1584                                                                         IEEE TRANSACTIONS ON COMPUTERS,              ...
1586                                                          IEEE TRANSACTIONS ON COMPUTERS,         VOL. 54, NO. 12,   D...
1588                                                           IEEE TRANSACTIONS ON COMPUTERS,    VOL. 54, NO. 12,   DECEM...
1590                                                          IEEE TRANSACTIONS ON COMPUTERS,    VOL. 54, NO. 12,   DECEMB...
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Joint Reliability of Medium Access Control and Radio Link ...

  1. 1. 1584 IEEE TRANSACTIONS ON COMPUTERS, VOL. 54, NO. 12, DECEMBER 2005 Joint Reliability of Medium Access Control and Radio Link Protocol in 3G CDMA Systems Mainak Chatterjee, Giridhar D. Mandyam, Senior Member, IEEE, and Sajal K. Das, Member, IEEE Abstract—In this paper, we study the reliability as offered jointly by the medium access control (MAC) and the radio link protocol (RLP) for third generation (3G) code division multiple access (CDMA) standards. The retransmission mechanism supported at the RLP layer has a considerable amount of delay (80-100 ms) associated with it; hence, it may not be able to support applications with strict delay requirements. On the other hand, if retransmissions are also performed at a lower layer, such as the MAC layer, then the performance of CDMA systems can be improved because of the fast retransmissions at the MAC layer. We show how the performance of RLP improves with respect to three metrics—mean delay, throughput, and RLP recovery—when a finite number of retransmissions is allowed at the MAC layer. Synthetically generated Web traffic is used as the application, the objects of which are fragmented into equal-sized RLP frames. We consider soft packet combining support at the receiver, which effectively lowers the frame error rate. We also consider the possibility of misinterpretations of acknowledgments. Simulation experiments are conducted to verify the performance as offered jointly by the MAC and the RLP, particularly for cdma2000 standard and wideband CDMA (WCDMA) systems. The improvement in the TCP throughput is also evaluated. Index Terms—Radio link protocol, MAC, retransmissions, packet combing, cdma2000, WCDMA. æ 1 INTRODUCTION W ITHthe proliferation and everyday use of the World Wide Web (WWW) applications, it has become desirable that such applications also be supported over error rates (BER), its performance severely degrades [8]. Any packet loss at the wireless link is interpreted as congestion by TCP, which then responds to it by reducing wireless access networks. Following the tremendous suc- the transmission window size, initiating the congestion cess of wireless voice services, service providers have control mechanism, and resetting the retransmission time started offering a variety of wireless data services such as [16]. This misinterpretation of channel related losses and audio, video streaming, file, and Web downloading. These consequent invocation of the congestion control mechanism wireless data services promise to offer significant revenue by TCP causes an unnecessary reduction in the TCP growth opportunity for service providers all over the world. throughput. Several schemes have been proposed to To bring WWW traffic to wireless mobile devices, it is alleviate the effects of noncongestion related losses over important that a suitable protocol or standard be chosen to wireless links—both for wireless local area networks (see cater to the growing demands of data services over wireless [19] and references therein) and cellular networks [8], [15], channels such that a wide variety of multimedia traffic with [6], [23], [27], [28]. The radio link protocol (RLP) is one such different quality of service (QoS) requirements can be scheme that has been used for cellular mobile networks for supported. No matter which wireless data technology or supporting data traffic [8]. radio interface dominates future wireless networks, the fact 1.1 Radio Link Protocols (RLP) remains that it will most likely rely on the Internet Protocol The RLP is generally employed within the data link layer, (IP) which is currently the most dominant internetworking i.e., between the physical layer and the TCP layer to help protocol. As for the transport layer, the transmission control shield the effect of the loss over wireless links from the protocol (TCP) is still the major suite for IP and provides TCP layer [8], [15]. As shown in Fig. 1, the RLP fragments reliable end-to-end transmission in wireline networks [9] in an upper layer packet (a TCP segment in this case) into which the channel error rates are extremely low. However, several equal-sized RLP frames before transmitting over the when TCP is used over a wireless network with high bit wireless channel. (The last frame can be padded if needed.) A physical layer header is added to the RLP frame before it . M. Chatterjee is with the Department of Electrical and Computer is transmitted at the physical layer. The fragmentation is Engineering, University of Central Florida, PO Box 162450 Orlando, FL done to increase the granularity of the transmission. In 32816-2450. E-mail: mainak@cpe.ucf.edu. other words, in case of any error, an RLP frame which is of a . G.D. Mandyam is with Nokia Research Center, 12278 Scripps Summit Drive, San Diego, CA 92131. E-mail: giridhar.mandyam@nokia.com. smaller size is affected rather than the entire TCP segment. . S.K. Das is with the Center for Research in Wireless Mobility and The RLP uses an Automatic Repeat reQuest (ARQ) error Networking (CReWMaN), Department of Computer Science and En- recovery mechanism to retrieve a lost RLP frame. The gineering, University of Texas at Arlington, PO Box 19015, Arlington, TX recovery process is initiated by the receiver by requesting a 76019. E-mail: das@cse.uta.edu. retransmission of only the missing or erroneous frame. For Manuscript received 8 July 2004; revised 20 June 2005; accepted 30 June 2005; example, let us consider that a TCP segment is fragmented published online 14 Oct. 2005. For information on obtaining reprints of this article, please send e-mail to: into L number of RLP frames which are transmitted one tc@computer.org, and reference IEEECS Log Number TC-0232-0704. after another. If, somehow, the ith frame, 1 i L, fails to 0018-9340/05/$20.00 ß 2005 IEEE Published by the IEEE Computer Society
  2. 2. CHATTERJEE ET AL.: JOINT RELIABILITY OF MEDIUM ACCESS CONTROL AND RADIO LINK PROTOCOL IN 3G CDMA SYSTEMS 1585 performance of TCP over 3G networks was evaluated in the presence of correlated fading channels. In all these standards and propositions, RLP has been the only layer below TCP to shield the losses by triggering retransmissions. Though this single layer of reliability from RLP shields losses for TCP, there can still be some performance limitations if the TCP applications have strict delay requirements. To deal with such demanding applica- tions or interactive services, it is necessary to incorporate a fast retransmission mechanism below the RLP. This was done by incorporating an ARQ mechanism at the MAC layer, thus providing two layers of retransmission reliability. The benefit of MAC layer ARQ is that retransmissions can be done very quickly, without notifying RLP at the upper layer. 