Apuntes Convergence+

  • Published on
    17-Nov-2014

  • View
    428

  • Download
    2

Embed Size (px)

Transcript

<p>TELEPHONY Legacy Hybrid and IP Telephony SystemsTelephone systems have begun the migration from sending voice across dedicated circuit switched lines using time division multiplexing to capturing voice and using IP packets to send the voice across a shared data network to the receiver. Hybrid telephony systems integrate key-systems and wide area connections to allow voice calls to move across traditional phone systems for local calls. If the call is destined for long distance, the voice is encapsulated and sent across the WAN link in packet form. The Internet Protocol (IP) is an OSI layer three protocol. This protocol uses an address to uniquely identify every host connect- ed directly to the Internet. The speakers voice is captured and segmented into separate blocks called packets. There are various reasons to migrate to an IP telephone system. First, the phone devices now have universal access. Wherever there is Internet access an IP phone may make calls to anywhere in the world.</p> <p>IP phones contain additional features not found on traditional phones. Cost reductions are made when companies migrate to a single infrastructure running both voice and data over the same wire. The number of technical experts required to operate a dual telephone/data structure will be reduced.</p> <p>Key SystemsThe smallest businesses usually begin with the same sort of single line telephone installed in most homes. However, as a business grows and adds staff, it needs the flexibility of multiple lines. For a business too big for a single telephone, but much too small for a largescale office switching system, key systems are the answer. Key systems are fairly simple on-site telephone systems geared to organizations with fewer than 100 telephones. Like a PBX, they switch calls to and from the public network and within users premises. However, key systems are simpler than a PBX, reducing the administrative workload for small businesses. Key System Components The first multiline business telephone system was called the 1A key telephone system. It consisted of a red hold button, four telephone line buttons, and an office intercom button. This system became the workhorse of small businesses, and many of these systems are still installed today. A key system provides multiple telephone extensions access to a group of single telephone lines. For example, if a small office has six single lines, it can use a key system to access</p> <p>any of those lines from each of its telephones. Each telephone extension would have six buttons (one for each line); this is known as a squared line configuration. To connect a telephone extension to a line, a caller simply presses one of the unlit line buttons and if a line is free the caller will hear dial tone. The concept of key systems is illustrated on the Key Telephone System Diagram</p> <p>Key Telephone SystemAll telephone sets in a key system were connected to a central device called a Key Service Unit (KSU), which connected each telephone set to a group of outside business lines. Today, new KSU-less systems offer all the functionality of KSU within each telephone set. The main point to remember about a key system is that it can sup- port only as many incoming or outgoing telephone calls, or call paths, as there are lines installed. In other words, if a customer has 100 telephone extensions in an office, but only 40 lines installed, the maximum number of simultaneous calls, coming in or going out, is limited to 40. If the 40 lines are all in use, outgoing callers must wait for a free line, while incoming callers receive a busy signal. In many business settings, such as large retail centers or factory floors, cordless key systems provide employees telephone service while allowing them freedom of movement. Wireless transmission is used to connect these mobile extensions to the main business lines, and to each other by means of intercom features. This type of technology is presented on the Cordless Key System Diagram</p> <p>Cordless Key SystemThe use of wireless telephones inside buildings requires special base stations with antennae located on every floor. There are gen- erally also special outside base stations with antennae for nearby outdoor areas between buildings on a campus. The base stations must be wired with twisted pair to specialized circuit packs within the telephone system cabinet. Specialized wireless telephones associated with key systems and PBXs are high profit margin peripherals. These telephones operate at higher frequencies than home telephones and have specialized features associated with particular key and PBX telephone systems. On-site wireless telephone systems use a cellular digital switching technology similar to Personal Communications Service (PCS). Calls are transferred between base stations when a user walks out of the range of a particular antenna. Some mobile telephone units can function both inside and outside of the business campus. They sense when they are out of the range of the base system, and automatically switch calls to a cellular telephone network.