Delay Performance of Radio Physical Layer Technologies as ...

Delay Performance of Radio Physical Layer Technologies as Candidates for Wireless Extensions to Industrial Networks C. Koulamas, A. Lekkas, G. Papadopoulos

G. Kalivas, S. Koubias

Industrial Systems Institute (I.S.I.) University of Patras, Rio Campus 26500 Patras – GREECE

Dept. of Electrical and Computer Engineering University of Patras, Rio Campus 26500 Patras - GREECE

Abstract - The benefits of wireless extensions in industrial networks are well recognized as long as the integration of the wireless and fieldbus domains will be capable to offer the realtime and dependability quality of the current wired industrial networking solutions. This paper presents a study, from a performance point of view, for the possible integration of the most popular 3G and WLAN radio technologies with fieldbus technologies. A brief examination of a number of dominant wireless protocols against the basic wireless fieldbus requirements is presented. For the case study, Profibus is used, on which, the basic message cycle performance is calculated for each of the UMTS, 802.11b and HIPERLAN protocols, under different integration approaches.

I. INTRODUCTION The potential usage of wireless communications inside the industrial environment has well recognized benefits, being a step beyond the evolution of fieldbuses, as well as, the only way – in a number of cases – to achieve the vision of a totally networked industry. Nevertheless, a number of dependability and performance aspects, still defer the wide adoption of wireless technologies inside the real automation environment, restricting their usage to autonomous and non-critical applications, mainly in the area of remote data collection. On the other hand, fieldbus technologies have shown great acceptance and spreading, supporting numerous hard real-time industrial applications of nearly any size of complexity and gravity. In parallel with the above situation inside industry, there is a huge evolution in the area of wireless LANs and WANs, which has already produced international standards, as well as working products and support in the market. Therefore, it is a belief that the penetration of the radio technology inside the automation area can be achieved by the seamless integration of existing fieldbus technologies with ever evolving technologies to be used in LAN/WAN communications. This paper focuses on the performance aspects of such integration, comparing the most popular and promising radio technologies as potential providers of an additional physical layer to an existing fieldbus, extending its usage beyond the cable. As an indicative fieldbus, Profibus is used, in a representative example topology, where the comparison is made among the existing wired solution and the most advanced wireless technologies for 3G and WLAN communications. Section 2 presents a number of This work was supported by the IST-11316-RFieldbus project

basic requirements for a wireless fieldbus, as these were derived from a user survey [1] accomplished in the context of the IST project R-Fieldbus, funded by the EU. Section 3 summarizes the most popular wireless technologies available today and, making a first level evaluation against the basic requirements, eliminates the discussion to a reduced set of applicable protocols. Section 4 describes an indicative wired and wireless fieldbus topology, taking Profibus as a fieldbus example. Section 5 exemplifies the reasons why a full-stack adoption of a wireless protocol would be inappropriate, in terms of performance, by studying the UMTS approach, while in section 6 a delay analysis is presented, applied to the physical layer bursts of the two most popular and promising technologies, namely the DSSS of 802.11b and the OFDM of HIPERLAN2 protocols. Finally, Section 7 concludes the paper. II. WIRELESS FIELDBUS REQUIREMENTS Any successful wireless fieldbus application should conform to two kinds of requirements, which are the radio technology requirements imposed by the wireless nature of communication and the industrial applications and user requirements, imposed by the current industrial practice and fieldbus technology. The radio technology requirements focus on the inherent ability of the system to communicate satisfactorily over the radio channel. Taking into account the harsh industrial environment, these requirements are: Path loss: The path loss represents the cumulative losses between the transmitter and the receiver. Path loss is affected by distance, type of transmission configuration (line-of-sight LOS, obstructed OBS), specifics of the environment (number of walls, wall thickness) and frequency of operation. The path loss has an impact on the required sensitivity of the radio, which will permit the system to operate properly. For a typical industrial environment, which covers an area of 100m x 100m, the expected path loss according to measurements is in the order of 100dB. For this reason we set this figure as one of the basic radio requirements for the wireless-fieldbus. Delay spread: The delay spread characterizes the multipath propagation of the channel. It gives us the variance (in time) of the received rays from the average delay. The multipath immunity of a receiver is measured with the maximum delay spread the receiver can tolerate. According to measurements in industrial environments of medium size, the delay spread can have values in excess of 200ns.

