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How to Turn
Analog Phones Into VoIP Phones ATAs and FXS ports let you connect
your analog telephones into VoIP networks.
By: John Shepler
Did you know that you can use regular telephones
and even FAX machines with VoIP telephone services? Even an enterprise
VoIP system can have a mix of VoIP and analog phones. The magic
is in the interfacing.
Anytime you want to connect two different networks together you
need a gateway. This is also true when connecting or interfacing
a digital network line carrying VoIP phone traffic with the traditional
telephone network. In this case the interface device may be called
a media gateway, VoIP gateway, FXS adaptor, ATA analog telephone
adaptor or broadband phone adaptor.
Analog Telephone Adaptors, or ATAs, do
pretty much what you would expect. They have a standard RJ-11
socket where you plug in the wire from your regular telephone.
It's just like plugging that phone into a wall socket. The ATA
mimics the connection to the telephone company so the phone doesn't
know the difference.
FSX and FXO Ports
The socket where you plug in the phone on the ATA is known as
an FXS port. FXS stands for foreign exchange station. This isPBX system lingo. You can get FXS adaptors for routers. These
are modules that plug into the back of the router and give you
one, two or more FXS ports. This is how an enterprise level system
can add ports to plug in regular phones. The FXS adaptors are
programmed so the VoIP system sees them as phone extensions.
By the way, the opposite of an FXS port
is an FXO port. FXO stands for foreign exchange office and connects
to the phone lines that run to a telephone company office. Never
get these port types mixed up. Sparks could fly!
ATA to Broadband Network
An ATA is also called a broadband phone adaptor because it converts
regular phone signals to and from broadband signals. On the back
of your ATA, you'll find one or two of the FXS ports we talked
about plus a standard RJ-45 network jack. This jack could be
labeled "LAN," "WAN connection," "Broadband"
or maybe "Ethernet." Its your digital network connection.
A standard Ethernet cable plugs into the RJ-45 port and goes
to a Router, DSL modem or Cable modem. An Ethernet switch might
be built into the ATA to provide two RJ-45 jacks. One connects
to the broadband line, the other goes to a computer that shares
the broadband connection with the telephone. The only other connection
is probably a jack for the AC power converter.
The ATA looks simple on the outside, but
what is going on inside? Understand that VoIP and analog telephones
are completely different species of phone service. Analog phones
run on varying voltages and currents. VoIP is based on digital
bits. To get from bits to volts, a special chip called a CODEC
is used. CODEC stands for Coder/Decoder. Coding converts analog
voice from the microphone to digital. Decoding converts digital
back into analog to drive the earpiece.
Mimicking Analog Telephone Lines
The FXS port circuitry also has to mimic the high voltage AC
ringing signal to the analog phone so that it will ring when
called. The FXS port provides the same 45 volt DC power to the
phone that it normally gets from the telephone company. Note
that most of the ringing circuits have enough power to drive
3 to 5 phones. Yes, you can connect multiple analog phones to
the same FSX port just like you would hook up multiple extensions
to a single phone line. The REN or ringer equivalence number
for each FXS port will tell you how many.
Dialing analog phones is done through a
keypad that generates somewhat musical tones sometimes called
"touchtones". The technical name for this is DTMF or
dual tone multi-frequency. Each of the 10 standard keys 0 through
9, plus the # and * keys, generate a unique tone made up of two
frequencies each. A DTMF detector senses these and converts them
to digital bits so the VoIP system knows what phone number you
are dialing. Most VoIP systems use H.323 or SIP standards for
control and signaling, although in enterprise wide systems you
may find other standards. Make sure that the adaptor you select
will work with the VoIP protocols you have implemented.
Voice Activity Detection Minimized Bandwidth
Analog telephone adaptors have some other functions unique to
VoIP telephony. VAD or voice activity detection senses when someone
is talking or when the line is quiet. Standard analog telephony
always has a complete circuit tied up for each call. VAD lets
a VoIP system only send packets when someone is talking. That
frees up bandwidth for other uses when both parties are on the
line but not speaking at the moment.
Generating Noise for Comfort
Comfort noise generation or CNG goes along with VAD. If nobody
is talking, the VAD circuit will stop sending packets through
the broadband connection. Your phone will go dead silent for
an instant and you might think it was disconnected. The comfort
noise generator creates a low hissing sound, like phone line
noise, in the background so you have the "comfort"
of knowing that your phone call is still connected.
Because VoIP calls are transmitted over
networks that can get congested, a dynamic jitter buffer is often
included to smooth out the voice signal. Excessive jitter is
caused by packets being sent by alternate routes or getting delayed
by other traffic. The jitter buffer helps to feed voice packets
to the CODEC at a more constant rate. That makes for a clearer
sounding voice signal.
Another source of voice distortion is echo. The regular PSTN
or public switched telephone network (the phone company) provides
circuits to cancel echoes on long distance phone lines. VoIP
has to provide its own mechanism to deal with annoying background
echoes, so echo canceling circuitry is included in the analog
to VoIP phone adaptors.
FAX over VoIP Limitations
FAX tones present a dilemma to many VoIP systems. Some CODECs
save bandwidth by compressing the voice signals so that many
more phones or computers can share the same network line. This
compression can completely distort FAX machine tones. Some ATAs
accommodate this by detecting FAX tones and then switching to
the most compatible CODEC, usually the G.711 standard, and shutting
off VAD and CNG for the duration of the FAX call. FAX can work
over VoIP, but only if the ATA and service provider support this.
Caller ID is another system that needs
special accommodation. The caller ID is transmitted over regular
phone lines using a system called FSK or frequency shift keying.
The FSK tones with the caller ID info get sent during the ringing
cycle before you pick up the phone. An ATA needs to generate
these same FSK tones based on the caller ID data supplied by
the VoIP system if you want to have this feature available.
Attaching to the Network
Other functions of an Analog Telephone Adaptor relate to the
network side of the connection. The ATA is a network peripheral
like a PC or printer. Most are set up to get their network address
assigned automatically by the router they are plugged into. This
is called DHCP for dynamic host configuration protocol. If no
router is being used or if the network uses all static IP addresses,
then the ATA needs to provide a way to reprogram its address
manually. Some ATAs do this by entering data on the telephone
keyboard. More sophisticated devices include a web based interface
for programming or downloading new software via a PC.
Nothing simple about it, is there? Fortunately,
the functions needed for analog telephones and VoIP are pretty
well standardized and designed into specialized circuits and
chips. A VoIP phone, also called a broadband phone, or SIP phone
when it uses the Session Initiation Protocol standard for VoIP,
contains many of the same functions, including CODECs, jitter
buffers, voice activity detection, and comfort noise generation.
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