Abstract:
A method and a system for operating a Mutual Broadcast Period (MBP) and Contention Access Period (CAP) for load control are provided. The proposed system and method is suitable for a short-range communication environment such as communication environment in or around the human body, and is for a mesh network communication environment in which one piconet is formed around the human body or a plurality of devices are connected. When signals carrying biometric information are periodically received from a plurality of sensor devices for medical purposes, the system and method may achieve efficient resource access by performing load control in a distributed manner, contributing to a reduction in access delay and power consumption and enabling appropriate QoS control.

Description:
PRIORITY 
     This application claims the benefit under 35 U.S.C. §119(a) of a Korean patent application filed in the Korean Intellectual Property Office on Jul. 5, 2011 and assigned Serial No. 10-2011-0066623, the entire disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The preset invention relates to a method and apparatus in which a device terminal accesses a coordinator terminal in a communication system. More particularly, the present invention relates to a system and method supporting efficient operations of a plurality of sensor devices that periodically transmit sensing information in a Body Area Network (BAN). 
     2. Description of the Related Art 
     Wireless Body Area Network (WBAN), which is under standardization as an international standard called Institute of Electrical and Electronic Engineers (IEEE) 802.15.6 TG6 BAN, aims to provide medical services such as telemedicine services over a communication network formed around three meters or less from the body, and to provide entertainment services in which wearable equipment for wearable computing or motion sensors are used. In addition, WBAN is under similar standardization as an international standard called IEEE 802.15.4j Medical BAN (MBAN), and 802.15.4j is defined as an amendment standard for using the existing 802.15.4 in a Medical BAN Service (MBANS) band of 2.36˜2.4 GHz. 
     WBAN generally includes a coordinator and a plurality of devices such as various types of sensors attachable to the body. 
     The main application of WBAN is to collect biometric information from medical sensors and to send the collected biometric information to medical institutions. A coordinator, which has a wire or wireless communication line connected to a medical institution server, sends data received from devices or sensors connected by WBAN to the medical institution server. For example, the coordinator may send the data received from the devices or sensors in an unprocessed form or after analyzing such data. 
     In the WBAN healthcare system, because small-sized devices equipped with a mobile power supply such as a battery are mainly handled, reducing (e.g., minimizing) the power consumption of the devices is an important system requirement. Generally, a low duty cycling technique may be applied, for low-power implementation. As an example, the small-sized devices may be sensors having poor power conditions. 
       FIG. 1  shows a data transmission process when it is operated by low duty cycling and when a beacon is used in an IEEE 802.15.4 WBAN according to the related art. 
     Referring to  FIG. 1 , when the data transmission process is operated by low duty cycling, the lower the duty cycling, the greater the number of nodes that have data during an inactive period. At the starting point of the next active period, the system attempts to transmit all of the data. 
     As described above, in the WBAN according to the related art, when data is transmitted by low duty cycling, many nodes may have data during an inactive period due to the low duty cycling. Consequently, transmission of all of this data is attempted in the next active period. 
     In this case, the WBAN according to the related art may deal with contention with the fixed initial backoff settings, for packet transmission. However, when the concentration of traffic is severe, it is difficult to solve this problem with the initial backoff settings which were made without recognizing this problem. 
     In addition, when a number of packet transmission attempts rapidly increases in the next active period, the packet transmission attempts are concentrated at the same time in a Contention Access Period (CAP). Accordingly, traffic may occur during the packet transmission. 
     Therefore, a need exists for an apparatus and method for controlling resource access by devices such that in a WBAN in which periodic data transmission is made, a plurality of devices may be prevented from causing a reduction in performance such as delays due to their excessive collisions in a Contention Access Period CAP 
     The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention are to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide a system and method for controlling resource access by devices such that in a Wireless Body Area Network (WBAN) in which periodic data transmission is made, a plurality of devices may be prevented from causing a reduction in performance such as delays due to their excessive collisions in a Contention Access Period (CAP). 
     In accordance with an aspect of the present invention, a coordinator in a Mutual Broadcast Period (MBP) and CAP operating system for load control is provided. The coordinator includes a Radio Frequency (RF) unit for broadcasting a beacon frame, and a controller for determining whether contention for data transmission in a CAP due to backlogged traffic increases, by recognizing the number of connected devices, for broadcasting a beacon frame including information about an MBP used for load control to each device through the RF unit before the CAP if the contention for data transmission increases, for determining whether a load control broadcast message for determining existence of data load is received in the MBP from the device without error, and for sending a response to the load control broadcast message to the device. 
