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?