Coherence multiplexing (CM) is a relatively unknown form of optical CDMA, which is particulary suitable in medium bit rate, short-range optical networks like LANs. The main purpose of the technique is to allow multiple users to transmit through a common optical fiber simultaneously. When this number is too large, however, the BER will become unacceptably high. Therefore a protocol is needed to control the traffic. In this paper several protocols are presented. An adapted version of synchronous TDMA, two new protocols and a central control unit will be proposed and discussed. Finally, the protocols will be compared with respect to performance and practical implementation aspects

Blokpoel, Robbin J.

Etten, Wim van

Meijerink, Arjan

English

van Etten, Wim

NARCIS 

Traffic control in coherence-multiplexed networks.

University of Twente Research Information

12th Annual Symposium of the IEEE/CVT, Nov. 3, 2005, Enschede,the Netherlands 1Traffic control incoherence-multiplexed networksRobbin J. Blokpoel, Arjan Meijerink, Wim van EttenUniversity of TwenteFaculty of Electrical Engineering, Mathematics and Computer ScienceTelecommunication Engineering GroupP.O. Box 217, 7500 AE, Enschede, The NetherlandsE-mail: a.meijerink@ieee.orgCoherence multiplexing (CM) is a relatively unknown form of optical CDMA, which isparticulary suitable in medium bit rate, short-range optical networks like LANs. Themain purpose of the technique is to allow multiple users to transmit through a commonoptical fiber simultaneously. When this number is too large,however, the BER will becomeunacceptably high. Therefore a protocol is needed to control the traffic. In this paperseveral protocols are presented. An adapted version of synchro ous TDMA, two newprotocols and a central control unit will be proposed and discu sed. Finally, the protocolswill be compared with respect to performance and practical imple entation aspects.1 Coherence multiplexing (CM)Coherence multiplexing (CM) [1] is a form of optical code divison multiple access(OCDMA) that enables distributed users to transmit through acommon optical fiber, us-ing broadband optical sources (like LEDs) and Mach-Zehnderint ferometers with verylarge path length differences. This is illustrated in Figure 1.transmitter i receiver rmodN-portcombinerN-portsplitterN transmitters N receiverscommonfiberTTx,i TRx,rφmod,i(t)Iout,r(t)Figure 1: A coherence-multiplexed optical communication systemEach transmitter transmits two versions of a broadband optical source signal. One of theseis phase-modulated by a signalφmod(t), whereas the other one remains unmodulated andacts as a reference signal. The two versions are delayed withrespect to each other bya valueTTx, which is different for each transmitter.TTx is always much larger than thecoherence timeτc of the light, so that the two versions that are transmitted through thecommon fiber are mutually incoherent and hence do not interfere. As a result, the modu-lating signalsφmod(t) are not visible in the total intensity of the composite optical signal inthe common fiber. In a particular receiverr, the modulating signalφmod,i(t) of transmitterican be extracted from the composite signal provided that itspath length differenceTRx,r isequal to the path length differenceTTx,i of transmitteri, so that the (unmodulated) optical12th Annual Symposium of the IEEE/CVT, Nov. 3, 2005, Enschede,the Netherlands 2signal that travels the upper path in transmitteri and lower path in receiverr interfereswith the (modulated) optical signal that travels the lower path in transmitteri and upperpath in receiver. The other CM channels are sufficiently suppressed when the differencesbetween the different values of the path length differencesis much larger thanτc.The main advantages of this technique are:• its potentially inexpensive implementation: the optical sources can be simple LEDsand the optical circuits only contain couplers, delay linesand modulators;• its robustness to fabrication inaccuracies, temperature drift and component aging:small fluctuations in the path length differences do not result in crosstalk betweenthe channels.The main disadvantages are:• its vulnerability to chromatic dispersion due to the large bandwidth of the opticalcarrier signal, which limits network span and bit rate;• the optical beat noise due to incoherent beating of light waves from interferingchannels, which limits bit