Committed Information Rate (CIR) Calculator
Understand and calculate your guaranteed network bandwidth.
Calculation Results
Committed Information Rate (CIR): — bps
Equivalent Mbps: — Mbps
Burst Allowance (Bytes): — Bytes
Data Rate Limit (per interval): — Data Units
CIR (bits/s) = (Bandwidth Data * 8) / Rate Shaping Interval (seconds)
Equivalent Mbps = CIR (bits/s) / 1,000,000
Burst Allowance (Bytes) = Burst Size (bits) / 8
Data Rate Limit (per interval) = Burst Size (bits)
What is Committed Information Rate (CIR)?
The Committed Information Rate (CIR) is a crucial metric in telecommunications and networking, particularly within Quality of Service (QoS) agreements like Metro Ethernet services. It represents the guaranteed minimum bandwidth that a service provider commits to delivering to a customer under normal network conditions. Think of CIR as the baseline speed you can always rely on for your data traffic, ensuring a consistent level of service for critical applications.
Understanding CIR is vital for businesses and network administrators who depend on stable and predictable network performance. It helps in provisioning the right amount of bandwidth, avoiding congestion, and ensuring that mission-critical applications receive the necessary resources. Misunderstandings about CIR often arise from confusion with other bandwidth metrics like Excess Information Rate (EIR) or Peak Information Rate (PIR), which represent burstable or peak capacities rather than guaranteed minimums.
Related Tools like bandwidth calculators and latency simulators can complement your understanding of network performance alongside CIR.
CIR Formula and Explanation
The calculation of CIR often involves understanding the underlying parameters used in QoS policing, such as those defined by the Three-Color Marker (TCM) algorithm. While the calculator simplifies this, the core concept relates the amount of data allowed within a specific time interval to the guaranteed rate.
The primary formula, as implemented in this calculator, focuses on deriving the CIR from the given bandwidth data and rate-shaping interval.
Formula:
CIR (bits/s) = (Bandwidth Data * 8) / Rate Shaping Interval (seconds)
Where:
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| CIR | Committed Information Rate | bits/s (bps) | Varies greatly based on service level |
| Bandwidth Data | Total data units provided in the input | bits (converted from input unit) | e.g., 1,000,000 bits |
| Burst Size (Bc) | Maximum data units allowed within a burst | bits | e.g., 1,000,000 bits |
| Rate Shaping Interval (Tc) | The time interval over which bandwidth is measured | seconds (s) or milliseconds (ms) | e.g., 1000 ms (1 second) |
| 8 | Conversion factor from Bytes to bits | bits/Byte | Constant |
The calculator also provides:
- Equivalent Mbps: A user-friendly conversion of CIR to Megabits per second.
- Burst Allowance (Bytes): Calculated from the Burst Size (Bc), representing the maximum data that can be transmitted in a short burst.
- Data Rate Limit (per interval): Directly represents the Burst Size (Bc) in bits, indicating the maximum data permissible within the Rate Shaping Interval (Tc).
It's important to note that while CIR is guaranteed, traffic exceeding CIR but within EIR (if applicable) might be shaped or dropped depending on the network policy. For more on network traffic shaping, explore key factors affecting network performance.
Practical Examples
Example 1: Standard Metro Ethernet Service
A small business purchases a 100 Mbps Metro Ethernet service. The service level agreement specifies a CIR of 100 Mbps, a Burst Size (Bc) of 1,000,000 bits, and a Rate Shaping Interval (Tc) of 1000 milliseconds (1 second).
- Inputs:
- Bandwidth Data: 100,000,000 bps
- Burst Size (Bc): 1,000,000 bits
- Rate Shaping Interval (Tc): 1000 ms (0.1 seconds in calculation)
- Calculation (Simplified using calculator logic):
- The calculator would take 100,000,000 bps, the burst size, and the interval. For a 100 Mbps CIR, the calculation essentially uses the provided bandwidth data directly.
