Dynamic Bandwidth Assignment Multiple Access for Efficient ATM-based Service Integration over LEO Satellite Systems Henry C. B. Chan and Victor C. M. Leung Department of Electrical and Computer Engineering The University of British Columbia Vancouver, B. C., Canada, V6T 1Z4
[email protected],
[email protected] ABSTRACT This paper proposes a novel dynamic bandwidth assignment multiple access (DBAMA) protocol employing a dynamic packet reservation technique for direct user terminal access to a ATM-based low earth orbit (LEO) satellite communications system. Performance analyses by simulations are presented. Methods to efficiently integrate ATM-based BISDN traffic are considered.
(TDMA) or combined TDMA/ FDMA schemes are not suitable because they are inefficient [1]. Motivated by previous packet reservation protocols for satellites [4][5][6], and also the PRMA [7] and DRMA [8] protocols for wireless PCNs, we propose a new Dynamic Bandwidth Allocation Multiple Access (DBAMA) protocol for integrating CBR, VBR and ABR services over a satellite channel in a LEO satellite system.
1. INTRODUCTION
The rest of the paper is organized as follows. Section 2 presents the DBAMA protocol. Section 3 presents the simulation results and discussions. Section 4 gives the conclusion.
Satellite systems are well-suited to provide global personal communication services and to complement the terrestrial broadband integrated services digital networks (BISDNs) employing asynchronous transfer mode (ATM) [1]. In recent years, a number of satellite projects such as Teledesic [2] and Iridium [3] are being developed to lay the foundation of a global satellite communication system. In particular, there has been growing interest in using LEO satellites with on-board processing and ATM switching capability (e.g., Teledesic [2]) to facilitate integration with terrestrial BISDNs. Fig. 1 shows the network architecture of such a system. An important research problem on ATM-based LEO satellite systems is how to integrate constant bit rate (CBR), variable bit rate (VBR) and available bit rate (ABR) services over bandwidth and power limited satellite channels. Traditional access methods such as frequency division multiple access (FDMA), time division multiple access
2. OVERVIEW OF DBAMA PROTOCOL As an example, we adopt the Teledesic system parameters [2] in our presentation. The propagation delay is about D=10 ms (round trip delay of 20 ms). Due to various operational reasons (e.g., power limitation), the uplink bandwidth is divided into 2 Mbps channels, while the downlink channels operate at 64 Mbps. As the downlink channels operate in the broadcast mode allowing user traffic to be multiplexed in an orderly manner, we shall focus on the access protocol for the uplink channels. Both uplink and downlink channels are assumed to be framed and slotted. We consider a frame duration of f=6 ms to provide nx64 Kbps services as explained later. Each slot is assumed to have a 48-byte payload and
7-byte header and trailer (i.e., 2 bytes more than an ATM cell), the extra overhead bits being used for error checking and guard time etc., in the satellite channel. The above slot format facilitates internetworking with ATM networks as an ATM cell can be easily formed by removing the 7-byte header and inserting the corresponding 5-byte ATM cell header. Each slot payload can either hold a cell, or be divided into 6 mini-slots of 8 bytes each for carrying reservation request from the stations to the satellite controller as explained later. Uplink access is based on the proposed DBAMA protocol. Users’ terminals are equipped with an access controller which is capable of signaling interactions with the network control centre via the satellite system for various call management functions, such as connection establishment and disconnection, and transmission of user data packets by the DBAMA protocol while a connection has been established. To implement the protocol, we assume that the satellite controller (SC) can broadcast/piggyback the type of the next uplink slot (i.e., whether it is a reserved slot or a slot of mini-slots) via the powerful downlink channel. Details of the slot assignment method will be addressed later. Note that the timing of the broadcast has to take into account propagation delay. If slot s in frame k becomes effective at t for the station farthest away from the SC, the SC needs to broadcast the slot status at t-D (D is the maximum propagation delay) in order to ensure that all stations receive the status message. In practice, all stations will be at a different distance from the satellite. We assume that appropriate synchronization/time-offset method will make them access the uplink slots correctly irrespective of the distance. To facilitate the discussion of the protocol, we first present the protocol for each type of service separately by assuming that they operate in different frequency channels. Later in the paper, we will address integrated traffic over a single channel.