1.3 Our Contributions Fig. 1. Fragmentation of TCP segments into RLP frames. Our main motivation behind this research is to study the reliability offered jointly by the RLP and the MAC layers of reach the receiver, the receiver would request for a 3G CDMA standards, particularly for cdma2000 1X-EV-DV retransmission of that frame. However, the receiver buffers [2] and wideband CDMA (WCDMA) [1] systems in the other frames ð1; 2; Á Á Á ; i À 1; i þ 1; Á Á Á LÞ and, on recep- supporting data traffic. It can be noted that two-layer tion of the ith frame, reassembles the TCP segment and reliability was not supported in the previous CDMA delivers it to the upper layer. The recovery from the standards such as IS-99 [21] and cdmaOne [36]. In this erroneous frames is done before the TCP timer expires so paper, we do not propose any additional reliability, but, that the TCP throughput remains unaffected. instead, study the performance that can be expected by the The RLP is usually sufficient to shield the physical layer incorporation of MAC layer retransmissions in 3G systems. impairment from the TCP. However, RLP might still fail to (Preliminary versions of these results can be found in [10], conceal losses due to two reasons: 1) Applications might [11].) More specifically: have strict delay requirements [33] and 2) the packet loss rate at the medium access layer (MAC) layer might be high. . We analyze the performance of both cdma2000 and If the packet loss rate is high, even for elastic data traffic, the WCDMA systems and demonstrate how the incor- RLP has no way to shield the physical impairment from the poration of MAC layer retransmissions can signifi- TCP. If real-time traffic is considered, the application is cantly improve the delay performance of the RLP. often assumed to be loss-tolerant. For example, if packets . We define three performance metrics: mean delay, throughput, and the fraction of packets recovered by are dropped in voice communication, human intelligence is the RLP for a different number of retransmission able to find the consistency in speech. Similarly, in video trials at the MAC layer. The performance analysis is transmission, if packets or frames are lost, the decoder at done assuming a fading channel and considering a the receiver conceals the losses by interpolating past and soft packet combining mechanism at the receiver. future frames. However, if channel conditions are extremely . We extend the analytical models to incorporate bad, error concealment becomes almost impossible. In such misinterpretations of acknowledgments (though a cases, interpolation or interpretation of a signal may not small fraction) at the MAC layer. only be difficult, but also impossible. Therefore, it is . We conduct simulation experiments to validate the recommended that as many damaged packets as possible analytical model. It is observed that the delay drops are salvaged, as quickly as possible. from 80-100 ms to less than 10 ms if the MAC layer is allowed at least one retransmission attempt, although 1.2 RLPs for CDMA Systems the decrease in delay is not substantial for more The performance of RLPs for various code division multiple than two retransmissions. The recovery rate from access (CDMA) systems have been studied over the years as missing frames at the RLP increases with an the standard evolved. Performance issues related to TCP increasing frame error rate and also when the and RLP interaction in the CDMA protocol stack have been misinterpretations of acknowledgments are high. investigated in [8]. The impact of TCP source activity on the . After studying the improvement in RLP perfor- call admission control for the cellular CDMA standard IS-95 mance, we study the effect on the TCP throughput was studied in [33]. Support of data services over the IS-95 with and without the two layers of retransmissions. physical channels using RLP was proposed in [22]. For IS-99 The rest of the paper is organized as follows: Section 2 (the first IS-95 data standard), the performance evaluation discusses the two-layer retransmission scheme supported by of TCP over RLP was shown in [21] and the performance for the 3G CDMA systems. The performance analysis is pre- circuit mode data services was shown in [15]. The sented in Section 3, where mean delay and the fraction of performance of TCP over RLP in the cdma2000 system [2] packets recovered by the RLP are evaluated. Section 4 was shown in [25]. A negative acknowledgment-based presents the simulation model and results with respect to hybrid ARQ scheme was proposed in [36]. In [27], the mean delay, throughput, and RLP recovery. In Section 5, we
  3. 3. 1586 IEEE TRANSACTIONS ON COMPUTERS, VOL. 54, NO. 12, DECEMBER 2005 show how misinterpretations of acknowledgments could possibly affect the performance of the cdma2000 and WCDMA systems. The improvement in the TCP throughput is shown in Section 6. Conclusions are drawn in the last section. 2 MAC RETRANSMISSIONS IN CDMA SYSTEMS A big challenge in third generation (3G) CDMA systems is how to handle a wide variety of multimedia services with different QoS requirements. To meet these demands, mod- ifications and enhancements are being made to cdma2000 and WCDMA standards. cdma2000 is being deployed in com- mercial networks to support voice and data on the same 1.25 MHz carrier [4]. A higher data rate evolution, called Fig. 2. Two layers of retransmission. cdma2000 1X-EV-DV, has been standardized by the Tele- communications Industry Association (TIA) and has been added to IMT-2000 [3]. As specified by the Third Generation site selection (FCSS), a feature under consideration Partnership Project (3GPP2) [2], cdma2000 1X-EV-DV pro- in HSDPA, provides mobility support for best effort vides integrated voice with simultaneous high-speed packet data services. One way to achieve this is by the data services such as video, video-conferencing, and other mobile terminal “echoing” information about pack- multimedia services at speeds of up to 4.8 Mbps. Further- ets it has recently received over the shared channel during a transition from one base station to another more, cdma2000 1X-EV-DV is backward compatible with so as to provide fast synchronization of the packet cdmaOne and cdma2000 1X, providing wireless operators a transmission queues between the new and the old seamless network evolution. base stations. On the other front, high-speed downlink packet access 2. The high-speed downlink shared channel in HSDPA (HSDPA) [10] is one of the main propositions for WCDMA and the forward packet data channel in cdma2000 systems. HSDPA allows instantaneous bit rates up to 1X-EV-DV, which carry payload for best effort data 10 Mbps for best-effort packet data services with certain users (in both time and code multiplexed modes) bounds on the delay and capacity. The HSDPA channel is a might contain several protocol data units (PDUs), resource that is shared among several users in the mobile not all of which come from the RLP. In fact, some communication system. By using a fast scheduler located at PDUs might come directly from the layer above the the base station, the HSDPA channel can be assigned to the RLP. As a result, the MAC-ARQ provides retrans- user transmitting with the current best channel (i.e., highest missions quickly in HSDPA, which employs a stop- data rate). The underlying idea is that only users with good and-wait hybrid ARQ method. In this method, each downlink channel conditions will use the HSDPA channels packet received by the receiver must be acknowl- and other users will not be allowed to use these channels if edged on a dedicated feedback channel to the they experience bad channel conditions. Some of the basic transmitter. This dedicated feedback takes the form principles used in HSDPA are fast link adaptation, fast of the reverse acknowledgment indicator with scheduling, and fast retransmissions of erroneously re- values of þ1 for an ACK, or À1 