Screens on IP telephones can act as Web browsers, allowing a user to open HTTP-encoded pages and, for example, click a telephone number link to complete a call to that number.
IP telephones may connect to a user’s personal digital assistant (PDA) through an infrared port, enabling the user to, for example, view his phone directory and touch a number on the IP telephone’s LCD screen to call that number.
If a line is busy, an IP telephone can offer the caller the option to leave an instant message on the called party’s IP telephone screen.
G.711 - known as a waveform codec because it obtains information from the analog waveform, and then uses this information to reassemble the waveform as accurately as possible at the receiving end. This does not manipulate the signal in any way. It simply tries to reconstruct it. This is a concern in packet-based networks. Requires 64 Kpbs. Requires a significant amount of throughput.
G.723 - uses a form of PCM known as differential pulse code modulation (DPCM). In DPCM, the codec samples the actual voice signal at regular intervals. Is able to predict voice samples, so required 6.4 Kbps, only one-tenth of G.711. Not as good voice quality but adequate for packet-based networks using VoIP and videoconferencing.
DPCM codecs - work well with human speech because, within very short time spans, our speech patterns are predictable.
Adaptive differential pulse code modulation (ADPCM) - in this codec, not only do the nodes base predictions on previously-transmitted bits, but they also factor in human speech characteristics to recreate wave-forms. The result is more accurate predictions.
G.726 uses ADPCM and can operate over a 16-, 24-, 32-, or 40 Kbps channel.
Real-Time Transport Protocol (RTP). Operates on top of UDP at the Transport Layer of OSI model. Indicates what order packets should be assembled by assigning each packet a time stamp. Cannot do anything to correct transmission flaws.
Resource Reservation Protocol (RSVP): A QoS technique that attempts to reserve a specific amount of network resources for a transmission before the transmission occurs. Emulates a circuit-switched connection.
Allows for two service types: Guaranteed service (will not suffer packet loss and minimal delay) and Controlled-load service expected transmission if network carried little traffic).
As a result of emulating a circuit-switched path, RSVP provides excellent QoS.
Because it requires a series of message exchanges before data transmission can occur, RSVP consumes more network resources than some other QoS techniques.