2.1 CBR Station Protocol Due to the use of a 6 ms frame, nx64 Kbps CBR services can easily be provided by requesting the SC to reserve the required number of slots per frame during call setup. 2.2 VBR Station Protocol We employ the commonly used bi-state voice model as an example of a VBR traffic source [7]. In general, this bi-state model also simulates the process of requesting an incremental bandwidth of 64 Kbps and can be adapted to represent video traffic [9]. Each voice station is represented by an on-off traffic source alternating between active and idle states [7]. The active and idle periods are assumed to be independent and exponentially distributed with means of
1 --β
= 1.5s and
1 --α
=
2.25s, respectively, for an activity factor of α r = ------------- . During an active period, a voice α+β source generates a 48-byte cell every frame. The packets are stored in a buffer before transmission. The buffer size is B cells so that if a voice cell cannot be transmitted within B frames, it will be pushed out by a new cell and hence lost. This gives a bounded access delay of 6B ms for all successfully transmitted cells. No cell is generated during idle periods. When a voice station starts an active period, it first acquires a mini-slot in the uplink channel to signal the SC that it wants to reserve a slot in the uplink direction. If more than one stations capture a mini-slot, a collision will occur in which case the minislot is wasted. If the SC successfully receives a captured mini-slot, it acknowledges the respective voice station and registers a slot request in a VBR request queue. Voice stations which do not receive an acknowledgment within 1.1D assumes that the previous capture is unsuccessful and starts
re-capturing. When a slot is available, the SC reserves it for the voice station at the head of the request queue in each frame so that isochronous transport service (with zero delay jitters) can be provided. When a voice station completes its active period, it informs the SC to release the reserved slot. To enhance bandwidth utilization, we propose that both the request bandwidth (i.e., the number of minislots per frame) and the capturing probability are adjusted dynamically as follows. •
Based on a similar concept proposed in [8], the number of mini-slots per frame is adjusted dynamically by turning any un-reserved slot to carry 6 mini-slots in the payload.
•
The capturing probability pv is adjusted dynamically based on the following equation: Nv – Nr – 1 pv =
∑
i=0
Nv – Nr – 1 – i 1 N – N r – 1 i ----------- v r (1 – r ) 1 + i i
where Nv is the number of voice stations, Nr is the number of reserved slots, and r is the activity factor of each voice station. The formula for the capturing probability is explained as follows. It is not difficult to show that if a station is active and there are i other capturing stations, the station should use a capturing probability of 1/(1+i) in order to maximize the chance of success. Unfortunately neither the SC nor the stations know the exact value of i. However, given that there are Nr reserved stations, the probability that there are i other capturing stations is: N – Nr – 1 – i N – Nr – 1 i v r (1 – r ) v i
noting that an active station without a reserved slot is a capturing station. Taking into account all possible values of i, the average capturing probability an active station
should use can therefore be found by the above equation. 2.3 ABR Station Protocol The purpose of ABR service is to support asynchronous data applications such as file transfer and email through the residual network bandwidth. When an ABR station becomes active with a data cell arriving at its empty buffer, it first captures a mini-slot with probability p d =0.3, based on the capturing probability proposed for PRMA [7]. In the mini-slot, it informs the SC its slot requirement based on the content of its buffer. Upon receiving a successful mini-slot, the SC acknowledges the respective ABR station and registers this slot requirement in a ABR request queue for subsequent slot assignments. Similar to VBR, the ABR station starts recapturing if an acknowledgment is not received within 1.1D. In all subsequent cell transmissions, the data station also piggybacks additional slot requirements to register them in the SC’s ABR request queue. 2.4 Service Integration We now discuss the service integration methods. As it is straightforward to incorporate CBR service, we focus on the integration of VBR and ABR traffic. The objective is to ensure that ABR stations only utilize the residual bandwidth of VBR traffic so that ABR traffic will not affect the performance of the time-sensitive VBR traffic while maximizing bandwidth utilization. We first formulate a general slot assignment strategy which forms the framework for all the methods. Consider that the SC can be equipped with a red-light indicator (RLI) with RLI=1 indicating that the ABR traffic is to be disabled in favor of the VBR traffic. The criteria for setting RLI will be determined by the different methods as described later. Based on the content of the VBR and ABR request queues and the value of RLI, the SC assigns each available (i.e., not
reserved) upstream slot as follows: •
If the VBR request queue is not empty, assign the slot to the first station of the VBR request queue
•
If the VBR request queue is empty and the ABR request queue is not, assign the slot to the first station of the ABR request queue if RLI=0, otherwise (i.e., RLI=1) assign the slot as exclusive mini-slots for the VBR stations.
