Category Archives: Telephony


NEBS is an acronym for Network Equipment-Building System. It is a standard used in telephony equipment. Many developers and sellers of telephony equipment create their products in compliance with the NEBS standards. This would prove to their customers that their products are properly built.

The main purpose of NEBS is to describe the environment of an RBOC (Regional Bell Operating Company) Central Office. The factors described by NEBS include criteria related to space and physical environment.

The criteria related to space include cable entrance facilities, operations support systems, power equipment, distributing and interconnecting frames, and cable distribution systems. The environmental criteria include illumination, acoustic noise, office vibration, air contaminants, equipment handling, fire resistance, and temperature and humidity.

Apart from market recognition, adhering to the NEBS standards gives vendors and developers other significant benefits. By designing equipment that is compatible with an RBOC Central Office, vendors will have lower development expenses. Developers of telephony equipment will also find it much easier to introduce NEBS-compliant devices into their networks.

NEBS has three major levels:

  • NEBS Level 1 indicates that hazards that may affect people and equipment are minimized. This level also indicates that network degradation is at a low rate.
  • NEBS Level 2 addresses the issues of equipment operability in a controlled environment. This level is the least used among the three due to its ambiguity.
  • NEBS Level 3 indicates that the equipment can be used by the network within its maximum life span. This means that the equipment will operate in spite of environmental extremes that may occur within the office.


Private Branch Exchange or PBX refers to a telephony switch that serves a specific office or business, as compared to an exchange that a carrier or phone company provides for different entities or the public.

PBX is also equivalent to PABX or Private Automatic Branch eXchange, and EPABX or Electronic Private Automatic Branch eXchange.

PBXs create connections between the organization’s internal telephones and link to the Public Switched Telephone Network PSTN) through trunk lines The term “extension” refers to an endpoint on the branch of the PBX; comprising telephones, modems, fax machines, etc.

Every telephone connects to the system in the PBX. When a user picks the receiver up and dials the access code, the system connects the user to an outside line.

PBXs vary as “key systems” as its users select their outgoing lines manually, while PBXs automatically connect to the outgoing number. Hybrid systems are a combination of the PBX and key systems.

Organizations use PBXs to lessen the telephone lines leased from a telephone company. Many medium- and large-scale companies use a PBX for the number to dial is usually composed of just a few digits.

The Centrex is an innovation on the PBX scheme, with switching occurring in a separate telephone office instead of within company premises.

Using the PBX system, the organization only requires a number of lines from the telephone service provider as the most number of people connecting to outside numbers at any given time.

Common PBX Systems:

  • Avaya’s Definity series
  • Northern Telecom’s Meridian series

Open Source PBX Systems

Many comprehensive PBX systems are open-source software. Significant networks supply product support and these systems are available at no cost to the user.


PSTN refers to the Public Switch Telephone Network. It is the collection of the world’s interconnected public telephone networks.

The PSTN is a circuit-switch network or a dedicated circuit established for voice traffic, such as a telephone call. This set-up contrasts with packet-switching networks, wherein messages are broken down into packets and sent individually. The Internet is one example of a packet-switching network which follows the TCP/IP protocol.

PSTN, which used to be a network of fixed-line analog telephone systems, is now almost completely digital. Nonetheless, most of its subscribers are still linked by analog circuits. Fixed-line and mobile phones are now included in this new system set-up.

History and Architecture

PSTN traces its beginnings to the invention of the telephone and the early development of the telephone service. The first telephones were all in private use and had no networks as they were wired in pairs.

Employing the same technology used by telegraph systems, every telephone was later connected to a local telephone exchange all wired together with trunks. These networks were in turn wired together in a hierarchy that spans cities, countries, continents, and oceans.

A network was created using analog voice connections using manual switchboards. They were replaced by automated telephone exchanges, and later on by digital switch technologies. Nearly all switches now employ digital circuits among exchanges, whilst two-wire analog circuits are still used to connect most phones.

Before the advent of the Internet, the PSTN was vital for data transmission using circuit switching, just as it was used by voice communications. However, the PSTN is now becoming just another application of the Internet, with the voice traffic shifted to voice over internet protocol. PSTN would eventually shift from circuit switching to packet switching.

PSTN Digital Circuit

The PSTN uses the Digital Signal 0 (DS0), a 64 kbit/s channel originated by Bell Labs. This digital circuit carries a typical phone call from a calling party to a called party. Its audio digital sound is digitized using a pulse code modulation that operates at a rate of 64 kbit/s. The call is then transmitted via telephone exchanges from one end to another and switched with a signaling protocol used with the exchanges.