     In accordance with another aspect of the present invention, a device in a MBP and CAP operating system for load control is provided. The device includes a RF unit for receiving a beacon frame broadcasted from a coordinator, and a controller for sending a load control broadcast message for determining existence of data load to the coordinator in an MBP based on information about the MBP upon receiving a beacon frame including information about an MBP used for load control from the coordinator before a CAP, for determining a type of a CAP depending on whether sending of the load control broadcast message is successful and whether packet transmission by other devices is successful, and for performing data transmission using a CAP corresponding to the determined CAP type. 
     In accordance with another aspect of the present invention, a method for operating a MBP and CAP for load control in a coordinator is provided. The method includes determining whether contention for data transmission in a CAP due to backlogged traffic increases, by recognizing the number of connected devices, broadcasting a beacon frame including information about an MBP used for load control to each device before the CAP, if the contention for data transmission increases, determining whether the load control broadcast message is received without error, and sending a response to the load control broadcast message to the device. 
     In accordance with another aspect of the present invention, a method for operating a MBP and CAP for load control in a device is provided. The method includes receiving a beacon frame including information about an MBP used for load control from a coordinator before a CAP, sending a load control broadcast message for determining existence of data load to the coordinator in the MBP based on information about the MBP, determining a type of a CAP depending on whether sending of the load control broadcast message is successful and whether packet transmission by other devices is successful, and performing data transmission using a CAP corresponding to the determined CAP type. 
     Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a data transmission process when it is operated by low duty cycling and when a beacon is used in an IEEE 802.15.4 Wireless Body Area Network (WBAN) according to the related art; 
         FIG. 2  shows a structure of a superframe including a Mutual Broadcast Period (MBP) according to an exemplary embodiment of the present invention; 
         FIG. 3  shows a superframe configured by dividing an MBP into Mutual Broadcast Zones (MBZs) and a Contention Access Period (CAP) into Contention Access Zones (CAZs) according to an exemplary embodiment of the present invention; 
         FIG. 4  shows a structure of one MBZ according to an exemplary embodiment of the present invention; 
         FIG. 5  shows a structure of a coordinator and a device according to an exemplary embodiment of the present invention; 
         FIG. 6  shows a structure of a beacon frame according to an exemplary embodiment of the present invention; 
         FIG. 7  shows a structure of an MBP field according to a first exemplary embodiment of the present invention; 
         FIG. 8  shows a structure of a superframe including the MBP field and having no GTS according to the first exemplary embodiment of the present invention; 
         FIG. 9  shows a structure of a superframe including the MBP field and having a GTS according to the first exemplary embodiment of the present invention; 
         FIG. 10  shows a structure of an MBP field according to a second exemplary embodiment of the present invention; 
         FIG. 11  shows a structure of a superframe according to the second exemplary embodiment of the present invention; 
         FIG. 12  shows a structure of an MBP field according to a third exemplary embodiment of the present invention; 
         FIG. 13  shows a structure of an MBP field according to a fourth exemplary embodiment of the present invention; 
         FIG. 14  shows a structure of a superframe according to the third exemplary embodiment of the present invention; 
         FIG. 15  shows a structure of a superframe according to the fourth exemplary embodiment of the present invention; 
         FIG. 16  shows a process of performing load control using an MBP according to an exemplary embodiment of the present invention; 
         FIGS. 17A and 17B  show a flow diagram of a Carrier Sense Multiple Access-Collision Avoidance (CSMA-CA) algorithm in an Exclusive CAP according to an exemplary embodiment of the present invention; 
         FIGS. 18A and 18B  show a flow diagram of a CSMA-CA algorithm in a Background CAP according to an exemplary embodiment of the present invention; 
         FIG. 19  shows a process of performing load control using an MBP in a coordinator according to an exemplary embodiment of the present invention; and 
         FIG. 20  shows a process of performing load control using an MBP in a device according to an exemplary embodiment of the present invention. 
     
    
    
     Throughout the drawings, in should be noted that like reference numbers are used to depict the same or similar elements, features, and structures. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. In addition, descriptions of well-known functions and constructions may be omitted for clarity and conciseness. 
     The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention is provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
     It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces. 
       FIG. 2  shows a structure of a superframe including a Mutual Broadcast Period (MBP) according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 2 , the superframe includes a beacon frame B, an MBP, a Contention Access Period (CAP), and a Contention Free Period (CFP). 
     In an exemplary embodiment of the present invention, the MBP is established between the beacon frame B and the Contention Access Period (CAP) as shown in  FIG. 2 . Thus, before entering the CAP, devices may exchange information with each other, thereby ensuring proper load control in the CAP. 
     Queue information about the number of packets accumulated thus far is sent in the MBP, thereby preventing transmission attempts from being concentrated at the same time in the CAP. 
     In the exemplary embodiment of the present invention, load control in the CAP is made naturally in a distributed manner depending on the information that the devices have exchanged in the MBP. 
