A Dynamic Resource Allocation and Media Access ... - CiteSeerX

A Dynamic Resource Allocation and Media Access Control Proposal for a Multi-tier Wireless ATM Network Bora A. Akyol BBN 10 Moulton Street Cambridge, MA 02138 Abstract | This paper presents a new protocol for controlling access to the shared radio medium in a multi-tier wireless ATM network. A multi-tier Wireless ATM network provides seamless connectivity to the users in multiple geographic environments ranging from local area networks to wide area wireless networks. Dynamic Resource Allocating Media Access (DRAMA) provides full support for multiple trac types with dierent priorities and for multiple connections per user. DRAMA bene ts from statistical multiplexing and allows bursts up to the full channel bandwidth for data trac. A full description of the protocol and simulation results are presented.

I. Introduction

The wireline telecommunications networks of the future may use asynchronous transfer mode (ATM) networking technology. ATM is designed to support isochronous and data trac and to guarantee quality of service (QoS). Wireless networks are also growing at a very rapid pace. Personal Handiphone Service (PHS) in Japan has taken on ve million subscribers by the end of 1996. Wireless users want not only voice but also data connectivity. A dynamic resource allocation and media access control algorithm for a multi-tier wireless ATM network is proposed in this paper. A multi-tier wireless ATM network may provide varying data rates depending on geographic location. For example, the data rates in an indoor environment may be 10-20 Mbits/sec. In campus environments 2-8 Mbits/sec may be provided. In wide area networks, user may gain access to a wireless ATM network but at a lower rate of 256-512 Kbits/sec. These environments are referred to as local area campus and wide area respectively. The need for a multitier network arises because of the scarcity of radio spectrum and power available for radio transmission in a portable unit. In the following sections, we describe a \Dynamic Resource Allocating Media Access" (DRAMA) algorithm that implements the media access control and resource allocation in a wireless ATM network. DRAMA accommodates multi-tier wireless ATM networking and multiple types of ATM trac and provides quality of service guarantees. It also bene ts from statistical multiplexing and allows data users full access to the channel bandwidth on burst basis when resources are available. This research was performed when B. Akyol was a student at Stanford University and was supported by Motorola Inc., Schaumburg, IL. and Paci c Bell, San Ramon, CA.

Donald C. Cox Stanford University Department of Electrical Engineering Stanford, CA 94305 Frame N

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Fig. 1. Time Slotted Frequency Multiplexed Radio Link

II. The DRAMA Algorithm

DRAMA is a resource allocation and media access control protocol for a wireless ATM network. Gaining access to a network resource can be divided into two tasks: Requesting the resource from the resource broker and allocation of the resource by the broker. The users are connected to the wireless ATM network via radio ports. The radio ports are controlled by radio port controllers. The radio ports are designed to be small and economical radio modems. The intelligence of the radio network is in the implementation of the radio port controller and the switching hardware. In this section we rst discuss the wireless ATM network environment, then present the implementation of the DRAMA algorithm in two parts: Media Access Control (MAC) and Resource Allocation (RA). A. The Wireless ATM Network In this section we discuss a DRAMA wireless ATM network. DRAMA functions in a time-slotted and frequency multi-plexed radio environment (Fig. 1). The users can access multiple time slots in a single frequency or multiple time-slots spanning multiple frequencies. The time-slotted

and frequency multiplexed radio channel is represented by an M by N channel matrix where M is the number of slots in each frequency and N is the number of total frequencies assigned to each radio port (See Fig. 3). The downlink frequencies are only accessed by the radio ports and the access is strictly controlled by the radio port controllers with no contention on the downlink radio channels. Hence, we limit our discussion in this paper to the uplink frequencies and aim to resolve the contention that is caused by user requests on the uplink channels. Received Requests

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Fig. 2. Resource Allocation in the Drama Algorithm

