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Chapter 12 Voice and Data Delivery Networks. Objectives. สามารถอธิบายความหมายเบื้องต้นของระบบโทรศัพท์ และข้อจำกัดของสัญญาณโทรศัพท์ได้ สามารถบอกความแตกต่างระหว่าง local exchange carrier และ interexchange carrier และสามารถระบุถึงการให้บริการของแต่ละชนิดได้
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Chapter 12 Voice and Data Delivery Networks
Objectives • สามารถอธิบายความหมายเบื้องต้นของระบบโทรศัพท์ และข้อจำกัดของสัญญาณโทรศัพท์ได้ • สามารถบอกความแตกต่างระหว่าง local exchange carrier และ interexchange carrier และสามารถระบุถึงการให้บริการของแต่ละชนิดได้ • สามารถอธิบายคุณลักษณะพื้นฐานของโมเด็มแบบ 56k ได้ • สามารถระบุประเภทของวงจรเช่า (leased lines) ที่มีการใช้งานอยู่ และคุณลักษณะพื้นฐานของวงจรเช่าแต่ละชนิดได้ • สามารถบอกคุณลักษณะสำคัญของ digital subscriber line และสามารถบอกความแตกต่างระหว่างระบบซีแมนติก (symmetric system) และระบบอะซีแมนติก (asymmetric system)ได้
Objectives (ต่อ) • สามารถอธิบายคุณลักษณะหลักของเคเบิ้ลโมเด็มได้ • สามารถระบุคุณสมบัติพื้นฐานของ frame relay ได้แก่ permanent virtual circuits, committed information rate และ switched virtual circuits ได้ • สามารถระบุคุณสมบัติพื้นฐานของ Asynchronous Transfer Mode (ATM) ประกอบด้วย บทบาทของ virtual path connection และ virtual channel connection รวมทั้งสามารถอธิบายข้อดีข้อเสียของระบบ ATM ได้ • สามารถอธิบายแนวคิดของ convergence และสามารถยกตัวอย่างที่ใช้ในอุตสาหกรรมเครือข่ายได้
Chapter Twelve - Voice and Data Delivery Networks Introduction Students used to go into either data communications or voice communications. Today, the two fields are merging. Voice systems transfer computer data and data networks support voice. Anyone studying the field of data communications and networks must learn some basic telecommunications too.
Chapter Twelve - Voice and Data Delivery Networks Basic Telephone Systems The local loop is the telephone line that runs from the telephone company’s central office to your home or business. The central office is the building that houses the telephone company’s switching equipment and provides a local dial tone on your telephone. If you place a long distance call, the central office passes your telephone call off to a long distance provider.
Chapter Twelve - Voice and Data Delivery Networks Basic Telephone Systems The country is divided into a few hundred local access transport areas (LATAs). If your call goes from one LATA to another, it is a long distance call and is handled by a long distance telephone company. If your call stays within a LATA, it is a local distance call and is handled by a local telephone company.
Chapter Twelve - Voice and Data Delivery Networks Basic Telephone Systems A trunk is a special telephone line that runs between central offices and other telephone company switching centers. A trunk is usually digital, high speed, and carries multiple telephone circuits. A trunk is typically a 4-wire circuit, while a telephone line is a 2-wire circuit.
Chapter Twelve - Voice and Data Delivery Networks Basic Telephone Systems A trunk is not associated with a single telephone number like a line is. A telephone number consists of an area code, an exchange, and a subscriber extension. The area code and exchange must start with the digits 2-9 to separate them from long distance and operator services.
Chapter Twelve - Voice and Data Delivery Networks Basic Telephone Systems When the telephone company installs a line, it must not proceed any further than 12 inches into the building. This point is the demarcation point, or demarc. Modular connectors, such as the RJ-11, are commonly used to interconnect telephone lines and the telephone handset to the base. When the handset is lifted off the base (off-hook), an off-hook signal is sent to the central office.
Chapter Twelve - Voice and Data Delivery Networks Basic Telephone Systems When the off-hook signal arrives at the central office, a dial tone is generated and returned to the telephone. When the user hears the dial tone, they dial (or press) the number. The central office equipment collects the dialed digits, and proceeds to place the appropriate call.