     According to exemplary embodiments of the present invention, the MBP is smaller than the CAP in time period. As example, with regard to a length of the MBP, the coordinator may inform the devices of the length of the MBP by adding a field indicating the length of the MBP in the beacon frame B and including information about the length of the MBP or information about a start point of the CAP in the added field. This MBP operates by Carrier Sense Multiple Access-Collision Avoidance (CSMA-CA) similarly to the CAP, but the MBP may be set so as to consider light contention situations such as making a length of a backoff slot short because the MBP does not require transmission of a lot of data. 
       FIG. 3  shows a superframe configured by dividing an MBP into Mutual Broadcast Zones (MBZs) and a CAP into Contention Access Zones (CAZs) according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 3 , the superframe includes a beacon frame B, an MBP, and a CAP. The MBP includes at least one MBZ and the CAP includes at least one CAZ. 
     Exemplary embodiments of the present invention propose MBZs and CAZs, for balanced load control. An MBZ corresponds to each of N zones obtained by dividing an MBP. Similarly, a CAZ corresponds to each of N zones obtained by dividing a CAP. MBZs correspond to CAZs on a one-to-one basis. 
     For example, assume that the coordinator sets 6 MBZs in an MBP and 6 CAZs in a CAP. 
     These MBZs and CAZs are used as a tool for load balancing, through load control. As an example, MBZ # 1  corresponds to CAZ # 1 , MBZ # 2  corresponds to CAZ # 2 , and in this way, MBZ # 6  corresponds to CAZ # 6 . 
     One MBZ will be described in detail with reference to  FIG. 4 . 
       FIG. 4  shows a structure of one MBZ according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 4 , one MBZ includes a plurality of mini backoff slots. When attempting to transmit data in an MBZ, a device transmits data at the boundary of a mini backoff slot in accordance with a slotted CSMA-CA operation. 
     According to exemplary embodiments of the present invention, the data that a plurality of devices attempt to send in one MBZ, includes queue information of each device, and a device detects transmission by other devices based on the slotted CSMA-CA operation, and transmits data by a backoff algorithm. However, when there are a large number of devices, the devices may not transmit data in an MBZ. 
       FIG. 5  shows a structure of a coordinator and a device according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 5 , a coordinator  100  includes a controller  101 , a Radio Frequency (RF) unit  102 , and a memory  103 , and a device  110  includes a controller  111 , an RF unit  112 , a sensing unit  113 , and a memory  114 . 
     According to exemplary embodiments of the present invention, if the controller  101  in the coordinator  100  expects an increase in contention by data transmission in a CAP due to backlogged traffic by recognizing the number of connected devices, the controller  101  informs the devices of the expected increase in contention by including MBP information in a beacon frame B. For example, the MBP information includes a period length and the number of MBZs. 
     Thereafter, upon receiving a queue information packet sent for load control from the device  110  without error, the controller  101  sends a response or an Acknowledgement (ACK) to the device  110 . If there is traffic that the coordinator  100  desires to send to the device  110 , the controller  101  sets a destination address as a broadcast address, sends a broadcast message to the broadcast address, and sends no ACK to the device  110  that has received the broadcast message. The queue information packet includes an indicator indicating a control packet sent in an MBP, and may further include a queue length associated with Quality of Service (QoS), a traffic type, a battery status, and the like. The queue information packet includes queue information in terms of other utilizations of an MBZ, and in fact, only for load control resource access, the queue information may be minimized or not. The queue information packet may be called a ‘load control broadcast message’. 
     The RF unit  102  in the coordinator  100  broadcasts a beacon frame B, and sends an ACK to the device  110  upon receiving a queue information message from the device  110 . 
     The memory  103  in the coordinator  100  stores information used for data transmission, and may store MBP information such as a period length and the number of MBZs. 
     Next, the controller  111  in the device  110  obtains MBP information from the coordinator  100 , finds the required amount of resources needed for packet transmission, and then determines the number of CAZs based thereon. Thereafter, the controller  111  arbitrarily selects one or multiple MBZs, the number of which corresponds to the found number of CAZs, from among all MBZs. 
     To transmit a packet in a CAP, the controller  111  first transmits a message packet including queue information to the coordinator  100  in a mini backoff slot at an MBZ point selected from a total of N MBZs in an MBP. 
     Upon receiving an ACK from the coordinator  100 , the controller  111  determines whether to use a CAZ corresponding to the MBZ as an Exclusive CAP, and stores it in the memory  114 . Upon receiving no ACK from the coordinator  100 , the controller  111  retries the packet transmission, considering that a Negative Acknowledgement (NACK) is received. If the transmission in the selected MBZ is not successful, the controller  111  selects again one of the remaining unselected MBZs. The term ‘Exclusive CAP’ as used herein may refer to a period in which the device  110  may transmit more data than an amount of data, which is set by default. 