DS0’s are also known as timeslots. These timeslots are conveyed from the initial multiplexer to the exchange over the access network. A number of defined reference points are contained in these access networks. The V reference point is one particular reference point that is used between a primary multiplexer and exchange.

Install Telephone Wiring

Gone are the days when the phone company would not only provide the telephone service, but install the phone wiring as well. Nowadays, this task is either done by paying the phone company or an electrician.

Not many people consider installing their telephone wiring themselves. Most people have the misconception that this task is an impossible feat for an average nontechnical person. Not many realize that the task is simple, safe, and does not require much technical know-how.

Complying with the Standard

It is best to determine the telephone wiring standard that is in use when installing additional wiring to an existing facility and follow that standard. When installing telephone wiring in a new building or home, however, the EIA/TIA T586A is the standard that is implemented.

Telephone Service Lines

The telephone line from the local telephone company reaches a home in two ways:

  1. Above ground wiring
  2. Phone service wire pairs are distributed via telephone poles using an aerial method of connection. Attached to the pole is a breakout box which has the main service lines for the area entering in one side and residential lines running out on the other. Each of these residential lines runs to a house in the neighborhood.

  3. Underground wiring
  4. Break-out boxes are set up in geographically logical points within the neighborhood before the building of houses began. Lines are then trenched back to the primary distribution can and to each house that is to be built. The wiring may need to be extended to reach the house once it has been built.

Residential Network Interface Device (NID)

The telephone wires installed within the neighborhood ends at a network interface device (NID) when connecting to each residence. The NID looks like an 8-inch wide and 12-inch long gray or tan box that features a door covering the customer and the telephone company compartments. The customer compartment can be accessed when checking for a dial tone, wiring connections inside the NID, and to trace lines that are coming out of the NID into the house.

Telephone Wires

Most telephone wires are one or more twisted pairs of copper wires, the most common of which is the 4-strand or two twisted pairs of copper wires. The first pair is made up of red and green wires, whilst the other is made up of yellow and black wires. This type of wire can carry two separate phone lines as one telephone needs only two wires. Telephone wires come in 22-gauge and 24-gauge, with the latter being the standard.

Modular Plugs

There are two types of most commonly used modular plugs:

  1. RJ-11
  2. This is the more common modular plug. It uses only two of the wires in a 4 strand wire. It is the connector used to plug the telephone into the wall and carries only one telephone line.

  3. RJ-14
  4. This plug uses 4 wires which make it ideal for handling two telephone lines.

Installing a Two-Line Wiring

In this procedure, the star or the homerun method will be used for the installation.

  1. Connect the tip wire (electrically positive wire) of the plain old telephone service line (POTS) to the tip wire of the jack. Do the same for the ring wire. There are a few possible combinations of colors for each pair. However, the most common pairs are composed of red/green and yellow/black. The former pair is used most of the time, wherein the red wire is the ring and the green wire is the tip.
  2. Run two or more lines within one wire in order to reduce the work and materials. At the end of the wire, break out the two lines using an adaptor. This allows line 1 to be connected to an RJ-11plug, and line 2 to another RJ-11 plug.
  3. For a small PBX or phone system, the POTS line (or trunk lines) is usually connected to the system Via RJ-11 plugs.
  4. NID’s terminating with modular jacks only requires the phone system to be connected wit RJ-11 plugs.
  5. NID’s with a terminal strip, or an area where the wire pairs that comprise each POTS line stop at an intersection, wherein the colored wires can be joined directly. The lines should be either directly connected to the PBX, or a modular jack should be connected to each of the POTS lines on the strip. The same procedure for connecting the POTS lines to phone jacks should be followed.

Dual-Tone Multi-Frequency

DTMF, or Dual-tone multi-frequency, is a type of signal used for telephone tone dialing. This type of dialing was trademarked as Touch-Tone, although it is no longer widespread today after it was canceled in 1984. Meanwhile, other telephone networks implement an internal multi-frequency system.

History of DTMF

Before DTMF became prevalent, telephone systems used the pulse system, also known as Dial Pulse in the US, and the loop disconnect in dialing telephone numbers. The latter works the same way like that of switching on and off a light switch as it connects and disconnects the line of the calling party. In this process, the dial pulses are counted to identify the caller number. However, this system proved to be inefficient for long distance calls as telegraphic distortion were recorded.

This is why the Dual-Tone Multi-Frequency was developed to instruct the telephone switching system of the number to be dialed. The DTMF uses eight different frequency signals that are transmitted in pairs.

More on Tone Frequencies

The eight DTMF tones basically represent the tones you hear whenever a standard telephone keypad is pressed. For each button, it produces a tone which is the sum of the row and column tones, hence, the term “dual tone”. For standard telephones, there are no ABCD keys.

