A Network Mngement System for Power-Line Communictions nd its Verifiction y Simultion Mrkus Seeck, Gerd Bumiller GmH Unterschluerscher-Huptstr. 10, D-90613 Großhersdorf, Germny Phone: +49 9105 9960-51, Fx: +49 9105 9960-19, Emil: Gerd.Bumiller@id-de.com Keywords: OFDM, TDMA, Repeters, Automtic Logon, Adptive Network Routing Astrct The network mngement system (NMS) developed y the compny GmH is used for the co-ordintion of communiction in networks where ll users shre the sme chnnel nd connection-oriented services re not required. Typicl pplictions of the NMS re remote metering, vlue-dded services for utility compnies, intelligent power mngement, security services nd home utomtion. The NMS is sed on mster-slve concept using repeters to increse the rnge of the network where every slve is possile repeter. Due to utomtic logon t ny time, the numer of slves in the network is dynmic nd hence, self-configurtion is provided. For dpttion to vrile chnnel conditions, dynmic routing is pplied. For purposes of evlution nd prmeteristion simultion model hs een developed which is sed on chnnel trnsfer mtrix nd two different models re pplied for this. Introduction For digitl dt trnsmissions over power-lines orthogonl frequency division multiplexing (OFDM) in comintion with FEC seems to e n pproprite modultion scheme, which provides optimum performnce if the whole chnnel ndwidth is llocted to only one communiction link, see e.g. [1]. Thus, if severl modems mking up network hve to communicte on the sme chnnel, only one cn trnsmit t the sme time, which requires time division multiple ccess (TDMA). Therefore, it is necessry to provide n intelligent network mngement which on the one hnd yields high communiction rte nd t the sme time tries to void collisions on the chnnel in order to mke n efficient use of chnnel cpcity. For purposes of verifiction of protocol nd testing different methods, trnsmission chnnel is required which cn e completely nlysed nd lso reproduced. In cse of field tests oth requirements cnnot e stisfied nd, moreover, lot of time nd effort is necessry, especilly if networks with lrge numer of slves re considered. Thus, it is hrdly possile to repet mesurements for different versions of protocol or different sets of prmeters. Therefore, simultion model ws designed. Furthermore, the NMS is lredy in use in our DLC-2 modem which ws introduced t ISPLC 99 in the pper Integrted PLC modem sed on OFDM [2]. The modem uses OFDM together with 64-stte convolutionl coder nd Viteri decoder on the physicl lyer. Due to differentil coding loding lgorithm is not required nd therefore it is especilly pproprite for point-to-multipoint trnsmissions. In the DLC-2 modem the lyers of the NMS s well s the ppliction lyer re implemented in micro controller unit (MCU) nd the physicl lyer is implemented in digitl signl processor (DSP). In the current version the protocol opertes with lock sizes of 48 to 168 ytes nd possile dt rtes up to 100kps.
Concept of the NMS The network mngement system is sed on mster-slve concept where ll modems shre the sme frequency nd nd thus TDMA scheme is pplied. The use of different frequency nds enles the coexistence of severl logicl networks on the sme physicl network. In order to expnd the physicl rnge of the network, two repeter levels re used nd every slve cn lso e repeter. There is no direct communiction etween two slves nd hence, the network structure represents virtul str topology, which is depicted in Figure 1. MASTER Power-Line Level 1 SLAVE 1 SLAVE 2 SLAVE 3 Level 2 SLAVE 4 SLAVE 5 SLAVE 6 Level 3 SLAVE 7 SLAVE 8 Figure 1: Virtul str topology of the NMS Slves re only llowed to trnsmit on demnd of the mster. As OFDM performs prllel trnsmission of its, decoding cn only strt fter the reception of complete lock. Therefore, responding is not possile within the next TDMA slot ut insted n interlef is specified fter which the slve is supposed to respond. Thus, system delys cused y run times of oth the physicl lyer nd the lyers of the network mngement re tken into ccount which is outlined in Figure 2. This figure shows n exmple of the mster s (M) communiction with slves (S) of levels 1-3 which mens tht 0, 1 or 2 repeters (R) re used, respectively. Telegrms lelled with their respective pth re depicted over the time xis which is normlised to the length of TDMA slot. As cn e seen, every slve responds its requests fter n interlef of 3 slots nd thus, the mster does not hve to wit for response of slve ut cn use the intermedite time for ddressing other slves so tht n effective use of chnnel cpcity is provided. processing NMS telegrm from mster repetition y repeter Demod Mod response from slve 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Figure 2: Exmple of scheduling of telegrms to slves of levels 1-3 t Dt trnsmission on the power-line tkes plce y mens of telegrms ech consisting of lock of OFDM signls. There re two types of telegrms: the user PDU (protocol dt unit) crrying dt for the ppliction lyer nd the service PDU contining informtion for network mngement purposes. Ech PDU is protected y
