An investigation of reservation random access algorithms for voice ...

International Journal of Wireless Information Networks, Vol. 2, No. 1, 1995

An Investigation of Reservation Random Access Algorithms for Voice-Data Integration in Microcellular Wireless Environments Allan C. Cleary 1'2 a n d Michael Paterakis I

We present the results of a simulation study that explores the performance of two promising reservation random access (RRA) protocols for transmitting voice packets over a common radio broadcast channel in a microcellular radio environment. We examine two inherently stable RRA voice protocols, RRA three cell and RRA two cell, with respect to voice transmissions under ideal and adverse cbannel conditions. In addition, we investigate the ability of both protocols to support efficient voice-data integration within the system. The RRA two-cell and RRA three-cell algorithms clearly mark the end of the voice contention period, thereby enabling all of the terminals within the microcell to differentiate between available voice and available data slots. Separating the two distinct types of transmissions and resolving the contending voice packets first thus enforces the priority of the voice traffic. In addition, each protocol can be combined with efficient, easy to implement, collision resolution random access protocols for transmitting data packets. Such a voice-data integration mechanism eliminates the potential voice degradation caused by competition between voice and data terminals for available slots. Our results show that the protocols provide stable and robust performance under adverse channel conditions and that they can be employed to sustain voice-data integration under heavy system loading. KEY WORDS: Multiple access; reservation random access; voice-data integration; wireless networks.

1. I N T R O D U C T I O N The radio channel bandwidth within t o d a y ' s cellular systems is limited, demand for access continues to surge, and presently no substantial bandwidth increase is predicted. System capacity can be increased by using microcells to increase frequency reuse and by using efficient multiple access protocols to exploit the variations in access and service required by disparate sources. In densely populated areas, microcells are expected to end the distinction between cordless and mobile telephones and to provide access to broadband ISDN public networks for large numbers of mobile voice and data terminals. Mobile communication within a microcell entails slow-moving vehicles (i.e., vehicles in city ~Department of Computer and Information Sciences, University of Delawam, Dover, Delaware 19716. 2Correspondence should be directed to Mr. Allan C. Cleary, 706 Market Street, Havre De Gmce, Maryland 21078.

traffic) and/or pedestrians equipped with inexpensive, lightweight, and low-powered communication devices (e.g., telephones, palmtop PCs, pagers, etc.). Since microcell diameters are predicted to be on the order of 100 m and since a mobile wireless network requires call setup and handoff capabilities, using microcells will increase the processing requirements of network control. It is expected that call control and call management functions will be decentralized and highly distributed [ 1]. Within each microcell, spatially dispersed source terminals share a radio channel that connects them to a fixed base station. The base station delivers feedback information and provides an interface to the mobile switching center and to the wired network infrastructute. The multiple access protocols must efficiently integrate the differing requirements of voice and data traffic. Voice packets must be delivered promptly to minimize delays that can be disconcerting within a con-

1068 9605/95/0100-0001$07.5010© 1995PlenumPublishingCorporation

2 versation. Therefore, voice packets that are not delivered within a specified maximum delay (e.g., up to 50 ms) are dropped. Speech can withstand approximately 1-2% of dropped packets without suffering a quality degradation noticeable t o humans [2]. Data applications, on the other hand, are more tolerant of delays (delays up to 200 ms are usually acceptable), but often require 100% delivery of correct packets (e.g., a file transfer). Multiple access protocols can be broadly classified as being conflict free or contention based. Conflict-free protocols divide the channel resource among the users in a fixed manner. They are most efficient under high loads, and they guarantee stable access under an acceptable delay (e.g., time division multiple access (TDMA)). In contrast, contention-based protocols allow users to access the channel on demand. They are most efficient with time-varying or "bursty" traffic, and since collisions are possible, a conflict resolution scheme is necessary (e.g., ALOHA). The coUision resolution mechanism must be carefully designed to avoid high packet delays or even unstable operation under high loads. For further details on conflict-free and contention-based protocols the interested reader is referred to [31. Reservation random access (RRA) protocols are designed to increase throughput over a dynamic multiaccess channel by combining a contention-based protocol with TDMA. Due to the strict packet delay requirements of voice traffic, two types of RRA protocols are usually proposed for use with voice terminals in future wireless networks [4-9]. The two types are similar in that the channel is divided into frames which contain slots that are classified as reserved or available. Terminals with packets and no reservation must contend for available slots. Terminals with reservations transmit their packets in a controlled way and without any interference. The two types differ in the contention mechanism employed. In one case, terminals with packets and no reservation use the packets to contend directly for available slots [4-8]. In the second case, a segment of the frame is partitioned into minislots where contention is performed with special reservation packets [9]. With RRA protocols, in which the terminals use the packets to contend directly for available slots, two approaches to voice-data integration are possible: allow the data terminals to eompete directly with the contending voice terminals, or eliminate competition between data and voice terminals by distinguishing between available voice and available data slots [6]. Packet reservation multiple access (PRMA) [4, 5] and the more recently proposed integrated packet reservation multiple

Cleary and Paterakis access (IPRMA) [10] are examples of the first case. IPRMA extends the PRMA voice reservation mechanism to data and provides a priority mechanism to ensure that voice packets have greater access to the available slots [10]. In the second case [6, 7], collisions between voice and data packets are eliminated by selecting a voice protocol that allows the data terminals to determine the end of the voice contention period. In both cases, the goal of the network is to provide adequate data throughput while maintaining the priority of voice traffic. RRA voice protocols which provide a mechanism that can be used to eliminate the competition between voice and data terminals for available slots are a promising alternative to PRMA [6, 7]. These protocols, by marking the end of the voice contention period, enable all of the terminals within the microcell to differentiate between available voice and available data slots (an option not available with PRMA). Separating the two distinct types of transmissions and resolving the contending voice packets first thus enforces the priority of the voice traffic. In addition, the voice protocols can be combined with efficient multiple access protocols (i.e., splitting algorithms) to inerease the data throughput without penalizing voice performance. Speech is modeled as alternating between periods of vocal activity (talkspurt) and silence. In [6, 7], it was assumed that, for all voice terminals, the transitions into (and out of) talkspurt occur only at the frame boundaries (i.e., all of the voice packets that were to be transmitted within a given frame were available at the start of the frame). Although this assumption simplifies the analysis, it is not realistic since the vocal activity of a speaker depends on the dynamics of a given conversation, as opposed to being a function of the system. In this paper, we use simulations to e~(tend the work in [6, 7] in several important ways: we consider the more natural assumption that voice terminals may transition into (out of) talkspurt at any time; we propose the RRA threecell algorithm as a promising new protocol for voice packet transmissions; and, since radio channels are inherently unreliable (they are subject to noise from the atmosphere, multipath effects, cochannel interference, etc.), we investigate the effect of adverse channel conditions on RRA voice protocols. The remainder of the paper is organized as follows. We present the system model in the next section. We describe the channel noise error models in Section 3 and the voice and data transmission protocols in Section 4. Representative simulation results are presented in Section 5, and we diseuss these results in Section 6. The paper is concluded in Section 7.