The standard DTMF tones have 16 keys; however, most telephones nowadays only use 12 out of these 16 keys. The other unused tones are still being used by telephone networks.

The difference in loudness for each frequency can go up to 3 decibels (dB), and is also known as twist. Each tone can run to at least 70 milliseconds, although it can also run to as low as 45 milliseconds, depending on the country and the type of DTMF receiver application.

Widespread Use of DTMF Today

Aside from telephone signaling, DTMF is also used for cable television broadcasting. This helps in signaling the start and stop times for insertion of local commercials during program breaks. Commercially speaking, this benefits the cable companies.

The DTMF tone sequences used to be widespread in cable channel companies all over the United States and in other parts of the world. However, the use of this method began to experience a slow decline with the advent of more advanced out-of-band signaling methods at the start of the 1990s.

Call Block

Call block is a telephone service that enables individuals to block incoming calls from a specific phone number. Telephone subscribers use call block when they do not want to accept calls from certain individuals for either security or personal reasons.

Call block is normally a paid service. Telephone companies require a monthly fee that normally ranges from three to five dollars, although others charge a higher amount. Since it is not a built-in service, subscribers have to ask their phone company to first activate their phone’s call block features before they can use it.

Call block has many alternative names. While majority of telephone companies call this service call block, other companies refer to it as call screening or call rejection.

The steps in blocking a call may differ from one company to another. Generally, the user needs to dial a two-number combination before he can specify the phone number he does not want to get calls from. If he does not know the phone number, he can dial a combination of numbers right after hanging up from that particular call. This service can easily be turned off through a set of number combinations.

Similar to other services, call block has a set of limitations. It cannot block overseas phone numbers. This service also cannot block payphones, mobile phones, and business lines. Using call block is not advisable for those who own rotary phones as it would be very difficult to enter the number they want to block.


Category 5 cable (CAT-5) is a computer network Ethernet cable standard as defined by the Electronic Industries and Telecommunications Industry Association (EIA/TIA). This 5th generation twisted pair Ethernet cabling system contains copper wire in four pairs. It has recently been replaced by the Category 5E (CAT-5E) specification.

CAT-5 structured cabling was designed for high signal integrity. It can carry many signals such as basic voice services among others. Local Area Network (LAN)–based computers mostly use CAT-5 cable connections. Networks that can use CAT-5 are ATM, token ring, 10Base-T, 100Base-T, and 1000Base-T.

CAT-5 cabling has four twisted pairs of wires in one cable jacket. These wires terminate in RJ45 connectors and preserve a high signal-to-noise ratio in spite of interference (crosstalk) from other pairs and external sources. This network cabling system can maintain frequencies of up to 100 MHz and speeds of up to 1000 Mbps. Typical CAT-5 cables have three twists per inch in each twisted pair of 24-gauge copper wire.

CAT-5 cable is produced in two forms: stranded form and solid form.

The stranded form is more flexible and less susceptible to breakage. This form is suitable for reliable connections with insulation-piercing connectors, but not for connectors of the insulation-displacement type.

The solid form is less expensive. It is suited to creating reliable connections with insulation-displacement connectors, but not for connectors of the insulation-piercing type.

Considering these characteristics, building wiring (wall socket to central patch panel) should be solid, while patch cable (movable cable that plugs into the wall socket and a computer) should be stranded core.

Category 5E (CAT-5E) is a newer version of CAT-5 which adds specifications for far-end crosstalk. Formally classified in 2001 as the TIA/EIA-568-B standard, it no longer uses the traditional CAT-5 specifications. The 1000Base-T networks were designed to use CAT-5 cables, but the enhanced specifications attributed to the CAT-5E cables and connectors make it a better choice for use within 100Base-T. The CAT-5E cable system does not allow longer cable distances within the Ethernet networks, despite the improvement in performance specifications. Its cable length is limited to 100 meters (328 feet) maximum.


ANI is short for Automatic Number Identification. This is a system used by telephone companies to determine the directory number (DN) of a subscriber currently making a call.

ANI can be likened to caller ID as both systems provide similar functions. However, ANI makes use of more advanced technologies to perform its processes. ANI uses multi-frequency signaling to transmit in-band data. ANI data can be transferred separately using an ISDN PRI (Integrated Services Digital Network–Primary Rate Interface). On the other hand, ANI is virtually impossible to block. This is in contrast to caller ID, which can be blocked by entering a set of numbers before dialing the telephone number one would call.