32-it CRC nd hence, the proility tht wrong PDU is considered to e correct is only 2-32 2.33 10-10. By use of sequence counter of the respective modems, every modem cn detect multiple receptions of the sme PDU or missing PDU. In cse of point-to-point PDUs confirmtion nd retrnsmissions re supported. In cse of rodcsts, severl repetitions of the sme telegrm y the mster s well s y selected repeters shll provide high coverge of slves. The numer of slves in the network is dynmic nd logon s well s logoff of the slves is performed utomticlly y the mster modem. Every modem in the network cn e identified y its unique seril numer nd during logon it is dditionlly ssigned unique network ddress y the mster which is vlid for the period of its logon. If no ppliction requires dt trnsmissions, the mster constntly updtes its network sttistics (logged-on modems, routing tles) y cyclic poll of ll slves. In the response PDU to those sttus polls the slve provides the mster with informtion out its current sttus, possile ville dt nd sttisticl informtion for routing purposes. The sttus poll period, i.e. the numer of cycles for polling ll slves, depends on the numer of slves in the network nd is used s reference for the computtion of network sttistics. Automtic Routing The use of repeters requires the determintion of the est routing pths, which is done y the network mngement protocol in the mster modem. For this purpose, every slve keeps tle with its fvourite repeter slves nd in response to sttus polls forwrds the est five entries to the mster. Bsed on the sttus poll period nd the counting of sent nd received telegrms, every slve cn compute its own frequency of trnsmission - tht it includes in its service PDUs - s well s the frequency of reception y ny other slve. In order to prefer short routing pths, weighting fctors re pplied to the counting depending on the numer of repeters used. In ddition to those reltive numers, the solute numer of received telegrms is lso tken into ccount wherey slves re preferred tht lredy work s repeters. This leds to the forming of repeter nodes which is fvourle in cse of rodcsts where s mny slves s possile shll e reched y using the smllest numer of repeters possile. By use of these vlues the repeter qulity of slve is determined s the reception proility from tht slve. Furthermore, the mster keeps two response quots for every slve representing the success of the mster s lst ttempts to ddress this slve directly nd over one repeter, respectively. As the second vlue does not tke into ccount which repeters were used, it my temporrily devite from the current sitution. In contrst to the reception proilities, the response quots do not ssume symmetric communiction chnnels, nd hence re closer to relity. In cse of telegrms to single slve, the mster hs to compre nd test eleven different pths including the direct pth, 5 pths using one repeter ech nd nother 5 pths using two repeters. For this purpose, every pth is judged y n evlution figure, computed y mens of the respective response quot, the reception proility of possily pplied repeter nd n dditionl weighting fctor used to prefer short pths. In cse of two repeters R1 nd R2, the first repeter R1 is still unknown nd hs to e determined nlogously to finding the est repeter for single-repeter-pth to trget slve R2. The d Hondt lgorithm is then pplied for the sequentil determintion of the currently est pth which chnges with the sttistics, which re updted y the mster in every cycle. This lgorithm is stopped in cse of success or if the mximum numer of ttempts is exceeded. Depending on rndom numer, the direct pth is chosen sometimes in the first ttempt, even if it is not the est one, to provide the chnce tht chnges in network conditions cn e noticed t ll. In cse of rodcsts multiple repetitions of the sme telegrm y the mster s well s y certin repeter slves re supposed to provide good coverge of slves. In this context, the question rises which repeters shll e used in order to mke do with low numer of repetitions. Using the sttisticl informtion in its routing tle the mster is le to estimte the proility of every slve tht it hs not yet received the rodcst. This proility is updted fter every telegrm sent. Thus, the mster chooses the respective pth such tht the proility of the slve with the currently highest proility is minimised wherey the proilities of the other slves re lso decresed. A few telegrms re lso trnsmitted using the direct pth. This procedure is repeted until either the proilities of ll slves hve reched minimum vlue or mximum numer of repetitions is exceeded.