This figure represents another basic requirement, which the selected technology should be able to satisfy. Range: range is defined as the maximum distance between the transmitter and the receiver within which the receiver can demodulate satisfactorily (i.e. obtain target BER) the transmitted information in a given environment. We have to note that the type of environment has strong impact on the useful range as path loss and delay spread can be quite different. As mentioned above, a typical industrial environment has a size of 100m x 100m. Thus, is adequate to cover a range of 70m with one omni-directional antenna. Of course, antenna techniques (smart antennas, antenna diversity) can be used to enhance system performance but the performance requirements must be satisfied under worst case receiving conditions. Range could be also categorized among the most important industrial applications and user related requirements, the rest of which are: Fieldbus equivalency: the real-time and the dependability quality of the wireless extensions shall be comparable with these of the currently used wired solutions. Fieldbus compatibility: as it has been already stated, the success of any wireless technology inside industry is expected to be reached if this technology integrates seamlessly with the current industrial networking infrastructure, providing an extension to it. Bit rate: the bit rate of 2Mbps is a representative figure, which gives a strong indication about the system capabilities and the services provided to an industrial application. Therefore, this figure is considered a basic radio requirement for the radio platform upon which a wireless fieldbus should be developed. III. RADIO TECHNOLOGY ALTERNATIVES The candidate radio technologies for the wireless fieldbus shall meet the requirements mentioned above. This section shortly addresses the most dominant wireless protocols against these basic requirements. 1. DECT The data rate of 2Mbps is not currently being supported by the standard, although there is a provision in DECT Packet Radio Service (DPRS) to extend the bit rate up to 2 Mbps without decreasing the overall performance. On the other hand, no explicit specification exists for the values of delay spread that can be accommodated, while the widespread penetration of DECT in cordless telephony implies high interference to a possible wireless fieldbus. 2. Bluetooth Both the range (10m) and the data rate (1Mbps) of the Bluetooth standard do not satisfy the basic radio requirements. The announcement of Mega-Bluetooth does not yet provide a solid ground on which to base the wireless Fieldbus technology.

3. 802.11b The 802.11b standard is an extension of the 802.11 standard. It is operating at the 2.4GHz bands designated for ISM applications. Two types of spread spectrum have been approved by the FCC in the unlicensed ISM band. Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS). The extension of the DSSS system builds on the data rate capabilities to provide 5.5 and 11 Mbps payload data rates in addition to the 1 and 2 Mbps data rates. By the use of RAKE receivers and Decision Feedback Equalizers, the 802.11b systems can obtain multipath diversity. For the physical data rate of 2Mbps these systems can manage delay spread higher than 200nsec. The typical operating range, as it is defined by the standard for 2Mbps and with omnidirectional antenna, is around 70 meters for indoor environments. The receiver sensitivity is specified to be less than –80dbm at 2Mbps and for BER lower than 10-5. The maximum allowable output power as measured in accordance with practices specified by the regulatory bodies is 100mW EIRP (20dbm) for Europe. According to this, 802.11b systems shall be able to overcome 100dbm path loss at 2Mbps data rate, providing also immunity for typical fade margin. The IEEE 802.11 FHSS Physical Layer is described by the standard for operation at 1Mbps, with optional 2 Mbps speeds in optimal quality conditions. Concerning the multipath diversity, currently employed frequency hopping systems employ 2 or 4 level FSK modulation and have a 20dB bandwidth of 1 MHz. The narrowband FH systems work satisfactorily in environments where the delay spread is in the range of 100-200 ns. The receiver’s sensitivity is defined by the standard at least –75dBm and since the transmitting power is 20dBm, we can conclude that FH systems satisfy marginally the path loss requirement of 100dBm. Comparing the two spread spectrum modulations, based on theoretical calculations DSSS offers a more robust wireless link than 1Mbps FHSS. More specifically, for a channel with a given SNR level, DSSS offers a link with much lower error rate. That is because DSSS employs QPSK modulation, which is more power efficient than GFSK. The reason for the moderate power efficiency of GFSK stems from the limited spectrum availability from FCC regulations which impact the deviation ratio of the modulation. For the same reason, DSSS can tolerate considerably more interference than FHSS systems. To improve FHSS performance more bandwidth should be allocated, while at the same time strong coding must be employed. However, these aspects are not foreseen by the current standard. As a result, for current implementations the difference between DSSS and FHSS in the minimum required SNR results in different allowable path loss (larger in DSSS). QPSK modulation yields a 4 to 10 dB SNR advantage

relative to GFSK. This SNR advantage translates into a considerable advantage in coverage area for a DSSS system operating at the same bit rate and transmitting power as an FHSS system. Given that a DSSS system at 2 Mbps has a range of 70 to 100 meters an FHSS system could not reach that target, which is not satisfactory for our requirements. 4. UMTS

The sensitivity of an OFDM modem is defined to be minimum –85dBm for the 6Mbps data rate. The transmitter output power is defined to be 23dBm. This results to a path loss of 108dbm, which meet the path loss requirement. Hiperlan2 is specified to have a minimum range of 30m in the higher bit rate mode of 54Mbps. However at lower bit rates, it is expected to cover an indoor range up to 150m.

