Utilization and Delay Performance Analysis of Carrier Sense CDMA Protocol

Transcription

1 -- Tsuzuki & Yamada ISPLC 2000 Utilization and Delay Performance Analysis of Carrier Sense CDMA Protocol Shinji TSUZUKI Yoshio YAMADA Department of Electrical and Electronic Engineering, Ehime University, Matsuyama, , Japan Phone Fax ABSTRACT Ethernet is based on camer sense multiple access with collision detection (CSMNCD) protocol. Although this protocol is widely used in local area networks (LANs), it has a problem. The throughput and packet-delay performances degrade rapidly when the offered loads of packets become high. As a solution to the problem, we have proposed camer sense spread spectrum with overload detection (CSSSIOD) protocol, which is a DS - CDMA (direct sequence - code division multiple access) based protocol with a carrier sensing function and changes the spreading factor adaptively depending on network loads to improve the conventional Ethernet's performance. In this paper, we show its optimized performances of the channel utilization and the delay time. key words: multiple-access techniques, spread spectrum techniques, local area network (LAN), carrier sense multiple access (CSMA), CDMA, multi-carrier modulation 1. Introduction, The growing success of digital services related to new directions in entertainment, high-speed Internet access, combined with trends in home automation, control, and security is leading to a new focus on digital home networks. Considering Japanese houses, the additional-wiring-free media, such as power-line carrier and RF, could be the promising candidates for the network media. As the suitable modulation technique for those media, spread spectrum (SS) and multi-carrier, such as Orthogonal Frequency Division Multiplexing (OFDM) and Discrete Multi Tone (DMT), have been studied. However, the multiple-access protocol suitable for these modulation techniques has not yet studied sufficiently. In this paper, the multiple-access protocol using SS modulation is introduced and its utilization and delay performance is analyzed. The carrier sense multiple access (CSMA) is often used in local area networks (LANs) and power line communication (PLC). For example, Ethernet is based on CSMA with collision detection (CSMNCD) protocol, and the Japanese regulation of PLC requires to support the CSMA function. Although this CSMA is widely used, it has a problem. The throughput and packet-delay performances degrade rapidly when the offered loads of packets become high. As a solution to the problem, we have proposed a protocol referred to as carrier-sense spread-spectrum with overload detection (CSSS/OD)[l], which is a direct sequence - code division multiple based

2 Yamada ISPLC 2000 protocol with a carrier sensing function and can work complementarily to the CSMNCD protocol [2]. The CSSS/OD protocol is exactly like the CSMNCD, except for the allowable number of simultaneous packet transmissions. In the protocol, one or more packets (up to a certain pre-selected maximum K,) can be transmitted by means of code division techniques. When an overload occurs, i.e., the simultaneous packet transmission of more than K,, the transmission is aborted and the affected packets are rescheduled just like in the CSMAICD protocol. At high offered loads, the CSSSIOD protocol achieves better channel utilization than that of the CSMNCD protocol. At low offered loads, the delay in the CSSSIOD protocol is, however, larger than that in the CSMAICD protocol due to the spread packet-length. Then the optimization is done by changing the allowed number of simultaneous packets, K,, by means of changing the spreading factor N,., adaptively depending on the loads to improve the CSMNCD protocol's performance. We show the optimized performances on an IEEE BASE2 network adopted as the physical layer's assumption, and find the load threshold values when the allowed number Kc, should be changed. The obtained performances are also compared to that of time-division multiple access (TDMA). The results can be applied to OFDM or DMT systems by changing the number of sub-carriers, instead of changing the spreading factor, adaptively depending on network loads. 2. Network Model and CSSSIOD Protocol Consider a slotted network that consists of a finite population of M, terminals, and the maximum signal propagation delay-time between terminals is a unit slot time. All packets are assumed to be of the same length, I slots. A transmission is received with equal power by all terminals. A terminal is said to be in the thinking state when it is in the process of generating a new packet. A thinking terminal changes to be a backlogged one after sensing the channel to be busy or just after it starts transmitting its packet. The backlogged terminals change to be in the thinking state only after the completion of a successful transmission, i.e., at the end of the transmission periods' slot.. Taking that Kc,, (Kc, I M,), is the maximum number of allowable simultaneous packet transmissions, when r (1 5 r I Kcs ) terminals start to transmit simultaneously, their transmission is considered to be successll. Otherwise, if r idle $0 (nonpersistent) Continue the trans. C r, Figure 1: Network access procedure for the slotted NP-CSSSIOD protocol. more than K, terminals simultaneously transmit, an overload is said to have occurred and hence that the

