The next generation of older people is outnumbering the younger ones. Statistics show that it will reach 761 million by 2025, more than double of what it was in 1990. This will put an enormous demand on the health care systems. The recent advancements in wireless networking enabled proliferation of wireless sensor networks. These sensor networks are capable of monitoring and controlling environments remotely, thereby reducing the workload of medical care. Body area networks (BAN) have revolutionized the remote patient monitoring system. Sensors implanted in the body collect the vital sign data and report any anomaly to a physician. A wireless technology is required to transmit these signals. Although many solutions are available today to address this, a globally standardized technology is necessary. We investigated the available IEEE standard 802.15.1(Bluetooth) and 802.15.4 (ZigBee) to decide the best fit for BAN. ZigBee/802.15.4 proved to be the most suitable for this application. It has an excellent battery life which is the most important factor in BAN as the sensors are implanted within the body. We evaluate the performance of slotted CSMA/CA mechanism and GTS mechanism of 802.15.4 standard. The beacon enabled mode is used with broadcast transmissions. We provide an accurate simulation model with respect to the specifications of IEEE 802.15.4 standard. The effect of the different attributes (Superframe order (SO), Beacon order (BO) and Backoff exponent (BE)) on the network performance is analyzed.