IEEE 802.11 standard and the enhanced amendments have defined several transmission rates for mobile stations to transmit and receive data frames. With the characteristic of modulation schemes, a higher network throughput can be expected by using higher level modulation scheme, but frame error probability will also become higher. Doubtlessly, it is an open issue of selecting a proper modulation scheme for a pair of mobile stations in time-varying indoor environment. This paper proposes a safe multiple access-rates transmission (SMART) scheme to enhance the reliability of data transmission in IEEE 802.11 multi-rate infrastructure wireless networks. SMART scheme provides reliable transmission by reserving a retransmission period which immediately follows the transmitted frame and is estimated by a lower transmission rate according to the transmitted frame. The performance of SMART scheme is evaluated by analytical model and simulation experiment, which show that the proposed scheme is significantly better than the IEEE 802.11 standard under the real environment with asymmetric traffic load. An enhanced SMART (ESMART) scheme is also proposed especially to improve bandwidth waste for long packet length at access point (AP). The performance measurements, such as goodput, throughput, average access delay, and service rate, are obtained and compared for the different schemes.

This paper presents an object-oriented model to handle the temporal relationship for all of the multimedia objects at the presentation platform. Synchronization of the composite media objects is achieved by ensuring that all objects presented in the upcoming "manageable" period must be ready for execution. To this end, the nature of overlays is first investigated for various types of objects. Critical overlaps which are crucial in synchronization are also defined. The objective of synchronization is to ensure that the media objects can be initiated precisely at the critical point of the corresponding critical overlap. The concept of manageable presentation interval is introduced and the irreducible media group is defined. The resource scheduling of each presentation group for media object pre-fetch time versus buffer occupancy is also examined. Accordingly, a new model called group cascade object composition Petri-net (GCOCPN) is proposed and an algorithm to implement this temporal synchronization scheme is presented.

This paper conducts performance evaluation for a code division multiple access (CDMA) system when channel bands of multiple neighboring CDMA/DSSS are overlapped in frequency domain. It is assumed that all systems adopt direct-sequence spread-spectrum (DSSS) technique and are BPSK modulated by the different carrier frequencies. Automatic power control (APC) is also applied in the interfered system such that the receiver gets the same power from all users. Without loss generality, an additive white Gaussian noise (AWGN) channel is also assumed during analysis. In this paper, the analytic solution of the signal to noise ratio (SNR) is first derived in which both CDMA systems are modulated by different carrier frequencies. This analysis is good for general cases; and the result shows an excellent computational performance. In particular, the result is very close to Pursly's result, when the systems have the same code length with no carrier difference.

Voice over Internet protocol (VoIP) is to transfer voice packets over IP networks, while voice signal is processed by using digital signal processing technology before being transmitted. VoIP quality cannot be expected, because it is hard to predict the influence of delay, packet loss rate, packet error, etc. It is difficult to rebuild the voice wave form, if a large amount of voice packets are lost. This paper mainly studies on how to maintain a better voice quality over hybrid fiber/coaxial (HFC) networks, if it is inevitable to drop packets. We particularly consider the data over cable service interface specification (DOCSIS) version 1.1 with the unsolicited grant service with activity detection (UGS/AD) for VoIP services. We propose a smallest successive times first (SSTF) scheduling algorithm to schedule VoIP packets for cable modem termination system (CMTS), which can support fair transmission and long-term transmission continuity for VoIP connections. We analyze voice quality about continuity of the transmitted VoIP packets, consecutive clipping times, and VoIP packet drop rate for all connections. Performance measurement shows excellent results for the proposed algorithm by simulation experiments and objective evaluation.

