Video Communications

Video communication is the transmission and reception of video (multiple images) using electrical or optical transmission signals. Telecommunications systems can transfer video signals in analog or digital form.

Analog Video
Analog video is the representation of a series of multiple images (video) through the use of rapidly changing signals (analog). This analog signal indicates the position, luminance, and color information within the video signal.

Sending a video picture involves the creation and transfer of a sequence of individual still pictures called frames. Each frame is divided into horizontal and vertical lines. To create a single frame picture on a television set, the frame is drawn line by line. The process of drawing these lines on the screen is called scanning. The frames are drawn to the screen in two separate scans. The first scan draws half of the picture and the second scan draws between the lines of the first scan. This scanning method is called interlacing. Each line is divided into pixels that are the smallest possible parts of the picture. The number of pixels that can be displayed determines the resolution (quality) of the video signal. The video signal television picture into three parts: the picture brightness (luminance), the color (chrominance), and the audio.

Digital Video
Digital video is a sequence of picture signals (frames) that are represented by binary data (bits) that describe a finite set of color and luminance levels. Sending a digital video picture involves the conversion of a scanned image to digital information that is transferred to a digital video receiver. The digital information contains characteristics of the video signal and the position of the image (bit location) that will be displayed.

Data Communications

There are two basic types of data communications: circuit-switched data and packet-switched data. Circuit-switched data provides for continuous data signals while packet-switched data allows for rapid delivery of very short data messages.

Circuit-switched Data
Circuit-switched data is a data communication method that maintains a dedicated communications path between two communication devices regardless of the amount of data that is sent between the devices. This gives to communications equipment the exclusive use of the circuit that connects them, even when the circuit is momentarily idle.

To establish a circuit-switched data connection, the address is sent first and a connection (possibly a virtual non-physical connection) path is established. After this path is setup, data is continually transferred using this path until the path is disconnected by request from the sender or receiver of data.

Figure below shows the basic operation that uses circuit-switched data. In this example, a laptop computer is sending a file to a company’s computer that is connected to the public switched telephone network (PSTN). The laptop computer data communication software requests the destination phone number from the user to connect to the remote computer. This telephone number (the address) is used connect a path through the PSTN switches until the call reaches the destination computer. The dialed number is first connected through local switch #1, port number 4236. This port number is assigned to a memory location in the switch that routes the data connection through a high-speed line, time slot 6 to an IXC switch. The IXC switch then assigns a memory location in its switch to a high-speed line, time slot 3 that connects to local switch #2. Local switch #2 assigns a memory location in its switch to port number 1249. This port connects to the remote computer. Once this path through the network is setup, it remains constant throughout the data communications session regardless of how much data is transferred between the laptop computer and the company’s computer.

Circuit-switched Data

Packet-switched Data
Packet data service provides data transfer in the form of short packets of information. The public telephone network was designed primarily to offer voice services. Shortly after the telephone network was introduced, circuit-switched (continuous) data services were offered. The operation requirements for circuit-switched and packet-switched data services are very different. Circuit-switched data has substantial time and is inefficient for serving sensing control and applications that require small amounts of information. Initially the standard telephone system had to be enhanced (functionally divided) to offer packet data service. However, with the digitization of communications systems, telephone systems operate more like packet data systems.

Typical applications for packet data service include Internet browsing, wireless email, train control system, route guidance, credit card processing and many other applications that benefit from the transmission of data in bursts when communicating.

Packet data systems provide effective use of the resources. Packet data systems only use network equipment resources when there is information to transfer. This provides the advantage of charging only for the amount of information used and increased system efficiency.

A packet is a group of digital bits that is transported and switched through a network of packet switches (often called routers) to their destination. The structure of these packets (digital bit sequence) is arranged in a specific format to allow the determination of the destination address for each packet in addition to the data that is being transported. Optionally, the packet structure may include other information such as the packet originator and error protection bits.

Transmitting data through a packet network involves dividing data files into small packets (typically under 100 bytes of information). A packet data system divides large quantities of data into small packets for transmission through a switching network that uses the addresses of the packets to dynamically route these packets through a switching network to their ultimate destination. When a data block is divided, the packets are given sequence numbers so that a packet assembler/disassembler (PAD) device can recombine the packets to the original data block after they have been transmitted through the network.

Figure below shows the basic operation that uses packet-switched data. In this example, a laptop computer is sending a file to a company’s remote computer that is connected to a packet data network. The laptop computer data communication software requests the destination address for the packets for the user to connect to the remote computer ( In this example, the source computer divides the data file into three parts and adds the packet address to each of the 3 data packets. The packets are sent through routers in the packet network that independently determine the best path at the time that will help the packet reach its destination (smart switches). This diagram shows the three packets take 3 different routes to reach their destination. When the 3 packets reach their destination, the remote computer reassembles the data packets into the original data file.