for a NACK. ceived packets. However, the receiver does not discard the received 2.1 Fast ARQ at the MAC soft information associated with the incorrectly received packet. Rather, it buffers the data and Let us now discuss how the RLP and MAC layers jointly coherently combines the buffered data with the provide the enhanced retransmission reliability. Fig. 2 shows received soft information of the retransmission of the the working of these two layers at the transmitter and receiver bad packet [34]. This type of packet combining ends. A TCP segment reaches the MAC layer after being provides increased reliability in CDMA systems. fragmented at the RLP. For every packet transmitted at the MAC layer, there is either an ACK or a NACK. This enhances 2.2 n-Phase ARQ the response time for retransmissions since the RLP is a Both HSDPA and cdma2000 1X-EV-DV use an n-phase stop- NACK-based protocol. The fast ARQ mechanism (also called and-wait MAC-ARQ [18]. By “n-phase,” we mean that MAC-ARQ) in HSDPA and cdma2000 1X-EV-DV is impor- multiple ARQ instances are employed in consecutive time tant to ensure that some performance loss can be recovered. slots (e.g., 5-ms frame durations). For instance, let us assume There are two reasons why RLP cannot provide the that three ARQ channels are used as shown in Fig. 3. Then, in functionality needed for MAC-ARQ: time slot t, the receiver will receive a packet corresponding to 1. In the process of selecting the base station with the phase 1. In time slot t þ 1, the receiver will receive a packet strongest signal for the cell selection, the RLP corresponding to phase 2. In time slot t þ 2, the receiver will terminates at the last network element (e.g., base receive a packet corresponding to phase 3. Again, in time slot station), resulting in network delays in servicing t þ 3, the receiver will receive a packet corresponding to retransmission requests at the RLP layer. Fast cell phase 1, and so on. The receiver must keep separate packets
  4. 4. CHATTERJEE ET AL.: JOINT RELIABILITY OF MEDIUM ACCESS CONTROL AND RADIO LINK PROTOCOL IN 3G CDMA SYSTEMS 1587 Fig. 3. An example of 3-phase ARQ. received from different phases for packet combining and In the following, we derive expressions for DMAC and packet acknowledgments. However, once any packet for any DRLP . The expression for DRLP will be conditioned to the fact phase is received correctly, the receiver may deliver the that the MAC layer failed even after the maximum number packets to the higher layers (e.g., RLP). of allowed retransmissions. Let us first briefly discuss the underlying channel characteristics and the frame error rates (FERs) observed due to packet combining. 3 PERFORMANCE MODELING Let us now calculate the mean delay and the fraction 3.1 Channel Characteristics (percentage) of packets recovered by the RLP, without In a wireless environment, the characteristics of the radio considering the time taken for a packet to get scheduled for channel are very important since the channel conditions transmission at the RLP. We consider the delay suffered by a vary with time and space. There are several factors that packet before being correctly received by the RLP at the affect the link attenuation. The hindrance may be due to receiver and then delivered to the upper layer. We assume multipath fading, shadowing, or any other noise source. that the transmission and propagation delays are negligible. The most commonly used additive white Gaussian noise That is, if a packet is successfully transmitted at the first (AWGN) channels do not represent the channel conditions trial, then the delay incurred is zero. This assumption can accurately and, hence, a more realistic view of the channel is be relaxed by adding an offset (with a statistical fluctuation) necessary for any involved analysis or experimentation. to the delay value. If N denotes the number of ARQ phases Moreover, the channel model should include the correlated and T the frame duration, it takes about N Â T ms for the errors, which is most often the case. Therefore, we consider ACK to reach the transmitter. Only then can the transmitter a 1-path Rayleigh fading model [26] for the losses, which remove the packet from its transmission buffer. If a packet generally has the worst behavior among multipath Rayleigh does not successfully reach the receiver at the first trial, models due to its inherent lack of diversity [38]. then, on the receipt of a NACK, it will undergo retransmis- 3.2 Packet Combining Mechanism sion. A delay will be incurred while the MAC attempts its In packet data services, when the receiver detects an fast ARQ mechanism to retransmit and recover the missing erroneous packet in error, it requests a retransmission. packet. Let the system parameter MAXRETRANS denote However, the receiver does not have to discard the soft the maximum number of retransmissions allowed at the information associated with the incorrectly received packet. MAC layer. If the packet is not recovered even after Rather, it can buffer the data and coherently combine it with MAXRETRANS retransmissions by the MAC layer, the RLP the received soft information on the retransmission of the triggers its own retransmission mechanism. same packet [34]. This is because there is a 3-db gain in the By definition, zero delay is incurred by a packet which Eb =N0 (Eb is the energy per bit and N0 is the spectral noise does not undergo any retransmission at RLP or MAC. A density) value if the retransmitted packet is soft-combined finite delay is incurred only when a packet fails and is with the stored packet under quasistatic channel conditions consequently retransmitted. The total delay (D) experienced [34]. This is because the retransmitted packet will double by a packet to successfully reach the receiver can potentially the power of the received signal; hence, the gain is 10 log 2 % have two components: 1) DMAC , the delay due to the MAC- 3 db. If the packet is not correctly decoded even after the ARQ mechanism and 2) DRLP , the delay due to the RLP combination, it is retransmitted for the second time (if retransmissions. If a packet is recovered by the MAC layer, MAXRETRANS ! 2). With the second retransmitted packet, then the RLP does not need to use its retransmission and, the gain in Eb =No would be 6-db if two copies of the same hence, DRLP ¼ 0. If the MAC layer fails to recover a packet, packet are retransmitted. That is, with every trial of then RLP invokes its retransmission and, hence, both the retransmission, there is approximately a gain of 3-db. This delay components will be nonzero. Thus, the total delay is type of packet combining mechanism provides increased reliability in CDMA systems. In order to ensure that the D ¼ DMAC þ DRLP : ð1Þ receiver does not try to combine packets from one ARQ
