FEATURE ARTICLE By David Rowe Embedded IP-PBX - Tracey
FEATURE ARTICLEby David RoweEmbedded IP-PBXSwitch Analog and VoIP CallsDavid Rowe describes the design of a µClinux-powered IP-PBX capable of switching bothanalog and VoIP calls. With an Analog Devices Blackfin processor, some custom hardware,and Asterisk PBX software, you can build a similar system.Ihave always been fascinated withembedded systems and telephony. Ireally like working “close to themachine” on embedded systems, and Ihave a parallel interest in speech andtelephony that dates back to my hamradio days as a teenager. One project Ihad been dreaming about for years isan embedded telephony platform forVoIP or IP-PBX applications.One problem with telephony is thatit requires a lot of processing power.Unfortunately, most embedded processors are not too powerful. So, a common design approach for embeddedtelephony consists of a general-purpose microcontroller combined with aspecial-purpose DSP chip to handlethe hard-core number crunching. Thistwo-CPU approach increases the system cost and complexity.About 18 months ago, I stumbledacross the Blackfin processor fromAnalog Devices. The Blackfin is apowerful host processor and a DSP(i.e., it can run µClinux applicationsand DSP code efficiently and simultaneously on one processor).A typical embedded processor willstruggle to run more than one or twochannels of G.729 speech compressionor echo cancellation software. TheBlackfin, running at 500 MHz, is capableof running 60 channels of G.729 speechcompression or echo cancellation. Thismakes it possible to build low-cost,high-performance embedded telephonysystems, like an IP-PBX that can supportmultiple analog and VoIP channels atthe same time. For about 500, you can12 Issue 208 November 2007now build an IP-PBX with features thatonly 10,000 PBXs had a few years ago.This work is part of the free telephonyproject (www.rowetel.com/ucasterisk).The goal of the project is to build opensource telephony hardware and software(e.g., an embedded Asterisk IP-PBX) at alow cost. Both the hardware and software are open source. You are free tocopy and reuse the hardware designs.IP-PBX OVERVIEWWhat is an IP-PBX and how does itwork? Figure 1 is a block diagram of theembedded IP-PBX that shows the majorhardware and software components.There are two types of analog ports:FXO ports that connect to your localtelephone exchange and FXS ports thatconnect to analog handsets. This jargon can be confusing. The way Iremember is the “S” in FXS stands for“station” handset.The analog ports convert the voiceand signaling information to digitalsignals that the IP-PBX can esDSPsoftwareInternetFigure 1—This is a block diagram of the Embedded IPPBX showing the major software and hardware components. The heart of the system is the Blackfin processor. This is connected to hardware such as theFXO/FXS telephone ports and Ethernet. The Blackfinprocessor runs the µClinux operating system andAsterisk application software.CIRCUIT CELLAR Examples of signaling information arethe ring detection and on/off hook status signals. Analog ports are surprisingly difficult to build because theyuse a mixture of rather old technologies. For example, a ring signal may be200 VPP (at low current), on and offhook status is indicated by loop current flowing, and the transmit andreceive audio is mixed together on justtwo wires, which leads to echo problems. All of this must be reliably andsafely connected to low-voltage digitalsystems. Fortunately, there are excellentchipsets available to help build costeffective and reliable analog interfaces.Telephone calls can also flowthrough the Ethernet port using VoIP.Both local (e.g., using SIP phones) andtrunked calls can be performed usingVoIP. The magic of an IP-PBX is thatanalog and VoIP calls can be tiedtogether. You can route a call from ananalog handset over the Internet tosave money on long-distance calls.What happens when you make a regular analog phone call? First, pick upan analog phone’s handset. This generates an “off-hook” event in the FXSport that tells the Asterisk software togenerate a dial tone in your phone.When you dial 9, DSP software decodesthe DTMF tones and presents Asteriskwith the digit. Asterisk then connectsyour FXS port to an FXO port so youcan reach the local exchange whereyou can dial a phone number as usual.What about VoIP calls? Well, sayyou use the same analog phone tomake the call, but this time you dialwww.circuitcellar.com