rate and number of channels that can be accommodated.CM is therefore particularly suited to low-cost, short-range applications with moderatenumber of channels and bit rate, like access networks, LANs and optical interconnects.2 Problem description and modelingFor a network with a given (fixed) bit rateRb, packet lengthDL and number of users(transmitter-receiver pairs)N, the bit error rate (BER) and hence packet error rate (PER)increase when the number of active usersM increases, because the power of the beat noiseincreases approximately proportionally toM2 [1]. For some network parameter values thiswill cause the network to become unusable when all the users ar imultaneously active(so whenM=N).In order to quantify this problem, two models are consideredfor the transmission behaviorof the users:1. The Full Load Model:M out of N users are continuously transmitting packets.(This is a suitable model in case of file downloading.)2. The Fractal Model: the time is divided in slots with a length that corresponds tothe length of a packet. At the beginning of each timeslot eachuser adds a packetto its queue with probabilityP and starts transmitting in case there are one or morepackets in the queue. The packet is removed from the queue in case of success-ful transmission, whereas it remains in case of a packet error. This is repeated ineach new timeslot. Note that the users are packet-synchronized in this model; it isassumed that this does not significantly affect the results.As an example consider the case whereM users are transmitting packets of lengthDL =12144 bits (like in Ethernet [2]) atRb = 2.5 Gbps in a network ofN = 20 users, accordingto the Full Load Model. The effective total network throughput Beff is then given byBeff = M Rb(1−PER) . (1)If a packet is assumed to be erroneous when one or more bits arecorrupted, and bit errorsare assumed to occur independently, then the PER is given byPER= 1− (1−BER)DL . (2)The relation between theBERand the other network parameters is given by [1]BER= Q(√2γs) , (3)12th Annual Symposium of the IEEE/CVT, Nov. 3, 2005, Enschede,the Netherlands 3where Q(.) is the Gaussian tail probabilityQ(z) ,12π∫ ∞zexp(−12x2)dx, (4)and the signal-to-noise ratioγs at the output of the (matched) receiver filter is given byγs =Rpd2P2in/RbRpd2P2inτc(4M2 +2M +1)+16eRpdPinLXM +32SthL2X. (5)The meaning of these symbols and the corresponding parameter valu s that were used inthe numerical examples in this paper are stated in Table 1.Table 1: Model parametersSymbol Description ValueN Number of transmitters 20M Number of active transmitters variable (0–20)Rb Data rate 2.5 GbpsRpd Responsivity of the photodiodes 0.8 A/WPin Optical source power 10 mWτc Coherence time 0.1 pse Electron charge 1.6·10−19 CLX Optical losses in the network 29 dBSthPower spectral density of the equivalent50 (pA)2/Hzinput noise current of the receiver amplifierDL Packet size 12144 bits0 5 10 15 2005101520→  M→  Beff [Gbps]Figure 2: Full Load situation without aprotocolThe resultingBeff using these parameters isshown as a function ofM in Figure 2. For lowM hardly any errors occur so that the effectivethroughput increases linearly withM. BeyondM=7, the PER increases rapidly, so that the ef-fective throughput collapses. (A similar graphcan be obtained forBeff as a function ofP incase of the Fractal Model.) Apparently, specialmeasures should be taken in order to preventnetwork breakdowns due to too many users be-coming active at the same time.3 Traffic controlFour protocols are proposed for controlling the traffic in a CMnetwork. Their perfor-mances are compared by running Matlab simulations based on the two traffic modelsdescribed in Section 2.3.1 Synchronous TDMAThe first protocol considered is an altered version of synchronous TDMA, adapted toCM. In this version the time is divided in slots with a size thatcorresponds to the packetlength, where in each timeslot, only a few users are allowed to transmit. There should be7 users per timeslot as a maximum, when the parameter values in Table 1 are used. This isbecause the maximum throughput is reached at that point, as can be seen from Figure 2.12th Annual Symposium of the IEEE/CVT, Nov. 3, 2005, Enschede,the Netherlands 4This leads to a practical disadvantage of TDMA: the ideal slotsize should be acquiredby testing or calculated with the parameters of the network.Moreover a synchronizationmechanism is needed