Introduction
In recent years, Internet obtained the swift development and the popularization application, but takes its core technologies the IP agreement system to obtain the widespread approval in the data net construction dominant status. At the same time, along with based on the IP technology in each kind of applied technology’s statement, VoIP technology’s statement, caused the data net correspondence gradually to integrate the traditional voice feature domain particularly.
VoIP (Voice over IP) is named the IP telephone (Internet Protocol Phone), is realizes the voice communication one advanced means of communication using the IP network, is one kind completely based on the IP network speech transmission technology. It uses the pronunciation gateway, will exchange the platform softly, the net defends and so on equipment the simulated signal digitization, then the data compression data packet, will transmit through the IP network the pronunciation destination address. After the destination address receives the data packet, reorganizes, after solving the compression, again the reduction simulated signal the data. Thus, a complete telephone conversation process realized in the IP network. How will this article discuss designs under the embedded μClinux environment undertakes the data compression, the decompression and the transmission in the IP network and so on important function equipment - - VoIP pronunciation gateway.
1 IP telephone system’s basic composition
The IP telephone system is composed generally by three parts: Telephone (phone), pronunciation gateway (gateway) and the net defend (gatekeeper), as shown in Figure 1.
The telephone is refers to may passes the (ISDN) company through the telephone network or one to arrive at the local gateway’s telephone terminal.
The pronunciation gateway is refers to through the IP network provides between the telephone to complete the voice correspondence the key equipment, namely the Internet network and the telephone network and pass between the (ISDN) net’s interface equipment. It completes the pronunciation compression (to compress 64 Kb/s voice signals low code rate voice signal) and the addressing and the call controls, and has the IP network and telephone (PSTN) or passes the (ISDN) interconnection the interface function.
The net defends is the network superintendent, is responsible for management functions and so on user registration, cost.
2 VolP pronunciation gateway
The IP telephone system design’s core is the VoIP pronunciation gateway. According to VoIP pronunciation gateway’s function demand as well as market mature technology, author use “hardware embedded operating system application layer software” the system construction designs the VolP pronunciation gateway.
Regarding the hardware platform, attaches take the VolP main chip as the core by the necessity auxiliary equipment. The main chip uses Texas Inslruments production TI TMS320VC549 DSP and Samsung Corporation ARM7TDMI nucleus double essence structure.
The VolP software part divides into two: Embedded operating system and application layer software. μClinux takes the embedded operating system using the opening source code’s real-time operating system, uses the modular design, develops the platform to be friendly. According to function demand, in application layer software including pronunciation package of processing module, telephone signaling gateway module, route addressing module and network management module.
2.1 hardware realize
The VolP pronunciation gateway by the main chip, the memory, the ethernet connection, the audio frequency connection and the I/0 expander and so on several parts is composed, hardware diagram as shown in Figure 2.
(1) main chip
Main chip diagram as shown in Figure 3. The main chip constitutes by TMS320VC549DSP and the ARlM7TDMI double essence.
The ARM7TDMI essence uses 50 MHz basic frequencies and the monocycle simplification instruction, in constructs 8 KB RAM, the DMA controller and the interrupt controller, and supports each kind of main line to expand. Its internal SIM (System Integrated Module) the unit integrated the rich general module, including 2 asynchronous serial ports, 2 32 counters and ethernet controller and so on.
TI Corporation’s 16 high speed fixed-point DSP TMS320VC549, its processing speed reaches as high as 100MIPS, and the function is very formidable, may support all high compression/low latency to arrange the decoding (G.711, G.729, G.729a and G.723.1), and supports the echo counter-balance, the pronunciation condition examination, the static sound compression, the vibration cushion and the comfortable noise completely has and so on functions, the guarantee unification, the high grade pronunciation is interactive.
