VoIP over Wireless LAN Brandon Wilson PI: Alexander L. Wijesinha

Linksys Cable/DSL Router w/ an 8-port switch Cisco Aironet 1200 series Access Point 4 Linksys Wireless-G USB adapters (Soft phones on Windows) 3-Com Wireless PCI adapters on Linux PCs Network configuration

Obtain a basic understanding of the protocol Examine the bandwidth consumption of one VoIP call (wireless to wireless) Research the effectiveness of the back-off algorithm (in the standard) Look closer at the tradeoff between call quality and background traffic throughput Research Objectives

Making a Call

Beacons Frequency 100ms Packet Size = 240 bytes Beacons/sec= 1/.1 = 10/sec (10*240 bytes*8 bits)/10 6 = Mbit / sec

1 Call Theoretical Bandwidth 50 Packets/sec Voice Data  G.711 Total bytes to transmit all voice packets = (100 * 380) + (102 * 158) =54563 bytes/sec Theoretical Bandwidth of one call = (54563*8) / 10 6 = Mbit/sec

FTP Server on the wired LAN and a FTP client on the wireless LAN Measure bandwidth as only TCP packets and corresponding acknowledgement packets TCP Traffic

TCP Alone

TCP with one call

TCP with two calls

Average TCP Bandwidth

UDP Traffic UDP packets generated on wireless LAN by PackETH With each added call, adjust UDP packet delay to retain call quality Measure Bandwidth as only UDP packets and their corresponding acknowledgements (ignore all other traffic)

UDP Only

UDP with one call

UDP with 2 calls

Average UDP Bandwidth

Conclusions TCP The results of the experiments showed that TCP traffic degrades to a significant extent when a VoIP call is added to the network A possible explanation for the decrease is the TCP congestion control mechanism or the access point may be giving priority to the VoIP traffic ~ 0.85 Mbit / sec drop in TCP Bandwidth which is very significant compared to ~ 0.45 Mbit / sec of bandwidth require by a VoIP call

Conclusions cont’d UDP Losses in bandwidth are far greater than the average bandwidth used by a VoIP call and show that the call, with assured voice quality, consumes a much greater portion of the bandwidth than expected 1 call  ~ 1 Mbit / sec bandwidth loss 2 call  ~ 1.5 Mbit / sec bandwidth loss

Future Work Future work will consist of repeating the same experiments on a g network and looking at the effects and behavior of the network when the only variation is the bandwidth is increased by about a factor of 5