# Quality of Service ## What is QoS Quality of Service or QoS refers to the ability of a network to provide different levels of service to different types of traffic. QoS mechanisms allow network administrators to prioritize certain types of traffic over others to ensure that critical or time-sensitive applications receive the bandwidth and low latency they need even when the network is congested. Without QoS all traffic is treated equally and time-sensitive applications like voice calls may suffer poor quality when the network is busy. ## Why QoS is Needed Different types of network traffic have very different requirements. Voice over IP requires low latency, low jitter, and low packet loss. A few milliseconds of delay in a phone call is unacceptable. Video conferencing has similar requirements. File transfers and email are not sensitive to latency and can tolerate delays without affecting the user experience. Without QoS when the network becomes congested all traffic slows down equally. Voice calls become choppy and unintelligible while file transfers just take a bit longer. QoS ensures voice and video traffic is prioritized so they remain functional during congestion. ## QoS Mechanisms Classification identifies different types of traffic so they can be treated differently. Traffic is classified based on protocol, port number, IP address, or DSCP markings in the IP header. Marking sets values in packet headers to indicate the priority level of the traffic. The DSCP field in the IP header and the 802.1p field in Ethernet frames are used for marking. Queuing manages how packets are stored and forwarded when the output link is congested. Priority queuing serves high-priority queues before lower-priority queues. Weighted fair queuing gives different amounts of bandwidth to different traffic classes. Policing limits the rate of traffic to a configured maximum and drops or marks excess traffic. Shaping buffers excess traffic and releases it at a controlled rate rather than dropping it. ## DSCP Marking The Differentiated Services Code Point or DSCP is a 6-bit field in the IP header used to mark packets with their priority class. Different DSCP values represent different traffic classes. Expedited Forwarding is used for voice traffic requiring very low latency. Assured Forwarding classes are used for different levels of business traffic. Best Effort is the default class for traffic with no special priority requirements. By marking packets at the network edge, QoS policies can be consistently applied across the entire network path.