Figure 2—Take a look at the FXO module schematic. It provides an interface between a telephone line and the Blackfin processor. Its most important task is to provide highvoltage isolation, which is achieved using the capacitive barrier formed by C1 and C2.long distance. In this case, Asterisk isprogrammed to route the call over theInternet. To save bandwidth, the DSPsoftware compresses the speech samples from the FXS port from 64 kbpsdown to 8 kbps. The Asterisk softwarethen puts the speech samples in packets and squirts them out the Ethernetinterface onto the Internet.A lot of the IP-PBX’s power is provided by the Asterisk software, whose primary sponsor is Digium. The ability torun a powerful operating system such asµClinux is also very useful. For example, you can telnet into the IP-PBXPhoto 1—Here you see the 4fx daughter board andmodules before assembly. Each of the four modules atthe bottom mate with matching connectors in the middleof the 4fx daughter board. Note the large vacant area inthe middle of the PCB. It is necessary to provide physical isolation between the FXO ports and the rest of thesystem.www.circuitcellar.comwhile it is running to debug and configure it.daughter board and modules disassembled. There are two modules of eachtype in the photo. Photo 2a featuresTHE HARDWAREThe hardware is built around fourPCBs. The first board is an AnalogDevices Blackfin STAMP card. It is adevelopment system that is available offthe-shelf from Digi-Key. It contains anAnalog Devices ADSP-BF537 Blackfinchip, 64 MB of RAM, 4 MB of flashmemory, and connectors that break outmany of the ports. It runs µClinux and issupported by www.blackfin.uclinux.org.One great feature is that an openhardware/software community existsaround the Blackfin. Reference designsfor Blackfin hardware (such as theSTAMP boards) and various daughterboards are freely available.On top of the STAMP sits a 4fx daughter board. (Why are boards alwaysgirls?) It holds some glue logic andsockets for the modules and also supports an MMC to provide extra flashmemory storage. The 4fx’s name comesfrom the fact that the daughter boardcan support up to four FXS or FXOmodules.The modules are the little boardsthat plug into the daughter board.There are two types of modules, FXSand FXO. Photo 1 shows the 4fxCIRCUIT CELLAR a)b)Photo 2a—The IP-PBX is assembled and configured forfour analog ports. From top to bottom, you can see theFXS/FXO modules, the 4fx daughter board, and the BF537STAMP. There is an optional MMC connector on the left.Green indicates an FXS port, red an FXO. b—The designis expandable to multiples of four ports by stacking additional daughter cards and modules. The wcfxs device driverautomatically detects the number and type of ports available. In this case, there are four FXS and four FXO ports.Issue 208 November 200713
Figure 3—Take a look at the FXS module schematic. It provides an interface between a telephone and the Blackfin processor. The DC/DC converter in the lower-right cornergenerates 48-V battery and 200-VPP ring voltages from a standard 12-V supply.the entire system assembled, poweredup, and ready to make calls. There aretwo FXS modules on the left and twoFXO modules on the right. The color ofeach LED indicates the type of moduleinserted via an autodetection algorithm. Itis also possible to stack multiple 4fxboards to expand the number of channelsthat can run on the system (see Photo 2b).In the next few sections, I’ll describethe schematic-level design for the FXS,FXO modules, and the 4fx card. TheBlackfin STAMP card schematics areavailable at www.blackfin.uclinux.org.FXO AND FXS MODULE DESIGNThe gEDA suite of GPL electronicdesign automation tools was used forschematic entry, PCB layout, and Verilog simulation throughout the project(www.geda.seul.org). The