A resource allocation that uses multiple time-slots spanning multiple frequencies is de ned as a frequency-time sliced allocation. The optimal resource allocation policy for accommodating dierent trac types in a time slotted-frequency duplexed environment is frequency-time sliced allocation[6]. Therefore, in DRAMA we use a frequency-time sliced resource allocation (RA) algorithm. A practical limitation on frequency-time sliced resource allocation is the frequency switching time required by most radio transceivers. A radio transceiver will require a non-zero time to switch between two frequencies while transmitting or receiving a burst. This creates unaccessible slots in the channel matrix. This problem is represented by a Frequency Switching Constraint on the resource allocation algorithm. The DRAMA algorithm performs ecient resource allocation considering frequency switching constraints and using a heuristic greedy resource allocation algorithm that is described later in this text. The existence of dierent trac types with dierent service criteria is accommodated by setting priorities. For our performance estimation, we give CBR trac higher priority than data trac. Our results indicate that this priority scheme is eective in ensuring success of prioritized trac. Note that DRAMA is capable of working with dierent priority policies including time of expiry, or quality of service. For requests within the same priority class, we employ a rst-come- rst-served (FCFS) priority scheme for allocating

resources to requests. Reference [1] gives details of our wireless ATM network architecture. B. Media Access Control in DRAMA Media Access Control is the process of gaining access to a shared media for information transmission. Radio channels are the shared media in a wireless ATM Network. Gaining access to shared radio channels is a two step process when the DRAMA algorithm is employed. The rst step is to send an access request to the radio port controller. The access request is sent in a special signaling and control channel(s) (See Fig. 1), i.e. a separate frequency. This signaling channel may be shared by all three tiers of the wireless ATM network. The DRAMA algorithm is based on a time-slotted transmission format for the signaling channels. The signaling channels are accessed using a slotted-ALOHA algorithm. The slot format of the signaling channel is dierent from the user channels which have considerably wider bandwidth. Use of a single signaling channel for access to multiple frequencies and multiple tiers of the network allows the network to bene t from statistical multiplexing on the signaling channel in contrast to having a smaller number of signaling slots in each user frequency and also allows the use of the same signaling formats and messages for all of the tiers in the network. This provides seamless connectivity when crossing network tiers. However, due to dierent trac parameters in dierent network tiers, signaling channel bandwidths may be dierent. In order to access the wireless network, a terminal transmits on the next available slot in the signaling channel and sends a short message indicating the type of service requested. A collision occurs when multiple user terminals access the same signaling slot to send a request1 . If there are no collisions in the slot; the request is received by the radio port controller and processed by the RA algorithm. If there is a collision, the user terminal does not receive an answer for its request from the radio port controller and the request times out. For a timed out request, the user terminal employs an exponential backo procedure for retrying the request[5]. If the user terminal is not successful in the 1 The capture eect where stronger signals override weaker ones may improve the results discussed in this paper.

maximum allotted time for the MAC operation it may try a dierent radio port or report failure to the upper layer protocols. C. Resource Allocation in the DRAMA Algorithm Before discussing the resource allocation part of the DRAMA algorithm, we digress brie y to de ne the Channel Chunk Matrix (CCM). The CCM is an M by N matrix related to the channel matrix de ned in section II-A and is created by the port controller for indexing and referencing the available wireless ATM network resources. The CCM is formed by grouping and counting the empty slots in all columns of the channel matrix and sorting them in the order of magnitude. For example, for a frequency channel of 5 slots, if the slots are (Idle,Idle,Idle,Busy,Idle), the associated column of the CCM will be ((3,0),(1,4)), where (3,0) denotes that there is an available chunk of 3 slots starting at position 0 in that frequency; similarly, (1,4) denotes a chunk of 1 empty slot starting at the 4th position (See Fig. 3). By using the CCM, the radio port controller rapidly allocates the available resources. The resource allocation process in DRAMA is given below: 1. Sort and Combine: While receiving the signaling frame, the radio port controller scans the received slots in the frame(Section II-B). All successfully decoded requests in the frame are combined with requests that were saved from previous frames. This process involves sorting the requests with respect to service class and order of arrival. Requests that were queued from previous frames get higher priority in the same priority class. 2. The resource allocation is then performed on a request by request basis (See Fig. 2): (a) Check Resource Availability: The request size is compared with the total number of available slots in the channel. If the request size is greater than the total number of available slots, the request is queued; otherwise, the algorithm advances to the next step. (b) Exact Match: The port controller scans the CCM for an exact match of the request size. If such a match is found, then the controller checks to see if frequency switching constraints2(See Section II-A) are satis ed for that user. If constraints are satis ed, the resource is allocated and both matrices are updated; otherwise, the process is repeated until an exact match is found or the channel matrix is completely scanned. If this step is unsuccessful, the port controller advances to the next allocation step. (c) Bigger Chunk: The CCM is scanned for any chunk that is larger than the request size. When such a chunk is found, it is checked for frequency switching constraints. If the constraints are satis ed, the chunk is allocated; otherwise this process is repeated until a feasible chunk is found or the channel matrix is completely scanned. If a usable chunk is not found then the port controller advances to the next allocation step. 2 The DRAMA algorithm supports multiple connections per user, these constraints need to be checked at every stage of the resource allocation process.