  5. 5. 1588 IEEE TRANSACTIONS ON COMPUTERS, VOL. 54, NO. 12, DECEMBER 2005 TABLE 1 TABLE 2 Effective FER Values for cdma2000 1X-EV-DV Effective FER Values for WCDMA phase with another, it is assumed that the outband the number of ARQ phases and T is the frame duration. signaling is sent on a forward control channel concurrently Therefore, the mean waiting time for all the packets at the with each frame that the receiver receives on the forward MAC layer is DMAC ¼ pNT , where p is the FER of the shared channel. wireless channel. The packets which will still fail to reach 3.3 Effective FER the receiver successfully will be recovered by the RLP. The We consider 1-path Rayleigh channel at 3 Km/h with 1 rate 2 packets being recovered by RLP would have a delay of M  8-PSK (phase shift key) modulation in the personal NT ¼ NT since M ¼ 1. This is essentially the RLP delay for communication system PCS 1800-1900 MHz frequency. triggering a retransmission. If M ¼ 0, the fraction of packets According to the simulation results reported in [29], the effective FER falls to approximately one-third of its value recovered by RLP is p. Since M ¼ 1, the fraction is given by for every transmission in a cdma2000 1X-EV-DV system. Cp  p, assuming the FER value drops by a factor of C (as The FER values for successive retransmissions as obtained noted earlier), which is different for cdma2000 and in [29] are shown in Table 1. Although the reduction in the WCDMA systems. Thus, the mean delay at the RLP is effective FER could not be generalized, it works well with DRLP ¼ Cp2  NT . The total mean delay is given by Rayleigh fading channels. For example, if the FER is 0.3 for the initial transmission, then, on successive retransmissions, D ¼ DMAC þ DRLP ¼ pNT þ Cp2 NT : ð2Þ the effective FER would be 0.095, 0.031, 0.0095, and so on, decreasing by approximately one-third every time. The The RLP recovery (R) measured by the fraction of packets corresponding FERs for WCDMA as obtained from [20] is recovered by RLP is given by shown in Table 2, for which the decrease in successive FERs R ¼ Cp2 : ð3Þ is approximately a little less than half. In general, we can assume that there is a drop in the FER by a factor of C for The factor C appears due to packet combining. every retransmission, where C % 0:33 for cdma2000 1X-EV- Case II (M ¼ 2). Since the ACKs of ðp  100Þ% of the DV (Table 1) and C % 0:45 for WCDMA systems (Table 2). retransmitted packets are yet to arrive, these packets need 3.4 Analysis of Delay and RLP Recovery to be retransmitted again (second MAC retransmission) Let us now derive expressions for mean delay and the fraction and the effective FER observed so far will be Cp. of packets recovered by the RLP when MAXRETRANS ¼ M. Therefore, the mean delay associated with the second Assume that p is the raw FER offered by the physical channel. retransmission is Cp  pNT . Thus, the mean delay at the When a packet is transmitted, the MAC usually waits for a MAC is DMAC ¼ pNT þ Cp  pNT . Since the fraction of certain number of ARQ phases (say, N) for the ACK. Let the packets taken care of by RLP is Cp2 when M ¼ 1, the throughput be defined as the ratio of the number of successful packets received to the total number of packets undergoing fraction for M ¼ 2 would be Cp  Cp2 and the delay at the transmission, including retransmissions. The RLP recovery, R, RLP would be DRLP ¼ ðCpÞ2 p  2NT . Hence, the total is defined as the fraction of packets recovered by the RLP mean delay is obtained as when the MAC fails to deliver a packet correctly. We will first derive the expressions for the total delay (D) and RLP D ¼ pNT þ Cp  pNT þ ðCpÞ2 p  2NT : ð4Þ recovery (R) for one transmission (M ¼ 1) and two transmis- The RLP recovery in this case would be sions (M ¼ 2) and then generalize them. Since the frame error rate is finite, the MAC layer will fail with certain probability R ¼ ðCpÞ2 p: ð5Þ and, hence, the average case delay analysis will have a DRLP component. Case III (M > 2). Following in a similar manner, we Case I (M ¼ 1). The waiting time for a packet unsuccess- calculate mean delay at the MAC for any arbitrary M fully received on the first transmission is N  T , where N is retransmissions as
  6. 6. CHATTERJEE ET AL.: JOINT RELIABILITY OF MEDIUM ACCESS CONTROL AND RADIO LINK PROTOCOL IN 3G CDMA SYSTEMS 1589 X M DMAC ¼ ðCpÞiÀ1 pNT i¼1 ð6Þ 1 Cp À ðCpÞM ¼ pNT : 1 À Cp The delay at the RLP is DRLP ¼ ðCpÞM p  MNT : ð7Þ Therefore, the total delay is given by 1 Cp À ðCpÞM D¼ pNT þ ðCpÞM p  MNT : ð8Þ 1 À Cp Fig. 4. Web page traffic scenario. Additionally, the RLP recovery is given by the RLP timer. Trlp is a system parameter which is R ¼ ðCpÞ p: M ð9Þ negotiated at the time of data connection establishment. A larger value of Trlp gives more scope for MAC to recover 3.5 Bound on M packets. However, it might delay delivery of segments to Though the number of retransmissions at the MAC layer is the upper layer (e.g., TCP), thus affecting the delay a design parameter, it is primarily dictated by the RLP requirements of the data application. So, depending on timer. The RLP starts a timer when it pushes a packet to the the application requirement and the corresponding TCP lower (MAC) layer. If a packet fails to reach the receiver by timeout value, the RLP timer is chosen. the expiration of the timer, the RLP invokes its own retransmission mechanism to recover the missing packet. 4 SIMULATION STUDY Therefore, we calculate the maximum number of retrans- missions that the MAC layer can afford before the RLP To validate the analytical model, we develop a UNIX-based timer expires and initiates its recovery process. In other simulator, perform extensive simulation experiments, and words, the delay at the MAC layer must be less than the measure the average delay and the fraction of packets RLP timer, Trlp . We seek the maximum number of recovered by the RLP for the cdma2000 system. Though retransmissions allowed at the MAC layer before the RLP multiple RLP frames can be mapped to a physical layer intervenes. Therefore, the condition to be satisfied is frame by the proper choice of codes from the code-tree [35], we assumed that one fixed-size RLP frame is mapped to a pNT þ ðCpÞ1 pNT þ ðCpÞ2 pNT þ Á Á Á þ ðCpÞM pNT Trlp ; physical layer frame. The hyper text transport protocol ð10Þ (HTTP) was used for the data application. We chose such a model because of its interactive parameters, i.e., the where M is the number of retransmission trials at the MAC. parameters that also reflect the end-user response. Though Since we are seeking the maximum number of retransmis- HTTP is considered a non-real-time application, the down- sions that the MAC can afford, we must find the maximum load time is important to the end user and, hence, some value of M, Mmax , for which the inequality is satisfied. delay bounds are needed. Therefore, Mmax is given by 4.1 HTTP Traffic Model Mmax ¼ maxfm : pNT þ ðCpÞ1 pNT þ ðCpÞ2 pNT þ Á Á Á Instead of investigating the nature of HTTP traffic, we þ ðCpÞm pNT Trlp g synthetically generate such traffic by using the results ( ) 1 À ðCpÞmþ1 obtained in [12]. The basic model of HTTP is shown in Fig. 4 ¼ max m : pNT Trlp in which a packet call represents the download of a Web page 1 À Cp & ' requested by a user. It usually has a main page followed by Trlp ð1 À CpÞ some embedded objects. A new request (packet call) is ¼ max m : ðCpÞmþ1 ! 1 À pNT immediately generated after the expiration of the viewing & ! Trlp ð1 À CpÞ period. The model is similar to an ON/OFF source, where the ¼ max m : m ! logCp 1 À À 1: pNT ON state represents the activity of a page request and the OFF ð11Þ state represents a silent period after all objects in that page are retrieved. The download time of a page follows Weibull Since Mmax must be an integer, we take the floor function distribution [12], the mean of which depends on the under- since the ceiling function will violate (10). Therefore, Mmax lying bandwidth of the wireless channel. We have considered is given by a data rate of 76.8 Kbps (9,600 bytes/s). Each object (main " ! # page and embedded objects) of the HTTP traffic is fragmen- Trlp ð1 À CpÞ Mmax ¼ logCp 1 À À1 : ð12Þ ted into multiple equal-sized RLP packets. Other statistics pNT and parameters used to generate the HTTP traffic are shown Taking the floor function also ensures that the time taken in Table 3. The way each RLP packet is quickly acknowledged for Mmax retransmissions at the MAC layer does not trigger is discussed next.
