Understanding in QoS

For a VoIP network to operate successfully, the voice traffic must have priority ove the data traffic as it traverses its way form one end of the network to the other.

There are many enemies of your VoIP traffic

• Lack of bandwidth
• Delay - Jitter
• Packet Loss

QoS is designed to keep voice traffic running smoothly during temporary moments of congestion
on the network.

Bandwidth requirements

• End-to-end delay = 150ms or less
• Jitter = 30ms or less
• Packet Loss = 1% or less

QoS mechanisms

• Best effort - This is the model every network uses by default. All traffic is treated on  FIFO basis.
• Integrated Services (IntServ) - It it only model that provides 'Guaranteed' bandwidth but it has scalability issues becauses each router must track every single traffic flow. If enough reservations are made, the network simply runs out of bandwidth
• Differentiated Services (DiffServ) - Most popular and flexible model to use for impletmenting QoS. It provides that a Per-Hop-Behavior, Traffic conditioning and SLA

References
http://middleware.tistory.com/entry/IntServ-%EB%B0%8F-DiffServ
Cisco Official Cert Guide