(d) Same Frequency: The port controller now attempts to allocate the request in a single frequency to avoid the inaccessible slot problem caused by the frequency switching. It uses the CCM to determine whether a frequency with enough slots exists. If such a frequency exists then the available slots in that frequency are checked for frequency switching constraints. If the constraints are met than the slots are allocated, otherwise the port controller moves to the next frequency. This step is repeated until a feasible frequency is found or all available frequencies are scanned. (e) Multiple Frequencies: Finally, the port controller attempts to perform a multiple frequency assignment. First, it nds the frequency with the highest available slots. Then all the feasible slots in that frequency are allocated; if the request is not completely allocated, this step is repeated until it is completely allocated or the channel matrix is completely scanned. The procedure discussed above will nd a feasible allocation with the least amount of frequency switching. 3. The procedure given in the previous step is repeated for all of the valid requests. At the end of the allocation process, all portables with pending requests are noti ed of the success or failure. We note here that failed requests may be queued for later allocation or until the request expires. The expiration time of a request is de ned by the trac type and the user application(s). III. Performance Estimates for the DRAMA Algorithm

A discrete-event simulator to test the eectiveness of the DRAMA algorithm has been implemented. In the following sections we describe the simulation and discuss performance measurements. Results of parameter variations follow the performance measurements. A. Simulation Environment The simulations discussed in this paper are an implementation of the DRAMA algorithm in a single radio port, multiple user environment. We chose to implement a single radio port in order to isolate the eects of interference from neighboring ports from the eects of the DRAMA algorithm. The co-channel interference from neighboring radio ports may be overcome by frequency reuse or by dynamic channel allocation methods[3], [4]. A unique feature of DRAMA is the fragmentation of data across time. For example, when a user has a large packet to transmit, only one resource allocation request is sent. If the request may not be accommodated in one user data frame, it is divided in time and transmitted in consequent frame(s). This increases the throughput of the system and avoids unnecessary transmissions on the signaling channel. The fragmentation of data may be turned on or o during the start of the simulation. The format of the user requests is given in Table I. The width of a request slot is determined by dividing the total size of a request by the request channel bit rate. The frame period of the request channel is an input parameter.

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0.4 0.35 0.3

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The Format of the Request Slots

0.25 0.2 0.15 0.1

A.1 Trac Modeling Modeling the user trac types and arrival patterns for wireless ATM networks is challenging because no networks exist for measurement. We brie y summarize our trac model assumptions. For voice telephone networks, the Poisson Process is widely-accepted as the arrival model for voice calls[8]. For local area ethernets, studies indicate that the \Self Similar" trac models are acceptable models of data trac[9]. For wireless voice networks, the Poisson process is still a good representation of the trac arrivals. The continuous bit rate (CBR) trac in the wireless ATM network represents applications that are initiated by people and have relatively long durations; hence, we assumed that the CBR requests from the users arrive according to a Poisson process with exponentially distributed connection durations. We assume three types of CBR trac diering in arrival rates, bandwidth and connection durations. We determined the bandwidth demands of CBR trac using bit rates required for voice, video conferencing, and distance learning applications. We used similar previously published studies for determining baseline parameters for our data trac models[7], [13], [10] For data requests, it is shown that Poisson processes may be used for independent telnet and ftp session arrivals but not for inter-packet arrivals[11]. Hence, a self-similar trac model as described in [14] is used to model data connection arrivals with uniformly distributed data packet train lengths. The performance of the DRAMA algorithm was tested using three dierent tiers of the wireless ATM network and a wide range of parameter variations in all tiers. The dierent tier environments are discussed next. B. Simulation Parameters Three dierent tiers of the wireless ATM network were considered in the DRAMA simulation: Wide Area Network: In this environment the users were limited to low bit rate CBR and low bit rate data trac. Campus Wide Network: In the campus environment, the users can use all three CBR and also high bit rate data trac types. Local Area Network: In the local area we concentrated mainly on data trac and some high bit rate CBR trac such as video conferencing. See Table II for details of these environments. The following criteria are used to evaluate the performance of the DRAMA algorithm:

