ATM (asynchronous transfer mode) is a dedicated-connection switching technology

ATM (asynchronous transfer mode) is a dedicated-connection switching technology that organizes digital data into 53-byte cell units and transmits them over a physical medium using digital signal technology. Individually, a cell is processed asynchronously relative to other related cells and is queued before being multiplexed over the transmission path.

Because ATM is designed to be easily implemented by hardware (rather than software), faster processing and switch speeds are possible. The prespecified bit rates are either 155.520 Mbps or 622.080 Mbps. Speeds on ATM networks can reach 10 Gbps. Along with Synchronous Optical Network (SONET) and several other technologies, ATM is a key component of broadband ISDN (BISDN).

The protocol for classical IP over ATM (sometimes abbreviated as CLIP/ATM) is a well-established standard spelled out in RFC 1577 and subsequent documents. Windows 2000 provides a full implementation of this standard.

The IP over ATM approach provides several attractive advantages over ELAN solutions. The most obvious advantages are its ability to support QoS interfaces, its lower overhead (as it requires no MAC header), and its lack of a frame size limit. All of these features are discussed in the following sections.
IP over ATM Architecture

IP over ATM is a group of components that do not necessarily reside in one place, and, in this case, the services are not usually on an ATM switch. In some cases, switch vendors provide some IP over ATM support, but not always. (For the purposes of this discussion, it is assumed the IP over ATM server services reside on a Windows 2000 server.)

The core components required for IP over ATM are roughly the same as those required for LANE, as both approaches require the mapping of a connectionless medium to a connection-oriented medium, and vice versa. In IP over ATM, these services are provided by an IP ATMARP server for each IP subnet. This server maintains a database of IP and ATM, and provides configuration and broadcast services, as described in the following section.
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IP over ATM Components

IP over ATM is a very small layer between the ATM protocol and the TCP/IP protocol. As with LANE, the client emulates standard IP to the TCP/IP protocol at its top edge while simultaneously issuing native ATM commands to the ATM protocol layers underneath.

IP over ATM is often preferred to LANE because it is faster than LANE. One key reason for this performance advantage is that IP over ATM adds almost no additional header information to packets as they are handed down the stack. Once it has established a connection, the IP over ATM client can generally transfer data without modification.

As with LANE, IP over ATM is handled by two main components: the IP over ATM server and the IP over ATM client. The IP over ATM server is composed of an ATMARP server and Multicast Address Resolution Service (MARS). The ATMARP server provides services to map network layer IP unicast addresses to ATM addresses, while MARS provides similar services for broadcast and multicast addresses. Both services maintain IP address databases just as LANE services do.

The IP over ATM server can reside on more than one computer, but the ATMARP and MARS databases cannot be distributed. You can have one IP over ATM server handle ATMARP traffic, and one handle MARS. If, however, you divided the ATMARP Server between servers, it would effectively create two different IP networks. All IP over ATM clients in the same logical IP subnet (LIS) need to be configured to use the same ATMARP server. Traditional routing methods are used to route between logical IP subnets, even if they are on the same physical network.

Windows 2000 includes fully integrated ATMARP and MARS servers. These services are described in more detail in the following sections.
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ATMARP Server

The IP over ATM client and ATMARP server go through a process similar to the LANE client and the LECS when a client joins the network and discovers other network members. As with LANE, once an address is found, native ATM takes over and TCP/IP packets are sent across a VC from endstation to endstation. There is, however, a major difference in how the IP over ATM client discovers the ATMARP server.
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ATMARP Server Discovery

Because the ATMARP server usually resides on a server rather than on an ATM switch, it is not possible to use ILMI or a well-known VC to discover its address. In fact, there is no default IP over ATM mechanism for server discovery. To start using IP over ATM, an administrator must find the ATM address of the appropriate ATMARP server and manually configure each IP over ATM client with this address. In a single ATM switch network, this is not much of a problem, but in larger networks it can become a demanding job. To ease configuration in smaller networks, Windows 2000 ATM ARP/MARS services and ATM ARP/MARS clients use a default address. For more information about deployment issues, see Windows 2000 Server Help.