Packet-switched Data

Public Data Networks (Internet)
Public data networks interconnect data communication devices (e.g. computers) with each other through a network that is accessible by many users (the pubic). To allow many different users to communicate with each other, standard communication messages and processes are used. The Internet is an example of a public data network (there are other public data networks) that uses standard Internet protocol (IP) to allow anyone to transfer data from point to point by using data packets. Each transmitted packet in the Internet finds its way through the network switching through nodes (computers). Each node in the Internet forwards received packets to another location (another node) that is closer to its destination. Each node contains routing tables that provide packet-forwarding information.

Each network in the Internet can have different transmission formats (e.g. different packet sizes, high-speed or low-speed data) but the all agree on how to receive and distribute IP packets. Internet service providers (ISPs) connect users (e.g. computers) to the Internet. ISPs are interconnected to each other through network service providers (NSP). NSPs are relatively large networks that may cross international boundaries. NSPs can connect to each other through network access points (NAPs). Because there are a limited number of NAPs, there are also private network access points (PNAPs). PNAPs are setup by the NSPs to relieve the congestion on the NAPs.

Figure below shows the Internet. This diagram shows that the Internet is composed of users (end points), Internet service providers (ISPs), network service providers (NSPs), and network access points (NAPs). Computers are connected to the Internet via an ISP. The ISP receives data from the computer, reformats it (if necessary), and forwards it to the destination computer in its network. If necessary, it may be routed to an NSP, which will route the data packets to their destination ISP to an NAP that will allow the packet to reach its destination. Eventually, packets reach their destination ISP that forwards the packets to the user.

Voice Communications

Voice communication is the transmission and reception of audio and other signals that can be represented by the frequency band used for voice signal transmission. Telephone systems transfer voice signals in a variety of forms through by wire, radio, light, and other electronic or electromagnetic systems. These forms include analog and digital voice signals. Options for voice communications include different voice quality of service levels and voice privacy options.

Voice Quality
Voice quality is a measurement of the level of audio quality, often expressed in mean opinion score (MOS). The MOS is number that is determined by a panel of listeners who subjectively rate the quality of audio on various samples. The rating level varies from 1 (bad) to 5 (excellent). Good quality telephone service (called toll quality) has a MOS level of 4.0.

The first telephone systems used analog signals to represent the voice. To overcome the cumulative noise limitations of analog signal transmission, digital transmission systems were created. These digital transmission signals represented voice signals by discrete levels that can be recreated eliminating the noise. As a result, in the 1960’s, many modern telephone systems began to offer digital voice communications.

The first digital voice services converted (digitized) the analog voice signal to a 64 kbps digital signal. This 64 kbps digital channel called a DS0) provided “toll quality” voice with a MOS score of 4.0 or above.

Generally, there is a tradeoff between system efficiency (bandwidth used) and the level of voice quality. To gain system efficiency (to add more customers per interconnection line), some telephone systems compress the voice using speech-coding (data compression) technology. The first compressed voice service uses adaptive pulse coded modulation (ADPCM) that further compresses the 64 kbps DS0 to 32 kbps ADPCM.

Other voice compressed voice service have been developed that can use low bit-rate standard or proprietary speech compression algorithms. These can further compresses the 64 kbps DS0 to below 16 kbps or even 8 kbps.

Voice Privacy
Voice privacy is a process that is used to prevent the unauthorized listening of communications by other people. Voice privacy involves coding or encrypting of the voice signal with a key so only authorized users with the correct key and decryption program can listen to the communication information.

Digital systems are inherently more secure than analog systems because they can easily use an encrypted mode of operation. This encrypted mode of operation “scrambles” voice data before it is sent to other users in the network. The encryption uses a key (mask value) that is calculated from some form of secret data. When the voice data is received, it must be decrypted using the same mask value that was used to encrypt it. Although an interceptor may be capable of receiving the data signals, they cannot learn the true data value unless the secret number that was added to it is also known.

While the telephone system can offer an encryption mode that encrypts the signaling between the end-user’s phone line and the telephone network, it is more common for the end-user to maintain their own voice encryption system. This does help to prevent unauthorized access to the telephone system. This also allows the end-user to have many different voice encryption algorithms. The voice encryption algorithms are typically stored on the end-user’s telephone devices.

Telecommunications Applications and Services

Telephone applications are the processes or programs that provide specific features and benefits for the customer that involve the transfer of information through communication systems. Telecommunications services are the underlying communications processes that provide information for telecommunications applications. It is common to use the word services in place of applications, especially when the service is very similar to the application. Examples of communication applications include voice mail, email, and web browsing.