CBR Failure Rate Data Failure Rate 0.05 0 0

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Fig. 4. Wide Area Network Failure Rates

Failure Rates: Requests that were successfully received by the radio port controller but no resources were allocated within the time-out period. The failure rates are determined by dividing the number of failed requests by the total number of requests of that trac type.  Average Access Delay: The time between a request being made by a user terminal and the response to the request being sent by the radio port controller.  Collisions: Percentage of requests that collide while accessing the signaling channel using the slotted-ALOHA algorithm. Based on the three tiers mentioned above, the parameter variations given in Table III were performed. In order to quantify the eects of the parameter variations, a uni ed measure of the demand on the system is needed. This parameter is usually referred to as the Oered Trac. In order to estimate the average oered trac we need to know the arrival rates of dierent trac types, the average number of slots per frame occupied by these trac types, and the average duration that is spent transmitting user information. The average oered trac per user is then calculated by multiplying the average arrival rate (i ) by the average duration (i ) and converting from erlangs to slots per frame by multiplying by the average occupied slots per frame for that trac type (si ) and summing over all trac types. This is given in equation 1. The average normalized oered traf c, denoted by TOffered?Normalized is calculated by multiplying the average oered trac per user by the number of users and dividing the result by the total available system resources where M is the number of slots per frequency and N is the number of frequencies in that radio port.



TOffered?Normalized =

(Ni=1 i  i  si )  Nusers M N

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C. Simulation Results In order to evaluate the performance of the drama algorithm we have performed extensive simulations. Simulation results are summarized in Table IV and plotted in gures 4 through 9.

Variable Number of Users Number of Frequencies Number of Slots End Time Signaling Channel Slot Width Signaling Channel Frame Period (slots) Arrival Rate Trac Ratio CBR LOW Bit Rate CBR MED Bit Rate CBR HIGH Slots Maximum Packet Size Frequency Switching Time Data Timeout CBR Timeout Duration of Low Trac Duration of Medium Trac Duration of High Trac Ratio of Low Trac Ratio of Medium Trac Ratio of High Trac

LAN 50 3 64 36000

Campus 50 5 64 10800

WAN 100 10 8 86400

5120 bits 512 bits 256 bits 4 16 8 0.1 0.009 0.0014 0.25 0.50 0.75 - 32 Kbits/s 32 Kbits/s 320 Kbits/s 128 Kbits/s 1.4 Mbits/s 256 Kbits/s 40960 bytes 2048 bytes 256 bytes 1 slot 1 slot 1 slot 1.0 sec 1.0 sec 1.0 sec 5.0 sec 5.0 sec 5.0 sec 180.0 180.0 180.0 360.0 600.0 180.0 600.00 600.0 180.0 0 0.6 1.0 0.9 0.3 0 0.1 0.1 0 TABLE II

Baseline Values for Simulation Environments

Variable Number of Users Number of Frequencies Number of Slots Number of Signaling Slots Arrival Rate (1/sec)

LAN Campus WAN 30 - 100 10 - 90 10 - 500 1-6 1 - 10 1 - 30 32-96 24 - 96 8 - 48 2-8 8 - 32 4 - 32 0.07 - 0.33 0.005 - 0.009 0.001 - 0.005 TABLE III

Summary of Performed Parameter Variations

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Fig. 6. Campus Wide Network CBR Access Delays

IV. Discussion of Results and Observations

in the literature, DRAMA performs better than or equal to those schemes with the bene t of supporting multiple trac types in a multi-tier environment[2], [7], [13], [10]. Our simulations show that DRAMA is a good, ecient and exible MAC/RA algorithm for wireless ATM technology[2].  The CBR and data access delays for lightly loaded networks are reduced by decreasing the signaling frame length.  The collisions observed in the signaling channels are inversely proportional to the signaling bit rate as can be seen

This paper presented the DRAMA algorithm for resource allocation and media access in a multi-tier wireless ATM network. We have observed the following:  DRAMA supports multiple users and multiple connections with dierent trac types per user and functions well in a wide-variety of environments. When the failure rate and delay values for DRAMA are compared to schemes proposed

