How WiFi Phones Work

Three people holding different wifi phones.
WIFI phones are great to save on monthly data charges. annick vanderschelden photograph / Getty Images

Fre­e stuff is good, and free stuff that you'd ordinarily have to pay for is even better. That's one of the reasons people have tried using Voice over Internet Protocol (VoIP) to make phone calls. With VoIP, you can make calls, even long-distance and international ones, for free. But until recently, VoIP had one pretty big limitation -- to make a call, you had to be at a computer.

New phones make it possible for people to place VoIP calls without being leashed to a computer. WiFi phones use the same wireless network technology that computers use, making VoIP a lot more portable.


This article will explore WiFi phones and their abilities, as well as the pros and cons of using them. We'll start with a quick review of VoIP and WiFi.

Understanding VoIP is easier if you have a basic understanding of how old-fashioned telephones work. Most people have used an ordinary "land line" phone so often that the process seems instinctive. You just pick up the phone, hear a dial tone, dial, talk and hang up.

­ But several things happen between you dialing the phone and the other person answering. To get from person to person, the call travels through the wires that make up the public switched telephone network (PSTN). Your local phone carrier routes your call through a series of physical switches that eventually connect your phone to the other person's. This is called circuit switching.

This isn't a particularly efficient way of making phone calls. The entire circuit between the two phones stays open during the conversation, even though only one person is talking at a time. It's also open when neither person is talking. (See How Telephones Work for more details on circuit switching).

Wi-fi phones are part of the smartphone phenomenon. Click here to read more about smartphones.

VoIP, on the other hand, uses packet switching. We'll look at packet switching and VoIP in more detail next.



Packet Switching and VoIP

VoIP uses packet switching rather than circuit switching. Instead of leaving an entire circuit open during the whole conversation, it breaks the conversation into small packets of data. It then transmits that data over the Internet. Instead of monopolizing a circuit, packet switching sends and receives data as needed.

Let's say you and a friend who uses the same VoIP service are going to talk to each other. You're using analog telephone adapters (ATAs), which let you plug your regular phones into your computers. Here's what would happen:


  1. You pick up the receiver, which sends a signal to the ATA. The ATA receives the signal and sends a dial tone to let you know that you have a connection to the Internet.
  2. You dial the phone number. The ATA converts the tones into digital data.
  3. Your computer sends the data to the VoIP company's call processor.
  4. The call processor maps the phone number by translating it to an IP address. A specialized mapping program called a soft switch connects your ATA to your friend's. It sends a signal to your friend's ATA, telling it to ring.
  5. Your friend picks up the phone, establishing a session between your computers. Each system knows to expect packets from the other.
  6. The system converts your voices into packets of binary data. The normal Internet infrastructure treats the just like it would for an e-mail message or a Web page.
  7. As you talk, the system transmits packets of data back and forth. The ATAs translate the packets into the analog audio signal that you hear.
  8. When you finish talking, you hang up the receiver, closing the circuit between your phone and the ATA. The ATA sends a signal to the soft switch to terminate the session.

It's an efficient process -- and it doesn't tie up the line when it doesn't need to. It can also be free. If you're using a free service, like Skype, and calling someone else who also uses the same service, your call costs you nothing. But you might want to make phone calls when you're away from your computer, or you might just want to walk around while you talk.

With 802.11 networking, or WiFi, VoIP can go wireless. A WiFi phone has an antenna that transmits information to a computer, base station or wireless router using radio waves. An antenna at a base station or router picks up the signal and passes it on the Internet. You can read more about this process in How WiFi Works.

Let's look at these phones in more detail.


WiFi Phones

From their user interface to how they work, WiFi phones are a lot like cell phones . Like a basic cell phone, a WiFi phone has a printed circuit board (PCB) that connects:

Both types of phones also send and receive signals as radio waves. The difference is that WiFi phones use different frequencies than cellular phones do. Cell phones use 824-MHz to 894-MHz frequency bands. WiFi phones that use the 802.11b or 802.11g standards transmit at 2.4 GHz. Phones that use the 802.11a standard transmit at 5 GHz.


When you make a call on a WiFi phone, you dial the number of the person you want to call, just like you would with a cell phone. If you're calling another VoIP user, you may enter a VoIP address instead of a phone number, depending on the service provider's requirements.

The phone translates the number you dial into packets of data. It uses radio waves to transmit the packets to a wireless receiver. The receiver passes the information over the Internet to the call processor like an ordinary VoIP call. When you begin your conversation, the phone transmits your voice in packets of data as well. Your voice travels just like it does in a VoIP call, although the specifics can differ from one provider to another.

ZyXEL P-2000W v2 WiFi phone
Photo courtesy ZyXEL

Many of these phones use a specific WiFi service or network. For example, Netgear makes a WiFi phone for Skype service, and UTStarcom makes a WiFi phone for Vonage service. In order to use either of these phones, you need to have an account with that provider, just like you need a service plan from a cell phone provider to use a particular phone. Either you or your service provider will configure the phone to work within the network.

Other phones work with a protocol rather than a particular network or provider. For example, ZyXEL and Linksys both make WiFi phones that use Session Initiation Protocol (SIP). SIP is one of several VoIP protocols -- you can find out more about them at After setting up a SIP phone, you can make calls simply by entering the other party's SIP address and pressing "Send."

SIP is an open protocol. Anyone can use it, and anyone who has a SIP address can contact anyone else who has one for free. SIP is a standard protocol for handling voic­e data, and many VoIP providers use it, even though they maintain a closed network. It can also control other forms of communication, like instant messaging and video conferencing.

Eventually, SIP will probably make communications tools interoperable. In other words, your computer, phone, PDA and other communications tools could all share the same address book and communicate with one another.


The Future of Wireless VoIP

WiFi phones have a lot of potential, but right now they're not for everyone. Someone who has a cell phone and needs a phone line to support their DSL connection doesn't necessarily need yet another phone. Anyone who lives in an area with little WiFi availability might not find many places to use a WiFi phone. People who don't have a network of friends using the same VoIP service may find it cheaper to use a land line or cell phone.

But as more cities develop city-wide WiFi networks, WiFi phones will become more practical and useful. What currently seems like a novelty or a toy could become a replacement for a land line, a cell phone or both. It could happen pretty quickly -- the WiFi phone market increased by 76 percent in 2005, and researchers believe it will double in 2006 [Source: Infonetics Research].


WiFi phones are already useful on a smaller scale in businesses and schools. Many businesses have company-wide WiFi networks and VoIP phone service, making WiFi phones a potential replacement for desk phones. Students who live on college campuses that have extensive WiFi networks can also use WiFi phones to make calls for little to no money.

In spite of all its promise, the technology is still new, and WiFi n­etworks aren't yet prevalent enough to make the phones practical for everyone. Users have also reported some quirks in newly-released phones that make using them a little tricky. For example, some phones reset every time the user moves to a new wireless network or every time an administrator makes a change to an existing network. Some WiFi hotspots require a Web browser to sign in. Phones without a built-in browser are useless in these locations.

Even though VoIP doesn't use a lot of bandwidth, other uses for public and corporate WiFi networks do. Sharing bandwidth with a lot of other traffic can result in poor voice quality or lost signals. Fortunately, quality-of-service requirements can be built in to new WiFi networks. With the right hardware and software, a hotspot can separate and prioritize the voice traffic, treating it as a separate signal and providing better voice quality.

While they're currently more like toys than tools, WiFi phones will probably become a lot more popular as WiFi networks spread. For more information on WiFi, telephones, cell phones and related technology, see the links on the next page.


Lots More Information

Related HowStuffWorks Articles

More Great Links

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