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An ATM cross-connecting node using optical CDMA
COMCOM 1556
Computer Communications 22 (1999) 849–857
An ATM cross-connecting node using optical CDMA L. Zhang*, C.-H. Eyoh, C.-H. Ng School of Electrical and Electronics Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Received 19 December 1997; received in revised form 4 November 1998; accepted 4 November 1998
Abstract An asynchronous transfer mode (ATM) cross-connecting node is proposed based on direct sequence optical code division multiple access (DS-OCDMA). No buffering is necessary to facilitate switching because code conversion is instead emulating the switching function. The switch is also capable of limited dynamic bandwidth allocation. It is free from timing jitters and switching delay is significantly reduced. The switch provides a new approach to asynchronous cross-connection in the ATM core network, locally or over a wider area. The performance with the new proposed switch is evaluated using prime and modified prime OCDMA codes. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Asynchronous transfer mode; Optical code division multiple access; Virtual path; Switching architecture
1. Introduction Asynchronous transfer mode (ATM) has been selected as the solution for broadband ISDN (B-ISDN). Generally, ATM cells can be mapped into different transmission frames [1] but the de facto standard of the physical medium used to support ATM is the synchronous digital hierarchy (SDH) format which is a time-division multiplexing (TDM) basis system. The clock signal in SDH must be distributed to the receivers for proper demultiplexing. However, timing jitters are introduced by signal regenerators, line repeaters and the timing recovery mechanism used for TDM multiplexer [2]. This has necessitated the provision of buffering bits to cushion the jitters effect. Additionally, interface between ATM and SDH is required to identify ATM cell boundaries which is normally carried out by scrambling the cell’s payload when cell flow is from the physical layer to the ATM layer and also remapping virtual paths (VPs) to digital paths when cell flow is from ATM to SDH. Clearly, this limits the flexibility and efficiency in the use of VPs because dynamic reconfiguration of VPs’ bandwidth as well as routing requires a corresponding change in the SDH digital network which imposes delay and introduces higher call blocking probability as well. In fact, SDH was developed to replace the plesiochronous digital hierarchy (PDH) by providing simpler synchronous multiplexing and demultiplexing of various tributaries with * Corresponding author. Tel.: 1 65-799-1285; fax: 1 65-792-0415. E-mail address: [email protected] (L. Zhang)
different bit rates, but this advantage is lost in ATM networks when ATM cells are multiplexed/demultiplexed between ATM layer and SDH. Therefore, delays are imposed at both multiplexing/demultiplexing and switching fabric buffering. However, the integration of lower or higher bit rate channels into the payload may require the use of padding bytes and in the case when a cell straddles two frames a time delay is incurred. The demultiplexing delay variation may be small but the switch buffering delay can be very high particularly for bursty traffic. Bursty traffic also leads to higher cell loss rate if the buffering mechanism is not optimally dimensioned. However, bursty traffic not only create problems in resource allocation but also generate substantial waste of bandwidth in SDH network, because certain SDH frames are made relatively empty during the silence periods. Even though higher SDH rates (up to OC768c) and hybrid SDH/DWDM are proposed [3], the limited speed of electronic processings and the access speed of electronic buffers will eventually become the bottleneck of ATM switching. The dynamic variations of cell loss and cell delay caused by the ATM/SDH network make quality of service (QoS) assurance difficult because call admission control usually makes pessimistic estimation during resource negotiations. Commercially, ATM is most favorably used in the backbone network, which is of interest in an integrated traffic environment. Wavelength division multiplexing (WDM) may also be used as the physical medium for ATM. WDM enables ATM to access a huge amount of bandwidth but offers very coarse bandwidth granulity due to cross-talk in very dense WDM systems
0140-3664/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0140-366 4(99)00052-3
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[4]. Hence, it is beneficial to exploit other asynchronous access techniques such as optical code-division multiple access. In this article, we present an ATM switch using OCDMA which each VP is assigned an OCDMA code, so that flexibility and efficiency of utilizing network resources can be enhanced. To maximize this flexibility, a VP connection (VPC) may span a number of hops of VPs. In fact, VPs only represent the logical connection across the network. Physically, CDMA codes are used in place of VPs to encompass the functions of multiplexing and switching using encoding and decoding of CDMA code based on lookup tables which are set up by VPCs at call admission control level. Hence, no buffering is required in the cross-connecting nodes, only code conversion is involved. In the process of call setup, call admission control (CAC) unit sends messages through the network to allocate the resources to satisfy the QoS requested by the call. At the same time VPs along the selected route together with bandwidth are physically allocated when the call is accepted [5]. The connection table for VPs is set up at cross-connecting nodes. Cell flows carrying the virtual path identifiers (VPIs) are switched at cross-connecting nodes based on the connection routing tables. By assigning the desired optical CDMA code to each corresponding VP, the code sequences can access the shared medium simultaneously. At the crossconnecting node, switching of code sequences is carried out by decoding/encoding of code sequences to transfer the traffic from input port to output port and no buffer is required. A number of optical codes have been discovered in Refs. [6–8]. However, direct sequence encoding is favored in most cases due to easier implementation. This article considers only the prime code and the modified prime code. These two classes of codes have symmetrical code structure that enables the use of more efficient recursive encoders to reduce component requirements as well as signal loss. Compared to their electronic counterparts, optical codes are amplitude modulated with ones and zeroes, resulting in stringent requirements on maintaining correlation properties. The design of the proposed cross-connecting node is heavily dependent on the particular class of the code considered. Prime code [10] is favored because of its recursive construction process and its derivative. We also consider modified prime code [7] which makes use of code synchronism to expand the original number of prime code sequence by P times, where P is a prime number. Using prime code as a reference, cross-connecting node architecture is proposed based on code conversion using look-up tables. Appropriate references are made to the modifications needed for modified prime code. The required control and management functions will also be briefly discussed. The rest of this article is organised as follows. In Section 2, the architecture of an ATM/OCDMA cross-connecting node is proposed. Details on the construction of two types
of closely related optical code are presented and their performances are evaluated in Section 3. Finally, some problems and potential solutions are analyzed in Section 4. 2. Cross-connecting node architecture Prime code is generated from a prime sequence constructed from the elements of galios field GF P {0; 1; …; j; …; P 2 1}; of a prime number P. Each prime sequence is mapped into a binary code sequence Cx. The resultant code sequence is P 2 chips in length and contains P pulses. A chip refers to the duration of a binary symbol of the optical code sequence. The maximum number of VPs that can be allocated to the prime code sequences is P. Each prime sequence Sx {Sx1 ; Sx2 ; …; Sxi ; …; Sx p21 } is derived from GF P by the following rule: i x × jmod P
;j 0; 1; 2; …; P 2 1;
1
(1)where x is an element of GF P: Hence, P, unique prime sequences can be derived. Note that each prime code sequence contains P elements. The code sequence can be segmented into P equally spaced segments each P chips in length and contains one element. Therefore, it is easy to see that each element of the prime sequence represents the position of a pulse relative to the beginning of the corresponding segment. It will become clear when the prime sequence is mapped into the signature code Cx {Cx0 ; Cx1 ; …; Cxi ; …; Cx P21 } using i Sxj 1 jP
;j 0; 1; …; P 2 1:
2
Table 1 illustrates the prime code sequences generated from GF(5). The disadvantage of using prime code is due to the limited number of orthogonal code sequences. However the symmetry provides simpler encoding mechanism. Time shifted version from each original set of prime sequences, can be used generate P 2 1 new prime sequences, that is, P 2 distinct binary codes can be derived from these P 2 prime sequences. The cross-correlation between binary codes from the same group is zero whereas between binary codes from different groups is less than or equal to one. As the binary codes from the same group have zero cross-correlation, interference cancellation can be implemented based on this property [12] to improve performance. Table 2 shows some of the modified prime code sequences derived from GF(5). The proposed cross-connecting node architecture as Table 1 Prime code sequences from GF(5) C0 C1 C2 C3 C4
10000 10000 10000 10000 10000
10000 01000 00100 00010 00001
10000 00100 00001 01000 00010
10000 00010 01000 00001 00100
10000 00001 00010 00100 01000
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851
Fig. 1. ATM optical cross-connect based on optical CDMA. The optical decoding and encoding process are code specific. Illustrated for synchronous modified prime code.