LAN

No CBR failures up to 90 percent oered traf c and less than 1 percent up to 100 percent offered trac (150 users supported by 3 channels of 10 Mbits/sec) Data Less than 1 Failure percent up to 90 Rate percent oered trac and below 3 percent up to 100 percent oered trac Average Less than 20 CBR msecs up to 80 Access percent oered Delay trac Average Less than 45 Data msecs up to 85 Access percent oered Delay trac Collision Less than 7 percent up to an oered trac of 100 percent

Campus

Less than 1 percent up to 88 percent oered trac (50 users supported by 5 channels of 2 Mbits/sec)

WAN

Less than 1 percent up to 83 percent oered trac (375 users supported by 10 channels of 256 Kbits/sec)

Less than 1 percent up to 90 percent oered trac

Less than 4.1 percent up to 83 percent offered trac

Less than 10 msecs up to 80 percent oered trac Less than 10 msecs up to 80 percent oered trac Less than 20 percent up to an oered traf c of 100 percent TABLE IV

Less than 40 msecs up to 80 percent oered trac Less than 100 msecs up to an oered trac of 80 percent Less than 1 percent up to an oered trac of 100 percent

Summary of Results 0.09

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Fig. 9. Local Area Network Average Collision Rates

in Fig. 9.  It is possible to support large number of users in a wireless ATM network with low signaling bit rates.  The failure rates observed with the DRAMA algorithm improve as the oered trac becomes burstier. References

[1] B. A. Akyol and D. C. Cox, \Signaling Alternatives in a Wireless ATM Network," IEEE J. on Selected Areas in Comm., Vol. 15, January 1997, pp. 35-49. [2] B. A. Akyol, \An Architecture for a Future Wireless ATM Network," Doctoral Dissertation, Stanford University, Stanford, CA, June 1997. [3] M. Cheng, J. C. Chuang, \Distributed measurement-based quasi xed frequency assignment for personal communications," ICC 95, Seattle, WA, USA, pp. 433-437 [4] J. C. Chuang, D. C. Cox and N. R. Sollenberger, \Pilotbased dynamic channel assignment scheme for wireless access TDMA/FDMA systems," Int. J. of Wireless Information Networks, Vol. 1, No. 1, Jan 1994, pp. 37-47. [5] D-G. Jeong, W-S. Jeon, \Performance of an Exponential Backo scheme for slotted-ALOHA protocol in local wireless environment," IEEE Trans. on Veh. Tech., Vol. 4, No. 3, August 1995, pp. 470-479. [6] M. J. Karol et al., \Performance Advantages of Time-FrequencySliced Systems," IEEE PIMRC' 95, Toronto, Canada, pp. 11041111. [7] M. J. Karol et al. \Distributed Queueing Request Update Multiple Access (DQRUMA) for wireless packet (ATM) networks," IEEE ICC 95 , pp. 1224-1231. [8] Queueing Systems, Leonard Kleinrock, New York, Wiley, 1974-76. [9] W. E. Leland, et al., \On the self-similar nature of ethernet traf c," IEEE/ACM Trans. on Networking, Vol. 2, No. 1, February 1994, p. 1-15. [10] A. Mahmoud et al., \Multiple access scheme for wireless access to a broadband ATM LAN based on polling and sectored antennas," IEEE PIMRC 95, Toronto, Canada, pp. 1047-1051 [11] V. Paxson, S. Floyd, \Wide Area Trac: The failure of Poisson Modeling," IEEE/ACM Trans. on Networking, Vol. 3, No. 3, June 1995, pp. 226-244. [12] D. Raychaudhuri, \Wireless ATM Networks: Architecture, System Design and Prototyping," IEEE Personal Comm. Mag., Vol. 3. No. 4, August 1996, pp. 42-49. [13] D. Raychaudhuri, N. Wilson, \ATM-based transport architectures for multiservices wireless personal communication networks," IEEE J. on Selected Areas in Comm., Vol. 12, Oct 1994. p 14011414. [14] W. Willinger et al., \Self Similarity Through High Variability: Statistical Analysis of Ethernet LAN Trac at the Source Level," IEEE/ACM Trans. on Networking, February 1997.