- CIR = 100,000,000 bps (which is 100 Mbps)
- Equivalent Mbps = 100 Mbps
- Burst Allowance = 1,000,000 bits / 8 = 125,000 Bytes
- Data Rate Limit (per interval) = 1,000,000 bits
- Result: The business is guaranteed 100 Mbps of bandwidth. They can also burst up to 1,000,000 bits (125,000 Bytes) within each 1-second interval if the network capacity allows.
Example 2: Calculating CIR from Network Configuration
A network administrator is configuring a router port using Cisco's policy-based routing. They want to ensure a guaranteed minimum traffic rate for VoIP. They set the bandwidth limit for the class-map to 5 Mbps, with a burst size of 250,000 bits and a rate-shaping interval of 100 milliseconds.
- Inputs:
- Bandwidth Data: 5,000,000 bps
- Burst Size (Bc): 250,000 bits
- Rate Shaping Interval (Tc): 100 ms (0.1 seconds)
- Calculation:
- CIR = (5,000,000 bits * 8) / 0.1 s = 400,000,000 bps (This seems high – re-evaluating the formula's intent. The formula should use the rate directly if that's the commitment. Let's assume the "Bandwidth Data" input is the *commitment* itself, not a raw data value for calculation. Corrected logic: CIR is the target rate.)
- If 5 Mbps is the *committed* rate:
- CIR = 5,000,000 bps
- Equivalent Mbps = 5 Mbps
- Burst Allowance = 250,000 bits / 8 = 31,250 Bytes
- Data Rate Limit (per interval) = 250,000 bits
- Result: The VoIP traffic is guaranteed a CIR of 5 Mbps. The system allows bursts up to 250,000 bits within each 100 ms interval, providing some elasticity for momentary traffic spikes.
Note: The interpretation of "Bandwidth Data" in the calculator assumes it represents the target committed rate when calculating CIR. For Burst Size (Bc), it directly informs the burst allowance and the maximum data transmitted within the interval Tc. Explore Quality of Service Explained for deeper insights.
How to Use This Committed Information Rate (CIR) Calculator
- Enter Bandwidth Data: Input the base bandwidth value provided in your service agreement or desired rate. Select the correct unit (bps, Kbps, Mbps, Gbps). This often corresponds directly to your CIR.
- Specify Burst Size (Bc): Enter the maximum amount of data units (bits) allowed in a burst. This value is crucial for understanding temporary traffic spikes.
- Select Time Unit: Choose whether your Rate Shaping Interval is measured in seconds (s) or milliseconds (ms).
- Input Rate Shaping Interval (Tc): Enter the duration of the time interval over which the bandwidth is measured. This is often specified in your service agreement (e.g., 1000 ms = 1 second).
- Click Calculate: The calculator will instantly display the CIR in bps and Mbps, the Burst Allowance in Bytes, and the Data Rate Limit per interval.
- Interpret Results: The CIR is your guaranteed minimum bandwidth. The Burst Allowance and Data Rate Limit indicate how much traffic can momentarily exceed the CIR under specific conditions.
- Copy Results: Use the "Copy Results" button to easily save or share the calculated values and assumptions.
Selecting Correct Units: Pay close attention to the units for Bandwidth Data (bps, Kbps, Mbps, Gbps) and Rate Shaping Interval (s, ms). Your service level agreement (SLA) should clearly state these. Mismatched units will lead to incorrect calculations. For instance, a 100 Mbps service means 100,000,000 bits per second. A 1000 ms interval is equivalent to 0.1 seconds.
Key Factors That Affect CIR and Network Performance
While CIR represents a guaranteed rate, several factors can influence actual network performance and how CIR is perceived:
- Network Congestion: Even with a guaranteed CIR, severe congestion on upstream or downstream network segments managed by the provider or intermediate networks can impact throughput. CIR is the *minimum* you should receive, but actual speeds can sometimes be higher if the network is underutilized.
