Quality of Service is decreased when packet loss, jitter, and latency increase
There are two styles of Quality-of-Service mechanisms to deal with the problem: CoS and QoS
CoS solutions are coarsely grained, best-effort approaches the rely on packet prioritization
CoS solutions are best in situations in which voice traffic represents 30% or less of the total traffic, such as enterprise networks
802.1p and DiffServ are the two most common CoS solutions for Voice over IP
QoS solutions are finely grained, guaranteed -delivery approaches that reserve bandwidth across the network
QoS solutions are best when there is limited bandwidth, or in carrier-grade networks
COPS is a system for centrally storing and maintaining QoS and CoS policies
RSVP is the most common QoS solution for enterprise-grade voice services over IP
Some ISPs offer QoS services to facilitate the use of the Internet and/or VPN for voice applications.
Many small-office and residential broadband routers support 802.1p, and a few even support DiffServ
Linux's kernel-based firewall, Netfilter, can be used as a DiffServ edge router if the Linux Traffic Control components are compiled into the Linux kernel
In Windows NT, 2000, and XP, the QoS Packet Scheduler provides packet prioritization for API-compliant Windows applications such as a softPBX
The Windows RSVP service provides an RSVP policy enforcement point for voice and video software running on Windows servers
pathping is a Windows utility that can help you determine how well WAN call paths support QoS measures