Investigation of RRA Algorithms for Voice-Data Integration 2. SYSTEM MODEL We model the wireless microcell as a star network with the base station as the central node. A shared radio channel, which may be impaired by noise and without capture, connects a finite number of spatially dispersed active voice terminals to the base station. The channel is divided into frames that are subdivided into slots. Generally, frame length and slot duration are design parameters. Here, we assume fixed-length packets that contain information (voice or data) and a header. Each slot can accommodate exactly one packet; and the frame length is selected such that a voice terminal in talkspurt generates exactly one packet per frame [6]. Each voice terminal is assumed to be equipped with a voice activity detector (VAD) that generates packets only during talkspurt (i.e., the terminal's vocoder suppresses silence periods). Because a voice terminal requires access to the channel only during talkspurt, the time periods corresponding to silence gaps within a conversation can be used to transmit packets from other source terminals (i.e., multiplexing takes place at the talkspurt level). To model speech activity we use a twostate (i.e., talk and silence), discrete-time, Markov chain with a mean talkspurt duration equal to 1 s and a mean silence duration equal to 1.35 s [1]. The resulting speech activity of approximately 43 % is reasonable, since generally only one speaker is active at any given time within a conversation, and speakers pause between phrases and even between syllables [11]. Unless otherwise stated, we assume that a voice terminal may transition from silence into talkspurt at any time. Once the voice terminal enters talkspurt, the time between each successive packet is equal to the duration of one frame. For example, if a terminal transitions into talkspurt in slot 5 of frame i, then it will generate new packets in slot 5 of each succeeding frame i + 1, i + 2, etc., for as long as it remains in talkspurt. The slots within a frame are classified as reserved or available according to the base station feedback broadcast at the end of each frame. We assume that reserved slots are deallocated immediately and that the base station correctly broadcasts the status (i.e., available or reserved) of each slot in the next frame. This implies that a voice terminal holding a reservation signals the base station upon transition from talkspurt to silence (e.g., by setting a control bit in the last talkspurt packet to alert the base station of the transition). If immediate deallocation is not practically feasible, silence in a reserved slot denotes the end of a talkspurt (i.e., one slot is wasted). Note that for all RRA protocols, the method used to deallocate reserved slots affects system

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performance [6]. For example, a delayed deallocation scheme decreases system throughput because, during the deallocation period, the contending terminals consider an available slot to be reserved and suppress their transmissions. The base station broadcasts a short feedback packet at the end of each time slot. Unless otherwise stated, the feedback packet indicates whether or not a collision occurred in the slot (collision versus no collision (C-NC)). Since the transmission delay within a microcell is negligible, we assume that the feedback information is immediately available to the terminals (i.e., before the next slot). It is assumed that all terminals continuously monitor the base station feedback (full feedback sensing). The more realistic assumption of limited feedback sensing (in which each terminal monitors the base station feedback channel only when it has packets to transmit) can be handled with minor modifications to the protocols under investigation [3, p. 302]. A contending voice packet involved in collisions (or noise errors) is retransmitted until it is dropped or successfully received, whichever happens first. If a noise error occurs when a voice terminal with a reservation transmits, then the base station labels that slot as being available and the terminal must recompete for another reservation. Notice that we have taken the position that a noise error in a reserved slot causes the base station to cancel the reservation and mark the slot as available. This position was also taken in [12]; it has the virtue of simplicity, and at low to moderate loads it does not degrade throughput. An alternative approach is to allow the base station to hold reservations when the transmission in a reserved slot results in a"collision" (i.e., the base station knows that the slot is reserved and infers that '%ollision" was simulated by noise). In the case of immediate deallocation, the difficulty with this alternative is that the base station could hold a reservation forever if a noise error occurs at the exact time that the terminal transitions out of talkspurt (i.e., the final transmission indicating the transition is garbled by noise). To overcome this, on detection of silence in a reserved slot, the base station would immediately mark the slot as being available. To model voice-data integration we make the following assumptions for voice and data traffic. Voice traffic has high priority and is generated by a constant number of independent active voice terminals. Data traffic has low priority and is generated by a large unknown number of data terminals (theoretically infinite). Each new data packet arrives at a new data terminal and the aggregate data traffic is assumed to be Poisson distributed [3, p. 275]. Our data traffic assumption is based

4 on the belief that the technology will create many unforeseen uses for data terminals over the wireless network.

Cleary and Paterakis

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3. A D V E R S E C H A N N E L C O N D I T I O N S In an ideal (error-free) slotted channel without capture, three outcomes are possible at the end of each time slot: no packets are transmitted (idle), exactly one packet is transmitted (success), or two or more packets are transmitted (collision). In this case, it follows that the base station feedback broadcast correctly identifies the event that occurred in a time slot. In practical applications, wireless communication systems are prone to atmospheric noise, cochannel interference, and distortion from multipath effects. In this work, we do not rigorously model the radio channel. For our purposes, noise introduces the possibility of errors into the system (i.e., the base station misconstrues what actually happened within the slot). This erroneous detection of information, and subsequent feedback, can degrade a protocol's performance, resulting in higher delays and/or instability. We assume a noisy uplink and an errorless feedback channel. This means that noise in a slot may cause an error at the base station, but the feedback broadcast indicating erroneous reception will be correctly received by all terminals within the microcell. We assume that error detection algorithms used by the base station eliminate the possibility of a collision being misconstrued as an idle or single packet transmission. As a result, two types of error are investigated; noise in an idle slot may be misinterpreted as a collision, or a success may be misidentified as a collision (i.e., a garbled packet header may be misinterpreted). Since the feedback channel is prone to the same adverse conditions as the uplink, our assumption of an errorless feedback channel is somewhat optimistic. However, because the feedback packets are short (e.g., 2 bits are required to differentiate between idle, success, collision), we would expect the number of erroneous packets on the feedback channel to be less than that observed on the uplink. In general, errors on the feedback channel will increase the number of wasted slots and the number of collisions in reserved slots, thereby causing the protocols performance to suffer. We use the following two models to introduce noise onto the channel. 1. Memoryless: Errors are modeled as a Bemoulli process (i.e., the errors are assumed to occur indepen-

kB Fig. 1. Two-state, discrete-time, Markov chain that describes transitions of the channel state.

dently in each time slot). Idle slots and slots containing a single packet transmission are misinterpreted as collisions with a constant probability Po and Pl, respectively. 2. Bursty: To introduce errors with memory, as shown in Fig. 1, we use a two-state (good or bad), discrete-time Markov chain to describe the condition of the channel [13]. In this work, when the channel is in the bad state, all idle slots and all slots containing a single packet transmission result in an error. No errors occur when the channel is in the good state. When the state transition probabilities are small, this model produces bursty errors (i.e., bunches of errors occur between relatively long errorless periods) [14]. Note that the ideal and memoryless error models are special cases. In the first case the channel state is always good, while in the second it is always bad. To use this model in the simulations, we choose values for the average duration of an error burst and for the overall probability of error. The value selected for the burst duration fixes the bad-to-good, PBc, and the bad-to-bad, PBB, state transition probabilities (because average error burst duration is equal to 1/PBn and PBB = 1 -- PBc). Since we assume that errors occur with probability 1 when the channel is bad, the overall probability of error is the probability that the channel is in the bad state, PB. Therefore, having selected the desired value for the overall probability of error, we use the following equation to solve for the good-to-bad state transition probability, PGB: PGB

PB = PBG + PcB and the good-to-good transition probability, P~c, is 1 P~B.

