Telephone Network HierarchyAnalog Sounds

ENGR 475 – TelecommunicationsHarding University

Jon White

Voice

Voice

Voice

Voice

Sound Barrier

Telephone Network• PSTN – Public Switched Telephone Network

– World’s public circuit switched voice assemblage.– Similar to how the Internet is the assemblage of the World’s IP

based packet switched networks

• POTS – Plain Old Telephone Service– Analog, full duplex, dial tone, subscriber dials– 2 wires for communication (Tip and Ring)– Voice travels over a 48 V DC source provided by the Telco.

• Local Loop– 2 to 25 miles of 19 AWG unshilded twisted pair

• Demarc or point of presence– Line in the sand

Telephone Network - Demarc

Telephone Network Components• Terminals

– Phones• Access

– Phone lines• Connectivity

– Intelligence to let phones use phone lines to call other phones

– Switches– PBX – Private Branch Exchanges

• Other Features– Call forwarding, re-dial, caller ID, voice mail, protocol

conversion

PBX – Private Branch Exchange

• Almost every corporation has one.• Handles internal switching.

– You don’t have to go outside your network when calling someone in your building.

• Performs other helpful features– Call forwarding.– Call holding.– Caller ID

• http://wandel.ca/homepage/pbx.html• With VOIP, your computer can do all this in

software.

Telephone Network• The Telephone Network must implement

areas in Connectivity:– Provide power to the phone when picked up– Announce when ringing

• The first signal we’ve talked about– Address resolution / Call routing

• How is this signal passed? In band or out of band? When you press a key, what do you hear?

– Provide a guaranteed QOS

Telephone Hiearchy• Ring Topology• Star Topology• Hybrid

Telephone Network

Toll Center = Tandem Office

End office = Central Office (CO)

Modern CO’s serve up to 100,000

customers.

Searcy’s CO

Telephone Network - CO

Telephone Network – Pedestal Box

Telephone Network – Wiring Closet

Telephone Network – Patch Panel

Telephone Network Terms• Exchange Area

– Local vs long distance• LEC – Local Exchange Carrier• ILEC – Incumbent LEC

– CLEC• BOC – Bell Operating Company

– RBOC• Trunks – fiber optical

– 10 to 1– TIE lines

• LATA – Local access and transport area• IXC – Inter-exchange Carrier

– Carry inter-LATA traffic

Telephone Network – LATA Areas

Telephone Network – After Divestiture

Telephone Network - POP

• POP – Point of Presence

• Where are they at?

Telephone Network - Connectivity• We’ve covered:

– Terminals– Access

• Connectivity– How a call gets routed to the proper place.– One of the most difficult parts.

• Whole world has to agree.• Hard to change once you’ve started giving out ID’s

• NANPA– North American Numbering Plan Association– Designed by AT &T in 1947– In charge of managing phone numbers

NANPA Map

NANPA Evolution – pg. 42• Phone numbers:

– Area Code – 3 digits– Exchange Code – 3 digits– Subscriber Code – 4 digits

• Area codes:– Don’t start with 1 or 0– 1-800, 0 for operator, 1 to dial long distance– Other area codes aren’t available, 411, 211,311,911

• Around 6 billion phone numbers are currently available.– How much storage space is required to store 6 billion phone

numbers using ASCII representation along with the switch they belong to?

– Number portability.

NANPA• Like IP addresses, we are running out of

telephone numbers.– Cell phones, fax, pagers, American Idol,

modems• http://telcodata.us/ - Your switch and what

your phone number tells people.• http://www.nanpa.com – Available numbers

in your area code, cool maps.

LATA – Local Access and Transport Areas• Confusing billing!• Occurred after the 1984 At &T breakup.• Now, there are differences in how long

distance your long distance call is.• Often, the LATA boundaries are arbitrary.• LATAs are smaller than area codes• If you live in Dallas, it now might cost more

to call Houston than Los Angeles

LATA Map

LATA Terms• Intrastate, IntraLATA

– Ought to be a local call– But, they can still charge “local” tariffs

• Intrastate, InterLATA– Same state, different LATA

• Interstate, InterLATA– Different state, different LATA

• Interstate, IntraLATA– Different state, same LATA– Does this ever occur?

• Telephone billing is confusing.

Conclusion• In what ways is the telephone network

different than the Internet? In what ways is it the same?

• Some of the distinctions in billing are rapidly fading away. Why is that?