Quality of Service (QoS)

Quality of Service (QoS) refers to the set of techniques and mechanisms used to manage network resources by prioritizing certain types of traffic over others. QoS ensures that critical applications receive the bandwidth and latency they require.

QoS Mechanisms

Classification identifies and marks traffic for special handling. Queuing places packets in different queues based on priority. Scheduling determines the order packets are transmitted. Shaping rates traffic to conform to specified limits. Policing drops or marks traffic that exceeds limits.

DiffServ and DSCP

Differentiated Services (DiffServ) uses the Type of Service (ToS) field in IP headers and DSCP (Differentiated Services Code Point) values to classify packets. Common Per-Hop Behaviors (PHBs) include Expedited Forwarding (EF) for low-latency traffic and Assured Forwarding (AF) for guaranteed delivery.

IEEE 802.1p CoS

At Layer 2, IEEE 802.1p uses the Priority Code Point (PCP) field in VLAN tags to mark frames for QoS treatment on switches.

Queuing Algorithms

Common queuing methods include FIFO (First In First Out), Priority Queuing (PQ), Class-Based Weighted Fair Queuing (CBWFQ), and Low Latency Queuing (LLQ) that prioritizes real-time traffic like VoIP.

End-to-End QoS

Effective QoS requires consistent implementation across all network devices from edge to edge, including traffic classification at the network boundary, queuing in the core, and congestion avoidance techniques.