A Free-Collision MAC Proposal for Networks

Transcription

1 12th Brazlan Workshop on Real-Tme and Embedded Systems 89 A Free-Collson MAC Proposal for Networks Omar Alment 1,2, Gullermo Fredrch 1, Gullermo Reggan 1 1 SITIC Group Unversdad Tecnológca Naconal FRBB Bahía Blanca Argentna 2 IIIE - Unversdad Naconal del Sur DIEC Bahía Blanca Argentna ealmen@crba.edu.ar, {gfred,ghreggan}@frbb.utn.edu.ar Abstract. Wreless technologes are a good choce for work n ndustral envronments, where t s necessary to nterconnect moble systems or t wants to avod sensors and controllers wrng n plant. However, these technologes present relablty and tmng problems nherent n the rado channels, mechansms for medum access, etc. The standard e provdes two alternatves for medum access (EDCA and HCCA) by dfferentatng traffc nto four Access Categores (ACs). Ths paper proposes a mechansm for controllng the medum access, so-called WRTMAC, developed from the EDCA scheme of standard e. The handlng of the arbtraton nter frame spaces (AIFS) has been modfed n order to make determnstc the medum access, even n terms of hgh traffc next to the saturaton of the system. 1. Introducton Wreless technologes have become a very attractve opton for ndustral and factory envronments. We can appont the reducton of tme and cost of nstallaton and the mantenance of cablng ndustral and ther changes. The damage on the wrngs and connectors due to the aggressve envronments of certan types of ndustres s another reason. The applcatons of ndustral control that nvolve some knd of moble systems, n whch data communcatons must meet requrements of real tme and relablty, can beneft from the wreless nterconnecton [Wllg A., Matheus K. and Wolsz A., 2005]. However, t s necessary to consder features of the wreless medum, such as the typcal weaknesses of a rado frequency channel (RF), the moblty of some statons, the uncertanty n the tme of physcal medum access of some protocols, etc. Despte other types of exstng wreless nterconnectons, we are nterested on wreless local area networks (WLAN) based on IEEE standard. The Medum Access Control protocol (MAC) s decsve n the performance of the network [Vanhatupa T., 2008]. The MAC mechansm can operate n two ways: Pont Coordnaton Functon (PCF) and Dstrbuted Coordnaton Functon (DCF). PCF, also called free of contenton, uses an Access Pont (AP) as a network coordnator. In DCF, wthout centralzed control, the nodes compete for the access to the physcal medum. In spte of the dfferences, both modes use the Carrer Sense Multple Access wth Collson-Avodance (CSMA/CA) mechansm to obtan the access to the medum and transmt. One of the weaknesses of the MAC protocol s that t not support dfferentated qualty of servce (QoS) for dfferent types of traffc. For that reason,

