

In its simplest form, a transmission facility is a communication between two end points. This communication path can also be referred to as:

The primary functions of switching systems are to provide:

1. Call setup and routing

2. Call supervision

3. Customer I.D. and phone numbers

These are accomplished by interconnecting facilities (Figure 1.8). Switching systems located at the central office (CO) that are used to provide dial tone and ringing are referred to as end offices or local switches. These switches can also be interconnected with other switches. Another type of switch, tandem, is used as a hub to connect switches and provide routing. (No dial tone is provided to the customer.)

Customer Premise Equipment (CPE)

CPE is the customer's interface with the network (e.g., the telephone set). The telephone set provides for the conversion of acoustical (voice) signals into electrical signals for transmission through the network. It also generates the addressing (the called number) and supervision signals (on-hook/off-hook, call status).

There are many other types of CPE available such as:

• Modems (used with computers)

• Facsimile machines

• Sensors (alarms)

• Video camera

Components for Transmission

Three components of any transmission system are the:

The transmitter

The receiver

The communication path

In its simplest form, the CPE or customer premises equipment, is the transmitter and receiver. The media (twisted pair copper, coaxial cable, optical fiber, radio waves) that connects the CPE is the path.

How a Telephone Works?

The telephone is typically located on the customer's premises. It serves as the customer's network access device. Basic parts of the telephone set are:

Ringer

- Always on line

- Alerting device (bell, buzzer) for incoming calls

Switch Hook

- Completes the loop (path) when lifted off-hook

Dual

-Tone Multi-Frequency Pad (DTMF) or Rotary Dial

- Signaling device that generates the pulses or tone required to identify the called number and billing information

Handset

- Contains the transmitter and receiver

Transmitter

Receiver

- Converts the analog electrical signals back into acoustical energy.

The transmitted consists of three parts:

Diaphragm with a dome

Chamber

Carbon granules or a conductor

This is how the transmitter works:

Vibrations of the voice sound waves cause the diaphragm to vibrate.

The attached dome causes the carbon granules to vibrate (compress or decompress) within the chamber. A current flows from the dome through the carbon granules in the chamber. The amount of current that flows depends on how tightly the carbon granules are packed. Thus, the voice sound wave energy is converted to an electrical energy wave for transmission over the network. The electrical signal is an analogous representation of your voice, hence the term "analog signal."

Diaphragm

Electromagnetic

Permanent magnet

When a varying electrical current flows through the electromagnet, the resulting magnetic field either attracts or opposes the magnetic field of the permanent magnet.

This causes the diaphragm to move closer or further away from the permanent magnet (vibrate), in step with the electrical waveform.

Requests use of the telephone switching system when the handset is lifted off-hook.

Indicates the switching system is ready for use by receiving a dial tone.

Generates and sends the telephone number of the address (by dialing the number or by means of a touch-tone keypad).

Indicates the status of a call by receiving tones, such as audible ringing, busy tone, or recorded message.

Indicates an incoming call by ringing a bell or some other device.

Converts acoustical energy into an analogous electrical signal for transmission to a distant party.

Converts electrical energy into an analogous acoustical signal representing the sounds of the sender's voice. Informs the system a call is finished when the handset is placed back on the switch hook.

 Editor's note: The telephone is an electrical instrument. Electricity works the phone itself: operates the keypad, makes it ring. Electric current does not convey the voice, however, sound simply varies that current. It's these electrical variations, analogs of the acoustic pressure originally spoken into the microphone that represent voice. We have two different points here: the first, about the current itself, and, secondly, about how that current is altered.

To sum up, electricity is indeed conveyed to the phone, whereupon 1) the current operates the telephone and 2) the current is varied by the voice to communicate. Got it? Tom]

Telephone Connection to the Central Office

Many customers' telephones are connected to the central office by a pair of wires within a cable

 Why two wires?

Because your telephone is an electro-mechanical instrument, it requires a battery source and a ground source.

The battery source is supplied from the central office equipment to your telephone set by a wire called the ring lead. The ground source is transmitted from the central office by a wire called the tip lead. Together, the tip and ring of the telephone set are commonly referred to as a cable pair.

Electrical Current in the Local Loop

Local telephone companies purchase their power from the same source as local residential customers (Figure 1.16).