shown in Fig. 1 consists of four functional blocks: (1) demultiplexing stage at the input ports including correlation receivers and threshold detectors; (2) optical-electrical photo-detection stage; (3) multiplexing stage at the output port where the regenerated signal is encoded into new code
sequences and routed to the corresponding output link; and (4) control stage including look-up table and code mapping module. Most of the functions are grouped into the switching modules. Up scaling of the switching capacity can be carried out using additional modules.
Table 2 Modified prime code sequence from GF(5)
2.1. Demultiplexing of Input OCDMA code sequences
C0,0 C0,1 C0,2 C0,3 C0,4
10000 00001 00010 00100 01000
10000 00001 00010 00100 01000
10000 00001 00010 00100 01000
10000 00001 00010 00100 01000
10000 00001 00010 00100 01000
C1,0 C1,1 C1,2 C1,3 C1,4
10000 01000 00100 00010 00001
01000 00100 00010 00001 10000
00100 00010 00001 10000 01000
00010 00001 10000 01000 00100
00001 10000 01000 00100 00010
C2,0 C2,1 C2,2 C2,3 C2,4
10000 00100 00001 01000 00010
00100 00001 01000 00010 10000
00001 01000 00010 10000 00100
01000 00010 10000 00100 00001
00010 10000 00100 00001 01000
C3,0 C3,1 C3,2 C3,3 C3,4
10000 00010 01000 00001 00100
00010 01000 00001 00100 10000
01000 00001 00100 10000 00010
00001 00100 10000 00010 01000
00100 10000 00010 01000 00001
C4,0 C4,1 C4,2 C4,3 C4,4
10000 00001 00010 00100 01000
00001 00010 00100 01000 10000
00010 00100 01000 10000 00001
00100 01000 10000 00001 00010
01000 10000 00001 00010 00100
This is carried out by the correlation of the multiplexed signal with the code sequence of the desired VP. The receiver’s output consists of a delta-shaped auto-correlation function, cross-correlations each bounded by the crosscorrelation constraint of the code sequences, as well as channel noise. The correlation decoder is functionally similar to an optical match filter whose transfer function is matched to that of the encoder. A fixed decoder is required for each code, but in practice tunable decoders can be used to improve utilization and also to provide limited bandwidth allocation capability. The correlation decoder is made up of parallel delay lines which are inversely matched to the delays used for the desired code sequence. The set of delay causes the weighted chips to coincide at the decision instance, which is the last chip position. Hence, the output light intensity traces out the correlation function of the desired code sequence when the sequence passes through the correlator. Consider the receiver is fed by N VPs’ code sequences, where VP 1 is the desired VP of the receiver. The receiver’s output can generally be expressed by the following equation: y t b1 c1 tc1 t 2 t1 1
N X
bk c1 tck t 2 tk 1 n t;
k1 ;k±1
3
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Fig. 2. Optical correlation Receiver (non-tunable). A wavelength converter is used to convert the clock to the spectrum of the OCDMA signals.
where b1 {…b121 ; b10 ; b11 …}; b1n [ {0; 1} represents the binary data sequence of VP 1 with symbol duration T. Likewise, b k is the data sequence of VP k k 2; 3; …; N: c1 t is the time function of code sequence for the desired VP 1 and ck t is the time function of interference caused by VP k. Also the relative delays between the referred to code sequence of VP 1 and the other code sequences is defined as t k. The first term on the right-hand side of Eq. (3) represents the auto-correlation function and the second term of summation indicates the cross-correlation from the other (N 2 1) VPs. n t consists of channel noise and photo-detection noise. The time function c1 t of code sequences is expressed by: c1 t
∞ X j2 ∞
a1j PTc t 2 jTc ;
4
where a1j [ 0; 1 is a periodic binary sequence with period N, which is the code length. We assume that each data bit is encoded with N chips, i.e. T NTc : PTc is defined as: ( 1; 0 , t , Tc : 5 PTc t 0; otherwise As the direct detection receiver does not require coherent signal, we assume that t 1 0 and thus the auto-correlation can be given by: uc1 tu W;
0 , t , NTc ;
6
where W is defined as the code sequence weight. Therefore, the cross-correlation between VP k and VP 1 can be expressed as: c1 tck t 2 tk
∞ X
∞ X
j2 ∞ l2 ∞
information is being transmitted on VP 1, error occurs if the multiple access interference (MAI) is strong enough to be detected. This property is the main influence on the systems performance. When modified prime code is used the following modifications are required. As the auto-correlation function of some known modified prime codes have very high side lobes and their cross-correlation can be as high as its autocorrelation peak [10]. This property can be derived from the shifting of code sequences shown in Table 2. For tk ± 0; the auto-correlations for the designed code sequence of VP 1 can be obtained from Eq. (3) i.e.: X b0;n c0;0 c0;n t 2 t0;n y t n0
1
X X
bm;n c0;0 cm;n t 2 tm;n ;
8
m±0 n±0
where c0,0 is the code that the receiver is matched to. Note that c0;i ;i 0; 1; …; P 2 1 are the code sequences belonging to the same group. Hence, they produce autocorrelation when tk ± 0: Owing to the large sidelobes, the transmission needs to be synchronised so that the decoder can be synchronized to perform detection at the last chip position. Wavelength converter is employed at the receiving end to provide the synchronising signal which is used to operate logic gates that closes the path for the correlated signals to pass through at the last chip position. A conventional decoder is shown in Fig. 2. The structure in dashed lines are used for the synchronized detection of modified prime codes. 2.2. Optical-electrical conversion
a1j akl PTc t
2 jTc
7
× PTc t 2 lTc 2 tk : A threshold detector is used to detect the peak of the autocorrelation function and reject cross-correlations. When no
The correlated signal is converted to the electrical baseband signal to filter interference from MAI and channel noise. The output of the threshold detector should ideally be optical pulses of chip duration. The pulses can be used as the chip waveform for encoding to maintain tolerance to pulse dispersion. However, in practice it should be
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853
Fig. 3. Tunable modified prime code encoder. The delay selector accepts the code sequence and mapped to the control signals for the switches based on the lookup table.
