Title - An Introduction to the ATM Protocol


Asynchronous Transfer Mode or ATM, is a communications technology primarily used in the backbone of high speed networks. It supports real-time voice and video as well as data establishing connections between the two endpoints. These connections may establish guarantees a quality of service (QoS) for that data transmission. However, unlike telephone switches that dedicate circuits end to end, unused bandwidth in ATM's logical circuits can be utilized whenever available. For example, idle bandwidth in a videoconference circuit can be used to transfer data.

ATM works by transmitting all traffic as fixed-length units called cells which are 53-bytes long. This fixed unit allows very fast switches to be built, because the processing associated with variable-length packets is eliminated (finding the end of the frame). The small ATM cells also ensure that voice and video can be intermixed because there are no long delays encountered because of large packets.

ATM transfers information in fixed-size. Each cell consists of 53 octets, or bytes. The first 5 bytes contain cell-header information, and the remaining 48 contain the "payload" (user information). Figure 1 illustrates the basic format of an ATM cell.


Figure 1. The ATM cell


The ATM standard actually consists of many different aspects required to effectively transmit information and manage the network. The various aspects of the standard include establishing connections, Quality of Service (QoS), User-to-Network Interface (UNI), Network-to-Network Interface (NNI), Management, and Interface Diagnostics.

ATM Reference Model

Computer users want to transmit messages and in order to do that, they need multiple layers of communications protocols. For example, users wanting to browse the web use TC/IP. These packets are carried over Ethernet and somewhere in the network, these packets may get converted into ATM cells. The ATM reference model, shown in Figure 2, is composed of the following OSI layers:

  • Physical layer --- Analogous to the physical layer of the OSI reference model, the ATM physical layer manages the medium-dependent transmission.

  • ATM layer --- Combined with the ATM adaptation layer, the ATM layer is roughly analogous to the data-link layer of the OSI reference model. The ATM layer is responsible for establishing connections and passing cells through the ATM network. To do this, it uses information in the header of each ATM cell.

  • ATM Adaptation Layer (AAL) --- Combined with the ATM layer, the AAL is roughly analogous to the data-link layer of the OSI model. The AAL is responsible for isolating higher-layer protocols from the details of the ATM processes.

Figure 2. Comparing the OSI reference model with the ATM reference model


For a more complete explanation of the OSI layers, please see our seminar Communications Layers.


The ATM physical layer performs the following functions: Bits are converted into cells; the transmission and receipt of bits on the physical medium are controlled; ATM cell boundaries are tracked; and cells are packaged into the appropriate type of frame for the physical medium.


The ATM Adaptation Layer (AAL) performs the function of taking packets from the upper layer protocols such as Ethernet or TCP/IP and segmenting them. There are actually several different ways larger packets may be segmented into the smaller ATM cells. The selection of the proper AAL is dependent on the physical links being used and the type of data being transmitted.


For more information on the process used to segment larger packets into cells and then reassemble the cells back into packets, please see the seminar ATM Adaptation Layers.

ATM Networks

An ATM network consists of a set of ATM switches interconnected by point-to-point ATM links or interfaces. ATM switches support two primary types of interfaces: user-network interface (UNI) and network-node interface (NNI). The UNI connects ATM end-systems (such as hosts and routers) to an ATM switch. The NNI connects two ATM switches.


Depending on whether the switch is owned and located at the customer's premises or publicly owned and operated by the telephone company, UNI and NNI can be further subdivided into public and private UNIs and NNIs. A private UNI connects an ATM endpoint and a private ATM switch. Its public counterpart connects an ATM endpoint or private switch to a public switch. A private NNI connects two ATM switches within the same private organization. A public one connects two ATM switches within the same public organization.


An additional specification, the Broadband Interexchange Carrier Interconnect (B-ICI), connects two public switches from different service providers. Figure 3 illustrates the ATM interface specifications for private and public networks.

Figure 3. ATM interface specifications differ for private and public networks

ATM Cell-Header Format

An ATM cell header can be one of two formats: UNI, or the NNI. The UNI header is used for communication between ATM endpoints and ATM switches in private ATM networks. The NNI header is used for communication between ATM switches. Figure 3 depicts the basic ATM cell format, the ATM UNI cell-header format, and the ATM NNI cell-header format.


Unlike the UNI, the NNI header does not include the Generic Flow Control (GFC) field. Additionally, the NNI header has a Virtual Path Identifier (VPI) field that occupies the first 12 bits, allowing for larger trunks between public ATM switches.


Figure 4. Two forms of ATM cells, UNI and NNI Formats

ATM Cell-Header Fields

In: addition to GFC and VPI header fields, several others are used in ATM cell-header fields. The following descriptions summarize the ATM cell-header fields illustrated in Figure 4:

  • Generic Flow Control (GFC) --- Provides local functions, such as identifying multiple stations that share a single ATM interface. This field is typically not used and is set to its default value.

  • Virtual Path Identifier (VPI) --- In conjunction with the VCI, identifies the next destination of a cell as it passes through a series of ATM switches on the way to its destination.

  • Virtual Channel Identifier (VCI) --- In conjunction with the VPI, identifies the next destination of a cell as it passes through a series of ATM switches on the way to its destination.

  • Payload Type (PT) --- Indicates in the first bit whether the cell contains user data or control data. If the cell contains user data, the second bit indicates congestion, and the third bit indicates whether the cell is the last in a series of cells that represent a single AAL5 frame.

  • Congestion Loss Priority (CLP) --- Indicates whether the cell should be discarded if it encounters extreme congestion as it moves through the network. If the CLP bit equals 1, the cell should be discarded in preference to cells with the CLP bit equal to zero.

  • Header Error Control (HEC) --- Calculates checksum only on the header itself.

More Information

The ATMForum has specifications available at: www.atmforum.com.


Additional ATM seminars:

ATM Connection Management - A description of how connections are established, Quality of Service (QoS), and Virtual Connections (PVCs and SVCs).

The ATM Adaptation Layers (AAL) - Large data packets are required to be segmented into the smaller ATM cells and later the ATM cells are reassembled back into packets. There are several different AALs that can be used based on the type of data being transported.


In Summary:

  • ATM is a layer 2 protocol that and able to transport an upper layer protocol (Layer 3-4).

  • ATM is connection oriented and can support Quality of Service (QoS).

  • There are several deferent types of ATM cells (NNI, UNI).


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