1 chapter 12 telecommunication systems data communications and computer networks: a business...

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Chapter 12

Telecommunication Systems

Data Communications andComputer Networks: A Business User’s Approach

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Data Communications and Computer Networks Chapter 12

IntroductionStudents 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.

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Basic Telephone SystemsPOTS is the plain old telephone system that connects most homes and small businesses.

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.

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Basic Telephone SystemsA 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.

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Basic Telephone SystemsThe 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.

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Basic Telephone SystemsThe 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.

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Basic Telephone SystemsA 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.

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Basic Telephone SystemsA 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.

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Basic Telephone SystemsWhen 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.

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Basic Telephone SystemsWhen 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.

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Basic Telephone Systems ServicesForeign 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.

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The Telephone Before and After 1984In 1984, the U.S. government broke up AT&T.

Before then, AT&T owned a large majority of all local telephone circuits and all the long distance service.

With the Modified Final Judgment of 1984, AT&T had to split off the local telephone companies from the long distance company.

The 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).

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The Telephone Before and After 1984Another 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), and long distance telephone companies became known as interexchange carriers (IEC, or IXC).

Calls that remain within a LATA are intra-LATA, or local calls. Calls that pass from one LATA to another are inter-LATA, or long distance.

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The Telephone Before and After 1984Before 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 top.

Users were connected to the Class 5 offices.

The longer the distance of a telephone call, the further up the tree the call progressed.

Today’s telephone structure is a collection of LECs, POPs, and IECs.

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The Telephone After 1996Another landmark ruling affecting the telephone industry was the Telecommunications Act of 1996.

This act opened up the local telephone market to competitors.

Now cable TV companies (cable telephony), long distance telephone companies, or anyone that wanted to start a local telephone company could offer local telephone service.

Local 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).

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The Telephone After 1996LECs are supposed to allow CLECs access to all local loops and switching centers / central offices.

If a local loop is damaged, the LEC is responsible for repair.

The 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.

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Other Players in the MarketAlternate 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

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PBXPrivate 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

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More PBX FeaturesVoice 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

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PBX ComponentsCPU, memory, telephone lines, trunks

Switching network

Supporting logic cards

Main distribution frame

Console or switchboard

Battery back-up system

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Automated AttendantPlays 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.

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Automatic Call DistributorWhen 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!)

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Automatic Call DistributorModern 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

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Interactive Voice ResponseIVR 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.

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Interactive Voice ResponseCommon examples of IVR include:

Call 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

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Key Telephone SystemUsed 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).

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Leased Line ServicesMost home computer users use POTS lines and conventional modems to connect to other computer systems.

What if you need a faster service, or need one that is always on? You can get a leased line service.

A basic leased line, or tie line, gives you a 56 kbps data transfer rate.

A T-1 (or T1) service gives you a 1.544 Mbps rate and is used by businesses to connect their in-house telephone systems (PBX) and data networks to the outside world.

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T-1 ServiceA 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).

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T-1 ServiceA 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.

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T-1 ServiceA 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).

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Integrated Services Digital Network (ISDN)ISDN is another leased service that provides a digital telephone or data connection into a home or business.

With ISDN you can have a digital telephone line and a 64 kbps data line, or one 128 kbps data line.

The basic rate interface (BRI) is the service for homes and small businesses, while the primary rate interface (PRI) is the service for larger businesses.

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ISDNBRI ISDN consists of two B channels and one D channel.

A B channel can carry 64 kbps of data or PCM-encoded voice.

The D channel is 16 kbps and carries signaling information.

The B channels are dialable, and the D channel can be always on.

Many users combine both B channels for a 128 kbps data channel.

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ISDNPRI ISDN is used by larger businesses and contains 23 B channels and one 64 kbps D channel.

PRI 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 service.

What could you use an always-on D channel for?

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Frame RelayFrame 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.

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Frame RelayA 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.

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Frame RelayConsider 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.

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Frame RelayThe user and frame relay service would 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.

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Frame RelayFor 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 seconds.

If 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 ms.

If the customer exceeds its CIR, and the network becomes congested, the customer’s frames may be discarded.

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Frame Relay vs. the InternetFrame relay has many advantages over the Internet, including guaranteed throughput and minimum delay, and better security.