conditioned prior to further retransmissions. A photo-detector with sample and hold circuit can be used to convert the optical pulse into electrical pulse of data bit duration. An optical source has an output rate of P times of information bit rate to generate chip pulses for encoding. When a code sequence is transmitted the recovered electrical signal controls logic gates which correspond to the data pattern being transmitted. The process also effectively regenerates the information bit stream for transmission over the next lap. In addition, the optical-electrical signal conversion allows electronic processing to be performed, either in serial or parallel. For example, an OCDMA adaptation layer error detection and correction procedure may be implemented to improve transmission characteristics. An electronics VC switch can also be integrated to transform the node into a transit-switching node. The addition of these signal processing functions will impose some delay on the switching time but these are deterministic and the performance impact will be minimal.
output VP. The encoder’s operation is similar to the demultiplexing decoder. A tunable encoder consisting of 2 × 2 optical space switches associated with delay lines. The encoder receives signal from the switch controller for setting the status of the 2 × 2 switches (i.e. cross or bar). The delays in terms of chip intervals are in power on the basis of two and the maximum required delay is P chip intervals. Fig. 3 shows a tunable encoder for the prime code and further details on the encoder can be found in Ref. [11]. For every bit of information a series of P pulses, each pulse with one chip interval, are passed through the encoder. Each of the P pulses will be delayed up to P chip intervals, where the output of the encoder is the concatenation of these P delayed pulses. The output of the encoder can be represented by Eqs. (4) and (5). The encoded signal will then be routed to the output link through a set of logic gates. The routing is carried out by the switch controller using a lookup table. 2.4. Node control
2.3. Switching and multiplexing Switching of ATM cells is carried out by encoding the bit stream of the cell into a new code sequence assigned to the
As mentioned in the last section, control signals are necessary to activate the switches in the encoder. A controller consists of I/O interface, a look-up table and a
Fig. 4. A reference implementation of the lookup table for optical conversion, i.e. VP switching.
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code-mapping module. The I/O interface communicates with network management functions to exchange information regarding code allocation or VP switching assignments. The look-up table as shown in Fig. 4 contains the information for each VP connection from the input ports to the output ports. It should be noted that the code assigned to VP, is logical and when the VP connection is terminated, the same code may be reassigned to other VP connections. Each encoder is able to tune to any code sequence by controlling the set of 2 × 2 switches. The code sequence for the output VP is programmable and the relevant control signal is transmitted to the encoder through the code-mapping module. The network management information can be transferred by operation and management (OAM) cells through the ATM network. The bandwidth of the assigned VPs are given by the product of traffic peak rate and the length of code sequence. As the highest chip rate is limited by dispersion and pulse spreading in the fiber the bandwidth is determined by the ratio of peak chip rate to code sequence length. However, it should be noted that the acceptable code sequence length has to be the square of prime numbers and the other lengths will compromise the correlation properties. We also note that increasing the VP bit rate will lead to less number of chips used to encode each information bit, but the product of traffic bit rate and the number of chips per bit remains constant, if the optical source is operating at peak chip rate. However, a variation of the VP bit rate must be met by corresponding reduction in the number of simultaneously transmitting VPs, hence, a feedback or feed-forward control algorithm is required for dynamic bandwidth allocation.
3. Illustrative code performance In OCDMA system a spreading code is transmitted in place of data bit using intensity modulation. During signal recovery the receiver’s decision statistics are converted to electrical signals through threshold detection. The threshold detector decides if binary ‘1’ is transmitted. The conversion process can be conveyed by the following expression [9], which equates the photoelectrons generated to the intensity of the incident photons, resulting in an electron intensity:
le t
hP t 1 Ld ; hf
9
where P t is the optical power of the incident lightwave on the photodetector. h and f are the Plank’s constant and the lightwave frequency, respectively. Ld is the photodetector’s dark current and h the efficiency of the photodetector. From the electron intensity the mean arrival rate for the compound poisson distributed detection process can be derived. The detected OCDMA signal can be view as a compound poisson process consisting of photon–electrons arriving a rate determined by Eq. (9). Compared to their wireless counterparts OCDMA systems are widely regarded as “positive”
systems. Hence, the auto-correlation results in addition of optical power and produces a very high peak when the relative delay t x is zero. The first term on the right in Eq. (3) represents the autocorrelation function while the latter indicates the crosscorrelation for the other (N 2 1) VPs, where N is the number of simultaneously transmitting VPs. Although the crosscorrelation is usually defined by its constraint however, their interference pattern is treated as random variables and their joint p.d.f. describes the probability of error conditioned on the data pattern. It is reasonable to assume that the VPs are independent and thus their sum is assumed to be gaussian distributed. The error probability in terms of SNR can be found from [10] as follows: SNR
P2 ; N 2 1s2
10
where P 2 gives the normalized power of the detected autocorrelation and s 2 the average variance of the interference from each user. s 2 is regarded as the noise component. It was found in Ref. [7] that the variance is approximately 0.29. Hence, the gaussian approximated probability of error is: p ! 2 SNR ; Pe F 2 F
2P p ; 1:16 N 2 1
11
where F x is the unit normal cumulative distribution function. To understand the trade-off between multiplexing more VPs and dropping performance we next consider a modified prime code system with N users. The bit error probability can be given by the following equation: Pe Pr erroruN users; bit ‘0’ sentP0 1 Pr erroruN users; bit ‘1’ sentP1 ;
12
where Pr(x) denotes the probability of event (x) and P0 P1 for equal probability that a data bit ‘0’ or bit ‘1’ is sent. The first term on the right in (12) refers to the probability that the intended user sent bit ‘0’ and errors occur when the N 2 1 interferers result in cross-correlation being greater than the threshold. Then the cross-correlation output can be modeled by the binomial distribution [10] as follows: ! Th X N 2 1 N212k k P0 Pr erroruN users; bit ‘0’ sent 1 2 P1 ; k k0 13 where the summation is the receiver’s output and Th the receiver’s threshold. Likewise, the second term in Eq. (12)
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855
Fig. 5. Relative performance of prime and modified prime code for 51, 155 and 622 Mbps user rate.
can be defined as follows: Pr erroruN users; bit ‘1’ sent; 1 2
P 1X N 21
N 21
kTh
k2P
the mean. Therefore, for the modified prime code [10] with the optimum threshold, the gaussian approximation can be simplified as the following error probability: 2P Pe F p : 16 N21
!
