Best Practices
|
March 9, 2023
|
15

# Understanding the Erlang-C Traffic Model Olaf Jacobson The Erlang-C traffic model is a widely used mathematical tool for predicting the performance of call centers, customer service centers, and other service-oriented facilities. The model is named after the Danish mathematician A.K. Erlang, who first introduced it in 1917 to calculate the capacity of telephone networks. Since then, it has evolved into a comprehensive traffic model for analyzing the traffic intensity, queueing behavior, and service level of a call center.

## Assumptions of the Model

The Erlang-C traffic model makes several assumptions about the call center and the customer behavior. These assumptions are crucial for the mathematical calculations used in the model, and they include:

### Poisson Arrival Process

The model assumes that the customer arrival process follows a Poisson distribution, meaning that customers arrive randomly and independently over time. This assumption implies that the rate of customer arrivals is constant, and the number of arrivals during any time interval follows a Poisson distribution.

### Exponential Service Time

The model assumes that the service time required to serve a customer follows an exponential distribution, meaning that the time to complete a service request is random and independently distributed. This assumption implies that the rate of service delivery is constant, and the number of service completions during any time interval follows an exponential distribution.

### Unlimited Queue Capacity

The model assumes that the queue has an unlimited capacity, meaning that there is no limit to the number of customers that can wait in the queue. This assumption is necessary for the mathematical calculations, as it simplifies the model and avoids the need to consider the effects of queue overflow or customer abandonment.

### First-Come-First-Serve (FCFS) Discipline

The model assumes that customers are served in the order of their arrival, following the FCFS discipline. This assumption implies that there is no prioritization or preemption of service requests, and all customers are treated equally in terms of service quality and waiting time.

## Calculations of the Model

The Erlang-C traffic model uses several mathematical calculations to predict the performance metrics of a call center, such as the average waiting time, the average queue length, and the probability of blocking or abandonment. The following are the key calculations used in the model:

### Traffic Intensity

The traffic intensity, denoted as ρ, is the ratio of the arrival rate (λ) to the service rate (μ) of the call center. Mathematically, ρ = λ / μ. The traffic intensity is a critical parameter that determines the behavior of the call center, as it indicates whether the system is underloaded, overloaded, or in a steady state.

### Average Waiting Time

The average waiting time, denoted as W, is the expected time that a customer spends waiting in the queue before being served. The Erlang-C traffic model calculates the average waiting time as W = (ρ^c / c!) * (c / (c * μ - λ)) * (1 - (ρ / c)), where c is the number of servers in the call center.

### Average Queue Length

The average queue length, denoted as Lq, is the expected number of customers waiting in the queue at any given time. The Erlang-C traffic model calculates the average queue length as Lq = (ρ^(c+1) / c!(c * μ - λ)) * (1 - (ρ / (c + 1))).

### Probability of Blocking

The probability of blocking, denoted as P_b, is the probability that a customer is unable to enter the queue due to the queue being full. The Erlang-C traffic model calculates the probability of blocking as P_b = (ρ^c / c!) / (Σ(i=0 to c)(ρ^i / i!)

### Probability of Abandonment

The probability of abandonment, denoted as P_a, is the probability that a customer who enters the queue will abandon before being served. The Erlang-C traffic model calculates the probability of abandonment as P_a = ((c * ρ)^c / c!) / (Σ(i=0 to c-1)((c * ρ)^i / i!) + ((c * ρ)^c / c!)*(c * μ / (c * μ - λ))).

The Erlang-C traffic model offers several advantages over other traffic models for call centers and customer service centers. These advantages include:

### Accuracy

The Erlang-C traffic model is highly accurate in predicting the performance metrics of a call center, such as the average waiting time, the average queue length, and the probability of blocking or abandonment. The model's accuracy is due to the model's assumptions and calculations, which are based on mathematical principles and statistical distributions.

### Simplicity

The Erlang-C traffic model is simple to use and understand, as it requires only a few parameters to make predictions about call center performance. The model's simplicity makes it a popular choice for call center managers and service designers who need to optimize call center operations.

### Flexibility

The Erlang-C traffic model is flexible and can be adapted to different call center environments, including single or multiple server systems, inbound or outbound call centers, and mixed inbound/outbound call centers. The model's flexibility makes it a versatile tool for analyzing call center performance and making informed decisions about call center management.

## Limitations of the Model

The Erlang-C traffic model also has some limitations that must be considered when using it to predict call center performance. These limitations include:

### Unpredictable Customer Behavior

The model assumes that customer arrivals and service times follow statistical distributions, which may not always be accurate in real-world call center environments. Customers may exhibit unpredictable behavior, such as increased call volume during peak periods or longer service times for complex requests, which can affect call center performance.

### Limited Queue Capacity

The model assumes that the queue has an unlimited capacity, which may not always be accurate in real-world call center environments. Limited queue capacity can lead to queue overflow or customer abandonment, which can affect call center performance and customer satisfaction.

### Simplistic Service Discipline

The model assumes that customers are served in the order of their arrival, following the FCFS discipline. This assumption may not always be accurate in real-world call center environments, as some service requests may require prioritization or preemption based on their urgency or importance.

## Conclusion

In summary, the Erlang-C traffic model is a powerful mathematical tool for predicting the performance of call centers and customer service centers. The model's assumptions and calculations are based on mathematical principles and statistical distributions, making it highly accurate, simple to use, and flexible. However, the model has some limitations that must be considered when using it to predict call center performance, such as unpredictable customer behavior, limited queue capacity, and simplistic service discipline. By understanding these limitations, call center managers and service designers can make informed decisions about call center management and optimize call center operations to improve customer satisfaction and business outcomes.

effortless scheduling

## See it to believe it

Choose a better way to organize your team's schedule. Get started for free or schedule a demo and discover what Soon is all about.