The UMTS standard makes provision of two different modes of operation in the physical layer, the FDD mode (WCDMA) and the TDD mode (TD-CDMA). Using spread spectrum modulation and RAKE receiver to compensate the multipath propagation, the multipath components that can be resolved are equal to the number of the RAKE demodulators in the receiver. Having this in mind, for an environment with 200ns delay spread, a spreading factor (SF) equal to 23=8 must be used. In fact, with SF=8 the tolerated delay spread by the system can be up to 300ns. Nevertheless, in the uplink of FDD multicode transmission is allowed only with SF=4. In view of this, it is very difficult to reach the desired delay spread while preserving the required bit rate, which is 960 kbps in this case. On the other hand, the uplink of TDD can carry a maximum of two simultaneous code channels by multicode transmission. Therefore, for SF=8 and two code channels, the slot physical bit rate will be 1.920 Mbps.

IV. INDICATIVE TOPOLOGY Two simple, but general and descriptive, possible topologies of a wireless fieldbus system are presented in Fig. 1. These topologies (the Base Station and the direct link version) will be used as an evaluation framework of wireless technologies against the functionally equivalent wired Profibus segment presented in Fig. 2. The communication scenario, which will be used to provide example numerical figures, assumes that master station M1 is continuously scanning a 246-byte entity (which is the longest Profibus data packet) from slave station S1 and a 1-byte entity (which is the shortest Profibus data packet) from each slave station S2 to S10. The basic equations relative to the Profibus performance are presented in (1) and (2), according to the EN50170 Standard [3], where: S1

Moreover, from the specifications of UMTS the station transmitted power can be from 10 to 30 dBm and the sensitivity for BER=10-3 is –105 dBm. From the base station (BS) point of view the transmitted power is from 2832dBm and the sensitivity for BER=10-3 and 12.2 kbps is – 109dBm. From these specifications, in the uplink a path loss is from 133 to 137dBm while for the downlink the path loss can be 119-139dBm.

S10

S2

S1 S10 S2

S9

S3 S9

S3 BS

M1 S8

S8

S4

S4

S5

S7

S7

S5

S6

M1

S6

a)

5. HIPERLAN Type1 Although range and bit rate seem to be satisfied by the standard, there is no specification for the ability to operate satisfactorily under severe multipath, a key characteristic of industrial environments. In fact, Hiperlan1, together with DECT, is the most vulnerable of all the examined technologies in large delay spreads as its modem techniques are not inherently associated with any form of diversity. 6. HIPERLAN Type2 HiperLan2 seems to be one of the most promising radio technologies of the future because it can achieve very high transfer rates, up to 54Mbps. Moreover, from simulations that were carried out and results from the BRAN meetings, it seems that HiperLan2 technology can achieve a BER of the order of 10-5 for a channel with RMS delay spread of 200ns in the lowest data rate mode (6Mbps).

b)

Figure 1:

a) Wireless fieldbus as a direct link network b) Wireless fieldbus with base stations

Standard PROFIBUS

M1

S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

Figure 2: A typical Profibus network with one (1) master and ten (10) slave nodes

• • •

TMC : is one message cycle time (requestresponse). TS/R : is the transmission time of the request telegram. TSDR : is the station delay of the responder, defined as the time which may elapse between the receipt of a request frame’s last bit until the

• •

•

transmission of a following (response) frame’s first bit. TA/R : is the transmission time of the acknowledgment or response telegram. TID : is the idle time of the initiator, defined as the time which expires at the initiator after receipt of a frame’s last bit until a new frame’s first bit is allowed to be transmitted. TSR : is the system reaction time, which, roughly, defines the minimum period of changing values in a scanned slave, in order for the master to accurately follow (scan) all these changes.

Message Cycle TMC =TS/R +TSDR +TA/R +TID +2TTD (1) System Reaction TSR = Σnumber_of_slaves TMC For the cycle time assumptions were made: • • • •

calculations,

the

(2)

following

Zero Transmission delays (TTD = 0) Zero message retransmissions Token transmission from M1 to itself ignored Other Profibus related transactions (i.e. GAP management) ignored

The timing calculations concerning the standard RS485 Profibus case of Fig. 2, and for the selected scenario, have as follows: Baudrate = 1.5 Mbps -> tBit = 0.667 µsec TS/R = 6 bytes * 11 bits = 66 tBit TA/R 1 = (9 + 246) bytes * 11 bits = 2805 tBit TA/R 2-10 = (9 + 1) bytes * 11 bits = 110 tBit TSDR = minTSDR = 11 tBit TSDI = minTSDI = 37 tBit Standard Message Cycle times (TMC) TMC 1 = 2919tBit =1.947 msec TMC 2-10 = 224 tBit = 0.149 msec Standard System Reaction time (TSR) TSR = TMC 1 + 9 * TMC 2-10 = 3.292 msec V. FULL STACK INTEGRATION APPROACH After the short discussion of section 3, three wireless technologies seem to be the most suitable to support fieldbus extensions, namely UMTS-TDD, 802.11b-DSSS and HIPERLAN2-OFDM, which will be further analyzed through the rest of the paper. For the integration of any of them and due to the fieldbus compatibility and service equivalency requirement, it must be evident that the fieldbus protocol stack must be preserved down to the layer which provides the real-time services, that is, the Data Link Layer. Therefore, we should expect great redundancy if we connected the fieldbus DLL to anything higher than the