2 90 Proceedngs e [IEEE Std e; Part 11, 2005] was developed to support two QoS mechansms: Enhanced Dstrbuted Coordnaton Access (EDCA) and Hybrd Coordnaton Functon Controlled Channel (HCCA). The EDCA scheme extends DCF, as t s known n the orgnal standard [IEEE Std ; Part 11, 2007], dfferentatng four prortzed Access Categores (AC) [Vttoro S. and Lo Bello L., 2007]. In spte of EDCA mproves the throughput and the response tme wth regard to DCF, the reduced amount of AC lmts the dfferentaton of traffc wth temporary restrctons [Ferré P., Doufex A., Nx A. and Bull D., 2004]. Ths paper proposes changes at the MAC level, based on the standard e, n order to adequate the EDCA mechansm for real-tme ndustral applcatons, generatng a number of ACs as devces and/or messages are there n the network [Perera da Slva M. and Becker Westphall C., 2005], makng determnstc the tme to access the medum. Ths mechansm has been called WRTMAC: Wreless Real- Tme Medum Access Control. 2. DCF and EDCA A wreless local area network (WLAN) type s a broadcast network, characterzed by the uncertanty n the medum access tme. DCF s a dstrbuted medum access control scheme, based on the CSMA/CA mechansm. A staton must sense the medum before startng a transmsson; f the medum remans dle durng a random tme, the staton transmts, otherwse ts transmsson must be postponed untl the end of the current one. DCF dstngushes two technques: the smplest, the staton transmts the frame when t s obtaned the access to the medum, and wats the acknowledge (ACK) from the recever; the other uses an exchange of RTS/CTS frames between sender and recever, pror to the dspatch of the data, n order to avod collsons due to the hdden nodes [Bensaou B., Wang Yu and Ch Chung Ko, 2000]. Ths work s based on the frst one. A collson s dffcult to detect n a wreless medum, so a gven amount of tme named nter-frame space (IFS) s used to control the access to the channel. When sensng ndcates that the medum s free, a staton must wat a tme named dstrbuted nter-frame space (DIFS) after the end of the prevous transmsson (Fgure 1). Then there s a watng tme, named backoff wndow (BW), whose duraton s a random quantty of slots tme (ST), between a mnmum of 0 and a maxmum equal to CW 1. CW s the value of the contenton wndow, whch begns wth a mnmum value CW mn, and doubles ths value after each collson up to a maxmum CW max. When the BW tmer reaches zero and f the medum remans free, the staton begns ts transmsson. If the medum becomes busy before BW expres, ths tmer s frozen untl the channel remans dle durng a DIFS tme. If BW expres n two or more statons at the same tme, there wll be a collson. After a frame was receved satsfactory, the recever staton must wat a tme short nter-frame space (SIFS) to send an ACK (Fgure 1). If the transmtter staton ddn t receve the ACK after a SIFS tme from the end of ts message, nterprets that a collson has occurred and wll be necessary retransmt. The collsons possblty of ths mechansm causes uncertanty about the tme needed to realze a transmsson.

3 12th Brazlan Workshop on Real-Tme and Embedded Systems 91 Fgure DCF Tmng The e standard ntroduces the EDCA mode (Fgure 2), whch proposed a dfferentated mechansm of QoS wth four ACs: AC_BK (Background) for the lowest prorty level (1-2), AC_BE (Best Effort) for the followng levels (0-3), AC_VI (Vdeo) for the prortes 4 and 5 and AC_VO (Voce) for the hghest (6-7). Accordng to ts prorty, a frame wll be located n one of those four categores. Each AC uses specfc values of arbtraton IFS (AIFS), CWmín and CWmáx [Wllg A., (2008)]. The dfference between DCF and EDCA s that, the frst does not dstngush types of traffc and, when the medum s free, all statons must wat for the same DIFS before startng ts BW tmer to contend for the medum access, usng all the same CW. However, each type of traffc n EDCA, parameterzed for ts AC, wll start ts BW tmer after sensng the medum dle for a whle AIFS. The AIFS value depends on the AC of the message; therefore an AC of hgher prorty wll have a lower value, havng more probablty to access the channel. Due to frames wth the same AC can coexst n several nodes, collsons can occur and they are resolved n a smlar way to DCF Fgure 2. EDCA Tmng The goal of WRTMAC s to develop a collson free MAC method that guarantee the response tme, defned as the tme measured from transmsson request untl the ACK recepton. The basc proposal establshes one AC for each type of message, assgnng a gven AIFS to each one. The watng tme pror to a transmsson s equal to DIFS plus the AIFS accordng to the type of message. 3. WRTMAC: a Real Tme varant to WLAN Basc scheme The objectve that has been establshed for WRTMAC s a real tme determnstc behavor. So, the maxmum latency to transmt a frame must be ensured and must be necessary to remove those probablstc elements own of DCF and EDCA. EDCA has been the startng pont for defnng WRTMAC, ntroducng varants to acheve the target. In that sense, have been establshed the followng patterns of operaton:

4 92 Proceedngs Each type of frame has assgned a certan prorty, dfferent from any other, n a smlar way to the bus CAN [Bosch Robert GmbH, 1991]. The prorty s ndcated by a numercal value from zero (maxmum prorty) and a certan postve number N for the mnmum. The total amount of prortes should be establshed by the amount of types of messages that should be handled n the context of a partcular applcaton. If two or more smultaneous requrements arse, always the frame of the hghest prorty must be transmtted. The logc for controllng the access to the channel has been desgned to avod the occurrence of collsons. However, collsons can occur after ntervals of prolonged nactvty, due to the drft between the local clocks of the nodes. The resoluton of these collsons should be done n a bounded and predctable tme. Also, t has been desgned a smple strategy to allow a free-collson operaton. Fgure 3 presents the bascs of WRTMAC. When a staton has a frame to send, t must wat untl the medum becomes dle. After a whle called Real-Tme Inter-Frame Space (RIFS), f the medum s stll free, the transmsson starts. If durng the wat, the channel becomes busy, the process wll be stopped and should be restarted when the medum becomes free. In WRTMAC each message has ts exclusve RIFS value. Its duraton s nversely proportonal to the prorty t represents. RIFS s called the watng tme (backoff) for the prorty message. Fgure 3. WRTMAC: Basc scheme WRTMAC determnsm s based on that, each message uses a RIFS arbtraton tme, fxed and dfferent from others. Ths tends to avod the occurrence of collsons, whle ensurng that, n case of contenton, the wnner wll be the hgher prorty message. Fgure 4 shows the order of three frames, wth prortes 2, 3 and 4, contendng for the medum access. The three nodes begn the wat, but as RIFS 2 s the shortest, Frame 3 and Frame 4 attempts must be aborted. They are restarted after the end of Frame 2 cycle. RIFS duraton s calculated based on the values of DIFS and ST. They are establshed by the selected physcal layer (PHY) of the standard, accordng to (1): RIFS = DIFS + ST (1) Table 1 shows the values of SIFS, DIFS and ST for dfferent varants of physcal layer (PHY):

5 12th Brazlan Workshop on Real-Tme and Embedded Systems 93 Fgure 4. Transmsson order accordng to prorty of messages Table : PHY varants PHY Frec. (GHz) Rate (Mbps) SIFS (µs) DIFS (µs) ST (µs) b g a The transmsson cycle of prorty, composed by RIFS, the transmsson tme of the frame (t FRAME ), SIFS and the transmsson tme of the ACK frame (T ACK ), s called C (2): C = RIFS + t + SIFS + t (2) FRAME The ACK nstructs the MAC entty of the transmtter that the frame sent, reached ts destnaton. In general, f ACK s not receved a retransmsson s not performed, but notfes the upper layers that the transfer has faled (or at least that there s no certanty that has been successful). The decson regardng what actons must be taken s left to the uppers layers; they know the logc and tmng constrants of the applcaton. WRTMAC s only responsble for provdng determnstc communcaton servce on the maxmum latency. Only t would be performed a unque retransmsson n case of collson, wthout affectng the determnstc behavor, as explaned n 3.4. One can see that WRTMAC allows mplement a Real-Tme scheme of Rate Monotoncs (RMS) type [Lu and Layland, 1973], assgnng prortes to messages n reverse order of ther perods. Knowng t FRAME for all messages of a certan real-tme system, and assumng that they are perodc, one can set the mnmum possble perod between transmsson requrements (T ) for a gven message m, n terms of all other messages m j of hgher prorty than m (where j<). Adaptng the classc formula used to analyze the schedulablty of a set of real-tme perodc tasks on a processor [Lehoczky J., L. Sha, and Y. Dng, 1989], the mnmum perod possble for a message of prorty, s (3): T T C j + C j< T j Where: T, T j, C, C j : Perod and transmsson cycle of prorty messages and j. Fgure 4 shows messages wth perods T 2 T 3 T 4. (3) s vald when the network s workng wth hgh traffc,.e. when there s always at least one transfer request pendng, awatng the end of the current transmsson. However, dependng on ACK (3)

6 94 Proceedngs the total amount of messages and ther perods, there may be long ntervals of slence. Ths requres a specal analyss, because the completon of a transfer s the event that reset the counter on each node and allows mantanng synchronsm. Exceedng the tme correspondng to RIFS longer wthout havng made any transmsson, all nodes must restart ther counters. Each node must mantan the tmng of actvty n the medum, even when there s no requrement, because even a moment pror to the expraton of ts RIFS s able to receve a request and transmt t n the current cycle Operaton on non-saturaton condtons When the network has extended moments of slence, all nodes must proceed as follows: wat a whle correspondng to the duraton of the message of the lowest prorty plus a ST, and then restart ther counters. In fact, as one takes nto account the tme ST elapsed after RIFS N, the counters RIFS are not restarted from zero, but wth an ntal value ST (equvalent to consder that the tmers are reset at the expry of RIFS N ). Fgure 5. WRTMAC: counters n non-saturaton condtons Fgure 5 shows the transmsson of a frame after cycles of nactvty. In real operatng condtons could succeed longer ntervals of slence, mantanng the same concept to restart the counters after each RIFSN perod of nactvty n the medum. In case of more extended nactvty, the drft between the locals clocks of each node can lead to a collson condton, as s dscussed n 3.4. Another stuaton to consder s prorty nverson, dscussed n 3.3. Both, the collson and the prorty nverson ntroduce delays, whch must be added to (3) to generalze ts expresson Prorty Inverson Prorty nverson s called the stuaton that occurs when the transmsson of a frame must wat untl the completon of a lower prorty. Fgure 6 shows the almost smultaneous transmsson request from Frame 2 and Frame 3, however, as the requrement of Frame 2 released an nstant after the end RIFS 2, ts transmsson must wat for the next cycle.

