Performance Analysis of Random Multiple Access Protocols used in Wireless Communication

Transcription

1 Performance Analysis of Random Multiple Access Protocols used in Wireless Communication Amardeep Kaur Abstract A Multiple Access Protocol is an access mechanism and a set of rules for successful transmission of information using a common medium. In this paper, the multiple access protocols studied are those used in communication systems in which the resource to be shared is the communication channel. For wireless communication efficient resource utilization relies upon ive sharing of the available medium. Contention based random multiple access protocols, specifically the repeated contention based random multiple access protocols such as pure-aloha, slotted ALOHA, Carrier Sense Multiple Access and Inhibit Sense Multiple Access protocols, are studied and the s presented. The protocols have been evaluated based on the quality of service parameters such as offered traffic, throughput, delay and capture. The simulations were carried out using MATLAB v7.. Keywords- Multiple Access Protocol; Pure-Aloha; Slotted- Aloha; non-persistent CSMA; ISMA; MATLAB. I. INTRODUCTION For wireless communications the multiple access protocols should be more robust with respect to changing conditions such as the hidden terminal problem, the near-far, and the s of multipath fading, shadowing and the of cochannel interference. In this paper several typical wireless communication protocols are introduced and evaluated for various conditions. It is assumed that the mobile terminals communicate with a centrally located base station using packet radio. The best known method for contention based random multiple access packet communication is ALOHA [3]. As traffic increases in an ALOHA system collisions among packets leads to a performance drop. The Carrier Sense Multiple Access (CSMA) protocol [4,5] offers higher capacity, but its performance is affected by the hidden terminals problem. A third variation known as Inhibit Sense Multiple Access (ISMA) [6-11], in which the central base station controls the flow of packets from the mobile terminals, reduces the two problems: (1) collisions among data packets; and (2) hidden terminals. The inhibit delay fraction [6-11] is a decisive parameter in designing a packet communication system using ISMA and is defined as a fraction of the packet duration necessary to inhibit other mobile packet transmissions by a broadcast outbound-signaling channel (base to terminal) in the event that the inbound multiple access channel (terminal to Mark A Gregory School of Electrical and Computer Engineering RMIT University Melbourne, Australia mark.gregory@rmit.edu.au base) is already busy. The ISMA protocols can be divided into two subclasses: (1) the non-persistent ISMA and (2) the p- persistent ISMA protocols. In the non-persistent ISMA protocols, if the inbound channel is idle, a user can transmit a packet; otherwise, the user will wait a random time and then try again. At first sight it seems that it totally avoids collision, however due to propagation delay between two users, it is possible that a user considers the inbound channel idle and starts transmission while another transmission is already in progress. A user is informed of a collision by the absence of an acknowledgment packet from the receiving station. Upon detecting a collision, the packet is rescheduled for transmission at a randomly selected time later. The remainder of the paper is organized as follows: Section II is the related background including a discussion of the basic performance parameters; Section III provides a list of the simulation parameters used; Section IV provides simulation s for Pure-Aloha, Slotted-Aloha, non-persistent CSMA and ISMA; and Section V provides a summary, conclusion and discussion of future work. II. BACKGROUND Each user terminal connects to an access point by using an access protocol. Performance parameters affected by the access protocol utilized may be analyzed to identify overall system performance. The performance parameters offered traffic; throughput and average transmission delay have been identified as key parameters and analyzed in this study. Offered Traffic. The total quantity of packets that include newly generated packets and retransmission packets at the access point in a time interval is called offered traffic and the normalized offered traffic by transmission data rate is shown in (1). G=T t /R (1) If no packets are generated, G=. R is the transmission data rate and T t (bit) is the total traffic to be transmitted.. The total quantity of packets that is successfully transmitted to the access point is called throughput and the normalized throughput by transmission data rate is shown in (2). The transmission data rate is R(bps), the quantity of information in a packet is T(bit) and n packets are successfully transmitted.