•
If both the VBR and ABR request queues are empty, assign the slot as exclusive mini-slots for the VBR stations if RLI=1, otherwise (i.e., RLI=0) assign the slot as mini-slots for both the VBR and ABR stations.
We now present the service integration methods as follows. •
Free access (FA): VBR and ABR stations access the slots freely. Basically RLI=0 all the time.
•
Slot-based request mini-slots (SB-RMS): A special mini-slot of 2 bytes called request-mini-slot (RMS) is attached to each slot which introduces an extra overhead to the satellite channel (effectively a slot is increased from 55 bytes to 57 bytes). Each capturing VBR station (i.e., active VBR station without a reservation) acquires all RMS with probability one by setting all its bits to one. Whenever the SC detects an idle RMS (i.e., no stations had tried to capture the RMS), it sets RLI to zero otherwise it sets RLI to 1.
•
Frame-based request mini-slots (SBRMS): Instead of attaching a RMS to every slot, a RMS is attached to the beginning of every frame. Unlike SB-RMS, a station only capture a RMS if it cannot capture a mini-slot within a frame time after it becomes active (i.e., suppose that an
active VBR station starts active at frame k and still cannot capture a mini-slot at frame k+1, it will keep on capturing RMS until it can acquire a mini-slot). Whenever the SC detects that an RMS is not idle, it sets RLI to 1 otherwise it leaves RLI at 0. This effectively disable the ABR traffic for one frame when congestion is detected. 3. SIMULATION RESULTS AND DISCUSSIONS To analyze the performance of the DBAMA protocol, we have conducted simulations using Simscript II.5. The duration of each simulation is 1 million frames. We first analyze a system with only voice stations by evaluating the cell loss ratio (CLR) and slot utilization of the system. A slot is defined as utilized if it carries a voice cell in its payload. Fig. 2 shows the CLR for DBAMA using different buffer sizes in comparison with that of the ideal DBAMA protocol (which employs ideal values for the capturing probability p v based on perfect knowledge of the number of capturing stations) under zero propagation delay. It can be shown that the minimum buffer size for the ideal case is B=2 in order to ensure that a cell which captures a mini-slot is always transmitted. Note that the result of the ideal case gives the best possible performance for a random access reservation protocol. As expected, as the buffer size increases, the CLR is lowered. However the CLR for buffer size of 5 cells is quite close to that of 6 cells. This indicates that a buffer size of 5 cells is a good compromise which will be adopted in the rest of this paper. In this case, DBAMA and ideal DBAMA can support about 56 and 57 voice stations, respectively, while maintaining a target PLR of less than 1% [7]. Therefore the DBAMA protocol gives performance close to the ideal result. If a TDMA system is used with the same slot size, the system can accommodate about 27 slots or 27 voice stations. Consequently DBAMA can support two times more voice stations than the
c o nv e n t i o n a l T D M A s y s t e m g iv i n g a multiplexing gain of 2. Fig. 3 compares the slot utilization of the DBAMA protocol and ideal DBAMA protocol. It can be seen that the two results are quite close. In the case of the DBAMA protocol, the slot utilization is about 84% when the number of voice stations is 56 (i.e., the maximum number of voice stations for a 1% PLR). This indicates that the protocol is highly efficient. To compare the above service integration methods, we consider a voice (VBR) and data (ABR) integration problem. We assume that