for TDMA.The effective throughput in the Full Load and Fractal case have been estimated by meansof simulation; the results are shown in Figure 5(a) and 5(b).The protocol turns out toperform well in the Fractal case. There the traffic increasesuntil the maximum possiblefor the network is reached; after that the traffic stays constant. The protocol is not efficientin the Full Load case: when there are several idle users, their part of the bandwidth iswasted.If there were differences in the PER between the users (for example due to differences infiber lengths or fiber coupling efficiencies), the users with alower PER would just suffera lower efficiency using this protocol. Under no condition a congestion will occur usingthis protocol so it is a possible solution to the problem described in Section 2.3.2 Adaptive Traffic Load Protocol (ATLP)A newly introduced protocol is the Adaptive Traffic Load Protocol (ATLP) [3]. Thisprotocol uses information about the PER to adapt its percentag of time in which it istransmitting. Each user adapts its load autonomously, onlybased on their own ATLP-state valueAs. In Figure 3 a flowchart of the protocol is shown.Is theresomething inthe queue?Determine whetherthe user is allowed to transmit(with probability As)TransmitDid anerror occur?Increase AsDecrease AsGo to nextpacket slotyesyesnonoyesnoFigure 3: Flowchart ATLPFirst of all the transmitter verifieswhether there is something in itsqueue. When this is not the case,it will just wait a packet time andthen start again. When there issomething in the queue, it willstart transmitting with probabil-ity As. In case of transmission itwill decreaseAs if an error occursor it will increase it when no erroroccurs. When it does not allow it-self to transmit, it will increaseAs.A traffic jam will tend to occur when many users have packets inheir queue and have ahigh As value. The number of errors will then tend to increase, because these users keepretransmitting their erroneous packets. However, the ATLPwill then force them to de-crease theirAs value, and hence the average number of users transmitting simultaneouslywill decrease, in order to reduce the number of errors. The actual traffic jam is mainlycaused by the users that have a highAs value. Therefore, the reduction ofAs is performedby multiplying it by a fixed factor of 0.9, so that a user with a largeAs value is moreseverely penalized than a users with a lowAs value.On the other hand, the users have to recover from this situation in order to be able totransmit again and hence empty their queue. This is done by increasingAs in fixed stepsof 0.01. This value is a compromize between preventing a relaps by too quickly increas-ing As, and taking too much time to recover (a value of 0.01 will takerecovery timecorresponding to at most a few hundred packets). This wayAs will tend to converge to avalue at which the network throughput is optimal and equal for each user, provided eachuser has the same parameters.12th Annual Symposium of the IEEE/CVT, Nov. 3, 2005, Enschede,the Netherlands 5The simulation results for ATLP can again be found in Figure 5. In the Fractal situationthe protocol turns out to perform worse than TDMA for higher values ofP. This isbecause it could happen by coincidence that there are more use s active than ideal, sincethe transmitters are using a stochastic procedure to control their traffic. This results in athroughput which is less than for TDMA. In case of Full Load the transmission time ofthe idle users is not wasted, so the performance is better than in TDMA.A practical advantage of this protocol is that the transmitters operate autonomously. Adisadvantage is that the transmitter uses information about the PER, which requires anacknowledgement scheme in the return link. Another problemarises when there are dif-ferences in PER, because the users with a high PER will converge to a lowerAs, resultingin a lower throughput, hence causing them to suffer twice as hard. Therefore this is not afair protocol.3.3 Adaptive Traffic Speed Protocol (ATSP)Is theresomething inthe queue?TransmitCalculaterunning averageof efficiencyUpdatetransmissionspeedGo to nextpacket slotyesnoFigure 4: Flowchart ATSPAnother new protocol is the Adaptive Traffic SpeedProtocol (ATSP) [3], which is schematically shownin Figure 4. This protocol adapts the transmis-sion speed according to a the running average ofthe transmission