ACQUIRING AND USING ASYNCHRONOUS TRANSFER MODE IN THE WORKPLACE
Asynchronous Transfer Mode (ATM) is a revolutionary new data
communications technology that utilizes a high-speed switching
protocol to quickly move cells (each containing 53 bytes) between
two or more distinct points in a network.  ATM uses cell
switching for the high-speed transmission of data over a variety
of subnetwork technologies; the media used and transmission rates
are not a function of ATM.  This bulletin assists users in
selecting and implementing ATM technology and enables users to
properly assimilate ATM products into the workplace.

Introduction
ATM works by accomplishing two simple functions:  cell-switching
(in hardware) and path (next hop) determination (virtual
connection management done primarily in software).  ATM offers
the capability of high-speed data transmission with minimum
delay, because only these two functions can reside at the network
node, eliminating the overhead of additional functionality.  The
connection which is created, called a virtual circuit (VC), is
similar in concept to a telephone connection, which remains in
effect until the connection is terminated.  Each VC is
characterized by a virtual circuit identifier (VCI), which
distinguishes one virtual circuit from another.  This concept is
similar to distinguishing one telephone conversation from another
on the same trunk line.

The ATM Switch
Users most often implement ATM connections via a device called an
ATM switch.  An ATM switch can handle multiple bit streams, in
the same way that a large telephone switch handles multiple
conversations from different points.  A bit stream is assigned to
a port (access point on the ATM switch), either statically or
dynamically.  Each port has a port identifier, which is known to
the ATM switch.  Different ports may support different data
rates, depending on the physical medium to which the port is
attached.  There is always a pairing of input port and output
port.  Each port can handle multiple conversations (VCs);
different conversations are identified by their VCI values. 
Semantically, a conversation may be a particular information
type, such as voice or video, or have another meaning of interest
to an application.

ATM Network
There are two kinds of ATM devices:  ATM switches and ATM access
devices.  A network using ATM consists of one or more ATM
switches, which connect different ATM access devices, where ATM
Adaptation Layer (AAL) functionality (and possibly "higher level"
functionality) is implemented.  (AAL enhances ATM services for
distinct classes of applications as necessary.)  ATM access
devices send ATM cells that can be processed by the ATM switches. 
The term "ATM network" is used in this bulletin to denote a
network using ATM as the underlying communications technology.  

Since the ATM network is a subnetwork, the AAL functions are
terminated at the ATM network boundaries, and so the AAL
functions are not necessarily end-to-end.  Thus a transport
protocol is necessary to have full end-to-end interoperability. 
For example, a Transmission Control Protocol/Internet Protocol
(TCP/IP) application may use an ATM network (including AAL
services) to provide this end-to-end interoperability. 
Accordingly, the "higher layers" mentioned above denote various
services or applications that may use the AAL.  Examples of these
are a connectionless service, a local area network (LAN)
application, a broadband ISDN application, or a user-defined or
proprietary application.   

ATM Interfaces
There are two levels of ATM interfaces:  a network-to-network
interface (NNI) and a user-to-network interface (UNI).  AAL
functionality, as well as "higher layer" functionality (e.g,
routing, transport, and application functionality), may be
implemented at the UNI.  At the UNI, many terminals can share a
given ATM access.  Different services are integrated at the UNI,
and each service may be associated with one dedicated terminal or
a multimedia terminal.  The interconnected terminals may form a
customer premises network (CPN).  

When a shared medium is used for ATM access, there is a need to
perform contention resolution and bandwidth sharing.  There are
many methods of implementing the UNI.  The UNI is contained in
ATM access devices.  The NNI governs the interface between
different ATM switches; it is more complex than the UNI and is
yet to be found in many switches.  Different switches may have
differing requirements; the NNI must reconcile these differences.

Features of ATM
ATM data is designed to be transported over high-quality digital
lines with minimal cell errors; it is a "fast packet" service. 
ATM is an ideal transmission mode for bursty traffic prevalent in
today's data networks.  It provides bandwidth (capacity) on
demand and high bandwidth to every host.  ATM reduces network
response time, allows efficient network usage, and provides for
scaling to very large or very fast networks.  

ATM can accommodate both a wide range of transmission rates (each
rate corresponding to a different information type, such as
voice, video, etc.) and unpredictable traffic patterns.  It can
support narrowband as well as broadband uses.  ATM connections
can be point-to-point or point-to-multipoint.  Small cell sizes
minimize delay, and cell sequencing means that cells are
delivered in order.  ATM eliminates the need for separate voice,
video, or data networks.  It provides circuit emulation
(simulating a normal circuit) and allows flexible networks to be
built using virtual path (VP) connections.

