Basic of telecommunication

Chapter 12: Telecommunications Systems

Data Cummunications & Computer Networks: A Business User's Approach, Third Edition

Objectives

Identify the basic elements of a telephone system, and discuss the limitations of telephone signalsDescribe the composition of the telephone industry before and after the 1984 Modified Final Judgment, and explain the differencesDescribe the difference between a local exchange carrier and an interexchange carrier, and list the services each offers


Objectives (continued)

Differentiate between the roles of the local telephone company before and after the Telecommunications Act of 1996List the types of leased lines that are available and their basic characteristicsOutline the features of ISDN, and distinguish a basic rate interface from a primary rate interfaceList the basic characteristics of frame relay, such as permanent virtual circuits, committed information rate, and switched virtual circuits


Objectives (continued)

Identify the main characteristics of Asynchronous Transfer Mode, including the roles of the virtual path connection and the virtual channel connection, the importance of the classes of service available, and ATMs advantages and disadvantagesIdentify the main characteristics of digital subscriber line, and recognize the difference between a symmetric system and an asymmetric system


Introduction

Students used to go into either data communications or voice communicationsToday, the two fields are mergingVoice systems transfer computer data and data networks support voiceAnyone studying the field of data communications and networks must learn some basic telecommunications too


The Basic Telephone System

POTS is the plain old telephone system that connects most homes and small businessesPOTS lines were designed to transmit the human voice, which has a bandwidth less than 4000 HzA telephone conversation requires two channels, each occupying 4000 Hz


The Basic Telephone System (continued)



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 secondIf you want to send information faster, you need a signal with a higher frequency OR you need to incorporate more advanced modulation techniquesPOTS cannot deliver faster signals



Local loop is the telephone line that runs from the telephone companys central office to your home or businessCentral office is the building that houses the telephone companys switching equipment and provides a local dial tone on your telephoneIf you place a long distance call, the central office passes your telephone call off to a long distance provider



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 companyIf your call stays within a LATA, it is a local distance call and is handled by a local telephone company



A trunk is a special telephone line that runs between central offices and other telephone company switching centersA trunk is usually digital, high speed, and carries multiple telephone circuitsA trunk is typically a 4-wire circuit, while a telephone line is a 2-wire circuit



A trunk is not associated with a single telephone number like a line isA telephone number consists of an area code, an exchange, and a subscriber extensionThe area code and exchange must start with the digits 2-9 to separate them from long distance and operator services



When the telephone company installs a line, it must not proceed any further than 12 inches into the buildingThis point is the demarcation point, or demarcModular connectors, such as the RJ-11, are commonly used to interconnect telephone lines and the telephone handset to the baseWhen the handset is lifted off the base (off-hook), an off-hook signal is sent to the central office



When the off-hook signal arrives at the central office, a dial tone is generated and returned to the telephoneWhen the user hears the dial tone, they dial (or press) the numberThe central office equipment collects the dialed digits, and proceeds to place the appropriate call



Foreign exchange service (FX) - customer calls a local number which is then connected to a leased line to a remote siteWide area telecommunications services (WATS) - discount volume calling to local and long distance sitesOff premises extensions (OPX) - dial tone at location B comes from the PBX at location A


The Telephone Network Before and After 1984

In 1984, the U.S. government broke up AT&TBefore then, AT&T owned a large majority of all local telephone circuits and all the long distance serviceWith the Modified Final Judgment of 1984, AT&T had to split off the local telephone companies from the long distance companyThe local telephone companies formed seven Regional Bell Operating Companies Today, there are only 4 left: Bell South, SBC, Qwest (US West), and Verizon (Bell Atlantic)


The Telephone Network Before and After 1984 (continued)



Another result of the Modified Judgment was the creation of the LATA (local access and transport area)Local telephone companies became known as local exchange carriers (LECs)Long distance telephone companies became known as interexchange carriers (IEC, or IXC)Calls that remain within a LATA are intra-LATA, or local callsCalls that pass from one LATA to another are inter-LATA, or long distance



