• KSII Transactions on Internet and Information Systems (TIIS)
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• v.18 no.2
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• pp.511-527
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• 2024

This paper proposes a block-based self-organizing time-division multiple access (BSO-TDMA) protocol for very high frequency (VHF) data exchange system (VDES) in shipborne ad-hoc networks (SANETs). The BSO-TDMA reduces the collisions caused by the simultaneous transmission of automatic identification system (AIS) messages by uniformly allocating channel resources using a block-wise frame. For this purpose, the BSO-TDMA includes two functional operations: (1) frame configuration and (2) slot allocation. The first operation consists of block division and block selection. A frame is divided into multiple blocks, each consisting of fixed-size subblocks, by using the reporting interval (RI) of the ship. Then, the ship selects one of the subblocks within a block by considering the number of occupied slots for each subblock. The second operation allocates the slots within the selected subblock for transmitting AIS messages. First, one of the unoccupied slots within the selected subblock is allocated for the periodic transmission of position reports. Next, to transmit various types of AIS messages, an unoccupied slot is randomly selected from candidate slots located around the previously allocated slot. Experimental simulations are conducted to evaluate the performance of BSO-TDMA. The results show that BSO-TDMA has better performance than that of the existing SOTDMA.

• Journal of Korea Multimedia Society
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• v.11 no.5
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• pp.685-691
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• 2008

Recently with the rapid evolution of the mobile computing and communication technologies, mobile VOD service becomes increasingly important for wireless mobile users. The VOD service is being widely used in various areas of application, such as education, entertainment and business, because it provides users convenience in easily having access to video information at any time in any places. However, in reality, the mobile system has many difficulties in providing the smooth VOD service owing to frequent transfers and cutoffs of clients. The importance of a technique to transmit broadcasting is being stressed as a method for providing stabler mobile VOD service to a large number of clients. This paper is aimed at showing how to reduce demands for server bandwidth and delay of earlier service through performance analysis by suggesting an effective VOD broadcasting transmission technique through channel division in the mobile atmosphere. Many researches have been made about regular broadcasting techniques in particular. This study divides the methods used for assigning channels which have been decided by the size of segments into a group of regular channels and assistant channels using wireless gap-fillers to provide effective VOD services to a large number of clients at the mobile environment using small bandwidth resources. The regular channels transfer regular streams, while assistant channels repeatedly transfer the first segment to reduce early service delay time to receive regular streams. In this way, the study suggests a technique to reduce server bandwidth demand and early service delay time. Through the proposed technique, the server bandwidth demand could be reduced by more than 30 percent and the study continuously shows reduced early service delay time through conducting performance analysis.

• Journal of Distribution Science
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• v.15 no.1
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• pp.105-114
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• 2017

Purpose - Recently, domestic pharmaceutical market is growing steadily, but top-tier companies are concentrating on sales growth. In this market, SMEs, which account for more than 80% of the entire market, suffer from the problem of lower margins and increasing inventory costs. According to the government's policy changes related to pharmaceuticals, it is pointed out that the management of existing customers and the control of salespeople are important issues for pharmaceutical companies. This study investigates the effect of the control system on the salesperson in domestic pharmaceutical distribution channel on customer-oriented selling behaviors and sales performance. Research design, data, and methodology - To verify the proposed research model and test hypotheses, the authors selected 244 MR(medical representatives)'s responses which have currently relationship with doctors or pharmacists. This study carefully investigated the reliability, content validity, convergent validity, and discriminant validity of the proposed model. Results - The authors find out the following results: capacity control, activity control, and self control have positive effects on customer-oriented selling behaviors and customer-oriented selling behaviors have a positive effect on sales performance. In addition, we present alternative model to check the direct effect between the control systems and the sales performance, but control system factors except self control have no direct influence. Conclusions - First of all, competency control and activity control increases the customer-oriented selling behavior of the salesperson. This means that the salesperson's sales skill, negotiation skill, customer access skill, presentation ability, monitoring, direction and evaluation are important and it is also important to control activities to check the number of visits to customers, report preparation, and customer service etiquette. Second, the fact that self-control of salesperson affects the customer-oriented selling behavior suggests that self-control is not controlled by external factors but rather establishes short/long-term goals. Therefore, it is important for sales organization to create an environment in which members can induce persistent incentives for self-control. Finally, output control did not affect customer-oriented sales behavior, which is less likely to form confidence or motivation to MRs when output control is perceived as a means of monitoring, supervising, or controlling rather than providing information to salespeople.

