The Asynchronous Transfer Mode (ATM) protocol is a connection-oriented protocol that is ideal for voice, video and data communications. ATM is a high-speed networking technology that transmits data in cells of a fixed length. ATM is a native connection-oriented transport protocol. It is composed of a number of related technologies including software, hardware, and connection-oriented media. A cell is a fixed-length packet containing 53 bytes of information. Since the number of bytes — and consequently the transit time — of the cell is constant, the cells can be switched at a constant interval.

An ATM endpoint establishes a connection or virtual circuit prior to sending any data on the network. It then sends cells along this path toward the destination. This virtual circuit is a direct path from one endpoint to another. While establishing the connection, the ATM endpoint also negotiates a Quality of Service contract for the transmission. This contract spells out the bandwidth, maximum delay, acceptable variance, and other parameters the virtual circuit (VC) provides, and this contract extends from one endpoint to the other. Since the virtual circuit is connection-oriented, the data arrives at the receiving end in proper order and with the specified service levels. ATM is an excellent compromise for the transmission of both voice and data on a network. ATM provides a guaranteed Quality of Service on a LAN, a WAN, and a public internetwork.

Figure B.9 illustrates the connection-oriented miniport driver architecture as implemented in ATM.


Figure B.9 ATM Using Connection-Oriented Miniports

ATM is supported by Windows 2000 architecture with the following components. This diagram is used for:

  • LANE (LAN Emulation)

  • IP over ATM

  • PPP over ATM

  • Native ATM through Winsock 2.0

For more information about ATM, see "Asynchronous Transfer Mode" in the Windows   2000Internetworking Guide .

LANE    LAN Emulation (LANE) is a method by which other protocols (not just TCP/IP) that only understand connectionless media can communicate over ATM. It allows ATM to utilize both legacy networks and applications. Traditional LAN-aware applications and protocols can communicate over an ATM network without modification.

LANE consists of two primary components: the LANE client (Atmlane.sys) and the LANE services. The LANE client allows LAN protocols and LAN-aware applications to function as if they were communicating with a traditional LAN. The LANE client communicates LAN commands to the network protocols and native ATM commands to the ATM protocol layer. The LANE services are a group of ATM components, usually on a switch that supports LAN emulation.

IP Over ATM    IP over ATM is a means of carrying IP packets over an ATM network. IP over ATM uses the connection-oriented properties of ATM to overcome the connectionless nature of IP. It functions in a manner similar to LANE. A central IP server (called an ATMARP server) maintains a database of IP and ATM addresses, and provides configuration and broadcast services. IP over ATM is a group of components that doesn't reside in one place. The services are not usually on an ATM switch. IP over ATM server services are provided with Windows 2000 and can reside on a Windows 2000 server.

IP over ATM is a small layer between the ATM protocol and the TCP/IP protocols. The client emulates standard IP to the TCP/IP protocol at its top edge and uses native ATM commands to the ATM protocol layers underneath.

IP over ATM is handled by two primary components: the IP over ATM server (Atmarps.sys) and the IP over ATM client (Atmarpc.sys). The IP over ATM server is composed of an ATMARP server and Multicast Address Resolution Service (MARS). The ATMARP server provides services that emulate standard IP functions, while MARS provides broadcast and multicast services. Both services maintain IP address databases.

ATM Over xDSL    Digital Subscriber Line (xDSL) technology is a means by which plain old telephone service (POTS) can be used to send ATM cells over a pair of copper wires to the central station of a phone company. ATM over xDSL offers high-speed network access from the home and small office environment. Several standards are being developed in these areas, including asymmetric digital subscriber line (ADSL) and universal ADSL (UADSL). These technologies use the local loop, the copper wires that connect the local central office in a user's neighborhood to the customer's phone jack. In many areas, this local loop connects directly to an ATM core network run by a telephone company.

ATM over xDSL service preserves the high-speed characteristics and QoS guarantees available in the core ATM network without changing protocols. This creates the potential for an end-to-end ATM network to the residence or small office.

Point-to-Point Protocol (PPP) over this end-to-end architecture adds functionality and usefulness. PPP allows necessary features such as authentication, encryption and compression. To support these architectures (such as residential broadband, PPP over ATM), Windows 2000 has additional components. Ndttsp.tsp is a TAPI service provider that allows NDIS proxy to interface with call control through TAPI. Ndproxy.sys provides call control over connection-oriented media.

Native ATM Access Through Winsock 2.0    Applications can directly use Winsock 2.0 to gain access the ATM protocols natively. Applications that using native ATM can access QoS guarantees such as bandwidth, and latency.

For more information about ATM, see "Asynchronous Transfer Mode" in the Windows   2000 Internetworking Guide .