use of opensource tools makes it easy for anyone tomodify the designs. Because the hardware designs are “open,” it is nice touse open-source tools where possible.14 Issue 208 November 2007The schematics for the FXS and FXOmodules are shown in Figures 2 and 3.Both modules are based on chipsetsfrom Silicon Laboratories. The circuitdesigns for the modules are derivedfrom the reference circuits provided inthe Silicon Laboratories datasheets.The most important function of theFXO module is to isolate the “lineside” of the port from the low-voltagedigital side. There are safety reasons forthe isolation. If lightning hits thephone lines, you will want a degree ofphysical isolation between the phoneline and the rest of the hardware. Thisisolation is rigorously tested duringFCC-68 compliance testing by the application of high voltages that simulatelightning. The Silicon Laboratories chipsachieve it with a capacitive isolationbarrier (C1 and C2). The Silicon Laboratories Si3050/Si3019 chipset used for theFXO module also performs other functions, such as line voltage and currentmonitoring, impedance matching, A/DCIRCUIT CELLAR and D/A conversion of the speech signal,ring detection, and DC termination.A screenshot of the PCB layout forthe FXO module is posted on the Circuit Cellar FTP site. The PCB measuresabout 50 25 mm. Note the “isolationbarrier” between the two chips, whichonly C1 and C2 are allowed to bridge.The PCB layout was carefully designedto keep the line side and low-voltageside physically isolated by a 3- to 4-mmgap. Even the ground plane is absent inthis area, because it would violate thephysical isolation requirements.The FXS module has similar functions to the FXO module (see Figure 3).In this case, the Silicon LaboratoriesSi3210/Si3201 chipset is used. Themain difference with the FXS moduleis that it generates ring and “battery”DC supply voltages. Silicon Laboratories employs a clever switched-modepower supply that is modulated togenerate the 200-VPP sinusoidal ringvoltage. Although the datasheetswww.circuitcellar.com
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Figure 4—This is the 4fx daughter board schematic.This board breaks out the TDM and SPI bus signalsfrom the STAMP card and feeds them to eachFXS/FXO module. U4 is a Xilinx CPLD thatexpands the number of SPI chip select signalsavailable from the STAMP.16 Issue 208 November 2007CIRCUIT CELLAR www.circuitcellar.com
claim that the DC/DC converter canwork well down to 5 V, I found that a12-V supply was required for theDC/DC converter to avoid excessiveanalog noise.The Si3210/Si3201 FXS chipset alsotakes care of other functions, such asline voltage monitoring, loop currentdetection, overload protection, impedance matching, and A/D and D/A conversion of the speech signal.The FXS module’s PCB layout isquite challenging because there arethree distinct signal areas in the small50 25 mm area. (A screenshot of theFXS module PCB is posted on the CircuitCellar FTP site.) The first area handlesline-level analog signals from the telephone handset. The second region is adigital interface containing a 2.048-MHzTDM bus and a SPI bus that connects tothe fast rise time (i.e., noisy) signals fromthe Blackfin. The third area is a switchmode DC/DC converter that converts alow-voltage 12-VDC rail to the –90 VDCrequired for the telephone “battery” supply and also generates the 200-VPP ringvoltage. To generate the high voltages ateven small currents, large pulsed currents must flow in the 12-VDC supplyline, which is a potential source of noise.It is important to prevent theDC/DC converter and digital sidefrom injecting noise into the sensitivelow-level analog section. Based on tipsfrom the Silicon Laboratories application notes, a few tricks were used. TheDC/DC converter ground was keptisolated from the ground plane andconnected only at a single point. Thisprevents large ground current spikesfrom entering the ground plane. Largecurrent pulses in the ground plane getconverted to voltages (because theground plane impedance is small butnonzero), which then get superimA2000011A1001100A0010101SPI DevicenCS0: sparenCS1: Port 1nCS2: Port 2nCS3: Port 3nCS4: Port 4nCS5: LED registerTable 1—This truth table relates addresses to the SPIdevice that you are currently talking to. Note that onlyfive devices are decoded. Four of the devices areFXS/FXO modules. Device 5 is a bank of bicolor LEDs.18 Issue 208 November 2007nCS1nCSADemuxnCS2A6, A7nCS5nCS3ComparenCS42SPI data3Shift registerennCSBenShift registerBicolor LEDs8Figure 5—The CPLD expands the number of SPI chipselect lines available using a demultiplexer and shift register. It also implements a SPI peripheral that drives thebicolor LEDs.posed on the low-level analog signals asunwanted noise. You can see the lack ofa ground plane in the upper-left handpart of the FXS module PCB screenshoton the Circuit Cellar FTP site.A ground plane was used throughout the analog section and in the digital section. The two ground planesare connected only at a single pointto prevent digital currents from flowing through the analog section andinducing noise. This point is as faraway from the DC/DC converter aspossible.Bringing the two module designs tolife was surprisingly straightforward.The first FXO module I built hadsome problems with clicks and popsin the audio; however, a good cleaningwith the flux remover fixed that! Theflux I use is conductive so any residuetends to upset circuits that depend onhigh-resistance values in certain areas.I then tried the FXS module and itworked on the first try. I was reallyhappy about that. I was placing callsabout 5 minutes after I applied power.Hardware development isn’t meant towork like that!4fx DAUGHTER BOARD DESIGNNow, take a look at the 4fx daughterboard schematic (see Figure 4). The FXOand FXS modules communicate withthe host processor via two serial buses.The signaling and control data flows viaa SPI bus. The actual transmit andreceive speech samples flow on a timeCIRCUIT CELLAR division multiplexed (TDM) bus.The 4fx breaks out the SPORT connector from the STAMP board and feedsthe SPI and TDM signals to each module. Because the STAMP has only alimited number of SPI chip-select signals available, a Xilinx CPLD (U4) isused to expand the number of SPIdevices that can be addressed. This isdescribed in the next section. TheCPLD also supports a “stacking”architecture where several boards canbe stacked on top of each other toobtain extra analog ports.To reduce EMI, the suppressioncomponents (e.g., ferrites and capacitors) for each port were placed on thedaughter board as close as possible tothe RJ-11 connectors. Each digital linein the daughter board also has seriestermination resistors. The resistors areinitially loaded as 0 Ω, but this can beincreased to combat EMI or ringingissues if required. A 74LV244 buffer(U2) is also used to reduce the edgerates of high-speed digital signals fromthe Blackfin STAMP card. Reducingthe edge rates reduces EMI becauseslow rise and fall times mean reducedhigh-frequency energy.XILINX CPLD FIRMWAREOn the 4fx card, a Xilinx CPLD isused to expand the number of SPIselect lines available and provide a register to drive the LEDs (see Figure 5).The STAMP SPORT connectorshave only a limited number of SPIselect lines available. However, on the4fx design, I have many SPI devices(e.g., four telephony ports and a register to drive the LEDs). I would alsolike to stack the 4fx cards to buildeight port telephony systems. So, theproblem is accessing multiple SPIdevices across multiple cards usingonly a small number of SPI lines available on the SPORT connector.D10011D00101LED1OffRedGreenOffTable 2—This is a truth table for LED status. Two digitallines are used to drive each LED, which enables you toselect the polarity as well as switch the LED off and on.www.circuitcellar.com