Chapter Twelve - Voice and Data Delivery Networks PBX Private branch exchange (PBX) - a common internal phone switching system for medium to large-sized businesses. Provides advanced intelligent features to users, such as: 4-digit internal dialing Special prefixes for WATS, FX, etc (private dialing plans) PBX intelligently decides how to route a call for lowest cost
Chapter Twelve - Voice and Data Delivery Networks More PBX Features Voice mail Routes incoming calls to the best station set (automatic call distribution) Provides recorded messages and responds to touch-tone requests (automated attendant) Access to database storage and retrieval (interactive voice response) VoIP
Chapter Twelve - Voice and Data Delivery Networks PBX Components CPU, memory, telephone lines, trunks Switching network Supporting logic cards Main distribution frame Console or switchboard Battery back-up system
Chapter Twelve - Voice and Data Delivery Networks Automated Attendant Plays a recorded greeting and offers a set of options. Lets the caller enter an extension directly (touch tone or voice) and bypass an “operator”. Forwards the caller to a human operator if the caller does not have a touch tone phone. Available as an option on a PBX.
Chapter Twelve - Voice and Data Delivery Networks Automatic Call Distributor When you call a business and are told all operators / technicians / support staff etc. are busy and that your call will be answered in the order it was received. Used in systems where incoming calling volume is large, such as customer service, help desk, order entry, credit authorization, reservations, and catalog sales. Early systems used hunt groups. Original systems routed call to first operator in line (kept person very busy!)
Chapter Twelve - Voice and Data Delivery Networks Automatic Call Distributor Modern systems perform more advanced functions, such as: Prioritize the calls Route calls to appropriate agent based on the skill set of the agent If all agents busy, deliver call to waiting queue and play appropriate message (like how long they may have to wait) Forward calls to another call center, or perform automatic return call
Chapter Twelve - Voice and Data Delivery Networks Interactive Voice Response IVR is similar to AA except: IVR incorporates a connection to a database (on a mainframe or server) IVR allows caller to access and/or modify database information. IVR can also perform fax on demand.
Chapter Twelve - Voice and Data Delivery Networks Interactive Voice Response Common examples of IVR include: Call your bank to inquire about an account balance University online registration system Brokerage firm taking routine orders from investors Investment fund taking routine requests for new account applications A company providing employees with info about their benefit plans
Chapter Twelve - Voice and Data Delivery Networks Key Telephone System Used within a small office or a branch office, a key telephone system (KTS) is an on premise resource sharing device similar to a PBX. For example, a key system might distribute 48 internal telephone sets over 16 external phone lines. The business would pay for the 16 individual lines but have 48 telephone sets operating. User selects outside line by pressing corresponding line button on key set (phone).
Chapter Twelve - Voice and Data Delivery Networks Basic Telephone Systems Services Foreign exchange service (FX) - customer calls a local number which is then connected to a leased line to a remote site. Wide area telecommunications services (WATS) - discount volume calling to local and long distance sites. Off premises extensions (OPX) - dial tone at location B comes from the PBX at location A.
Chapter Twelve - Voice and Data Delivery Networks Other Players in the Market Alternate operator services - pay phones, hotel phones Aggregators - an aggregator pulls a bunch of small companies together and goes after phone discounts Reseller - rents or leases variety of lines from phone companies, then resells to customers Specialized mobile radio carriers - mobile communication services to businesses and individuals, including dispatch, paging, and data services. ARDIS and RAM Mobile Data two good examples
Chapter Twelve - Voice and Data Delivery Networks Limitations of POTS POTS lines were designed to transmit the human voice, which has a bandwidth less than 4000 Hz. A telephone conversation requires two channels, each occupying 4000 Hz.
Chapter Twelve - Voice and Data Delivery Networks Limitations of POTS A 4000 Hz analog signal can only carry about 33,600 bits per second of information while a 4000 Hz digital signal can carry about 56,000 bits per second. If you want to send information faster, you need a signal with a higher frequency or you need to incorporate more advanced modulation techniques. POTS cannot deliver faster signals. What will?
Chapter Twelve - Voice and Data Delivery Networks The 56k Digital Modem A 56k modem (56,000 bps) achieves this speed due to digital signaling as opposed to analog signaling used on all other modems. A 56k modem would actually achieve 64k except: 1. the local loop is still analog, thus analog signaling 2. the analog to digital conversion at the local modem introduces noise/error Combined, these shortcomings drop the speed to at best 56k.
Chapter Twelve - Voice and Data Delivery Networks The 56k Digital Modem A 56k modem does not achieve 56k either because the FCC will not let the modem transmit at the power level necessary to support 56k, so the best the modem can do is approximately 53k A 56k modem will not even achieve 53k if the connection between your modem and the remote computer contains an additional analog to digital conversion, or if there is significant noise on the line.
