Robustel M1000 Pro V2 Spezifikationen PDF herunterladen (Seite 42)

RobustelGoRuggedM1000ProUserGuide

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RT_M1000Pro_UG_v01.01 27.12.2011 41/59

Confidential

4.2 GPRSGeneral

Whenusingdataservicesvia GPRS,GPRSISPsoffervarious contractoptions,especiallyregardingthepricing (basic

price,basicdatavolume,billingunit).Pleasecontacttheaccordi ngprovidersforfurtherinformation.

In general, GPRS ISPs bill every time a connection is terminated and daily at midnight (the ISP will terminate the

connectionatthistime),andallaccumulateddataareroundedtothebillinguni t.

Wethereforerecommendtoselectaratewhichoffersthesmallestpossiblebillingunit.

ManyGSM/GPRSprovidersofferso‐calledM2Mrates,whichhaveanexact1kbbillingmatrix.

Note:

Thetransmittedamountsofdatanotonlyconsistofthesumoftheapplicationuserdata.Theyareratherpackedinto

TCP/IPorUDP/IPpackets,whichalsogeneratenetworkloadsandthereforeaddtothetotalcosts.

Ingeneral,aTCP/IPheaderhas40bytes,andeachTCP/IPpacketsentwillgenerateareplyacknowledgement(ACK)

packetof 40byteswithnodatainsideofit.Ifapacketisnotacknowledged,theTCP/IPpacketcontainingthedata

will be resent. The rate of retransmission of packets depends on the reliability of the underlying network and the

configurationoftheTCPstack.

Additionally, theapplication itself may attempt to resendthe data, so evenif the TCP/IP stack discardsthe packet

duetoanetworktimeoutcondition,theapplicationitselfmaysendthedataagain,causinganewpackettoattempt

topropagateacrossthenetwork.

IngeneralaUDP/IPpacketwillhave28bytesofheaderdata,howeverUDPpacketsareonlysentonetime,andthere

isnoACK.UDPthereforehasatleasta50%advantageinoverheadonahighlyreliablenetwork.Iftheapplicationor

datadevicewillresendorre‐queryfordata,UDPcanoffersignificantsavingsintermsofnetworkefficiency.

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EDGEisonlyanextensi onofGPRS(EGPRS).Thefollowingdescriptions(exceptthedatarates)alsoapplytoEDGE.

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4.2.1 IPAddresses/Accessibility

TheIPaddressoftheGPRSterminaldeviceisdynamicallyassignedbytheproviderandistemporary,i.e.duringthe

nextdial‐inattheprovideranotherIPaddresswillbeassigned.

In many GPRS networks the IP address cannot be accessed from outside (private IP, routing enabled), as the

providers perform the addressing for the switch from GPRS networks to the “normal Internet” via a NAT table

(NetworkAddressTranslation).

This also provides a security aspect, as the GPRS device is not accessible by so called "scanners" because the IP

addresscannotbeaccessed.AsGPRSisbilledbytheamountofdata,thispreventsunwantedandcostlydatatraffic.

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Thisalsomeansthatthefollowingfunctionsmaynotbepossible:

 PingingtheGPRSdevicefromoutside

 EstablishingTCP/IPconnectionstotheGPRSdevicefromoutside

 SendingUDP/IPpacketstotheGPRSdevicefromoutside

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Allconnections (channels) must be opened starting with the GPRS device. This means that the GPRS device or the

EthernetdevicebehinditcanonlyreactasClient.

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