d Arrival Rates As shown in Table 3.10, voice frames are typically 92 bytes long and arrive every 20 millisec- onds (the frame rate is determined by the syllabic rate). Video frames are 1464 bytes long and arrive every 40 milliseconds (assuming a 25 frame per second video rate). A 92-byte voice packet arriving every 20 milliseconds implies a voice data rate of 36.8 kbits/second (92 X 8 X 50). A 1464-byte video packet arriving every 40 milliseconds implies a video data rate of 292.5 kbits/second. Table 3.10 Voice frame lengths, video frame lengths and arrival rates Voice frames 92 bytes Every 20 milliseconds Video frames 1464 bytes Every 40 milliseconds (25 fps) Data frames 1500 bytes Fast data frames 3000 bytes Making Telecoms Work A combined voice and video call would have a combined data rate of 329 kbits per second. This is, however, the rate to support unidirectional voice and video. Although people do not (generally) speak at the same time, the MAC layer has to provision bidirectional periodic bandwidth (known as voice and video transmission opportunities) SO the bandwidth occupancy is effectively doubled to 73.6 kbits/second to support a bidirectional voice call, 585 kbits per second to support two way video and 658 kbits per second to support two-way voice and video. This suggests a capacity of 13 voice channels per Mbit, 1.7 video channels per Mbit or 1.5 voice and video channels per Mbit (2-way video calls). 3.6.2 Data Throughput - Distance and MAC Overheads Data throughput is dependent on the modulation used and channel coding. In 802.11 a and g, between 48 and 54 Mbps of gross data rate is available if 64 QAM is used in a lightly coded (3/4) channel, but this is dependent on having a strong C/I (carrier to interference ratio). As the C/I worsens, the gross data rate reduces to 23/36 Mbps (16 QAM), then 12-18 Mbps (QPSK), then 6-9 Mbps (BPSK) and the channel coding overhead increases from 3/4 to 1/2 (one error protection bit for each data bit). This is shown in Table 3.11. Although there are more channe