× P0P1N212k Pk2P 1 ; 14 where P , k , P 1 N 2 1: The Eq. (14) gives the probability that the cross-correlation output is below the threshold. In direct sequence OCDMA the bandwidth requirement is dependent on the maximum chip rate. The data bandwidth can be found from the processing gain. For prime and modified prime code the code length is P 2 chips. Prime code is able to support P VPs. However, modified prime code is able to allocate P 2 distinct code sequences to the VPs. Hence we can assume that N is large, binomial distribution can then be approximated by the gaussian distribution. Combining of Eqs. (13) and (14) with the mean m N 2 1P1 and m 0 2P 1 N 2 1P1 ; respectively, where both Eqs. (13) and (14) have the same variance s2 N 2 1P1 1 2 P1 ; then we have: " ! ZTh 1 1 2 x2m2 = 2s2 p e 12 dx Pe 2 2∞ 2ps2 15 !# Z∞ 1 2 x2m 0 2 = 2s2 p e dx : 1 12 Th 2ps2 It is shown that for the symmetrical cross-correlation output distribution the optimum threshold is found to be
Fig. 5 plots the bit error rate for both classes of code with defined ATM bit rate of 51, 155, and 622 Mbps. From the figure modified prime code clearly performs better. ATM cell header has a forward error correction (FEC) scheme using CRC-8 code and the segment and reassembly (SAR) at AAL layer has the error detection scheme using CRC-10. Fig. 6 shows the performance of cell error with the aforementioned error protection mechanism i.e., ! PX 1H P 1 H Pje 1 2 Pe P1H2j Pr a cell in error j j5 ! ! !! P1H P H 1 2 P4e 1 2 Pe P1H24 4 2 2 ! ! ! ! !! P1H P H P H 1 2 2 3 2 1 1 2 × P3e 1 2 Pe P1H23 ;
17
where P and H are the ATM payload and header size, respectively. The error correcting codes are able to correct one error and detect two bit errors. We have assumed link
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Fig. 6. Performance comparison between prime and modified prime code in terms of cell error rate.
control exist to allow recovery of up to two bit errors. The first term of Eq. (17) denotes the probability the number of errors exceeds the correcting ability of the error correction mechanism. The second and the third terms take into account situations where the total number of errors exceed the recovery limit but not when they are considered individually. Fig. 6 clearly shows that as the VP bandwidth decreases the performance margin between the codes widens. For example, if the acceptable upper bound on cell error rate is 10 26, the prime code systems are able to support up to 10 VPs of bandwidth 51 Mbps simultaneously whereas modified prime code systems are capable of supporting 12 VPs with the same bandwidth while maintaining the same cell error rate. The major difference between prime and modified prime code becomes even more obvious as shown by the plots for VPs with bandwidth of 25 Mbps. For the same upper bound of cell error rate the prime code system is able to support up to 19 VPs, in contrast, the modified prime code systems can carry as much as 25 VPs. Hence, it has been shown that the modified prime code are more suitable for lower bit rate application and the prime code has the lower capacity.
4. Conclusion The design of a new cross-connecting node using optical code-division multiplexing technique is presented. Switching and multiplexing are implemented using code conversion based on lookup table. A delay that is not more than the longest duration of a information bit may incur during
transit but this compares favorably with the delay caused by multiplexing/demultiplexing and buffering fabric used in TDMA system. Compared to their byte synchronizing, optical CDMA provides a new approach to the physical media access and drastic reduction in call setup procedures. Problems with timing jitter can safely be defused with little or no synchronism between the network entities. With optical signal processing the potential for growth is bounded only by conventional means, e.g. bandwidth of fiber, nonlinear effects and optical pulse dispersion. Optical CDMA channels are inherently noisy, as every users are simultaneously accessing the physical media. The noise floor of such systems no doubt is higher than that other access methods, but this can be improved with several techniques, e.g. optical hard-limiter, multi-user detection [13] and more robust error recovery mechanism at higher layers. The performance of these systems improves as the processing gain or the number of simultaneous transmissions is limited, hence a trade off in capacity against BER cannot be avoided. However a number of articles have been published on the integration of optical CDMA and WDM technologies [14,15], which will vastly increase the number of simultaneously transmitting VPs. The proposed cross-connect can be used for both local and wide area networks. It can be used just as a network access technology or with full ATM implementation at very high speed. With a gateway node consisting of optical CDMA and electronic virtual channel (VC) switching components, the interconnecting to an optical CDMA based core network is fast and easy, while the delay variance can be reduced and accurately predicted. The proposed optical CDMA cross-connecting node provides vast opportunity
L. Zhang et al. / Computer Communications 22 (1999) 849–857
in further research an optical ATM network and more importantly itself as a derivative of TDM. Thus directly benefiting from breakthroughs and advances in TDM systems. The ability to adapt these improvements to OCDMA system will potentially lower the cost of development.
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Dr Liren Zhang is currently an Associate Professor in the School of Electrical and Electronic Engineering, Nanyang Technological University (NTU). He received his B.Eng. degree from Shandong University of Technology in 1982, M.Eng. degree from the University of South Australia in 1988, and Ph.D. from the University of Adelaide, Australia in 1990, all in electrical engineering. From 1990 to 1995, he was a Senior Lecturer in the Department of Electrical and Computer Systems Engineering, Monash University, Australia. Dr Zhang has vast experience as an engineer, academic and researcher in the field of multimedia communications, switching and signaling, teletraffic engineering, network modeling and performance analysis for ATM networks, high speed data networks, mobile networks and satellite networks.
Chih-Hong Eyoh received the B.Eng.(Hons) in electrical and electronics engineering from the University of Strathclyde, Glasgow, Scotland, in 1995. He is currently pursuing his Ph.D. degree at the Nanyang Technological University (NTU), Singapore. His research interests include broadband networking, ATM and CDMA communications.
Chee-Hock Ng is currently an Associate Professor in the School of Electrical and Electronic Engineering, Nanyang Technological University (NTU). He received his B.Eng.(Hons) degree from the National University of Singapore in 1981 and M.Sc. (Computer Engineering) degree from the University of Southern California in 1987. He has vast experience as an engineer, academic and researcher in the field of data communications, networking and network performance analysis. He was the Head of Resource Management Group in one of the organizations in the Ministry of Defence, Singapore. At NTU, he has conducted numerous short courses in the area of networking to computer professionals in the industry. He is also the author of a book “Queueing Modelling Fundamentals” published by John Wiley in 1996.