The power is received into the central office in an area called the power plant.

Alternating current is received by the rectifier which converts it to direct current.

Direct current is then used to power the equipment used in the net work.

Direct current is used in the local loop because:

AC and analog are both represented by a sine wave

Would anyone care to tell me what they might have been thinking? -- Ken Solomon adds the following;

"As far as I know DC was used to prevent hum, and to allow service to continue in the event of a power outage and because people were afraid of AC. It is used, after all, in the electric chair. Also, Bell used wet cells and probably would have killed himself using an AC generator. And, even though AC would have induced hum the interference and cross talk using DC was terrible anyway until a two-wire loops were introduced."

"In the early days iron wires were used and they were strung from roof top to roof top (this is pre poles). Since iron is a fairly poor conductor it picked all kinds of squawks, screeches, buzzing, parts of conversations (from adjacent lines) and other noises. Also, a single strand grounded wire will act as an antenna picking up atmospheric noises as well. Later, pure copper was used but it proved too soft so they used iron coated with copper which worked much better since it had many times the conductivity of iron alone. Of course if you hook up two modern phones in your house with modern conductors and a solid power supply you shouldn't have any interference. But plug in some cheap intercoms and you'll pick up (or induce) noise. BTW, did you know that the best place to ground the phones was in the soft earth surrounding the outhouse?"]

In most central offices the power is sent from the rectifier through the batteries and then out into the local loop via the central office equipment.

During a power outage batteries are used as the main source of power for the local loop.

A generator is started to provide power to the CO equipment.

Underground tanks are used to store the fuel.

In the event of generator failure, the batteries in the CO will power the CO equipment. Most battery back-up lasts 6-8 hours.

Voice Frequency Range

The tones produced by the human vocal cords range from about 30 vibrations per second to 20,000 vibrations per second.

A voice signal is characterized as an analog signal, or a sine wave, with the following characteristics:

Continuously changing value

No abrupt discontinuities

All values between the extremes are allowed

Sine waves are measured in hertz, or cycles per second.

One hertz (Hz) = one cycle per second (CPS)

Continuous

All values allowed

This is known as the voice band of frequencies.

Anything above or below the voice frequency range is filtered out.

The electrical signals produced and transmitted by the telephone network rise and fall, just as the human voice does.

In common terminology: Voice Signal = Electrical Signal = Analog Signal

Feeder Cable

Feeder (or F1) cable is the largest cable used in the local loop, usually 3600 pair copper cable (Figure 2.3). Demand from the distribution cables is aggregated to determine the proper size for the feeder cable. Feeder cable comes out of the Central Office and goes to the SAI. It is usually placed in conduit, accessed by manholes and sometimes referred to as underground cable.

Serving Area Interface (SAI)

A cross connect point is used to distribute the larger feeder cable into smaller distribution cables. The feeder cable is terminated on pins mounted on the back of the terminal panel inside the SAI. The distribution cables are terminated on adjacent panels. To accomplish the cross-connection between the two cables a jumper is run on the front side of the panel.

Wire Termination

There are three different ways to terminate the cables and jumpers . A binding post is a threaded stud with a nut. The insulation is stripped off and the wire is wrapped once around the stud. Then the nut is screwed down on top of the wire. A wire wrap is performed by inserting a wire into a wire wrap gun, which will wrap the wire very tightly around a pin. A punch down is performed by inserting a wire into the punch down pins. It requires a special tool that forces the wire between the pins.

The wire insulation is cut and the wire is crimped between the pins.

Distribution Cable

Distribution (or F2) cable is a smaller version of feeder cable. It contains a smaller number of twisted

pair wires. Its sheathing varies based on its placement (i.e., buried versus aerial).

Distribution Cable Termination

Depending upon the existing facilities in a given area, and city ordinances, distribution cable, drop wire cross-connect device can be either an aerial terminal, pedestal, or handhole.

Drop Wire

The drop wire terminates at the Subscriber Network Interface (SNI).

Subscriber Network Interface (SNI)

The Subscriber Network Interface is the device which serves as the demarcation point between local exchange carrier (LEC) responsibility and customer responsibility for telephone service. It is usually a gray box with modular telephone jacks inside (Figure 2.6) The SNI provides the LEC with a place to troubleshoot service problems.

