With both ICG3 cards connected to the 3845 router simulating calls at the central office, the router didn't break a sweat, taking a mere 30 percent CPU utilization. Call quality was excellent, with a PSQM score of 0 (the range is 0 to 6.5, with 0 the highest and 4 intelligible) and an MOS of 4.6 (on a scale of 1 to 5, with 5 being the highest).
The same scores were achieved when I connected one ICG3 to the 3845 and the other to the 2821 and turned on the Shunra Storm for network emulation. At 100-Mbps and T3 speeds, there was still plenty of overhead to handle up to 180 simultaneous calls, with more than four calls per second.
Shrink the Pipes
Things got more interesting when I simulated a T1 at 1,544 Kbps. As the pipe got smaller, I needed to reduce the number of simultaneous calls from 180 to 24, and then to 20. With TCP overhead, bandwidth per call ranged from 64 to 80 Kbps. When the T1 link was saturated with calls and other traffic, call quality was reduced to minimal levels. Dropping to 20 calls gave me good PSQM and MOS scores.
The quality of the calls improved more when I applied QoS from the GUI. I launched Cisco's SDM 2.0 over SSL using Internet Explorer with Java 1.4.2 support. Once SDM was up, I selected the appropriate WAN interface to apply QoS with the help of a wizard. QoS consists of a low-latency queuing service for traffic classes, including real time (voice, video, signaling), critical applications (transactional, management, routing) and best effort (e-mail, HTTP). I took the default.
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