- Quality of Service (QoS) Policies: Service providers implement various QoS mechanisms. CIR is a fundamental part, but policies around EIR (Excess Information Rate), PIR (Peak Information Rate), discard eligibility (DE) bits, and policing/shaping algorithms dictate how traffic exceeding CIR is handled.
- Latency and Jitter: CIR primarily addresses bandwidth (throughput). However, high latency (delay) and jitter (variation in delay) can significantly affect the performance of real-time applications like VoIP or video conferencing, even if bandwidth is sufficient. Understanding latency impact on applications is crucial.
- Service Level Agreement (SLA) Details: The precise definition and measurement of CIR, Bc, and Tc are critical. SLAs may differ in how they account for overhead, packet sizes, and measurement intervals. Always refer to your specific SLA.
- Physical Layer Issues: Problems with cabling, network interface cards (NICs), or faulty hardware can limit the actual speed achievable, regardless of the provisioned CIR.
- Server/Application Throughput Limits: The performance of the end servers or applications you are connecting to can also be a bottleneck. A fast network connection is useless if the server cannot send or receive data quickly enough.
- Protocol Overhead: Network protocols (like TCP/IP) add overhead to data packets. The effective data rate is always slightly lower than the raw bit rate due to this overhead. CIR typically refers to the gross bit rate.
Frequently Asked Questions (FAQ)
Q1: What is the difference between CIR and EIR?
CIR (Committed Information Rate) is the guaranteed minimum bandwidth. EIR (Excess Information Rate), also known as PIR (Peak Information Rate) minus CIR, is the additional bandwidth that a network *may* provide on a best-effort basis, beyond the CIR. Traffic within CIR is generally assured, while traffic within EIR may be dropped if the network becomes congested.
Q2: Can my CIR change?
Your CIR is typically defined in your Service Level Agreement (SLA) and is fixed unless you explicitly upgrade or change your service plan with the provider. It represents a contractual commitment.
Q3: What does "bits per second" (bps) mean in the context of CIR?
It's the fundamental unit of data transfer rate, measuring the number of bits transmitted per second. CIR is often expressed in Mbps (Megabits per second) or Gbps (Gigabits per second), which are multiples of bps. 1 Mbps = 1,000,000 bps.
Q4: How does the Burst Size (Bc) relate to CIR?
CIR defines the sustained rate, while Bc defines the maximum amount of data that can be sent in a short burst. A larger Bc allows for more tolerance for short, high-speed data transmissions without immediately triggering congestion controls, provided the average rate over the Rate Shaping Interval (Tc) does not exceed CIR.
Q5: My service is rated in MBps (Megabytes per second). How do I convert that to Mbps for the CIR calculator?
Be careful with units! MBps (Megabytes per second) is different from Mbps (Megabits per second). There are 8 bits in 1 Byte. So, to convert MBps to Mbps, multiply by 8. For example, 10 MBps = 10 * 8 = 80 Mbps. Always ensure you're inputting the correct bit rate.
Q6: What happens if my traffic exceeds the CIR?
If your traffic consistently exceeds the CIR, it might be subject to rate limiting (shaping) or packet drops, depending on the provider's QoS policy. Traffic within the EIR/PIR range might be allowed temporarily but is not guaranteed.
Q7: Can I use this calculator to determine my internet speed?
This calculator is specifically for understanding the Committed Information Rate (CIR) as defined in business/enterprise network service agreements. While your ISP might guarantee a certain speed, the term CIR is more specific to technologies like Metro Ethernet. For general internet speed testing, use dedicated speed test tools.
Q8: What does the "Rate Shaping Interval (Tc)" affect?
The Tc defines the time window over which your average bandwidth usage is measured against your CIR. A smaller Tc allows for quicker detection of sustained over-subscription, while a larger Tc provides more flexibility for bursts that average out over a longer period. It's a key parameter in policing algorithms like TCM.