Showing papers in "Mobile Networks and Applications in 1997"

Bit-sequences: an adaptive cache invalidation method in mobile client/server environments

Abstract: In this paper, we present Bit-Sequences (BS), an adaptive cache invalidation algorithm for client/server mobile environments. The algorithm uses adaptable mechanisms to adjust the size of the invalidation report to optimize the use of a limited communication bandwidth while retaining the effectiveness of cache invalidation. The proposed BS algorithm is especially suited for dissemination-based (or “server-push”-based) nomadic information service applications. The critical aspect of our algorithm is its self-adaptability and effectiveness, regardless of the connectivity behavior of the mobile clients. The performance of BS is analyzed through a simulation study that compares BS's effectiveness with that of a hypothetical optimal cache invalidation algorithm.

A mobile transaction model that captures both the data and movement behavior

Abstract: Unlike distributed transactions, mobile transactions do not originate and end at the same site. The implication of the movement of such transactions is that classical atomicity, concurrency and recovery solutions must be revisited to capture the movement behavior. As an effort in this direction, we define a model of mobile transactions by building on the concepts of split transactions and global transactions in a multidatabase environment. Our view of mobile transactions, called Kangaroo Transactions, incorporates the property that transactions in a mobile computing system hop from one base station to another as the mobile unit moves through cells. Our model is the first to capture this movement behavior as well as the data behavior which reflects the access to data located in databases throughout the static network. The mobile behavior is dynamic and is realized in our model via the use of split operations. The data access behavior is captured by using the idea of global and local transactions in a multidatabase system.

Error control schemes for networks: an overview

Abstract: In this paper, we investigate the issue of error control in wireless communication networks. We review the alternative error control schemes available for providing reliable end-to-end communication in wireless environments. Through case studies, the performance and tradeoffs of these schemes are shown. Based on the application environments and QoS requirements, the design issues of error control are discussed to achieve the best solution.

Performance study of access control in wireless LANs—IEEE 802.11 DFWMAC and ETSI RES 10 Hiperlan

Abstract: Currently two projects are on their way to standardize physical layer and medium access control for wireless LANs—IEEE 802.11 and ETSI RES 10 Hiperlan. This paper presents an introduction to both projects focussing on the applied access schemes. Further we will present our simulation results, analyzing the performance of both access protocols depending on the number of stations and on the packet size, evaluating them regarding their capability to support QoS parameters, regarding the impact of hidden terminals and their range extension strategy.

Per-user profile replication in mobile environments: algorithms, analysis, and simulation results

Abstract: We consider per-user profile replication as a mechanism for faster location lookup of mobile users in a personal communications service system We present a minimum-cost maximum-flow based algorithm to compute the set of sites at which a user profile should be replicated given known calling and user mobility patterns We show the costs and benefits of our replication algorithm against previous location lookup approaches through analysis We also simulate our algorithm against other location lookup algorithms on a realistic model of a geographical area to evaluate critical system performance measures A notable aspect of our simulations is that we use well-validated models of user calling and mobility patterns

Performance evaluation of optimum combining and maximal ratio combining in the presence of co-channel interference and channel correlation for wireless communication systems

Abstract: Antenna diversity is an important technique to combat fading and reduce co-channel interference (CCI). In this paper, we present an analytical approach to derive bit error rate (BER) for Optimum Combining (OC) and Maximal Ratio Combining (MRC) in the presence of CCI. The paper has two parts. In the first part, the analysis of BER for OC with two co-channel interferers and MRC with an arbitrary number of interferers is presented for M-array antenna systems under the assumption that the channels of users are independent of each other. In the second part, the analysis of BER for OC and MRC in the presence of one dominant co-channel interferer is presented for dual antenna systems by assuming that the channels of the desired user or co-channel interferer are correlated. For DPSK signal, an exact BER expression is derived. The work presented here also yields an upper bound for BPSK or QAM signal based on the results of Foschini and Salz (1983).