3 Tsuzuki & Yamada ISPLC 2000 particular transmission is said to be unsuccessful. All the affected packets are aborted and retransmitted at a later random time. In this slotted non-persistent (NP) CSSSIOD scheme, a terminal with a packet ready for transmission senses the channel and proceeds as follow; (a) If the channel is sensed idle, the transmission of the packet is initiated at the beginning of the next slot. (b) If the channel is sensed busy (i.e., the presence of a transmission(s)), the packet is rescheduled for transmission to some later random time and repeats procedures (a) and (b). (c) If the overload condition is sensed during a transmission, an overload is declared. The terminal aborts the transmission immediately and initiates an overload-consensus-reenforcement procedure which is similar to the collision consensus reenforcement in the conventional CSMNCD protocols[4]. It then schedules the retransmission of the packet to some later random time. (d) At the time the backlogged packet is to be retransmitted, it senses the channel and repeats the above described algorithm. These procedures are also shown in Fig.1. Note that when we change the word 'overload' to 'collision', the above procedures are the same as those of the CSMNCD protocol. o, in the figure is the probability of a thinking state terminal starting a transmission, (0 l CF, terminal starting a retransmission, (0 I v l 1). 3. Analysis of Utiliition and Delay Performance 5 1). V denotes the probability of a backlogged Figure 2 shows the idle and transmission periods in the slotted NP-CSSSIOD protocol. The summation of these slqt.dua... overload Transmission I ltransmissionl Idle (successful) - t ' Idle -@nsuccessfu I I+ a cycle a cycle I Figure 2: Idle and transmission periods of the NP-CSSSIOD protocol. two periods forms a cycle period. The packet length I equals Tpk2 or Yph, ( Tpkt 2 Ypkt 2 2 1, when the transmission is successful or unsuccessful, respectively. We first define S,, (0 I S, < K,), as the expected number of successfully and simultaneously transmitted packets per unit slot, where the packets are I, spread in time domain (not in f;equency domain), CSMA/C~) as shown in Fig.3, under a restriction of keeping - csm~ the same signaling bandwidth, in other words, keeping the same chip rate irrespective of its spreading factor. If the packet length for the Gss= N.x K S~Z CSMAtCD protocol is denoted by T,,,, T,,, Figure 3: Packet length.

4 ISPLC 2000 is identical to the information data length. However, the packet length for CSSSJCD, T,, is N$ times longer than the information data length. We note that when Nsf =1, CSSSIOD is equivalent to CSMAICD so that K, =l. The average packet delay normalized by Tcsma, denoted by Dp,, is then given as, Dp, = N$ XN~I IS pr 7 (1) where Nbl is the mean number of backlogs. The channel utilization P,,(0 I Pcu < I), 1.e ',, the actual total information (unspread) data rate transmitted successfully (which is normalized by the maximum channel capacity), can be given as since the data rate per packet is l/nq compared to the chip rate of spread packets. The detail derivation of Spr and Nbl is shown in [2]. The offered load g is given as Next, the utilization and delay performances of TDMA are given as follows. In the contention systems, e.g., CSMA, multiple users share a common channel in a way, that can lead to collisions. On the other hand, controlled systems such as TDMA are well controlled not to occur the collisions. Users in TDMA systems transmit sequentially in fixed assigned order. Figure 4 shows the transmission procedure of TDMA. The transmission order is prearranged, then individual terminal of M, transmits a packet using a correspondiing slot according to the given order. A slot length is set to be equal to the packet length of TMa. The average packet delay Dp, normalized by Tcsma is given as, I The first term is a transmission time for a packet of the terminal. The second term indicates the average waiting time till the next transmission order comes, and is constant regardless of the offered load g. The utilization increases in proportion to g, that is, Bcu = glmt. (5) frame Note that, it is assumed that the offered load 1 -- g',(0 5 g'l 1),of each slot is the same, i.e., g = M,g'. 4. Numerical Results and Discussion 3 terminal frame Figure 4: Controlled access method by TDMA. The utilization and delay performances of the CSSSIOD protocol were calculated under the assumption of the IEEE BASE2 network with M, = 30 as an example. The slot time is 2.15~ sec from the lobase2 standard if all terminals are connected to the same segment. Because an unsuccessful transmission period which