2 S= Tn/R (2) If no transmission packets are generated and all packets are destroyed due to collisions, S becomes equal to and if all packets are transmitted successfully S becomes equal to 1. Average Transmission Delay. The period until a packet is generated at an access terminal, transmitted to the access point and received at the access point is called the average transmission delay. The average transmission delay depends on the length of the packet so the normalized average transmission delay by the length of the packet is shown in (3). D= P/R (3) Where P is the number of bits in each packet and R is the total rate of transmission in bits/sec. III. SIMULATION The elements used to evaluate the access protocols used in this study are offered traffic G, throughput S, and average transmission delay D. In the ideal access protocol, throughput is limited as shown in (4). S= G (if G<1) 1 (if G>=1) (4) In the simulation, created in MATLAB v7. [12], it was assumed that propagation loss and shadowing are constant. Until the number of successfully transmitted packets becomes equal to the required number, the simulation continues. Within MATLAB several subprograms based upon a common sequence of steps were created to permit the performance evaluation of the access protocols to be studied. The various simulation parameters used are shown in Table-I and the simulation environment is depicted in Table-II. Variable name brate Srate plen Ttime Dtime alfa sigma Capture r bxy tcn Mnum Mxy mcn Mplen Mstate mgtime mtime Mstime TABLE I. SIMULATION PARAMETERS Note Bit rate (bps) Symbol rate (sps) Length of packet (symbol) Transmission time of packet (sec) Normalized propagation delay Decline fixed number of propagation loss Standard deviation of shadowing (db) Capture : :nothing 1:consider Service area radius (m) Position of the access point (m) Capture ratio (db) Number of the access terminal Position of the access terminal (x,y,z) (m) C/N at the access point when transmitted from area edge (db) Length of a packet (symbol) State of the access terminal Packet generation time in the access terminal (sec) State change time of the access terminal(sec.) Packet transmitting time of the access terminal (sec) G Tint Rint Spnum spend Splen Tplen Wtime TABLE II. Offered traffic Expectation value of the packet generation interval (sec) Expectation value of the packet resending interval (sec) Number of successfully transmitted packets Number of packets that simulation is finished Data amount of successfully transmitted packets (symbol) Data amount of packets that is tried the transmission (symbol) Transmission delay time (sec) SIMULATION ENVIRONMENT Variable Note Name r Service area radius (m) 1m bxy Position of the access point (m) 5m Mnum Number of the access terminal 1 Srate Symbol rate (sps) 256 ksymbol/sec plen Length of packet (symbol) 128 symbol alfa Decline fixed number of 3 propagation loss sigma Standard deviation of shadowing 6 db (db) mcn C/N at the access point when 3 db transmitted from area edge (db) tcn Capture ratio (db) 1 db IV. RESULTS Pure-ALOHA. Pure-ALOHA is a protocol in which access terminals transmit packets when ready to do so. In this case, all of the access terminals do not bother whether the communication channel is busy or not. The throughput in this case is shown in (5) [1,2]. S= Ge 2G (5) Theoretically the maximum throughput is.184 when G=.5 if an infinite call-source model is assumed. When the capture is not considered, the throughput is close to the theoretical value even if the number of users is large (~1). Moreover, when the capture is considered, the throughput is larger than when the capture is not considered, because in Pure-ALOHA, collision often occurs so the capture is the reason to increase throughput. In addition, the average transmission delay is reduced. Slotted-ALOHA. Slotted-ALOHA is a simple modification of Pure-ALOHA. Messages are required to be sent in the time slot between two synchronization pulses, and can be started only at the beginning of a time slot. This change reduces the rate of collisions to a half. As a of reducing the rate of collision by half, the throughput becomes double and average transmission delay becomes half of that of Pure- ALOHA. Also the influence of the Capture is affected. Non-persistent CSMA. In this protocol, the existence of carrier wave on the communication channel is sensed by the access terminals so it is possible to judge whether other access