Performance and implementation of clustered-OFDM for wireless communications

Abstract: An elegant means by which high-speed burst wireless transmission can be accomplished with small amounts of overhead is through a novel technique referred to as clustered-OFDM (Cimini et al., 1996). By using OFDM modulation with a long symbol interval, clustered-OFDM overcomes the complex and costly equalization requirements associated with single carrier systems. Moreover, the need for highly linear power amplifiers typically required in OFDM systems is alleviated through the use of multiple transmit antennas combined with nonlinear coding. The clustering technique also leads to a natural implementation of transmit diversity. This paper reports on preliminary results on the performance of a clustered-OFDM system as well as the design and implementation of a clustered-OFDM transmitter. The prototype transmitter can deliver 7.5 Mbps, and it is expected that this data rate could be easily tripled with existing technology in a second generation system. The paper also describes the architectural trade-offs made in order to reduce the hardware complexity of the boards as well as some experimental results showing the operation of the transmitter.

Zippering: managing intermittent connectivity in DIANA

Abstract: This paper describes an approach for handling intermittent connectivity between mobile clients and network-resident applications, which we call zippering. When the client connects with the application, communication between the client and the application is synchronous. When the client intermittently connects with the application, communication becomes asynchronous. The DIANA (Device-Independent, Asynchronous Network Access) approach allows the client to perform a variety of operations while disconnected. Finally, when the client reconnects with the application, the operations performed independently on the client are replayed to the application in the order they were originally done. Zippering allows the user at the client to fix errors detected during reconciliation and continues the transaction gracefully instead of aborting the whole transaction when errors are detected.

A protocol for causally ordered message delivery in mobile computing systems

Abstract: There is a growing trend in developing applications for mobile computing systems in which mobile host computers retain their network connections while in transit. This paper proposes an algorithm that enforces a useful property, namely, causal ordering, that delivers messages among mobile hosts. This property ensures that causally related messages directed to the same destination will be delivered in an order consistent with their causality, which is important in applications that involve human interaction such as mobile e-mail and mobile teleconferencing. Such applications are envisioned by the proponents of Personal Communications Services (PCS). Without this property, users may receive and read original messages and the corresponding replies out of order. Our algorithm, when compared with previous proposals, provides an alternative with a low handoff cost, medium message overhead, and low probability of unnecessary inhibition in delivering messages.

Design and performance evaluation of an RRA scheme for voice-data channel access in outdoor microcellular environments

Abstract: In PCS networks, the multiple access problem is characterized by spatially dispersed mobile source terminals sharing a radio channel connected to a fixed base station. In this paper, we design and evaluate a reservation random access (RRA) scheme that multiplexes voice traffic at the talkspurt level to efficiently integrate voice and data traffic in outdoor microcellular environments. The scheme involves partitioning the time frame into two request intervals (voice and data) and an information interval. Thus, any potential performance degradation caused by voice and data terminals competing for channel access is eliminated. We consider three random access algorithms for the transmission of voice request packets and one for the transmission of data request packets. We formulate an approximate Markov model and present analytical results for the steady state voice packet dropping probability, mean voice access delay and voice throughput. Simulations are used to investigate the steady state voice packet dropping distribution per talkspurt, and to illustrate preliminary voice-data integration considerations.

A contention/reservation access protocol for speech and data integration in TDMA-based advanced mobile systems

Abstract: The performance of third generation mobile systems is greatly influenced by the multiple access protocols used in the radio access system. The paper introduces a multiple access protocol, SIR (Service Integration for Radio access), which has the potential for accommodating the requirements of speech and bursty data traffic in an efficient way. SIR is evolved from an access protocol (PRMA++) studied within the framework of the TDMA-based version of the European evolving standard for third generation mobile systems. In particular, SIR uses the same frame structure and in-band signalling but introduces a contention-free handling of data bandwidth requests while meeting speech service requirements via basic PRMA++ mechanisms.

Performance of ARQ Go-Back-N protocol in Markov channels with unreliable feedback

Abstract: In this paper, an ARQ Go-Back-N protocol with time-out mechanism is studied. Transmissions on both the forward and the reverse channels are assumed to be subject to Markovian errors. A recently developed approach, based on renewal theory, is further extended and the steady state average number of packets in the ARQ system is evaluated. This allows us to determine analytically both throughput and transmission delay of the system. Simulation results, that confirm the analysis, are also presented. Based on the analysis, the trade-off involved in the choice of the time-out parameter is identified and discussed.

