The principal conclusions of the study were:
- Overall, the one-way listening speech quality provided by the combination of Microsoft’s client and USB handset was consistently better than that provided by Cisco’s IP phones and Call Manager, whether using G.711 or G.729.
- More detailed analysis showed that the mean transmission bandwidth of the Microsoft solution typically fell between 24kbps and 80kbps, which are the bit-rates of the Cisco solution with G.711 and G.729 operation respectively. The peak bandwidth of the Microsoft solution was 100kbps for some IP conditions.
- In general, the maximum recommended value for round-trip delay is approximately 400ms, i.e. a one-way delay of 200ms. The delays observed for both Clients with low IP impairment conditions are within, or close to, this budget, although it should be noted that control of the delay budget within VoIP networks remains essential.