wireless PHY. Considering Profibus, the existence of the token-based MAC will overrule i.e. the whole CSMA mechanism of the 802.11 MAC. Moreover, other wireless MAC mechanisms like association / disassociation protocols or constant duration frames, would interfere with the real-time traffic managed by the fieldbus MAC in a way, which, at least, would increase the complexity of the solution, if not the performance. Nevertheless, for the UMTS case, regulations prohibit the usage of any of its transmission techniques unless the whole protocol stack is used, always according to the UMTS standard. That means, at least for UMTS we have to analyze only the possibility of the full wireless protocol stack integration. Due to the great complexity of the protocol, in the following subsection the analysis is worked out in the mapping interface of the transport channels, taking into account the Radio Resource Control (RRC) operation. It will be shown that, even this mechanism alone, results to a roughly unacceptable performance.

1. UMTS – TDD Due to the varying characteristics of packet data traffic in terms of packet size and packet intensity, a triple-mode packet transmission scheme is used for UMTS. With this scheme, packet transmission can either take place on: • • •

Common Channels (RACH/FACH) Dedicated Channels (DCH) Shared Channels (DSCH/USCH)

In the following subsections we evaluate the performance of a message cycle according to the delays introduced by the protocol, using each of the above channels, and for both topologies, direct link and link through a base station. We have to mention that in this case, the direct link topology is achieved by placing the master station functionality inside the BS, that is, in the UTRAN, while the base station version is achieved by using all stations, the master and the slaves, as User Equipment (UEs), according to the UMTS architecture terminology. A. Common-channel packet transmission Common-channel packet transmission is typically used for short infrequent packets, where the link maintenance needed for a dedicated channel would lead to unacceptable overhead. Also the delay associated with a transfer to a dedicated channel is avoided. Common-channel packet transmission should therefore be limited to short packets that only use a limited amount of capacity. On the other hand the capacity of the common channels is limited and inefficient because of the high overhead for UE identification in each transport block. The channels used for common-packet transmission are: • Random Access Channel (RACH): a contention based uplink channel used for transmission of

For our time analysis we have presumed the best-suited frame structure according with the packet length needs, that is, if a data packet is larger than the maximum allowed data bits per slot, the appropriate number of continuous slots of the same direction (uplink or downlink) is allocated, leading to an asymmetric frame structure (see Figure 3). The TDD mode frame contains 15 timeslots with duration 2560*Tc (Tc=1/3.84Mchips/s), therefore the timeslot duration is 0.667ms. In order to satisfy the PhL requirement of delay spread of up to 200ns, we select spreading factor (SF) equals to 8. The timeslot format of RACH for SF=8, contains a maximum data field of 464 bits.

Figure 3: An Asymmetric Frame Structure of UTRA TDD mode

slot, as mentioned before, is 464 bits (58 bytes). Apparently for data packets greater than 58 bytes we need additional data timeslots, which imply additional delay.

R D a AC ta H #1 F D AC at H a # R 2 D a AC ta H #3 F D AC at H a #4

•

relatively small amounts of data, e.g. for initial access or non-real time dedicated control or traffic data. Forward Access Channel (FACH): common downlink channel without closed-loop power control used for transmission of relatively small amount of data.

4slots

Figure 5: Timing scenario for the case where Common channels are being used (Master is a UE)

The biggest PROFIBUS packet has a length of 255 bytes, therefore 5 successive timeslots would be needed for data transfer #3 through RACH (255/58≈5) and the total time required would be: TRACH +TFACH+5TRACH+TFACH=8*0.667=5.336ms ii. Direct Link In this case the Master can be a BS as well. This means that the Master is able to poll directly the Slave. Fig. 6 and 7 depict the procedure in direct link. The total time needed in this case is: 2slots=2*0.667=1.334ms=TS/R+TA/R

i. Link through Base Station The mobile stations, or else, the User Equipment (UE) in UMTS terminology, communicate through the Base Station. The Base Station apart from the data transfer between the different UEs, is also responsible for the Radio Bearer association de-association mechanisms and for mobility management as well.