  7. 7. 1590 IEEE TRANSACTIONS ON COMPUTERS, VOL. 54, NO. 12, DECEMBER 2005 TABLE 3 TABLE 4 Statistics for HTTP Simulation Parameters 4.2 Fast ARQ at the MAC packets are recovered and the third retransmission is We emphasize that the analysis is flexible enough to handle hardly required. Fig. 7 shows the throughput of the newer advanced packet data access methods for both system which is the combined throughput due to MAC upstream and downstream traffic. Though the analysis and RLP. It can be seen that the throughput decreases as does not specifically follow 3GPP and 3GPP2 assumptions, the FER increases. This is obvious because larger FER the analysis can be readily adapted to new physical layer damages more packets during transmission and, thus, the setups that use RLP and hybrid-ARQ. For example, the number of retransmitted packets is also high. Fig. 8 authors in [17] describe the latest 1X-EV-DV evolution presents the efficiency with which the RLP recovers the (enhanced reverse link) with frame durations as low as missing packets. If the MAC-ARQ mechanism is turned 10 ms. Though the frame duration can vary depending on off, effectively implying MAXRETRANS = 0, all the the modulation and coding schemes, the largest frame missing packets are recovered by the RLP. When duration is set to 20 ms. MAXRETRANS = 1, there is a considerable drop in the We consider a configuration of the cdma2000 1X-EV-DV RLP recovery because the MAC does most of the recovery system where the transmitter transmits one RLP packet in with just one retransmission. However, there is not much each 5 ms physical layer frame and waits for the ACK. If the improvement if the maximum number of such retransmis- ACK does not arrive in 20 ms (equal to four ARQ phases, sions at the MAC layer is more than 1. With MAXRETRANS which is specific to cdma2000) [14], then the frame is = 2 or 3, the RLP recovery is almost zero because virtually retransmitted immediately. It can be seen from Fig. 5 that all the packets are recovered with three retransmissions. frames with sequence numbers 0; 1; 2; 3; Á Á Á are being transmitted. Frames 0 and 2 are undergoing retransmission 5 MISINTERPRETATION OF ACK/NACK because of nonreceipt of ACKs. The ACK timers of these packets are again reinitialized. In a practical scenario, such So far, we have not considered the probability that the ACK or NACK packets might get corrupted. In reality, there is a as the downlink, it is not necessary that the receiver deliver small probability that some of the ACKs and NACKs from the ACK/NACK precisely at the slot boundaries, each of the MAC-ARQ will be misinterpreted [22], [39]. The which is 5 ms. The actual physical layer boundary is more decoder at the transmitter might interpret a NACK as an precise. This is due to the fact that, normally, when using ACK and therefore would not transmit the packet, assum- coherent receivers in the reverse link, the base station ing correct reception. The RLP at the receiver will detect the suffers from some processing delay. The allowed number of missing packet and trigger its own retransmission mechan- retransmission trials is varied between 1 and 3. If a packet is ism to recover the packet. On the other hand, if an ACK is not successfully received or combined at the receiver even decoded as a NACK, then a retransmission will be triggered after the maximum number of MAC retransmissions, then by the MAC if the number of retransmissions has not the RLP retransmission is triggered. Other parameters used reached the maximum limit (i.e., MAXRETRANS). If the for the simulation are listed in Table 4. number of allowable retransmissions at the MAC is 4.3 Analytical and Simulation Results exhausted and a packet has not been ACKed, then the Fig. 6 shows the total delay (D) for various MAXRE- RLP will initiate the recovery process. No matter which TRANS values. Not much difference is observed between layer does the recovery, it results in unnecessary (duplicate) MAXRETRANS = 2 and MAXRETRANS = 3. This is due retransmissions. It can be noted that any kind of wrong to the fact that, after two transmissions, most of the interpretation at the MAC can only be detected by the RLP and, thus, the reliability of the RLP cannot be ignored. This is where the real benefit and importance of RLP is manifested. The percentage of error recovery by the RLP depends on the percentage of misinterpretations, which we call the “falseACK.” 5.1 Analytical Modifications Due to the presence of the falseACKs, we need to modify the analytical expressions for D and R derived in Fig. 5. Number of ARQ phases (N) = 4 timing diagram. Section 3.4. Let f denote the percentage of falseACKs. The
  8. 8. CHATTERJEE ET AL.: JOINT RELIABILITY OF MEDIUM ACCESS CONTROL AND RADIO LINK PROTOCOL IN 3G CDMA SYSTEMS 1591 Fig. 6. Mean delay. Fig. 7. Throughput. fraction of packets not reaching the RLP at the first trial is p pðCpÞ2 ð1 À fÞ2 . Therefore, the additional fraction of packets simply because p is the FER and one packet gets mapped to recovered by the RLP in this case is one physical layer frame. Out of these, a fraction f will be AddM¼2 ¼ fp þ fpðCpÞð1 À fÞ þ pðCpÞ2 ð1 À fÞ2 : ð14Þ misinterpreted as ACK and the fraction ð1 À fÞ will under- go retransmission at the MAC layer, leading to an FER of In general, the additional fraction of packets recovered Cp. The fraction of packets not correctly reaching after the by RLP is derived as first retransmission (if MAXRETRANS = 1) is pðCpÞð1 À fÞ. X M À1 Therefore, for M ¼ 1, due to the misinterpretations, the RLP AddM ¼ fpðCpÞi ð1 À fÞi þ pðCpÞM ð1 À fÞM : ð15Þ i¼0 recovers an additional fraction of packets given by The summation term corresponds to the misinterpretations AddM¼1 ¼ fp þ pðCpÞð1 À fÞ: ð13Þ and the second term corresponds to the fraction of packets If M ¼ 2, the fraction of packets misinterpreted on failing to reach the receiver even after MAXRETRANS the second retransmission is fpðCpÞð1 À fÞ. The fraction retransmissions. Therefore, the RLP recovery expression in of packets not delivered correctly by the MAC is (9) is modified as
  9. 9. 1592 IEEE TRANSACTIONS ON COMPUTERS, VOL. 54, NO. 12, DECEMBER 2005 Fig. 8. RLP recovery. R ¼ ðCpÞM p þ AddM : ð16Þ percentage is very small and is usually around 2 percent Clearly, the number of packets recovered by RLP [39], so we varied f from 1 percent to 5 percent. Fig. 9 shows increases with misinterpretations. The delay expression that the mean delay due to simulation as well as analysis (from (8)) for MAXRETRANS = 2. We observe that the delay obtained in (8) will modify to is more as compared to Fig. 6, where f ¼ 0. Fig. 10 shows X M the percentage of RLP recovery for MAXRETRANS = 2 and D¼ ðCpÞiÀ1 pNT þ ðAddM Þ Â MNT : ð17Þ this percentage is more than that for MAXRETRANS = 1. i¼1 This is because almost all the packets are recovered with Note that, if f ¼ 0, (16) and (17) will reduce to (9) and (8), two retransmissions at the MAC layer and the RLP mostly respectively. recovers the ones that were misinterpreted. It is expected that the packet recovery by RLP will be more at larger 5.2 Results with falseACK values of FER and higher percentage of ACK misinterpreta- We conducted simulation experiments by incorporating a tions. However, we did not consider FER greater than certain percentage of ACK misinterpretations. This 30 percent because, at that high FER, there will be other Fig. 9. Mean delay (MAXRETRANS = 2).