Investigation of RRA Algorithms for Voiee-Data Integration 4. T R A N S M I S S I O N P R O T O C O L S

4.1. Voice Terminals At a given time, an active voice terminal may be silent, contending (in talkspurt with no reservation), or in talkspurt and holding a reservation. A voice terminal finds itself contending when it transitions from silence into talkspurt or when a channel error causes its reserration to be released prematurely. The contending terminal uses its packet to compete for available slots within a frame. Upon successful transmission into an available slot, the terminal receives a reservation for the corresponding slot in each successive frame. A voice terminal maintains its reservation until it exists talkspurt, unless its transmission is erroneously detected as a "collision" by the base station. In either case, the base station immediately marks the slot as available. Our primary interest is in RRA voice protocols that can be used to eliminate the competition between voice and data terminals for available slots. For example, in Fig. 2, assume that each frame in a system has a duration equal to 10 slots. Further assume that, in the frame depicted, the slots containing an A are available and the slots containing an R a r e reserved (based on results from previous frames). The voice contention period begins at the start of each frame (slot 0) and ends when the RRA algorithm marks it as ended or when the frame ends. To eliminate the competition for available slots between voice and data terminals, only voice terminals are permitted to transmit first (giving rise to the voice contention period), and only data terminals may transmit into the available slots that follow the voice contention period. For the frame shown, slots 2, 5, and 6 are available voice slots, while slots 7 and 9 are available data slots. Note that for any frame the frame duration is constant since it is a system parameter, but the duration of the voice contention period will vary since it is a function of the RRA protocol employed, the number of contending voice terminals, the condition of the channel,

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and the distribution of the available slots within that frame. As described in Section 2, a voice terminal in talkspurt generates one packet per frame, and the maximum voice packet delay is equal to the duration of one frame. Therefore, a voice terminal that fails to successfully transmit a new packet arrival within a time equal to the duration of one frame, drops that packet. Voice packets are dropped when a packet expires while the terminal is contending or when a noise error occurs in a reserved slot and the packet expires before the terminal can obrain a new reservation. A contending voice terminal that drops its packet and remains in talkspurt continues to compete for available slots with fresh packets until it either succeeds or exits talkspurt. We consider the following RRA voice protocols for voice tmftic.

4.1.1. RRA Two-Cell Stack (RRA Two Cell) [6] Each voice tenninal uses a counter, r, as follows. At the beginning of every frame all of the contending terminals initialize r to 0 or 1 with equal probability, while all of the other terminals (silent or with reservation) set r = 0. During the voice contention period, a contending voice terminal with r = 0 transmits in the first available slot. Let x be the feedback for that transmission (collision (C) versus no collision (NC)). Then the transitions in time of the counter, r, are as follows: If r = 0 and x = NC, then the packet is successfully transmitted. If r -= 0 and x = C, then r = 0 with probability 0.5 and r = 1 with probability 0.5. I f r = l a n d x = NC, t h e n r = 0. I f r - = 1 a n d x = C, t h e n r = 1. We have slightly modified the algorithm presented in [6]. Notice that during the voice contention period any voice terminal that transitions from silence into talkspurt, or loses its reservation prematurely, has unblocked access (because the terminal is contending and r = 0, the terminal will transmit in the first available voice slot). As illustrated in Fig. 3, we can use a two-ceU stack to depict the operation of the protocol. Without loss of generality, we show the available slots as consecutive (realistically, as in Fig. 2, reserved slots would be interspersed between them). The state of the stack, at the start of each available slot, is shown above the slot (the number of lines within a cell corresponds to the number of terminals with counter values equal to the cell num-

6

Cleary and Paterakis mission. Then the transitions in time of the counter r a r e as follows:

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ber), and the feedback (collision (C) versus no collision (NC)) resulting from the action taken by the terminals is shown within each slot. The arrows underneath the horizontal line represent the contending voice terminals (terminals with a voice packet and no reservation). Two consecutive noncollisions denote an empty stack, thereby uniquely marking the end of the voice contention period. Consequently, all of the terminals within the system can distinguish between available voice and available data slots. In part (A), the stack above the first available slot results from the three contending terrninals initializing their respective counters prior to the start of the voice contention period. Both terminals in the bottom cell of the stack transmit in the first available slot, and a collision ensues. The subsequent transitions in time of the stack (based on probabilities chosen for illustrative purposes) follow the rules described above. Part (B) illustrates a simple case of unblocked access. The RRA two-cell algorithm commences operation with no contending terminals. Then, during the first available slot, a voice terminal transitions into talkspurt. The terminal transmits successfully in the second available slot. In this case, the unblocked access has no effect on the duration of the voice contention period (if another voice terminal were present in the bottom cell, a collision would ensue and the voice contention period duration would increase).

4.1.2. RRA Three-Cell Stack (RRA Three Cell) Each voice terminal uses a counter, r, as follows. At the beginning of every frame all of the contending terminals initialize r to 0, 1, or 2 with probability 1/3, while all of the other terminals (silent or with reservation) set r = 0. During the voice contention period, a contending voice terminal with r = 0 transmits in the first available slot. Let x be the feedback for that trans-

If r = 0 and x = NC, then the packet is successfully transmitted. If r = 0 and x = C, then r = 0 with probability 1/3, r = 1 with probability 1/3, and r = 2 with probability 1/3. I f r > 0 a n d x = NC, t h e n r = r - 1. I f r = l a n d x = C, t h e n r = 1. I f r = 2 a n d x = C, t h e n r = 2. Notice that during the voice contention period any voice terminal that transitions from silence into talkspurt or loses its reservation prematurely has unblocked access (because the terminal is contending and r = 0, the terminal will transmit in the first available voice slot). We can depict the protocol's operation as a threecell stack, where in a given slot the bottom cell contains the transmitting terminals (those with r = 0) and the middle and top cells contain the witholding terminals (those with r = 1 and r = 2, respectively). The empty stack is identified by three consecutive noncollisions in available slots, and it uniquely identifies the end of the voice contention period within a frame. As in the RRA two-cell protocol, we can exploit this feature to eliminate the competition between voice and data terminals for available slots.

4.1.3. RRA-ControlIed ALOHA Ideal (RRA-C Ideal) We modify the ideal version of RRA-controlled A L O H A presented in [6] such that it " m a r k s " the completion of the voice contention period. Because the simulator knows the true number of contending terminals, the voice packet transmission probability, p, is correctly adjusted at the beginning of each available voice slot as follows:

p=~1 where C,, is the true number of contending voice terminals. When C,, = 0, the voice contention period is complete and the protocol halts. Notice that during the voice contention period any voice terminal that transitions from silence into talkspurt or loses its reservation prematurely immediately joins the backlog (C,, is incremented by 1). For this version of the RRA-C ideal protocol, a performance penalty to separate voice and data is arbitrarily charged. In this work, RRA-C ideal uses at least one available voice slot per frame (i.e., if there are no contending voice terminals when the first available slot

Investigation of RRA Algorithms for Voice-Data Integration is reached, that slot is wasted (it will be empty) and all of the following available slots within the frame are used for data access).