     While not transmitting its queue information packet in the selected MBZ, the controller  111  receives queue information packets from other devices in a listening state, and upon receiving ACKs for the queue information packets from the coordinator  100 , the controller  111  stores the queue information packets in the memory  114 . This may be used when the controller  111  adjusts CSMA-CA variables in a CAP. 
     The controller  111  performs listening only, in the unselected MBZs. If there is no queue information packet that the device  110  has transmitted in the MBZ and received an ACK therefor, the controller  111  determines to use a CAZ corresponding to the MBZ as a Normal CAP. The term “Normal CAP’ as used herein may refer to a period in which the device  110  may transmit data in the amount of data, which is set by default. 
     If the device  110  fails to transmit the queue information packet in all MBZs, the controller  111  determines to use the full CAP as a Background CAP. The term ‘Background CAP’ as used herein may refer to a period in which the device  110  may transmit data in the remaining period among the entire data transmission period. 
     The RF unit  112  in the device  110  is configured to transmit and receive information. For example, the RF unit  112  receives a beacon frame broadcasted from the coordinator  100 , transmits a queue information packet to the coordinator  100  in each MBZ corresponding to each CAZ, and receives an ACK from the coordinator  100 . 
     The sensing unit  113  in the device  110  outputs sensed data to the controller  111 . 
     The memory  114  in the device  110  stores information needed for data transmission, and may store the queue information packet received from the coordinator  100 . The memory  114  stores in advance CAP information corresponding to the transmission results of the queue information packet. For example, the CAP information includes an Exclusive CAP, a Normal CAP and a Background CAP. 
     As a result, exemplary embodiments of the present invention may enable efficient resource access by performing load control in a distributed manner for data transmission/reception, contributing to a reduction in access delay and power consumption and enabling appropriate QoS control. 
     A structure of the above-described beacon frame B will be described in detail below with reference to  FIG. 6 . 
       FIG. 6  shows a structure of a beacon frame according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 6 , the beacon frame includes a field for Frame Control, a field for Sequence Number, an Addressing field, an Auxiliary Security Header, a field for Superframe Specification, a Pending address field, a Beacon Payload, an MBP field, a Frame Check Sequence (FCS). The beacon frame may also include a GTS field. 
     According to exemplary embodiments of the present invention, it is appropriate for the MBP field to include variable fields, similarly to GTS fields, rather than the Superframe Specification field giving information, because the MBP field is a field newly added to the existing specification. Details and structure of the MBP field will be described in detail below with reference to  FIGS. 7 to 15 . 
       FIG. 7  shows a structure of an MBP field according to a first exemplary embodiment of the present invention. 
     A MBZ/CAZ Count field (with 4 bits, having a value of 0 to 15) indicates the number of MBZs/CAZs. 
     A MBZ Length field indicates a length of one MBZ on a slot basis. 
     A CAZ Length field indicates a length of one CAZ on a slot basis. 
     Referring to  FIG. 7 , if MBZ/CAZ Count is 3, MBZ length is 1, and CAZ length is 4, then slots # 0  to # 2  operate as an MBP, slots # 3  to # 14  operate as CAZs corresponding to MBZs, and the remaining slot # 15  may operate as a Normal CAP. 
     Two different types of superframes including the MBP field according to a first exemplary embodiment of the present invention may be represented as shown in  FIGS. 8 and 9 .  FIG. 8  shows a type of a superframe without GTS, and  FIG. 9  shows a type of a superframe to which GTS is applied. 
       FIG. 8  shows a structure of a superframe including the MBP field and having no GTS according to the first exemplary embodiment of the present invention.  FIG. 9  shows a structure of a superframe including the MBP field and having a GTS according to the first exemplary embodiment of the present invention. 
     Referring to  FIG. 8 , the superframe includes a beacon frame, an MBP, and a CAP. The MBP includes at least one MBZ and the CAP includes at least one CAZ. 
     Referring to  FIG. 9 , the superframe includes a beacon frame, an MBP, and a CAP, and a GTS. For example, the GTS may be included in a Circuit Emulation over Packet (CEP). The CEP may include a plurality of GTSs. 
       FIG. 10  shows a structure of an MBP field according to a second exemplary embodiment of the present invention. 
     Referring to  FIG. 10 , the MBP field includes an MBZ/CAZ Count, an MBZ Ending Slot, and a CAZ Ending Slot. 
     MBZ Ending Slot indicates the last slot among the existing 16 available slots, which is to be used for an MBP. 
     CAZ Ending Slot indicates a slot of the last CAZ in a CAP. 
