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Voice over IP
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Introduction *

How VOIP works *

Advantages to VOIP *

Disadvantages to VOIP *

Requirements for VOIP *

How it all fits together *

Making it work *

Appendix A - VOIP References *


Voice over IP (VOIP) is a low cost solution to control business telephone costs. Typically, business telephone service costs much more than residential service for the same features. Alternative technologies, utilizing a broadband Internet connection to make and receive telephone calls, can significantly reduce business telephone costs while providing enhanced features.

This paper introduces Voice over IP (VOIP) as an alternative to standard business or even residential telephone service and describes its operation.


How VOIP works

VOIP utilizes the infrastructure of the Internet to transmit voice information from one "telephone" to another "telephone" using the Internet as the transmission network. This is different from the standard voice transmission network and utilizes different technologies.

All internet transmitted data, regardless of its origin, is transmitted over the internet as digital data in blocks known as "packets." Packets are routed from a source computer to a receiving computer using addressing information that is embedded in the data packets that is examined by intermediate computers known as "routers," which direct the transmission of the data packet to the proper destination based upon the destination IP address.

An IP address is a series of numbers that uniquely identifies a computer or device attached to the Internet. Every computer or device attached to the Internet is required to have a unique IP address.

A "telephone call" over the Internet consists of digitizing analog speech, incorporation of the digitized speech into data packets which are transmitted across the Internet as any other data packet, and reconstitution into analog sound at the receiving end.

To make an internet telephone call to a standard voice telephone that is not directly connected to the Internet, a network switch is required to receive the digitized voice packets and retransmit them as a normal telephone call on the standard dedicated voice transmission network (also known as the "plain old telephone system" or POTS) to the receiving party’s telephone line.

The technology works in both directions and is almost completely transparent to the people at both ends of the conversation.

While transmitting digitized voice packets anywhere in the world is essentially free, jumping over to the ubiquitous POTS requires a telephone company network switch, whose owner (called an Internet phone company) charges for this service.

Nevertheless, using the Internet for telephony greatly reduces the cost of a telephone call due to the economic model that underlies the Internet. This model results in effectively tax free, long distance charge free, un-timed calls to anywhere in the USA and Canada, all for a reasonable monthly charge to use the internet phone company’s internet-to-POTS switch.


Advantages to VOIP

Besides the obvious cost savings (possibly exceeding 75%) in making telephone calls, Internet phone companies also usually include premium features such as call waiting, caller ID, call forwarding, voice mail, and web-based account management tools at either no additional, or reduced, charges.

Implementation of VOIP almost always includes a hardware firewall that protects computers connected to the Internet from hackers.

In most cases, an existing phone number can be transferred from the old service to the internet phone company.


Disadvantages to VOIP

The most obvious one has to do with telephone directory listings. The local telephone company typically provides a "free" listing in the business white pages/yellow pages for a business account. In most cases, the local telephone company will not be providing service for phone numbers that are connected to the Internet, and will not be providing this free listing. "Pay-for" advertising in the "yellow page" publications would not be affected.

Another disadvantage concerns "911" services. Federal regulation requires that all POTS providers and cell phone companies to provide subscriber and location data for "911" calls to be displayed for the emergency operator, usually run by a local government agency and funded by a monthly fee included in the telephone bill. Cell phone manufacturers are required to utilize GPS to locate cell phones calling "911," Internet telephony, however, has no geographical ties to phone lines, does not utilize GPS, and has no innate way to provide location information to an emergency operator.

Internet phone companies can provide this geographical data for "911" calls to the local government agency, though they universally recommend that "911" calls be made from a standard land-line telephone.

A third disadvantage is that the internet phone only works as longs the broadband internet connection remains functional. A broadband connection requires electricity at the terminal location. A power outage will disrupt any broadband connection that is not separately maintained via an uninterruptible power supply (also known as a "stand alone battery" such as manufactured by APC).

This is not a problem for telephones attached to the POTS, because the telephone company supplies it own voltage to operate the phones.


Requirements for VOIP

The physical requirements for VOIP are these:

Broadband Internet connection

Telephone Adaptor

Isolation from the local telephone company’s circuits

Analog telephone handset (some 2 line sets are also compatible)

Obviously, the most important requirement is a broadband internet connection. Broadband connections are provided by cable companies (digital cable service), telephone companies (DSL, T1, etc.), and radio/microwave broadband internet connections. Currently, satellite (ie., satellite uplink dish) internet connections are not compatible with VOIP equipment because of the proprietary data compression algorithms used in satellite uplink and downlink. Further, the speed of light delay to and from a geosynchronous orbiting satellite would prove to be very annoying people trying to talk.

Broadband connection data uplink and downlink speeds of greater than 80 kilobits per second per telephone circuit (while a call is in progress) are generally considered to be the minimum requirement for "decent" voice transmission quality.

A "Telephone Adapter" (or "TA," and also known as an "Analog Telephone Adapter" or "ATA") is a piece of hardware that is used to digitize the voice and establish the IP session to the internet phone company’s network switch. While it is possible to use a computer’s microphone and speakers and special software for telephony over the Internet, the obvious limitation that the computer has to be turned on to make or receive phone calls makes this unwieldly.

A TA eliminates the need for a computer to be up and running and accepts a standard 4-wire RJ11 telephone cable to support either premise telephone wiring or a direct connection of a standard analog telephone.

In addition, the TA usually includes a built in "router" that provides firewall isolation for the computers connecting to the Internet, as well as a Local Area Network (LAN) switch or hub. This allows efficient Internet connection sharing. The internet phone company usually provides or rent the TA, or they can be purchased at retail for a reasonable price.

The third requirement is to ABSOLUTELY insure that in-house or premise wiring is DISCONNECTED from the telephone company’s external telephone circuit (or line) before connecting to VOIP. There must be no cross connection between the local telephone company’s circuits and the broadband connection to the Internet. Instructions to disconnect the local telephone company’s phone circuit from premise wiring at the local telephone company’s Network Interface Device are typically provided by both the internet phone company and the local telephone company.

The final requirement is one or two common variety analog telephone handset. Almost all commonly available wireless telephones and most two line phone sets will work with VOIP.


How it all fits together

The TA is usually a cable or DSL router that connects to the cable company’s or DSL provider’s supplied terminal (or "modem"). The customer’s computer is then connected to the TA, as are the one or two standard (RJ11) telephone cables, which connect to either a wall outlet or a standard analog telephone, depending on whether telephone extensions are present or not. More than one computer usually can be connected to the TA to create a Local Area Network (LAN).

The above diagram illustrates a sample installation. The telephone on the left is directly connected to the TA, while the one on the right is connected to the premise distribution wiring which is connected to the TA via RJ 11 cable to a wall jack.


Making it work

The key to making VOIP work is the correct initialization of the TA to work with the internet phone company and configuring IP addresses on the router to use for the computer or premise LAN connected computers sharing the broadband connection.

ZoeS Network Consulting can design and install voice over IP for our customers and integrate it into their Local Area Networks.


Appendix A - VOIP References

Voice over IP Telephony Referneces:

Voice over Internet Working Group:

VOIP Protocols and Standards:



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Last modified: August 30, 2011