A Short Telecommunications Glossary

Automated Attendent   Accepts and interprets input from caller and responds with further options.

• Input from caller can be either touch tone or voice input. Voice input usually has a limited vocabulary and is less reliable than touch tone input.
• Forwards to a human operator if caller chooses an option that the automated attendant cannot handle or if the caller does not have touch tone phone.
• Options presented by an automated attendent should be short and logical.
• An automated attendent is either a front end or part of a PBX.

Automatic Call Distributor (ACD)   A PBX function that distributes calls entering an incoming call center to an operator or agent.


• Examples: customer service, help desk, order entry, credit authorization, reservations, insurance claims, catalog sales.
• A hunt group is the system that the ACD uses to assign calls to agents. Early ACDs forwarded call to first available agent. More recently, ACDs forward call to the line that has been idle the longest or based on the skill set of the agent.
• If all agents are busy, the ACD automatically places calls in a queue.
• ACDs can also forward calls to another call center, or can automatically return calls.
• ACD can also be coupled with automated attendents or ANI.
• Modern ACDs have status monitors and keep call statistics.

Automatic Number Identification (ANI)   Identifying the calling number of a phone call before the voice trunk or line is actually connected to the telephone by using SS7 information.

BORSCHT   An acronym describing some of the services of the CO for the subscriber loop. Some of these are handled by the line cards.


• Battery Feed.   The subscriber loop supplies -48 V DC.
• Overvoltage Protection.   Surge protection of up to 1000 V is provided.
• Ringing.   When the CO wants to ring a phone to show an incoming call, it sends a 90 V AC 20 Hz. signal.
• Supervision.   When the phone is taken off hook, the line card in the CO detects the completed circuit.
• Coding.   Conversion between analog (local calls) and digital (long distance).
• Hybrid.   Analog, 2-wire for local calls, digital, 4-wire for long distance.
• Testing.   Extra equipment is attached to the local loop for diagnosing trouble with the line.

Central Office (CO)   Receives calls from subscriber loops and connects them to other subscriber loops or forwards them another CO. Some of the components of a CO are the following:


• Common Control.   CPU that coordinates the functions of the wire center.
• Data Store.   Database containing customer records and routing information.
• Service Circuits.   Circuits that provide services to the subscriber loop lines, such as ringing, dial tone, and digit collection.
• Switching Matrix.   Switches that connect lines to lines, lines to trunks, or trunks to trunks.
• Line Cards.   Computer cards that perform BORSCHT functions.
• Trunk Cards.   Computer cards that perform functions similar to line cards. However, since trunks have no phone attached to them, and trunks carry digital signals, no ringing or coding is necessary.
• Main Distribution Frame.   Connects the cables coming in from the outside to the line and trunk cards.

Cellular Communications Systems   See Some Notes on Cellular and Personal Communications Systems

Centrex   A service of the CO that provides PBX functionality. All the equipment is housed in the CO.

Computer Telephony Integration (CTI) Integrating the functionality of a PBX with a database server. See the textbook [4], Page 380 for more information.

Demarcation Point (DEMARC)   When a phone company installs a line, it must not proceed any further than 12 inches into the building. This ending point is called the DEMARC. The modular connector can be:

• RJ-11C: standard 2-pair interface, six pins (common telephone jack).
• RJ-45
: single line, two wire, digital connection, eight pins (ISDN).
• RJ-48C: signal line, four wire, eight pins, digital connection 1.544 Mbps T1 line).


RJ means Registed Jack.

Digital Subscriber Line (DSL)   See Some Definitions for LANs and Page 376 of the textbook [4] for more information.

Direct Distance Dialing (DDD) Foreign Exchange   A leased line or virtual connection from a PBX (Site A) to a CO in another area code (Site B). A calls from Site B to Site A is then billed as a local call.

Interactive Voice Response (IVR)   More sophisticated then an Automated Attendent. In addition, IVR


• can connect to a database. Database can be accessed and updated, depending on caller response.
• can provide fax on demand (e.g. fax caller confirmation or receipt).
• can capture credit card or id information from touch tone phone.

Interexchange Carrier (IXC)   A company providing long distance service. According to the MFJ, LECs must provide equal access to IXCs. A CO that has not upgraded its equipment to provide such equal access is called a Nonconforming End Office.

Integrated Services Digital Network (ISDN)   See Some Definitions for LANs and Page 365 of the textbook [4] for more information.

Key System Unit (KSU)   Used in a small office or branch office. It provides resource sharing similar to a PBX, but without the more intellegent features. A PBX is preferred over a KSU when the business has ten or more trunks coming in from the CO.


North American Numbering Plan (NANP)  

• Pre 1979: NBX/NNX-XXXX
• Pre 1995: NBX/NXX-XXXX
• Post 1995: NXX/NXX-XXXX
where N is a digit from 2 to 9, B is 0 or 1, and X is any digit.