Telephone services include voice, data, and video transmission. Voice services can be categorized into quality of service and voice privacy. Data services use either circuit-switched (continuous connection) data or packet-switched (dynamically routed) data. Video transmission is the transport of video (multiple images) that may be accompanied by other signals (such as audio or closed-caption text).

Telecommunication services may be provided (distributed) to one or more users of information. Distribution of services can be categorized into broadcast, multicast, and point-to-point delivery. Broadcast service delivers the same information to all users in a network. Multicast distribution service distributes information to specific users within a network. Point-to-point service transfers information between two specific users or devices within a network.

The transfer of information between users can be unmodified or modified. Telecommunication services that only involve the transport of information are called bearer services. Services that require information processing (such as store and forward) in the network are called teleservices. Services that combine bearer services and teleservices into a new unique services are called supplemental services.

System features and services are typically provided by call-processing software in the telephone network that interacts with end-user equipment. As a result, some telephone equipment may be only able to operate some system features (e.g., call forwarding) while some service features require device capabilities and software (e.g., calling number identification presentation). To help ensure the correct operation of services and device interaction, industry standards are created.


The regulation of telecommunications systems and services are developed to help or improve the ability of citizens in a country to reliably communicate with each other at reasonable cost. Telecom rules and regulations are usually imposed by a government agency. These rules are designed to maintain the quality and cost of public utility services.

In the United States, the Telecommunications Act of 1996 was created to allow competition into the telecommunications industry. It provides a national framework for the deregulation of the local exchange market, a deregulation that was already taking place on a state-by-state basis through the actions of state regulatory commissions. Its summary impact on the local exchange market is to require current LECs to remove all barriers to the competition (e.g., interconnect, white and/or yellow pages access, co-location, and wholesaling of facilities restrictions) in return for LEC access to the long distance market.

Telephone companies in the United States are regulated by the government, but not owned by the government. For most European countries and many other countries, local telephone service is provided by government owned post telephone and telegraph (PTT) operators. In some European countries, the post (mail) network has been separated from the operation of telephone and telegraph networks. In some countries, the telephone and telegraph systems have become privatized, and are no longer owned by the government.

Networks, as a rule, connect to long distance networks through a separate toll center (tandem switch) for inter-exchange connection,. In the United States, this toll center is called a point of presence (POP) connection.

To connect local systems to each other (long distance), inter-exchange service is provided. In the US, these are called inter-exchange carriers (IXCs). In the United States, from 1984 until 1997, IXC and LEC operating companies were legally required to refrain from engaging in directly competitive business operations with each other. Since 1997, one business entity can engage in both IXC and LEC business if it satisfies certain competitive legal rules. In Europe and throughout the rest of the world, the same PTT operators also usually provide inter-exchange service within their country. In any case, governments regulate how networks are allowed to interconnect to local and long distance networks...

Telephone and Device Numbering

Each device within a network must have its own unique address. Some of the different types of addresses that are available include telephone numbers and data network addresses.

International Numbering Plan (ITU)
The International Telecommunications Union (ITU), a division of the United Nations, has defined a world numbering plan recommendation, “E.164.” The E.164 numbering plan defines the use of a country code (CC), national destination code (NDC), and subscriber number (SN) for telephone numbering. The CC consists of one, two or three digits. The first digit identifies the world zone. The number of digits used for telephone numbers throughout the world varies. However, no portion of a telephone number can exceed 15 digits. There are several “E” series of ITU numbering recommendations that assist in providing unique identifying numbers for telephone devices around the world.

North American Numbering Plan (NANP)
An 11 digit-dialing plan is used within North America. It contains 5 parts: international code, optional intersystem code (1 +), geographic numbering plan area (NPA), central office code (NXX), and station number (XXXX). The NPA code defines a geographic area for the serving telephone system (such as a city). The NXX defines a particular switch that is located within the telephone system. Finally, the station code identifies a particular line (station) that the switch provides service to. Figure below shows the telephone numbering systems.

Internet and Data Network Numbering
Most data network addresses are hierarchical where the beginning of the address identifies the entire network and each progressive address number (or group of numbers) identifies more specific parts within the network.

Data networks are usually composed of several interconnected links. These links can be of different technologies with each of their end points identified by a unique numbering system. Figure 1.14 shows how different types of data network addressing systems can co-exist. This diagram shows a data connection that is composed of several parts. An end-user is connected to an application server through a company Ethernet network. The computers network interface card (NIC) has an address unique to the Ethernet hub. The Internet address is included as part of the data message after the Ethernet address. The company’s network is connected to an ISP by a high-speed frame relay connection. The frame relay access device (FRAD) has a unique identifier to the ISP. The ISP connects the data connection via asynchronous transfer mode (ATM) to the ASP.