• Journal of Korea Multimedia Society
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• v.14 no.9
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• pp.1165-1174
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• 2011

In data broadcasting systems, servers continuously disseminate data items through broadcast channels, and mobile client only needs to wait for the data of interest to present on a broadcast channel. However, because broadcast channels are shared by a large set of data items, the expected delay of receiving a desired data item may increase. This paper explores the issue of designing proper data allocation on multiple broadcast channels to minimize the average expected delay time of all data items, and proposes a new data allocation scheme named two level bin-packing(TLBP). This paper first introduces the theoretical lower-bound of the average expected delay, and determines the bin capacity based on this value. TLBP partitions all data items into a number of groups using bin-packing algorithm and allocates each group of data items on an individual channel. By employing bin-packing algorithm in two step, TLBP can reflect a variation of access probabilities among data items allocated on the same channel to the broadcast schedule, and thus enhance the performance. Simulation is performed to compare the performance of TLBP with three existing approaches. The simulation results show that TLBP outperforms others in terms of the average expected delay time at a reasonable execution overhead.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.39C no.4
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• pp.351-361
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• 2014

Compared to a terrestrial communication, the high BER(Bit Error Ratio) and low channel bandwidth are the major factor of throughput degradation due to characteristics of underwater channel. Therefore, a MAC protocol must be designed to solve this problem in UWASNs(Underwater Acoustic Sensor Networks). MAC protocols for UWASNs can be classified into two major types according to the contention scheme(Contention-free scheme and Contention-based scheme). In large scale of sensor networks, a Contention-based scheme is commonly used due to time-synchronize problem of Contention-free scheme. In the contention-based scheme, Each node contends with neighbor nodes to access network channel by using Back-off algorithm. But a Slot-Time of Back-off algorithm has long delay times which are cause of decrease network throughput. In this paper, we propose a new scheme to solve this problem. The proposed scheme uses variable Slot-Time instead of fixed Slot-Time. Each node measures propagation delay from neighbors which are used by Slot-time. Therefore, Slot-Times of each node are optimized by considering node deployment. Consequently, the wasted-time for Back-off is reduced and network throughput is improved. A new mac protocol performance in throughput and delay is assessed through NS3 and compared with existing MAC protocol(MACA-U). Finally, it was proved that the MAC protocol using the proposed scheme has better performance than existing MAC protocol as a result of comparison.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.40 no.8
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• pp.1631-1636
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• 2015

With the advance of wireless communication technology in terrestrial area, underwater communication is also evolving very fast from a simple point-to-point transmission to an elaborate networked communications. Underwater acoustic channel has quite different features comparing with the terrestrial radio channel in terms of propagation delay, Doppler shift, multipath, and path loss. Thus, existing technologies developed for terrestrial communication might not work properly in underwater channel. Especially medium access control (MAC) protocols which highly depend on propagation phenomenon should be newly designed for underwater network. CSMA/CA has drawn lots of attention as a candidate of underwater MAC protocol, since it is able to resolve a packet collision and the hidden node problem. However, a received signal could be degraded by the interferences from the nodes locating outside the receiver's propagation radius. In this paper, we study the effects of interference on the CSMA/CA based underwater network. We derived the SNR with the interference using the sonar equation and analyzed the degradation of the RTS/CTS effects. These results are compared with the terrestrial results to understand the differences. Finally we summarized the design considerations in CSMA/CA based underwater network.

• Journal of Digital Contents Society
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• v.10 no.1
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• pp.61-71
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• 2009

While the broadband network and multimedia technology are being developed, the commercial market on digital contents as well as IPTV has been widely spreading. Especially IPTV is providing various useful services since it makes using multimedia contents easier and more convenient by combining the strong points of both TV and PC system. Until now, IPTV system has been focused on VoD service as a general usages. However, various IPTV services are recently demanded since they can have special features for some specific places and occasions. So in this paper, we propose a design of the download based IPTV system using user-specific channel management. At first, program contents previously scheduled by users are downloaded and are played at settop-box according to the prescribed time table. This system has some advantages of giving users easy access to any wanted media contents in anytime that is IPTV's characteristics. And we can increase stability of services by using time slot distributed download for contents. Because this service can be constructed and maintained at relatively low cost, we confirm that this service model is more suitable to some companies with small infrastructure and special purposes. Accordingly, in this paper, we look into necessary elements and define some protocols for system in order to help other developer build this system.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.36 no.3A
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• pp.215-222
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• 2011