ATM can support a unified addressing capability among these
systems.  It can carry data, audio, or full-motion video.  ATM
can be public or private, and can be included in wired or
wireless systems.  Total throughput is increased by adding more
ATM switches and more links at higher rates.  The simplicity of
both the algorithm for forwarding ATM cells and the constant cell
size leads to efficient hardware implementation.  ATM is based on
a network of switches and dedicated links to each host, so that
the aggregate bandwidth of an ATM network increases as hosts are
added.  Data transfer by a large number of hosts can occur in an
ATM network.  Since there is a large amount of bandwidth on an
ATM network, a user should be able to send information on demand
with minimal delay.  

ATM Products
In chronological order, ATM product offerings are likely to be:

  -  ATM local area networks (LANs)
  -  campus ATM (connecting LANs)
  -  ATM wide area networks (WANs)
  -  broadband ISDN and cell-relay services offered by
     subscription from public carriers.  

ATM LAN products exist now, but from only a few vendors.  An ATM
LAN comprises one or more ATM switches, dedicated high-speed
circuits from each switch to networking elements, and the
networking elements (such as workstations) themselves.   All of
the equipment is owned by a single organization, the distances
involved are small, and the ATM switch may "broadcast" a cell to
multiple destinations.  ATM LANs differ from traditional LANs
(e.g., Ethernet) in that they do not use shared media.  In
addition, ATM switches can be directly connected to LAN servers,
LAN switches, or Ethernet hosts.  ATM LANs can be used for
client-server computing involving workstations and servers. 
Inhibitors to deployment are incompatibilities between ATM and
NICs (network interface cards) for PCs and workstations, and the
lack of development of a widespread user market for ATM LANs. 
These factors may set widespread deployment of ATM LAN systems
back several years.  

Since campus ATM systems interconnect ATM LANs, hubs, bridges,
and routers throughout a campus, fewer campus ATM products exist
than LAN ATM products.  Widespread campus ATM products may also
not be available for several years.  

ATM in the WAN refers to carrier service offerings based on
switching ATM cells from a network-entry access location to a
remote-destination access location.  The address information
could be connection-oriented or connectionless.  Widespread ATM
WAN product offerings are a number of years away.  The inhibitors
to deployment include economic feasibility and a possible
shortage of applications which could benefit from ATM WAN
technology.  

The introduction of widespread broadband ISDN and cell-relay
services will not occur until near the end of the decade, and
will await the evolution of broadband ISDN and the stabilization
of access rates offered by all public carriers.  In addition,
signalling concerns and concerns over the variety of physical
media may delay introduction of ATM on public carriers. 

Acquiring ATM Products
Users must make decisions on a number of important issues when
procuring and installing ATM products.  The choices that users
make will affect the operation of the ATM product for them. 
Following is a list of concerns and the factors to be 
considered for each concern.  

ATM Access
The characteristics of ATM access are important to users because
user applications have specific transfer requirements and need to
know what capabilities to request of an ATM network through the
UNI, including type and number of connections.  In most cases,
users will not procure ATM switches directly, but will have
access to ATM switch service provided by others.  Users need to
determine the types of physical media in their ATM environments,
because this affects cost and performance.  

Users should buy ATM access equipment appropriate to their
configuration, application and functional requirements, and
appropriate to interfaces with other technologies coexistent with
ATM.  Users need to look at the state of ATM product offerings to
meet the above requirements.  For example, users may want to
replace private network connections with access to public ATM
services; users should ensure that those services are available
before considering such a replacement.

Users should consider the number and type of ATM access ports
(bit streams) supported by the ATM access device.  ATM access
ports can be directly connected to multimedia workstations and
central servers, or can be connected indirectly to hosts through
shared-access LANs such as Ethernet.  ATM access devices can have
other ports (connected to diverse devices) in addition to ATM
access ports.  ATM access devices could be connected to gateways,
routers, or hubs.  Synchronous interfaces can be built into an
ATM access device.
  
Users need to determine the possible destinations of ATM
connections in their ATM access devices.  For example, ATM access
devices can link to local telephone companies and cable system
providers.  In an enterprise network, there are a number of
different devices connected via an ATM network to an ATM access
device, and the ATM access device must recognize these other
devices.  The more types of devices recognized, the more flexible
is the ATM access device.

ATM Switches
When procuring an ATM switch, users should specify the number of
input and output ports supported by the switch, as well as the
speeds supported for each port.  There are several ATM switch
types, and not all of them interwork.  For signalling purposes,
an ATM switch needs to know to which kind of ATM switch it is
connected.  Users should determine to which switch types their
ATM equipment will be connected, and ensure that the switches can
effectively interwork.  All ATM switches support permanent
virtual circuit (PVC) and UNI capability, but more and more
switches are supporting switched virtual circuit (SVC) and NNI
capability; users should specify if these additional capabilities
are needed.

ATM switches must (if the application demands) be able to handle
SVC calls, and must be able to handle PVC calls if necessary. 
Applications where SVC capability is important include
videoconferencing and transmitting trading information, as well
as transmitting 3-D images over multiple routes to multiple
destinations.