  10. 10. CHATTERJEE ET AL.: JOINT RELIABILITY OF MEDIUM ACCESS CONTROL AND RADIO LINK PROTOCOL IN 3G CDMA SYSTEMS 1593 Fig. 10. RLP recovery (MAXRETRANS = 2). Fig. 11. Throughput (MAXRETRANS = 2). severe consequences with maintaining the link. Fig. 11 misinterpretations was varied from f ¼ 1% to f ¼ 5%. shows the degradation in the system performance in terms Fig. 12 depicts the mean delay for MAXRETRANS = 2. of throughput when M ¼ 2. Fig. 13 shows the degradation in the system performance in Comparing the simulation and analytical results, we terms of the throughput for MAXRETRANS = 2. The observe that they are in good agreement for FER < 0:2. percentage of RLP recovery for MAXRETRANS = 2 is However, they differ by less than 1 percent for FER > 0:2. One 2 shown in Fig. 14. The RLP recovers more packets with possible reason might be the assumption on the FER for MAXRETRANS = 1 than with MAXRETRANS = 2. This successive retransmissions. The FERs used for simulation happens because, as explained earlier, almost all the packets (see Table 1) do not strictly fall by 1 for cdma2000 systems, 3 are recovered with two retransmissions at the MAC layer. which was assumed for obtaining the analytical results. The RLP mostly recovers the ones which were misinter- 5.3 WCDMA Performance preted. It can be expected that the RLP recovery will be Simulation experiments were also conducted for WCDMA more for even larger FER values. The value of C ¼ 0:45 for systems, using the same parameters as those used for WCDMA (see Table 2) can be used for analytical purpose. cdma2000 simulations, except that the WCDMA frame To compare the performance of cdma2000 1X-EV-DV and duration was 2 ms (3 Â 1.67 ms). The percentage of WCDMA systems, we summarize in Table 5 the mean delay,
  11. 11. 1594 IEEE TRANSACTIONS ON COMPUTERS, VOL. 54, NO. 12, DECEMBER 2005 Fig. 12. Mean delay (MAXRETRANS = 2). Fig. 13. Throughput (MAXRETRANS = 2). RLP recovery, and throughput of these two standards for segment is lost. Thus, the TCP segment loss probability, MAXRETRANS = 2 and misinterpretation fraction f ¼ 5%. T CPloss , is given by T CPloss ¼ 1 À ð1 À pÞL ; ð18Þ 6 TCP THROUGHPUT ENHANCEMENT where p is the frame error probability at the physical layer. With the two layers of retransmission, we evaluate the If, however, we assume an underlying RLP (1, 2, 3) in throughput improvement at the TCP layer. Recall from operation, i.e., one retransmission at the first trial, two Section 1.1 that TCP segments are fragmented into multiple retransmissions at the second, and three retransmissions at RLP frames. If we assume that a TCP segment of size T bytes the third trial, then the effective frame loss probability at the is fragmented into equal-sized RLP frames, then the RLP layer is p7 . Hence, with the RLP layer, the TCP segment T number of such frames is L ¼ dRe, where R is the payload loss probability is modified to of each RLP frame. The actual size of the RLP frame would L be R plus some header information. For a TCP segment to T CPloss ¼ 1 À ð1 À p7 Þ : ð19Þ be reassembled successfully, all the L frames must be Furthermore, if we assume that the MAC layer is also in received correctly. If one or more RLP frames fail, the TCP operation, allowing only one retransmission attempt (i.e.,
  12. 12. CHATTERJEE ET AL.: JOINT RELIABILITY OF MEDIUM ACCESS CONTROL AND RADIO LINK PROTOCOL IN 3G CDMA SYSTEMS 1595 Fig. 14. RLC Recovery (MAXRETRANS = 2). TABLE 5 Performance of cdma2000 1X-EV-DV and WCDMA MAXRETRANS = 1) to recover any lost or corrupted RLP assumed to be 0.2 seconds, T0 ¼ 2 Â RT T , and b ¼ 2, as per frame, then the effective frame loss probability at the RLP the traces obtained in [30]. MSS is assumed to be 1,460 layer is ðp7 Þ2 and (19) is modified to bytes. RLP frames were assumed to be of constant size of L 80 bytes. The transmit window size for the TCP is assumed T CPloss ¼ 1 À ð1 À p14 Þ : ð20Þ to grow without limit. The improvement in TCP throughput We will assume that the TCP throughput is given by [30]: is due to two reasons. First, the fragmentation of the TCP segments into RLP frames prevents the entire TCP segment ST CP ¼ from being retransmitted, if lost. Second, due to faster pffiffiffiffiffiffiffiffiffiffiffiffi À pffiffiffiffiffiffiffiffiffiffiffiffi MSS Á; ð21Þ retransmissions and soft combining, the effective frame loss 2bT CP 3bT CP loss þT min 1;3 loss ÞT CP 2 RT T loss ð1þ32T CPloss 3 0 8 probability is drastically reduced. where MSS is the maximum segment size, RT T is the round trip time for the TCP ACKs, T0 is the TCP 7 CONCLUSIONS retransmission timer, and b is a system constant. T0 is evaluated as an exponentially moving average of the As the demand for real-time wireless data services instantaneous RT T s. increases, more efficient and faster protocols have to be Substituting the value of T CPloss obtained from (18), (19), designed for timely delivery of data packets. In this paper, and (20), we can obtain the TCP throughput when 1) both we demonstrated how cdma2000 1X-EV-DV and WCDMA RLP and MAC are off, 2) RLP is on and MAC is off, and promise to be such protocols which can deliver real-time 3) both RLP and MAC are on, respectively. Fig. 15 shows data over the wireless medium. This is made possible with the relative performance of these three cases. The RT T is the help of the joint reliability offered by both the MAC and
