Steve Brown from Network Instruments, a relatively new blogger has a post describing common VoIP myths. He only has 3 though, so I'll add a couple which I'll continue with number 4...
#4) VoIP's main purpose is to save on long-distance charges. It's all about the almighty dollar. (or yen, pound, rupee, Euro, etc.)
While that may be true for residential VoIP, for business VoIP there are many more reasons, including converged applications, lower TCO due to easier moves/adds/changes, simple 3-4 digit dialing between multiple sites, and more.
#5) VoIP calls have poorer voice quality than traditional landline calls.
With proper QoS in place, VoIP calls can actually have SUPERIOR voice quality than traditional landlines since landlines use G.711 which maxes out at 64kbps and is usually limited to 300 Hz to 3400 Hz frequency range. This limitation explains why traditional telephony speech sounds weak, unnatural, and lacks a certain "crispness". Using wideband VoIP codecs on the other hand along with a wider frequency spectrum results in a cleaner, crisper, and more natural sounding speech.
#4) VoIP's main purpose is to save on long-distance charges. It's all about the almighty dollar. (or yen, pound, rupee, Euro, etc.)
While that may be true for residential VoIP, for business VoIP there are many more reasons, including converged applications, lower TCO due to easier moves/adds/changes, simple 3-4 digit dialing between multiple sites, and more.
#5) VoIP calls have poorer voice quality than traditional landline calls.
With proper QoS in place, VoIP calls can actually have SUPERIOR voice quality than traditional landlines since landlines use G.711 which maxes out at 64kbps and is usually limited to 300 Hz to 3400 Hz frequency range. This limitation explains why traditional telephony speech sounds weak, unnatural, and lacks a certain "crispness". Using wideband VoIP codecs on the other hand along with a wider frequency spectrum results in a cleaner, crisper, and more natural sounding speech.
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#6) VoIP calls consume fixed bandwidth.
Bandwidth of a voip call is not fixed as in PSTN. There are factors such as codec, packet-time and silence suppression which affects the bandwidth of voip calls. Different codecs have different bandwidths (g711 64 kbps, g729 8 kbps, g923 5.3-6.3 kbps). Packet-time of voip call affects the number of packet-headers transmitted (10 msec packet-time means 100 packet-headers per second while 40 msec packet-time means 25 packet-headers). In a normal call, 40-60% of the time the party is in listening-mode means silent. So, if silence suppression is enabled 40-60% less bandwidth is consumed. There are still other factors which affects the bandwidth of voip call, such as underlying network (ethernet, ATM, etc.)
Myth # 7) If you implement QoS, all your VoIP worries are over.
QoS is critical, but it's not sufficient. You have to have a way to understand the user experience talking on the phone. Metrics like delay-to-dial tone, quality statistics for all calls and the like are some of the metrics you must have to be successful.