Computer Communications 22 (1999) 849–857
An ATM cross-connecting node using optical CDMA L. Zhang*, C.-H. Eyoh, C.-H. Ng School of Electrical and Electronics Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Received 19 December 1997; received in revised form 4 November 1998; accepted 4 November 1998
Abstract An asynchronous transfer mode (ATM) cross-connecting node is proposed based on direct sequence optical code division multiple access (DS-OCDMA). No buffering is necessary to facilitate switching because code conversion is instead emulating the switching function. The switch is also capable of limited dynamic bandwidth allocation. It is free from timing jitters and switching delay is significantly reduced. The switch provides a new approach to asynchronous cross-connection in the ATM core network, locally or over a wider area. The performance with the new proposed switch is evaluated using prime and modified prime OCDMA codes. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Asynchronous transfer mode; Optical code division multiple access; Virtual path; Switching architecture
1. Introduction Asynchronous transfer mode (ATM) has been selected as the solution for broadband ISDN (B-ISDN). Generally, ATM cells can be mapped into different transmission frames [1] but the de facto standard of the physical medium used to support ATM is the synchronous digital hierarchy (SDH) format which is a time-division multiplexing (TDM) basis system. The clock signal in SDH must be distributed to the receivers for proper demultiplexing. However, timing jitters are introduced by signal regenerators, line repeaters and the timing recovery mechanism used for TDM multiplexer [2]. This has necessitated the provision of buffering bits to cushion the jitters effect. Additionally, interface between ATM and SDH is required to identify ATM cell boundaries which is normally carried out by scrambling the cell’s payload when cell flow is from the physical layer to the ATM layer and also remapping virtual paths (VPs) to digital paths when cell flow is from ATM to SDH. Clearly, this limits the flexibility and efficiency in the use of VPs because dynamic reconfiguration of VPs’ bandwidth as well as routing requires a corresponding change in the SDH digital network which imposes delay and introduces higher call blocking probability as well. In fact, SDH was developed to replace the plesiochronous digital hierarchy (PDH) by providing simpler synchronous multiplexing and demultiplexing of various tributaries with * Corresponding author. Tel.: 1 65-799-1285; fax: 1 65-792-0415. E-mail address: [email protected] (L. Zhang)
different bit rates, but this advantage is lost in ATM networks when ATM cells are multiplexed/demultiplexed between ATM layer and SDH. Therefore, delays are imposed at both multiplexing/demultiplexing and switching fabric buffering. However, the integration of lower or higher bit rate channels into the payload may require the use of padding bytes and in the case when a cell straddles two frames a time delay is incurred. The demultiplexing delay variation may be small but the switch buffering delay can be very high particularly for bursty traffic. Bursty traffic also leads to higher cell loss rate if the buffering mechanism is not optimally dimensioned. However, bursty traffic not only create problems in resource allocation but also generate substantial waste of bandwidth in SDH network, because certain SDH frames are made relatively empty during the silence periods. Even though higher SDH rates (up to OC768c) and hybrid SDH/DWDM are proposed [3], the limited speed of electronic processings and the access speed of electronic buffers will eventually become the bottleneck of ATM switching. The dynamic variations of cell loss and cell delay caused by the ATM/SDH network make quality of service (QoS) assurance difficult because call admission control usually makes pessimistic estimation during resource negotiations. Commercially, ATM is most favorably used in the backbone network, which is of interest in an integrated traffic environment. Wavelength division multiplexing (WDM) may also be used as the physical medium for ATM. WDM enables ATM to access a huge amount of bandwidth but offers very coarse bandwidth granulity due to cross-talk in very dense WDM systems
0140-3664/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0140-366 4(99)00052-3
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[4]. Hence, it is beneficial to exploit other asynchronous access techniques such as optical code-division multiple access. In this article, we present an ATM switch using OCDMA which each VP is assigned an OCDMA code, so that flexibility and efficiency of utilizing network resources can be enhanced. To maximize this flexibility, a VP connection (VPC) may span a number of hops of VPs. In fact, VPs only represent the logical connection across the network. Physically, CDMA codes are used in place of VPs to encompass the functions of multiplexing and switching using encoding and decoding of CDMA code based on lookup tables which are set up by VPCs at call admission control level. Hence, no buffering is required in the cross-connecting nodes, only code conversion is involved. In the process of call setup, call admission control (CAC) unit sends messages through the network to allocate the resources to satisfy the QoS requested by the call. At the same time VPs along the selected route together with bandwidth are physically allocated when the call is accepted [5]. The connection table for VPs is set up at cross-connecting nodes. Cell flows carrying the virtual path identifiers (VPIs) are switched at cross-connecting nodes based on the connection routing tables. By assigning the desired optical CDMA code to each corresponding VP, the code sequences can access the shared medium simultaneously. At the crossconnecting node, switching of code sequences is carried out by decoding/encoding of code sequences to transfer the traffic from input port to output port and no buffer is required. A number of optical codes have been discovered in Refs. [6–8]. However, direct sequence encoding is favored in most cases due to easier implementation. This article considers only the prime code and the modified prime code. These two classes of codes have symmetrical code structure that enables the use of more efficient recursive encoders to reduce component requirements as well as signal loss. Compared to their electronic counterparts, optical codes are amplitude modulated with ones and zeroes, resulting in stringent requirements on maintaining correlation properties. The design of the proposed cross-connecting node is heavily dependent on the particular class of the code considered. Prime code [10] is favored because of its recursive construction process and its derivative. We also consider modified prime code [7] which makes use of code synchronism to expand the original number of prime code sequence by P times, where P is a prime number. Using prime code as a reference, cross-connecting node architecture is proposed based on code conversion using look-up tables. Appropriate references are made to the modifications needed for modified prime code. The required control and management functions will also be briefly discussed. The rest of this article is organised as follows. In Section 2, the architecture of an ATM/OCDMA cross-connecting node is proposed. Details on the construction of two types
of closely related optical code are presented and their performances are evaluated in Section 3. Finally, some problems and potential solutions are analyzed in Section 4. 2. Cross-connecting node architecture Prime code is generated from a prime sequence constructed from the elements of galios field GF P {0; 1; …; j; …; P 2 1}; of a prime number P. Each prime sequence is mapped into a binary code sequence Cx. The resultant code sequence is P 2 chips in length and contains P pulses. A chip refers to the duration of a binary symbol of the optical code sequence. The maximum number of VPs that can be allocated to the prime code sequences is P. Each prime sequence Sx {Sx1 ; Sx2 ; …; Sxi ; …; Sx p21 } is derived from GF P by the following rule: i x × jmod P
;j 0; 1; 2; …; P 2 1;
1
(1)where x is an element of GF P: Hence, P, unique prime sequences can be derived. Note that each prime code sequence contains P elements. The code sequence can be segmented into P equally spaced segments each P chips in length and contains one element. Therefore, it is easy to see that each element of the prime sequence represents the position of a pulse relative to the beginning of the corresponding segment. It will become clear when the prime sequence is mapped into the signature code Cx {Cx0 ; Cx1 ; …; Cxi ; …; Cx P21 } using i Sxj 1 jP
;j 0; 1; …; P 2 1:
2
Table 1 illustrates the prime code sequences generated from GF(5). The disadvantage of using prime code is due to the limited number of orthogonal code sequences. However the symmetry provides simpler encoding mechanism. Time shifted version from each original set of prime sequences, can be used generate P 2 1 new prime sequences, that is, P 2 distinct binary codes can be derived from these P 2 prime sequences. The cross-correlation between binary codes from the same group is zero whereas between binary codes from different groups is less than or equal to one. As the binary codes from the same group have zero cross-correlation, interference cancellation can be implemented based on this property [12] to improve performance. Table 2 shows some of the modified prime code sequences derived from GF(5). The proposed cross-connecting node architecture as Table 1 Prime code sequences from GF(5) C0 C1 C2 C3 C4
10000 10000 10000 10000 10000
10000 01000 00100 00010 00001
10000 00100 00001 01000 00010
10000 00010 01000 00001 00100
10000 00001 00010 00100 01000
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851
Fig. 1. ATM optical cross-connect based on optical CDMA. The optical decoding and encoding process are code specific. Illustrated for synchronous modified prime code.