  13. 13. 1596 IEEE TRANSACTIONS ON COMPUTERS, VOL. 54, NO. 12, DECEMBER 2005 Fig. 15. TCP throughput. RLP layers. The MAC layer retransmissions can recover [10] M. Chatterjee, G.D. Mandyam, and S.K. Das, “Fast ARQ in High Speed Downlink Packet Access for WCDMA Systems,” Proc. missing or damaged packets very quickly without notifying European Wireless, pp 451-457, Feb. 2002. the upper layer (e.g., RLP) and thus reduce the delay from [11] M. Chatterjee, S.K. Das, and G.D. Mandyam, “MAC Layer about 100 ms to less than 10 ms. Due to the soft packet Retransmissions in 1XTREME,” Proc. 13th IEEE Int’l Symp. Personal, Indoor and Mobile Radio Comm. (PIMRC), vol. 3, pp. 1452- combining mechanism, the effective FER experienced by a 1456, 2002. packet is lowered for every successive retransmission. We [12] H.K. Choi and J.O. Limb, “A Behavioral Model of Web Traffic,” analyzed the system performance in terms of delay and the Proc. Int’l Conf. Network Protocols (ICNP), pp. 327-334, 1999. fraction of packets recovered by the RLP for a finite number [13] S. Choi, Y. Choi, and I. Lee, “IEEE 802. 11 MAC-Level FEC with Retransmission Combining,” IEEE Trans. Wireless Comm., Jan. of retransmissions at the MAC layer. The analysis is 2005. corroborated with simulation experiments using syntheti- [14] Y.-J. Choi and S. Bahk, “Scheduling for VoIP Service in cdma2000 cally generated HTTP traffic. We also considered misinter- 1x EV-DO,” Proc. IEEE Int’l Conf. Comm. (ICC) 2004, vol. 3, pp. 1495-1499, 2004. pretations of the ACKs at the MAC layer. Results [15] A. Chockalingam and G. Bao, “Performance of TCP/RLP Protocol demonstrated that the fast retransmission mechanism at Stack on Correlated Fading DS-CDMA Wireless Links,” IEEE the MAC layer enhances the overall TCP throughput. Trans. Vehicular Technology, vol. 49, no. 1, pp. 28-33, Jan. 2000. [16] D.E. Comer, Internetworking with TCP/IP, vol. 1. Prentice-Hall, 1991. ACKNOWLEDGMENTS [17] R.T. Derryberry and Z. Pi, “Reverse High-Speed Packet Data Physical Layer Enhancements in cdma2000 1xEV-DV,” IEEE The authors are grateful to the anonymous referees for Comm. Magazine, pp. 41-47, Apr. 2005. insightful comments which greatly helped them improve [18] M. Dottling, J. Michel, and B. Raaf, “Hybrid ARQ and Adaptive the quality of the paper. Modulation and Coding Schemes for High Speed Downlink Packet Access,” Proc. 13th IEEE Int’l Symp. Personal, Indoor, and Mobile Radio Comm. (PIMRC), vol. 3, pp. 1073-1077, 2002. REFERENCES [19] H. Elaarag, “Improving TCP Performance over Mobile Net- works,” ACM Computing Surveys, vol. 34, no. 3, pp. 357-374, Sept. [1] http://www.3gpp.org, 2005. 2002. [2] http://www.3gpp2org, 2005. [20] F. Frederiksen and T. Kolding, “Performance Modeling of [3] http://www.itu.org, 2005. WCDMA/HSDPA Transmission/HARQ Schemes,” Proc. IEEE [4] TIA/EIA/IS-2000-2, “Physical Layer Standard for cdma2000 Vehicular Technology Conf. (VTC), pp. 472-476, Fall 2002. Spread Spectrum Systems,” Mar. 2000. [21] A.S. Joshi, M.N. Umesh, A. Kumar, T. Mukhopadhyay, K. Natesh, [5] TIA/EIA/IS-95-B, “Mobile Station—Base Station Compatibility S. Sen, and A. Arunachalam, “Performance Evaluation of TCP Standard for Dual Mode Spread Spectrum Systems,” 1999. over Radio Link Protocol in TIA/EIA/IS-99 Environment,” Proc. [6] TIA/EIA/IS-707-A.2, “Data Service Options for Spread Spectrum IEEE Int’l Conf. Personal Wireless Comm., pp. 216-220, 1999. Systems: Radio Link Protocol,” Mar. 1999. [7] A. Bakre and B.R. Badrinath, “I-TCP: Indirect TCP for Mobile [22] J.M. Harris and M. Airy, “Analytical Model for Radio Link Hosts,” Proc. 15th Int’l Conf. Distributed Computing Systems, pp. 136- Protocol for IS-95 CDMA Systems,” Proc. IEEE 51st Vehicular 143, May 1995. Technology Conf. (VTC), vol. 3, pp. 2434-2438, Spring 2000. [8] G. Bao, “Performance Evaluation of TCP/RLP Protocol Stack over [23] C. Geßner, R. Kohn, J. Schniedenharn, and A. Sitte, “Layer 2 and CDMA Wireless Links,” ACM Wireless Networks J., vol. 2, pp. 229- Layer 3 of UTRA-TDD,” Proc. IEEE 51st Vehicular Technology Conf. 237, 1996. (VTC), vol. 2, pp. 1181-1185, Spring 2000. [9] R. Caceres and L. Iftode, “Improving the Performance of Reliable [24] H. Kaaranen, A. Ahtiainen, L. Laitinen, S.K. Naghian, and V. Transport Protocols in Mobile Computing Environments,” IEEE J. Niemi, UMTS Networks: Architecture, Mobility, and Services. Wiley Selected Areas in Comm., vol. 13, pp. 850-857, June 1995. & Sons, 2001.
