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Chapter 11. HomeRF > Media Access Control Basics

11.1. Media Access Control Basics

The HomeRF MAC is designed to handle both asynchronous data and TDMA (Time Division Multiple Access) voice packet transmissions. On the other hand, the point coordination function part of IEEE 802.11 MAC is designed to handle timing-sensitive applications through a polling system. HomeRF can be viewed as a specialized version of IEEE 802.11 wireless Ethernet whose MAC is a combination CSMA/CA of IEEE 802.11 and TDMA of DECT and that uses the FHSS modulation method of IEEE 802.11. To guarantee the periodic delivery of voice packets, a superframe is defined for HomeRF. A frequency hop starts every superframe. Original voice packet transmissions are allocated near the end of the superframe, and retransmission of voice packets is allowed right after the frequency hop at the beginning of the superframe to benefit from the frequency and time diversity. Asynchronous data transmission occurs after a DIFS. The reception of every asynchronous data packet is acknowledged by a short packet within a SIFS. On the other hand, all voice packets are separated by a SIFS. Therefore, asynchronous data transmission can start a DIFS after the frequency hop if there is no retransmission voice packets. Different HomeRF MAC frames are defined for management tasks, asynchronous data transmissions, and TDMA connections. For security purposes, HomeRF uses a different encryption algorithm than the WEP of IEEE 802.11.

11.1.1. Simultaneous TDMA and CSMA/CA

HomeRF provides both data and voice transmission capabilities based on 802.11 FHSS GFSK signaling methods. To maintain constant connections, voice packets need to be delivered periodically with minimal jitter; therefore, a periodic superframe is defined to accommodate the needs of voice connections. The period of the superframe depends on the voice packet size and the throughput of each voice connection. For example, if the voice packet size is 640 bits and the throughput of each voice channel is 32 kbps, the period of the superframe is 640/32 × 103 = 20 ms. Two voice packet time slots are required for each full-duplex voice connection. Because each voice packet occupies a relatively small portion of time, multiple pairs of voice packet slots, corresponding to multiple voice connections, can be located in each superframe. To ensure reliability, retransmission voice slots are also allocated after a frequency hop to avoid an unfavorable transmission environment. After retransmission voice slots, the remaining time of the superframe is available for asynchronous data transmission. Figure 11.1 shows the general structure of a HomeRF superframe.


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