4.1.4. Packet Reservation Multiple Access (PRMA) [4,5] Upon transition into talkspurt, a voice terminal taust compete for channel access. A contending terminal may transmit its packet only if the slot is available and the terminal has permission to transmit. Permission is issued by a pseudo random number generator with probability, p, in each time slot. Once the contending terminal successfully transmits its packet into a time slot, the terminal receives a reservation for that slot in each successive frame (until it exits talkspurt or loses its reservation due to channel error). The permission probability, p, is a system design parameter. It should be noted that the performance of PRMA is sensitive to the choice o f p [5]. In this work, we set p equal to the constant value of 0.35, since this value was suggested in [1] for a system with parameters similar to those that we use in our simulations.

4.2. Data Terminals Data traffic differs from speech. Many data applications do not require multipacket messages, nor do they have strict delay requirements. Our use of the infinite node assumption for data terminals means that every new data packet arrival is viewed by the system as a new data terminal. In this work, data terminals are not provided with reservations. We consider the following two data protocols for voice-data integration.

4.2.1. Two-Cell Stack [15] A blocked access mechanism is established, which specifies the first time transmission rule for newly generated packets. A collision resolution period (CRP) is defined as the interval of time that begins with an initial collision and ends with the successful transmission of all packets involved in that collision. New packet arrivals may not transmit during a CRP. Even when a CRP is completed, only a subset of the waiting packets is generally allowed to transmit. This subset is formed by data packets that arrive within an allocation interval of maximum length A. The parameter ,5 is generally chosen to limit the expected number of packet arrivals in the allocation interval to about one. All data packets arriving within the allocation interval transmit in the first available data slot. If zero or one packet is transmitted, the interval is resolved in one slot. Otherwise, collision

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resolution is performed according to the rules in Section 4.1 part (1). Two consecutive noncollisions in available data slots mark the end of a CRP. The maximum data throughput of 0.429 packet per slot is achieved by setting the size of the allocation interval equal to 2.33 slots [15]. Observe that this algorithm differs from the RRA two-cell voice protocol; where we have unblocked access, an implied allocation interval of up to one frame, and the first time transmission is based on the initial "split" (i.e., contending voice terminals adjust their respective counters at the beginning of each frame). The advantages of the two-cell stack algorithm are its operational simplicity, relatively high throughput, and the two-slot flag that clearly marks the end of a CRP. In addition, the protocol was shown to be robust (i.e., has a positive throughput, and, unlike other collision resolution random access protocols, does not lead to deadlocks) in the presence of base station feedback channel errors in which an empty slot is mistaken as a collision [16].

4.2.2. First Come First Serve (FCFS) [3] FCFS is a tree, blocked-access protocol. A node involved in a collision is split into one of two subsets according to its packet arrival time. The subset with the earlier packet arrival times is resolved first. The FCFS algorithm provides high throughput by including two improvements to the traditional tree-splitting algorithm (for details on traditional tree-splitting algorithms, see [3, p. 290]). First, when a collision is followed by an idle slot, the second subset splits before transmitting in order to avoid a sure collision. Second, if back-to-back collisions occur, the second subset is removed from the CRP and it is considered as part of the next allocation interval. FCFS requires ternary base station feedback (collision versus idle versus success). The end of a CRP is clearly marked by two successive successful transmissions. The maximum data throughput of 0.4871 packet per slot is achieved by setting the size of the allocation interval equal to 2.6 slots [3]. The advantages of the FCFS protocol are high throughput, operational simplicity, and the two-slot feedback flag that marks the end of a CRP. The disadvantages are the lack of robustness in the presence of base station feedback errors in which an empty slot is mistaken as a collision (for details, see [3, p. 302]), and the assumption that the data terminals can measure packet arrival times with infinite precision. In practice, arrival times are represented with a finite number of binary bits. As a result, closely spaced packet arrivals will appear to occur simultaneously. This problem is easily

8

Cleary and Paterakis Table I. ExperimentalSystem Parameters

solved by having each node generate additional (less significant) bits, as needed for splitting, with a random number generator [3].

Speech rate (bps) Channel rate (bps) Packet size (bits) Speech/data (bits) Header (bits) Frame duration (ms) Slots per frame Voice delay limit (slots) Mean talkspurt duration (s) Mean silence duration (s)

5. R E S U L T S We investigate voice traffic over an ideal channel, voice traffic under adverse channel conditions, and voice-data integration under both ideal and adverse channel conditions. Our study focuses on steady-state performance. We examine voice system capacity (i.e., the maximum number of simultaneously active (with conversation) voice terminals constrained by a voice packet dropping probability less than approximately 1%) and the ability of the RRA voice protocols to accommodate data traffic. The parameters in Table I (similar to parameters in [1, 6[) were used to simulate the system. All voice terminals within the microcell are assumed to be active throughout the entire simulation period. We make this assumption because statistical fluctuations in the presence of talkers are much slower than statistical fluctuations in the generation and transmission of voice (the holding time per voice conversation is on the order of hundreds of seconds, while the holding times of voice packets are on the order of tens of milliseconds [17]). All simulation results are based on 10 independent runs of 50,000 frames each. Initially, all terminals are silent, and we include a warm-up period of 1000 frames to reduce start-up effects. The 97.5 % t-confidence intervals

8,000 270,000 552 472 80 59 28 28 1.0 1.35

are constructed in the usual way (for details see [18, p. 288]). First we investigate the performance of the four RRA voice protocols operating under ideal channel conditions. For purposes of comparison, Fig. 4 shows the voice packet dropping probability versus the number of voice terminals in the microcell for the case in which all voice packet talk/silence transitions occur at the frame boundaries only; Fig. 5 illustrates what happens when we relax that assumption. It is worth noting that the theoretical system capacity, with no upper bound on packet dropping probability, is about 65 simultaneously active voice terminals (i.e., the number of slots per frame divided by the speech activity rate, or 28/0.425 -- 65.9). All subsequent results are based on the more natural assumption that an active voice terminal in the silent state may transition into talkspurt at any time. Next we investigate the operation of the four RRA

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Investigation of RRA Algorithms for Voice-Data Integration

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voice protocols under adverse Channel conditions. Figure 6 presents the voice packet dropping probability versus the number of voice terminals when a nonideal channel is subject to memoryless errors. The numbers in the caption are the probability of error in an idle slot, P0, and the probability of an error in a slot containing a

value of the overall probability of error is equal to 0.01. The value of the average error burst duration is input at the start of the simulation, and it remains constant throughout that run. Fixing the overall probability of error and the average error burst duration determines the channel state transitions (as described in Section 3, part 2). For example, in Fig. 7 all data points are based on an overall error probability of 0.01 with an average error

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Fig. 6. RRA voice protocols operating over a nonideal channel subject to memoryless errors (P0 = 0.01, Pl = 0.0l).