BS 2

Master 1 1

UE1 Slave1

Figure 6: Direct Link

UE2 2

Slave1 3

BS

For data packet responses of 255 bytes length the number of timeslots needed is: 1TFACH + 5TRACH=5*0.667+0.667=4.002ms

4

Master1

Figure 4: Master-Slave communication through BS

In the link through BS case, 4 data slots need to be exchanged between the master and the slave through the Base station (see Figure 4). The data slots are mapped to common physical channels (RACH for the uplink direction and FACH for the downlink direction). With RACH, multicode transmission is unfeasible, therefore the capacity is very limited. Figure 5 depicts the timeslots needed for the message cycle completion. For the completion of the message cycle the total time needed is: 4slots=4*0.667=2.668ms Since RACH, in contrast to FACH, does not support multicode transmission the maximum packet length per

F D AC at H a #1 R Da AC ta H #2

1

UE1

2slots

Figure 7: Timing scenario for the case where Common channels are being used (Master in the UTRAN)

B. Dedicated-channel packet transmission In this mode, an initial Random-Access request (through the RACH) is used to set up a dedicated channel for the data packet transmission (Radio Bearer Establishment). Once the UE has established RB connection on dedicated channel, data packets may be transmitted without any further scheduling. Considering the fact that, in TDD mode, the available dedicated channels are only 8 (since SF=8), and that for 2Mbps operation we have to use multi-code (two codes per peer connection), only 4 dedicated connections are available per cell. In the most probable

#3

#4 R B co re m l D pl eas C e e C te H

#2

R B th rel D rou eas C gh e C H

#1

R B th se r FA ou tup R C gh co B S H m e R pl tup AC et H e R B AC c o Se K mp tu FA thr lete p C oug H h

R B FA se C tup H B co S m et R pl up AC e t e R H B AC com Se K p tu FA thr lete p C oug H h R

R B S th et u R rou p r AC gh e q H

It can be seen that the overhead introduced by the establishment and de-establishment procedures compared with the packet transfer is enormous. More specifically 9 slots are used for the association de-association of dedicated channels between the Base station and the UE1 and UE2 and only 4 slots are being used for real data transfer. The calculated time for a short Profibus response is 13slots=13*0.667=8.671ms

7slots

C. Shared-channel packet transmission We have seen in the previous sub-sections that the possibility to transfer data on the common channels RACH and FACH is limited by their capacity and from the high overhead for UE identification in each transport block. The allocation and de-allocation of the dedicated channels, on the other hand, is too slow for the bursty characteristics of packet data transmissions. To overcome these problems, a shared channel operation using short time, automatically releasing resources on the physical layer was introduced. The channels used for shared transmission are: •

Uplink Shared Channel (USCH): The uplink shared channel is an uplink transport channel shared by several UEs carrying dedicated control or traffic data

•

Downlink Shared Channel (DSCH): The downlink shared channel is a downlink channel shared by several UEs carrying dedicated control or traffic data.

13slots

In the downlink direction, physical channels shall use SF=16. Multiple parallel physical channels can be used to support higher data rates. Since the SF=16, the total number of channels which can be used is 16. For the specific timeslot format the maximum number of bits per slot is 276bits (34 bytes). For a data packet with length 255 bytes, 8 different code channels can be used in parallel in one timeslot. The resulting total time needed for the transmission of a 255 bytes data packet is: 14timeslots=14*0.667=9.338ms ii. Direct Link In the case where the Base station can be a Master as well, the overall time needed is reduced as expected. In

#2

Figure 9: Timing scenario for the case where Dedicated channels are being used (Master in the UTRAN)

Figure 8: Worst time scenario for the case where Dedicated channels are being used (Master is a UE)

Considering the maximum Profibus packet, calculations are slightly different. In the uplink direction, and for SF=8, the maximum number of bits per slot that can be transferred is 552 bits. For multicode transmission (2 channels are used in parallel), the maximum number of bits per slot is 2x552=1104 bits (138 bytes per slot). That means, if a packet has length larger than 138 bytes, another one timeslot must be used.

#1

R B c o re m l D pl eas C et e C e H

In the case where the response of the Slave has a length of 255 bytes, according to the previous paragraph, we need 2 timeslots for the data transfer of packet #2 during the uplink. The total time needed is: 8slots=8*0.667=5.336m

R B th rel D r ou eas C gh e CH

i. Link through Base Station Figure 8 depicts the total number of timeslots needed for the radio bearer establishment connection for both UEs (UE1 & UE2), the completion of the message cycle (data transfer #1 - #4) and finally the RB release, which is initiated by the UTRAN.

Figure 9 the slots needed for the message cycle completion between the Master and the Slave (worst-case scenario), are depicted. The total time required is 7slots=7*0.667=4.669ms

th RB ro ug se h tup FA C H R th co B S ro m e ug pl tup h ete R AC H c R t h om p B S ro le et ug te up h A FA C C K H

scenario, where the cell contains more than 4 UEs, apparently the 4 available dedicated connections cannot be maintained by the same UEs all the time. One of the UEs must release the connection each time and initiate an establishment again at the next time that it will require access. The conclusion is that for a domain with more than 4 UEs the allocation and de-allocation of the dedicated radio RBs is inevitable because the dedicated connections cannot be maintained by the same UEs infinitely.

