Telephony Overview

Network Access

Network access options come in a wide range of combinations and variations. This section discusses the primary ones.

Let's start with the telephone, called a station set by telecommunications people. Regardless of the kind of communication network a business chooses, the basic instrument for access to the network is the telephone station set.

The station set achieves network connection through two general functions: providing onhook/offhook and dialing signaling, and acting as a transmission endpoint (sending and receiving the voice signal).

How Does a Station Set Work?

Up to now, we've said very little about the telephone itself. That is because the phone is the simplest part of the telephone network. The phone's basic functions are:

• Trigger onhook/offhook signals to the CO. 

• Transmit dialed digits (either pulse or tone).

• Transmit a voice signal.

• Receive a voice signal.

Like other network elements, the station set needs a power supply. Single line phones draw power from the CO. Corporate telecommunications sets, such as those used with key systems and PBXs, usually draw power from the system unit.

The latest development in telephone set technology is the digital phone. A primary feature of digital phones is that PCM is performed within the set, enabling conversion of the voice to a digital signal before it leaves the set. If the receiving phone is also digital, it reconverts the digital signal to an analog signal for the listener. Provided that there are no analog elements in the network, the result is an end-to-end digital transmission. The advantages of a digital phone for the user are debatable, although there are no real disadvantages other than cost. Sound quality is generally somewhat better. Certain features made feasible by digital technology may be very appealing.

For the network provider, however, end-to-end digital connectivity is ideal because it simplifies the network's job. The same is true for the makers of PBXs and key systems. By using a completely digital technology, manufacturers can offer greater reliability and more features at a reasonable price.

Although the technology is fairly well developed, digital sets are still very expensive. They will not be common for some time--probably not until the rest of the telephone network is entirely digital. At present, many digital communication systems use analog station sets and make the analog/digital conversions in the system unit.

The corporate user can obtain network services and features in one of two ways. One is to buy the local telco's Centrex offering. The other is to purchase Customer Premises Equipment (CPE) that provides controlling internal features with access to the CO and the network. Common examples are the PBX (Private Branch Exchange) and the key system. Each of these options is discussed below.

Whether connected to Centrex, a key system, or PBX, the phone provides access to system software. Whatever system is chosen, seemingly unlimited features are available to the user, with more always in the works. Many new features take advantage of computer technology.  

What Is a Key System?

As you know, the connection between the Central Office and the customers' single line and key equipment is called a business line. These lines are usually loop start. A loop start line is always an open connection between the customer premise and the Central Office. That is, the Central Office is always trying to send dial tone out over the line. These business lines are very similar to the residential lines that you have at home.

Key Systems are supported by a KSU or a Key Service Unit. This Key Service Unit supplies power and intelligence to all of the stations. It is important to recognize that Key Systems require power and electricity in order to function. In the event of a power failure these KSUs will shut down unless there is a back up power source.

Some vendors offer customers an optional battery back up system. This system will generate power into the KSU from anywhere between 2-6 hours. If no battery back up has been purchased, a power failure will result in leaving the customer without service.

It is important to understand that when you lose power it generally does not mean that you lose dial tone. When you are at home and the electricity goes out, what is one of the first things that you do? Most people pick up the phone and call a friend or neighbor to ask if they have lost power too. How many of you have those small trimline telephones in your homes that have the keypad right on the handset? Well, when the electricity goes out at your home and you pick up that handset, it is still illuminated and you still have dial tone. The light on that key pad is powered by the dial tone being generated at the Central Office.

For those customers who have not invested in a back up power supply for their KSU, you should recommend some type of power failure procedures. An easy solution would be to have   a single line set installed with the main number on it. The line would not only be crosswired into the KSU but also into this single line set. In the event of a power failure, your customer would have the ability to answer calls on the single line set and take messages. If the line has call forwarding capabilities, they could call forward all of the incoming traffic to another location until the power issues have been resolved.

One and two line sets are KSU-LESS. These sets, like the ones you have at home, are powered by dial tone. Any speed dial buttons are energized by a battery that is usually stored in the bottom of the set. New batteries can last approximately two years depending on quality of both batteries and set type. Depending on the vendor of the set, you may need to reprogram the speed dial keys when you change the battery. You should always follow the instructions provided with packaging of the set. Very few KSU-LESS sets can support speed dial keys without a battery to charge them.

The whole concept of using a Key System is to allow users to share lines. In order to notify another user of a call, you must activate the intercom path. Users refer to these as intercom number, internal extensions and extension numbers. Regardless of what name end users call them, intercoms do not require dialtone from the Central Office. Intercoms are supplied by the KSU.

