IP Phone

An IP phone uses Voice over IP technologies allowing telephone calls to be made over an IP network such as the internet instead of the ordinary PSTN system. Calls can traverse the Internet, or a private IP Network such as a that of a company. The phones use protocols such as Session Initiation Protocol, Skinny Client Control Protocol or one of various proprietary protocols such as that used by Skype. IP phones can be simple software-based Softphones or purpose-built hardware devices that appear much like an ordinary telephone or a cordless phone. There also exist the possibility to reuse ordinary PSTN phones as IP phones, with analog telephony adapters (ATA). One of the primary motivations for implementing such a system is the lower calling cost. When calling other IP phones over the internet one only pays for the usually fixed cost internet bandwidth.

It may have many features an analog phone doesn't support, such as e-mail-like IDs for contacts that may be easier to remember than names or phone numbers.

Elements of an IP phone

  1. Hardware
  2. DNS client
  3. STUN client
  4. DHCP client (not commonly used)
  5. Signalling stack (SIP, H323, Skinny, Skype, or others)
  6. RTP Stack
  7. User interface

Hardware of a stand alone IP phone

The overall hardware may look like a telephone or mobile phone. An IP phone has the following hardware components.

  • Speaker/ear phone and microphone
  • Key pad / touch pad to enter phone number and text (not used for ATAs).
  • Display hardware to feedback user input and show caller-id/messages (not used for ATAs).
  • General purpose processor (GPP) to process application messages.
  • A voice engine or a Digital signal processor to process RTP messages. Some IC manufacturers provides GPP and DSP in single chip.
  • ADC and DAC converters: To convert voice to digital data and vice versa.
  • Ethernet or wireless network hardware to send and receive messages on data network.
  • Power source might be a battery or DC source. Some IP phones receive electricity from Power over ethernet.

Other devices

There are several WiFi enabled mobile phones and PDAs that come pre-loaded with SIP clients or are at least capable of running IP telephony clients. Some IP phones may also support PSTN phone lines directly.
Analog telephony adapters
These are usually rectangular boxes that are connected to the internet or Local area network using an Ethernet port and have sockets to connect one or more PSTN phones. Such devices are sent out to customers who sign up with various commercial VoIP providers allowing them to continue using their existing PSTN based telephones.

Another type of gateway device acts as a simple GSM base station and regular mobile phones can connect to this and make VoIP calls. While a license is required to run one of these in most countries these can be useful on ships or remote areas where a low-powered gateway transmitting on unused frequencies is likely to go unnoticed.

Stun Client

A STUN client is used on some SIP-based IP phones as firewalls on Network interface sometimes block SIP/RTP packets. Some special mechanism is required in this case to enable routing of SIP packets from one network to other. STUN is used in some of the sip phones to enable the SIP/RTP packets to cross boundaries of two different IP networks. A packet becomes unroutable between two sip elements if one of the networks uses private IP address range and other is in public IP address range. Stun is a mechanism to enable this border traversal. There are alternate mechanisms for traversal of NAT, STUN is just one of them. STUN or any other NAT traversal mechanism is not required when the two sip phones connecting are routable from each other and no firewall exists in between.

DHCP Client

DHCP client may be used to configure the TCP/IP parameters and server details if network segment uses dynamic IP address configuration. DHCP client then provides central and automatic management of IP phones configuration.

Common features of IP phones

  • Caller ID
  • Dialing using name/ID: This is different from dialing from your mobile call register as the user does not need to save a number to a sip phone.
  • Locally stored and network-based directories
  • Conference and multiparty call
  • Call park
  • Call transfer and call hold
  • Preserving user name/ number when choosing a different service provider (not widely supported).
  • Applications like weather report, Attendance in school and offices, Live news etc.

Disadvantages of IP phones

  • Requires internet access to make calls outside the Local area network unless a compatible local PBX is available to handle calls to and from outside lines.
  • IP Phones and the routers they connect through usually depend on mains electricity, unlike PSTN phones which are supplied with power from the telephone exchange.
  • IP networks, particularly residential internet connections are easily congested. This can cause poorer voice quality or the call to be dropped completely.
  • IP Phones, like other network devices can be subjected to Denial of service attacks as well as other attacks especially if the device is given a public IP address
  • Due to the latency induced by protocol overhead they do not work as well on satellite internet and other high-latency internet connections.


See also

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