7 12th Brazlan Workshop on Real-Tme and Embedded Systems 95 Fgure 6. Prorty nverson: Frame 3 s transmtted before Frame 2 As the requrement Frame 3 came before the expry of RIFS3, t s transmtted and completes ts cycle C 3. Thus Frame 2 was blocked for a whle B 2, whose maxmum value s B 2 = C 3 RIFS 2. If all frames are consdered of lower prorty than 2, the maxmum block tme of Frame 2 s: B = máx C j ) RIFS j 2 (4) 2 ( 2 > In general, for any frame of prorty, tme blockng by prorty nverson s: B = máx( C ) RIFS j (5) j > Another deadlock occurs when a frame must wat untl the next cycle to be transmtted, because ts request arrved a moment after the expraton of ts RIFS, havng requrements that cause a prorty nverson (Fgure 7). Fgure 7. Deadlock of a frame of prorty 2 to the end of RIFS N In ths case, the deadlock tme s B 2 = RIFS N RIFS 2. As s lower than the prorty nverson blockng, remans vald (5). Based on these consderatons, (3) s extended as follows (6): T T C j + C + j< Tj In (6), t sn t takng nto account the occurrence of collsons, whch s dscussed n 3.4. B (6)

8 96 Proceedngs 3.4 Collson by drft of local clocks When takes place an dle nterval of duraton greater than or equal to RIFS N, all the nodes must restart ther RIFS tmers wth a perodcty RIFS N. In case of almost smultaneous requests of consecutve prortes, due to the asynchrony that could exst between clocks of dfferent nodes, t could have cancelled (or reduced almost totally) the dfference of one ST between adjacent prorty levels, gvng rse to a collson (Fgure 8). Fgure 8. Collson by drft of local clocks The way n whch each node detects a collson depends on the tme gap between the ends of the collded transmssons. Nevertheless, after detected a collson and resynchronzed the RIFS tmers, each node that has been nvolved n the collson, restarts the process to try a new transmsson. Ths s the unque stuaton for whch a retransmsson s allowed,.e., after a collson followng a slence longer than RIFS N. Fgure 9. Collsons Type 1 (left) and Type 2 (rght) Fgure 9 shows both types of collsons. In Type 1, Frame B fnalzes an amount of tme after Frame A, enough to enable node A to sense carrer durng the SIFS after ts transmsson. Node A restarts ts RIFS tmer wth the end of transmsson of B. However, B detects the collson by the lack of ACK, and therefore t assumes that the new begnnng of ts RIFS tmer s the end of ts own transmsson. The remanng nodes that have not been nvolved wth the collson, restart ther tmers when the medum becomes dle,. e. after the end of Frame B. In ths case, the end of transmsson of Frame B s the event that allows the clock synchronzaton between all the nodes. In Type 2, the nvolved frames fnalze wth a tny dfference of tme, whch not allows the detecton of the later endng frame by the other node. Then, all the nodes synchronze wth the last end of transmsson (Frame A n the example), wth the excepton of the node that ends n frst place ts collded transmsson (Frame B n the