     Although a first exemplary embodiment of the present invention is similar to a second exemplary embodiment of the present invention, when the CAZ Ending Slot is not defined, a CAP from the next slot of MBZ Ending Slot to the final CAP slot of the Superframe Specification field will be divided by a number in the MBZ/CAZ Count field in the same length. 
     When the CAZ Ending Slot is defined, if the CAZ Ending Slot is greater than the final CAP slot, it is regarded as the same value as that of the final CAP slot, and a CAP from the next slot of MBZ Ending Slot to CAZ Ending Slot will be divided by a number in the MBZ/CAZ Count field in the same length. In this case, a CAZ length will not be a multiple of a superframe slot. An MBP from 0 to MBZ Ending Slot is also divided by a number in the MBZ/CAZ Count field in the same length and used as MBZ field. 
     When the MBP field according to a second exemplary embodiment of the present invention is used, for each MBZ/CAZ, a CAP from the next slot of MBZ Ending Slot to a superframe slot designated by CAZ Ending Slot will be divided by a number in the MBZ/CAZ Count field in the same length. 
     A superframe including the MBP field according to the second exemplary embodiment of the present invention may be represented as shown in  FIG. 11 . 
       FIG. 11  shows a structure of a superframe according to the second exemplary embodiment of the present invention. 
     Referring to  FIG. 11 , the superframe includes a beacon field, an MBP, a CAP, and a GTS. The MBP field includes at least one MBZ, and the CAP includes at least one CAZ. As an example, the GTS may be included in a CFP. 
     In addition, MBP Duration (MD) determined by an MBP Order value rather than based on the superframe slot as in the above-described first and second exemplary embodiments of the present invention may be configured with an MBP. An MBP Order field is a field used to adjust a size of the superframe, and an MBP Order value may be determined by the user or the coordinator&#39;s algorithm, like the superframe order or beacon order, and may have 2 bits in an exemplary embodiment of the present invention. 
     The configured MBP fields may be represented as shown in  FIGS. 12 and 13 . 
       FIG. 12  shows a structure of an MBP field according to a third exemplary embodiment of the present invention, and  FIG. 13  shows a structure of an MBP field according to a fourth exemplary embodiment of the present invention. 
     Referring to  FIG. 12 , the MBP field includes an MBP Order, an MBZ/CAZ Count, and a CAZ Length. 
     Referring to  FIG. 13 , the MBP field includes an MBP Order, an MBZ/CAZ Count, and a CAZ Ending Slot. 
     Similarly to Superframe Duration (SD) and Beacon Interval (BI), MD may be calculated by Equation (1) below.
 
 MD =aBaseSuperframeDuration*2 MO  symbols  (1)
 
     An MBP is inserted as a new period ahead of a CAP before a start of a superframe slot # 0  depending on the calculated MD, and other fields in the MBP field according to the third and fourth exemplary embodiments of the present invention are the same as those in the MBP field according to the first and second exemplary embodiments of the present invention. 
     Superframes including the MBP fields according to the third and fourth exemplary embodiments of the present invention may be represented as shown in  FIGS. 14 and 15 . 
       FIG. 14  shows a structure of a superframe according to the third exemplary embodiment of the present invention, and  FIG. 15  shows a structure of a superframe according to the fourth exemplary embodiment of the present invention. 
     Referring to  FIG. 14 , the superframe includes a beacon field, an MBP, a CAP, and a GTS. The MBP may include at least one MBZ, and the CAP may include at least one CAZ. As an example, the GTS may be included in a CFP. 
     Referring to  FIG. 15 , the superframe includes a beacon field, an MBP, a CAP, and a GTS. The MBP may include at least one MBZ, and the CAP may include at least one CAZ. As an example, the GTS may be included in a CFP. 
     An operation of the device will be described in detail below with reference to  FIG. 16 . 
       FIG. 16  shows a process of performing load control using an MBP according to an exemplary embodiment of the present invention. 
     According to exemplary embodiments of the present invention, if a plurality of devices attempt transmission of a queue information packet in one MBZ, all or some of the attempting devices may succeed in the attempts, or all of the attempting devices may fail in the attempts. 
     When all of the attempting devices succeed in the attempts, all of the devices operate in a CAP corresponding to an Exclusive CAP (E-CAP). However, when there are a large number of devices, only some of the devices may succeed in the attempts generally, because it will be unlikely that all of the devices may succeed in the attempts. Some devices having succeeded in the attempts may operate in a CAP corresponding to an Exclusive CAP, but the remaining devices having failed in the attempts may not use the associated CAZs. If all of the devices have failed in their respective attempts, the devices having made the attempts may not use the associated CAZs. However, the devices, which have been performing listening instead without making the attempts, may use the associated CAZs as a Normal CAP. The devices, which have finally failed in transmission in an MBP because they have failed in transmission in all MBZs where they attempted the transmission, may use the entire CAP as a Background CAP. 