The best way to understand how intercoms work is to think of them as walkie-talkies. All of the station users in a Key System environment have both a telephone and a walkie talkie. When a station user answers an incoming call on the telephone, they place it on hold and use the walkie talkie (or intercom) to notify the specific station user that a call is holding. In review, lines are shared...intercoms are used for internal notification as well as internal communication.

Key Systems can be pooled or squared. Let's begin with Squared Key Systems. In a standard squared Key System environment, all lines appear on all telephones. Each station is then programmed for specific ringing patterns. "Immediate ringing" would be programmed into attendant positions, so that they can answer and screen calls and then notify the specific end  user that a call is holding on a designated line. "Delayed ringing" would be programmed into the sets of back-up personnel so that they can pick up calls that are going unanswered at the attendant position. "No ring" is usually programmed into executive telephones so that they are not disturbed by incoming calls but still have access to the lines that are holding for them.

You can also have a Key System with selected access. In this particular environment the lines are in the same key position on each set, but some sets are denied access to particular lines. Those keys are most commonly useless on a denied set and cannot be used for another line or feature key. Those end users that have access are notified through intercom that a call is holding on a designated line.

Pooled Key System environments are a little bit more complicated. the typical scenario is a few multi-line sets combined with a few single line sets. Technically, several lines are programmed into a general pool for outgoing calls. Multi-line sets have line key access to this pool. What that means is that when they need to make an outgoing call, they press a line key labeled "POOL" on their multi-button set which selects a line from the pool group and then they can dial an outside line number.

Single line users in this environment have no line key on their telephones to access the pool. When they want to make an outgoing call they enter a code which accesses the pool and gives them dialtone to make an outgoing call. Usually, the code in this environment is "9". This access code of "9" can be somewhat confusing to end users in a Centrex environment. Single line users will have to dial a "9" to access the pool and then another "9" to signal the Central Office that they are trying to place a call outside of the Centrex . This gets complicated when you prepare training materials. It is commonly necessary to prepare several Centrex user guides in a multi-location environment with key Systems involved in the design.

It is worth while to review the system design with the vendor contact and determine the capabilities of the existing KSU. In some instances, depending on equipment capabilities and space for system growth in the unit, the Centrex "9" can be "buried". The way this works is that every time a user selects a line in the outgoing pool, the KSU inserts the first "9". This design is truly dependent upon the age of the installed equipment, the availability of the card that does the insertion of the "9" and the customer's budget. This design varies in price and complexity depending upon the customer's equipment.

A more complex design in a pooled Key System environment is to have multi-line sets with different station designs. To be more detailed, a multi-line set may have a pool key, DID termination and several feature keys or speed dial keys.

No matter how simple or complex a customer's key system is, it is important to remember that even a simple installation or change in the system can effect the way outgoing calls are placed. The more you understand the Key System environment that your customers have, the more detailed you need to be when proposing Network solutions.

What is a PBX?

The Private Branch Exchange (PBX) is a CPE switch. It communicates with the CO over one or more trunks. Because it is a switch, it can handle certain signaling functions that station sets or key systems cannot. The PBX provides users with an internal communication network and with access to outside networks, such as other PBXs and the public network.   Most PBXs today use digital technology. Many use digital switching. A few are completely digital, using digital station sets and communicating with other switches over digital trunks.
Most, however, continue to use analog sets and trunks.

PBX Dialing

For outgoing calls, call addressing is the same as for a single-line CPE. Internal calls, however, do not have to follow the same system. Since most PBXs provide access to the public network, typically, internal numbers are derived from the numbers assigned by the public network.

For example, let's say you are in a PBX private network. You call another station set within that same network, a phone having the call address 349-1212. The first three digits, 349, designate a public network exchange switch. Since your call will not go through the public switched network, you need dial only the last four digits, 1212--or even a completely different internal number, if that is the way your PBX network works.

To reach a phone outside the PBX network, your call will have to go through the CO. Typically, you will dial "9" for outside access, then the full seven to ten digit number.

PBX Features

Like Centrex, PBX systems provide features for internal use, such as call forwarding, call waiting, hold, and rollover.

Other features include:

Links to computers, providing immediate database retrieval. For example, a user can dial a number by selecting a number on the computer screen.

Automatic Call Distribution (ACD), which allows the switch to automatically direct incoming calls to designated receivers in the order of an established priority. For large call centers such as telemarketing operations, a PBX can be a standalone ACD unit.

Directory systems. With a directory system, the user can key in the first few letters of a name to bring up a list of dialing options or speed dial using a directory database.

On-site management of telephone network and features. This is an important issue. Typically, SMDR management and moving telephone sets around is easier with a PBX than with Centrex. Another issue involves how much control a user has over the PBX's operation and features. The current trend is toward "open architecture", where manufacturers provide detailed information about their PBX's programming, enabling third party vendors and users to program custom features.