The RRA-ISA multiple access protocol with and without simple priority schemes for real-time and data traffic in wireless cellular systems

Abstract: A multiple access protocol, based on a Reservation Random Access (RRA) scheme, is derived for a wireless cellular network carrying real-time and data traffic. Given a TDMA framed channel and a cellular structure, the aim of the protocol is that of maximizing the one-step throughput over an entire frame. This is achieved by deciding on the access rights at the cell base station, which then broadcasts this information at the beginning of the frame. The decision is made on the basis of binary channel feedback information (collision/no collision) over the previous frames, as well as of long term averages of packet generation rates at the mobile stations, assuming independence in the presence of packets at the latter. The resulting protocol has therefore been termed Independent Stations Algorithm (ISA), and the overall scheme RRA-ISA. As in other RRA protocols, time constrained (e.g., voice) traffic operates in a dynamic reservation mode, by contending for a slot in the frame with the first packet of a burst, and then keeping the eventually accessed slot for the duration of the burst; packets of the time constrained traffic unable to access a slot within a maximum delay are dropped from the input buffer. No such constraint is imposed on data traffic. Together with the “basic” version of the access algorithm, three other variants are presented, which exploit three simple different priority schemes in the RRA-ISA “basic” structure, in order to give a prominence to the voice service. The aim of these variants is to improve the performance in terms of the maximum number of stations acceptable in the system, by slightly increasing the data packets delay. All the proposed schemes are analyzed by simulation in the presence of voice and data traffic. Several comparisons show a relevant performance improvement (in terms of data delay and maximum number of voice stations acceptable within a cell) over other protocols that use ALOHA as a reservation mechanism (RRA-ALOHA or PRMA schemes).

Methods for packet combining in HARQ systems over bursty channels

Abstract: The performance of ARQ systems can be improved by combining current and prior transmissions at the receiver. Two techniques for combining outputs in a packet-based communication system are presented. In both techniques the fundamental unit of retransmission is a packet, and the fundamental unit of combining is a codeword. The techniques are analyzed for a bursty channel and a system that employs Reed--Solomon coding and bounded-distance errors-and-erasures decoding. Performance results show that the packet-combining schemes provide significant gains in throughput and reductions in error probability when compared with a system that does not employ combining.

A formal method for synthesizing optimized protocol converters and its application to mobile data networks

Abstract: As mobile information networks are expanding rapidly, we expect to integrate voice, paging, electronic mail and other wireless information services. Interworking units that perform protocol conversion at the boundaries of different networks will play an important role. In this paper, we propose an efficient algorithm for constructing optimized protocol converters to achieve interoperability between heterogeneous data networks. This algorithm first derives constraints from two given protocols, and apply the constraints to channel specifications, thus removing message sequences that do not contribute to system progress. Then, an optimized converter is generated from a given service specification, the two protocol specifications and the modified channel specifications. A reduction relation is used to compare the service specification and the constructed internetworking system in order to deal with the problem of nondeterministic services. Compared with related works, our method has two advantages: (1) it generates an optimized converter; (2) it can be applied to the case that the service specification is nondeterministic. The application of the method to mobile networks is given by an example.

An efficient multicast protocol for PCS networks

Abstract: In a Personal Communication Services (PCS) network, mobile hosts communicate with other mobile hosts through base stations on a wired (static) network. The mobile hosts connect to different base stations through wireless links and the base stations to which mobile hosts are connected change depending on the current location of the mobile hosts. In this environment, the problem of efficiently delivering a multicast message from one mobile host to a group of other mobile hosts becomes challenging. In this paper, we present a multicast protocol that delivers multicast messages from a mobile host to a group of other mobile hosts without flooding the wired network. The multicast protocol is built on top of a user location strategy that should follow one of the three models of user location described in the paper. The basic multicast protocol proposed guarantees exactly-once message delivery to all mobile hosts in the multicast group and also ensures that multicast messages are delivered in FIFO order from the point of view of the base station that originates the multicast message (referred to as BS-FIFO). More importantly, an extension of the basic protocol is provided that, unlike earlier work, delivers multicast messages in FIFO order from the point of view of the mobile host that initiates the multicast message (referred to as MH-FIFO). The modifications to be made to the multicast protocol to accommodate each of the three models of user location is also described.