Number Portability
Number portability involves the ability for a telephone number to be transferred between different service providers. This allows customers to change service providers without having to change telephone numbers. Number portability involves three key elements: local number portability, service portability and geographic portability.

The first part of the telephone number (NPA-NXX) usually identifies a specific geographic area and specific switch where the customer subscribes to telephone service. If a telephone number is assigned to another system (different NXX) in the same geographic area (same NPA), the interconnecting carriers (IXCs) connecting to that system must know which local system to route the calls based on the selected local service providers. In this case, the IXC must look up the local telephone number in a database (called a database dip) prior to delivering the call to the end customer.

Figure below shows an example of local number portability (LNP). In this diagram, a caller in Los Angeles is calling to someone in Chicago. The call is routed through the LEC in Los Angeles and routed through a long distance provider who needs to connect the call into a local telephone company in Chicago. Because there are several local providers in Chicago, the IXC must look into a database to see if the number has been ported to a different service provider. This is the next to last switch before the call reaches the end office switch (called “N-1”). This switch uses the dialed digits to find which local carrier is providing service in the Chicago area. When the IXC finds which exchange is serving the number, the call is routed to the correct local switching office and the call is completed.

example of local number portability

Service portability allows a customer to take their telephone number to a different type of service provider. Service portability involves determination of the type of service provider (e.g., wireless or wired) who is responsible for completing the call using the area code and NXX. The interconnection and call processing for different types of service providers varies.

Geographic portability involves the transfer of telephone numbers outside the normal geographic boundaries of the service provider’s area. Geographic portability allows a customer to keep their same area code when they move to new cities or other distant geographic regions...

Wireless Networks

Wireless networks are primarily designed to transfer voice and or data from one point to one or more other points, (multipoint). Many networks make use of some wireless technologies as a transport medium even though we do not consider them to be wireless networks. Examples of wireless networks include cellular, personal communication service, (PCS), paging, wireless data, satellite, and broadcast radio and television.

Cellular and PCS
Cellular and PCS systems are comprised of a set of radio towers that are strategically distributed over a geographical area in order to provide a continuous service coverage area. A mobile switching center (MSC) provides the switching and control functions necessary to connect calls from the public switched telephone network (PSTN) to the individual mobile telephones. The MSC also manages the radio resources within the entire network.

Several service providers (carriers) within a particular geographical area can provide wireless services simultaneously as long as they use a different set of radio frequencies that do not interfere with each other. Specific sets of radio frequencies are allocated for use in cellular communications systems. Typically a government agency is responsible for assigning these frequencies and licensing them to specific service providers.

Paging is a wireless system that is capable of delivering message to one or more people whose exact whereabouts are unknown by the sender of the message. Users typically carry a small paging receiver that displays a numeric or alphanumeric message displayed on an electronic readout. It also could be sent and received as voice message or other data. Pager systems may be classified based upon their capabilities as numeric, alphanumeric, tone, or voice.

Paging networks are comprised of terrestrial based antennas all interconnected by means of some other type of physical network. Satellite transmission systems are often used to distribute the messages to multiple towers for retransmission. The use of satellite interconnection systems permits regional, nationwide, or global paging service. Some paging systems are capable of providing two-way communications.

Each pager in the network has a unique identifier (address) that is stored to the pager’s memory. This address is also stored in the paging network. When an incoming telephone call accesses the pager customer’s account, the network transmits the address of the pager on its radio tower. In addition to sending a specific paging address, the system may send other digits, text, or even voice information that follows the addressing message.

Wireless Data
Wireless data networks transfer data between network access points (wireless data devices) through radio transmission and are primarily designed to transfer data from one point to one or more points (multipoint). These networks may be composed of a variety of communication systems including: mobile data terminals, radio access nodes, packet switching networks. Wireless data networks provide a variety of services including: equipment status monitoring, dispatch services, vehicle and goods tracking, credit card validation, and include wireless Internet access.

Broadcasting is a process that sends voice, data, or video signals simultaneously to group of people or companies in a specific geographic area or who are connected to the broadcast network system (e.g., satellite or cable television system). It is typically associated with radio or television radio transmission systems that send the same radio signal to many receivers in a geographic area. Broadcasting can also be applied to wired distribution or point-to-point networks where all users that are connected to the network can receive the same information signal.

Figure below shows the different types of wireless networks. This diagram shows a private land mobile radio system, television broadcast system, paging system, mobile telephone system, and satellite communication system. Although all wireless networks can transmit information from one point to another, different types of networks better suited to provide specific types of services (e.g., paging compared to television broadcasting).

