What is Asynchronous Transfer Mode (ATM)?

Asynchronous Transfer Mode (ATM) is a high-speed networking technology used to transfer data, voice, and video over a network. It uses fixed-length packets called cells, each of which is 53 bytes in size. The first 5 bytes of each cell contain control information, while the remaining 48 bytes contain user data. The control information is used to route the cell through the network and to ensure that it is delivered to the correct destination.

ATM networks can transmit data at speeds ranging from 155 Mbps to 10 Gbps and its technology is widely used in telecommunications networks, particularly for long-distance and high-bandwidth applications.

Dissecting Asynchronous Transfer Mode

Asynchronous Transfer Mode (ATM) was developed in the late 1980s and early 1990s to fulfill the high-bandwidth needs of multimedia applications, which were beyond the capabilities of other contemporary technologies like frame relay and circuit-switched networks.

Originally established as a telecommunications network standard, it aimed to offer a unified platform for transmitting voice, video, and data across various network topologies.

Numerous large corporations implemented ATM-based networks in the early 1990s. Despite its promise and potential for high-bandwidth multimedia applications, ATM was ultimately superseded by more cost-effective, more scalable and less complex technologies such as Ethernet and IP-based networks. As a result, many companies opted to transition away from ATM and adopt these alternative solutions for their networking needs.

Key Aspects of Asynchronous Transfer Mode

The fundamental elements of ATM include the ATM network architecture, ATM switching, and ATM traffic management.

The ATM network architecture encompasses the physical and logical structure of an ATM network, along with the interconnection of its different network components. ATM switching refers to the process of transferring data between ATM network devices, while ATM traffic management is devoted to optimizing network performance, alleviating congestion, and upholding a reliable quality of service.

ATM Network Architecture

ATM utilizes a three-layer architecture to facilitate the transfer of data over high-speed networks.

• Physical Layer - First layer of the ATM network architecture and is responsible for transmitting data over the physical medium, which can be either electrical or optical signals. This layer ensures that data is transmitted efficiently and reliably over the physical medium.
• ATM Layer - Responsible for the segmentation and reassembly of data into 53-byte fixed-length cells. ATM cells are used for transmission as they allow for efficient routing and transmission of data. By breaking down data into small, fixed-length cells, ATM Layer reduces the delay caused by packetization and other factors that may cause jitter, delay variation, or packet loss.
• ATM Adaptation Layer (AAL) - Sits above the ATM Layer. The AAL is responsible for adapting different types of traffic, such as voice, video, and data, into ATM cells. It ensures that the transmission is optimized for the type of traffic being transmitted. This layer adapts different types of traffic to ATM cells, ensuring that the transmission is optimized for the type of traffic being transmitted.

The ATM cell structure is comprised of a 5-byte header and a 48-byte payload. The header contains information such as the virtual path identifier (VPI) and virtual circuit identifier (VCI), which are used for routing the cell through the network.

ATM Switching

ATM Switching routes ATM cells through switches using Virtual Path (VP) and Virtual Circuit (VC) connections and is responsible for forwarding ATM cells from one network element to another. The switch examines the VPI and VCI of each cell to determine the appropriate path through the network.

VP and VC connections are used to establish a logical path between the sender and receiver. A VP connection is used to create a logical path between two points in the network. A VC connection is used to create a specific connection between two endpoints. The use of VP and VC connections allows for efficient routing of ATM cells through the network.

ATM Traffic Management

ATM Traffic Management is responsible for regulating the flow of ATM cells through the network. It ensures that the network is not congested and that the quality of service is maintained.

There are three main functions of ATM Traffic Management: Connection Admission Control (CAC), Traffic Shaping, and Congestion Control.

• CAC - controls the number of connections established in the network. It determines whether there is enough bandwidth available to establish a new connection.
• Traffic Shaping - Regulates the flow of traffic. It ensures that the transmission rate of ATM cells is within the limits set by the network.
• Congestion Control - prevents network congestion. It ensures that the network is not overwhelmed with traffic, which can lead to dropped cells and poor quality of service.

ATM-based networks continue to be used in certain applications, particularly in some specialized industries such as aviation and defense. Its legacy also lives on in other networking technologies, with some of its concepts and principles having been incorporated into modern network protocols such as MPLS (Multiprotocol Label Switching).

Asynchronous Transfer Mode Services

ATM Services refer to the types of services that can be offered using the Asynchronous Transfer Mode (ATM) technology. ATM supports four types of service categories:

1. Constant Bit Rate (CBR) - Provides a fixed bandwidth for constant bit rate applications with low delay and jitter, making it ideal for real-time applications such as voice and video. In the telecommunications industry, CBR is commonly used in applications such as voice-over-IP (VoIP) and video conferencing.
2. Variable Bit Rate (VBR) - Offers a flexible bandwidth for applications that require a variable bit rate depending on the amount of data being transmitted, making it ideal for multimedia applications in the entertainment industry that can tolerate some degree of delay or jitter, such as compressed video.
3. Available Bit Rate (ABR) - Provides variable bandwidth for applications that require occasional bursts of data, and does not guarantee a fixed amount of bandwidth. This makes it suitable for financial applications where bandwidth requirements can fluctuate based on demand. ABR is also used in applications that require a variable amount of bandwidth, such as compressed video or multimedia applications, and can tolerate some degree of delay or jitter.
4. Unspecified Bit Rate (UBR) - Is a best-effort service that does not provide any QoS guarantees. UBR is used in non-critical applications, such as email and web browsing, where delay and packet loss can be tolerated, and no specific QoS guarantees are required.

Each of these service categories has its own unique characteristics and requirements, which make it suitable for certain applications and not for others.