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I decode a large number of SPIdevices using two SPI select linescalled nCSB and nCSA. nCSB isasserted and a byte is sent that selectsthe SPI device you want to address.The format is:Listing 1—Take a look at the floating-point DTMF detector inner-loop code. This is the core of a DTMF tonedetector. It executes 64,000 times for every channel running on the IP-PBX. The final line implements a typicalDSP operation: multiply two numbers and then add and subtract from the result.static inline void goertzel sample(goertzel state t *s, short sample){float v1;float fsamp sample;D7 D6 D5 D4 D3 D2 D1 D0A7 A6 X X X A2 A1 A0A [7:6] selects the card from othercards in a stack. Two jumpers determine the address of the card. Eachcard in the stack must have a unique2-bit address to be successfully decoded. A [2:0] selects the SPI device onthe card (only five devices are decoded) (see Table 1).nCS [4:1] is routed via I/O pins todevices external to the CPLD. nCS5 isan internal SPI device and is not routed to an I/O pin. Once the SPI devicehas been selected, nCSA is asserted totalk to the actual SPI device. nCSAmay be asserted as many times asrequired (e.g., to perform a blocktransfer on the SPI device).Note that once the device has beenselected, SPI transfers proceed normally (i.e., assert nCSA and read/write asdesired). When access to another SPIdevice is required, nCSB is assertedand the byte sent can be used to selectanother device (or card).The LEDs appear as an 8-bit SPIregister at address five on the card.When you write to the register, thevalue determines the status of eachAsteriskdsp.cDTMF detectorµClinuxUser modeKernel modeZapteldevice driverWcfxsdevice driverFXS porthardwareAnalogphonesFXO porthardwareLocalexchangeFigure 6—The IP-PBX runs the µClinux operating systemand Asterisk PBX software. To run Asterisk on the Blackfin,the DTMF detector was modified to use fixed-point arithmetic. The wcfxs FXS/FXO port device driver was also modified to support the Blackfin serial port and SPI hardware.20 Issue 208 November 2007}v1 s- v2;s- v2 s- v3;s- v3 s- fac * s- v2 - v1 fsamp;Listing 2—This is fixed-point DTMF detector inner loop code. The DSP operation has been broken down intotwo lines. Note the casting to 16-bit integers and scaling via shifts. These operations give you fine control overhow the C compiler treats these operations. The main challenge with fixed-point DSP is keeping the dynamicrange of signals inside a 16-bit range.#define AMP SCALE 8#define FAC SCALE 14static inline void goertzel sample(goertzel state t *s, short sample){int16 t v1 fix;int mpy;}v1 fix s- v2 fix;s- v2 fix s- v3 fix;mpy (int16 t)( ((int)s- fac fix * (int)s- v2 fix) FAC SCALE );s- v3 fix mpy - v1 fix (sample AMP SCALE);LED (see Table 2). And similarly forthe other LEDs:D[3:2] LED2D[5:4] LED3D[7:6] LED4SOFTWAREThe Silicon Laboratories chips arecommonly used with Asterisk on PCIcard-based line interface hardware.Open-source drivers for the chips exist,dramatically simplifying development.Figure 6 shows the software architecture of the Asterisk implementation used for the embedded IP-PBX.Asterisk is a user-mode applicationthat communicates with a kernelmode driver called zaptel. Zaptel communicates with lower-level driversthat talk to the actual hardware.The Asterisk wcfxs driver was chosenas the starting point for the embedded IPPBX. The driver was originally written tocommunicate with the Silicon Laboratories chips operating on a PCI bus. To portthe driver to the Blackfin, the PCI interface component was carefully removedand replaced with code that interfaces toCIRCUIT CELLAR the Blackfin’s SPORT (TDM serial port)and SPI hardware.Surprisingly, it is actually easier(and cheaper) to interface the SiliconLaboratories chips to the Blackfincompared to an x86 PC because noPCI bridge is required. The driver isalso simpler.Most of the software changes wereconfined to the wcfxs driver, althoughsome changes to the Asterisk application were also required, such as architecture-specific word-alignment issuesand several Linux-µClinux portingissues. One area that needed optimization was the DTMF detection routines.The code was written to run on a general-purpose x86 CPU where it is assumedfloating-point support is available in theform of a hardware floating-point unit(FPU). The Blackfin does not have anFPU and is in fact optimized for fixedpoint operation. Thus, the original floatDTMF code ran very slowly, consuming31 MIPS per channel.The problem was traced to the“inner loop” of the DTMF detector(see Listing 1). The code implements adigital filter and is called eight timeswww.circuitcellar.com
