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What Is ATM?

Updated: March 28, 2003


What Is ATM?

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The cost of maintaining separate, specialized networks for computer, voice, and video is high. To reduce networking costs, ATM enables integration of all of these services on a single network and the combination of existing networks into a single infrastructure. In particular, Windows operating systems provide rich connectivity using Asynchronous Transfer Mode (ATM) while maintaining support for legacy systems.

To support ATM, Network Driver Interface Specification (NDIS) has been updated with ATM commands. Because many applications do not yet use ATM services, Windows Server 2003 includes support for LAN Emulation (LANE) for LAN applications, such as Ethernet. Similarly, IP over ATM support has been added, eliminating the additional header cost of LAN packets. Winsock 2.0 native ATM has also been added to support the many applications that use Windows Sockets (Winsock).

ATM Overview

ATM is a connection-oriented, unreliable (does not acknowledge the receipt of cells sent), virtual circuit packet switching technology. Unlike most connectionless networking protocols, ATM is a deterministic networking system — it provides predictable, guaranteed quality of service. From end to end, every component in an ATM network provides a high level of control. ATM technology includes:

  • Scalable performance. ATM can send data across a network quickly and accurately, regardless of the size of the network. ATM works well on both very low and very high-speed media.

  • Flexible, guaranteed Quality of Service (QoS). ATM allows the accuracy and speed of data transfer to be specified by the client. This feature distinguishes ATM from other high-speed LAN technologies such as gigabit Ethernet. The QoS feature of ATM also supports time dependent (or isochronous) traffic. Traffic management at the hardware level ensures that quality service exists end-to-end. Each virtual circuit in an ATM network is unaffected by traffic on other virtual circuits. Small packet size and a simple header structure ensure that switching is done quickly and that delays due to high traffic are minimized.

  • Unobstructed speed. ATM imposes no architectural speed limitations. Its pre-negotiated virtual circuits, fixed-length cells, message segmentation and re-assembly in hardware, and hardware-level switching all help support extremely fast forwarding of data.

  • Integration of different traffic types. ATM supports integration of voice, video, and data services on a single network. ATM over Asymmetric Digital Subscriber Line (ADSL) enables residential access to these services.

Connection Types: LAN vs. ATM

Traditional LANs, such as Ethernet and Token Ring, use a connectionless, unreliable approach, that cannot guarantee successful transmission when sending information across the network. Likewise, TCP/IP data transfers between networks are connectionless and unreliable. ATM, which is a connection-oriented, circuit-based technology, differs from the traditional approaches to networking.

Traditional LAN

In a traditional LAN, each client uses a network adapter card, which has a software driver. Above that driver is a protocol driver, such as TCP/IP. The protocol driver bundles information into frames of varying size and gives each bundle an appropriate header. As a result, when the adapter gains access to the media, the data packets are sent on the media to a destination hardware address. Traditional LAN technologies do not guarantee that data arrives on time or in the proper order. While Ethernet and Token Ring can detect errors, they provide no service guarantees and are not responsible for the recovery of missing or corrupted data packets.

Because the end stations are joined by a common medium, each end station on the traditional LAN recognizes the frames, or packets, of data put on the wire by each of the others, regardless of whether the frame is passed sequentially from one station to the next (as in a ring topology) or broadcast to all stations simultaneously (as with Ethernet). Each station has an adapter card, which processes the frame and examines the destination address. If the address applies to that computer, the frame is checked for errors. If there are no errors, the adapter initiates a hardware interrupt and passes the frame to the network adapter driver. The following figure, Traditional LAN: Connectionless Data Transmittal of a Packet, shows an example of a traditional LAN.

Traditional LAN: Connectionless Data Transmittal of a Packet

Connectionless Data Transmittal of a Packet

Because a traditional LAN is connectionless, it cannot provide mechanisms that can guarantee successful transmission. For example, it cannot determine the status of the destination adapter to ensure that it can receive a frame. It cannot ensure that bandwidth is available throughout the transmission. Unanticipated blockage, due to the media access control scheme of shared access technologies, can hinder a traditional LAN technology from supporting time-sensitive applications such as video or voice traffic. Traditional LANs can use upper-level protocol drivers to verify packet transmission (retransmitting, if necessary), partition big messages into smaller ones, and use time stamps for synchronization. However, these services add time to the transmission, and none of them provides end-to-end QoS guarantees.