shown in Fig. 1 consists of four functional blocks: (1) demultiplexing stage at the input ports including correlation receivers and threshold detectors; (2) optical-electrical photo-detection stage; (3) multiplexing stage at the output port where the regenerated signal is encoded into new code
sequences and routed to the corresponding output link; and (4) control stage including look-up table and code mapping module. Most of the functions are grouped into the switching modules. Up scaling of the switching capacity can be carried out using additional modules.
Table 2 Modified prime code sequence from GF(5)
2.1. Demultiplexing of Input OCDMA code sequences
C0,0 C0,1 C0,2 C0,3 C0,4
10000 00001 00010 00100 01000
10000 00001 00010 00100 01000
10000 00001 00010 00100 01000
10000 00001 00010 00100 01000
10000 00001 00010 00100 01000
C1,0 C1,1 C1,2 C1,3 C1,4
10000 01000 00100 00010 00001
01000 00100 00010 00001 10000
00100 00010 00001 10000 01000
00010 00001 10000 01000 00100
00001 10000 01000 00100 00010
C2,0 C2,1 C2,2 C2,3 C2,4
10000 00100 00001 01000 00010
00100 00001 01000 00010 10000
00001 01000 00010 10000 00100
01000 00010 10000 00100 00001
00010 10000 00100 00001 01000
C3,0 C3,1 C3,2 C3,3 C3,4
10000 00010 01000 00001 00100
00010 01000 00001 00100 10000
01000 00001 00100 10000 00010
00001 00100 10000 00010 01000
00100 10000 00010 01000 00001
C4,0 C4,1 C4,2 C4,3 C4,4
10000 00001 00010 00100 01000
00001 00010 00100 01000 10000
00010 00100 01000 10000 00001
00100 01000 10000 00001 00010
01000 10000 00001 00010 00100
This is carried out by the correlation of the multiplexed signal with the code sequence of the desired VP. The receiver’s output consists of a delta-shaped auto-correlation function, cross-correlations each bounded by the crosscorrelation constraint of the code sequences, as well as channel noise. The correlation decoder is functionally similar to an optical match filter whose transfer function is matched to that of the encoder. A fixed decoder is required for each code, but in practice tunable decoders can be used to improve utilization and also to provide limited bandwidth allocation capability. The correlation decoder is made up of parallel delay lines which are inversely matched to the delays used for the desired code sequence. The set of delay causes the weighted chips to coincide at the decision instance, which is the last chip position. Hence, the output light intensity traces out the correlation function of the desired code sequence when the sequence passes through the correlator. Consider the receiver is fed by N VPs’ code sequences, where VP 1 is the desired VP of the receiver. The receiver’s output can generally be expressed by the following equation: y t b1 c1 tc1 t 2 t1 1
N X
bk c1 tck t 2 tk 1 n t;
k1 ;k±1
3
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Fig. 2. Optical correlation Receiver (non-tunable). A wavelength converter is used to convert the clock to the spectrum of the OCDMA signals.
where b1 {…b121 ; b10 ; b11 …}; b1n [ {0; 1} represents the binary data sequence of VP 1 with symbol duration T. Likewise, b k is the data sequence of VP k k 2; 3; …; N: c1 t is the time function of code sequence for the desired VP 1 and ck t is the time function of interference caused by VP k. Also the relative delays between the referred to code sequence of VP 1 and the other code sequences is defined as t k. The first term on the right-hand side of Eq. (3) represents the auto-correlation function and the second term of summation indicates the cross-correlation from the other (N 2 1) VPs. n t consists of channel noise and photo-detection noise. The time function c1 t of code sequences is expressed by: c1 t
∞ X j2 ∞
a1j PTc t 2 jTc ;
4
where a1j [ 0; 1 is a periodic binary sequence with period N, which is the code length. We assume that each data bit is encoded with N chips, i.e. T NTc : PTc is defined as: ( 1; 0 , t , Tc : 5 PTc t 0; otherwise As the direct detection receiver does not require coherent signal, we assume that t 1 0 and thus the auto-correlation can be given by: uc1 tu W;
0 , t , NTc ;
6
where W is defined as the code sequence weight. Therefore, the cross-correlation between VP k and VP 1 can be expressed as: c1 tck t 2 tk
∞ X
∞ X
j2 ∞ l2 ∞
information is being transmitted on VP 1, error occurs if the multiple access interference (MAI) is strong enough to be detected. This property is the main influence on the systems performance. When modified prime code is used the following modifications are required. As the auto-correlation function of some known modified prime codes have very high side lobes and their cross-correlation can be as high as its autocorrelation peak [10]. This property can be derived from the shifting of code sequences shown in Table 2. For tk ± 0; the auto-correlations for the designed code sequence of VP 1 can be obtained from Eq. (3) i.e.: X b0;n c0;0 c0;n t 2 t0;n y t n0
1
X X
bm;n c0;0 cm;n t 2 tm;n ;
8
m±0 n±0
where c0,0 is the code that the receiver is matched to. Note that c0;i ;i 0; 1; …; P 2 1 are the code sequences belonging to the same group. Hence, they produce autocorrelation when tk ± 0: Owing to the large sidelobes, the transmission needs to be synchronised so that the decoder can be synchronized to perform detection at the last chip position. Wavelength converter is employed at the receiving end to provide the synchronising signal which is used to operate logic gates that closes the path for the correlated signals to pass through at the last chip position. A conventional decoder is shown in Fig. 2. The structure in dashed lines are used for the synchronized detection of modified prime codes. 2.2. Optical-electrical conversion
a1j akl PTc t
2 jTc
7
× PTc t 2 lTc 2 tk : A threshold detector is used to detect the peak of the autocorrelation function and reject cross-correlations. When no
The correlated signal is converted to the electrical baseband signal to filter interference from MAI and channel noise. The output of the threshold detector should ideally be optical pulses of chip duration. The pulses can be used as the chip waveform for encoding to maintain tolerance to pulse dispersion. However, in practice it should be
L. Zhang et al. / Computer Communications 22 (1999) 849–857
853
Fig. 3. Tunable modified prime code encoder. The delay selector accepts the code sequence and mapped to the control signals for the switches based on the lookup table.