Internet has the advantage of being practically everywhere, cheaper, and simpler to create connections (no PVCs necessary). And Internet tunnels (VPNs) are attractive.

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Voice over Frame RelayFrame relay is also capable of supporting voice communications.

The high transfer speeds of frame relay adequately support the needs of interactive voice.

If a company requires multiple voice circuits, frame relay is an interesting solution.

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Switched Virtual CircuitsFrame 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.

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Asynchronous Transfer ModeAsynchronous transfer mode (ATM) is a very high speed packet delivery service, similar in a number of ways to frame relay.

Both send packets of data over high speed lines.

Both require a user to create a circuit with a provider.

One noticeable difference between ATM and frame relay is speed - ATM is capable of speeds up to 622 Mbps while frame relay’s maximum is typically 45 Mbps.

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Asynchronous Transfer ModeSimilar 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 ATM’s strengths (besides its high speeds) is its ability to offer various classes of service.

If a company requires a high-speed, continuous connection, they might consider a constant bit rate service.

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Asynchronous Transfer ModeA less demanding service is variable bit rate (VBR).

VBR can also support real time applications (rt-VBR), as well as non-real time applications (nrt-VBR), 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.

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Asynchronous Transfer ModeAdvantages of ATM include very high speeds and the different classes of service.

Disadvantages include potentially higher costs (both equipment and support) and a higher level of complexity.

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Digital Subscriber LineDigital 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.

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Digital Subscriber LineDSL, 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.

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Digital Subscriber LineA 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.

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Digital Subscriber LineA 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)

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Computer Telephony IntegrationComputer telephony integration (CTI) is the emerging field that combines more traditional voice networks with modern computer networks.

Consider a system in which a customer calls a customer support number. The customer’s telephone number appears on the customer support rep’s terminal and immediately pulls up the customer’s data. The rep answers the phone by clicking on an icon on the screen and helps the customer. The rep transfers the call by clicking on another icon on the computer screen.

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Computer Telephony IntegrationCTI can also integrate voice cabling with data cabling.

The company PBX talks directly to the LAN server. The PBX can direct the LAN server to provide a telephone operation to the user through the user’s computer.

The telephones may still be connected to the PBX or they may be connected to the LAN via the LAN wiring.

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Computer Telephony IntegrationCTI 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

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Computer Telephony IntegrationCTI applications could include the following:

• Third party call control

• PBX Graphic User Interface

• Call filtering

• Customized menuing systems

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Telecommunication Systems In Action - A Company Makes a Service ChoiceBetter Box Corporation has offices in Seattle, San Francisco, and Dallas, with headquarters in Chicago.

Better Box wants to connect Chicago to each of the other three offices.

Better Box needs to download 400 kbyte files in 20 seconds. This requires a transmission speed of 160,000 bps.

Better Box could use three separate T-1 lines, use a frame relay service, or use asynchronous transfer mode.

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Telecommunication Systems In Action - A Company Makes a Service ChoiceBetter Box Corporation might also consider dial-up access lines, ISDN BRI service, and leased 56k lines.

Dial-up, ISDN BRI, and leased 56k lines will not meet the company’s requirements for a 160 kbps download.

Typical various prices for these services are shown on the next table.

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Telecommunication Systems In Action - A Company Makes a Service ChoiceTo provide T-1 service to all four offices:

Seattle to Chicago: $6325 ($1200 + $2.50 per mile)

San Francisco to Chicago: $6625

Dallas to Chicago: $3500

Total interLATA T-1 costs = $16,450 / month

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Telecommunication Systems In Action - A Company Makes a Service ChoiceTo provide frame relay service:

Three ports at 256K = 3 x $495

One port at 768K = $1240

Three 256K PVCs = 3 x $230

Four intraLATA T-1s = 4 x $350

Total charge = $4815 / month

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Telecommunication Systems In Action - A Company Makes a Service ChoiceTo provide asynchronous transfer mode service:

Four ports at 1.544 Mbps ABR = 4 x $1750

Three channels = 3 x $250

Three paths = $2 per mile x 5140 miles = $10,280

Four intraLATA T-1s = 4 x $350

Total ATM charges = $19,430 / month

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