Lines and Trunks (Ch. 7) PBX (Ch. 9) Key Telephone Systems (Ch. 10)

Lines and Trunks (Ch. 7)PBX (Ch. 9)

Key Telephone Systems (Ch. 10)

ENGR 475 – TelecommunicationsHarding 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• 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• 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• http://cgi.ebay.com/NEW-SOHO-PBX-PABX-AUTOMATED-PHONE-

EXCHANGE-4-x-16_W0QQitemZ230023614851QQihZ013QQcategoryZ11908QQrdZ1QQcmdZViewItem

• 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• 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