This hardware design uses the double essence structure, caused the VoIP pronunciation gateway has established good balanced in the power loss and between the application order of complexity. Uses the double essence the superiority to be as follows:
① Enhances pronunciation processing timeliness, avoids the traditional single essence design power loss too big shortcoming. If the entire job completes by the ARM7TDMI nucleus, the burden is overweight, the real-time performance comes under the influence, will create the pronunciation drop in quality, and has limited the correspondence conversation way, cannot satisfy the massive user correspondence the request. Presently completes the real-time voice signal compression, the DTMF signal examination by the DSP processor and produces, work and so on echo cancellation, both lightened the ARM nucleus burden, simultaneously and has provided many telephone channels, reduced the power loss, causes the gateway for multiuser to provide possibly serves into.
② Realizes the complex system function. Uses 32 ARM nuclei to take the control supervisor center, can stabilize the movement embedded real-time operating system, can support each kind of network protocol stack like TCP/IP, enables the pronunciation data to transmit effectively in the network.
(2) memory
The system including 1 piece of 8 MB Flash, uses in saving the embedded real-time operating system essence and the application procedure code compressed file, but also has 1 piece of 512KB SDRAM. When system initiation, the ARM nucleus’s calls in Flash in compressed file SDRAM to carry on the decompression, then execution.
(3) audio frequency decoder
The codec encoder-decoder chip is responsible for the PCM data which comes to the DSP solution compression evacuation carries on the decoding, after and gives filter’s simulation voice signal the subscriber’s premises; At the same time, the codec encoder-decoder chip will also be responsible to simulate the voice signal to carry on the PCM code, will then deliver the DSP chip to carry on compression processing. In the design, the codec encoder-decoder chip uses IDT Corporation’s 4 channel PCM to arrange decoding chip IDT821034. This chip has the programmable gain establishment, the master clock frequency is 2.048MHz, supports 128 programmable time slots, a law/μ law to be possible most greatly to choose, built-in characteristics and so on digital filter, serial control interface and low power loss.
(4) ethernet controller
Uses ethernet chip which produces by Realtek Corporation, it has the 10/100M auto-adapted receiving and dispatching function, supports the full-duplex to operate, conforms to 802.3 agreements. Is responsible by it to process the related ethernet data frame the transmission and the receive, codes, the collision detection, the frame head’s production and the examination, the CRC verification code production and the confirmation as well as the data reproduction including Manchester and so on.
(5) other periphery
In the system also includes LCD modules and so on liquid crystal display, keyboard, AC-DC power source.
2.2 softwares realize
Figure 4 is software system’s system design and the level and the module division diagram. Below carries on the introduction to each level and the module.
2.2.1 embedded operating systems and driver design
The embedded operating system uses μClinux, contains the File module, the GUI module and the NET module and so on. μClinux aims at the Linux operating system which the non-memory management unit processor has custom-made, has the source code to be public, the essence may cut out and easy transplant and so on characteristics. This system carries on the development with the C language, μClinux the formidable application library of subroutines μCLibc has provided the support to the application software level’s development, has saved the system development time. The File module is the filing system module. The GUI module is a graph administration module, supports LCD and the touchscreen, uses in developing the man-machine contact surface. The NET module is the network module, has realized the TCP/IP agreement stack, uses in providing the network voice communication the function.
According to hardware platform request, but also needs to develop the adaptation hardware platform the driver, including BSP board level package, serial port driver and ethernet mouth driver. The BSP board level package is passing through the hardware level, the operation system-level and the application procedure level multi-layer, with μClinux provides the service together for the application procedure. A BSP board level package of compilation realizes μClinux the system transplant key. Wraps in the BSP board level develops and completes the hardware initialization, interrupt processing and the production, the hardware timer management, this locality and main line’s functions and so on memory mapping. In the serial port actuation design, has realized the single character transmission, the string of character transmission, the single character read and so on basic function, achieves through the serial port receiving and dispatching string of character goal. The net mouth actuation was relatively simple, has realized the chip initialization, a network package of transmission, a network package of receive and the chip replacement and so on basic function, achieved through the network mouth transmission and a receive network data package of goal.