Leased Line   A private line leased by the phone company for a monthly charge (no per minute connect charge). The line is connected continuously. At present, a Virtual Private Network provides the same functionality and is cheaper.

Local Access and Transport Area (LATA)   An area set up by the MFJ defining the service area of a LEC. Callers wishing to connect to a different LATA must go through a IXC.

Local Exchange Carrier (LEC)   The company that provides local phone service to a LATA. Since the Telecommunications Act of 1996, an LEC from outside the LATA, called a Competitive Local Exchange Carrier (CLEC) can compete with the incumbant LEC for service. These CLECs do not have to be traditional phone companies. They can be

• Value Added Carriers   A company that adds value to a system (like voice recognition, voice messaging) and resells it. Most traditional phone companies do this already.
• Alternate Operator Services   Pay phones, hotel phones, airplane phones, pay for a local line to the CO and then charge extra for local service.
• Aggregators   Organize business from small phone companies and then buy service from AT&T, getting a volume discount.
• Resellers   Rent a variety of lines or trunks from phone companies, then resell to customers.
• Specialized Mobile Radio Carriers   Provide mobile communication services like dispatch, paging, and data services.
• Satellite Carriers   Geostationary satellite systems provide satellite TV and global phone service.
  
• Cable TV Companies   In addition to TV, they can provide data and phone service, although they have been slow to enter this market.
  
• Packet Data Network Providers   Voice over Frame Relay or ATM is slowly growing.

Off Premises Extension   A connection from a PBX (Site A) to a phone (Site B) through a CO. The phone at Site B appears as an extension on the PBX.

Party Line   A subscriber loop with more than one phone on it. A tuned ringer can be used on each phone.

Personal Communications Systems   See Some Notes on Cellular and Personal Communications Systems

Plain Old Telephone Service (POTS)   Plain Old Telephone Service used for data transfer over a modem, as compared to newer digital telephone services such as DSL or ISDN.

Point of Presence (POP)   The point where a customer can connect to an IXC if it bypasses the LEC.

Private Branch Exchange (PBX)   An internal, intelligent, switching system for medium to large sized businesses. A PBX allows n phones to share m phone lines where n is larger than m. Some PBX features are

• Provide switching between station sets or access to CO. Users dial an access code, like 9 to get dial tone from C0.
• 4-digit special prefixes for WATS, FX, etc.
• Collects dialed digits and decides how to route call for lowest cost (e.g. over FX line or WATS line).
• Voice mail.
• Automatic Call Distribution.
• Classes of service for users. Certain features like call monitoring or international dialing are allowed to certain stations or with password.
• Can support digital or analog phones.
• Can act as a database server in some situations.
• Direct Inward Dialing   Each phone has a number which looks like a direct connection to the CO. A call coming in from the outside is routed directly to the phone by the PBX; no operator is necessary.
• Direct Outward Dialing   The caller dials an access code like 9 to get an outside line. No operator is necessary.
Here are the components of a typical PBX: CPU, memory, telephone lines, trunks, switching network, console, logic cards, battery back up system, cabinets to house everything.

Public Switched Telephone Network (PSTN)   Two COs are connected by a 5-level hierarchy of switching centers:


• Level 1
  Regional Center.
• Level 2
  Sectional Center.
• Level 3
  Primary Center.
• Level 4
  Toll Center.
• Level 5
  Central Office.
In addition:
• Subscriber loops are only connected to Level 5 COs.
• Level 1 to 4 centers are also called Tandem Centers.
• Each Level 1 center is connected to every other Level 1 center.

Remote Call Forwarding (RCF)   Automatically forwards a local phone call to a PBX to a number in a different LATA. This is to make a number in another city look like a local number connected to the same CO as the PBX. There is a monthly charge for the RCF connection as well as DDD per minute connect charge.

Signaling System 7 (SS7)   SS7 was first introduced by the Consultative Committee on International Telegraphy and Telephony (CCITT) in 180 with revisions in 1984, 1988, and 1992. It was first deployed in 1990. SS7 is a switched data network, independent of the trunks that transmit the actual digital voice and data signals. It was developed to intellegently route calls faster than the previously in-band signaling could, and to prevent the extensive phone fraud that was possible with in-band signaling. According to [2]: "Compared to in-band signaling, SS7 signaling allows:

• Faster call setup times,
• More efficient use of voice circuits,
• Support for services that require signaling during a call, and
• Better control over fraudulent network usage."

Here is a diagram showing the relationship between the voice trunks and the SS7 system. CO means Central Office, STP means Signal Transfer Point (used to establish and connect calls), and SCP means Signal Control Point (used to supply database information to the STPs).
When a long distance call is made, the initiating CO sends an Initial Address Message (IAM) through the STPs to the destination CO. This IAM contains
• The calling number (used for caller ID),
• The called number,
• Information about call (voice, data, ISDN)
• Trunks needed to complete call.
Messages are also send to all the intermediate COs along the path needed to complete the connection. The terminating CO sends an Address Complete Message back to the initiating CO and then rings the call.