Wireless Networks

Cable Television Networks

Cable television systems provide video and data services through a system of high bandwidth coaxial cables and fibers. The cable network includes a head-end amplifier that combines the broadcast and data signals for transmission to the subscribers. High-speed Internet access is obtained by including a cable modem termination system (CMTS) function within the head-end that connects to a 10/100Mbps ethernet router. The head-end is connected to fiber or coax trunks that carry the signals into the neighborhoods where they are tapped off to provide service to the residence.

Earlier cable TV systems provided only one-way broadcast type services such as standard and premium channel television. Upgrading these earlier systems to support the two-way communications necessary to offer Internet access, pay-per-view, voice and video-on-demand services requires large capital investments. Many cable TV carriers have merged with large telecommunications companies in order to take advantage of the enormous market potential that exists. Cable TV systems can deliver high-speed Internet access at costs that are far below that of digital subscriber line (DSL).

Picture on top shows a typical cable television network. This diagram shows that cable television systems can be simple one-way video distribution systems to advanced two-way high-speed digital networks. The head-end is the initial distribution center for a cable television (CATV) system. The head end is where incoming video and television signal sources (e.g., video tape, satellites, local studios) are received, amplified, and modulated onto TV carrier channels for transmission on the CATV cabling system. The cable distribution system is a cable (fiber or coax) that is used to transfer signals from the head end to the end-users. The cable is attached to the television through a set-top box. The set-top box is an electronic device that adapts a communications medium to a format that is accessible by the end-user.

Inter-Exchange Carriers (IXC)

Inter-Exchange Carrier (IXC) networks are used to link telephone networks within geographical service area to each other. AT&T, Sprint, MCI, and Qwest are examples of well-known IXCs.

In order to provide the bandwidth necessary to carry the volume of long-distance voice and data traffic at reasonable cost, most IXCs have deployed large bundles of fiber-optic cables that interconnect their switching systems. Burying thousands of miles of fiber cable is costly. However, each pair of fibers is capable of providing many Gbps of bandwidth.

The explosion of the Internet and the demand for advanced multi-media services continues to drive the demand for increased bandwidth at low cost. To increase the capacity of fiber cables, new fiber optic technology has emerged. By utilizing a technology known as dense wavelength division multiplexing, DWDM, each fiber can carry 80 or more separate light-waves. As of 2001, some DWDM technologies were capable of providing over 1 Tbps (1,000 Gbps) of bandwidth, enough to transmit in one second the contents of 150,000 encyclopedias. Advances in optical networking equipment and light-wave amplification technologies will continue to add bandwidth the fiber networks.

Picture on top shows the typical inter-exchange carriers (IXCs) connections. This diagram shows that there are many different IXCs. Each of these IXCs must interconnect to the local telephone companies at a defined point of presence(POP)...

Private Telephone Systems

Private telephone systems are independent telephone systems that are owned or leased by a company or individual. Private telephone networks include key telephone systems (KTS), private branch exchange (PBX) and computer telephone integration (CTI).

Key telephone systems provide customers with telephones that have multi-line capability. The key telephone system is composed of multi-line telephones and a key service unit (KSU). Each multi-line telephone has several buttons (switches) to allow the user to select from several telephone lines. The KSU coordinates the activity of each line (e.g., ring or call hold).

The private branch exchange (PBX) is a computer-controlled switch that is an extension of the carrier’s central office switch. These switches are used primarily to switch a large number of “private” lines within an organization onto a smaller number of trunk lines connected to the carrier’s central office switch. Using direct inward dialing (DID), the PBX allows one caller to directly dial another line within the organization without the involvement of the switching equipment at the central office. The PBX can provide services to an organization that include, call pick up groups, call forwarding, conference calling, call detailing and automatic routing.

Computer telephony integration (CTI), allows external computers to automate the call processing activities of the PBX in real-time and consists of a set of application program interfaces that can be used to provide advanced services and complex switching functions. Figure 1.8 shows the different types of private telephone systems. The most simplistic private telephone network is a single telephone attached to a business line. Key systems allow each telephone in a business to answer and originate calls on several business lines. A PBX system allows many extensions within a business to call each other and the PSTN...

Local Exchange Carriers (LEC)

Local exchange carriers (LECs) or post and telegraph and telecommunications (PTT) companies provide telephone services directly to residential and business customers located within a localized geographic area. Typically, these telephone companies provide services via copper lines that extend from a local carrier’s switching facilities to the end customer’s premises equipment (CPE). This is referred to local loop.