2.2.2 application software module
The application layer software is composed of system application layer agreement stack H.323 and each functional module.
System’s application layer agreement stack H.323 realizes through the VoIP gateway real-time transmission data function. The H.323 communication protocol stack is responsible for in the IP telephone seek for the called subscriber, the establishment reply, and according to each other’s data-handling capacity transmission data, carries on the real-time speech transmission in the network. Application layer agreement stack H.323 is the system application layer software’s foundation, the application layer software module carries on the development in this agreement stack’s foundation.
Application layer software function module including pronunciation package of processing module, telephone signaling gateway module, route addressing module and network management module.
The pronunciation package of processing module is mainly moves on the digital signal processor (DSP) chip, may realize the following function.
① Pronunciation code and decoding.
②Static sound examination. Through to does not have when the voice the noise carries on the examination, thus judges the signal which receives whether to include the voice signal, if has not examined the voice signal, will feed back to “a pronunciation package of processor”, will let it send out will include “the static sound” the information packet, will thus maximum limit save the correspondence band width.
③Echo counter-balance. Realizes in the voice communication echo counter-balance based on international standard G.165/G.168, improves the pronunciation the correspondence quality.
④Auto-adapted pronunciation restoration. Through a buffer, docking receives postpones a pronunciation package of signal to carry on the pronunciation to restore, thus achieves anti-”the time delay”, anti-”the latency vibration” goal; And may “auto-adapted” the adjustment latency value, serves the speech transmission latency smallest purpose, the improvement voice communication quality. Simultaneously supports “the interpolation” the algorithm, namely, when the pronunciation wraps in the transmission process loses, can use “the interpolation” the technology to carry on the restoration.
⑤Pronunciation package of processing. To carries on “the pronunciation pack” after the code voice signal processing, or is the pronunciation package which docking receives carries on “unpacking” processing, thus completes the seal agreement processing.
The telephone signaling gateway module mainly realizes the function which the signaling transforms. The VoIP pronunciation gateway has provided with the public switched telephone network (PSTN) connection, therefore the VoIP pronunciation gateway must realize an important function is the signaling explanation and the transformation, namely comes from public switched telephone network (PSTN) the call to control the signaling to translate the signaling form which the IP telephone network interior may distinguish, then carries on the transmission through the IP network.
The route addressing module mainly completes the function which the route addressing and the route manage. It each area’s telephone district number mapping for the corresponding gateway’s IP address, deposits in the database, when user dial long-distance telephone call, the gateway basis telephone district number’s database material, the establishment corresponding gateway’s IP address, and joins this IP address to the IP data packet, simultaneously chooses the best route to reduce the transmission delay, transmits the IP data packet after Internet to the goal gateway.
The network management module (SNMP/WEB/CLI) provides a pronunciation management the connection, realizes to the VoIP pronunciation gateway’s disposition and the maintenance. The network management information is based on international standard ASN.1 and SNMP (simple network management protocol) the request establishment, has developed three kind of pattern management in view of the system: Based on WEB management, based on SNMP (simple network management protocol) management and based on command line management. This causes the user is simpler to the VoIP pronunciation gateway’s disposition and the maintenance and is effective.
Conclusion
This article has analyzed based on the embedded VoIP pronunciation gateway system’s architecture, and has analyzed the system software and hardware modular structure, used the ethernet agreement to be rich and the open strong characteristic, used TCP and the H.323 agreement design, has met the practical application need fully. At present, VoIP equipment from based on PC primary product development for large-scale, redundant reliable, high performance telecommunication level network operation equipment. Technically, synthesizes the existing telephone network signaling technology, may the voice, the data, the facsimile as well as the multimedia fusions to a unified network. The IP telephone while reduces the network infrastructure cost, has provided the extremely rich new service function for the user.