9 12th Brazlan Workshop on Real-Tme and Embedded Systems 97 example). But ths gap s smaller than that t s needed to cause a new collson, because t dd not allow the sensng of the later end-of-frame by the frst fnshng node. After collson detecton and clock synchronzaton, the pendng transmssons wll be dspatched accordng to ther RIFS, followng the rules of WRTMAC. 3.5 Worst-case delay due to a collson Fgure 10 shows that, after a long dle perod a request arrves for the sendng of Frame, but t comes mmedately after the expraton of RIFS. Then, the node must wat untl the next cycle to try agan. Snce there are no pendng requests of prortes lower than, all the RIFS the tmers wll be restarted after the expraton of RIFS N. Untl that moment, the delay accumulated by Frame s: D = RIFS N RIFS (7) Durng the next cycle starts the transmsson of Frame, but a collson occurs. Fgure 10 shows the worst-case collson delay for Frame, because s nvolved the longest frame (the effect would be the same f Frame s the longest one). Fgure 10. Worst-case delay due to a collson After the collson, a new cycle starts. If Frame contnues beng the hghest prorty among those that are awatng transmsson, the delay due to the collson s: Col = D + RIFS + max(t FRAME ) (8) Replacng D from (7), and as the resultng delay does not depend on frame prorty, t s desgnated genercally as Col: Col = RIFS N + max(t FRAME ) (9) Now t should be modfed the formula (6) to nclude the delay due to a collson. However, the prorty nverson blockng, B, and the delay due to a collson, Col, are mutually exclusve. It could happen one or the other but not both. Thus, the mnmum possble perod between requests of the th-message s: T T C j + C + j< Tj max( B, Col) The formula (10) allows the schedulablty analyss for a partcular system, based on the amount of messages, ther duraton and perodcty. (10)

10 98 Proceedngs Although the Rate Monotonc schedulng requres that prortes must be assgned n nverted order wth respect to the request perods, f some (or eventually all) messages have the same perod, t must be assgned dfferent RIFS to each one, to allow the medum access arbtraton Collson-Free operaton Gven that dle ntervals greater than RIFS N can lead to the occurrence of collsons, one way to avod them could be to avod the occurrence of such long ntervals wthout transmssons. A smple strategy could be that, the node desgnated for the transmsson of the lowest prorty frames, always should perform a transmsson. Therefore, f at RIFS N tmer expraton t does not have a pendng request for the transmsson of a message, t must send a dummy frame, whose sole purpose s to occupy the medum, allowng the synchronzaton of all nodes wth the end of ths transmsson. Usng ths smple strategy, becomes vald the formula (6) to establsh the mnmum possble perod for a gven message. 4. Performance evaluaton Gven the ntal motvaton that led to the development of WRTMAC, ts applcaton n ndustral control systems, usually based on small perodcal messages, they was evaluated the maxmum number of messages that could be drven by a network of ths type and / or the shortest feasble perod for each one. Also, the utlzaton factor was calculated, defned t as the fracton of the total tme that the medum s used to transport data. It has been consdered that the network s used for the transmsson of a gven set of messages of equal sze and perod. It has been selected the b physcal layer at 11 Mbps, wth long preamble (192 mcroseconds); message payload of 50, 100 and 1500 bytes (plus 36 bytes of header), and 14 bytes of ACK. Also, t has been evaluated two optons: one of ths based on Formula (10) wth collsons and the other based on Formula (6) collson-free. The utlzaton factor has been calculated for the freecollson mode. The results are showed n Table 2. It s observable that there are no sgnfcant dfference between payloads of 50 and 100 bytes (typcal szes for supervsory and control systems). Also, there s a small mprovement by applyng the free-collson model. Hence, t could be smplfed the MAC mechansm, elmnatng the handlng of collsons and ts assocated retransmssons. Moreover, t can be seen that for long messages (1500 bytes) the mnmum perod does not ncrease proportonately (eg. 77 ms for 64 messages of 100 bytes each; 143 ms for 64 messages of 1500 bytes each). It s allowed to estmate that ths network could be used wth a mxng of short messages (montorng and control) and long messages (data, mages, etc.), wth a small mpact on the real-tme performance.

