Picking Wanting: This article proposed from the application angle one kind of new local area network IP telephone’s software and hardware design proposal, namely takes the hardware platform by at75C220 development board, uses H.323 under the mClinux operating system the agreement stack development local area network IP telephone terminal, and has enumerated some highly effective processing method to the realization key question.
Key word: IP telephone; Gateway; Real-time operating system; High-level simplification set of instructions computer;
Foreword
Because the computer network technological development and the renewal are quicker than PBX, the equipment price range of fall is big, establishes one take Internet (or Intranet) as the foundation IP telephone network, substitutes the tradition telephone equipment system to become enterprise’s goal.
Local area network IP telephone
The local area network IP telephone use unified network service equipment and the wiring transmit the voice and the data. In traditional PBX (private branch exchange) in the system, the voice call through enters the office with the work PBX connection’s a series of standard voice line, namely through one kind of special purpose equipment on standard telephone wiring receive and unblocking voice business volume. However in the LAN telephone voice network, the voice call through carries on the receive and unblocking based on IP PBX, but this kind of IP PBX is and the existing data net connection. IPPBX may be one kind of independent solution, may also decompose is several kind of scattered installments. Concrete function diagram as shown in Figure 1.
Figure 1 local area network IP telephone functional module chart
This IP telephone’s principal advantage:
May facilitate the migration and increases the terminal device;
Supports the multimedia terminals;
Is advantageous in the development computer telephone integration technology;
Hardware platform
Usually hardware’s basic design request is: High density, low power loss and low cost. But VoIP many performance correspond the specific application demand, such as: (1) system division, including an integrated data package of set and route; (2) definition product function software performance; (3) satisfies the high usability network management ability.
LAN telephone product work under the pure IP way, therefore telephone after turning on WAN, in the network management aspect should be compatible. This plan’s VoIP telephone is the terminal product which under one kind of gateway moves, does not need to be possible to turn on directly through the gateway this equipment in the ethernet, disposes the good related address message to be possible to use. Its development based on Atmel Corporation’s AT75C220 development board. AT75C220 is specially the high performance processor chip which applies for each kind of Internet designs. Its core part - ARM7TDMI microprocessor running ability is 40MIPS, uses in voice processing OakDSPCore cooperating the processor specially (Figure 2 the voice processing chip), running ability may reach 60MIPS. Between both the information uses the highly effective twin port mailbox mechanism alternately. This had decided AT75C220 can synthesize general CPU the control function and the special-purpose CPU information processing function, enhanced system’s integration rate, simultaneously the ARM processor’s operational order was quite simple, reduced the development order of complexity.
At75C220 development board besides has uses in connecting computer’s standard serial interface and the connection telephone handle’s RJ11 connection, but also two standard RJ45 ethernet connection uses in connecting the net to defend. ARM7TDMI the core processor control entire VoIP telephone’s movement, and provides the general I/O connection to use in connecting the digit dialing keyboard, the liquid-crystal display and the ringing circuit. AT75C220 structure diagram as shown in Figure 2.
Figure 2 AT75C220 structure diagram
On at75C220 development board is loaded with the memory application procedure to dodge saves (flash). The computer use super terminal procedure may carry on the information through the development board’s on serial interface with at75C220 chip to be interactive, realizes to the flash application procedure programming and other software management function. Develops AT75C220 application software’s PC machine to start moves the siap-mClinux operating system. After the application procedure development finished, will provide the tool through 220software set to contain the application procedure and the operating system essence img document downloads to 220 development boards on flash, completed system’s development finally.
VoIP telephone’s structure as shown in Figure 3. Its user interface part and ordinary telephone not different, the monitor uses for to demonstrate the related information, like initiates when the call dials the number, comes the electricity demonstration and so on. In addition, the VoIP telephone may be connected by the serial port with the personal digital assistant (PDA) equipment, realizes the software to promote, functions and so on automatic dialing. The voice connection’s function is realizes the voice simulated signal and standard 64Kbit/s between the PCM signal interconversion. The network interface is responsible for the voice grouping the transmission and the receive as well as each kind of call signaling exchange. The VoIP telephone defends connected through on the RJ45 plug and local area network’s net.
Figure 3 IP telephone constructional drawing
The software realizes
Should realize relies on the network protocol stack and real-time operating system (RTOS). The most application systems need RTOS, so that simultaneous working many flows and call. Uses RTOS should have following characteristic to satisfy the communication protocol the complexity: The system essence is small; The interrupt processing time is short; The continuous running time is long; Many millisecond levels or microsecond level timer’s handling ability. ARM- mClinux is the very outstanding embedded real-time operating system, it provided many kinds of has used in the real time system development, the debugging, and the movement function, like the multi-duty mechanism, the essence might cut out, the network function, the real storage management games and so on. At the same time, the Linux essence source code is completely public, this to reduces the development cost, to enhance the software development aspects and so on multiplying is without doubt very advantageous.
