本章介绍基本的通话系统及其发展历程,包括拨号调制解调器,DSL(数字用户线路),Cable Modem(电缆调制解调器),T1 Leased Line Services(T1专线服务)、Frame Relay(帧中继)、CTI(将传统语音网络与现代计算机网络相结合以及将语音布线与数据布线相结合) & UC(Unified Communications)等内容。

有只羊发布于2018/10/05

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1.Chapter 11 Voice and Data Delivery Networks

2.2 Introduction Basic Telephone Systems Dial-up Modem ISDN DSL Cable Modem T1 Leased Line Services Frame Relay ATM CTI & UC

3.3 Basic Telephone Systems (I) POTS is the ‘plain old telephone system’ Transmit voice at bandwidth less than 4000 Hz Two 4000 Hz channels required for telephone conversation A 4000 Hz analog signal can only carry 33,600 bps, while a 4000 Hz digital signal can carry about 56,000 bps Higher frequency needed for faster transmission. POTS cannot deliver faster signals

4.4 Local loop runs from the central office to your home or business Central office houses the switching equipment and provides a local dial tone on your telephone Central office passes your long distance call off to a long distance provider The country is divided into a few hundred local access transport areas (LATAs) long distance call goes from one LATA to another, handled by a long distance telephone company Local call stays within a LATA, handled by a local telephone company Basic Telephone Systems (II)

5.5 Trunk runs between central offices and other telephone company switching centers Trunk is usually digital, high speed, and carries multiple telephone circuits Trunk is typically a 4-wire circuit, while a telephone line is a 2-wire circuit 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 Basic Telephone Systems (III)

6.6 Modified Final Judgment of 1984 made AT&T split off the local telephone companies from the long distance company Regional Bells (baby bells) service local phone. Started with 7, only 3 remained (AT&T, CenturyLink , Verizon) Telecommunications Act of 1996 opened up the local telephone market to competitors Cable TV companies (cable telephony), long distance telephone companies, or anyone that wanted to start a local telephone company could offer local telephone service History

7.7 Private branch exchange (PBX) - common internal phone switching system for medium to large-sized businesses. Provides advanced intelligent features to users, such as: 4-digit, special prefixes for WATS, FX, etc. (private dialing plans) PBX collects dialed digits and intelligently decides how to route this call for lowest cost Additional advanced 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 PBX

8.8 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

9.9 Automatic Call Distributor Automatic Call Distributor: perhaps you’ve experienced this when you call a business, 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!) 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

10.10 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 Common examples of IVR include: Calling 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 Interactive Voice Response

11.11 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 Example – 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)

12.12 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

13.13 Other Players in the Market Alternate operator services Pay phones, hotel phones 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

14.The 56k Dial-Up Modem A 56k modem (56,000 bps) achieves this speed due to digital signaling as opposed to analog signaling used on all other modems Would actually achieve 64k except: Local loop is still analog, thus analog signaling Analog to digital conversion at the local modem introduces noise/error Combined, these shortcomings drop the speed to at best 56k Does not achieve 56k either FCC will not let modem transmit at power level necessary to support 56k, so the best modem can do is approximately 53k Will not even achieve 53k if connection between your modem and remote computer contains an additional analog to digital conversion, or if there is significant noise on line 14

15.15 DSL (I) 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 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 DSL service can be: Symmetric – downstream and upstream speeds are identical Asymmetric – downstream speed is faster than the upstream speed

16.16 DSL (II) DSL service Often connects a user to the Internet 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 then 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 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

17.17 A leased service that provides a digital telephone or data connection into a home or business Can have a digital telephone line and a 64 Kbps data line, or one 128 Kbps data line Basic rate interface (BRI) for homes and small businesses Consists of two B channels and one D channel One B channel carry 64 Kbps of data or PCM-encoded voice. D channel is 16 Kbps and carries signaling information The B channels are dialable, and the D channel can be always on The 2 B channels can be combined for a 128 Kbps data channel Primary rate interface (PRI) for larger businesses Used by larger businesses and contains 23 B channels and one 64 Kbps D channel Essentially equivalent to a T-1, but with ISDN the 23 channels are dialable! Appropriate ISDN modems / multiplexors are necessary to support this service ISDN