11 12th Brazlan Workshop on Real-Tme and Embedded Systems 99 Table 2. Mnmum perod and utlzaton factor, based on quantty and sze of messages Payload (bytes) N of messages Mnmum Perod (ms) Utlzaton Factor (%) Wth collsons Collson-free % % % % % % % % % % % % % % % As can be expected, the utlzaton factor ncreases wth the payload sze, because the nfluence of the overhead due to headers, ACK, etc. s reduced. However, the utlzaton factor decreases sgnfcantly as the total number of messages ncreases. Ths s due to the growng overhead assocated wth the dfferent RIFSs needed to arbtrate the medum access. Based on ths stuaton, a way to mprove the network performance could be to group several messages, sharng the same RIFS. However, as RIFS s used to arbtrate the medum access, the RIFS sharng could be possble only when there are more messages than nodes. In these cases, as t s not possble a collson between messages orgnated n the same node, t could be possble that several messages share the same RIFS value. Ths wll be analyzed n a future work. 5. Conclusons WRTMAC (Wreless Real Tme Medum Access Control) s a proposal that mplements a MAC based on EDCA concepts of the standard e, n order to acheve a mechansm for dstrbuted wreless medum access, allowng predctable access tme to the medum of the devces n the network. Changes are proposed to make sutable the EDCA mechansm, n order to generate as Categores Access (AC) as devces and/or messages are n the network. Ths wll be determnstc the access tme to the medum. Ths proposal could brng the EDCA mechansm for real-tme ndustral applcatons. It was shown that WRTMAC allows the mplementaton of a Rate Monotonc Schedulng (RMS) scheme, snce t s possble to know the mnmum feasble perod of a message, accordng to all messages. The proposal also shows that, despte collsons can occur, the collded frames wll be retransmtted, and the collson wll be solved n a bounded tme. Moreover, WRTMAC can operate n a free-collson mode.

12 100 Proceedngs Furthermore, when evaluatng the performance of WRTMAC over traffc patterns typcal of ndustral applcaton networkng, t was noted that ths mechansm provdes an adequate performance wthout tmes uncertantes. In addton t was shown that the network could be used by combnng short and long messages (for montorng, control and general data, mages, etc.), wthout greatly real-tme performance degradaton. As t was analyzed, the ncrease n the number of RIFS dmnshes the utlzaton factor of the network, due to the RIFS overhead. Hence, future works wll be orented to share the same RIFS between several messages orgnated n the same node. The goal wll be to develop a methodology to establsh the mnmum number of RIFS needed for a gven set of messages. References Wllg A., Matheus K. and Wolsz A. (2005), Wreless Technology n Industral Networks, Proceedngs of the IEEE, Vol. 93, No. 6 (June), pp Vanhatupa T. (2008), Desgn of a Performance Management Model for Wreless Local Area Networks, PhD Thess. IEEE Std e; Part 11 (2005): Wreless LAN Medum Access Control (MAC) and Physcal Layer (PHY) Specfcatons and Amendment 8: Medum Access Control (MAC) Qualty of Servce Enhancements. IEEE Std ; Part 11 (2007): Wreless LAN Medum Access Control (MAC) and Physcal Layer (PHY) Specfcatons. Vttoro S. and Lo Bello L. (2007), An Approach to Enhance the QoS Support to Real- Tme Traffc on IEEE e Networks, 6 th Intl Workshop On Real Tme (RTN07) Psa Italy. Ferré P., Doufex A., Nx A. and Bull D. (2004), Throughput Analyss of IEEE and IEEE e MAC, WCNC, IEEE Communcatons Socety. Perera da Slva M. and Becker Westphall C. (2005), Performance Analyss and Servce Dfferentaton n the MAC SubLayer of IEEE e Ad Hoc Networks, Proceedngs of the Advanced Industral Conference on Telecommuncatons, IEEE. Bensaou B., Wang Yu and Ch Chung Ko (2000), Far Medum Access n based Wreless Ad-Hoc Networks, IEEE/ACM The frst Annual Workshop on Mobl Ad hoc Networkng e Computng (MobHoc 00), Boston, EUA. Wllg A.(2008), Recent and Emergng Topcs n Wreless Industral Communcatons: A Selecton, IEEE Transactons On Industral Informatcs, V4 Nº 2. Robert Bosch GmbH (1991), CAN Specfcaton Lu and Layland (1973), Schedulng algorthms for multprogrammng n a hard realtme envronment, Journal of the ACM, Vol.20 Nº 1, pp Lehoczky J., L. Sha, and Y. Dng (1989), The rate monotonc schedulng algorthm: Exact characterzaton and average case behavor. Proc. IEEE Real-Tme Systems Symposum, pp