Before 1984, the telephone network in the U.S. resembled a large hierarchical tree, with Class 5 offices at the bottom and Class 1 offices at the topUsers were connected to the Class 5 officesThe longer the distance of a telephone call, the further up the tree the call progressedTodays telephone structure is a collection of LECs, POPs, and IECs


Telephone Networks after 1996

Telecommunications Act of 1996: opened up the local telephone market to competitorsNow cable TV companies (cable telephony), long distance telephone companies, or anyone that wanted to start a local telephone company could offer local telephone serviceLocal phone companies that existed before the Act are known as incumbent local exchange carriers (ILEC) while the new companies are competitive local exchange carriers (CLEC)


Telephone Networks after 1996 (continued)

LECs are supposed to allow CLECs access to all local loops and switching centers / central officesIf a local loop is damaged, the LEC is responsible for repairThe LEC is also supposed to provide the CLEC with a discount to the dial tone (17-20%)LECs can also provide long distance service if they can show there is sufficient competition at the local service level


Other Players in the Market

Alternate operator services - pay phones, hotel phonesAggregators - pulls a bunch of small companies together and goes after phone discountsReseller - rents or leases variety of lines from phone companies, then resells to customersSpecialized mobile radio carriers - mobile communication services to businesses and individuals, including dispatch, paging, and data servicesARDIS and RAM Mobile Data two good examples


PBX

Private branch exchange (PBX) - common internal phone switching system for medium to large-sized businessesProvides advanced intelligent features to users, such as:4-digit internal dialingSpecial prefixes for WATS, FX, etc (private dialing plans)PBX intelligently decides how to route a call for lowest cost


More PBX Features

Voice mailRoutes 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 Components

CPU, memory, telephone lines, trunksSwitching networkSupporting logic cardsMain distribution frameConsole or switchboardBattery back-up system


Automated Attendant

Plays a recorded greeting and offers a set of optionsLets the caller enter an extension directly (touch tone or voice) and bypass an operatorForwards the caller to a human operator if the caller does not have a touch tone phoneAvailable as an option on a PBX


Automatic Call Distributor

Automatic Call Distributor: perhaps youve 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 receivedUsed in systems where incoming calling volume is large, such as customer service, help desk, order entry, credit authorization, reservations, and catalog salesEarly systems used hunt groupsOriginal systems routed call to first operator in line (kept person very busy!)


Automatic Call Distributor (continued)

Modern systems perform more advanced functions, such as:Prioritize the callsRoute calls to appropriate agent based on the skill set of the agentIf 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


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 informationIVR can also perform fax on demand


Interactive Voice Response (continued)

Common examples of IVR include:Calling your bank to inquire about an account balanceUniversity online registration systemBrokerage firm taking routine orders from investorsInvestment fund taking routine requests for new account applicationsA company providing employees with info about their benefit plans


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 PBXFor example, a key system might distribute 48 internal telephone sets over 16 external phone linesThe 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)


Leased Line Services

Most home computer users use POTS lines and conventional modems to connect to other computer systemsWhat if you need a faster service, or need one that is always on? You can get a leased line serviceA basic leased line, or tie line, gives you a 56 kbps data transfer rateT-1 (or T1) service provides 1.544 Mbps rateUsed by businesses to connect their in-house telephone systems (PBX) and data networks to the outside world


T-1 Service

A T-1 service is a digital, synchronous TDM stream used by businesses and telephone companiesA T-1 service is always on and always transmittingOne T-1 service can support up to 24 simultaneous channelsThese 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)


T-1 Service (continued)

A T-1 service requires 4 wires, as opposed to a 2-wire telephone lineA 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 T-1 or a T-1


T-1 Service (continued)