What Is Centrex?

Centrex was first introduced around 1960. It was developed by AT&T Bell Laboratories and deployed by the Regional Operating Companies (RBOC) that were part of AT&T at the time (pre-divestiture). Centrex was really developed to serve very large customers with Central Office based services instead of PBX services.

Centrex was created with an added advantage: It has the benefits of direct inward and outward dialing without calls having to be handled by an attendant. (This saves on labor related costs and allows quicker and easier completion of calls.) Centrex features can currently match or exceed those offered in the most advanced PBX's.

The name "Centrex" is derived from "Central Exchange" which is a reference to the switching of calls performed by a Central Office. The term Centrex is an assigned product name for the special call handling features that are provided by the local operating company Central Office equipment. These features are extended over the network to business customers who have subscribed to the service. Centrex capabilities far exceed the basic dialtone or local exchange service that is the core of the telecommunications network. Various types and quantities of features are available with the Centrex product in each state. 
To understand Centrex you need to be familiar with where the Centrex service is generated. An operating company Central Office is usually an above ground reinforced structure (building) that houses the equipment which transfers or switches calls to their destinations. Each bell operating company Central Office handles a geographic service area.

Due to advanced technology over the last few years, the various switching gear has migrated over the years from primarily mechanical (analog) equipment to highly electronic (digital) computer based processing. Centrex service is provided from assigned facilities in the Central Office. In electromechanical offices, a customer's Centrex system is actually "built" in the program of the main processor.

Each Central Office also has transmission facilities for a host of other types of services such as foreign exchange, other common carrier terminations and tie lines. These services can be accommodated by Centrex copper cables, and on an increasing basis, fiber optic cables flow into and out of each Central Office.

Computer Telephone Corp. has installed over 18,000 lines of Centrex throughout the country over the last two years. CTC's team approach to design and installation lead to customer specific designs. Because equipment compatibility is a significant piece of system design, CTC provides complimentary Vendor negotiations. The entire process allows a fully developed inventory analysis through Account Management.

With Centrex, the telco manages the network and decides which features to offer. All the user has to do is provide CPE and lease the desired lines and features from the network.

The advantage of leasing Centrex service over owning a PBX or key system is the basic difference between lease and purchase economics. With Centrex, initial purchases include only telephone sets and any necessary premise wiring. Everything else is provided on a monthly lease basis. All maintenance (except for wiring and CPE on the user side of the demarcation point) is taken care of by the network.

ISDN

Combines digital voice and data services in the same transmission. With ISDN, the user can make simultaneous voice and data calls. For example, let's say that you, an ISDN user, have a document displayed on your computer screen. You want to discuss it with your partner in another location. Given the proper equipment, you could use the voice line to talk and the data line to display the same document on your partner's screen.

ISDN with Centrex today, has developed into a cost effective alternative in certain voice applications. A station set which requires multiple appearances of a single number (or a hunt group) would require multiple phone lines or cable pair. With ISDN a single cable pair can provide multiple appearances as well as busy lamp capabilities.

ISDN can be available through Centrex, termed ISDN BRI, for Basic Rate Interface. BRI provides a single phone with a voice and data line or two voice lines. ISDN can also be part of a PBX system, termed PRI, for Primary Rate Interface. PRI systems typically have 23 voice and data channels.

Many telcos have ISDN capability now. In addition, many Centrex and PBX switching systems have been designed for migration to ISDN. Whether ISDN becomes a common communication tool depends on a variety of factors, including the development of other similar communication packages. A more detailed discussion of ISDN can be found in the MasteryPOINT ISDN module.

The increasing use of digital equipment in the networks and in the newer PBX and key systems promises even more sophisticated call control features for the customer. The increasing use of optical fiber as a transmission medium will also provide a basis for more features and better service.

Comparison of Services

Let's finish this section with a quick comparison of the various CPE options available.

Centrex is the basic telco option for business customers. It is important to consider those customers with a need for: THE FOUR LEGS (Direct Inward Dialing, Multi-location, Growth & Cost)  PBX systems are more common among large corporations. Such companies often require control of internal communication to offices outside the local telco area, a function that a PBX network can perform well. PBXs generally require a telecommunications staff to manage the system.

Key systems are most commonly used by small businesses with simple needs.

Control often becomes a deciding factor. If you have the resources, you can choose to manage your own system in-house. If your staff is small or your purchasing budget is limited, you can lease, and let the telco worry about system operations. The wide range of choices provides the customer with more bargaining power.

In addition to the standard options of Centrex, PBXs, and key systems discussed above, telcos and vendors continue develop new communication packages. For example, some companies use packages that combine sophisticated key systems and Centrex. Another example of a new type of package is the offering termed Integrated Systems Digital Network, or ISDN.

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