Committed Information Rate (CIR) Calculator
Calculate your network’s Committed Information Rate (CIR) to determine guaranteed bandwidth for your Frame Relay or other WAN connections. Enter your connection details below to get accurate CIR measurements and visualization.
Committed Information Rate (CIR) Results
Comprehensive Guide to Committed Information Rate (CIR) Calculators
The Committed Information Rate (CIR) is a critical parameter in wide area network (WAN) connections that defines the guaranteed bandwidth a service provider commits to deliver. Understanding and calculating CIR is essential for network engineers, IT managers, and businesses that rely on consistent network performance.
What is Committed Information Rate (CIR)?
CIR represents the average rate (in bits per second) at which a Frame Relay, ATM, or MPLS network agrees to transfer data under normal conditions. It’s a fundamental component of service level agreements (SLAs) between customers and network service providers.
The CIR is typically measured over a specific time interval (Tc) and is associated with two key parameters:
- Committed Burst Size (Bc): The maximum amount of data (in bits) that can be transmitted at the access speed during the time interval Tc
- Excess Burst Size (Be): Additional data that may be transmitted beyond Bc during Tc, but isn’t guaranteed
How CIR is Calculated
The basic formula for calculating CIR is:
CIR (bps) = Bc (bits) / Tc (seconds)
Where:
- Bc is the Committed Burst Size in bits
- Tc is the Measurement Interval in seconds
For example, if Bc = 1,000,000 bits and Tc = 1 second, then CIR = 1,000,000 bps or 1 Mbps.
The Relationship Between CIR and Access Speed
The access speed (also called port speed or line rate) is the maximum physical capacity of the connection. CIR is always equal to or less than the access speed. The ratio between CIR and access speed determines how much of the connection’s capacity is guaranteed.
| Access Speed | CIR | Utilization Ratio | Typical Use Case |
|---|---|---|---|
| 1.544 Mbps (T1) | 1.544 Mbps | 100% | Premium dedicated connections |
| 1.544 Mbps (T1) | 768 kbps | 50% | Standard business connections |
| 1.544 Mbps (T1) | 384 kbps | 25% | Budget-conscious implementations |
| 44.736 Mbps (T3) | 10 Mbps | ~22% | Enterprise backbone connections |
Committed Burst Size (Bc) and Excess Burst Size (Be)
The committed burst size (Bc) represents the maximum amount of data that can be transmitted at the access speed during the measurement interval Tc. This allows for temporary bursts of traffic without violating the CIR agreement.
The excess burst size (Be) is additional capacity that may be available but isn’t guaranteed. Traffic within Bc is delivered with high priority, while traffic between Bc and Be may be delivered if network resources are available (this is called the “excess burst” or “burstable” capacity).
The relationship between these values can be expressed as:
Maximum Burst = Bc + Be
Measurement Interval (Tc)
The measurement interval (Tc) is the time period over which the CIR is calculated. It’s typically calculated as:
Tc = Bc / CIR
In practice, Tc is often set to a standard value like 1 second for simplicity, but it can vary based on the service provider’s implementation.
CIR in Different Network Technologies
Frame Relay
In Frame Relay networks, CIR is a fundamental parameter that defines the guaranteed bandwidth. Frame Relay uses the following key parameters:
- CIR: Committed Information Rate
- Bc: Committed Burst Size
- Be: Excess Burst Size
- Tc: Measurement Interval
Frame Relay networks mark frames as Discard Eligible (DE) when they exceed the CIR. During congestion, DE frames are the first to be discarded.
ATM (Asynchronous Transfer Mode)
In ATM networks, CIR is implemented through traffic contracts that include:
- PCR (Peak Cell Rate): Maximum cell rate
- SCR (Sustained Cell Rate): Similar to CIR
- MBS (Maximum Burst Size): Similar to Bc
ATM uses more complex traffic shaping mechanisms but the concept of guaranteed bandwidth remains similar to Frame Relay’s CIR.