An ANI message consists of several pieces of information. It consists of the key pulse(KP), the information digit, the directory number, and the start signal (ST). The information digit can have a number of possible values. Automatic identification is represented by 0, while operator identification is signified by 1. Identification failure is denoted by 2.

During the early years of ANI, it was used by telephone companies to determine the exact bill they would charge to their subscribers. This ensured that the payments being given were accurate and all telephone services acquired were registered. Later on, certain telephone companies started to make ANI available to commercial customers requiring its features. One of the most prominent services where ANI is used is the 911 emergency number. Through this core technology, 911 staff can quickly determine the number of the phone used by individuals who need urgent help.


What is TTY?

TTY is an acronym that stands for Text Telephone. It is also known as TTD or Telecommunications Device for the Deaf. However, TTY is more widely accepted because it is also used by people with speech impairments.

A TTY is a device designed specially for deaf people, and people who are speech or hearing impaired. It enables them to use the telephone. It features a keyboard where users type their messages, which are then displayed on a text monitor. These messages can be sent back and forth between two TTY devices or between devices that share the same protocol.

How Does TTY Work?

A TTY indicates an incoming call with a flashing light, or through a vibrating wristband that resembles a watch. The device itself is often the size of a small laptop with a keyboard that holds about 20 to 30 character keys. It features a small screen that uses LEDs or an LCD to display the text. Some devices include a small roll of paper on which text could also be printed.

The letters in the typed message are turned into electrical signals that could travel over a regular telephone line to a compatible device. Upon reaching their destination, these signals are converted back into electronic text which can be displayed on the screen, printed out, or both.

Communicating With a Hearing Person

A hearing person can still communicate with a deaf person through an ordinary voice phone with the use of the Telecommunications Relay Service (TRS). The TRS employs special operators who types down the message so that the hearing impaired recipient can read it. Conversely, the recipient could type a response and the TRS operator will read the message over the phone for the hearing person. This procedure also done for hearing and speech impaired callers.

What are the Advantages of TTY Devices?

TTY devices make telephone communication more accessible for people with hearing and speech impairments. It provides the comfort of just picking up the phone and chatting away — a luxury which most hearing persons take for granted. It has also created a convenient means of contacting the police or the fire station in case of emergencies.


What is SS7?

SS7, or Signaling System No. 7, is a series of telephone signaling protocols defined by the Telecommunication Standardization Sector (ITU-T) of the International Telecommunication Union (ITU). SS7 specifies the procedure and protocol used for signaling and control for a variety of network services and capabilities, used to set-up most of the world’s public switched telephone network. SS7 is often referred to as Common Channel Signaling System No. 7 (CSS7 or C7) in North America. Meanwhile, the same standard is referred to in the United Kingdom as C7 (CCITT number 7), number 7 and CCIS7.

What are the Uses of the SS7 Network and Protocol?

The SS7 network and protocol have the following uses:

  • Setting up and tearing down telephone calls;
  • Wireless services, such as those used in personal communications services (PCS), mobile subscriber authentication, and wireless roaming for mobile telephony;
  • Toll-free calls (800/888) and toll wireline services (900);
  • Call features, such as call forwarding, three-way calling, and calling party name/number display;
  • and Worldwide telecommunications.

SS7 Technology

SS7 technology is a type of packet switching, which dynamically assigns routes based on accessibility and “least cost” algorithms. One advantage of SS7 over the Internet, which uses a vast Web of interconnecting facilities and routing equipment, is that its networks are private and logically self-contained. This feature makes the SS7 network secure and reliable.

SS7 Signaling Modes

SS7 networks are designed to run in two modes: Associated Mode and Quasi-Associated Mode.

Associated Mode is capable of signaling moves from switch to switch through the public service telephone network (PSTN). It follows the path used by the associated facilities carrying the telephone call. The Associated Mode is suitable and more efficient for small networks. However, it is not the predominant choice for signaling mode in North America.

Quasi-Associated Mode is capable of signaling moves from the originating switch to the terminating switch. It follows a path through a separate SS7 signaling network that consists of signal transfer points (STP’s). The Quasi-Associated Mode is suitable and more efficient for large networks and is a predominant choice for signaling mode in North America.

SS7 Networks

An SS7 network is composed of several types of links (A, B, C, D, E, and F) and three signaling nodes (SSP, STP, and SCP). Each node is identified on a network by a number called a point code.

Signaling Links

The messages in the SS7 network are exchanged between signaling links, which are 56 or 64 kbps bidirectional channels. Signaling occurs out-of-band on dedicated channels, instead of in-of-band on voice channels. This attribute provides for faster call set-up time, more efficient use of voice circuits, support for intelligent network and improved control over deceptive network usage.