56

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Fig. 7. RRA voice protocols operating over a nonideal channel subject to bursty errors (average burst duration is equal to 0.10 frame).

channel conditions. T h e v o i c e throughput corresponds to the m e a n n u m b e r o f successful v o i c e packet trans-

Finally, we investigate the viability o f eliminating the competition for available slots b e t w e e n v o i c e and data terminals by c o m b i n i n g the R R A three-cell, R R A two-cell, and R R A - C ideal protocols with the F C F S and the two-cell stack data protocols to e x a m i n e their ability to provide for v o i c e - d a t a integration. Tables I I - I V present the v o i c e - d a t a system throughput a c h i e v a b l e w h e n the three R R A v o i c e protocols operate at their respective m a x i m u m v o i c e system capacity level u n d e r various

missions per frame. T o quantify the efficiency p r o v i d e d by the three R R A v o i c e algorithms, Tables V and VI show v o i c e - d a t a system results o v e r an ideal channel w h e n the n u m b e r o f v o i c e terminals in the system is 41, the data rate is 2.5, 3.0, and 3.5 packets per frame, and the data p r o t o c o l is F C F S (since it is the most efficient available). The data packet delay is the t i m e b e t w e e n

Table II. Maximum Voice-Data Throughput, Constrained by a Packet Dropping Probability of Approximately 1%, over an Ideal Channel RRA voice algorithm

Max no. vo. terminals

Voice packet drop (%)

Voice thr. (packets/frame)

FCFS (packets/frame)

Two-cell stack (packets/frame)

RRA three cell RRA-C ideal RRA two cell

46 43 40

1.02 _+ 0.00 1.07 + 0.00 1.02 _ 0.00

19.29 _+ 0.01 18.03 + 0.01 16.78 + 0.02

2.95 + 0.01 4.42 + 0.01 4.50 + 0.01

2.58 _+ 0.01 3.88 _+ 0.01 3.95 + 0.00

Table III. Maximum Voice-Data Throughput, Constrained by a Packet Dropping Probability of

Approximately 1%, over a Nonideal Channel Subject to Memoryless Errors RRA voice algorithm

Max no. vo. terminals

Voice packet drop (%)

Voice thr. (packets/frame)

Two-cell stack (packets/frame)

RRA three cell RRA-C ideal RRA two cell

39 24 29

1.00 + 0.00 1.03 _+_0.00 1.01 _ 0.00

16.37 + 0.01 10.09 + 0.01 12.19 + 0.01

3.70 + 0.01 7.22 + 0.01 5.86 + 0.01

Investigation of R R A Algorithms for Voice-Data Integration

11

Table IV. Maximum Voice-Data Throughput, Constrained by a Packet Dropping Probability of Approximately 1%, over a Nonideal Channel Subject to Bursty Errors of O. 1 Frame Duration RRA voice algorithm

Max no. vo. terminals

Voice packet drop (%)

Voice thr. (packets/frame)

Two-cell stack (packets/frame)

RRA three cell RRA-C ideal RRA two cell

34 24 24

0.99 _+ 0.00 1.02 +_ 0.00 0.96 +_ 0.00

14.29 + 0.01 10.09 _+ 0.01 10.08 +_+_0.01

4.57 + 0.00 7.24 _+ 0.00 6.77 + 0.00

Table V. Comparison of Voice-Data Throughput over an Ideal Channel" RRA voice algorithm

Voice packet drop (%)

Voice thr. (paekets/frame)

Avail. data slots (slots/frame)

FCFS data thr. (packets/frame)

Data delay (frames)

RRA-C ideal RRA two cell RRA three cell

0.94 + 0.00 1.03 _+ 0.00 0.56 _+ 0.00

16.83 _+ 0.01 16.83 + 0.01 16.89 _+_+0.02

10.33 _ 0.01 9.20 :t: 0.01 8.49 _+ 0.02

3.5 _+ 0.00 3.5 + 0.00 3.5 _+ 0.00

1.52 + 0.02 3.51 _+ 0.05 6.76 + 0.17

aFor all RRA voice protocols, the number of voice terminals is equal to 40 and the data packet arrival rate is equal to 3.5 packets per frame.

Table VI. Additional Comparisons of Voice-Data Througput over an Ideal Channel" RRA voice algofithm

FCFS data thr. (packets/frame)

Data delay (frames)

FCFS data thr. (packets/frame)

Data delay (frames)

RRA-C ideal RRA two cell RRA three cell

3.0 _+ 0.00 3.0 ___0.00 3.0 _+ 0.00

0.93 _+ 0.01 1.81 _+ 0.03 2.94 _+ 0.05

2.5 _+ 0.00 2.5 _+ 0.00 2.5 _+ 0.00

0.62 _+ 0.01 1.11 _+ 0.01 1.55 _+ 0.02

aFor all RRA voice protocols, the number of voice terminals is equal to 40, and the data packet arrival rate is equal to 3.0 and 2.5 packets per frame, respectively.

packet arrival and successful transmission; it consists of the time spent in each frame waiting for voice packet resolution and the time spent c o m p e t i n g for available data slots.

6. D I S C U S S I O N Important performance metrics for voice traffic include throughput (we express it in packets/frame), access delay, and packet dropping probability. In any frame, the voice throughput is equal to the n u m b e r of successful voice packet transmissions (in available and reserved slots). F o r the system and protocols u n d e r investigation, voice throughput, access delay, and packet dropping probability will increase as the n u m b e r o f active voice terminals in the system increases. Therefore, voice system capacity (the m a x i m u m n u m b e r o f voice terminals constrained by less than approximately 1%

packet dropping probability) is a useful, and c o m m o n l y used, measure of c o m p a r i s o n for R R A voice protocols operating in wireless microcellular systems. W h e n integrating data with voice transmissions, an important performance measure is the m a x i m u m data throughput w h e n the voice system capacity is maximized for a given R R A voice protocol. A related performance measure is the efficiency o f an R R A voice protocol. All else being equal, a more efficient protocol will, on average, require a lower n u m b e r o f available slots to resolve voice contention.

6.1. Voice Transmission over an Error-Free Channel U n d e r the a s s u m p t i o n that talk/silence transitions occur at the frame boundaries only, Fig. 4 suggests that, constrained by a packet dropping probability of approximately 1%, the P R M A , R R A three-cell, and R R A two-