     Referring to  FIG. 16 , it is assumed that a superframe is divided into 6 CAZs in a CAP and 6 MBZs in an MBP, and as the coordinator broadcasts this information to devices, the devices recognize the information in advance. 
     For example, in a case in which first and fourth devices delivered a Queue (Q) information packet to the coordinator in MBZ # 1  among 6 MBZs in an MBP, second, third and fifth devices determine to transmit data using a Normal CAP N-CAP at CAZ # 1  in a CAP, when no ACK is received from the coordinator. 
     In a case in which second and fifth devices delivered a Q information packet to the coordinator in MBZ # 2  among 6 MBZs in an MBP, the second and fifth devices determine to transmit data using an Exclusive CAP E-CAP at CAZ # 2  in a CAP upon receiving an ACK from the coordinator. 
     In a case in which first and fifth devices delivered a Q information packet to the coordinator in MBZ # 3  among 6 MBZs in an MBP, only the fifth device determines to transmit data using an Exclusive CAP E-CAP at CAZ # 3  in a CAP, if no ACK is received at the first device from the coordinator and an ACK is received at the fifth device from the coordinator. 
     In a case in which a first device delivered a Q information packet to the coordinator in MBZ # 4  among 6 MBZs in an MBP, only the first device determines to transmit data using an Exclusive CAP E-CAP at CAZ # 4  in a CAP if an ACK is received at the first device from the coordinator. 
     In a case in which third and fourth devices delivered a Q information packet to the coordinator in MBZ # 5  among 6 MBZs in an MBP, first, second and fifth devices determine to transmit data using a Normal CAP N-CAP at CAZ # 5  in a CAP if no ACK is received from the coordinator. 
     In a case in which third and fourth devices delivered a Q information packet to the coordinator in MBZ # 6  among 6 MBZs in an MBP, only the third device determines to transmit data using an Exclusive CAP E-CAP at CAZ # 6  in a CAP, if an ACK is received at the third device from the coordinator and no ACK is received at the fourth device from the coordinator. 
     The fourth device, which has failed in transmission of a Q information packet at all of 6 MBZs in an MBP, determines to transmit data using a Background CAP (B-CAP) in a CAP. 
     As described above, an operation in a CAP is based on the CSMA-CA resource access scheme which is defined according to each of the Exclusive CAP, Normal CAP and Background CAP determined in an MBP in advance. Although the detailed operation in each period will not be described herein, it is general that an Exclusive CAP may be set for a device to attempt resource access more strongly than usual, and a Background CAP may be set for a device to attempt resource access more weakly than usual. In this regard, the Exclusive CAP, Normal CAP and Background CAP may have, for example, the following variables and algorithms. Specifically, CSMA-CA algorithms, in which the foregoing is reflected, will be described with reference to  FIGS. 17A ,  17 B,  18 A and  18 B. Operations on the CSMA-CA algorithms in  FIGS. 17A ,  17 B,  18 A and  18 B are the same as an operation of the general CSMA-CA algorithm, and variables and algorithm setting values by the Exclusive CAP and Background CAP will be applied as described below. 
       FIGS. 17A and 17B  show a flow diagram of a CSMA-CA algorithm in an Exclusive CAP according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 17A , at step  1701  it is determined whether a CSMA-CA operation is a slotted CSMA-CA operation. If it is a slotted CSMA-CA operation, then the process proceeds to step  1708 . At step  1708 , the NB may be set such that NB=0, and CW may be set such that CW=2. Upon setting NB and CW, the process proceeds to step  1709 . At step  1709 , it is determine whether battery life extension is required. If battery life extension is not required, then the process proceeds to step  1710  at which BE may be set such that BE=less of (2, macMinBE) and thereafter the process proceeds to step  1712 . If at step  1709 , it is determined that battery life extension is required, then BE may be set such that BE=macMinBE at step  1711 . Thereafter, the process proceeds to step  1712 . At step  1712 , the back off period boundary is located and the process proceeds to step  1713 . At step  1713 , a delay for a random number of backoff periods is performed. For example, the delays may be such that a delay of random(2 BE −1) unit backoff periods is performed. After the delay, the process proceeds to step  1714  at which a CCA on backoff period boundary is performed and the process proceeds to step  1715 . At step  1715 , it is determined whether a channel is idle. 
