Telephone Network
Hierarchy
Analog Sounds
ENGR 475 – Telecommunications
Harding 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?