Auto Configurtion of the Network The numer of slves in the network s well s the network topology re dynmic, nd for this reson the network cnnot e configured from the eginning, ut insted, uto-configurtion is required. There re three sttes for slves which re depicted in Figure 3. When new slve modem is plugged to the physicl network, it is in stte frequency scn. In this stte it scns ll frequency nds nd tries to find n pproprite mster y listening to ll telegrms in the respective nd nd evluting them to get informtion out the respective network. Bsed on different criteri, like network qulity nd equl ssignment of slves to the different networks, it finlly mkes decision for certin frequency nd enters stte logon. In this stte it tries to log on to the network. If this is not possile within fixed timeout it returns to stte frequency scn. In cse of success it is ssigned unique network ddress y the mster which is vlid s long s the slve is logged in, nd proceeds to stte logged on. For purposes of uto logon the mster regulrly sends uto logon requests. These re sent s rodcst PDUs using selected repeters. In order to void collisions on the chnnel s fr s possile, the slves which wnt to log on ech respond fter rndom dely within fixed timeout. If the mster detects collisions on the chnnel, nywy, he cn reduce the numer of slves which re llowed to nswer the request y specifying few its in their seril numer nd thus reduce the proility of collisions. As the rndom delys cn only e multiples of the slot intervl the system cn e considered s slotted ALOHA system during periods of uto logon. frequency scn loginprocedure logintimeout found frequency logged- on logoutprocedure logon Figure 3: Stte digrm of slve modem Simultion For purposes of evlution nd prmeteristion of the NMS, simultion model hs een developed s shown in Figure 4. Every modem in the network is represented y dt genertor mking up the ppliction lyer, NMS nd two physicl lyer simultors for oth the trnsmitting nd the receiving prt. All prts of the simultion which my vry for the different modems, e.g. dt genertors nd physicl lyer reception simultors, re implemented such tht they cn e specified differently. The dt genertors representing the ppliction lyer hve to e dpted ccording to the suject of the current simultion. The NMS is designed for use in TDMA system where trnsmission is only llowed within fixed time slots. Therefore, the simultion does not need to replicte the physicl lyer ut cn e sed on the ssumption of slotted system with slot intervl which corresponds to the durtion of n OFDM symol. This is lso justified in cse of uto logon periods where the system cn e considered s slotted ALOHA system.
The network mngement protocol uses 32-it CRC for error detection nd throws wy erroneous telegrms. This is simulted y throwing wy telegrms rndomly with proility depending on the chnnel properties etween trnsmitter nd receiver. Since the proility tht n erroneous telegrm is considered to e correct is only 2.33 10-10 this cse does not hve to e considered in simultion. In network of N modems, these chnnel properties, which re not necessrily reciprocl, re descried y chnnel trnsfer mtrix of formt N N. As system vlues were not ville, chnnel model hd to e designed which will e descried lter. control flow dt flow simultion time dt genertor properties simultion control chnnel trnsfer mtrix dt genertor evlution physicl lyer receiving simultor NMS physicl lyer trnsmission simultor NMS memory chnnel memory Figure 4: Simultion model (grey: glol elements; white: elements existing once for every user) The line of ctions during one system period cn e descried s follows. First, the simultion control sequentilly clls the dt genertors of ll modems. If nyone wnts to send, the NMS is ctivted to process the dt nd forwrd them to the physicl lyer trnsmission simultor. Afterwrds, the physicl lyer receiving simultors of ll modems re clled sequentilly. Due to this modulr structure of the simultion nd the use of the sme interfces s in system implementtion the NMS will not recognise ny difference etween working in simultion nd working in rel system. The physicl lyer receiving simultor is clled in every cycle for every modem. It checks if collision hs occurred on the chnnel. Besides, it determines if premle ws detected ccording to the respective proility, nd sed on the length of the PDU, the it error rte nd rndom process it decides whether the PDU ws received correctly or not. Chnnel Model For the simultion chnnel trnsfer mtrix is required nd for this purpose two different models re pplied. The first one is line model where ll slves re connected to the min t equl distnces using rnch lines of equl lengths, s depicted in Figure 5. This model cn e considered s specil cse of n rrngement s it my pper in relity in cse of row of terrced houses. As it is deterministic nd regulr the respective results cn esily e checked nd therefore it is pproprite for the verifiction of the protocol.