3 terminals are transmitting their packets or not. This is one of the protocols that attempts to avoid collisions. In this protocol when packets are generated in an access terminal, it starts the carrier sense. If the of carrier sense is idle, the packet is transmitted to the access point immediately. However if it is busy, the access terminal stops the carrier sense, waits for a while and then starts the carrier sense again. This waiting time is the key factor to realize a system with high throughput Figure 2. and average delay of Pure-ALOHA with Capture Figure 1. and average delay of Pure-ALOHA without Capture Although each access terminal performs carrier sense, the collisions still occur which may be due to the non-zero propagation delay. Also in a wireless environment the carrier sense cannot be done at some access terminals even if an access terminal transmits packets, because some obstacles exist between the access terminals. This issue is called the hidden terminal problem. The throughput S is shown in (6) [1,4,5] Figure 3. and average delay of Slotted-ALOHA without Capture S= Ge -ag / {G(1+2a)+e ag } (6) Where a is normalized propagation delay. The throughput is determined under an ideal communication environment where an infinite call-source model is assumed, and no hidden terminal exists. The simulation was carried out with the normalized propagation delay a of.1 or.1. The maximum throughput depends on the normalized propagation delay. If a is small the maximum throughput of non-persistent CSMA is larger than the Pure-ALOHA. However, if a is large, the performance is close to Pure-ALOHA. This is because the other terminals transmit their packets during a large propagation delay time even if an access terminal transmits its packet after sensing the carrier, as a collisions occur.

4 Average Delay Time (packet) 2 15 The throughput is determined under an ideal communication environment where an infinite call-source model is assumed..8 (a=.1) Average Delay Time (packet) (a=.1) Figure 4. and average delay of Slotted-ALOHA with Capture If the Capture is considered, a transmitted packet sometimes survives because of the received power difference between transmitted access terminals. Therefore the throughput increases. Also average transmission delay is reduced as compare to Slotted-ALOHA. ISMA is a system that can solve hidden access terminal problems by informing the communication channel of busy or idle conditions from the access point to the access terminal. In ISMA, the access point sends a busy signal to all access terminals when the access point is receiving packets from the access terminals. On the other hand, the access point sends an idle signal when the access point is not receiving any packets. When each access terminal receives the idle signal, each access terminal must decide whether to transmit packets to the access point or not. When each access terminal receives the busy signal, the packet transmission of each access terminal is inhibited. Therefore this protocol is called ISMA. In the slotted non-persistent ISMA, an uplink is slotted by the period of one packet T. When each access terminal generates its packet, the access terminal performs inhibit sense. If it receives an idle signal, the access terminal transmits a packet at the next time slot. if it receives busy signal access terminal does not transmit and waits until the next time slot, which is decided randomly and then starts the inhibit sense. By taking a normalized propagation delay d, the busy slot must be more than (1+d)T and the idle slot must be at least more than dt. The throughput S is shown in (7) [1]. S= dg -dg / (1+d-e -dg ) (7) Figure 5. and average delay of Non-persistent CSMA without Capture In the simulation analysis the normalized propagation delay d was.2 and.2. The maximum throughput depends on the normalized propagation delay. Further if the Capture is considered, a transmitted packet may survive because the received power for arriving packets were different from each other. Therefore the throughput is increased. Similarly the average transmission delay has been decreased (a=.1)

5 Average Delay Time (packet) 15 1 carried out by developing a simulation environment using MATLAB. The protocol performance comparison necessitated utilizing a number of parameters and in some cases default values were selected so as to permit other parameters to be varied and the s considered (a=.1) Figure 6. and average delay of Non-persistent CSMA with Capture (d=.1).8.6 (d=.1) 1.4 Average Delay Time (packet) (d=.1) Figure 7. and average delay of Slotted Non-persistent ISMA without Capture V. CONCLUSION AND FUTURE WORK This paper discussed methods to evaluate throughput and average transmission delay of typical multiple access protocols used in wireless communication. The protocols studied included Pure-ALOHA, Slotted-ALOHA, Non-persistent- CSMA and Slotted Non-persistent ISMA. The study was Figure 8. and average delay of Slotted Non-persistent ISMA with Capture Future work may include further analysis and variation with the goal of identifying new variations of the protocols that improve performance and reduce the s of wireless transmission. REFERENCES (d=.1) 1 [1] Harada H. and Prasad R., Simulation and software radio for mobile communication, Vol. 1, Artech House, USA, 22 [2] Prasad R., CDMA for wireless personal communication Norwood, MA, Artech House, USA, 1996 [3] Abramson N., The ALOHA system-another alternative for computer communication, Proc. Fall Joint Computer Conferece, Vol. 37, 197 pp [4] Kleinrock, L. and Tobagi F.A., Packet switching in radio channels, Part I- Carrier Sense Multiple Access Nodes and Their - Delay characterstics, IEEE Trans. Comm., Vol. 23, Dec 1975, pp

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