How to Maintain Network Data Rates Using the Committed Information Rate (CIR)

Illustration of an explosion.
Is this within specification?
SOURCE: Public Domain Pictures

You might think it’s a great deal if your internet service provider (ISP) gives you a 100 Mbps line speed, yet the service-level agreement (SLA) speed is 20 Mbps. You may also think the excess line bandwidth from the ISP should make up for any deficiencies. However, this assumption is false. Your network infrastructure will have to process more continuous traffic as data sent can only occur at one speed. In essence, you’ll need more resources for the same performance. Not to fear, for the committed information rate (CIR) is here to save the day!

In this article, I’ll explain what CIR is and how you can implement it in your company. Let’s dive in. 

What Is the Committed Information Rate (CIR)?

Illustration of a computer with a gauge in the red by the side of it.
A need for speed can mean adding limits!
SOURCE: Public Domain Pictures

CIR is a Quality of service (QoS) tool that enables you to limit your network traffic’s average speed at the access point router to meet your ISP’s limited speeds.

You can conduct this QoS in two ways: policing traffic and shaping traffic. Policing traffic involves a strict denial of the data packets presented to your router. On the other hand, shaping traffic works by queuing any data per clock cycle that exceeds the set rate. In short, providing an average speed is in line with your ISP’s commitment, but the data will transfer at the connection speed. In addition, using a policing strategy would deny non-complying data connections and can hinder your company’s performance. To this end, CIR is a logical choice for data governance.

CIR is the average speed limit we’re setting for policing and shaping QoS. Policing drops traffic exceeding the defined CIR speed. On the other hand, shaping is gentle and adds data to a queue or bucket and sends each allotment per clock cycle. In essence, shaping is far more efficient for overall performance as data doesn’t drop. For shaping, you’ll require bucketing as the ISP’s line rate is constant due to the physical process involved in sending data. For instance, you can’t alter the speed of light in fiber optics, but intermittently sending data gives the same overall effect.

So, how can you calculate the committed information rate? Let’s find out.

How to Calculate the Committed Information Rate

To calculate CIR, you’ll first need to understand the terms used in calculating it. You must consider three values: CIR, Bc, and Tc. To calculate CIR, you’ll divide Bc by Tc. Here’s the formula:


So, let’s break it down. Here, CIR is the average traffic speed limit, Bc is the committed burst, and Tc is the specified timing interval. The committed burst is the number of bits, for shaping, or the number of bytes, for policing, deposited in a token bucket during the timing interval. Likewise, the timing interval refers to the number of tokens deposited in the token bucket.

Here’s an example:

A typical usage of this formula involves you calculating Bc, as you often know the limit you need to enforce as well as the clock cycle. To calculate Bc, simply rearrange the above formula:


In our example, let’s assume we need to enforce a CIR of 20 Mbps, and your clock cycle is 0.25 ms. Remember to keep your units consistent, so 0.25 ms equates to 0.00025 s in this case. Next, simply apply the formula.




So, we must cap our committed bursts at 5,000 bits to give us a 20 Mbps connection to meet our SLA. Remember that the above equation defines Bc in bits or bytes.

If you’re still confused, don’t worry. Read on!

Data Sending Analogy

Image of lights overexposed on a highway at night.
Data transmits at the line speed, not the SLA-defined speed!
Source: Adriana Guda on Pexels

This section uses a simple analogy to describe CIR and how to calculate and apply it practically.

Imagine you need to drive your car along a highway with a speed limit of 60 mph on your daily commute to the office. Your vehicle only travels at 100 mph, so police officers stop and caution you every few minutes. This is our token bucket.

Each time this happens, your car is either at 100 mph or, with good brakes, at 0 mph. However, you’ll still average 60 mph over your entire journey. In brief, this is how CIR works—it’s an average rate and not a line rate.

It’s also important to calculate exactly what you need. I’ll go over this in more detail in the next section.

Calculating What You Need

Remember that your ISP can only send at one speed (the line rate). To this end, you buffer data that spills over the clock cycle limit at the router. This limit is the committed burst (Bc) value defined above.

The Bc value is the fixed number of tokens transmitted per clock cycle. For instance, if you calculated it as 1,000, you can only send 1,000 tokens per clock cycle. The excess tokens will go into the bucket. Depending on what you’re discussing, you may be dealing with bits or bytes, so the token and bucket analogy comes in handy.

Overall, CIR enables you to transmit at a line rate but not to send data all the time. It improves efficiency in the long run. As such, and on average, time equates to the required limited value and stops you from unnecessarily overworking your infrastructure.

Now that you know what CIR is, let’s look at a few scenarios where using it might prove beneficial!

Frame Relay Networks and CIR

Network infrastructure isn’t as simple as having one router and a group of computers directly connecting to it. Instead, you often find the mainframe connecting to frame relay networks. These networks physically divide network traffic to help improve data security and network performance. Mainframes contain the perimeter router for the site in a majority of businesses. 

These frame relay networks usually need to optimize data speeds and bandwidths. You can use CIR in this case. Simply use it on every router in your company, including software-defined wide area networks (SD-WANs). In short, you must ensure that every part of your network works at the same CIR.

You can also use CIR in virtual circuits (VCs). Let’s talk about those next.

Virtual Circuits and CIR

Virtual circuits, also known as virtual connections, transfer data using a packet-switching network. You can think of this as running a connection between endpoints through a series of network domains. An example could be broadcasting servers worldwide that you need to use to route a video stream to Fiji. First, a setup call creates a connection between endpoints, followed by data transfer. Although the connection type is different, the same principles apply. In essence, you’ll need to use CIR to limit parts of this network type that exceed a defined traffic limit for an efficient network.

Time for a recap!

Final Thoughts

To conclude, you should consider using the committed information rate (CIR) to limit traffic to match the line your ISP provided. Failing to do this makes your infrastructure work harder, including ad hoc queuing. You can use CIR to satisfy the committed burst requirements without having any surplus tokens. To this end, creating data pulses is key to meeting ISP speed caps. 

Do you have more questions about the committed information rate (CIR)? Check out the FAQ and Resources sections below!


What is an internet service provider (ISP)?

Internet service providers are companies that provide enterprises and private users with internet services. ISPs include the direct infrastructure company or secondary and tertiary companies. Non-primary companies act as intermediary companies and generate profits on the markup.

Should I implement a quality of service strategy in my business?

Quality of service reduces excessive traffic in your network. Therefore, successfully rolling out an effective QoS strategy enables you to reduce your infrastructure requirements. You can also use infrastructure-as-a-service to mitigate the need to address QoS.

How does using TCP optimize my network traffic?

Transmission Control Protocol (TCP) enables you to send data more securely with less packet loss than other network protocols like UDP. It checks and verifies received data packets, which makes it slower than UDP. Overall, TCP is excellent for FTP file transfers and VOIP call services.

What benefits do I receive for moving to the cloud?

Migrating your business operations to the cloud provides you with many benefits. One of them is never needing to invest in infrastructure since you only pay for what you need/use in the cloud. In turn, this gives you the convenience of having all backend admin tasks handled by the solution provider.

When should I use a frame relay network?

Also known as an Intermediate Distribution Frame (IDF), you can use a frame relay network to improve network performance over long distances and create siloed networks. Essentially, doing this creates a physical divide between business teams. Administrators can also add dedicated routing and firewall solutions to help protect siloed teams.


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