All manufacturers base their IP telephone on proprietary design
schematics and circuitry, but there are common design elements across
the unique terminals. IP telephone basics include:
- User interface
- Voice interface
- Network interface
- Processor complex and associated logic
The accompanying diagram illustrates the internal design elements
of an IP telephone instrument (Figure 13-5.)
The user interface elements provide four classic telephone user function
interfaces: keypad for dialing numbers; a variety of keys for line
and feature access; a display for user prompts, caller feedback, messages,
and other call processing information; serial interface to allow
communications to an external device, such as a PDA, to allow synchronization
of telephone information; speed dialing; and customer programming.
An audible indicator (ringer) is also included to announce incoming
calls.
The voice interface converts input analog voice signals into 8-bit digital
word bit samples. Speech signals are sampled at an 8-KHz rate to
create a 64-Kbps digital bit stream to the processor by using a standard
PCM codec. Voice signal compression and IP encoding functions are performed
by processor complex elements. The processor complex performs
voice processing, call processing, protocol processing, and network management
software functions. The complex consists of a DSP for voice-related
functions and a MCU for the remaining control and management
functions. The DSP and MCU each have associated memory. DSP
memory usually includes RAM and ROM elements; MCU memory usually
includes RAM and Flash elements. The Flash memory element supports
software upgrades.
The network interface allows the transmission and reception of voice
packets to and from the telephone terminal based on 10BaseT or
10/100BaseT Ethernet running TCP/IP protocols. Some IP telephones
may be equipped with multiple RJ-45 Ethernet connector ports and an
integrated Ethernet hub/switch to support connections to the customer
premises LAN and desktop PC clients. Newer IP telephones also may be
designed with a USB connector port.
Basic IP telephone software modules include a variety of user interface
drivers (display, keypad, ringer, user procedures), voice processing
modules, telephony signaling gateway modules, network management
modules, and system service modules. The voice processing software
modules include a PCM interface unit; a tone generator (call progress
tones, in-band DTMF signaling digits); a line echo canceler unit (ITU
G.168-compliant echo cancellation on sampled, full-duplex voice port
signals); an acoustic echo canceler for terminals equipped with a speakerphone;
VAD; voice codec unit (compression and packeting of the 64-
Kbps digital stream received from the station user based on a variety of
algorithms, such as G.711, G.723.1, G.729/A, etc.); packet playout unit
(compensation for network delay, jitter, and packet loss); packet protocol
encapsulation unit (based on RTP, which runs directly on top of the
UDP); voice encryption (to ensure privacy); and a control unit (coordinates
the exchange of monitor and control information between the
voice processing module and the telephony signaling and network management
modules).
The telephony signaling gateway subsystem in an IP telephone performs
the basic functions for call setup and teardown procedures. Software
modules used by this subsystem include call processing, address
translation and parsing, and network signaling. The most widely implemented
network signaling standard used by currently available IP-PBX
systems is H.323 protocol. H.323 is an ITU standard that defines several
signaling and protocol specifications for multimedia communications
between LAN-based terminals and network equipment. The main H.323
standards used for VoIP in an IP telephone are H.225–Call Signaling
Protocol (based on Q.931), H.245–Control Protocol; RAS Protocol; and
RTCP. An emerging network signaling standard not currently used by
any commercially available IP-PBX, but being planned for by most suppliers,
is SIP. SIP is the protocol developed and promoted by the Internet
Engineering Task Force (IETF) and is forecasted to be widely implemented
in network hosted services, such as IP-Centrex, and may
eventually replace H.323 as the primary signaling protocol used by
premises communications systems.
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