Lines and Trunks (Ch. 7)
PBX (Ch. 9)
Key Telephone Systems (Ch. 10)
ENGR 475 – Telecommunications
Harding University
Jonathan White
Lines
• End Point
– End in terminals (telephones, modems,
faxes)
– New term: FX0 – Foreign Exchange
Office
• Connection to analog POTS
– 2 wire, typically analog
– Called DS-0
– 64 Kbps
– Nothing special
Trunks
• Connection between points
– Carry many lines
– Connects 2 intelligent switching systems
• PBX/Centrex/CO
– Digital
– Multiplexed
– Expensive
DID
• Direct Inward Dialing
– Everyone has their own unique telephone
number at a business
– However, you don’t have enough
incoming/outgoing lines to support all
users at once.
• Handled by the PBX (routing)
• You don’t have to go through an
operator.
DOD
• Direct Outward Dialing
• Again, you don’t have to go through an
operator.
• Handled by the PBX
WATS
• Wide Area telephone service
• Same thing as 800/888/877/866
service
• A way to reduce long distance costs
PBX - Review
• Private Branch Exchange
– Owned by an individual company
– Performs call routing and switching
– Allows the business to save money
because not as many connections to the
PSTN are provided.
– Keeps calls between employees internal
to the network.
• How many lines would a call between
employees normally use?
PBX - Review
• How does the individual telephone
numbers within a PBX system work?
– Takes the cooperation of the Telco and
the organization.
• Some trunks can be dedicated strictly
to Direct Inward Dialing.
• Page 179 graphic
PBX Functions
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DID / DOD
Auto Attendant
Call transfer
Speed Dialing
Voice mail
Follow-me
Call forwarding on absence
Call forwarding on busy
Music on hold
Automatic ring back
PBX Functions
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Night service
Automatic call distributor
Call waiting
Call pick-up
Call park
Conference call
Call accounting
Voice paging (PA system)
Custom greetings
Shared message boxes
PBX Cost
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http://cgi.ebay.com/NEW-SOHO-PBX-PABX-AUTOMATED-PHONEEXCHANGE-4-x16_W0QQitemZ230023614851QQihZ013QQcategoryZ11908QQrdZ1QQcmd
ZViewItem
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Can be found cheaply
Others can be very expensive
Installation and maintenance can be expensive
Asterisk
• Open source implementation of a PBX
• Runs on Windows/Linux/UNIX and
performs all the features of a
proprietary PBX:
– DID/DOD, voice mail, conference calling,
interactive voice response (phone
menus), and automatic call distribution.
• Runs on a standard desktop computer
– Needs a special PCI card to attach to the
PSTN or other analog phones (a modem
will not work).
Asterisk
• Can also be used to run a VOIP
network with connections to the PSTN
• With VOIP, this can be incredibly
cheap.
Asterisk PCI card
Asterisk PCI Cards
• PCI cards exist in several flavors:
– 1 inward/outward phone line ($20)
– 2 inward/outward ($225)
– 4 inward/outward ($380)
– T1 connection (24 inward/outward) ($890)
• Very comparable in price to the
cheapest of proprietary PBX’s
Asterisk Analog PCI Hubs
• Act as the Main Distribution Frames
– Kind of like a router or hub
• Allow the users of analog telephones to
connect to the PBX server
• Necessary if you want to run a non VOIP
network
• Cost: 12 port ($1400)
– Not many people actually run analog Asterisk
• http://www.voicetronix.com/hda.htm#features
Non PCI Switches
• Called Channel Banks
• Connects many phone lines to 1 main
trunk
• Not incredibly expensive.
• Provide the multiplexing/demultiplexing
Channel Banks
VOIP Asterisk
• If the PBX is already running digitally,
why not run digital phones using
VOIP?
– Phones can be computers
– Or special VOIP phones
• Instead of buying a $1400 switch, you
can buy an Ethernet switch for
incredibly cheap.
Skype
• Allows computer to computer
conversations that only cross the
Internet.
– There’s one main difference between
Skype and Asterisk.
• Skype is free or almost free.
• Skype has very good security.
Centrex
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Owned by the Telco
Same features as a PBX
Features are leased by businesses
Each user has their own external
phone line.
PBX/Centrex Boxes
Key Telephone Systems
• Multi-line telephone system used in
SMALL office environments
• Very old technology
• Similar to a PBX.
• Many telephone lines come into
certain phones. A light comes on if the
line is busy. A user takes control of a
line by pressing a button.
Key Telephone Systems
• Provided intercom capabilities.
• Often 6, 12, or 25 lines would come
into a phone.
• In North America, these type of phones
were known as 1A2.
• They are still manufactured and
supported today, though they are
rapidly becoming outdated.
Key Telephone Systems