Deadlock detection in communicating finite state machines by even reachability analysis

Abstract: A network of communicating finite state machines (CFSM) consists of a set of finite state machines which communicate asynchronously with each other over (potentially) unbounded FIFO channels by sending and receiving typed messages. As a concurrency model, CFSMs has been widely used to specify and validate communications protocols. CFSMs is also powerful and suitable for modeling mobile communication systems—a CFSM can naturally model a mobile station in a wireless communication system. The unbounded FIFO channels are ideal for modeling the communication behavior among mobile stations. Fair reachability is a very useful technique in detecting errors of deadlocks and unspecified receptions in networks of (CFSMs) consisting of two machines. The paper extends the classical fair reachability technique, which is only applicable to the class of two-machine CFSMs, to the general class of CFSMs. For bounded CFSMs, the extended fair reachability technique reduces by more than one half the total number of reachable global states that have to be searched in verifying freedom from deadlocks. The usefulness of the new reachability technique, called even reachability, is demonstrated through two examples.

A CDMA-based radio interface for third generation mobile systems

Abstract: This paper deals with the use of a CDMA-based radio interface in third generation mobile systems (Universal Mobile Telecommunications System—UMTS, and Future Public Land Mobile Telecommunications System—FPLMTS). The paper is not intended as a detailed analysis of the radio interface performance, but as an overview of the main issues arising in a typical CDMA-based mobile system, discussing the different available technical solutions. First of all, the basic requirements of the radio interface in a third generation mobile system are outlined. In particular, the support of variable bit rate transmission, the adaptability to the different propagation and service environments and the flexibility are felt to be important topics to be discussed. Then, the main characteristics of the CDMA access technique are depicted, in relation with the above mentioned requirements, focusing in particular on the DS-CDMA radio interface designed within the RACE II—CODIT Project. In that context the paper describes some of the technical solutions proposed for the provision of advanced features such as macrodiversity, multibearer transmission and variable bit rate services.

Design and performance evaluation of a MAC protocol for wireless local area networks

Abstract: We propose and analyze, from a performance viewpoint, a Medium Access Control (MAC) protocol for Wireless Local Area Networks (WLANs). The protocol, named Prioritized-Access with Centralized-Control (PACC), supports integrated traffics by guaranteeing an almost complete utilization of network resources. The proposed protocol combines random access for signalling, with collision-free access to the transmission channel. The transmission channel is assumed to be slotted, with slots grouped into frames. Access to transmission slots is controlled by a centralized scheduler which manages a multiclass queue containing the users' requests to access the transmission channel. Three classes of users are assumed: voice traffic (voice), data traffic with real-time constraints (high-priority data), and classical data traffic (low-priority data). A priority mechanism ensures that speech users have the highest priority in accessing the idle slots, since speech packets have a more demanding delay constraint. The remaining channel bandwidth is shared fairly among the high-priority data terminals. The low-priority data terminals use the slots left empty by the other classes. Specifically, access to transmission slots is controlled by the centralized scheduler by managing a transmission cycle for each class of terminals. The voice-terminals cycle has a constant length equal to one frame, while the lengths of the data-terminals cycles are random variables which depend on the number of active voice and data terminals. In this paper we show that the proposed scheme can support the same maximum number of voice terminals as an ideal scheduler, while guaranteeing an almost complete utilization of network capacity. In addition, via a performance analysis, we verify that by limiting the number of real-time data terminals in the network this class of traffic can be statistically guaranteed access delays in the order of 200--300 msec. Hence, the QoS the network gives to the real-time data terminals makes this service suitable for real-time applications such as alarms or low bit rate video.

Link layer retransmission schemes for circuit-mode data over the CDMA physical channel

Abstract: In the last few years, wide-area data services over North American digital (TDMA and CDMA) cellular networks have been standardized. The standards were developed under three primary constraints: (i) compatibility with existing land-line standards and systems, (ii) compatibility with existing cellular physical layer standards that are optimized for voice, and (iii) market demands for quick solutions. In particular, the IS-95 CDMA air interface standard permits multiplexing of primary traffic (e.g., voice or circuit data) and secondary traffic (e.g., packet data) or in-band signaling within the same physical layer burst. In this paper, we describe two radio link protocols for circuit-mode data over IS-95. The first protocol, Protocol S, relies on a single level of recovery and uses a flexible segmentation and recovery (FSAR) sublayer to efficiently pack data frames into multiplexed physical layer bursts. We next describe Protocol T, that consists of two levels of recovery. Protocol T has been standardized for CDMA circuit-mode data as IS-99 (Telecommunications Industry Association, 1994). We provide performance comparisons of the two protocols in terms of throughput, delay and recovery from fades. We find that the complexity of the two level recovery mechanism can buy higher throughput through the reduced retransmission data unit size. However, the choice of TCP (and its associated congestion control mechanism) as the upper layer of recovery on the link layer, leads to long fade recovery times for Protocol T. The two approaches also have significant differences with respect to procedures and performance at handoff and connection establishment.