The SCPs store information needed to route calls intelligently. For example, they store

• 800 and 900 number translations,
• private dialing plans like VPNs or RCF,
• selective call forwarding,
• selective call rejection, and
• calling card identification.

According to [3]: "SS7 is an internationally standardized general-purpose common-channel signaling system with the following primary characteristics:
• Optimized for use in digital telecommunication networks, utilizing 64-kpbs digital channels.
• Designed to meet present and future information transfer requirements for call control, remote control, management, and maintenance
• Designed to be a reliable means for the transfer of information in the correct sequence without loss of duplication.
• Suitable for operation over analog channels and at speeds below 64kbps
• Suitable for use on point-to-point terrestrial and satellite links."

Although SS7 was designed before the OSI Model, the first three layers of the two models are roughly parallel, as shown in this table:

	OSI Model	SS7 Model
Layer 3	Network	Signaling Network and Signaling Connection Control Part
Layer 2	Data Link	Signaling Link
Layer 1	Physical	Signaling Data Link

• Layers 1, 2, and 3 form the Message Transfer Part.
• The Signaling Data Link (Layer 1 of MTF) defines the physical and electrical characteristics of the network,
• The Signaling Link (Layer 2 of MTF) defines the format of the message signal unit format (frames in OSI terminology). It provides for error free transmission from one STP to another. Some of the fields of the frame contain
  • Flags to signal the beginning and end of a frame,
  • Sequence number to order frames correctly at the destination,
  • Length indicator tells how much data is included in the frame,
  • Routing information,
  • CRC to provide error detection.

• The Signaling Network (Layer 3 of MTF) provides message routing and network management functions. It has the capability of automatically rerouting a connection in the case of congestion or link failure.
• The Signaling Network Control Part handles Transaction Capabilities Application Part (TCAP) functions like routing 800 and 900 numbers, calling card validation, and functions of wireless phone networks like connecting a cellular phone to the closest base station.
• In addition to real time management of phone connections, the higher levels of SS7 handles the routing of ISDN connections.

Subscriber Loop   The loop connecting a telephone to a CO. The subscriber loops are arranged in a star configuration, and each loop should not be more than 3 miles in length. Load coils are placed in a subscriber loop to counteract the natural capacitance of the wires.

Switching System 7 (SS7)   To be added later.

Tandem Office   A Level 2, 3, 4, or 5 switching office.

T-Carrier System   A popular digital voice and data transmission system, of which DS1 and T1 lines is an example. See Some Details of T-Carrier Systems for more details.

Telephone   Invented in 1874. The parts of a modern (since early 1960s) phone are

• Receiver   Converts analog signals to sound by moving a thin membrane with an electromagnet.
• Speaker   Converts sound to an analog signal by passing electricity through a chamber of carbon granules. When the carbon is compressed by the changing air pressure of the voice, the resistance changes, thereby changing the electrical signal.
• Key Pad   Twelve buttons are available, each of which can be pressed, sending a 2-frequency signal to the CO that is interpreted by a Service Circuit. See Standard Tone Dialing Frequencies for more details.
• Hook   When the receiver is taken off-hook the subscriber loop is completed so that a line card at the CO can recognize that the phone is off-hook.
• Handset Cord   4-wires, 2 for receiver and 2 for transmitter, RJ-11C connector. One receiver wire tied to one transmitter wire so you can hear yourself talk, or recognize a dead circuit.

Trunk   A line connecting two switching centers, a switching center to a PBX, or two PBXs. A trunk

• does not have a telephone number associated with it.
• carries voice or data signals between switching centers.
• can carry either analog or digital signals.
• repeaters or amplifiers required every few miles.
• Digital trunks use TDM to carry multiple voice signals (see T-Carrier System).

Virtual Private Network (VPN)   A connection that looks like a private leased line to the customer, but is actually part of the PSTN. The two possibilities for such a connection are

• On-to-On   Both ends of the connection are predetermined PBXs.
  
• On-to-Off   One end is a predetermined PBX, but the other end is an ordinary long distance dialing connection.
On-to-on is slightly cheaper than on-to-off.

Voice Mail   A voice message storage system, often controlled by a PBX.


• Contains logical mailboxes assiciated with extension numbers.
• Each mainbox can store multiple voice messages.
• Possibly multiple greetings for each mailbox.
• Mailbox owner can record greetings, hear, delete, and forward messages.
• Can send a message to multiple destinations.

Voice Processing   See Automated Attendent.

Wide Area Telecommunications Services (OUTWATS)   Provides discounted rates for outbound calls to certain geographic areas. It began in the 1970s, but other pricing schemes have largely replaced it. Full time WATS can be used by large businesses. They pay a monthly fee, but no per minute connect charge.

INWATS   Commonly known as 800 service. The receiver of the call pays for the call. 900 service is also available, where the caller pays a charge (per call or per minute) for the service provided by the call.

Videoconferencing   To be completed later.

References