Central Office Termination

Now let's see how the feeder cable gets connected to the switch which will provide the dial tone for the customer.

Cable Vault

The feeder cable, in conduit, travels underground from the first manhole into the cable vault or cable entrance facility. Inside the cable vault the feeder cable enters a splice case where it is di vided into 100-pair riser cables . At this point, the (black) feeder cable is spliced to a (gray) cable called a riser cable. Riser cable is fire retardant; feeder cable is not. Because the cable vault is usually below ground, the feeder cable travels horizontally into the vault from the street. Once feeder cable is spliced to riser cable, it usually will travel vertically up the walls of the cable vault through the ceiling/floor above to the protector blocks (either on an MDF or protector frame).

Conventional Main Distribution Frame (MDF)

The conventional main distribution frame (MDF) provides the point of cross-connection between the OSP cable pairs and the office equipment (switch).

One side of a conventional main distributing frame is known as the vertical side or outside plant (OSP) cable termination. The cables coming from the local loop are terminated on protector blocks on the vertical side of the MDF. The MDF protector blocks terminate 100-cable pairs.

Telecommunications Act of 1996

Great Seal of the United States

Other short titles Communications Decency Act of 1996

Long title An Act to promote competition and reduce regulation in order to secure lower prices and higher quality services for American telecommunications consumers and encourage the rapid deployment of new telecommunications technologies.

Nicknames Communications Act of 1995

Enacted by the 104th United States Congress

Effective February 8, 1996

Previously, the Communications Act of 1934 (“1934 Act”) was the statutory framework for U.S. communications policy, covering telecommunications and broadcasting. That act created the Federal Communications Commission (FCC or “Commission”), which was to implement and administer the economic regulation of the interstate activities of the telephone monopolies and the licensing of spectrum used for broadcast and other purposes. However, the Act explicitly left most regulation of intrastate telephone services to the states.

In the 1970s and 1980s, a combination of technological change, court decisions, and changes in U.S. policy permitted competitive entry into some telecommunications and broadcast markets. In this context, the Telecommunications Act was designed to open up markets to competition by removing unnecessary regulatory barriers to entry.

The Act's stated objective was to open up markets to competition by removing regulatory barriers to entry: The conference report refers to the bill “to provide for a pro-competitive, de-regulatory national policy framework designed to accelerate rapidly private sector deployment of advanced information technologies and services to all Americans by opening all telecommunications markets to competition”. Congress attempted to create a regulatory framework for the transition from primarily monopoly provision to competitive provision of telecommunications services.

Enactment

The Act was approved by the 104th Congress on January 3, 1996, and signed into law on February 8, 1996, by President Bill Clinton. It was the first bill signed in cyberspace and the first bill signed at the Library of Congress.

Framework

The 1996 Act sought to foster competition among companies that use similar underlying network technologies (e.g., circuit-switched telephone networks) to provide a single type of service (e.g., voice). For example, it creates separate regulatory regimes for carriers providing voice telephone service and providers of cable television, and a third for information services.

Preemption: One key provision allowed the FCC to preempt state or local legal requirements that acted as a barrier to entry in the provision of interstate or intrastate telecommunications service.

Interconnectedness: Since communications services exhibit network effects and positive externalities, new entrants would face barriers to entry if they could not interconnect their networks with those of the incumbent carriers. Thus, another key provision of the 1996 Act set obligations for incumbent carriers and new entrants to interconnect their networks with one another, imposing additional requirements on the incumbents because they might desire to restrict competitive entry by denying such interconnection or by setting terms, conditions, and rates that could undermine the ability of the new entrants to compete.

Intercarrier compensation: Under these conditions, many calls will arise between parties on different networks. While it might be possible to have the calling party pay its carrier and the called party pay its carrier, for various reasons it has been traditional in the United States for the calling party’s carrier to pay the called party’s carrier for completing the call this is called intercarrier compensation and, in turn, recover those costs in the rates charged to its subscribers. The 1996 Act requires that intercarrier compensation rates among competing local exchange carriers (CLECs) be based on the “additional costs of terminating such calls”. However, the framework created by the 1996 Act set different intercarrier compensation rates for services that were not competing at that time but do compete today.