Like Dedicated Channels, data transfer through Shared Channels requires a Radio Bearer establishment of the USCH and DSCH. With this procedure the transport block of data from the upper layers are mapped to the Shared Channels of the Physical layer. Since the establishment procedure has been performed, the UE makes a Capacity Request to the BS using the USCH. The BS station decides to allocate physical resources to the logical channel SHCCH between the UE and the BS, sending a PhyShChAllocation message to its peer entity in the UE. After the completion of this procedure the UE can send data through the USCH. At the end of packet transmission the UTRAN will release automatically the RB. The next time that a UE will need a shared connection with UTRAN, only a Capacity Request message will be required. In Figure 10, it is depicted the time needed (in timeslots) for the completion of the procedure described above, for the first and the second time respectively that UE1 and UE2 require access to the shared channels.

Table 1: Profibus over UMTS-TDD calculated times in msec UMTS: UMTS: Master in the UTRAN Master as a UE Shared Ch. Common Dedicated Shared Common Dedicated Ch. Ch. Ch. Ch. Ch. Mobile 1 cell

RS485

TMC (246)

1.947

4.002

5.336

4.002

5.336

9.338

support 12.006

TMC (1)

0.149

1.334

4.669

3.335

2.668

8.671

11.339

TSR

3.292

16.008

47.357

34.017

29.348

87.377

(1)

only 7.337 6.67 67.367

Table 2: Profibus over UMTS-TDD performance UMTS: UMTS: Master in the UTRAN Master as a UE Shared Ch. Common Dedicated Shared Common Dedicated Ch. Ch. Ch. Ch. Ch. Mobile 1 cell

RS485

TMC (246)

a

2.055 a

2.741 a

2.055 a

2.74 a

4.80 a

support 6.17 a

only 3.77 a

TMC (1)

b

8.953 b

31.336 b

22.38 b

17.90 b

58.19 b

76.10 b

44.76 b

26.54 c

1

20.46 c

TSR

c

4.863 c

14.385 c

10.33 c

8.91 c

()

1

( ): TSR depends on the number of stations that change cell during the scanning of the 10 slaves

#7

PhyShChAlloc through DSCH

Capacity Req USCH

#6

PhyShChAlloc through DSCH

cycle is 8 timeslots. is:8*0.667=5.336ms.

The

total

time

needed

The overhead added by the establishment mechanism applies only for the first time. Only the PhyScChAllocation message shall be repeated every time we need access to the Shared Channels. The delay for the second time of data transfer through Shared channels is: 5*0.667=3.335ms.

#8

10slots

First time

Capacity Req USCH

#3

PhyShChAlloc DSCH

8slots

#2

PhyShChAlloc DSCH

Capacity Req USCH

#1

PhyShChAlloc DSCH

PhyShChAlloc DSCH

Figure 10: Timing scenario for the case where Shared channels are being used (Master is a UE)

th RB ro u g se h tup FA C H R c B ro om Se ug p tu h lete p R AC co RB H th mp S ro le etu ug te p h A FA C C K H

Second time

th

#5

PhyShChAlloc through DSCH

Capacity Req USCH

#4

PhyShChAlloc DSCH

#3

PhyShChAlloc DSCH

Capacity Req USCH

#2

PhyShChAlloc DSCH

RB Setup complete ACK FACH

R

17slots First time

PhyShChAlloc through DSCH

#1

B th se ro FA u tup R C gh co B S H m e R ple tup A C te H

R co B S m e R R ple tup B A AC com Se CH te K p tu FA thro lete p C ug C H ap h th acit U rou y R Ph SC gh e q yS H th hC D rou hA SC gh llo c H

R B th s e ro FA u tup C gh H

R B S th etu R rou p re A C gh q H

i. Link through Base Station When UE1 requires access for the first time, the total time needed for the establishment of a radio bearer and the completion of the message cycle is 17timeslots=17*0.667=11.339ms

#4

5slots Second time

The second time that the UE1 will require data transmission through shared channels, no allocation will be required but only one slot for Capacity Request and one slot for PhyScChAllocation (see Figure 10). The total time needed in this case is: 10timeslots=10*0.667=6.67ms In the case where the Slave’s response has a length of 255 bytes, two timeslots are needed for the uplink and one for the downlink since multicode transmission is supported. The total number of timeslots needed in this case is 11timeslots = 11*0.667ms=7.337ms The above time occurs in the case where RB setup has been established, otherwise the overall time needed is: 18timeslots=18*0.667ms=12.006ms

Figure 11: Timing scenario for the case where Shared Channels are being used (Master in the UTRAN)

In the case where the data packet has a length of 255 bytes and no RB setup has been established, the total time needed is: 9timeslots=9*0.667=6.003ms. Otherwise, if RB setup has been established the overall time is: 6timeslots=6*0.667=4.002ms Table 1 shows the absolute timings (in msec) of the message cycles for the 1 byte (TMC (1)) and the 246 byte (TMC (246)) scanned variables in the scenario presented in Section 4. Table 2 shows the normalized values which indicate the ratio of each UMTS time value to the corresponding value of the standard RS485 Profibus. VI. LAYER 1 (PHY) SERVICE APPROACH

ii. Direct Link Finally the case where the Master device can be a BS at the same time is examined. The timeslots needed for the first and the second time that access to shared channels is going to be required are depicted in Figusr 11. The total number of timeslots required for the completion of message