In this paper, we propose a power control scheme for effective serving cell selection in Relay environment of 3GPP (3rd Generation Partnership Project) LTE (Long Tenn Evolution)-Advanced system. A conventional serving cell selection scheme which does not use channel states of backhaul link has a problem that this scheme does not select serving cell supporting maximum throughput. Also, conventional proposed serving cell selection schemes that eNB or RN transmits channel states of backhaul link have problems that conventional schemes need to additional data transmission, serving cell selection process complexity is increased because UE considers channel states of backhaul link, and received signal is degraded because strong interference which is transmission signal from RN. Therefore, for solve these problems, we propose power control scheme that RN control transmission power according to received SINR (Signal to Interference plus Noise Ratio) of backhaul link. By extensive computer simulation, we verify that the power control Relay scheme is attractive and suitable for the Relay environment.

• Journal of The Geomorphological Association of Korea
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• v.28 no.3
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• pp.13-26
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• 2021

In mountain gully, channelized debris flow is an important phenomenon in the process of topographical change. Social infrastructure as roads may be damaged by channelized debris flows, but there has been little information about their occurrence and movement to prepare for the risk of the debris flow. Most of the channelized debris flows occur during heavy rains in mountainous valleys that are difficult to access, so there are not many field data. In this study, the topographical characteristics of the catchment, the rainfall and runoff related to the debris flow, the sedimentary pattern and the cross-sectional change of the channel bed, and the underflow velocity of the gravel bed have been investigated and analyzed in the Singi gully where the channelized debris flows occurred. In the catchment, there was almost no sediment runoff because the vegetation combine with the debris landforms and covered the surface. Therefore, the obvious cause of the channelized debris flows is the collapse of the slope and bed of the gully. Even if the gravel, cobbles, and boulders of the channel bed were lost by debris flow, the thalweg change due to debris flow may not be significant because they are supplied from the gully side slope normally. After the gabion structures were installed, the debris flow increased the thalweg change, bed erosion and side slope of the gully. Various sedimentary structures in the gully were classified according to the factors supporting the sedimentation. The hypsometric curve of the gully reflects the debris landforms and vegetation characteristics of the watershed and the sediment runoff due to debris flow, etc. The relationship between the flow velocity and the hydraulic gradient was non-linear under the condition that the porous medium with gully bed gravels is saturated with water. These results may be used as basic data for channelized debris flow research.

• Journal of KIISE:Information Networking
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• v.35 no.2
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• pp.99-111
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• 2008

In recent years, the needs for WLANs(Wireless Local Area Networks) technology which can access to Internet anywhere have been dramatically increased particularly in SOHO(Small Office Home Office) and Hot Spot. However, unlike wired networks, there are some unique characteristics of wireless networks. These characteristics include the burst packet losses due to unreliable wireless channel. Note that burst packet losses, which occur when the distance between the wireless station and the AP(Access Point) increase or when obstacles move temporarily between the station and AP, are very frequent in 802.11 networks. Conversely, due to burst packet losses, the performance of 802.11 networks are not always as sufficient as the current application require, particularly when they use TCP at the transport layer. The high packet loss rate over wireless links can trigger unnecessary execution of TCP congestion control algorithm, resulting in performance degradation. In order to overcome the limitations of WLANs environment, MAC-layer LDA(Loss Differentiation Algorithm)has been proposed. MAC-layer LDA prevents TCP's timeout by increasing CRD(Consecutive Retry Duration) higher than burst packet loss duration. However, in the wireless channel with high packet loss rate, MAC-layer LDA does not work well because of two reason: (a) If the CRD is lower than burst packet loss duration due to the limited increase of retry limit, end-to-end performance is degraded. (b) energy of mobile device and bandwidth utilization in the wireless link are wasted unnecessarily by Reducing the drainage speed of the network buffer due to the increase of CRD. In this paper, we propose a new retransmission module based on Cross-layer approach, called BLD(Burst Loss Detection) module, to solve the limitation of previous link layer retransmission schemes. BLD module's algorithm is retransmission mechanism at IEEE 802.11 networks and performs retransmission based on the interaction between retransmission mechanisms of the MAC layer and TCP. From the simulation by using ns-2(Network Simulator), we could see more improved TCP throughput and energy efficiency with the proposed scheme than previous mechanisms.