Electronic Systems Technology ESTEEM192S Wireless Modem User Manual part 7

Electronic Systems Technology Wireless Modem part 7

user manual part 7

CHAPTER 7THEORY OF OPERATIONINTRODUCTIONESTEEM HOW IT WORKSSPECTRUM UTILIZATIONPACKET PROTOCOLSFLOW CONTROLDATA PRIVACYEFFECTIVE BAUD RATE
CHAPTER 7THEORY OF OPERATIONRevised: 11 Feb 99 7-2 INTRODUCTIONESTeem wireless modem products provide a "WirelessSolution" by eliminating conventional hardwiring of leasedphone lines.All of the ESTeem models come with the industry standardRS-232C, RS-422, and RS-485 asynchronouscommunications ports to give the user a new dimension to"Local Area Networking".Our packet burst, frequency agile communications productsallow the user to create a "Radio Area Network" of up to255 users on a single frequency. The packet burstcommunications technique was chosen to give the systemvery high data integrity in high noise industrialenvironments. The ESTeem incorporates a method of errorchecking that provides received data accuracy of greaterthan one part in 100 million.Internal Digi-Repeater features allow the user to increaseoperating range by relaying transmission through amaximum of three ESTeems to reach the destinationESTeem. An ESTeem can operate as an operating node, arepeater node, or both simultaneously for added flexibility."Private Data Communications" is provided by the use of aninterleaving technique of the modulated data, user definablecommands for unit addressing, network addressing, andsecurity lock-out of software programming. If highersecurity is required, the ESTeem is compatible withasynchronous Data Encryption Standard (DES) encryptiondevices.The ESTeem has programmable software commands toallow the user to easily configure the unit for anyapplication or mission. The ESTeem setup parameters aresaved in non-volatile memory.When you buy ESTeem products you are getting equipmentdesigned by the company that holds the United States andCanadian patent for the wireless modem. We are proud tosay that we design, develop and manufacture our productsin the United States. Each ESTeem is subjected to arigorous quality control bench test before shipping to insureour customers have out-of-the-package reliability. We alsohave a dedicated Customer Support Staff, Field EngineeringServices and Factory Training classes to make sure thatyour application problems are solved.HOW IT WORKSNow, as you can probably guess, the ESTeem is asophisticated piece of technology, however the concept iseasy to comprehend if you understand packet radio. All packet systems, whether hardwired or radio, share thesame principle of operation; data is taken from yourstandard RS-232C, RS-422, or RS-485 asynchronous portand is transmitted in "Blocks". Think of this block as an"Electronic Envelope" that we call a packet. The size of thepacket can be defined by the user from 1 to 2000 bytes ofinformation. Reducing the size of the packet allows theESTeem to operate better in high EMF noise environments,because by reducing the packet size you reducetransmission exposure time on the radio waves therebyincreasing your probability of a successful transmission. Once this packet of data is formed, it’s transmitted in a"burst," one ESTeem to another, hence the term "packetburst communications". Now, if more than one packet isrequired to send the data then the ESTeem goes into fullautomatic mode and transmits additional packets. Before an ESTeem transmits its packet it listens to ensurethat the air waves are clear before transmitting. This listenbefore transmit scheme is called "carrier sensed multipleaccess," or CSMA. When a "packet" has been transmitted, every modem inradio range on the same frequency hears it. To design amodem to communicate with a network of modems it has tobe "address specific" so only the modem you want to talk toaccepts your information. It's like yelling into a crowd of255 people but you want only the person’s name you calledto acknowledge (ACK). Well, very simply, that's how theESTeem works. Once the address you're calling receivesyour packet, it's checked for accuracy. Accuracy is probably the single most important part of anycommunication device. The ESTeem uses Forward ErrorCorrection (FEC) and a 16 bit Cyclic Redundancy Check(CRC) which is a very sophisticated method of checking thedata integrity of the packet once its been received. TheCRC insures data integrity greater than one part in onehundred million. Once the CRC is completed on thereceived packet, the data is outputted to the user and apositive acknowledgment (ACK) is transmitted back to thesender. It's safe to assume that the data you receive is good data oryou get nothing at all using the CRC technique. If no ACKis returned after a given delay, the sender assumes thepacket was not received and "retries" the transmission. The
CHAPTER 7THEORY OF OPERATIONRevised: 11 Feb 99 7-3 number of retries are user definable from 1 to 255, allowingthe unit to automatically retry sending the packet. SPECTRUM UTILIZATIONThe ESTeem uses a "listen before transmit" or CarrierSensed Multiple Access (CSMA) scheme. This means onlyone unit in a network is allowed to transmit at a time. Byfixing each user's communication window and allowing thecomputer in the ESTeem to be the Air Traffic Controller,many individual users can share one frequency. TheESTeem firmware can support up to 255 ESTeems on asingle channel or frequency. The CSMA technique is a very efficient way to manageyour network of ESTeems and prevent communicationbottlenecks. In addition, an anti-collision software schemeis used to recover data if two or more units transmit atexactly the same time. When this feature is added thetechnical term for this technique is now called CSMA-CD(collision detection). PACKET PROTOCOLSBy using CSMA no polling station or token is required inthe ESTeem network. When an ESTeem has information tosend it will check to see if the channel is clear beforetransmitting its packet and await an (ACK). The ESTeem isa Master/Master system, meaning any ESTeem cancommunicate with any other ESTeem.FLOW CONTROLThe ESTeem supports hardware and software flow control,which allow different devices on the network tocommunicate at different baud rates. In addition to flowcontrol the ESTeem also has a 4000 byte data buffer onboth the receive and transmit buffers in the unit.DATA PRIVACYData privacy in the ESTeem is provided by three levels ofdata encoding in the firmware and by the user being able todefine over four security and communications parameters(Unit Address, Network ID, and Operating Frequency) thatallow communications access to the modem giving over 100million combinations. If higher security is required, theunits are compatible with asynchronous Data EncryptionStandard (DES) encryption peripherals.EFFECTIVE BAUD RATEThe maximum input baud rate to the modem is 57,600baud, asynchronous, full duplex, but this is misleading sincethe ESTeems actually communicate to each other simplexover the RF link, at 85.5kbps, 171kbps, 344kbps or688kbps selectable by the user. The effective baud rate is afunction of the above plus the packet length variable in theESTeem (definable from 1 to 2000 bytes. The effectivebaud rate will degrade as the pack length variable isreduced.

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