1. 802.11b DSSS PhL The 802.11b DSSS physical layer includes the High Rate PLCP Preamble (Physical Layer Convergence Protocol), the High Rate PLCP Header and the PSDU

3000

2500

2000

Tmc (µsec)

RS485 1.5Mbps 802.11b Direct Link 2Mbps 1500

802.11b Base Station 2Mbps 802.11b Direct Link 5.5Mbps 802.11b Base Station 5.5Mbps

1000

500

243

232

221

210

199

188

177

166

155

144

133

122

111

89

100

78

67

56

45

34

23

1

12

0

Response Data Length (#bytes)

Figure 12: Profibus TMC over 802.11b DSSS

(PLCP Service Data Unit). The short PLCP header uses the 2Mb/s Barker code spreading with DQPSK modulation and the PSDU is transmitted at 2Mb/s, 5.5 Mb/s or 11 Mb/s. The protocol overhead delay introduced by the physical layer is 96µsec. In the case of the TMC calculations, we must add this 96µsec overhead in each Profibus telegram transmission, but this duration along with the inter-frame-space time (IFS) overlaps with the TSDR and TID timing requirements of the Profibus protocol, leading to a usage of the following values: IFS = 2 * Rx/Tx turnaround time = 10 µsec TS/R = 6 bytes * 8 bits = 48 tBit + 96µsec TA/R 1 = 2040 tBit + 96 µsec TA/R 2-10 = 80 tBit + 96 µsec TSDR = max(11tBit –106µsec, IFS) = IFS = 10 µsec TSDI = max(37tBit –106 µsec, IFS) = IFS = 10 µsec

The results are presented in Tables 3 and 4, where Table 3 shows the absolute timings (in msec) of the message cycles for the 1 byte (TMC (1)) and the 246 byte (TMC (246)) scanned variables in the scenario presented in Section 4, and Table 4 shows the normalized values which indicate the ratio of each DSSS time value to the corresponding value of the standard (wired) Profibus. Moreover, in Fig. 12, the TMC times are presented for both wired and DSSS Profibus and for the complete range of data telegram lengths, while preserving the assumption that the request telegram (SRD) contains no data, in order for the situation to be closer to the rest of the EN50170 protocols, e.g. it could be the sequence of a WorldFIP Bus Arbitrator (BA) request for the broadcasting of a certain variable identifier and the broadcasting itself. 2. HIPERLAN Type2 (OFDM) PhL

Table 3: Profibus over 802.11b DSSS PHY calculated times in msec 802.11b PHY 802.11b PHY RS485 (DSSS) (DSSS) Profibus with Direct Link with Base Station 1.5Mb 2Mb 5.5Mb 2Mb 5.5Mb TMC (246) 1.947 1.256 0.59 2.512 1.18 TMC (1)

0.149

0.276

0.235

0.552

0.470

TSR

3.292

3.740

2.705

7.480

5.41

Table 4: Profibus over 802.11b DSSS PHY performance 802.11b PHY 802.11b PHY RS485 (DSSS) (DSSS) Profibus with Direct Link with Base Station 1.5Mb 2Mb 5.5Mb 2Mb 5.5Mb TMC (246) A 0.6450 a 0.303 a 1.29 a 0.606 a TMC (1)

b

1.8523 b

1.577 b

3.7046 b

3.154 b

TSR

C

1.1360 c

0.821 c

2.272 c

1.642 c

The HIPERLAN2 physical layer is based on bursts which consist of a Tpreamble and a Tpayload part. The duration of the Tpreamble part is 8, 12 or 16 µsec, according to the type of the burst, while the Tpayload part carries the PSDU with a bit rate based on the modulation and the coding rate scheme selected, ranging from 6 to 54Mbps. For our case of the TMC calculations, we assume a usage of a broadcast and an uplink burst for the base station (access point) topology and a direct link burst for the direct link topology, resulting to a constant 16µsec Tpreamble component. The Tpayload component is given by equation (3), where: • • •

NLENGTH : is the total number of bits in the PSDU NDBPS : is the number of data bits per OFDM symbol 4 µsec : is one OFDM symbol duration. Tpayload =NLENGTH / NDBPS  * 4 µsec

(3)

3000

2500

Tmc (µsec)

2000

RS485 1.5Mbps 1500

HIPERLAN2 Direct Link 6Mbps HIPERLAN2 Base Station 6Mbps

1000

500

235

222

209

196

183

170

157

144

131

118

92

105

79

66

53

40

27

1

14

0

Response Data Length (#bytes)