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 MbpsIf a channel is used for voice, each byte is one byte of PCM-encoded voiceIf a channel is used for data, each byte contains 7 bits of data and 1 bit of control information (7 * 8000 = 56 kbps)


Integrated Services Digital Network (ISDN)

ISDN is another leased service that provides a digital telephone or data connection into a home or businessWith ISDN you can have a digital telephone line and a 64 kbps data line, or one 128 kbps data lineThe basic rate interface (BRI) is the service for homes and small businesses, while the primary rate interface (PRI) is the service for larger businesses


ISDN (continued)

BRI ISDN consists of two B channels and one D channelA B channel can carry 64 kbps of data or PCM-encoded voiceThe D channel is 16 kbps and carries signaling informationThe B channels are dialable, and the D channel can be always onMany users combine both B channels for a 128 kbps data channel


ISDN (continued)

PRI ISDN is used by larger businesses and contains 23 B channels and one 64 kbps D channelPRI ISDN is essentially equivalent to a T-1, but with ISDN, the 23 channels are dialable!The appropriate ISDN modems / multiplexors are necessary to support this serviceWhat could you use an always-on D channel for?


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 distancesA business only has to connect itself to the local frame relay portHopefully this connection is a local telephone callOnce the data reaches the local frame relay port, the frame relay network, or cloud, transmits the data to the other side


Frame Relay (continued)



A connection between two endpoints is called a permanent virtual circuit (PVC)PVCs are created by the provider of the frame relay serviceUser uses a high-speed telephone line to connect its company to a port, which is the entryway to the frame relay networkThe high-speed line, the port, and the PVC should all be chosen to support a desired transmission speed



Consider a company that has four office locations and currently has six leased lines interconnecting the four locationsTo install frame relay, the company would ask for six PVCs in place of the six leased linesThe company would also need four high-speed telephone lines and four ports connecting the four locations to the frame relay cloud


Frame Relay Setup


Frame Relay Setup (continued)


Committed Information Rate (CIR)

The user and frame relay service would agree upon a committed information rate (CIR)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 framesThe burst rate cannot be exceeded for longer than 2 seconds


CIR (continued)

Example: If a company agrees to a CIR of 512 kbps with a burst rate of 256 kbps, the company must stay at or below 512 kbps, with an occasional burst up to 768 kbps, as long as the burst does not last longer than 2 secondsIf the company maintains their end of the agreement, the carrier will provide something like 99.99% throughput and a network delay of no longer than 20 msIf the customer exceeds its CIR, and the network becomes congested, the customers frames may be discarded


Frame Relay vs. the Internet

Frame relay has many advantages over the Internet, including guaranteed throughput and minimum delay as well as better securityInternet has the advantage of being practically everywhereCheaper 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 communicationsHigh transfer speeds adequately support the needs of interactive voiceIf 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 customerGood for short-term connections, but more expensive


Asynchronous Transfer Mode (ATM)

Asynchronous transfer mode (ATM) is a very high speed packet delivery service, similar in a number of ways to frame relayBoth send packets of data over high speed linesBoth require a user to create a circuit with a providerOne noticeable difference between ATM and frame relay is speed ATM is capable of speeds up to 622 Mbps while frame relays maximum is typically 45 Mbps


ATM (continued)

Similar to frame relay, data travels over a connection called a virtual channel connection (VCC)To better manage VCCs, a VCC must travel over a virtual path connection (VPC)One of ATMs strengths (besides its high speeds) is its ability to offer various classes of serviceIf a company requires a high-speed, continuous connection, they might consider a constant bit rate service


ATM (continued)

Variable Bit Rate (VBR): less demanding serviceCan also support real time applications (rt-VBR), as well as non-real time applications (nrt-VBR), but does not demand a constant bit streamAvailable bit rate (ABR): used for bursty traffic that does not need to be transmitted immediatelyABR traffic may be held up until a transmission opening is availableUnspecified bit rate (UBR): for lower rate traffic that may get held up, and may even be discarded part way through transmission if congestion occurs