Traditional Internetworking

If the destination address is remote rather than local, the chances of transmission failure increase. If a router on an Ethernet network detects a broadcast meant for another network, the router accepts the packet and passes it on using TCP/IP. A TCP/IP datagram is packet-switched to its destination individually. The header of each contains a globally significant switching address. This address allows a routing decision to be made each time the packet is forwarded, and packets to the same destination might follow completely different paths to get there, jumping over networks that use different underlying technology. No connection is required, but no delivery is guaranteed. The following figure, Two Packets Taking Different Routes Through a Traditional LAN, shows an example of two packets taking different routes through a traditional LAN.

Two Packets Taking Different Routes Through a Traditional LAN

Two Packets Taking Different Routes Through a LAN

Like an Ethernet data transfer, a routed data transfer cannot offer guarantees because bandwidth is never reserved ahead of time. The packets being sent over TCP/IP are simply transmitted on the wire and routed. While this allows flexibility in routing around obstructions, network performance can vary a great deal depending on conditions at the routers and on the amount of network traffic.

ATM Networks

In contrast to connectionless transmission protocols, ATM is connection-oriented. An ATM endpoint establishes a defined path known as a virtual channel (VC), also called virtual circuit, to the destination endpoint prior to sending any data on the network. It then sends a series of same-size frames, called cells, along the virtual channel towards the destination.

While establishing the connection, the ATM endpoint also negotiates a QoS contract for the virtual channel. The QoS contract spells out the bandwidth, maximum transit delay, acceptable variance in the transit delay, and so forth, that the VC provides, and this contract extends from one endpoint to the other through all of the intermediate ATM switches.

The path of ATM traffic is established at the outset, and the switching hardware merely needs to examine a simple header to identify the proper path. Beyond specifying a path, ATM allows a location to establish a full duplex connection (traffic travels in both directions) with multiple locations at the same time. Note, however, that ATM is an unreliable transmission protocol because it does not acknowledge the receipt of cells sent. As with LANs, missing or corrupted information must be detected and corrected by upper-layer protocols.

The following figure, “ATM Virtual Channel and Packet Transmission,” illustrates ATM virtual channel and packet transmission.

ATM Virtual Channel and Packet Transmission

ATM Virtual Channel and Packet Transmission

Network Speed

Unlike Ethernet networks, ATM has no inherent speed limit, and its efficiency is not affected by the distance that the data has to travel. In addition, ATM establishes the pathway for a particular series of packets at the outset and ATM switches make minimal switching decisions thereafter. To travel across the ATM network, data is segmented into same-size cells, and encapsulated with a header that contains information about switching, congestion, and error-checking.

Cells are transmitted in order, and the ATM network uses Virtual Path Identifier and Virtual Channel Identifier (VPI/VCI) numbers in the ATM header to forward them efficiently. A switch reads the header, compares the VPI/VCI to its switching table to determine the correct output port and new VPI/VCI, and then forwards the cell. All the addressing information that the ATM switch needs is contained in the header and is always found in the same place. This makes the forwarding task simple to implement in hardware by, reducing latency. Moreover, with ATM from end to end, there is no data translation required if a packet must travel from a LAN through a WAN to reach a destination LAN. The following figure, “ATM Fixed-Length Cells,” shows two ATM end stations sending fixed-length cells from A to B (although ATM traffic is bi-directional).

ATM Fixed-Length Cells

ATM Fixed-Length Cells

Because ATM uses small (53-byte), fixed-length cells that require less logic to process, the network spends no time determining where a particular cell begins and ends. The small cell size ensures that delays in forwarding cells are minimized. Because the cell size is so predictable, buffer usage and analysis algorithms can be simplified and optimized.

Traditional LAN technologies, such as Ethernet, have inherent speed limitations Either the underlying infrastructure (the cable) or the segment length must be changed to support fast traffic. However, unlike Ethernet and Token Ring, ATM has no such imposed limitations. If you can invent a faster physical layer — if you can design a quicker method of transmitting data from one place to another over one wire or many wires — ATM can work over that physical layer and at those new speeds. In addition, ATM allows information with different requirements and from different nodes to be transmitted nearly simultaneously without conflict.

ATM places fixed-length cells on the media when the data is produced according to the parameters of a negotiated connection. ATM can simultaneously handle the needs of isochronous (time-dependent) traffic, such as voice and video, and non-isochronous traffic, such as LAN data.

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