</p> <p>Limitations of Key Systems A key system provides a cost-effective way for a small business to share a moderate number of telephone lines. However, key systems offer fairly unsophisticated functionality</p> <p>and features. In addition, their main advantage, simplicity, becomes a liability as a business adds telephone lines past a certain point. As we have seen, each telephone in a key system has a button to access each telephone line, plus a hold button, CO telephone lines, and intercom lines. Therefore, if a business needs 18 business lines, its telephone sets would have at least 20 buttons. By the time a business requires 20 or more lines, each telephone has become quite complex, hard to use, and expensive. Can you imagine an extension telephone with 50 or more buttons on it? Therefore, we can upgrade a true key system only so far before we need to try a different approach.</p> <p>Hybrid SystemsWith the integration of computer technology inside telephone systems, key telephone systems became more and more advanced. Gradually, they began to include features previously found on only full PBX systems. Thus, the term hybrid describes a telephone system that includes features of both a key system and PBX. A characteristic of a hybrid key system is the grouping of outside trunks into pools, by function or organization. For example, certain trunks are allocated to a particular department. Electronic Key Telephone Systems (EKTS) often cross the line into the PBX world, providing switching capabilities, as well as impres- sive functionality and feature content. EKTS is a key telephone system in which electromechanical relays and switches have been replaced by electronic devices, often in the telephone sets and central cabinet. The inner workings of the central cabinet of an EKTS more resemble a computer than a conventional key system.</p> <p>IP Telephony SystemsIP Telephony (Internet Protocol) is a means for handling your phone calls and faxes over the Internet as opposed to a traditional phone line. It is becoming the preferred technology for large organizations because it saves money, is easy to maintain, and produces a superior ROI compared to PBX systems. IP Telephony is less expensive to install than a PBX phone system. Less structured cabling is required. Once a drop is wired, it can be used for data and voice.</p> <p>Maintenance costs are lower compared to a PBX system. Technicians trained in converged networks are able to maintain both data and voice systems. In fact, you can handle routine maintenance in house, such as add-ons, moves and changes. Monthly operating costs are less because the system does not require dedicated lease lines for voice and data.</p> <p>Employees favor IP Telephony as it offers better tools to archive voice mail and keep track of faxes. In addition, faxes and voice mail can be retrieved off the Internet anywhere in the world. Features are visible on your computer making it relatively simple to operate. The system provides call accounting, giving you documentation on employee performance, carrier billing and call trends. Call NIC for a total solution. We are a single source for all of your technology requirements. Services include consulting, design, project management, system installation, maintenance, and support for all network systems http://www.nicweb.com/en/ services/network_systems/ip_telephony.html</p> <p>Voice Transmission FundamentalsWhen a caller lifts the handset off hook the PBX signals the router to seize a trunk. The PBX then forwards the dialed digits to the router. The routers dial plan maps these digits to an IP address and initiates a call establish request to the remote router. The end nodes are responsible, on a VoIP call, for call connection and signaling. The ITU-T Q.931 recommendation manages the call setup and teardown. Steps to initialize the call include: SETUP of a connection, CALL PROCESSING determines the remote terminal received the call, ALERTING informs the calling party the remote terminal is ringing, CONNECT tells the calling party the remote terminal is now off hook, RELEASE COMPLETE happens when either end of the conversation hangs up the call The call control sequence may use RTP Real-Time Transport Protocol in conjunction with RTCP Real-Time Transport Control Protocol to manage the audio and/or video streams as requested. RTP running atop UDP carries the voice and/or video stream. RTCP controls the connection and runs atop TCP for tracking and control. H.245 control signaling negotiates channel usage and may negotiate agreement between all the endpoints of a conference call on technologies such as codecs, speeds, and parameters. The call may also use various protocols to set a quality of service for the voice packets. Running on the transport layer is the RSVP Resource Reservation Protocol for QoS. The packets may have quality of service set on layer three with DSCP Differentiated Service Code Points. And, on layer two the IEEE 802.1Q for quality of service/precedence levels for the frames themselves.