Until the early 1990’s, most countries had a single company that provided local telephone services. This company was either owned or highly regulated by the government. To increase competition and reduce telephone service prices to consumers, some governments have begun to allow other companies to provide basic (local) telephone service. These competitive local exchange company (CLEC) or competitive access providers (CAPs) provide alternative connections to the public switched telephone networks (PSTN). The established telephone companies are now called the incumbent local exchange carriers (ILECs),

Traditional incumbent local exchange company (ILEC) may be connected with one or more competitive local exchange companies (CLECs) who provide local telephone service in a defined geographic area. End customers (the houses) in the geographic area are connected to the End Office (EO) switching center by copper wire (local loop). The local loop length is approximately 1k to 10k feet from the EO. Rarely will the distance of the local loop exceed 20k feet.

The end office interconnects calls between local customers. Each end office switch can usually supply service up to 10,000 customers. In larger areas (such as a city), established LECs may have several EO switches. The EO switches are interconnected using a higher level tandem switch. If is a significant amount of calls regularly processed between end offices, they may be directly connected via high-speed communication lines (trunks).

Figure 1.7 shows that CLECs must be connected to the ILEC to allow calls to be routed between the different networks. There may be several CLECs in a specific local telephone service area.

Local Telephone Service Networks

Local telephone service areas are connected to each other by inter-exchange switches. These higher level (meaning above tandem) are often operated by long-distance service providers called inter-exchange carriers (IXCs). The IXCs interconnect their long distance switches to the local network through a point of presence (POP). The POP is the toll center that allows separation of billing for local and long distance service.

There are several other types of competitive access companies that are starting to provide local telephone service. These include cable television companies, Internet telephony service providers (ITSPs) and wireless local loop (WLL) providers.

Network Control

Network control is the transmission of signaling messages that perform call-control functions such as supervision, call setup routing, provisioning (authorizing) of services, and call processing control. Networks are either common to all users or privately leased by a customer for some specific application. The term “network” also refers to a group of two or more broadcast stations or cable systems interconnected physically and organizationally so as to broadcast the same program schedule simultaneously without any switching functions.

In the early telephone systems, network control routing of a telephone connection was manually monitored and processed by human operators. Human operators would supervise the call by listening for request tones (ringing sounds) and manually coordinate the connection by talking to end customers (who originate calls) and other operators (for cross-connections). When the call setup process had been agreed (all the switching points established), the connection was made through physical connections (patch panels).

To provide for more efficient network control, telephone control signals (tones) were created to allow the transfer or call control information on the same audio lines as the voice signals for call setup. These control tones would either be mixed with the audio or temporarily replace the audio signals. This type of audio signal control is called in-band signaling.

As the design of telephone networks advanced, it was necessary to add more intelligence to the call setup (e.g., automatic forwarding of telephone calls), it became necessary to shift the control signaling to circuits outside the audio path. This allowed more rapid call setup and better overall control over the communications connection. When the control signals are separated from the actual communication channel, these are called out-of-band signaling.

Provisioning of a network is a process within a company that allows for establishment of new accounts, activation and termination of features within these accounts, and coordinating and dispatching the resources necessary to fill those service orders. Provisioning involves customer care and billing systems.

Picture below shows how different types of networks can be controlled. This diagram shows that a network can have no control (distribution only), can use intelligent databases to control dumb switches, or it can use intelligent switches to route information through a dumb network.

Interconnection Networks

Local access networks often connect to other networks (such as the PSTN, PTT or Internet) via switching systems or gateway connection. Various types of gateway connections can connect the local telephone switching system (often called the “End Office” or “Central Office”) to other public (e.g. Internet) or private (e.g. corporate) networks. A gateway transforms data that is received from one network into a format that can be used by a different network. It usually has more intelligence (processing function) than a bridge as it can adjust the protocols and timing between two dissimilar computer systems or data networks. A gateway can also be a router when its key function is to switch data between network points.

Interconnections to other public telephone networks are classified by type of connection. Basically, the lower the connection type number, the more simple (and more limited) is the connection. The connection types include the basic customer type POTS (type 1) and inter-switch types 2. Type 1 POTS connection provides for basic signaling and low speed (audio) connection. The higher types of connection include various capabilities such as types of information services available (operator assist, emergency number support). In the United States, the typical interconnection types include those designated as type 2A, 2B and other variants of type 2, each serving a specific purpose. Type 2 interconnections link the LEC into a tandem (standard local switch interconnect) office. When using the type 2 connections, the CO appears as a standard end office switching facility.

Networks commonly increase their data transmission capacity or quality levels as you move towards the top of the network (away from the endpoints). For cable networks, this is called feeders and for telephone networks, these are called high-speed backbone interconnects. High-speed interconnection lines between switches and tie lines are called trunks.

Figure 1.5 shows some of the different types of interconnection networks. These vary from distribution networks (no switching functions such as cable television), centrally controlled networks (such as the public telephone system), and packet switching networks (such as the Internet).