Regarding the network protocol stack, this plan uses the standard the H.323 agreement stack, in order to carry on the interconnection with the public network. Looking from the TCP/IP lamination viewpoint, H.323 is an application layer agreement race, it contained has been suitable for each kind of media correspondence and the signaling control agreement, the establishment foundation is TCP or the UDP agreement. According to the actual use demand, this article key discussion’s agreement has speech coding aspect G.723.1, G.711 and so on and the call controls signaling H.225 and H.245 and so on. Under the CPU normal designed capacity, system all advancements will block in the respective news formation, only will then be at the lowest IDLE advancement to be at the running status, in the news formation the news total is in the quite low quantity level. Increases data area which and inside information the advancement possibly must increase uses in common, corresponding must introduce the complex management mechanism.
Based on the above software request, this VoIP telephone must be able with to conform to the H.323 standard gateway and the net defends the intercommunication, and realizes between the ordinary telephone’s voice correspondence function which the VoIP telephone and the VoIP telephone and the VoIP telephone and the gateway bring. In addition, the VoIP telephone should also have certain sound recording and replay ability.
Speaking of the IP telephone, the main software realizes, in the telephone and the net defend between correspondence as well as telephone’s intercommunication, its call flow like chart 4.
Figure 4 H.323 call signaling flow
The above process’s channel realizes the mechanism is TCP or UDP. The IP telephone after the success registers the net defends, its connection process completely is similar between the ordinary telephone’s connection, picks machine the backward net to defend sends “the call to request” (udp), after the net to defend receives, sends “whether to move the calling digit dialing” (tcp), if does not permit, prompts on-hook; If permits, the calling after receiving the dial tone starts the digit dialing, defends to the net delivers “the digit dialing information” text (tcp), the net defends to the calling transmission “is called the idle information” text (tcp), is separately to the calling and is called to deliver the ring-back tone and the ringing tone. After the waiting is been called picks machine, is called to the net to defend delivers “is called to pick machine” text (udp), the net defends to is called to deliver “the stop ringing” (tcp), the net defends to the calling delivers “is called to pick machine” text (tcp), enters talks over the telephone the (udp) condition. Related voice’s processing must involve to 220 voice processing module.
The gateway defends the transmission to the net the text: No matter is the high-level subscriber’s premises defends the transmission to the net to register the text, the IP telephone or the switchboard lateral net defends the transmission the call processing text, can lay aside first in the named mqRecvBottom news formation, then the basis text uses the transfer method sends out the following three formations: Signaling TCP transmits _mqSendTCP; Signaling UDP transmits _mqSendUDP; The voice transmits _mqSendData (to be possible to defend repeater through net, may also between two gateways transmission, then transmit again directly) through socket.
Several key question solutions
Picks on-hook examination and the index dial number receive
AT75C220 provides certain users from the definition general I/O connection. Only need through certain hardware connection and the software programming, may realize picks on-hook status the examination the receive which dials a number with the user.
Incoming call ringing
Whether there is in at75C220 internal register has instructs the incoming call bit position specially. The application software examines this bit regularly, once discovers has incoming call, programs the definite general I/O mouth through one to send out the ringing pilot signal beforehand immediately, this signal may enable the signal as the ringing circuit.
Voice processing module
The voice processing function controls OakDSPCore by ARM7TDMI to complete together.
When carries on the IP telephone conversation, front end will simulate 64Kbit/s which the electric circuit sends the PCM digit voice signal to carry on the compression, delivers to the network interface module; Meanwhile the compression voice signal which sends the network interface module carries on the decompression, forms 64Kbit/s the PCM digit voice signal to deliver to the simulation front end.
Through to the OakDSPCore nimble programming, the VoIP telephone may realize the pronunciation message sound recording and the playbacking function conveniently.
When user from index dial digit dialing, starts OakDSPCore through the programming the DTMF signal generating device function, produces the corresponding double sound multi-frequency signal in the user receiver. If the need, this DTMF signal may also make sense the news number equally after the compressed encoding transmits.
The voice compression and the decompression use the G.723.1 algorithm. In order to carry out the effective pronunciation compression, must consider the important factor. First, when all channels work, must guarantee performance not any drop. The data packet must be may dispose, guarantees the biggest flexibility. In addition, this topic uses the G.723.1 algorithm has used the voice activity detection vad technology, the VAD technology is the auto-adapted gain control foundation, can further realize the band width.