A spread-slotted random-access protocol with multi-priority for personal and mobile communication networks carrying integrated traffic

Abstract: This paper describes and analyzes a medium access protocol for wireless personal communication networks serving integrated traffic. A multi-priority mechanism, which is implemented and controlled explicitly by the users and without any involvement from the network is incorporated into the protocol. We examine the proposed protocol under reservation and non-reservation policies and evaluate its performance when handling integrated voice and data traffic. The proposed protocol is suitable to be used in Personal Communication Systems aiming to provide integrated (voice/data) services.

Traffic impacts of international roaming on mobile and personal communications with distributed data management

Abstract: In this paper, we propose four network interconnection scenarios and the related signaling aspects for the international roaming traffic in mobile and personal communications. With or without international gateway relay nodes summarized from the proposed scenarios, we also derive three international roaming network sets {IR1, IR2, IR3} for the observed signaling traffic model with two-level databases. Based on the proposed performance metrics, analysis results show that under some reasonable assumptions the signaling traffic of international roaming is significantly impacted by the related gateway relay nodes and databases. In addition, the studied cache data management strategy at VLR can reduce the impacts of the signaling traffic of query operation, but the caching approach cannot improve the performance of update operation. Furthermore, the performance of proposed standalone STP scenario is better than that of integrated STP scenario for the international network connection.

Retransmission control and fairness issue in mobile slotted ALOHA networks with fading and near-far effect

Abstract: The effects of different retransmission control policies on the performance of a mobile data system employing the slotted ALOHA protocol are investigated, with the emphasis on the unfairness between close-in and distant users due to the near-far effect. An analytical multi-group model is developed to evaluate both the user and the network performance of the mobile slotted ALOHA network under two classes of retransmission control policies, namely the Uniform Policy and the Nonuniform Policy. The Uniform Policy requires that all users adopt the same retransmission probability, whereas the Nonuniform Policy allows more distant users to have larger retransmission probability in order to compensate for the unfairness caused by the near-far effect. The performance of a slotted ALOHA network with a linear topology in a Rician fading channel under the two policies is compared by the multi-group model and simulation. The Nonuniform Policy is found to be more effective in alleviating the unfairness of user throughput over a wider range of the data traffic load than the Uniform Policy, which is effective only when the data traffic load is very light. Thus, a mobile data network can enjoy the network performance improvement derived from the near-far effect while the unfairness between close-in and distant users can be greatly mitigated without resorting to power control.

The n -ary stack algorithm for the wireless random access channel

Abstract: The emergence of wireless and personal communication networks has brought random access protocols for packet radio networks back to the research forefronts. Most such protocols are based on the ever popular ALOHA protocol. Unfortunately, this protocol is inherently unstable and requires sophisticated schemes to stabilize it. Another class of random access schemes, called limited sensing or stack algorithms, has been proposed that is stable and allows for the dynamic incorporation of new stations into the network. In this paper, we will review the simple to implement n-ary stack algorithm, and we will study its performance under various system parameters in the presence of capture, and also in the presence of feedback errors. Finally, we will investigate its maximum system throughput under various traffic generation processes.

Protocol discovery in multiprotocol networks

Abstract: Interoperability requires that communicating systems support compatible protocols. Maintaining compatible protocols is problematic in heterogeneous networks, especially in a wireless infrastructure where hosts can move from one protocol environment to another. It is possible to improve the flexibility of a communication network's operation by deploying systems that support multiple protocols. These multiprotocol systems require support mechanisms that enable users to effectively access the different protocols. Of particular importance is the need to determine which of several protocols to use for a given communication task. In this work, we propose architectures for a protocol discovery system that uses directory services and protocol feedback mechanisms to determine which protocols are supported. We describe the issues related to protocol discovery and present protocol features necessary to support multiprotocol systems.