Network Access Lines

Since the early 1900’s, copper telephone lines have been the primary method of connecting customers to the telephone systems or computer networks. Copper wire pairs typically use twisted pairs of copper. The twisting of wires reduces the effects of electrical noise from distorting the desired audio signals. In essence, when the noise is received on one twist, the same noise is received on the other twist. The voltage goes positive on one line while it also goes positive on the other. Basically, the two noise signals are at the same level and they cancel each other (balance). Coax lines use one wire (a shield) to surround the other wire to help contain electrical energy from leaking out.

Telephone lines usually start from the central office’s switching center in the form of bundles of many wire pairs (trunks). These trunks connect the central switching office to distribution cables (cables with a reduced number of wire pairs) that eventually are connected to individual homes or businesses. Trunks may contain thousands of pairs of wires while local distribution cables only contain 25 to 100.

Cables are produced in rolls with a limited length (often 500 feet long). The installation of telephone cables requires several splices points as the large trunk cables connect to the distribution cables that connect to the drop cables to the home.

The switch wires are connected to the local loop lines in a main distribution frame (MDF). This trunk cable is connected to (3) 200 pair distribution cables that supply circuits to nearby neighborhoods. As the cables enter into a neighborhood, they are connected at splice points to smaller distribution cables until a final distribution cable that only holds 25 pairs reaches a telephone pole located near a house. At the telephone pole, usually 2 pairs of wires are tapped to the drop line that enters into the house (to allow up to 2 separate phone lines). These 2 pairs of wire are attached to a network interface device (NID) that protects (isolates) the wiring in the home from the telephone network wiring. Once in the home, twisted pairs of wires are looped from the NID to telephone jacks within the house. This illustration also shows that there is significant potential for different types and sizes of wire and many splice points. This inconsistency can dramatically affect the ability to transfer high-speed digital signals.

End User Equipment

End user equipment, (often called “terminals”) are the interface between the customer and the network. Terminals may translate electrical or optical signals to forms understandable by people or may be translation devices for other electronic equipment (such as computers).

Most of the telephone equipment in use during the year 2000 converted electrical analog (audio signals) into acoustic energy that the customer can hear. The basic function of analog telephone service is called plain old telephone service (POTS). The standard telephone (also known as a 2500 series phone), continuously monitors the voltage on the telephone line to determine if an incoming ring signal (high voltage tone) is present. When the ring signal is received, the telephone alerts the user through an audio tone (on the ringer). After the customer has picked up the phone, the hook switch is connected. This reduces the line connection resistance (through the hybrid) and this results in a drop in line voltage (typically from 48 VDC to a few volts). This change in voltage is sensed by the telephone switching system and the call is connected. When the customer hangs up the phone, the hook switch is opened increasing the resistance to the line connection. This results in an increase in the line voltage. The increased line voltage is then sensed by the telephone switching system and the call is disconnected.

Basic Communication System

A basic communications system consists of end user equipment, network access connections, network interconnection devices (e.g. switches) and a control system that coordinates the network. A carrier or service provider is company that is engaged in transferring electrical signals or messages for hire through one or more telecommunications systems.

Customers (users) request and may receive telecommunications services from the telecommunications network. Because customers request and receive services, a customer is sometimes called a service subscriber or end user. A telecommunications service provider offers communications for a fee directly to the public, or to such classes of users as to be effectively available directly to the public, regardless of the facilities used.

A network operator is a provider of telecommunication services. A network operator manages the network equipment parts of a communications system to allow authorized customers to transfer and/or process information via the network. The network operator may provide services directly to end customers or may only manage network equipment and another company (service provider) may manage the provision of services to customers.

End-user equipment converts various types of information from a user (such as audio or computer data) into a signal that can be transferred via a communications system. Since the late 1800’s, different types of systems had very specific types of end user devices. For example, public telephone systems have a telephone, data communication systems have a channel service unit (CSU), and wireless systems have a mobile telephone. As technology has evolved, end user equipment devices began to combine functionality. This can be found in voice telephone systems that can transfer digital data by using a modulator/demodulator (modem).

Access connections are the link between the end user equipment and the wide area network, WAN, owned by the service provider. Access connections can be provided via pairs of copper wires, radio links, or fiber connections. Twisted pairs of copper wires can carry low frequency audio signals such as voice and high-speed digital signals (e.g., 11 Mbps DSL). Radio access can carry low speed information signals (such as wide area cellular) or can be high-speed data transmission (such as microwave directional signals). Each strand of fiberoptic cable (and there may be several hundred fibers per cable) can carry more than 1 Terabit per second of data (1,000,000 million bits per second).