  14. 14. CHATTERJEE ET AL.: JOINT RELIABILITY OF MEDIUM ACCESS CONTROL AND RADIO LINK PROTOCOL IN 3G CDMA SYSTEMS 1597 [25] F. Khan, S. Kumar, K. Medepalli, and S. Nanda, “TCP Giridhar D. Mandyam received the BSEE Performance over cdma2000 RLP,” Proc. IEEE 51st Vehicular degree (magna cum laude) from Southern Technology Conf. (VTC), vol. 1, pp. 41-45, Spring 2000. Methodist University (SMU) in 1989, the MSEE [26] W.C.Y. Lee, Mobile Cellular Telecommunications. McGraw-Hill, degree from the University of Southern Califor- 1995. nia (USC) in 1993, and the PhD degree from the [27] H. Lin and S.K. Das, “Performance Study of Link Layer and MAC University of New Mexico in 1996. He is the Layer Protocols in Supporting TCP in 3G CDMA Systems,” IEEE director of the Radio Systems Group in the Trans. Mobile Computing, vol. 4, no. 5, pp. 489-501, Sept./Oct. 2005. Radio Communications Laboratory of Nokia [28] H. Lin and S.K. Das, “Performance Study of CDMA Systems with Research Center and the head of the Nokia RLP and MAC Layer Retransmissions,” Proc. IEEE Int’l Symp. Research Center, San Diego. At SMU, he was a Modeling, Analysis, and Simulation of Computer and Telecomm. University Scholar and Hyer Society Scholar. He held positions at Systems (MASCOTS), pp. 313-320, Oct. 2002. Rockwell International, Qualcomm Inc., and Texas Instruments before [29] P. Luukkanen, Z. Rong, and L. Ma, “Performance of 1XTREME joining Nokia Research Center (NRC) in Dallas in 1998. In 2002, he System for Mixed Voice and Data,” Proc. IEEE Int’l Conf. Comm. became the director of the Radio Systems Group at NRC’s Radio (ICC), vol. 5, pp. 1411-1415, 2001. Communications Laboratory. In 2004, he became the first head of the [30] J. Padhye, V. Firoiu, D. Towsley, and J. Kurose, “Modeling TCP newest Nokia Research Center division in San Diego, California. While Throughput: A Simple Model and Its Empirical Validation,” Proc. at NRC, he continued his participation in CDMA standardization work, ACM SIGCOMM pp. 303-314, 1998. which he started while at Texas Instruments. He was a contributor to the [31] S. Parkvall, E. Dahlman, P. Frenger, P. Beming, and M. Persson, development of the cdma2000 wireless standard. More recently, he has “The Evolution of WCDMA towards Higher Speed Downlink led efforts in building experimental radios and participated in research Packet Data Access,” Proc. IEEE 53rd Vehicular Technology Conf. into “Beyond 3G” technologies. He is the inventor or coinventor of 10 (VTC), vol. 3, pp. 2287-2291, 2001. issued US patents. He has also published more than 50 conference and [32] M. van der Schaar, S. Krishnamachari, S. Choi, and X. Xu, journal papers and four book chapters. In addition, he was a guest editor “Adaptive Cross-Layer Protection Strategies for Robust Scalable for a special issue of the Eurasip Journal on Applied Signal Processing Video Transmission over 802. 11 WLANs,” IEEE J. Selected Areas in entitled “3G Wireless Communications and Beyond.” He is a coauthor of Comm., vol. 21, no. 10, pp. 1752-1763, Dec. 2003. the book Third-Generation CDMA Systems for Enhanced Data Services [33] S.K. Sen, J. Jawanda, K. Basu, N.K. Kakani, and S.K. Das, “TCP (Academic Press, 2002). Dr. Mandyam is a senior member of the IEEE. Source Activity and Its Impact on Call Admission Control in CDMA Voice/Data Network,” Proc. Fourth ACM/IEEE Int’l Conf. Sajal K. Das received the BTech degree in 1983 Mobile Computing and Networking (MobiCom) 1998, pp. 276-283, from Calcutta University, the MS degree in 1984 1998. from the Indian Institute of Science, Bangalore, [34] S. Souissi and S.B. Wicker, “A Diversity Combining DS/CDMA and the PhD degree in 1988 from the University System with Convolutional Encoding and Viterbi Decoding,” of Central Florida, Orlando, all in computer IEEE Trans. Vehicular Technology, vol. 44, no. 2, pp. 304-312, May science. He is a professor of computer science 1995. and engineering and also the founding director [35] Y.C. Tseng and C.M. Chao, “Code Placement and Replacement of the Center for Research in Wireless Mobility Strategies for Wideband CDMA OVSF Code Tree Management,” and Networking (CReWMaN) at the University of IEEE Trans. Mobile Computing, vol. 1, no. 4, pp. 293-302, Oct.-Dec. Texas at Arlington (UTA). His current research 2002. interests include resource and mobility management in wireless and [36] M.N. Umesh, A.S. Joshi, A. Kumar, and T. Mukhopadhyay, “A sensor networks, mobile and pervasive computing, wireless multimedia NAK Based Hybrid Type II ARQ Scheme for cdmaOne/cdma2000 and QoS provisioning, mobile Internet protocols, distributed processing, Systems,” Proc. IEEE Vehicular Technology Conf. (VTC), vol. 5, and grid computing. He has published more than 350 research papers, pp. 2596-2600, Fall 1999. directed numerous funded projects, and holds five US patents in [37] Y. Xiao, H. Li, and S. Choi, “Protection and Guarantee for Voice wireless mobile networks. He received the Best Paper Award from ACM and Video Traffic in IEEE 802.11e Wireless LANs,” Proc. IEEE MobiCom ’99, ICOIN ’01, ACM MSWIM ’00, and ACM/IEEE PADS ’97. INFOCOM, vol. 3, pp. 2152-2162, Mar. 2004. He was also a recipient of UTA’s Outstanding Faculty Research Award [38] A.J. Viterbi, CDMA: Principles of Spread Spectrum Communication. in Computer Science (2001 and 2003), College of Engineering Addison-Wesley, 1995. Excellence in Research Award (2003), and University Award for [39] X. Zhaoji and B. Sebire, “Impact of ACK/NACK Signalling Errors Distinguished Record of Research (2005). He is the coauthor of the on High Speed Uplink Packet Access (HSUPA),” Proc. IEEE Int’l book Smart Environments: Technology, Protocols and Applications Conf. Comm. (ICC), May 2005. (John Wiley, 2005). He is the editor-in-chief of the Pervasive and Mobile Computing journal and serves on the editorial boards of five international Mainak Chatterjee received the PhD degree journals, including IEEE Transactions on Mobile Computing, IEEE from the Department of Computer Science and Transactions on Parallel and Distributed Systems, and ACM/Springer Engineering at the University of Texas at Wireless Networks. He has served as general chair of IEEE WoWMoM Arlington in 2002. Prior to that, he completed ’05, IWDC ’04, IEEE PerCom ’04, CIT ’03, and IEEE MASCOTS ’02; the BSc degree in physics (Hons) from the general vice chair of IEEE PerCom ’03, ACM MobiCom ’00, and HiPC University of Calcutta in 1994 and the ME degree ’00-01; program chair of IWDC ’02 and WoWMoM ’98-99; TPC vice in electrical communication engineering from the chair of CIT ’05 and ICPADS ’02; and as TPC member of numerous Indian Institute of Science, Bangalore, in 1998. IEEE and ACM conferences. He is the vice chair of two IEEE technical He is currently an assistant professor in the committees (TCPP and TCCC) and a member of the advisory boards of Department of Electrical and Computer Engi- several cutting-edge companies. He is a member of the IEEE and the neering at the University of Central Florida. His research interests IEEE Computer Society. include economic issues in wireless networks, applied game theory, resource management and quality-of-service provisioning, ad hoc and sensor networks, CDMA data networking, and link layer protocols. He serves on the executive and technical program committee of several international conferences. . For more information on this or any other computing topic, please visit our Digital Library at www.computer.org/publications/dlib.