MPLS (Multiprotocol Label Switching)
In MPLS networks, CIR is implemented through traffic engineering and QoS (Quality of Service) mechanisms. MPLS can provide:
- Guaranteed bandwidth similar to CIR
- Traffic prioritization
- More flexible bandwidth allocation than traditional Frame Relay
Practical Applications of CIR Calculations
- Network Planning: Determine required bandwidth for new applications or services
- Cost Optimization: Right-size connections to balance performance and cost
- SLA Verification: Ensure service providers are meeting their commitments
- Capacity Planning: Forecast future bandwidth needs based on current utilization
- Troubleshooting: Identify bottlenecks in network performance
Common CIR Values and Their Implications
| CIR Value | Typical Access Speed | Utilization Ratio | Performance Characteristics | Typical Cost |
|---|---|---|---|---|
| 256 kbps | 1.544 Mbps (T1) | 16.6% | Basic email and web browsing | $ |
| 512 kbps | 1.544 Mbps (T1) | 33.2% | Moderate business applications | $$ |
| 768 kbps | 1.544 Mbps (T1) | 50% | Good for most business needs | $$$ |
| 1.544 Mbps | 1.544 Mbps (T1) | 100% | Premium performance, no contention | $$$$ |
| 10 Mbps | 44.736 Mbps (T3) | 22.4% | Enterprise backbone | $$$$$ |
Factors Affecting CIR Performance
- Network Congestion: During peak periods, even traffic within CIR may experience delays if the network is congested
- Service Provider Policies: Some providers may implement more aggressive policing of CIR limits
- Burst Characteristics: Applications with bursty traffic patterns may benefit from proper Bc and Be configuration
- QoS Implementation: Proper Quality of Service configuration can optimize CIR utilization
- Physical Layer Issues: Line quality and error rates can affect actual throughput
Best Practices for CIR Management
- Monitor Utilization: Regularly track your actual bandwidth usage against your CIR
- Right-Size Your Connection: Balance CIR with access speed to meet needs without overpaying
- Understand Your Traffic Patterns: Match CIR to your actual usage patterns, especially burst requirements
- Negotiate SLAs: Work with your provider to get appropriate CIR guarantees
- Implement QoS: Prioritize critical traffic to make best use of your CIR
- Plan for Growth: Regularly review and adjust CIR as your needs evolve
- Consider Redundancy: For critical applications, consider multiple connections with appropriate CIR
CIR vs. Other Bandwidth Metrics
It’s important to understand how CIR relates to other common bandwidth metrics:
- Access Speed: The physical maximum capacity of the connection (always ≥ CIR)
- Peak Information Rate (PIR): The absolute maximum rate, including bursts (similar to access speed)
- Maximum Burst Size (MBS): The largest burst of data that can be sent at line rate
- Sustained Information Rate (SIR): Similar to CIR in some technologies
- Available Bit Rate (ABR): In ATM, a service category that adapts to available bandwidth
Troubleshooting CIR Issues
When experiencing performance issues that might be related to CIR:
- Verify your actual CIR value with your service provider
- Check for Discard Eligible (DE) bits being set on your frames/cells
- Monitor utilization during peak periods
- Test with different traffic patterns (constant vs. bursty)
- Check for physical layer errors that might affect throughput
- Review QoS configurations on your routers
- Consider upgrading your CIR if consistently exceeding limits
The Future of CIR in Modern Networks
While CIR remains an important concept, modern networks are evolving:
- SD-WAN: Software-defined WAN solutions often use different approaches to bandwidth management
- Cloud Connectivity: Direct cloud connections may use different pricing models
- 5G Networks: Wireless technologies are changing how bandwidth is allocated
- Network Function Virtualization (NFV): Virtual network functions can provide more flexible bandwidth management
However, the fundamental concept of guaranteed bandwidth remains important, even as the implementation methods evolve.