Figure 13: Profibus TMC over HIPERLAN2 OFDM

HIPERLAN2 PhL defines also a maximum Tx/Rx switch time of 6µsec, which will be used as the value for the TSDR and TSDI parameters of equation (1). Tables 5 and 6 depict the delay performance of a 6Mbps HIPERLAN2 operation, that is, with BPSK modulation and ½ coding rate, resulting to a NDBPS equal to 24. Fig. 13 is the equivalent of Fig. 12 applied to HIPERLAN2 for the whole range of Profibus data telegram lengths. Table 5: Profibus over HIPERLAN2 OFDM PHY calculated times in msec RS485 HIPERLAN2 PHY HIPERLAN2 PHY Profibus with Direct Link with Base Station (1.5Mbps) (OFDM 6Mbps) (OFDM 6Mbps) TMC (246) 1.947 0.392 0.784 TMC (1)

0.149

0.068

0.136

TSR

3.292

1.004

2.008

Hiperlan/2 PhL is a very powerful technology that can satisfy the basic radio requirements presented in Section 2. In addition its delay performance is superior to the rest radio technologies. However, as Profibus packets are small, a very small part of OFDM abilities will be utilized. Finally there are no solid proven products yet of Hiperlan2 OFDM technology to support a full blown system deployment. 802.11b Phl ( DSSS) technology satisfies all the basic radio requirements while at the same time exhibits comparable delay performance characteristics with RS485 (Tables 3,4). In addition, it incorporates advanced receiver techniques to ensure robust performance in harsh industrial environments. Finally, it is a mature technology, which has been proven in numerous indoor and outdoor environments of big variety including industrial sites. VIII. REFERENCES

Table 6: Profibus over HIPERLAN2 OFDM PHY performance RS485 HIPERLAN2 PHY HIPERLAN2 PHY Profibus with Direct Link with Base Station (1.5Mbps) (OFDM 6Mbps) (OFDM 6Mbps) TMC (246) a 0.2 a 0.4 a

[1]

TMC (1)

b

0.456 b

0.912 b

[2]

TSR

c

0.305 c

0.61 c

[3] VII. CONCLUSION [4] A number of dominant wireless protocols were examined as candidates for this integration, while a more detailed performance analysis was presented for UMTS, 802.11b and HIPERLAN2, since these satisfy the range, bit-rate, delay spread and path-loss requirements of the harsh industrial environment. UMTS is the most advanced technology, which matches the target bit rate of 2 Mb/s in our application. However, as stated previously the whole protocol stack has to be used according to regulations, which imposes prohibitive delay overhead.

[5]

[6]

RFieldbus project IST-1999-11316, Deliverable D1.1, “Requirements for the RFieldbus System”, Technical Report, Apr. 2000. RFieldbus project IST-1999-11316, Deliverable D1.2, “Assessment and Selection of the Radio Technology”, Technical Report, Sep. 2000. “General Purpose Field Communication System, Volume 2” – Profibus, European Norm EN 50170, 1996. DPRS - ETSI EN 301 649 V0.5.1, Final Draft: "Digital Enhanced Cordless Telecommunications (DECT); DECT Packet Radio Service (DPRS)", 1999-11 DECT - ETSI EN 300 175-4 V1.4.2: "Digital Enhanced Cordless Telecommunications (DECT); Common Interface (CI); Part 4: Data Link Control (DLC) layer", 1999-06 BLUETOOTH SPECIFICATION Version 1.0 B: "Specification of the Bluetooth System",

[7]

[8]

[9] [10] [11]

Specification Volume 1, Document No.: 1.C.47/1.0 B, December 1999 Bilstrup, U., Wiberg, P.-A., “Bluetooth in Industrial Environment”, in Proceedings of the 2000 IEEE International Workshop on Factory Communication Systems, pp. 239-246, September 2000. Carl Andren (Harris Semiconductor): "A Comparison of Frequency Hopping and Direct Sequence Spread Spectrum Modulation for IEEE 802.11 Applications at 2.4 GHz", 1997 ETSI TS 125 201 V3.0.2 “UMTS-Physical layer General description”, 2000-03 ETSI TS 125 213 V3.2.0 “UMTS-Spreading and modulation (FDD)”, 2000-03 ETSI TS 125 223 V3.2.0 “UMTS-Spreading and modulation (TDD)”, 2000-03

[12] [13] [14] [15] [16] [17]

ETSI TS 125 302 V3.4.0, “UMTS-Services provided by the physical layer”, 2000-03 INTERNATIONAL STANDARD ISO/IEC 880211 ANSI/IEEE Std 802.11, First edition, ISBN 07381-1658-0, 1999 ETSI TS 25 221 "Physical channels and mapping of transport channels (TDD)" ETSI TS 25 222 "Multiplexing and channel coding (TDD)" ETSI TR 101 683 V1.1.1 (2000-02), ”Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; System Overview” ETSI TS 101 475 v1.1.1 (2000-04), “Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Physical (PHY) layer”