Advantages and Disadvantages of ATM

Advantages of ATM include very high speeds and different classes of serviceDisadvantages include potentially higher costs (both equipment and support) and a higher level of complexity


Digital Subscriber Line

Digital subscriber line (DSL) is a relative newcomer to the field of leased line servicesDSL can provide very high data transfer rates over standard telephone linesUnfortunately, less than half the telephone lines in the U.S. are incapable of supporting DSLThere has to be a DSL provider in your region


DSL (continued)

DSL, depending on the type of service, is capable of transmission speeds from 100s of kilobits into single-digit megabitsBecause DSL is highly dependent upon noise levels, a subscriber cannot be any more than 5.5 kilometers (2-3 miles) from the DSL central officeService can be symmetric, in which downstream and upstream speeds are identical, or asymmetric in which downstream speed is faster than upstream speed


DSL (continued)

A DSL service often connects a user to the InternetA 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 businessesA 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


DSL (continued)


DSL (continued)

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)


DSL (continued)


Computer Telephony Integration

Computer telephony integration (CTI): emerging field that combines more traditional voice networks with modern computer networksConsider a system in which a customer calls a customer support number:Customers telephone number appears on customer support reps terminal and immediately pulls up customers dataRep answers phone by clicking on an icon on the screen and helps the customerRep transfers the call by clicking on another icon on the computer screen


Computer Telephony Integration (continued)

CTI can also integrate voice cabling with data cablingPBX talks directly to the LAN server PBX can direct the LAN server to provide a telephone operation to the user through the users computerTelephones may still be connected to the PBX or they may be connected to the LAN via the LAN wiring


Computer-Telephony Integration (CTI)CTI applications could include the following:*Unified messaging*Third party call control *Interactive voice response *PBX Graphic User Interface*Integrated voice recognition *Call filtering and response*Fax processing and fax-back *Customized menuing systems*Text-to-speech and speech-to-text conversions


Telecommunication Systems in Action: A Company Makes a Service Choice

Better Box Corporation has offices in Seattle, San Francisco, and Dallas, with headquarters in ChicagoBetter Box wants to connect Chicago to each of the other three officesBetter Box needs to download 400 kbyte files in 20 secondsRequires a transmission speed of 160,000 bpsBetter Box could use three separate T-1 lines, use a frame relay service, or use asynchronous transfer mode


Telecommunications Systems in Action: A Company Makes a Service Choice



Better Box Corporation might also consider dial-up access lines, ISDN BRI service, and leased 56k linesDial-up, ISDN BRI, and leased 56k lines will not meet the companys requirements for a 160 kbps downloadTypical various prices for these services are shown on the next table



To provide T-1 service to all four offices:Seattle to Chicago: $6325 ($1200 + $2.50 per mile)San Francisco to Chicago: $6625Dallas to Chicago: $3500Total interLATA T-1 costs = $16,450 / month



To provide frame relay service:Three ports at 256K = 3 x $495One port at 768K = $1240Three 256K PVCs = 3 x $230Four intraLATA T-1s = 4 x $350Total charge = $4815 / month



To provide asynchronous transfer mode service:Four ports at 1.544 Mbps ABR = 4 x $1750Three channels = 3 x $250Three paths = $2 per mile x 5140 miles = $10,280Four intraLATA T-1s = 4 x $350Total ATM charges = $19,430 / month


Summary

Telephone system1984 Modified Final JudgmentLocal exchange carrier vs. interexchange carrierTelecommunications Act of 1996Leased linesISDN and basic rate interface vs. primary rate interfaceFrame relayAsynchronous Transfer ModeDigital subscriber line and symmetric vs. asymmetric system


Basic of telecommunication

Documents

business users approach

basic telephone systempots

telephone line

telephone industry

computer data

local telephone company

plain old telephone

basic elements