Chapter Twelve - Voice and Data Delivery Networks The 56k Digital Modem A 56k modem is based upon one of two standards: V.90 - Upstream speed is maximum 33,600 bps V.92 - Newer standard which allows maximum upstream speed of 48 kbps (under ideal conditions) and can place a data connection on hold if the telephone service accepts call waiting and a voice telephone call arrives
Chapter Twelve - Voice and Data Delivery Networks Digital Subscriber Line Digital subscriber line (DSL) is a relative newcomer to the field of leased line services. DSL can provide very high data transfer rates over standard telephone lines. Unfortunately, less than half the telephone lines in the U.S. are incapable of supporting DSL. And there has to be a DSL provider in your region.
Chapter Twelve - Voice and Data Delivery Networks Digital Subscriber Line DSL, depending on the type of service, is capable of transmission speeds from 100s of kilobits into single-digit megabits. Because DSL is highly dependent upon noise levels, a subscriber cannot be any more than 5.5 kilometers (2-3 miles) from the DSL central office. A DSL service can be symmetric, in which the downstream and upstream speeds are identical, or asymmetric in which the downstream speed is faster than the upstream speed.
Chapter Twelve - Voice and Data Delivery Networks Digital Subscriber Line A DSL service often connects a user to the Internet. A DSL service can also provide a regular telephone service (POTS). The DSL provider uses a DSL access multiplexer (DSLAM) to split off the individual DSL lines into homes and businesses. A user than needs a splitter to separate the POTS line from the DSL line, and then a DSL modem to convert the DSL signals into a form recognized by the computer.
Chapter Twelve - Voice and Data Delivery Networks • Digital Subscriber Line • A DSL service comes in many different forms: • ADSL - Asymmetric DSL • CDSL - Consumer DSL (trademarked version by Rockwell) • DSL.Lite - Slower form than ADSL. • HDSL - High-bit rate DSL • RADSL - Rate adaptive DSL (speed varies depending on noise level)
Chapter Twelve - Voice and Data Delivery Networks Cable Modems Cable modems allow high speed access to wide area networks such as the Internet. Most cable modems are external devices that connect to the personal computer through a common Ethernet card. Cable modems can provide data transfer speeds between 500 kbps and 2.5 Mbps
Chapter Twelve - Voice and Data Delivery Networks The T-1 A T-1 line is a digital service offered by the telephone companies and can transfer data as fast as 1.544 Mbps (both voice and computer data). To support a T-1 service, a channel service unit / data service unit (CSU/DSU) is required at the end of the connection.
Chapter Twelve - Voice and Data Delivery Networks The T-1 A T-1 service is a digital, synchronous TDM stream used by businesses and telephone companies. A T-1 service is always on and always transmitting. One T-1 service can support up to 24 simultaneous channels. These channels can be either voice or data (PBX support). A T-1 service can also be provisioned as a single channel delivering 1.544 Mbps of data (LAN to ISP connection).
Chapter Twelve - Voice and Data Delivery Networks The T-1 A T-1 service requires 4 wires, as opposed to a 2-wire telephone line. A T-1 can be either intra-LATA (local) which costs roughly $350-$400 per month, or inter-LATA (long distance) which can cost thousands of dollars per month (usually based on distance). A customer may also be able to order a ¼ T-1 or a ½ T-1.
Chapter Twelve - Voice and Data Delivery Networks The T-1 A T-1constantly transmits frames (8000 frames per second). Each frame consists of one byte from each of the 24 channels, plus 1 sync bit (8 * 24 + 1 = 193 bits). 8000 frames per second * 193 bits per frame = 1.544 Mbps. If a channel is used for voice, each byte is one byte of PCM-encoded voice. If a channel is used for data, each byte contains 7 bits of data and 1 bit of control information (7 * 8000 = 56 kbps).
Chapter Twelve - Voice and Data Delivery Networks Frame Relay Frame relay is the leased service that can provide a high-speed connection for data transfer between two points either locally or over long distances. A business only has to connect itself to the local frame relay port. Hopefully this connection is a local telephone call. Once the data reaches the local frame relay port, the frame relay network, or cloud, transmits the data to the other side.
Chapter Twelve - Voice and Data Delivery Networks Frame Relay A connection between two endpoints is called a permanent virtual circuit (PVC). PVCs are created by the provider of the frame relay service. The user uses a high-speed telephone line to connect its company to a port, which is the entryway to the frame relay network. The high-speed line, the port, and the PVC should all be chosen to support a desired transmission speed.
Chapter Twelve - Voice and Data Delivery Networks Frame Relay Consider a company that has four office locations and currently has six leased lines interconnecting the four locations. To install frame relay, the company would ask for six PVCs in place of the six leased lines. The company would also need four high-speed telephone lines and four ports connecting the four locations to the frame relay cloud.
