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?