How a desk phone runs on one network cable
An IP phone, or VoIP desk phone, is a small computer built for calls, and it gets both its data and its power through one network cable. When you plug it in, it gets an address, downloads its settings and signs in to the phone system. When you talk, it sends your voice as small packets of data, 50 a second in each direction.
Purple: power and data on the cableKey
- Purple: power and data on the cable
Signed in · ready for a call
Press a button, or click the phone to look inside it.Press a button, or tap the phone to look inside it.
Anatomy of a desk phone: an internet phone on an office desk, the cable that feeds it and the network cabinet on the wall. The drawing is not to scale.
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Follow one call
Press Play to watch the call, or scroll down and the drawing follows you one step at a time. Try it, under the drawing, makes your own call.Press Play to watch the call, or Next to go one step at a time. Try it, under the drawing, makes your own call.
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Step 1 of 6
You plug in one cable
This desk phone needs no power plug. You plug one network cable into its back, and the other end runs through the wall to the network switch in the office's cabinet. The switch sends the phone its data and its power down that same cable, and the screen lights up.
How can a data cable carry power?
A network cable holds four pairs of thin copper wires. Power over Ethernet, a standard of the IEEE, sends a low direct current along them on top of the data. Its requirements put the switch's output between 44 and 57 volts.
Before it sends any power, the switch checks what sits at the other end. The standard had to work with a powered device and an unpowered one, and harm neither, so a laptop plugged into the same socket is safe.
Sources: IEEE 802.3af task force: requirements for DTE power via MDI (2000); IEEE 802.3af task force: objectives for DTE power via MDI (1999)
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Step 2 of 6
It gets its address and settings
A phone that has just woken up knows nothing about your office. It sends a question to the whole network, and a server answers with an address for the phone. The answer can also say where the phone's settings are kept. The phone downloads them: your name, your extension and the phone system to sign in to.
Does someone type the settings in by hand?
Usually not. An IETF framework from 2011 describes how a phone gets them with minimal or no User and Administrative intervention. The phone gets an address, asks a server for its profile and downloads it over HTTP or HTTPS, the same protocols your browser uses.
The address is lent, not given. DHCP, the protocol that hands it out, assigns an address for a limited period of time, and the phone asks to keep it before the time runs out.
Sources: IETF RFC 6080: a framework for SIP user agent profile delivery (2011); IETF RFC 2131: Dynamic Host Configuration Protocol (1997)
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Step 3 of 6
It signs in to the phone system
With its settings in hand, the phone sends a short message through the router and over the internet to the phone system: this extension is now at this address. The message is a SIP registration. SIP, the Session Initiation Protocol, is the language phones and phone systems use to set up calls. From now on, the phone system knows where to send your calls, and the screen says Signed in.
What happens when the phone moves to another desk?
It signs in again from its new address. A registration only lasts a while. It carries an expiry time in seconds, and the phone renews it before the time is up. Your number follows the phone, not the socket.
The phone can sign in over an encrypted connection. SIP set out a secure address for this in 2002, starting with sips:, which keeps who calls whom hidden from anyone listening on the line.
Sources: IETF RFC 3261: SIP, the Session Initiation Protocol (2002)
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Step 4 of 6
Your computer plugs into the phone
Many desks have one network socket. So the phone has a second socket on its back, for your computer, and a small network switch inside that passes the laptop's data on to the wall. When the cable is busy, a network set up for calls lets voice go first, because a late packet of voice is a gap you hear and a late email is not.
How does the network know which packets are voice?
The phone marks them. An IETF guide from 2006 sets out classes of traffic that a network treats in different ways. Its Telephony class is meant for internet calls: its packets carry the code EF, short for Expedited Forwarding, and wait in a queue of their own that goes first.
So your laptop and your phone share the cable, and a large download on the laptop does not have to slow your call.
Sources: IETF RFC 4594: configuration guidelines for DiffServ service classes (2006); IETF RFC 3246: an Expedited Forwarding per-hop behavior (2002)
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Step 5 of 6
Your voice leaves as packets
You press the speaker key and dial. The sound chip turns your voice into numbers, and the processor packs them into small packets, usually one every 20 thousandths of a second. That makes 50 packets a second going out, and 50 coming back with the other person's voice, all on the same cable. Look inside the phone below.
Look inside The circuit board
- 1Network port
- 2Power circuit
- 3Computer port
- 4Switch chip
- 5Processor
- 6Sound chip
- 7Loudspeaker
50 packets a second leave this board while you talk, each with 20 milliseconds of your voice.
How much of your voice fits in one packet?
20 milliseconds of it. The IETF's profile for sending sound over the internet says a packet should hold 20 ms by default. With the standard codec, which needs 64,000 bits a second, that is 160 bytes of your voice in each packet.
Longer packets would spend less space on addresses, but the same profile warns that they add delay and make a lost packet more noticeable.
Sources: IETF RFC 3551: RTP profile for audio and video conferences (2003); IETF RFC 3550: RTP, a transport protocol for real-time applications (2003); ITU-T G.711: pulse code modulation of voice frequencies (1988)
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Step 6 of 6
The keys show who is on the phone
Beside the screen sit the line keys, buttons with a light and a name on each, one for you and one for each colleague you keep an eye on. Your phone asked the phone system to tell it when Finn's line changes. When Finn takes a call, a short message comes down the cable and his key lights red, so you know not to pass a caller to him.
How does the phone know Finn is busy?
It subscribed to his line. SIP has a pair of messages for this, SUBSCRIBE and NOTIFY, and a 2005 standard uses them to report the state of someone's calls as they ring, connect and end.
Phone makers call the row of keys a busy lamp field, or BLF.
Sources: IETF RFC 4235: an INVITE-initiated dialog event package for SIP (2005)
How it came to be
- 1997
Phones that ask for an address
The IETF publishes DHCP in its current form. A device that joins a network asks a server for an address and borrows one for a limited time, which is how a desk phone still starts its day.
- 2000
Power for internet phones
On 30 January, the IEEE approves a project to send power down the network cable. The Telecommunications Industry Association had asked for it, for the internet phones it was writing standards for.
IEEE 802.3af task force: project authorization request (2000)
- 2002
The language of the call
SIP gets the form it still has. It is the protocol a desk phone uses to sign in to the phone system and to set up every call.
- 2003
Power over Ethernet
In June, the IEEE Standards Board approves 802.3af, the first standard for power over the network cable. A desk phone no longer needs a power plug.
- 2004
A lock for the voice
The IETF publishes SRTP, which encrypts the packets of a call so that nobody on the way can listen in. It protects the voice. The sign-in and the messages that set up a call need their own encryption, the sips: address of 2002.
IETF RFC 3711: the Secure Real-time Transport Protocol (2004)
- 2005
A light for a colleague's line
A new SIP standard lets one phone follow the calls of another, which is what lights a colleague's line key.
IETF RFC 4235: an INVITE-initiated dialog event package for SIP (2005)
- 2011
Phones that set themselves up
The IETF describes how a SIP phone fetches its settings by itself, so a new phone can come out of its box, be plugged in and sign in.
IETF RFC 6080: a framework for SIP user agent profile delivery (2011)
Where Voxbi fits
Voxbi, a cloud phone system hosted in the European Union, works with standard SIP desk phones, including the ones already on your desks.
See it in Voxbi