     Referring to  FIG. 17A , if the channel is determined to be idle at step  1715 , the process proceeds to step  1716  at which an Exclusive CAP is set less than a Normal CAP in terms of setting values: macMinBE and macMaxBE, and in a BE incremental equation in step  1716 , BE may be set such that BE=min(BE+0.5, macMaxBE), and maxCSMAbackoffs is set large. In an NE incremental equation in step  1716 , NB may be set such that NB=NB+0.5. When NB or BE is used, their integers may be taken and used. After step  1716 , the process proceeds to step  1717  at which it is determined whether NB is greater than macMaxCSMABackoffs. If NB is not greater than macMaxCSMABackoffs, then the process returns to step  1713 . If NB is greater than macMaxCSMABackoffs, then the process ends in failure. 
     If the channel is determined to not be idle at step  1715 , then the process proceeds to step  1718  at which CW may be set such that CW=CW−1. After the CW is set, the process proceeds to step  1719  at which it is determined whether CW=0. If CW is determined to not equal 0, then the process returns to step  1714 . However, if CW is determined to equal 0, then the process ends in success. 
     Referring to  FIGS. 17A and 17B , if at step  1701  it is determined that the CSMA-CA operation is not slotted, then the process proceeds to step  1702 . At step  1702 , the NB may be set such that NB=0 and ME may be set such that BE=macMinBe. Thereafter, the process proceeds to step  1703  at which a delay is performed. As an example, the delay may be for a number of unit backoff periods corresponding to random(2 BE −1). Thereafter, the process proceeds to step  1704  at which a CCA is performed. After performing the CCA, the process proceeds to step  1705  at which it is determined whether the channel is idle. If the channel is determined to be idle at step  1705 , then the process proceeds to step  1706  at which NB may be set such that NB=NB+0.5 and BE may be set such that BE=min(BE+0.5, macMaxBE). Thereafter, the process proceeds to step  1707  at which it is determined whether NB is greater than macMaxCSMABackoffs. If it is determined that NB is not greater than macMaxCSMABackoffs, then the process returns to step  1703 . If NB is determined to be greater than macMaxCSMABackoffs, then the process ends in failure. 
     Conversely, if at step  1705  it is determined that the channel is idle, then the process ends in success. 
     According to exemplary embodiments of the present invention, a Normal CAP is the same as macMinBE, macMaxBE, and maxCSMAbackoffs values used by the existing CSMA-CA algorithm in a CAP. BE has min([BE+1], macMaxBE) and NB has NB+1. 
       FIGS. 18A and 18B  show a flow diagram of a CSMA-CA algorithm in a Background CAP according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 18A , at step  1801  it is determined whether a CSMA-CA operation is a slotted CSMA-CA operation. If it is a slotted CSMA-CA operation, then the process proceeds to step  1808 . At step  1808 , the NB may be set such that NB=0, and CW may be set such that CW=2. Upon setting NB and CW, the process proceeds to step  1809 . At step  1809 , it is determine whether battery life extension is required. If battery life extension is not required, then the process proceeds to step  1810  at which BE may be set such that BE=less of (2, macMinBE) and thereafter the process proceeds to step  1812 . If at step  1809 , it is determined that battery life extension is required, then BE may be set such that BE=macMinBE at step  1811 . Thereafter, the process proceeds to step  1812 . At step  1812 , the back off period boundary is located and the process proceeds to step  1813 . At step  1813 , a delay for a random number of backoff periods is performed. For example, the delays may be such that a delay of random(2 BE −1) unit backoff periods is performed. After the delay, the process proceeds to step  1814  at which a CCA on backoff period boundary is performed and the process proceeds to step  1815 . At step  1815 , it is determined whether a channel is idle. 
     Referring to  FIG. 18A , if the channel is determined to be idle at step  1815 , the process proceeds to step  1816  at which a Background CAP is set greater than a Normal CAP in terms of setting values: macMinBE and macMaxBE. In a BE incremental equation in step  1816 , BE may be set such that BE=min(BE+2, macMaxBE), and maxCSMAbackoffs may be set small. In an NB incremental equation in step  1816 , NB may be set such that NB=NB+2. 
     NB corresponds to the number of retries due to a backoff made at one access attempt. CW is the number of backoff periods needed to check whether the channel is in an idle state. BE is related to the number of backoff intervals for which a device should wait before performing channel sensing, and the device may select any number from among numbers of 0 to 2BE−1 before its operation. 
     After step  1816 , the process proceeds to step  1817  at which it is determined whether NB is greater than macMaxCSMABackoffs. If NB is not greater than macMaxCSMABackoffs, then the process returns to step  1813 . If NB is greater than macMaxCSMABackoffs, then the process ends in failure. 
     If the channel is determined to not be idle at step  1815 , then the process proceeds to step  1818  at which CW may be set such that CW=CW−1. After the CW is set, the process proceeds to step  1819  at which it is determined whether CW=0. If CW is determined to not equal 0, then the process returns to step  1814 . However, if CW is determined to equal 0, then the process ends in success. 