5 Tsuzuki & Yamada g ( offered load ) g ( offered load ) (a) Effect of the probability of retransmission. ( N$ (b) Effect of the spreading factor. (v is fixed to is fixed to 1) 0.75) Figure 5: Effect of v and NXf to the delay performance. ' s,o b 2h 2k 3h i5 g ( offered load ) g ( offered load ) (a) Switching points for the optimization (b) Optimized performances of CSMAICD, CSSSIOD and TDMA Figure 6: Delay performance. includes a procedure for a collision detection and a transmission of the jam signal is standardized to be 110 bits time, yp, is set to 5 slot-time [3]. T, was fixed to 100 slot, as an example, which is equivalent to a data-time of about 269 Byte, where the preamble and the protocol header were not included. Here, when the simultaneous transmission is successful, it is assumed that a given spreading code is different from each other and the destination terminal is not duplicated. Since the spreading code for CDMA was assumed to be perfectly orthogonal in this paper, the maximum allowable simultaneous transmission packet number K, was assumed to equal with the spreading factor N$, i.e., K, = Nq, to obtain the upper bounds' performances. In order to study an influence on the delay performance by the re-transmission probability v, the average packet delay D,, for a given offered load was calculated, where Nsf is fixed to 1, i.e., CSMAICD. The selected results to achieve the best delay performance at a certain g when v is given as 0.05, 0.01, 0.15, 0.2, 0.3,

6 ISPLC 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, are shown in Fig.S(a). g of Eq.(3) is given by letting v be fixed and o, change. Because a terminal does not transmit a packet again immediately even if the packet collides under the condition of a low offered load and a small value of v, the average packet delay D,, is large. Accordingly, the value of v must be large in low offered loads in order to optimize the delay performance. Conversely when the offered load increases, the value of v needs to be reduced. In order to study an effect of the spreading factor N$, the retransmission probability v was fixed to and the spreading factor was changed as Nsf =I, 4, 8,16,32. The obtained results of D,, are shown in Fig.S@). Since the packet length becomes longer g ( offerd load ) (a) CSMAICD when the spreading factor becomes larger, the delay i I 1 D,, also becomes longer in low offered loads when compared to that with the smaller spreading factor. However in high offered loads, the delay performance is improved by making Ng large because the maximum number of allowable simultaneous packet transmissions K, becomes large and the occurrence probability of overload decreases. Next, the average packet delay performances were calculated with all combinations of the parameters g ( offerd load ) Ng =4,8,16,32 and v = 0.5, 0.6,0.75, And the (b) CSSSIOD ( Nsf 14 ) selected results which achieve the best delay performance for a given offered load are shown in Fig.6(a). In the figure, the line of Ng =I is the 0.8 n.- Ei optimized performance of Fig.S(a). By changing Ng and V at switch points shown in the figure to draw an 0.4 envelope curve with increase of the offered load, the delay performance can be optimized. That is to say is, in this example, with increase of offered loads, the parameters are changed as (Nsf,V)= (4, 0.75) + (8, g ( offered laad ) 42 ( c) CSMAICD, CSSSIOD, and TDMA 0.75) + (8, 0.6) + (1 6, 0.999) + (1 6, 0.75) -+ Figure 7: Utilization performance.