conditioned prior to further retransmissions. A photo-detector with sample and hold circuit can be used to convert the optical pulse into electrical pulse of data bit duration. An optical source has an output rate of P times of information bit rate to generate chip pulses for encoding. When a code sequence is transmitted the recovered electrical signal controls logic gates which correspond to the data pattern being transmitted. The process also effectively regenerates the information bit stream for transmission over the next lap. In addition, the optical-electrical signal conversion allows electronic processing to be performed, either in serial or parallel. For example, an OCDMA adaptation layer error detection and correction procedure may be implemented to improve transmission characteristics. An electronics VC switch can also be integrated to transform the node into a transit-switching node. The addition of these signal processing functions will impose some delay on the switching time but these are deterministic and the performance impact will be minimal.
output VP. The encoder’s operation is similar to the demultiplexing decoder. A tunable encoder consisting of 2 × 2 optical space switches associated with delay lines. The encoder receives signal from the switch controller for setting the status of the 2 × 2 switches (i.e. cross or bar). The delays in terms of chip intervals are in power on the basis of two and the maximum required delay is P chip intervals. Fig. 3 shows a tunable encoder for the prime code and further details on the encoder can be found in Ref. [11]. For every bit of information a series of P pulses, each pulse with one chip interval, are passed through the encoder. Each of the P pulses will be delayed up to P chip intervals, where the output of the encoder is the concatenation of these P delayed pulses. The output of the encoder can be represented by Eqs. (4) and (5). The encoded signal will then be routed to the output link through a set of logic gates. The routing is carried out by the switch controller using a lookup table. 2.4. Node control
2.3. Switching and multiplexing Switching of ATM cells is carried out by encoding the bit stream of the cell into a new code sequence assigned to the
As mentioned in the last section, control signals are necessary to activate the switches in the encoder. A controller consists of I/O interface, a look-up table and a
Fig. 4. A reference implementation of the lookup table for optical conversion, i.e. VP switching.
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code-mapping module. The I/O interface communicates with network management functions to exchange information regarding code allocation or VP switching assignments. The look-up table as shown in Fig. 4 contains the information for each VP connection from the input ports to the output ports. It should be noted that the code assigned to VP, is logical and when the VP connection is terminated, the same code may be reassigned to other VP connections. Each encoder is able to tune to any code sequence by controlling the set of 2 × 2 switches. The code sequence for the output VP is programmable and the relevant control signal is transmitted to the encoder through the code-mapping module. The network management information can be transferred by operation and management (OAM) cells through the ATM network. The bandwidth of the assigned VPs are given by the product of traffic peak rate and the length of code sequence. As the highest chip rate is limited by dispersion and pulse spreading in the fiber the bandwidth is determined by the ratio of peak chip rate to code sequence length. However, it should be noted that the acceptable code sequence length has to be the square of prime numbers and the other lengths will compromise the correlation properties. We also note that increasing the VP bit rate will lead to less number of chips used to encode each information bit, but the product of traffic bit rate and the number of chips per bit remains constant, if the optical source is operating at peak chip rate. However, a variation of the VP bit rate must be met by corresponding reduction in the number of simultaneously transmitting VPs, hence, a feedback or feed-forward control algorithm is required for dynamic bandwidth allocation.
3. Illustrative code performance In OCDMA system a spreading code is transmitted in place of data bit using intensity modulation. During signal recovery the receiver’s decision statistics are converted to electrical signals through threshold detection. The threshold detector decides if binary ‘1’ is transmitted. The conversion process can be conveyed by the following expression [9], which equates the photoelectrons generated to the intensity of the incident photons, resulting in an electron intensity:
le t
hP t 1 Ld ; hf
9
where P t is the optical power of the incident lightwave on the photodetector. h and f are the Plank’s constant and the lightwave frequency, respectively. Ld is the photodetector’s dark current and h the efficiency of the photodetector. From the electron intensity the mean arrival rate for the compound poisson distributed detection process can be derived. The detected OCDMA signal can be view as a compound poisson process consisting of photon–electrons arriving a rate determined by Eq. (9). Compared to their wireless counterparts OCDMA systems are widely regarded as “positive”
systems. Hence, the auto-correlation results in addition of optical power and produces a very high peak when the relative delay t x is zero. The first term on the right in Eq. (3) represents the autocorrelation function while the latter indicates the crosscorrelation for the other (N 2 1) VPs, where N is the number of simultaneously transmitting VPs. Although the crosscorrelation is usually defined by its constraint however, their interference pattern is treated as random variables and their joint p.d.f. describes the probability of error conditioned on the data pattern. It is reasonable to assume that the VPs are independent and thus their sum is assumed to be gaussian distributed. The error probability in terms of SNR can be found from [10] as follows: SNR
P2 ; N 2 1s2
10
where P 2 gives the normalized power of the detected autocorrelation and s 2 the average variance of the interference from each user. s 2 is regarded as the noise component. It was found in Ref. [7] that the variance is approximately 0.29. Hence, the gaussian approximated probability of error is: p ! 2 SNR ; Pe F 2 F
2P p ; 1:16 N 2 1
11
where F x is the unit normal cumulative distribution function. To understand the trade-off between multiplexing more VPs and dropping performance we next consider a modified prime code system with N users. The bit error probability can be given by the following equation: Pe Pr erroruN users; bit ‘0’ sentP0 1 Pr erroruN users; bit ‘1’ sentP1 ;
12
where Pr(x) denotes the probability of event (x) and P0 P1 for equal probability that a data bit ‘0’ or bit ‘1’ is sent. The first term on the right in (12) refers to the probability that the intended user sent bit ‘0’ and errors occur when the N 2 1 interferers result in cross-correlation being greater than the threshold. Then the cross-correlation output can be modeled by the binomial distribution [10] as follows: ! Th X N 2 1 N212k k P0 Pr erroruN users; bit ‘0’ sent 1 2 P1 ; k k0 13 where the summation is the receiver’s output and Th the receiver’s threshold. Likewise, the second term in Eq. (12)
L. Zhang et al. / Computer Communications 22 (1999) 849–857
855
Fig. 5. Relative performance of prime and modified prime code for 51, 155 and 622 Mbps user rate.
can be defined as follows: Pr erroruN users; bit ‘1’ sent; 1 2
P 1X N 21
N 21
kTh
k2P
the mean. Therefore, for the modified prime code [10] with the optimum threshold, the gaussian approximation can be simplified as the following error probability: 2P Pe F p : 16 N21
!