12 cell protocols produce similar voice system capacity results over an ideal channel (48, 49, and 47 voice terminals, respectively). Each protocol falls within a 4-8 % range of the RRA-C ideal protocol (51 voice terminals). Under this assumption, all the voice packets that are to be transmitted within a given frame are available when the frame begins (i,e., all voice packets are available at the start of the voice contention period). Note that, when exactly one contending voice terminal is present at the beginning of a frame, the duration of the voice contention period for RRA-C ideal, RRA two cell, and RRA three cell is exactly orte, two, and three available slots, respectively, while PRMA averages 2.86 available slots (i.e., 1/0.35) to successfully transmit the packet. When active voice terminals in the silent state are permitted to transition into talkspurt at any time, Fig. 5 indicates that, constrained by a packet dropping probability of approximately 1%, PRMA, RRA three-cell, RRA-C ideal, and RRA two-cell accommodate 48, 46, 44, and 40 voice terminals respectively. A comparison with Fig. 4 shows that the voice system capacity for PRMA remains about the same, while RRA three-cell, RRA two-cell, and RRA-C ideal accommodate about three, eight, and seven fewer voice terminals, respectively. This suggests that relaxing the assumption used in [6, 7] significantly decreases the voice system eapacity of RRA two cell, while the effect on RRA three cell is minimal. When the condition that talk/silence transitions occur only at frame boundaries is relaxed, we must use care when comparing the three RRA protocols that mark the end of the voice contention period to PRMA. The RRA two-cell and RRA three-cell protocols execute a collision resolution algorithm and then halt. For purposes of this study, the RRA-C ideal protocol " m a r k s " the end of voice contention and then immediately halts. Therefore, once the end of the voice contention period is marked, all of the remaining available slots within that frame are open only to data terminals (or, alternatively, any voice packet that arrives within the frame and after the marking must wait until the next frame to compete for available voice slots). On the other hand, PRMA uses the entire frame for its voice contention period because there is no identifiable end to the PRMA contention algorithm. The strong voice system capacity of RRA three cell, and the disappointing performance of RRA two cell and RRA-C ideal, under the relaxed assumption is primarily due to the expected length of its voice contention period. On average, RRA three cell has the longest voice contention period (i.e., it operates over a longer duration within a frame) because its minimum collision resolution period is three available slots versus two and one

Cleary and Paterakis for RRA two cell and RRA-C ideal, respectively. Since voice packets arrive throughout the frame, the longer voice contention period of RRA three cell increases the probability that a newly generated voice packet will be given unblocked access. Conversely, RRA two cell and RRA-C ideal have a shorter voice contention period, thereby increasing the probability that a newly generated packet will be forced to wait until the hext frame to compete for available voice slots. It is also worth noting that the counters used in the RRA three-cell collision resolution mechanism are adjusted with probabilities similar to the permission probability used for PRMA (i.e., 1/3 for RRA three cell versus 0.35 for PRMA). The positive effect of a longer voice contention period on voice system capacity is also observed when we compare the RRA two-cell and RRA-C ideal curves in Fig. 5. As the number of active voice terminals increases to 36, RRA two cell has a lower packet dropping probability than RRA-C ideal. Then, from that point on, RRA-C ideal performs bettet. Since increasing the number of active voice tenninals within the microcell causes the expected number of talk/silence transitions to increase, on average the number of voice terminals contending for the available slots within a frame will increase. Although the average duration of the voice contention period will increase for both RRA two cell and RRA-C ideal, the RRA-C ideal algorithm resolves the contending voice terminals more efficiently. It should be noted that if we allowed RRA-C ideal to operate over the entire frame (rather than mark the end of voice contention and halt at a given point), its voice system capacity would be larger than that of PRMA and the shape of its curve would be similar to that of its counterpart in Fig. 4. In Figs. 4 and 5 our PRMA simulations show that the packet dropping probability increases sharply once the system becomes overloaded (after approximately 49 voice terminals). The abrupt increase of the packet dropping probability is caused by the inherent instability of the ALOHA-based contention algorithm, and it leads to dropped packets and/or blocked calls in this system. This behavior was also observed in the approximate analysis in [6] for the case where all talk/silence transitions occur at the frame boundaries. 6.2. Voice Transmission Under Adverse Channel Conditions If microcellular systems are to provide value in practical applications, errors over the radio channel must be constrained to reasonably low levels. Using error correcting coding and selection diversity, it is reasonable to expect that the packet header error rate on the

Investigation of RRA Algorithms for Voice-Data Integration wireless channel can be maintained at or below 0.01. In addition, since the RRA-C ideal, RRA two-cell, and RRA three-cell protocols are reset at the start of each frame (and since undelivered voice packets that age beyond the delay constraint are dropped), channel errors will not cause them to reach a state of deadlock. Note that deadlock is an important consideration when investigating the operation of pure collision resolution protocols (i.e., protocols that do not use reservations and frame structures in the time domain) over noisy channels. Therefore, we focus out attention on the effect that a nonideal channel with an average error probability of 0.01 has on the voice system capacity. To study the voice system capacity of the four RRA voice protocols operating over a nonideal channel with memoryless errors, we set the probability of error equal to 0.01 for slots that contain single packet transmissions and equal to 0.01 for slots that are idle. Figure 6 illustrates that, constrained by a packet dropping probability of approximately 1%, PRMA, RRA three cell, RRA two cell, and RRA-C ideal accommodate 46, 39, 29, and 24 voice terminals, respectively. A comparison with Fig. 5 shows that PRMA performs well, while RRA three cell, RRA two cell, and RRA-C ideal suffer an increasingly noticeable performance degradation (i.e., PRMA accommodates about 2 fewer voice terminals, RRA three cell about 7 fewer voice terminals, and RRA two cell and RRA-C ideal about 11 and 19 fewer voice termihals, respectively). It is worth mentioning that our PRMA results are consistent with the results reported in [12]. For the RRA three-cell and RRA two-cell collision resolution schemes, assuming no new transitions into talkspurt, an error during an idle slot will add three and two slots to the voice contention period, respectively, while an error in a slot containing a single packet transmission will add two and one slots, respectively. For the ALOHA-based protocols, PRMA and RRA-C ideal, an error during an idle slot has no effect, and an error in a slot containing a single packet transmission will add the number of slots required to retransmit the packet successfully. Since, under ideal channel conditions, a longer voice contention period tends to enhance voice system capacity, the decrease in performance of the RRA three cell, RRA two cell, and RRA-C ideal protocols is attributable to the combination of the channel errors and the policy of eliminating competition between voice and data terminals for the available slots (i.e., only data terminals may transmit into the available slots that follow the end of the voice contention period). In particular, if a transmission by a terminal holding a reservation is erroneously received by the base station, two possibilities exist: if the error occurs during the

13

voice contention period, then the packet recompetes immediately, but if the error occurs after the voice contention period ends, then that packet ages, and possibly expires, while the terminal taust wait to transmit in the voice contention period of the next frame. The quality of radio channels in outdoor microcells is expected to be bettet than that observed in conventional cellular systems because the direct line-of-sight path and short propagation leads to Rician rather than Rayleigh fading characteristics [19]. However, we anticipate that bursty errors of short duration (several slots) will arise within the microcell (e.g., cochannel interference). When the average error burst duration is equal to 0.1 frame (2.8 slots), Fig. 7 shows that PRMA, RRA three cell, RRA two cell, and RRA-C ideal accommodate 38, 34, 24, and 24 voice terminals, respectively. Comparing the results in Fig. 7 with the results obtained for the ideal channel, Fig. 5, indicates that the bursty errors cause the voice system capacity of each protocol to suffer noticeably. PRMA accommodates about 10 fewer voice terminals, RRA three cell about 12 less, and RRA two cell and RRA-C ideal about 16 and 20 fewer voice terminals, respectively. When we compare the results in Fig. 7 to the memoryless error results, Fig. 6, PRMA accommodates about eight fewer voice terminals, RRA three cell and RRA two cell each accommodate about five fewer voice terminals, and RRA-C ideal shows no change. It is of interest to examine the performance of the four RRA voice protocols over a nonideal channel subject to error bursts of increasing duration. In Fig. 8 we plot voice system capacity (constrained by approximately 1% packet dropping probability) versus the average duration of error burst. As the average error burst duration increases from 0.1 to 0.25 frame, the voice system capacity of PRMA and RRA three cell decreases by an additional 50 % and 18 %, respectively, while RRA two cell and RRA-C ideal show no significant change. As we increase the average error burst duration from 0.25 to 1 frame, PRMA becomes essentially inoperable, while the RRA three-cell, RRA two-cell, and RRA-C ideal voice system capacity remain fairly constant (about 28, 24, and 24 voice terminals, respectively), Note that we consider error bursts with an average duration of one frame to be the worst case for the system under investigation because, on average, all the voice packets present at the start of the burst will be dropped and the system is effectively reset, since any terminal that holds a reservation and transmits during the error burst is forced to recompete. Because of their inherent stability, the three RRA voice protocols that mark the end of the voice contention period perform significantly better than PRMA as