there are 54 voice stations and 10 data stations in the network. Each data packet consists of 8 cells and the data packet interarrival time follows an exponential distribution. The size of each data station buffer is 64 cells. Fig. 4 shows the voice CLR for different aggregate data rates. Note that ideally the CLR should not be affected by the data traffic (i.e., the ideal graph is flat). The aggregate data rate is obtained by varying the data packet inter-arrival time accordingly. It is found from the simulations that no data cells were lost for all methods when the aggregate data rate is below 0.1 Mbps. This indicates that congestion starts to occur at around 0.1 Mbps. Under 0.1 Mbps, all methods (even FA) provides similar CLR close to the 1% target. Above 0.1 Mbps, the CLR for FA increases dramatically causing serious performance degradation to the voice traffic. Below 0.1 Mbps, SB-RMS has higher CLR than other methods because of its overhead (2 more bytes per slot) which effectively reduces the available bandwidth. SB-RMS and FBRMS have the least increase in CLR indicating that they are less affected by the data (ABR) traffic. Fig. 5 shows the access delay of the data cells for different aggregate data rates. FA gives the lowest access delay because it favors data rather than voice when congestion
occurs which is undesirable. Above 0.1 Mbps (the congestion point), on the other hand, SBRMS and FB-RMS have higher access delay because they favor voice instead of data, which is in fact a desirable result. 4. CONCLUSIONS We have presented a DBAMA protocol for integrating ATM-based traffic over a satellite channel of a LEO satellite system. It combines the advantages of centralized control and distributed access. We have proposed some methods to provide ABR services. Simulation results indicate that DBAMA gives performance close to the ideal result, the protocol is efficient in terms of bandwidth utilization, and the performance of the VBR traffic is virtually unaffected by the amount of ABR traffic in the network by using a new FB-RMS method. REFERENCES [1] I. F. Akyildiz and S. H. Jeong, “Satellite ATM networks: A Survey,” IEEE Comm. Mag., vol. 35, no. 7, pp. 30-43, Jul. 97. [2] http://www.teledesic.com [3] http://www.iridium.com [4] T. Le Ngoc and S. V. Krishnamurthy, “Performance of combined free/demand assignment multiple access schemes in satellite communications,” Int’l J. of Sat. Commun., vol. 14, no.1, pp. 11-21, Jan. 1996. [5] V. C. M. Leung, “Packet reservation protocols for multichannel satellite networks,” IEE ProceedingsI, vol. 140, No. 6, pp. 453-461, Dec. 1993. [6] C.J. Powell and V.C.M. Leung, “Demand assignment multiple access and dynamic channel allocation strategies for integrating mobile radiodispatch and telephone services over mobile satellite systems”, IEEE Journal on Selected Areas in Communications, vol. 10, pp 1020-1029, Aug. 1992. [7] D. J. Goodman, R. A. Valenzuela, K. T. Gayliard and B. Ramaurthi, “Packet reservation multiple ac-
cess for local wireless communications,” IEEE Trans. on Comm., vol. 37, pp. 885-890, Aug. 1989.
Figure 3. Utilization of DBAMA and ideal DBAMA 100
[8] X. Qiu and V. O. K. Li, “Dynamic reservation multiple access (DRMA): A new multiple access scheme for personal communication system,” ACM/Baltzer Wireless Networks, vol. 2, pp. 117128, 1996.
90
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[9] B. Maglaris, D. Anastassiou, P. Sen, G. Karlson and J. Robbins, “Performance models of statistical multiplexing in packet video communications,” IEEE Trans. on Commun., vol. 36, pp. 834-844, Jul. 1988.
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