Signaling Types

Signaling types are classified according to their use in the SS7 signaling network:

A link

An Access link connects a signaling end point to a signaling transfer point (STP). Messages originating from or destined to the signaling end point are transmitted on this link.

B link

A Bridge connects an STP with another STP.

C link

A Cross link connects STP’s, performing the same functions into a mated pair. This link is used when an STP has no other available route to destination signaling point caused by link failure. SCP’s may also be set up in pairs to improve its reliability.

D link

A Diagonal link connects a secondary STP pair to a primary STP pair.

E link

An Extended link connects an SSp to an alternate STP. These alternate signaling paths are often used when an SSp’s STP is unreachable with an A link.

F link

A Fully Associated link connects two signaling end points.

Signaling Points

Signaling points in an SS7 network are identified by a numeric point code. These codes are transmitted via signaling messages exchanged between signaling points to identify the source and destination of each message.

There are three types of signaling points in an SS7 signaling network:

SSPService signaling point

SSP are switches that originate, terminate, or tandem calls. It sends signaling messages to other SSP’s to set up, release, and manage voice circuits necessary to complete a call. It may also determine how to establish a route for a specific call by sending a query to a central database.

STPSignal transferring point

An STP is a packet switch that routes incoming messages to an outgoing signaling link based on the information in the ss7 message. As it functions as a network hub, STP’s eliminates the need for direct links between signaling points. STP’s may perform global title translation, and act as a firewall to screen messages exchanged with other networks.

SCpService control point

SCP’s and STP’s are often set up in mated pairs in separate physical locations to ensure a system-wide service in the event of an isolated failure.

Caller-ID Block

Caller ID block is a means of making a call to a number without revealing the number you are using. Essentially, when you call someone and use this technique, the caller will see a message such as “Caller ID Blocked” in his caller ID screen.

Maintaining your privacy is the primary reason why you may need to block a phone’s caller ID feature. There may be times when you need to contact individuals without letting them know your number. You might choose to block a phone’s caller ID feature when using a borrowed number to prevent your contacts from registering the number to your name.

There are several ways to block the caller ID of a number. Dialing a caller ID block code before dialing the number is the first option. The code may differ from one country to another so you need to check the exact one being used in your country. Although this blocking technique is generally effective, there are certain cases where the caller ID of your contacts will still display your phone number.

Another way is to request your phone company for services that can block the caller IDs of all your contacts. By default, all the outgoing calls made through your phone will block all your contacts’ caller IDs. However, not all companies may provide this service.

You can also block caller ID by using a service that routes your call to a third party system. For instance, you can use a prepaid phone card to make calls. You can also use online calling services such as the ones offered by Skype and Yahoo.


Registered Jack-11 or RJ-11 is a type of physical connector interface frequently used for telephone wire terminals.

It is a 4- or 6-wire connector primarily used to connect telephone equipment in the United States. However, even if this connector has a total of 6 connector positions, usually only 2 or 4 connectors are actually being used.

RJ-11 is the most common type of Registered Jack. It is used to plug the handset into the telephone and the telephone into the wall.

In addition, it is used for single line POTS (Plain Old Telephone Service) telephone jacks in most homes and offices.

RJ-11 often uses a 6P4C (6-position, 4-conductor) jack which has 4 wires, 2 of which are unused, functioning to a central junction box. Two of its 6 possible contact positions are used to attach the Tip (ground or positive side) and Ring (battery or negative side) of a phone circuit.

It can also be wired with a 16P2C (6-position, 2-conductor) variety of modular jack, though this is not frequently done.

The RJ-11 connector wiring appears in 2 varieties: Unshielded Twisted Pair (UTP) cable and Untwisted or flat-satin cable.

The extra wires are utilized for various applications such as:

  • an anti-tinkle circuit to prevent a pulse-dial or loop disconnect dial telephone from ringing the bell on other extensions;
  • a ground (reference point in an electrical circuit) for selective ringers; and
  • a dial light requiring a low voltage

In the powered version of RJ-11, Pin 5 (orange) and Pin 2 (white with orange stripe) may carry either low voltage Alternating Current (AC) or Direct Current (DC) power from a central transformer plugged into a wall next to one jack. This delivers power to each of the other jacks throughout the house.

In most cases, the telephone line itself can directly supply ample power for most phone terminals. However, old phone terminals with incandescent lights require more power than the phone line can deliver. Hence, there is a need to use Pins 5 and 2.

The positive terminal in RJ-11 is Pin 3 (blue) while the negative terminal is Pin 4 (white with blue stripe).

Additional Reading on RJ11