14

Cleary and Paterakis 50 I ~RRA-2 Cell I - - l i ~ RRA-3 Cell[ 4, RRA-Cl I X PRMA I

45 m m e-

40

ù~ 35 z0 U

~,

2s

& v

u 20 ~ 15 U

-X 5 0 0

I

I

I

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0,25

0.5

0.75

1

Avg Error Burst Duration (Frame)

Fig. 8. Summaryof the voice system capacity achievedby the protocols operating over a nonideal channel subject to bursty errors.

the average error burst duration increases in Fig. 8. In the case of RRA three cell and RRA two cell, stability is due to the collision resolution mechanism employed, whereas the stability of the RRA-C ideal algorithm derives from adaptively setting the transmission and retransmission probabilities before the start of every available voice slot. The decreasing voice system capacity of PRMA is caused by the inherent instability of the ALOHA-based contention algorithm. In other words, as we increase the average duration of the error burst, a significant backlog will develop during the burst, and after the error burst is completed the PRMA contention algorithm is unable to reduce the backlog quickly (because of collisions), causing the voice packets to expire. 6.3. Voice-Data Integration

We focus on the RRA three-cell, RRA two-cell, and RRA-C ideal protocols because they clearly mark the end of the voice contention period. Recall that this marking allows all of the terminals within the system to distinguish between available voice and available data slots. By separating the two distinct types of transmissions, we eliminate direct competition between contending voice and data terminals for the available slots. Therefore, the addition of data traffic has no effect on the operation of these RRA voice protocols (i.e., each will perform exactly the same as it would in a voicetraffic-only environment.) Table II presents voice and data throughput (packets/frame) results for the system when the three RRA

voice protocols are combined with the FCFS and twocell stack data protocols. The system is operated at the maximum voice capacity levet (constrained by approximately t% packet dropping) for each RRA algorithm over an ideal channel; the data throughput is the maximum achievable for each data protocol. When combined with FCFS, RRA-C ideal supports a system load of about 80%, while RRA three cell and RRA two cell support 79% and 76%, respectively. When combined with the two-cell stack data algorithm, the system loads for RRA-C ideal, RRA three cell, and RRA two cell are approximately 78 %, 78 %, and 74 %, respectively. Because the two-cell stack data protocol is robust in the face of channet errors, it is a promising alternative to the FCFS algorithm for handling the data traffic. As in Section 6.2, we consider a nonideal channel with an average error probability of 0.01. Tables III and IV show the maximum data throughput achieved by the two-cell stack data algorithm when the system is operated at the maximum voice capacity level (constrained by approximately 1% packet dropping) for each RRA protocol over a nonideal channel subject to memoryless and bursty (0.1 frame duration) errors, respectively. In the memorytess error case, RRA-C ideal supports a system load of 62 %, while RRA three cell and RRA two cell support about 72% and 64%, respectively. In the case of bursty errors, the system loads for RRA-C ideal, RRA three cell, and RRA two cell are approximately 62%, 67 %, and 60 %, respectively. Over the ideal channel, the RRA-C ideal protocol performs slightly better than RRA three cell with respect

lnvestigation of RRA Algorithms for Voice-Data Integration to system load. The RRA two cell supports about the same amount of system load, but it provides for significantly lower voice throughput than the other two protocols. Under adverse channel conditions, the RRA three-cell protocol clearly provides the best results in terms of system load and voice throughput. When we compare the bursty error results to the memoryless error results, the two stack-based protocols show about a 5 % drop in system loading, while RRA-C ideal shows no change. This behavior follows from the earlier observation (seen in Figs. 6 and 7) that the bursty errors decrease the voice system capacity sustainable by the RRA three and RRA two cell more drastically than do the memoryless errors (whereas the RRA-C ideal voice system capacity is the same in both cases). To explore the relationship between the efficiency of each RRA voice protocol and the data throughput and delay, we simulated the system operating with 40 voice terminals and data arrival rates of 3.5, 3.0, and 2.5 packets per frame (40 is the number of voice terminals that all of the RRA algorithms considered in this paper can handle under ideal channel conditions and constrained by a packet dropping probability of 1%). We used the FCFS collision resolution protocol to transmit the data packets, since it is the most efficient (and since operation is over an ideal channel). This combination of active voice terminals and data packet arrival rates provides system loads of approximately 73%, 71%, and 69% respectively. Recall that we defined efficiency in terms of the number of available voice slots used by an RRA protocol to resolve voice contention (i.e., a more efficient protocol has a shorter average voice contention period). Alternatively, we can represent efficiency by the average number of available data slots that a pmtocol provides per frame (i.e., a more efficient protocol provides a greater number of available data slots). In Tables V and VI we list the protocols in decreasing order of efficiency to show that efficiency has a direct bearing on data throughput and data delay. We explain the results in Tables V and VI using the average number of available data slots per frame (shown in Table V). Combined with RRA-C ideal, FCFS can sustain a maximum data packet arrival rate of approximately 5.03 packets/frame (i.e., 10.33 available data slots/frame × 0.4871 packets/slot). Combined with RRA two cell and RRA three cell, FCFS can sustain maximum data rates of approximately 4.48 and 4.14 packets per frame, respectively. (Note that this agrees with Table Il, where FCFS combined with RRA two cell, operating with 40 active voice terminals, provided a maximum data throughput of 4.5 packets/frame.) Therefore, in the case of 3.5 data packet arrivals per

15

frame (Table V), FCFS, when combined with RRA three cell, operates fairly close to its maximum sustainable data packet arrival rate. This causes the large data packet delay of 6.76 frames (about 399 ms). Finally, it is important to note that there is a clear trade-off between efficiency and voice system capacity. Of the three protocols, RRA three cell provides the best voice system capacity because it is the least efficient (i.e., its longer average voice contention period means that it operates over a larger portion of the frame). In systems where voice traffic has a higher priority than data traffic, and the design goal is to eliminate the competition between voice and data transmissions for available slots, our results suggest that a less efficient voice contention mechanism is the best approach.

