DSCP TOS CoS Precedence Conversion Chart

In networking, DSCP, TOS, CoS, and Precedence are terms related to Quality of Service (QoS), which is used to prioritize certain types of traffic to ensure the performance of critical applications over a network. Below you can find DSCP TOS CoS Precedence Conversion Charts.

What is ToS vs CoS vs DSCP vs Precedence?

Don’t mix up TOS (Type of Service) and COS (Class of Service). TOS is found in the header of an IP packet (layer 3), and COS is found in the header of 802.1Q (layer 2) and is used for Quality of Service on layer 2 switching.

• DSCP (Differentiated Services Code Point): This is part of the IP header and uses 6 bits to classify traffic. DSCP values can be used to provide different levels of service to network traffic.
• TOS (Type of Service): Originally defined in the IP header to specify the priority and request a route for high throughput, low delay, and high reliability. It has evolved over time and is now largely replaced by DSCP.
• CoS (Class of Service): This is a 3-bit field found in an Ethernet frame’s VLAN tag (802.1Q). It’s used to prioritize traffic on LANs, especially for trunk links between switches.
• Precedence: This is part of the older TOS field in the IP header and uses the first 3 bits of the TOS byte. It was used to define the priority of the packet, with higher values indicating more important packets.

Here’s a bit more detail on how they work:

• IP Precedence: The first 3 bits of the TOS byte were used to define precedence. The higher the value, the more important the IP packet is. In case of congestion, routers would drop low-priority packets first.
• Type of Service Bits: The next bits (after the precedence bits) in the TOS byte were intended to specify the type of service, such as delay, throughput, and reliability. However, these bits were not widely used and have been superseded by DSCP.
• DSCP Values: DSCP replaced the TOS byte with a more granular and flexible system. It allows for 64 different traffic classes, which can be used to ensure that critical traffic gets the bandwidth and low-latency treatment it needs.

In summary, these terms are all about prioritizing traffic to ensure that the most important data gets through the network efficiently, even when the network is congested. They are essential for networks that carry a mix of voice, video, and data traffic, where some types of traffic need to be prioritized over others to maintain quality. Below is a DSCP TOS CoS and Precedence Conversion Chart.

DSCP to ToS conversion table

DSCP Usage best practices

Commonly used DSCP values and typical applications:

1. Best Effort (0): This is the default DSCP value and is used for standard, non-critical traffic that doesn’t require special handling. Most Traffic.
2. Expedited Forwarding (EF – 46): This value is used for low-latency, low-loss, and low-jitter traffic, such as voice over IP (VoIP) and Video Conferencing.
3. Assured Forwarding (AF): These values provide different levels of assurance for delivery. Apply these to Business Applications:
   • AF11 (10): Low priority with low drop probability.
   • AF12 (12): Low priority with medium drop probability.
   • AF13 (14): Low priority with high drop probability.
   • AF21 (18): Medium priority with low drop probability.
   • AF22 (20): Medium priority with medium drop probability.
   • AF23 (22): Medium priority with high drop probability.
   • AF31 (26): High priority with low drop probability.
   • AF32 (28): High priority with medium drop probability.
   • AF33 (30): High priority with high drop probability.
   • AF41 (34): Highest priority with low drop probability.
   • AF42 (36): Highest priority with medium drop probability.
   • AF43 (38): Highest priority with high drop probability.
4. Class Selector (CS): These values are used for Backward Compatibility with IP precedence:
   • CS1 (8): Typically used for background traffic.
   • CS2 (16): Used for standard traffic.
   • CS3 (24): Used for critical applications.
   • CS4 (32): Used for real-time applications.
   • CS5 (40): Used for signaling.
   • CS6 (48): Used for network control.
   • CS7 (56): Used for network control with the highest priority.

What is the value of CoS in DSCP?

The value of Class of Service (CoS) in relation to Differentiated Services Code Point (DSCP) comes from how these two Quality of Service (QoS) tools work together to provide consistent traffic prioritization across networks. CoS is defined at Layer 2 of the OSI model, using the 802.1Q VLAN tag and its 3-bit Priority Code Point (PCP) field to classify Ethernet traffic into eight priority levels.

DSCP, by contrast, operates at Layer 3 in the IP header, where its 6-bit field supports up to 64 traffic classes for more control of packet handling. By mapping CoS values to DSCP values, network engineers can maintain QoS policies consistently as traffic moves from the LAN to routed WAN or internet environments. Consistency is essential for real-time applications such as VoIP, video conferencing, and low-latency services, because end-to-end traffic prioritization directly affects performance and user experience.

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