× P0P1N212k Pk2P 1 ; 14 where P , k , P 1 N 2 1: The Eq. (14) gives the probability that the cross-correlation output is below the threshold. In direct sequence OCDMA the bandwidth requirement is dependent on the maximum chip rate. The data bandwidth can be found from the processing gain. For prime and modified prime code the code length is P 2 chips. Prime code is able to support P VPs. However, modified prime code is able to allocate P 2 distinct code sequences to the VPs. Hence we can assume that N is large, binomial distribution can then be approximated by the gaussian distribution. Combining of Eqs. (13) and (14) with the mean m N 2 1P1 and m 0 2P 1 N 2 1P1 ; respectively, where both Eqs. (13) and (14) have the same variance s2 N 2 1P1 1 2 P1 ; then we have: " ! ZTh 1 1 2 x2m2 = 2s2 p e 12 dx Pe 2 2∞ 2ps2 15 !# Z∞ 1 2 x2m 0 2 = 2s2 p e dx : 1 12 Th 2ps2 It is shown that for the symmetrical cross-correlation output distribution the optimum threshold is found to be
Fig. 5 plots the bit error rate for both classes of code with defined ATM bit rate of 51, 155, and 622 Mbps. From the figure modified prime code clearly performs better. ATM cell header has a forward error correction (FEC) scheme using CRC-8 code and the segment and reassembly (SAR) at AAL layer has the error detection scheme using CRC-10. Fig. 6 shows the performance of cell error with the aforementioned error protection mechanism i.e., ! PX 1H P 1 H Pje 1 2 Pe P1H2j Pr a cell in error j j5 ! ! !! P1H P H 1 2 P4e 1 2 Pe P1H24 4 2 2 ! ! ! ! !! P1H P H P H 1 2 2 3 2 1 1 2 × P3e 1 2 Pe P1H23 ;
17
where P and H are the ATM payload and header size, respectively. The error correcting codes are able to correct one error and detect two bit errors. We have assumed link
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L. Zhang et al. / Computer Communications 22 (1999) 849–857
Fig. 6. Performance comparison between prime and modified prime code in terms of cell error rate.
control exist to allow recovery of up to two bit errors. The first term of Eq. (17) denotes the probability the number of errors exceeds the correcting ability of the error correction mechanism. The second and the third terms take into account situations where the total number of errors exceed the recovery limit but not when they are considered individually. Fig. 6 clearly shows that as the VP bandwidth decreases the performance margin between the codes widens. For example, if the acceptable upper bound on cell error rate is 10 26, the prime code systems are able to support up to 10 VPs of bandwidth 51 Mbps simultaneously whereas modified prime code systems are capable of supporting 12 VPs with the same bandwidth while maintaining the same cell error rate. The major difference between prime and modified prime code becomes even more obvious as shown by the plots for VPs with bandwidth of 25 Mbps. For the same upper bound of cell error rate the prime code system is able to support up to 19 VPs, in contrast, the modified prime code systems can carry as much as 25 VPs. Hence, it has been shown that the modified prime code are more suitable for lower bit rate application and the prime code has the lower capacity.
4. Conclusion The design of a new cross-connecting node using optical code-division multiplexing technique is presented. Switching and multiplexing are implemented using code conversion based on lookup table. A delay that is not more than the longest duration of a information bit may incur during
transit but this compares favorably with the delay caused by multiplexing/demultiplexing and buffering fabric used in TDMA system. Compared to their byte synchronizing, optical CDMA provides a new approach to the physical media access and drastic reduction in call setup procedures. Problems with timing jitter can safely be defused with little or no synchronism between the network entities. With optical signal processing the potential for growth is bounded only by conventional means, e.g. bandwidth of fiber, nonlinear effects and optical pulse dispersion. Optical CDMA channels are inherently noisy, as every users are simultaneously accessing the physical media. The noise floor of such systems no doubt is higher than that other access methods, but this can be improved with several techniques, e.g. optical hard-limiter, multi-user detection [13] and more robust error recovery mechanism at higher layers. The performance of these systems improves as the processing gain or the number of simultaneous transmissions is limited, hence a trade off in capacity against BER cannot be avoided. However a number of articles have been published on the integration of optical CDMA and WDM technologies [14,15], which will vastly increase the number of simultaneously transmitting VPs. The proposed cross-connect can be used for both local and wide area networks. It can be used just as a network access technology or with full ATM implementation at very high speed. With a gateway node consisting of optical CDMA and electronic virtual channel (VC) switching components, the interconnecting to an optical CDMA based core network is fast and easy, while the delay variance can be reduced and accurately predicted. The proposed optical CDMA cross-connecting node provides vast opportunity
L. Zhang et al. / Computer Communications 22 (1999) 849–857
in further research an optical ATM network and more importantly itself as a derivative of TDM. Thus directly benefiting from breakthroughs and advances in TDM systems. The ability to adapt these improvements to OCDMA system will potentially lower the cost of development.
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Dr Liren Zhang is currently an Associate Professor in the School of Electrical and Electronic Engineering, Nanyang Technological University (NTU). He received his B.Eng. degree from Shandong University of Technology in 1982, M.Eng. degree from the University of South Australia in 1988, and Ph.D. from the University of Adelaide, Australia in 1990, all in electrical engineering. From 1990 to 1995, he was a Senior Lecturer in the Department of Electrical and Computer Systems Engineering, Monash University, Australia. Dr Zhang has vast experience as an engineer, academic and researcher in the field of multimedia communications, switching and signaling, teletraffic engineering, network modeling and performance analysis for ATM networks, high speed data networks, mobile networks and satellite networks.
Chih-Hong Eyoh received the B.Eng.(Hons) in electrical and electronics engineering from the University of Strathclyde, Glasgow, Scotland, in 1995. He is currently pursuing his Ph.D. degree at the Nanyang Technological University (NTU), Singapore. His research interests include broadband networking, ATM and CDMA communications.
Chee-Hock Ng is currently an Associate Professor in the School of Electrical and Electronic Engineering, Nanyang Technological University (NTU). He received his B.Eng.(Hons) degree from the National University of Singapore in 1981 and M.Sc. (Computer Engineering) degree from the University of Southern California in 1987. He has vast experience as an engineer, academic and researcher in the field of data communications, networking and network performance analysis. He was the Head of Resource Management Group in one of the organizations in the Ministry of Defence, Singapore. At NTU, he has conducted numerous short courses in the area of networking to computer professionals in the industry. He is also the author of a book “Queueing Modelling Fundamentals” published by John Wiley in 1996.