Voice and Data Delivery Networks. 2 Telephone Lines, Trunks, and Numbers The local loop is the telephone line that runs from the telephone company’s central.

Voice and Data Delivery Networks

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Telephone Lines, Trunks, and Numbers

• The local loop is the telephone line that runs from the telephone company’s central office to your home or business– Central office – 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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Telephone Lines, Trunks, and Numbers (continued)

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Telephone Lines, Trunks, and Numbers (continued)

• 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 call and is handled by a local telephone company

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Telephone Lines, Trunks, and Numbers (continued)

• Trunk – special telephone line that runs between central offices and other telephone switching centers– Usually digital, high-speed, and carries multiple

telephone circuits– Typically a 4-wire circuit, while a telephone line is

a 2-wire circuit

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Telephone Lines, Trunks, and Numbers (continued)

• 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

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Telephone Lines, Trunks, and Numbers (continued)

• When 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 handset is lifted off base (off-hook), an off-hook signal is sent to the central office

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Telephone Lines, Trunks, and Numbers (continued)

• When off-hook signal arrives at central office, a dial tone is generated and returned to telephone

• When user hears the dial tone, they dial (or press) number

• The central office equipment collects dialed digits, and proceeds to place appropriate call

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PBX

• 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 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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PBX (continued)

• 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

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PBX (continued)

• PBX components: – CPU, memory, telephone lines, trunks– Switching network– Supporting logic cards– Main distribution frame– Console or switchboard– Battery back-up system

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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

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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 call 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 Distributor (continued)

• 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

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Interactive Voice Response

• IVR is similar to automated attendant 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 Response (continued)

• 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

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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)

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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

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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

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The Telephone Network Before and After 1984

• In 1984, U.S. government broke up AT&T• Before then, AT&T owned large majority of all local

telephone circuits and all the long-distance service• With Modified Final Judgment of 1984, AT&T had to split

off local telephone companies from long-distance company– The local telephone companies formed seven Regional Bell

Operating Companies• Today, there are only 4 left: BellSouth, SBC, Qwest (US

West), and Verizon (Bell Atlantic)

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The Telephone Network Before and After 1984 (continued)

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The Telephone Network Before and After 1984 (continued)

• Another result of the Modified Judgment was creation of 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 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 Network Before and After 1984 (continued)

• Before 1984, telephone networks 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 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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Telephone Networks After 1996• Another landmark ruling affecting the telephone industry

was the Telecommunications Act of 1996– Opened up local telephone market to competitors

• Now cable TV companies (cable telephony), long-distance telephone companies, or anyone that wants to start a local telephone company can 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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Telephone Networks After 1996 (continued)

• LECs 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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Limitations of Telephone Signals

• 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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Limitations of Telephone Signals (continued)

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Limitations of Telephone Signals (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 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?

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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

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The 56k Dial-Up Modem (continued)

• 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

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The 56k Dial-Up Modem (continued)

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The 56k Dial-Up Modem (continued)

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The 56k Dial-Up Modem (continued)

• Based upon one of two standards:– V.90

• Upstream speed is maximum 33,600 bps

– V.92 • Newer standard • Allows maximum upstream speed of 48 kbps (under

ideal conditions) • Can place a data connection on hold if the telephone

service accepts call waiting and a voice telephone call arrives

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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

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DSL Basics• 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

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DSL Basics (continued)• 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

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DSL Basics (continued)

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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

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Cable Modems (continued)

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T-1 Leased Line Service

• T-1 – digital service offered by the telephone companies that 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

T-1 Leased Line Service

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T-1 Line

Data

CSUDSU

T-1 MuxDSX-1Interface

Customer SuppliedFrom Telco ?

Loopbackcontrol

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T-1 Leased Line Service (continued)

• A T-1 service – Is a digital, synchronous TDM stream used by

businesses and telephone companies– Is always on and always transmitting– Can support up to 24 simultaneous channels

• These channels can be either voice or data (PBX support)

– Can also be provisioned as a single channel delivering 1.544 Mbps of data (LAN to ISP connection)

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T-1 Leased Line Service (continued)

• A T-1 service (continued)– Requires 4 wires, as opposed to a 2-wire

telephone line– 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 1/4 T-1 or a 1/2 T-1

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T-1 Leased Line Service (continued)

• Constantly 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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CSU (Channel Service Unit)

• First (last) piece of equipment on a T-1 line• Can perform various loop-back tests• CSU can also generate “keep alive” signal when the attached

DTE fails to deliver a valid stream of data or DTE is disconnected

• CSU can also collect error statistics for the phone company

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DSU (Digital Service Unit)

• Shapes the T-1 signal being sent• Prepares the customer data to meet the requirements of

the DSX-1 interface• Suppresses long strings of zeros with special coding• Provides the terminal (user) with remote and local

loopback tests• DSU if often built into the terminal equipment or

multiplexor and should eventually disappear

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ISDN

• Allows digital transmission of voice and data over traditional copper lines

• We have already seen the ISDN frame layout in the chapter on Multiplexing

• Three basic types of ISDN– Basic rate– Primary rate– Broadband

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Basic Rate ISDN

• Entry level service• 144 kbps service• 2 – 64 kbps bearer (data) channels (DS-0) and 1 – 16 kbps

delta channel (for signaling or data) (also known as 2B+D)• User can bond both data channels together for a 128 kbps

channel• Rarely used in the US; some degree of use in Japan /

England / Europe; fairly popular in Germany (29% of all subscriber lines as of 2003; 20% of all ISDN lines worldwide) (source:Wikipedia)

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Primary Rate ISDN

• In Europe, this consists of 30 B-channels of 64 kbps each plus 1 D-channel of 64 kbps and is carried over an E-1

• In North America, this service is 23 B-channels and 1 D-channel and carried over a T-1 (J-1 in Japan)

• PRI-ISDN is popular through-out the world and is used to connect PSTN to company PBXs

• In US, PRI-ISDN is used on connection of non-VoIP PBXs to PSTN