7 Tsuzuki & Yamada ISPLC 2000 (32, 0.999) to optimize the delay performance. And if a strict control is not needed, V can be 0.75 or when Nsf I 16 or Ng = 32, respectively. Notice that, plots of (N$, V ) = (16,0.75) and (32,0.999) are interrupted at g = 22.5 and 30, respectively. Since our analysis was based on the finite population of M, terminals, offered loads reached the upper limit values of 22.5 and 30 given by Eq.(3) when the traffic is congested to become N,, M,. As for the plot of ( N,/,V ) = (32,0.999), the delay time becomes suddenly small atg% 30. Since the maximum allowable simultaneous transmission packet number was set to 32 although the total transmitter number was 30, all terminals are finally allowed the simultaneous transmission without wait time, so that Nb,/Sp, becomes nearly equal to 1 and Dp, G Nd (= 32). Figure 6(b) shows the delay optimized performances of the CSMAICD and CSSSIOD protocol, and TDMA. The performance of CSSStOD protocol could be brought close to that of TDMA compared to that of CSMAICD. Note that the performance of CSSSIOD is better than that of TDMA when the offered load is relatively low, that is, g is less than 7.5 in this example. Next, optimization of the utilization performance is examined. In order to study an influence to the utilization performance by the retransmission probability V, the utilization performance P, for V = 0.05,0.01, 0.15,0.2,0.3,0.4,0.5,0.6,0.7,0.8,0.9 was calculated when the spreading factor was 1, that is CSMAICD. The selected results which achieved the best performance depending on the offered loads are shown in Fig.7(a). When the value of V is small at low offered loads, the utilization shows good performances, since the number of simultaneous retransmission packets decreases so that the collision is avoided. That is to say, to optimize the utilization performance the value of V must be small in low offered loads, and the value of V needs to be big in high offered loads. The reason why some plots are intempted in the figure is the same as that of Fig.6(a). Similarly, the utilization performances of CSSSIOD for the offered load g with all combinations of Nsf =4,8,16,32 and V = 0.2,0.3,0.4,0.5,0.75,0.999 were calculated. The selected results which achieved the best performance are shown in Fig.7(b). It is shown that the values of offered loads at which the utilization becomes maximum shift to higher values successively when the spreading factor increases so that the maximum allowable simultaneous transmission packet number is also made large. Therefore the optimum performance shown in Fig.7(c) can be obtained by changing the spreading factor and V to draw an envelope curve in the same way as Fig.6(b). In this example, with increase of offered loads the values of spreading factor and retransmission probability are made changed such as (Nsf,V) = (4, 0.2) + (4, 0.3) + (8, 0.3) + (8, 0.4) + (8, 0.5) + (1 6, 0.75) + (32, 0.999). Note that, the interruptions of plot and the rapid increase at g = 30 are because of the same reason mentioned above. Figure 7(c) shows the optimized utilization performances of the CSMAICD and CSSSIOD protocol, and I

8 ISPLC TDMA. This figure shows that the CSSSIOD protocol provides the better performance for high offered loads compared to that of CSMAICD. Moreover, the CSSS/OD protocol performs better than TDMA in a wide area of g=22.5 or less. However, since both of the utilization and delay performances cannot be optimized simultaneously at the same offered load, the switching points of N,/ and v must be differentiated according to the situation, that is, whether the application needs the delay or utilization performance. 5. Conclusions In this paper, a method to obtain the optimized utilization and delay performances of CSSSIOD is discussed under the condition that the total number of terminals is 30 and the maximum allowable simultaneous transmission packet number and the spreading factor are the same. The result shows that the proposed protocol achieves the better performance than that of CSMAICD by switching the spreading factor and the retransmission probability adaptively depending on network loads. By permitting the simultaneous transmission of the optimized number, the proposed method could be a compromising protocol between the conventional contention-protocol such as CSMA and the transmission-bandwidth allocation protocol using CDMA. Although the proposed switching method of the spreading factor was based on the stationary value of offered loads in the analysis, the measurement method of an instantaneous value of offered load must be studied in practical use. Moreover, the spreading-code assignment was assumed to be done ideally in this paper, but the degree of performance deterioration by duplication needs to be studied. Acknowledgment: The authors wish to thank Dr. F.N.Mumba and Mr. S.Ohtsuka for their help of the calculation. The authors would like to be appreciate the support by The Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research.

9 References [I] F.Mumba, S.Tsuzuki, Y.Yamada, and S.Tazaki, "Throughput Analysis of Slotted Nonpersistent and One-Persistent CSSSIOD (Carrier Sense Spread Spectrum with Overload Detection) Protocols," IEICE Trans. Fundamentals, vol.e78-a, no.9, pp , Sept [2] S.Tsuzuki, S.Ohtsuka, F.N.Mumba, Y.Yamada, and S-Tazaki, "A Carrier Sensing CDMA Protocol Coexistable with a CSMAICD Protocol, " IEEE Malaysia International Conference on Communication, Kuala Lumpur, pp.s , Nov [3] S.Tsuzuki, Y.Yamada, and S.Tazaki, "A Study of CSMAICD -- CSSSIOD Switching Protocol's Implementation, " Proc IEICE General Conference, A-5-13, p. 197, Mar (In Japanese) [4] F.A.Tobagi and V.B.Hunt, "Performance Analysis of Carrier Sense Multiple Access with Collision Detection, " Computer Networks, 4, pp ,oct.--nov