Interconnection systems connect of all the various types of equipment. Interconnection systems may include signal taps, splitters, bridges, gateways, switches, and routers to move the information from one part of the network to another along its path between originating and destination points. The interconnection can be completely dumb such as the form of signal taps and splitters that only direct part of the signal energy to multiple points. Some interconnection devices such as bridges and gateways adapt the format of the information to another form (e.g., different packet length or type of packet) between dissimilar networks. Active devices such as switches and routers can direct signals from one source to various other paths depending on call setup information (e.g., telephone number) or an address contained in a data packet (such as a signal router that transfers Internet packets of data).

System control and coordination functions ensure that the various resources of the network are coordinated in their actions by detecting equipment and network status. Commands are issued to direct the various network elements in order to configure the network parts and to maintain a high level of network service. .Network operators can centrally coordinate system control or multiple network operators can independently and dynamically control it. An example of a centrally managed control system is the signaling system number 7 (SS7) packet control system that coordinates the public telephone network. The SS7 network contains packet switching points and databases that are controlled by the public telephone network operators. Distributed network control is demonstrated by how the Internet is dynamically managed. The Internet is composed of thousands of independent networks that use intelligent routing devices within each network to forward packets throughout the Internet.

Billing and Customer Care

Billing and custom care systems convert the transfer of bits and bytes of digital information within the network into the money that will be received by the service provider. To accomplish this, billing and customer care systems provide account activation and tracking, service feature selection, selection of billing rates for specific calls, invoice creation, payment entry and management of communication with the customer.

Billing and customer care systems are the link between end users and the telecommunications network equipment. Telecommunications service providers manage networks, setup the networks to allow customers to transfer information (provisioning), and bill end users for their use of the system. Customers who need telecommunication services select carriers by evaluating service and equipment costs, reviewing the reliability of the network, and comparing how specific services (features) match their communication needs. Because most network operations have access to systems with the same technology, the billing and customer care system is one of the key methods used to differentiate one service provider from another.

There are many different types of services to be supplied and billed. These include traditional voice, short messaging, fax, data communications, and information services. Billing systems process the usage of network equipment that is used during the call (events) into a single call detail billing record. The billing process involves receiving billing records from the system and other networks, determining the billing rates associated with the billing records, calculating the cost for each billing record, gathering these records periodically to produce invoices, sending invoices to the customer, and recording payments received from the customer.

Customer care systems, also known as provisioning systems, provide customer service representatives (CSRs) with information that assists and standardizes communication with the customer.

Billing system and customer care system costs are substantial. In addition to the initial acquisition cost of computers and software, operational costs are very high. Of the service provider’s staff, 20%-30% are employed to provide billing and customer care support.

There are many billing standards that have been developed for telecommunications networks. Billing standards are also converging because the services offered by different types of network operators (e.g., cable television compared to local telephone companies) are beginning to overlap.

Real time billing allows service operators to collect billing information at the same moment that services are being consumed. This type of billing allows the service provider to measure and respond to usage trends more rapidly and provides extra services such as prepaid phone cards and improved customer service.

Generally proprietary systems are developed in order to integrate the company’s current back-office and customer service systems in those situations where customer off the shelf (COTS) billing and service packages do not fit the company’s needs.

Future trends and challenges for billing systems include new types of services to bill for; telephone number portability that complicates account identification numbers and increased customer self care to reduce the burden (and cost) of billing systems. Self-based customer care services allow customers to access and maintain their own account information.

Key Telecommunications Services [Telecom]

Telecommunications services can be divided into three key categories;
- voice
- data
- and video.

Each of these categories has specific characteristics such as maximum transmission delay time, minimum and maximum transmission rates, and acceptable transmission error types and rates.

Voice services involves receiving of audio signals, processing audio signals into various formats (analog and digital), storing and transporting these signals, and converting the signals back into a form that is similar to its original form. The characteristics of voice networks are very small transmission delay (below 100 msec typical), maximum of 64 kbps for each digital voice channel, and reasonable tolerance to errors. Examples of voice services are Telephony, Voice Messaging, Call Processing, and Computer and Telephony Integration, CTI.

Data services provide transport of digital information from one point to one or more points. The characteristics of data networks are moderate transmission delays (above 1 sec may be acceptable), minimum of 28 kbps for each dial-up digital customer and 1 Mbps for each broadband customer, and very low tolerance to errors. Examples of data services include switched connections (circuit switched channels / dial-up, dedicated lines (leased lines/circuits), packet switching (e.g. Internet), and multicast and broadcast (one to many) data transfer.

Video services transport high information content signals (video) from one point to one or more points. The characteristics of video networks are very long transmission delay (above 15 seconds for digital broadcast acceptable), minimum of 1 Mbps for each digital video channel (3.2 Mbps for DVD), and reasonable tolerance to errors. Examples of video services include television, closed circuit TV (CCTV), video on demand (VOD), videoconferencing, and interactive multimedia.

Telecom Made Simple

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