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chapter 7. Voice Networks. CHAPTER OVERVIEW. Voice networks are increasingly visible in organizations because of their potential for convergence with data networks.
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chapter 7 Voice Networks
CHAPTER OVERVIEW • Voice networks are increasingly visible in organizations because of their potential for convergence with data networks. • By unifying voice and data networks into a common infrastructure, organizations can realize substantial maintenance and support cost reductions.
CHAPTER OBJECTIVES • Identify the business purpose of voice networks. • Describe how data is transmitted over a plain old telephone system local loop. • Identify the business purpose and features of a private branch exchange. • Define PBX technology and discuss PBX switching topologies and design considerations. • Identify the business purpose of an IP-PBX. • Describe the converged and packet-switched topologies of an IP-PBX.
CHAPTER OBJECTIVES (cont’d) • Define and describe voice over IP (VoIP). • Define computer telephony integration and identify its business purpose and features. • Discuss cellular wireless voice networks and describe the anatomy of a cellular wireless connection. • List and describe three cellular wireless access methods. • Identify and describe three cellular wireless topologies and connectivity to the PSTN. • List and discuss cellular wireless data services.
VOICE NETWORKS – AN INTRODUCTION • Voice networks convey the human voice between remote locations. • Voice networks use electrical transmission techniques. • Voice networks were in existence long before modern data networks. • Voice networks have traditionally used analog transmission techniques where data networks generally use digital methods. • Voice networks have traditionally been constructed and maintained separately from data networks.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • From the late 1870s – 1950s, voice networks were specifically used for the analog transmission of the human voice. • In the 1960s and 1970s, organizations with mainframe computers began using telephone companies’ analog voice networks to share computing resources and to transmit digital data between remote locations.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • Modern voice networks can be based on analog or digital techniques or a combination of both. • Modern voice networks can be as simple as a pair of wires that connect a home to the phone company. • Modern voice networks can be as state-of-the-art as an IP service that uses a LAN or WAN infrastructure.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • Earliest voice networks appeared after the introduction of the first commercial telephone in the late 1870s. • These early telephone networks connected a person’s home directly with another person’s home to create a point-to-point connection. • Subsequent connections to others’ homes required the installation of additional point-to-point connections.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • All these point-to-point connections were inefficient to install and maintain. • This led to the development in 1878 of the first telephone exchange, which is also known as a telephone central office(CO). • A CO is a physical facility owned by the telephone company, and it acts as a hub to which all telephone subscribers in a specific geographic region connect. • When a subscriber makes a call, the call connects to the CO, and then the call is connected to another subscriber.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • Modern connections between a home or office and a CO is provided by a local exchange carrier (LEC). • The connection between home or office and the CO is known as a local loop. • Local loops consist of a pair of twisted copper wires that are like the UTP cabling used in data networks. • Local loops in combination with one or more COs form the basic voice network for local calls.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • Local loops and COs in combination with long-distance technologies comprise a voice network that is commonly referred to as the plain old telephone system (POTS). • Formally, POTS is better described as the public switched telephone network(PSTN). • POTS provides the basic foundation for the voice network that spans across the U.S. and the entire world. • POTS is technically that part of the PSTN that services regular analog telephones.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • PSTNrefers to the entire network of analog and digital equipment and services that provide telephone service to subscribers.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • A CO is analogous to a data network switch. • A local loop is analogous to the UTP cable that is used to connect a computer to a switch in a data network. • If calls need to be routed to another local CO, digital trunks (high-speed circuits) multiplex multiple calls from one CO to another.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • If a call is long-distance, point of presence (POP) equipment owned by an inter-exchange carrier (IXC or IEC) transmits the call along the appropriate long-distance communications path. • Long-distance calls are received at a remote POP, switched to the destination LEC, and then connected to the remote subscriber.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • Business Purpose of Voice Networks • Transmission of human voice as well as data between point A and point B. • Organizations use POTS and the PSTN for simple analog voice and data communications over POTS to switched digital services such as ISDN. • The PSTN services voice and data transmission across trunks to organizations’ private branch exchange (PBX) systems. • Cellular wireless also relies on the PSTN for connecting voice and data transmissions between points A and B.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • Digital data transmissions across POTS use modems. • A modem modulates a simple carrier wave into different frequencies, amplitudes, and phases, and then demodulates the signal at the receiving end. • Modem development spawned the beginning of data communications over regular telephone lines.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • Growing PC use in the 1980s along with modem technologies and connections to other PCs fostered the continued growth of data communications over voice networks. • In the 1990s, faster modems and Internet access across regular phone lines resulted in more data than voice traffic across analog phone circuits.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • How Modems Work – the basics • A modem in a sending computer places data bits on a modulated carrier wave. • The digital data travels across the analog local loop to the telephone company’s CO. • The data travels through the CO or across the PSTN to the destination computer’s modem.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • How Modems Work – The technology • POTS was designed to convey the continuously varying pitches and tones of the human voice between two remote locations. • Those continuously varying pitches and tones look like a sound wave when represented graphically, with numerous peaks and valleys. • The sound wave changes continuously as a person talks to accommodate the speaker’s voice.