efficiency (1−PER) of that user,which is calculated after every packet transmissionand compared to a target value. The length of therunning average is 1,000 packets. The target ef-ficiency is chosen as 0.90, because the maximumthroughput for Full Load occurs when 7 users aretransmitting (see Figure 2), and it can be calculatedthat the efficiency is 0.90 in that case. When the running average is higher than the target,then the speed is divided by 0.999; else it is multiplied by 0.999 This factor and the lengthof the running average are chosen in such a way in order to react quickly to changes onone hand and prevent oscillations on the other hand. Hence, every user is always allowedto transmit, because traffic jams are solved by changing the transmission speed. This waythe speed will converge to the best transmission speed for each individual user.The simulation results for ATSP can again be found in Figure 5. In the Fractal model theperformance for high values ofP is significantly worse than for TDMA. This is becausethe throughput is lower when all users are continuously active using a low speed thanwhen part of the users are active at a higher speed. For example: in case of TDMA withP = 100%, 7 users will be transmitting simultaneously atRb = 2.5 Gbps resulting in aneffective throughput of approximately 16 Gbps, whereas in case of ATSP withP= 100%,all users will be transmitting so that the transmission speed is reduced to an average ofapproximately 0.6 Gbps, resulting in an effective throughput of approximately 11 Gbps.When several users are idle ATSP performs relatively well, because it will allow the activeusers to shift to higher speeds.Practical disadvantages of ATSP are that the transmitter requir s information about thePER, and that transmitters and receivers need to be able to operate at different speeds. Anadvantage occurs when there are differences in PER between us rs; then each user willconverge to its individually best transmission speed.12th Annual Symposium of the IEEE/CVT, Nov. 3, 2005, Enschede,the Netherlands 63.4 Central control unit (CCU)With a central control unit (CCU) everything could be arrangedideally: the capacity isdivided equally between the users and no bandwith is wasted du to idle users. Differencesin PER can be handled too by assigning different transmission speeds [3].This seems to be the best solution (see Figure 5), but it has a severe topology requirement:the CCU needs to be able to communicate with every user.0 5 10 15 2005101520→  M→  Beff [Gbps]no protocolTDMAATLPATSPCCU(a) Full Load situation0 20 40 60 80 10005101520→  P [%]→  Beff [Gbps]no protocolTDMAATLPATSPCCU(b) Fractal situationFigure 5: Performance comparison of the proposed traffic control protocols4 ConclusionsIt can be seen that ATLP and ATSP are better in the case when there ar idle users andTDMA is the best in the Fractal simulation. ATSP can also be proven to be the bestsolution when there are differences in PER between different users. Although the CCUis the best in every case, the unit itself is a very severe topology requirement. ThereforeATSP can be considered as the best solution overall.All the protocols prevented persistent traffic jams while using the ability of CM to allowseveral users to transmit simultaneously. The protocols con idered should be suited forother CDMA systems where the performance decreases with increasing number of activeusers too.References[1] Arjan Meijerink, Geert Heideman, Wim van Etten, “Balanced optical phase diversityreceivers for coherence multiplexing,”J. Lightwave Technol., Vol. 22, No. 11, pp.2393–2408, November 2004.[2] The Institute of Electrical and Electronics Engineers,IEEE Standard for InformationTechnology-Telecommunications and Information ExchangeBetween Systems-Localand Metropolitan Area Networks–Specific Requirements-Part 3: Carrier Sense Mul-tiple Access with Collision Detection (CSMA/CD) Access Method and Physical LayerSpecifications(IEEE 802.3), 2002.[3] Robbin J. Blokpoel,Traffic control in coherence-multiplexed networks, BSc thesis,University of Twente, Enschede, The Netherlands, June 2005.

Trafﬁc control in coherence-multiplexed networks, BSc thesis,

Wim van Etten, “Balanced optical phase diversity receivers for coherence multiplexing,”

http://purl.utwente.nl/publications/53149

Traffic control in coherence-multiplexed networks

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