Slve 1 Slve 2 Slve 3 Slve 4 Slve 5 Slve 6 Slve 7 Slve N-1 + Mster Figure 5: deterministic line model The second one is stochstic plne model where ll slves re distriuted eqully within plne t limited distnce to the mster, s shown in Figure 6. The network consists of 4 mins originting from the mster on the specified xes nd slves re locted t the end of rnch lines which re orthogonl to the mins. This model is more relistic nd therefore pproprite for the optimistion of prmeters. Arrngements s they normlly pper in relity cn e expected to e less regulr thn in Figure 5 ut more structured thn in Figure 6 nd therefore comintion of oth models cn e ssumed. Hence, if the NMS works well for oth models it is supposed to work in rel systems, too. Im Slve 3 Slve 2 Slve 1 Slve 4 Slve 5 Mster Re Slve 8 Slve N-1 Slve 6 Slve 7 Slve 9 Figure 6: stochstic plne model The chnnel is descried y word error rte (WER) nd synchronistion error rte (SER) which mke up the prmeters of the chnnel trnsfer mtrix nd which re derived s follows. If we ssume equl trnsmit power S s well s equl power of ckground noise N for ll modems nd constnt ttenution incresing only with the distnce d, the SNR etween ny two modems S n nd S n is only function of distnce:
SNR S d Sn, S n' = 10 log = S N 100m N Sn, Sn' d Sn, Sn' 100m The SNR hs to e trnsformed to the E /N 0 vlue which is done y pplying correction fctor Cor which depends on the current trnsfer mode nd thus lso on ndwidth efficiency. This E /N 0 vlue is further used to determine the it error rte (BER). For this purpose, simultion results were used to derive functionl dependency etween BER nd E /N 0. Bsed on the formul for uncoded trnsmission this led to the use of the error function, such tht E BER N 0 = BER ( SNR + Cor ) = 1 2 1 2 ( SNR Cor ) + erf 2β where the dditionl prmeters α nd β re used for pproximtion to existing results of physicl lyer simultions. If we ssume sttisticl independence of it errors, the WER for words of length N W cn e expressed s N ( ) W 1. WER = 1 BER This ssumption, of course, represents the worst cse, ecuse using Viteri decoder in the physicl lyer leds to urst errors nd thus to etter WER. Since pproprite simultion results re not ville for the determintion of the SER, model is pplied which is lso sed on the use of the error function. In comprison to the OFDM lock the trnsmit power of the synchronistion sequence is higher. This is due to the Crest fctor which is smller in cse of the synchronistion sequence (see [2]) nd cn e tken into ccount y dding respective gin Crest to the SNR. Furthermore, urst losses cused y impulse noise re simulted y specifying vlue SER min. Thus, the SER is pproximted y use of the error function s α SNR + Crest 1 erf + δ SER = ε SERmin if else > SER min with prmeters δ nd ε for pproximtion to results of field tests. Simultion Results As n exmple of simultion results the evlution of the uto logon procedure shll e presented. For this purpose, the plne model ws considered during simultion period of 500.000 cycles where 499 slves simultneously strted in stte logon nd tried to log on. This sitution, of course, is not the norml cse ut my pper if the mster hs suffered temporry power supply interrupt. After out 121.000 cycles 473 slves were in stte logged-on, 90 of which were directly reched y the mster, 127 y using one repeter nd 256 y using two repeters ech. The remining 26 slves were outside the receiving rnge nd therefore no logon ws possile. Tht mens, if we ssume n OFDM symol durtion of 13ms, it would hve tken only out 26 minutes to log on those 473 slves lthough most of them t lest needed one repeter nd hence, uto logon s well s utomtic routing ws required. In comprison to this, mnul configurtion of the sme network including determintion of repeters nd respective optimistion would hve required few weeks of work, we guess.
Conclusion In this contriution we hve presented network mngement system (NMS) which is sed on mster-slve concept using repeters to increse the rnge of the network where every slve cn lso e repeter. As slves re logged on utomticlly y the mster, the numer of slves in the network is dynmic nd therefore, selfconfigurtion of the network is pplied. Besides, the NMS is le to cope with chnging conditions on the communiction chnnel y use of dynmic routing. Furthermore, simultion model hs een developed where chnnel properties re descried y chnnel trnsfer mtrix nd for this purpose two different chnnel models were pplied. The simultion hs een used to verify the functionlity of the protocol nd to optimise prmeters. Severl simultion runs concerning uto logon procedure s well s utomtic routing, stility of the NMS nd other fetures hve shown promising results nd thus confirm the concept of the NMS nd demonstrte its suitility even for lrge networks. Moreover, the NMS is lredy implemented in our DLC-2 modem nd currently field tests re eing prepred. Besides, we re currently working on further implementtions on more powerful chips for 300kit/s solution for oth the CENELEC nd nd the USA-Asi mrket. References [ 1 ] Comprison nd Optimistion of Differentil Encoded Trnsmission on Fding Chnnels, L. Lmpe nd R. Fischer, University Erlngen-Nürnerg, Germny [ 2 ] Integrted PLC-Modem sed on OFDM, M. Deinzer nd M. Stöger, iad GmH, Germny, Pper presented on ISPLC 99