for each input sample (eight filters arerequired for each DTMF detector).Because each channel has 8,000 inputsamples per second, the total number offunction calls is 8 8,000 the numberof channels per second. So, it is important to make sure the “inner loop” coderuns as efficiently as possible.The trick is to replace the floatingpoint code with equivalent fixed-pointcode (see Listing 2). The code mapsdirectly to the assembler instructionset of the Blackfin and compiles downvery efficiently. The scale factors werechosen to keep the dynamic range ofthe variables within a range easily represented by a 16-bit integer. The fixedpoint port was tested with a unit testfrom the Spandsp library that puts theDTMF detector through its paces(www.soft-switch.org).The result was that the fixed-pointDTMF detector worked just as well asthe floating-point version, but it consumed about 1.75 MIPS compared to the31 MIPS required for the floating-pointversion. Standard vanilla C code wasused (see Listing 2). With a little Blackfin assembler replacing the C code, theperformance could be improved evenfurther. However, with 500 MIPS available on the Blackfin, 1.75 MIPS for theDTMF decoder is probably fast enough.development. Refer to the project website for more information.If you would like to get started withembedded IP-PBX development, I havea fully assembled and tested IP04 IPPBX for 450 plus shipping. IAuthor’s Note: Thanks to the Linux,Blackfin, Asterisk, and gEDA communities. My wife Rosemary did a greatdeal of the schematic entry for the FXSmodule, and Jerry Zeng from AnalogDevices was very helpful in checkingthe design and brainstorming theCPLD requirements for the 4fx.David Rowe has 20 years of experiencein the development of DSP-based telephony and satellite communications hardware/software. He has a wide mix ofskills, including software, hardware, andproject management. He earned a Ph.D.in DSP Theory. David has held executivelevel positions in the satellite communications industry (www.dspace.com.au)and has built and successfully exited asmall business (www.voicetronix.com).However, he has decided that he is betterat debugging machines than people, sohe currently hacks telephony hardwareand software full time.PROJECT FILESTo download code, go to ftp://ftp.circuitcellar.com/pub/Circuit Cellar/2007/208.RESOURCESBlackfin Linux Project, www.blackfin.uclinux.org.gEDA Project, www.geda.seul.org.D. Rowe, “Free Telephony Project:Open Embedded Telephony,” 2007,www.rowetel.com/ucasterisk.S. Underwood, “Spandsp Project,”www.soft-switch.org.SOURCESBlackfin STAMPAnalog Devices, Inc.www.analog.comSi3050/Si3019 and Si3210/Si3201 chipsetsSilicon Laboratories, Inc.www.silabs.comGO BUILDIn this article, I explored the architecture of an embedded IP-PBX anddescribed in detail the schematic-levelhardware design and PCB layout.Using embedded techniques and opensource hardware and software, it ispossible to build a low-cost IP-PBXwith features (VoIP, IVR, and flexibility) that rival those of 10,000 PBXs.Other areas of the project that havenot been mentioned due to space limitations include the Asterisk softwareconfiguration, a custom DSP motherboard and configuration of the PBX. Formore information, visit the project website (www.rowetel.com/ucasterisk).The software and hardware designsfor the project are open-source andcontributions by corporations or individuals are welcome. Currently, ateam of companies and individuals isworking on the project in areas likeDSP, µClinux software, and hardwarewww.circuitcellar.comCIRCUIT CELLAR Issue 208 November 200721
(e.g., an embedded Asterisk IP-PBX) at a low cost. Both the hardware and soft-ware are open source. You are free to copy and reuse the hardware designs. IP-PBX OVERVIEW What is an IP-PBX and how does it work? Figure 1 is a block diagram of the embedded IP-PBX that shows the major hardware and software components. There are two types of analog .
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