ATM (Asynchronous Transfer Mode)

Asynchronous Transfer Mode (ATM) is one of the primary Layer 2 WAN transport protocols. Originally developed as high-speed public WAN transport for voice, video, and data, ATM was later modified to include transport over private networks by the ATM Forum.

ATM faces the same problems as the other Layer 2 transport protocols, namely the establishing connections and the formatting information. ATM was developed before the availability of quality of service (QoS) and traffic engineering. At the time of its development, ATM was one of the only protocols capable of providing differentiated service for customers or traffic types.

ATM Devices
ATM networks consist of ATM switches and ATM endpoints. ATM switches are responsible for moving cells through the ATM network. ATM endpoints�' including workstations, routers, data service units (DSUs), LAN switches, and video coder-decoders (codecs)�'all require an ATM interface adapter to access the ATM network.

ATM Network Interfaces
ATM networks consist of ATM switches interconnected by point-to-point ATM links. The links connecting the switches come in two forms, user-network interfaces (UNIs), which connect ATM endpoints to ATM switches, and network- node interfaces (NNTs), which connect ATM switches.

UNIs and NNIs are further classified by the type of network that the switch resides in (public or private). This figure shows examples of several interfaces.

ATM Features
ATM implements two features that make it both useful and interesting (well, interesting in a network geek kind of way). Anyway, the two features are asynchronous transmission and fixed cell size.

Asynchronous Transmission
The asynchronous part of ATM refers to the ability of the protocol to use a more efficient version of time-division multiplexing (TDM). Multiplexing is combining multiple data streams onto a single physical or logical connection. Time-division means that each data stream has an assigned slot in a repeating sequence. Although this process is more efficient than giving each data stream its own physical connection, it still has some inefficiencies, as shown in this figure.

With standard TDM, each end station essentially owns a time slot, preventing any other end station from using it, even when it is idle.

With standard TDM, each end station essentially owns a time slot, preventing any other end station from using it, even when it is idle.

With asynchronous TDM, each end station has a primary time slot where it has priority when it has data to send; however, other end stations can use it when the time slot is idle.

Fixed Cell Size
ATM delivers information in fixed-size units called cells. Every ATM cell, regardless of the type of information (voice, video, data), is exactly 53 bytes (or octets) with 48 bytes of information and 5 bytes of header (overhead) information. This arrangement is different from that of other protocols that can increase the cell size and actually increase the header or overhead traffic on a network. However two distinct advantages in using fixed cell sizes outweigh the cost of the additional overhead.

The main advantage is that the serialization delay or the delay of processing cells is deterministic (regular and predictable), which aids in the management of the network. This feature also makes ATM better suited for voice and video traffic, which is time sensitive. With fixed-length cells, there is no danger of a voice cell getting “stuck in traffic” behind a large data cell.

ATM Connections
ATM is a connection-based service that uses two primary types of circuits, permanent virtual circuits (PVCs) and switched virtual circuits (SVCs). ATM is also capable of using a connectionless service, but it is relatively rare.

A connection-based service means that the network must request, establish, and confirm a connection before it sends any user information (such as voice, video, or data).

PVCs typically make direct connections between sites. Similar to a leased line, the connection remains open even when idle. PVCs guarantee availability, but you must configure them manually.

SVCs are set up and torn down dynamically. The connection is only open while the network actively sends information. SVCs use a signaling protocol between the ATM endpoint and the ATM switch to establish a dynamic connection. SVCs are more flexible than PVCs but require additional overhead. An SVC is similar to a phone call.

Virtual Connection
As previously mentioned, ATM is a connection-based protocol. The network establishes ATM connections using a combination of virtual paths and virtual channels.

An ATM connection actually consists of a number of virtual channels, which can take different physical paths through the network. ATM then bundles virtual channels into logical groups called virtual paths. Several virtual paths can also be bundled into a larger logical grouping called a transmission path. All of this bundling is transparent to the user. This figure illustrates this concept.

LAN Emulation
LAN emulation (LANE) is a standard that allows end stations connected via an ATM network to communicate as if they were connected across an Ethernet or Token Ring LAN, but at a much faster speed.

LANE uses LAN emulation clients (LEC) and LAN.