7. C O N C L U S I O N S We have investigated RRA voice protocols that provide a mechanism to achieve voice-data integration in future microcellular systems. To extend the work presented in [6, 7], we incorporated the more natural assumption that a silent voice terminal can transition into talkspurt at any time (as opposed to occurring at the frame boundaries only), we introduced the promising RRA-three cell protocol, we investigated the operation of RRA voice protocols under adverse channel conditions, and we coupled RRA voice protocols with efficient multiple access protocols for data transmissions to study voice-data integration. Our primary focus was on the RRA two-cell and RRA three-cell protocols; we used simulations to compare them to PRMA and RRA-C ideal with respect to voice transmissions under ideal and adverse channel conditions and to examine their ability to support voicedata integration within out experimental system. Our results suggest that the inherently stable RRA three-cell protocol is the most promising of the two. RRA three cell provides stable and robust voice performance and, by marking the end of the voice contention period, allows terminals within the microcell to differentiate between available voice and available data slots. The RRA three-cell protocol achieves voice-data integration by eliminating competition between voice and data terminals for available slots. By separating the two distinct types of transmissions and resolving the contending voice packets first, the priority of the voice traffic is enforced while ensuring that data transmissions are provided with a best effort. We have shown that voice-data integration can be achieved by combining RRA three cell with very efficient, easy to implement, collision resolution random access protocols (e.g., two-cell stack

16 [16]) to s u c c e s s f u l l y t r a n s m i t t h e d a t a p a c k e t s . T h e r e fore, we believe that the RRA three-cell protocol cons i d e r e d in this p a p e r is a n a t t r a c t i v e a l t e r n a t i v e to P R M A

Cleary and Paterakis 19. R. J. C. Bultitude and G. K. Bradley, Propagation characteristics on microcellular urban mobile radio channels at 910 MHz, IEEE Journal on Selected Areas of Communications, Vol. 7, No. 1, pp. 31-39, 1989.

f o r u s e in f u t u r e m o b i l e m i c r o c e l l u l a r c o m m u n i c a t i o n systems.

REFERENCES 1. D. J. Goodman, Cellular packet communications, IEEE Transactions on Communications, Vol. COM-38, No. 8, pp. 12721280, 1990. 2. B. Mukherjee, Integrated voice-data communication over high speed fiber optic networks, IEEE Computer Magazine, Vol. 24, No. 2, pp. 49-58, 1991. 3. D. Bertsekas and R. Gallager, Data Networks, 2nd ed., Prentice Hall, Englewood Cliffs, NJ, 1992. 4. D. J. Goodman, R. A. Valenzuela, K. T. Gaylaird, and B. Ramamurthi, Packet reservation multiple access for local wireless communications, IEEE Transactions on Communications, Vol. COM-37, No. 8, pp. 885-890, 1989. 5. D. J. Goodman and S. X. Wei, Efficiency of packet reservation multiple access, IEEE Transactions on Vehicular Technology, Vol. 40, No. 1, pp. 170-176, 1991. 6. S. Jangi and L. Merakos, Performance analysis of reservation random access protocols for wireless access networks, IEEE Transactions on Communications, Vol. COM-42, Nos. 2-4, pt. 2 Feb-Apr 94, pp. 1223-1234. 7. M. Paterakis and A. C. Cleary, On the voice-data integration in third generation wireless access networks, European Transactions on Telecommunications, Vol. 5, No. 1, pp. 11-18, 1994. 8. I. Rubin and S. Shambayati, Performance evaluation of a reservation random access scheine for packetized wireless systems with call control, Proceedings of Globecom "92, Orlando, Florida, pp. 16-20, 1992. 9. N. M. Mitrou, T. D. Irinos, and E. N. Protonotarios, A reservation multiple access protocol for microcellular mobile communication systems, IEEE Transactions on Vehicular Technology, Vol. 39, pp. 340-351, 1990. 10. W. C. Wong and D. J. Goodman, Integrated data and speech transmission using packet reservation multiple access, Proceedings o f l C C '93, Geneva, Switzerland, pp. 172-176, 1993. 11. P. T. Brady, A technique for investigating on-oft patterns of speech, Bell Systems Technical Journal, Jan. 1965. 12. W. C. Wong, Packet reservation multiple access in a metropolitan microceltular radio environment, IEEE Journal on Selected Areas in Communication, Vol. 11, No. 6, pp. 918-925, 1993. 13. E. Gilbert, Capacity ofa burst-noise channel, Bell Systems Technical Journal, Vol. 39, pp. 1253-1265, 1960. 14. I. Kessler and M. Sidi, Splitting algorithms in noisy channels with memory, IEEE Transactions on Information Theory, Vol. IT-35, No. 5, pp. 1034-1043, 1989. 15. M. Paterakis, L. Georgiadis, and P. Papantoni-Kazakos, A full sensing window random access algorithm for networks with strict delay constraints, Algorithmica, Vol. 4, pp. 313-328, 1989. 16. M. Paterakis and P. Papantoni-Kazakos, A simple window random access algorithm with advantageous properties, IEEE Transactions on Information Theory, Vol. IT-35, No. 5, pp. 1124-1130, 1989. 17. T. Suda and T. Bradley, Packetized voice/data integrated transmission on a token passing ring local area network, IEEE Transactions on Communications, Vol. COM-37, No. 3, pp. 238-244, 1989. 18. A. M. Law and W. D. Kelton, Simulation Modeling and Analysis, 2nd ed., McGraw Hill, New York, 1991.

Allan C. Cleary (S'93) was bom in Kinston, North Carolina, in 1959. He received the B.S. degree from Clemson University and the M.S. degree from the University of Massachusetts-Lowell in 1983 and 1985, respectively, both in chemical engineering. In 1992 he received the M.S. degree from the University of Delaware in computer and information sciences. Since 1985 he has been employed as a chemical engineer at the U.S. Army Edgewood Research Development and Engineering Center. In addition, he is pursuing the Ph.D. degree at the University of Delaware in computer and information sciences. His research interests include stochastic processes with emphasis on modeling, simulation, and pefformance evaluation of communication networks.

Michael Paterakis (S'86-M'88) was bom in Athens, Greece, in 1961. He received bis Diploma degree from the National Technical University of Athens, his M.Sc. degree from the University of Connecticut, and bis Ph.D. degree from the University of Virginia, in 1984, 1986, and 1988, respectively, all in electrical engineering. He is an associate professor in the Department of Computer and Information Sciences at the University of Delaware, where he has been since September 1988. He was a visiting professor at the Department of Electronic and Computer Engineering, Technieal University of Crete, Greece (June-July 1991). He served on the technical program committee of the 1992 IEEE INFOCOM Conference, Florence, Italy, and of the 1994 IEEE INFOCOM Conference, Toronto, Canada. His research interests include comnputer communication networks with emphasis on modeling and performance evaluation of broadband highspeed networks, on multiple access wireless communication systems, and on packet radio networks; queueing and applied probability theory and their application to computer communieation systems. Dr. Paterakis is a member of the Greek Chamber of Professional Engineers, and of the Greek Association of Electrical and Electronic Engineers.