     Referring to  FIGS. 18A and 18B , if at step  1801  it is determined that the CSMA-CA operation is not slotted, then the process proceeds to step  1802 . At step  1802 , the NB may be set such that NB=0 and ME may be set such that BE=macMinBe. Thereafter, the process proceeds to step  1803  at which a delay is performed. As an example, the delay may be for a number of unit backoff periods corresponding to random(2 BE −1). Thereafter, the process proceeds to step  1804  at which a CCA is performed. After performing the CCA, the process proceeds to step  1805  at which it is determined whether the channel is idle. If the channel is determined to be idle at step  1805 , then the process proceeds to step  1806  at which NB may be set such that NB=NB+2 and BE may be set such that BE=min(BE+2, macMaxBE). Thereafter, the process proceeds to step  1807  at which it is determined whether NB is greater than macMaxCSMABackoffs. If it is determined that NB is not greater than macMaxCSMABackoffs, then the process returns to step  1803 . If NB is determined to be greater than macMaxCSMABackoffs, then the process ends in failure. 
     Conversely, if at step  1805  it is determined that the channel is idle, then the process ends in success. 
     According to exemplary embodiments of the present invention, even in the same CAZ, differentiated access may be performed based on the queue information exchanged in an MBP in advance, taking into account inter-device QoS. 
       FIG. 19  shows a process of performing load control using an MBP in a coordinator according to an exemplary embodiment of the present invention. 
     In step  1900 , the coordinator  100  determines whether contention for data transmission in a CAP due to backlogged traffic has increased. If the contention has increased, the coordinator  100  proceeds to step  1902 . Otherwise, the controller  100  performs common data transmission in step  1901 . 
     In step  1902 , the controller  100  generates a beacon frame including MBP information and broadcasts it to devices. 
     The coordinator  100  determines in step  1903  whether a queue information packet for load control is received from each device without error in an MBP. If the queue information packet is received without error, the coordinator  100  proceeds to step  1905 . Otherwise, the coordinator  100  sends no response (or ACK) to each device in step  1904 . 
     In step  1905 , the coordinator  100  sends a response to the queue information packet received without error, to each device. 
       FIG. 20  shows a process of performing load control using an MBP in a device according to an exemplary embodiment of the present invention. 
     In step  2000 , the device  110  receives a beacon frame including MBP information from the coordinator  100 . 
     In step  2001 , the device  110  finds the required amount of resources needed for packet transmission based on the received MBP information, and then determines the number of CAZs depending on the found required amount of resources. 
     In step  2002 , the device  110  determines the number of MBZs, which corresponds to the determined number of CAZs. The number of MBZs is equal to the number of CAZs. 
     In step  2003 , the device  110  transmits a queue information packet for load control to the coordinator  100  in an MBZ corresponding to the time point selected from among the determined number of MBZs. 
     In step  2004 , the device  110  determines a CAP type depending on whether its transmission of a queue information packet is successful and whether packet transmissions by other devices are successful. For example, the device  110  may determine any one of an Exclusive CAP, a Normal CAP, and a Background CAP depending on whether its transmission of a queue information packet is successful. 
     In step  2005 , the device  110  performs a data transmission/reception operation using the determined CAP. 
     For example, upon receiving a response message to the queue information packet from the coordinator  100 , the device  110  determines a type of CAP as an Exclusive CAP, determining that its transmission of a queue information packet is successful, and performs data transmission using the determined Exclusive CAP. 
     If transmissions of a queue information packet by other devices are failed, the device  110  determines a type of CAP as a Normal CAP, and performs data transmission using the determined Normal CAP. 
     If transmissions of a queue information packet in an MBP are all failed, the device  110  determines a type of CAP as a Background CAP, and performs data transmission using the determined Background CAP. 
     As is apparent from the foregoing description, in operations of a coordinator and a device, devices participating in packet transmission/reception may receive the packets which are transmitted and received in an MBZ. For example, if another device receives the packet whose address is designated as an address of a specific device, such as unicast, in an MBZ, then the device may demodulate the packet regardless of its original destination so that the packet transmitted/received between the coordinator and the specific device may be delivered to other devices, making it possible to determine whether the packet transmission is successful. In addition, as a destination address of a queue information packet or a response packet is set as a broadcast address, the packet may be delivered not only to the device but also to the coordinator, making it possible to determine whether the packet transmission is successful. In this case, the coordinator broadcasts a response to the received packet. 
     In this manner, exemplary embodiments of the present invention may enable efficient resource access by performing load control in a distributed manner, for data transmission/reception, thus contributing to a reduction in access delay and power consumption and enabling appropriate QoS control. 
     While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.