18.18 Cable Modems Allow high-speed access to wide area networks such as the Internet Most are external devices that connect to the personal computer through a common Ethernet card Can provide data transfer speeds between 500 kbps and 25 Mbps

19.19 A digital, synchronous TDM stream used by businesses and telephone companies To support a T-1 service, a channel service unit / data service unit (CSU/DSU) is required at the end of the connection Always on and always transmitting Support up to 24 simultaneous channels. These channels can be either voice or data (PBX support) Can be provisioned as a single channel delivering 1.544 Mbps of data (LAN to ISP connection) Can order a ¼ T-1 or a ½ T-1 T-1 constantly transmits 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) T-1 Leased Line

20.20 High-speed (45 Mbps) connection for data transfer between two points either locally or over long distances A business connect itself to the local frame relay port through a high-speed line The frame relay network transmits the data to the other side Permanent virtual circuit (PVC) is a connection between two endpoints PVCs are created by the provider of the frame relay service The high-speed line, the port, and the PVC should all be chosen to support a desired transmission speed The user and frame relay service agree upon a committed information rate (CIR) The CIR states that if the customer stays within a specified data rate (standard rate plus a burst rate) the frame relay provider will guarantee delivery of 99.99% of the frames The burst rate cannot be exceeded for longer than 2 seconds Frame Relay (I)

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22.22 Frame Relay (II) Frame Relay vs. the Internet Frame relay has many advantages over the Internet, including guaranteed throughput and minimum delay as well as better security Internet has the advantage of being practically everywhere Cheaper and simpler to create connections (no PVCs necessary) Internet tunnels (VPNs) are also attractive Voice over Frame Relay (VoFR) Frame relay is also capable of supporting voice communications High transfer speeds adequately support the needs of interactive voice If a company requires multiple voice circuits, frame relay is an interesting solution Frame Relay Switched Virtual Circuits Frame relay can also provide switched virtual circuits (SVC) An SVC can be created dynamically by the customer Good for short-term connections, but more expensive

23.23 Very high-speed packet delivery service, capable of speeds up to 622 Mbps (in 53-byte cells) Able to offer various classes of service (QoS) constant bit rate service for a high-speed, continuous connection A less demanding service is variable bit rate (VBR) VBR can also support real time applications, as well as non-real time applications, but do not demand a constant bit stream Available bit rate (ABR) is used for bursty traffic that does not need to be transmitted immediately. ABR traffic may be held up until a transmission opening is available Unspecified bit rate (UBR) is for lower rate traffic that may get held up, and may even be discarded part way through transmission if congestion occurs ATM

24.MPLS and VPNs Frame relay and ATM are declining in popularity due to more people using the Internet But you can’t just send potentially important data over the Internet without doing something first One thing businesses are doing is applying MPLS labels to the IP packets The use of MPLS routes data packets quickly through the Internet And as we have also seen, VPNs (virtual private networks) create secure tunnels 24

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26.26 Convergence Big issue in the voice and data delivery industry Phone companies are buying other phone companies Older technologies are falling by the wayside as newer technologies take over a larger share of the market Newer devices are incorporating multiple applications Computer telephony integration is one large example of convergence

27.27 CTI Combines traditional voice networks with modern computer networks. Can also integrate voice cabling with data cabling. CTI applications could include the following: Unified messaging Interactive voice response Integrated voice recognition and response Fax processing and fax-back Text-to-speech and speech-to-text conversions Third party call control PBX Graphic User Interface Call filtering Customized menuing systems

28.Unified Communications Just as CTI is a convergence of multiple technologies and applications, unified communications is the convergence of real-time and non-real-time communications For example, convergence of telephony, instant messaging, video conferencing, voice mail, e-mail, and presence information into one or more applications 28