ABR (Available Bit Rate)
Available Bit Rate (ABR) is a traffic contract type used in Asynchronous Transfer Mode (ATM) networks. It is one of the four traffic contract types supported by ATM technology, with the other three being Constant Bit Rate (CBR), Variable Bit Rate (VBR), and Unspecified Bit Rate (UBR). ABR is a connection-oriented service, which means that it requires the establishment of a virtual circuit between the source and destination endpoints before the data transfer can occur.
The ABR service is designed to support bursty and variable bit rate traffic, such as data and multimedia applications, where the traffic load can vary over time. It provides a quality of service (QoS) guarantee to the user by ensuring that a certain minimum bandwidth is always available for the transfer of data, while also ensuring that the network resources are utilized efficiently.
ABR uses a feedback-based mechanism to regulate the flow of data between the source and destination endpoints, based on the network conditions. This feedback mechanism allows the source endpoint to adjust its data transmission rate based on the network congestion, which helps to avoid network congestion and improve overall network performance.
The ABR service is widely used in LAN, WAN, and MAN networks, where it provides an efficient and reliable way to transport data over the network. In this article, we will discuss the key features of ABR, including its service model, congestion control mechanism, and performance characteristics.
ABR Service Model
The ABR service model is designed to support bursty and variable bit rate traffic, where the traffic load can vary over time. The ABR service provides a QoS guarantee to the user by ensuring that a certain minimum bandwidth is always available for the transfer of data, while also ensuring that the network resources are utilized efficiently.
The ABR service model defines two parameters, the minimum cell rate (MCR) and the sustainable cell rate (SCR), which are used to control the flow of data between the source and destination endpoints. The MCR parameter specifies the minimum bandwidth that is guaranteed to the user, while the SCR parameter specifies the maximum bandwidth that can be used by the user.
The ABR service also defines a third parameter, the peak cell rate (PCR), which is used to specify the maximum rate at which the source endpoint can transmit data. The PCR parameter is used to prevent the source endpoint from flooding the network with data, which could lead to network congestion.
The ABR service model is connection-oriented, which means that it requires the establishment of a virtual circuit between the source and destination endpoints before the data transfer can occur. The virtual circuit is established using the signaling protocol, which is responsible for setting up and tearing down the connection.
Congestion Control Mechanism
The ABR service uses a feedback-based mechanism to regulate the flow of data between the source and destination endpoints, based on the network conditions. The feedback mechanism allows the source endpoint to adjust its data transmission rate based on the network congestion, which helps to avoid network congestion and improve overall network performance.
The feedback mechanism is based on the use of explicit rate feedback (ERF) and forward explicit congestion notification (FECN). ERF is used by the destination endpoint to inform the source endpoint of the available network bandwidth, while FECN is used by the network to inform the source endpoint of the network congestion.
When the source endpoint receives ERF from the destination endpoint, it adjusts its data transmission rate based on the available network bandwidth. If the available network bandwidth is greater than the MCR parameter, the source endpoint can increase its data transmission rate up to the PCR parameter. If the available network bandwidth is less than the MCR parameter, the source endpoint reduces its data transmission rate to the MCR parameter.
When the source endpoint receives FECN from the network, it reduces its data transmission rate to avoid contributing to network congestion. This helps to prevent network congestion from occurring in the first place, which is important for maintaining good network performance.
Performance Characteristics
The ABR service provides a number of performance characteristics that make it suitable for transporting bursty and variable bit rate traffic over the network. These performance characteristics include low delay, low delay variation, and high throughput.
Low Delay: The ABR service provides low delay for data transfer, which is important for real-time applications such as voice and video. The feedback-based mechanism used by ABR allows the source endpoint to adjust its data transmission rate based on the available network bandwidth, which helps to ensure that the data is delivered in a timely manner.
Low Delay Variation: The ABR service also provides low delay variation for data transfer, which is important for real-time applications. Delay variation, also known as jitter, can cause problems for real-time applications such as voice and video, which require a consistent and predictable delay. The feedback-based mechanism used by ABR helps to minimize delay variation by ensuring that the data is transmitted at a consistent rate.
High Throughput: The ABR service also provides high throughput for data transfer, which is important for applications that require a large amount of data to be transferred quickly. The feedback-based mechanism used by ABR helps to ensure that the network resources are utilized efficiently, which allows for a high throughput for data transfer.
Conclusion
In conclusion, Available Bit Rate (ABR) is a traffic contract type used in Asynchronous Transfer Mode (ATM) networks. ABR is designed to support bursty and variable bit rate traffic, such as data and multimedia applications, where the traffic load can vary over time. ABR provides a QoS guarantee to the user by ensuring that a certain minimum bandwidth is always available for the transfer of data, while also ensuring that the network resources are utilized efficiently.
ABR uses a feedback-based mechanism to regulate the flow of data between the source and destination endpoints, based on the network conditions. The feedback mechanism allows the source endpoint to adjust its data transmission rate based on the network congestion, which helps to avoid network congestion and improve overall network performance.
The ABR service provides a number of performance characteristics that make it suitable for transporting bursty and variable bit rate traffic over the network. These performance characteristics include low delay, low delay variation, and high throughput. Overall, ABR is a reliable and efficient way to transport data over the network, and it is widely used in LAN, WAN, and MAN networks.