</p> <p>Encoding, Decoding, and Compression</p> <p>It takes time to convert a voice signal from analog to digital. A similar delay, or latency, occurs when the remote end converts the digital signal back to analog. Voice compression reduces the number of bits in a transmission by removing redundant character strings in the digital character stream. However, compression algorithms increase latency by requiring large voice stream samples before they can compress and packetize voice signals</p> <p>Digitizing the VoiceAs the benefits of a digital telecommunications infrastructure became apparent, it was necessary to take the analog voice and convert it to a digital format. The purpose for digitizing analog voice signals is so they are compatible with a digital telecommunications network. Analog to Digital The first part of the Analog-to-Digital Conversion Diagram (A) represents the original analog waveform. Part (B) represents digital pulses that control the sampling rate of the analog waveform. The digital pulses open a gate for the duration of their pulse widths, reading the amplitude of the analog waveform for this period of time. The sampled analog waveform appears as pulses (C) that are each correlated to a specific number (D). This number represents one sample of the voice signal. The binary representation of this number (E) is transmitted digitally across a circuit. At the receiving end, the reverse process takes place to convert the digital signal back to the original analog waveform A typical sampling rate (number of times a byte is generated) is 8,000 times per second, approximately twice the bandwidth required for an analog voice signal. Research has determined that a sampling rate of at least two times the highest frequency compnent of the original signal results in accurate representation of the original intelligence. This is called the Nyquist Theorem. A coder/decoder (codec) is the device that takes the analog voice signal and converts it to digital (binary) format for transmission over a digital circuit. The Analog-to-Digital Conversion Diagram illustrates this concept. Analog-to-digital conversion is also called A-to-D conversion or ADC. The most common example of this is found in a codec. This device takes the analog voice signal and converts it to digital (binary) format for transmission over a digital path, such as a T1. The output of a codec is combined with other outputs and multiplexed onto a high-speed digital network DTMF Signaling As you saw earlier, the first step-by-step switches were designed to work with rotarydialed telephones. Those telephones used dial pulse signaling, which produced short, regular interruptions of the direct current flowing between a telephone and switch. The</p> <p>number of interruptions, or pulses, corresponded to the value of the digit. In other words, when you dial the number 5, you hear five clicks As CO switching went digital, telephone sets also improved the way they transmitted telephone numbers. The dual tone multifrequency (DTMF) system, commonly called Touch Tone, uses a pad of 12 buttons. When pressed, each button sends out a combination of two pure tones not found in nature: one high- frequency and one low-frequency. The DTMF Touchpad and Tones Diagram illustrates this concept</p> <p>DTMF Touchpad and TonesBy assigning one tone to each row and column, only seven unique tones are needed to identify each of the 12 buttons. These tones can easily be detected by a telephone switching system.</p> <p>How a Call is MadeWhen you pick up a telephone handset, a sequence of predefined operations occurs that provides you the ability to use the telephone network. Now that you understand the basic components of the telephone system, let us see how they work together to complete a typical telephone call. Dial Tone When you lift the receiver, placing the telephone in the off-hook position, the telephones internal switch closes the local loop circuit with the CO switch. This allows electrical direct current to flow through the circuit; the presence of this current signals the CO switch that you need a telephone connection. In telephony terms, we say the CO switch has detected the off-hook condition.</p> <p>The switching module of the CO switch then tests the line and determines its suitability for call processing. If the line tests good, the switch provides dial tone to the callers telephone. The off-hook signal also alerts the switch to receive incoming touch tones. If the switch does not receive these tones in a timely manner, it sends a recorded message that reminds the customer that the telephone is off the hook. Entering a Telephone Number As soon as the CO switch detects the tones that represent the first digit, it removes dial tone from the line. The switch continues to detect tones and record the corresponding digits, while checking that the number of digits is cor...</p>