## 1 Introduction  
Recently, there is an increased need for high data rate wireless multimedia services such as uncompressed high-definition television (HDTV), instantaneous music, data, and video file transmission for both personal and home networking purposes. For this purpose, the IEEE 802.15.3 task group has been working on a promising standard including both medium access control (MAC) and physical layer (PHY), to realize short range and high data rate (1 Gbps or above) applications in WPAN. In this paper we discuss mainly about MAC layer issues in WPAN. The IEEE standard 802.15.3 MAC layer [1] is based on a centralized, connection oriented topology which divides a large network into several smaller ones termed as \"piconets\". As shown in Figure 1, a piconet consists of a Piconet Network Controller (PNC) and DEVs (Devices). The DEV, a sensor node, is made to be low power and low cost. In a given piconet one DEV is required to perform the role of PNC (Piconet Coordinator), which provides the basic timing for the piconet as well as other piconet management functions, such as power management, Quality of Service (QoS) scheduling, and security. Using the formation of child and neighboring piconets users can increase the range of network span. The WPAN starter piconet is called a \"parent piconet\" and child/neighbor piconets are called \"dependent piconets\". These piconets differ in the way they associate themselves to the parent piconet. The IEEE 802.15.3 standard supports different power saving modes as well as multiple acknowledgement (ACK) policies (NO ACK, Imm-ACK, and Dly-ACK). IEEE 802.15.3 is very robust, stable and fast, and may coexist with other wireless technologies such as IEEE 802.11. In the IEEE 802.15.3 MAC protocol, at the start, communications are connection based under the supervision of PNC, however, at later stage connections and data transfer can be made in peer-to-peer fashion. In IEEE 802.15.3 MAC protocol, the channel time is divided into superframes, where each superframe beginning with a beacon. The superframe is made of the three major parts: the beacon, the optional contention access period (CAP), and the channel time allocation period (CTAP), as shown in Figure 2.  
**Beacon**: The beacon is broadcast at the start of super frame by the PNC. All the information about timing allocation and management information are included in the beacon. Figure 1: Piconet Structure in IEEE 802.15.3. Figure 2: Superframe Structure of IEEE 802.15.3. **Contention Access Period (CAP):** During the CAP period, DEVs can send command or asynchronous data packets using the CSMA/CA as the multiple access method. **Channel Time Allocation Period (CTAP):** The channel time allocation period (CTAP) is composed of channel time allocations (CTAs), including management CTAs (MCTAs) and regular CTAs. This period is the same as the TDMA method and used to transmit commands, isochronous streams, and asynchronous data packets. A wireless channel is usually vulnerable to errors. Hence, an error control mechanism is an essential part of any MAC protocol design. A good error control mechanism provides a certain level of reliability in terms of communication robustness and dependability for higher network layers. In accordance with that, IEEE 802.15.3 standard defines three types of acknowledgment mechanisms for CTAs and CAPs as follows:  
**No-ACK:** In No-ACK (No-Acknowledgement) mechanism, ACK is not sent after a reception of message. This mechanism is useful for only high data rate applications where guaranteed delivery is not required or acknowledgement is handled by higher layers or by some other mechanism. **Imm-ACK:** In Imm-ACK (Immediate Acknowledgement), mechanism, each received data frame is individually acknowledged after the successful reception. This mechanism provides simpler and more stable operation compared to No-ACK, but at the cost of reduced data rate.