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • How Modems Work – The technology • Each change in tone or speed conveys specific meaning to both the talker and the listener. • As long as both parties understand the language and nuances of speech, the vocal information is transmitted and understood. • Data communications across POTS works in much the same way. • But computers by themselves don’t generate any sound waves to convey information.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • How Modems Work – The technology • A modem generates a sound wave known as a carrier wave that initially doesn’t convey any data. • When a computer has data to send through a modem and onto a local loop, the carrier wave is modulated in numerous ways so that data bits can be transmitted. • Several types of modulation exist: amplitude, frequency, and phase.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • Amplitude Modulation • Amplitude generally refers to the height of a carrier wave. • When a computer needs to transmit a binary 1, for example, a modem that uses amplitude modulation modifies the carrier wave with a higher amplitude. • Amplitude modulation is susceptible to external interferences such as high-voltage lines, electric motors, and electrical storms.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • Frequency Modulation • Also known as frequency shift keying. • Changes the number of waves per second. • Higher frequency sections can represent binary 1s and lower frequency sections can represent binary 0s. • Shifting frequencies thousands of times per second can allow thousands of binary 1s and 0s to be transmitted in a short period of time.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • Phase Modulation • Also known as phase shift keying. • Interrupts the continuous flow of the carrier wave by changing the direction, or angle, of the wave. • A direction of down and to the right is known as a phase of 180 degrees. • A direction of up and to the right is known as a phase of 0 degrees. • With each change in phase data bits can be transmitted across the circuit.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • Baud Rate and Bit Rate • Bit rate defines the number of bits per second that can be transmitted across a circuit. • A baud is a signaling event, which corresponds to a change in the amplitude, frequency, or phase of the carrier wave. • Multiple bits per baud can be achieved with modern modulation techniques such as quadrature amplitude modulation (QAM) or trellis-coded modulation (TCM)
VOICE NETWORKS – AN INTRODUCTION (cont’d) • Baud Rate and Bit Rate (cont’d) • The only time bit rate equals baud rate is if only a single bit is transmitted with each baud. • QAM and TCM support multiple bits per baud, so in modern modems the bit rate is always higher than the baud rate.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • Most modern modems incorporate technologies that combine both analog and digital transmission techniques. • The V.90 standard for 56Kbps modems uses analog transmission of digital data for upstream data rates of 33,600 bps and digital transmission of digital data for downstream data rates that approach 56Kbps. • Newer standards such as V.92 provide for digital transmission of digital data in both directions over an analog local loop.
VOICE NETWORKS – AN INTRODUCTION (cont’d) • Digital Telephone Lines and Modems • Modems are not required to connect a computer to a digital line. • Instead, other interface devices are required depending on the type of digital line used.
PRIVATE BRANCH EXCHANGE • A private branch exchange (PBX) is a private version of the PSTN central exchange. • A PBX can be implemented by an organization to control and manage voice network use, functionality, and costs. • With PBX implementation, control, management, and cost of a substantial part of an organization’s voice network passes from the telephone company to the organization itself.
PRIVATE BRANCH EXCHANGE (cont’d) • The only parts of the voice network that aren’t internalized are the hardware and services that connect an organization’s internal voice network to the PSTN or to a metropolitan area or wide area network.
PRIVATE BRANCH EXCHANGE (cont’d) • PBX Business Purpose and Features • PBXs have existed for over 100 years. • First PBXs were switchboards installed at business locations in the late 1800s and early 1900s. • Business purpose was to reduce the cost of telephone service to organizations that had multiple telephones.
PRIVATE BRANCH EXCHANGE (cont’d) • PBX Business Purpose and Features (cont’d) • Business subscribers and the telephone company both benefited with the installation of a PBX. • Businesses could reduce the rental costs of having multiple local loops. • Telephone company could reduce the expense of installing and supporting additional local loops for businesses.
PRIVATE BRANCH EXCHANGE (cont’d) • Early PBX Systems • Were manually operated switchboards. • They were installed, owned, and maintained by the telephone company for a fee. • Business subscriber enjoyed the benefits of internal call switching and control. • Business subscriber also enjoyed a cost structure that was offset by reduced local loop charges.
PRIVATE BRANCH EXCHANGE (cont’d) • Early PBX Systems (cont’d) • The business was charged for the trunk lines that connected the PBX to the telephone company’s CO. • The number of trunk lines needed to be sufficient to service the maximum number of outgoing calls – a number that was usually less than the number of internal calls.
PRIVATE BRANCH EXCHANGE (cont’d) • Modern PBX Systems • Include the features of early PBXs such as efficiencies of internal call switching and control as well as cost savings. • Include services that are designed to improve voice network performance and communications efficiency.
PRIVATE BRANCH EXCHANGE (cont’d) • PBX Technology • Prior to the mid-1970s, PBXs were either operator-attended switchboards or electro-mechanical devices that automatically serviced analog telephones. • Since the mid-1970s, analog PBXs have faded, being replaced by digital PBXs. • Digital PBX technologies have paralleled the development and implementation of digital switching equipment at telephone company COs.
PRIVATE BRANCH EXCHANGE (cont’d) • PBX Technology (cont’d) • Software-based PBX features have paralleled the development of computers and programming tools. • Modern PBXs are sophisticated computers. • Modern PBXs run software that handles call processing and switching. • Modern PBXs include software that provides the services listed previously.
PRIVATE BRANCH EXCHANGE (cont’d) • PBX Technology (cont’d) • Modern PBXs are based on digital transmission technologies. • Modern PBXs can also service analog signals from analog telephones and computers equipped with modems. • Modern PBXs are also modular in design so that they can be expanded as business requirements change.
PRIVATE BRANCH EXCHANGE (cont’d) • PBX Technology (cont’d) • PBX is comprised of: • Common control complex. • A switching matrix. • Station cards. • Trunk cards.