In wireless ATM mobile communication environments, an abrupt increase in the number of mobile users in a specific base station (BS) will overload the BS and turn it into a so-called hot-spot cell, which causes an unbalanced traffic load condition if the traffic load in the neighboring BSs is light. In order that efficiently allevates to utilize a limited bandwidth, it is necessary to use a good mobile handoff strategy to alleviate unbalanced traffic load condition. This paper proposes a complete traffic shedding algorithm (CTSA) for soft handoff efficiently to alleviate the overloaded traffic in a hot-spot cell by adjusting handoff parameters, which virtually reduces the service coverage of the heavy loaded BS without adjusting the transmission power. Mobile terminals (MT) located in the outer area of the hot-spot cell will be handed off early to the neighboring BSs with light traffic load; and MTs in the neighboring BSs will not be handed off to the hot-spot cell, so that the traffic load of the hot-spot cell can be decreased. After analyzing the proposed algorithm and comparing the other methods, variable threshold soft handoff (VTSH) and enhanced soft handoff (ESH), by simulation, the performance of CTSA is better than the others. CTSA obviously increases system throughput and decreases soft handoff call dropping ratio, new call blocking ratio, and total call refusing ratio.

This paper conducts performance evaluation and performs simulation for a code division multiple access (CDMA) system when channel bands of multiple neighboring CDMA/DSSS are overlapped in time domain. It is assumed that all systems adopt direct-sequence spread-spectrum (DSSS) technique and are BPSK modulated by the different carrier frequencies. Automatic power control (APC) is also applied in the interfered system such that the receiver gets the same power from all users. Without loss generality, an additive white Gaussian noise (AWGN) channel is also assumed during analysis. In this paper, the analytic solution of the signal to noise ratio (SNR) is first derived in which both CDMA systems are modulated by different carrier frequencies. We have the results by simulation with Δ f = 0 and Δ f = 1 MHz, respectively. This analysis is good for general cases; and the results show an excellent computational performance. In particular, the result is very close to Pursley's result, when the systems have the same code length with no carrier difference.

In ATM networks, call processing on switches can be greatly simplified by using the concept of virtual path (VP); and good resource management strategies ensure that virtual channel connections (VCC) can be rapidly and efficiently established. In order to have good system performance, several methods of constructing virtual paths and strategies of allocating and managing resources should be considered. In this paper, several multicast strategies with dynamic routing are used and applied to the metropolitan LATA network. For the VP-based network, dynamic routing is also applied, and those strategies are discussed and investigated to show the versatility of the approach. Some results using dynamic multicast routing, such as call blocking rate, VP utilization, and VP adjustment rate, are obtained for the different strategies by simulation experiments.

In order to achieve the prioritized quality of service (QoS) guarantee, the IEEE 802.11e EDCAF (the enhanced distributed channel access function) provides the distinguished services by configuring the different QoS parameters to different access categories (ACs). An admission control scheme is needed to maximize the utilization of wireless channel. Most of papers study throughput improvement by solving the complicated multidimensional Markov-chain model. In this paper, we introduce a backoff model to study the transmission probability of the different arbitration interframe space number (AIFSN) and the minimum contention window size (CWmin). We propose an adaptive control scheme (ACS) to dynamically update AIFSN and CWmin based on the periodical monitoring of current channel status and QoS requirements to achieve the specific service differentiation at access points (AP). This paper provides an effective tuning mechanism for improving QoS in WLAN. Analytical and simulation results show that the proposed scheme outperforms the basic EDCAF in terms of throughput and service differentiation especially at high collision rate.

The interworking between high speed networks is receiving more attention recently. Among them, Frame Relay/Asynchronous Transfer Mode (FR/ATM) interworking is foreseen in the near future to provide seamless service across geographic areas. In the heterogeneous network interworking environment, an interworking function facility called the interworking unit (IWU) is generally interposed between these networks to perform the essential protocol conversion. As an intermediate network component, IWU will have great influence on network performance in terms of data latency. The performance issues of IWU are identified and addressed in this paper. These issues include frame processing schemes, frame shaping schemes, and cell processing schemes. The cell processing performance issues are further investigated via simulation experiments. Connectionless (CL) data service is also expected to be one of the most important applications for FR/ATM interworking. Some alternative methods for providing CL service over FR and ATM are described. We propose an enhanced protocol stack in the IWU to interoperate with the ATM network which uses the direct method to provide CL service. To proveide VPN service, we propose a feasible VPN communication structure based on the FR and ATM network interworking.