Extending NCP for protocols using public keys

Abstract: One of the greatest obstacles to wide-spread deployment of wireless mobile systems is security. Cryptographically strong protocols and algorithms are required to enable secure communication over links that are easy to monitor and control by an attacker. While good cryptographic algorithms exist, it is difficult to design protocols that are immune to malicious attack. Good analysis techniques are lacking. This paper presents extensions to a technique for specifying and analyzing nonmonotonic cryptographic protocols that use asymmetric keys. We introduce new actions and inference rules, as well as slight modifications to the Update function. An important observation is that reasoning about the origin of messages is quite different when dealing with asymmetric key protocols. We also introduce the notion that keys in certificates should be bound to the principals receiving them. We extend the technique to meet the binding requirements and show how the flaw in the Denning and Sacco public key protocol, which was discovered by Abadi and Needham, is revealed. We demonstrate the extended technique using one protocol of our own and the Needham and Schroeder public key protocol. We also introduce and analyze a fix to a known weakness in Needham and Schroeder's protocol using our extended technique. Finally, we present several applications of these techniques to protocols for mobile computing over wireless networks.

Communication protocol design to facilitate re-use based on the object-oriented paradigm

Abstract: The main motivation for the present work stems from the wide gap which exists between the research efforts devoted to developing formal descriptions for communication protocols and the effective development methodologies used in industrial implementations. We apply Object-Oriented (OO) modelling principles to networking protocols, exploring the potential for producing re-useable software modules by discovering the underlying generic class structures and behaviour. Petri Nets (PNs) are used to derive re-useable model elements and a slightly modified TTCN notation is used for message sequence encoding. This demonstrates a formal, practical approach to the development of a protocol implementation through OO modelling. Our utilisation of PNs in the context of object based modelling allows for isolation of the behavioural characterisation of objects into a separate design plane, treated as a meta-level object control. This separation permits greater execution flexibility of the underlying object models. It is that very aspect of our modelling approach which can be utilised in software implementations where dynamically determined “re-programming” (i.e., change of procedures) is needed. For example, one of the requirements in wireless networking software is the ability to cope with ever-changing transmission/reception conditions and that, in turn, creates greatly varying error rates. Similarly, handoff procedures create situations where dynamically determined change of operational modes is required. To illustrate the modelling concepts, the paper addresses the problem of inter-layer communication among multiple protocol entities (PEs), assuming the standard ISO/OSI Reference Model. A generalised model called the Inter-Layer Communication (ILC) Model is proposed. An example of a PE based on the Alternating-Bit Protocol (ABP) is also discussed. The final example demonstrates how meta-level object control (PNs) allows for the dynamic selection of different ARQ based algorithms.

Transmitting time-critical data over heterogeneous subnetworks using standardized protocols

Abstract: Current communication networks consist of subnetworks of different types. Therefore a common network protocol has to be used for the transmission of data in such a heterogeneous network. Since some time the requirement of mobility in communication networks is showing up. For that reason wireless networks are playing an increasing role as subnetworks. On the other hand there is the need for multiplexed transmission of time-critical and non time-critical (normal) data within a heterogeneous network. In this paper we discuss the problem of multiplexed transmission of time-critical and of non time-critical data over a wireless type subnetwork using a common standardized network protocol. Many of the available wireless subnetworks are of low or medium transmission speed and guarantee a fixed transmission bandwidth at the access point. We describe a mechanism to transmit time-critical data in such a type of subnetwork using a connectionless transport and a connectionless network protocol. The concurrent transmission of non time-critical data using a connection oriented transport and the same connectionless network protocol is assumed to be of lower priority; it is scheduled in a way to fill the remaining capacity, which has not been reserved for the transmission of time-critical data. In our discussion we concentrate on the standardized ISO/OSI protocools CLNP as connectionless network protocol, CLTP as connectionless transport protocol and TP4 as connection oriented transport protocol. We propose a header compression protocol and a fragmentation protocol for use on low bandwidth subnetworks.

A data request scheduling policy for PCS networks

Abstract: This paper proposes a scheduling policy called latest preempted, earliest resumed (LPER) for personal communications services (PCS) systems that support voice services as well as the circuit mode data services The policy gives priority to the voice services over the data services The scheduling of voice requests is performed as if no data requests exist Thus, if no idle channel is available when a voice request arrives, a data channel (if exists) is interrupted and the channel will be used for the voice request The interrupted data request will be resumed when an idle channel is available In LPER, the data channel selected for interruption is the one that serves the latest outstanding data request (ie, other outstanding requests enter the system earlier than this request) When an occupied channel is released, LPER resumes the earliest request that entered the system An analytic model is proposed to study the performance of LPER and provides guidelines to select the input parameters for the PCS systems