Ports on ATM switches should be flexibly assigned, and must keep
traffic counts on different service classes so that usage-based
billing can be supported.  ATM switches can have non-ATM
interfaces (e.g., Ethernet) directly connected to them.  ATM
switches must also segregate traffic with differing delay-
tolerance characteristics (for example, file transfer vs.
interactive video).  The time delays supported by ATM switches
vary widely from nanoseconds (jitter) to microseconds, and could
pose problems for continuous bit-rate traffic, such as voice and
videoconferencing.  Users need to specify acceptable delay-
tolerance characteristics for ATM switches. 

Security
When procuring and installing ATM equipment, security is
important because users want to ensure that data that passes
through an ATM switch has its integrity preserved, is subject to
access control restrictions, and is subject to authentication and
identification restrictions.  The scope of security for ATM
includes aspects of network security, as well as administrative
and operational security.  ATM products are still in their
infancy, so a number of features have yet to be defined.

High-speed ATM traffic poses difficulties with network security. 
Current methods of data encryption are unable to keep up, because
in real-time processing, encryption times are too slow for the
data rates.  With ATM, (possibly replicated) cells can be sent on
different paths to a destination, thus "scattering" the data;
this is a possible network security mechanism.  Users should
ensure that their network (and other) security concerns are
satisfied in ATM products.

Management
Users want to access resource management, performance,
accounting, and configuration information for ATM networks.  They
will want to know, for a call placed through an ATM access
device, did the call complete?  The payload ID field of an ATM
cell header distinguishes management information from user and
other signalling information in an ATM cell.  Users should ensure
that management concerns (such as resource management) are
satisfied by placement of appropriate information in this field. 
Resource management involves the proper allocation of memory,
processors, and devices to support the requirements of an ATM
network.

Performance is important in ATM networks.  ATM networks must be
able to ensure consistent performance in the presence of widely
varying traffic characteristics, because many high-speed
applications require guaranteed throughput.

ATM access devices are responsible for guaranteeing good
performance for a mix of traffic types in an ATM network.  ATM
access devices can perform traffic management.  Traffic
prioritization and load balancing are examples of performance
metrics for ATM access devices.  Other performance metrics
include device data rate (throughput) and full (peak) ATM cell
rate, both in number of cells/second, number of PVC and SVC
connections supported per module, and the time it takes to set up
an ATM connection.  ATM access devices can also offer different
connection qualities of service.  Users should specify a desired
quality of service.  ATM access devices should be efficient, in
that all existing paths should be completely filled with cells
before new connection requests occur.  ATM connections are
refused if insufficient bandwidth is available at connection
request time, so users should specify the bandwidth reservation
capability.    

ATM switches may be implemented with inadequate congestion
management capabilities.  The buffers may be too small and they
may discard data when the network becomes too congested.  There
is also concern about traffic delays.  An alternative to a
congestion management algorithm is over-provisioning of existing
lines.  Configuration may involve setting up buffer management
parameters.  Buffer management ensures that different types of
traffic will be handled according to the quality of service each
requires.  Users should specify appropriate buffer management
capabilities. 

The Future of ATM
ATM technology is a unifying communications paradigm; it
insulates protocols from underlying changes in the physical
medium.  Use of the ATM technology promises:

  -  cost savings
  -  speed improvements
  -  flexibility gains in the design and operation of networks
  -  consolidation of voice, data, video and other information
     types

Whether the user's need is for wide area connectivity or LAN
links, ATM promises to bring the swift and sure transport of
information.  Any application or service can be supported by
cells on an ATM network.  ATM makes the interface between LANs,
WANs, and MANs transparent to users.  ATM provides better
application integration and speeds up application development. 

By far the most important advantage that ATM offers over
competing technologies is its open-ended growth path.  As a LAN
architecture, ATM is not locked into a single physical medium or
speed.  Instead, it can utilize any combination of speeds and a
variety of media, including metallic and optical.  There are many
opportunities for cost reduction in designing ATM networks.  ATM
can be implemented without affecting the local user environment. 
The ATM technology separates the basic transfer mechanism (that
is, cell relaying) from policies governing routing, quality of
service, and security, facilitating the efficient interconnection
of administrative domains.  

ATM networks provide access to centrally stored video information
through the network, two-way or multipoint videoconferencing, and
remote classroom instruction using video.  For distributed
computing, ATM is being used as the interconnection network for a
distributed memory multiprocessor.  ATM networks can also be used
in:
  -  LAN interconnectivity
  -  publishing (newspapers, journals)
  -  healthcare (such as telemedicine)
  -  multimedia libraries
  -  financial applications
  -  government
  -  entertainment   

Cable companies are experimenting with ATM for delivering real-
time interactive TV and video on demand.  Some ATM LAN
applications are:

  -  transport of real-time video
  -  development of new transport protocols
  -  parallel computation on ATM-connected workstation clusters
  -  development of distributed multimedia applications

For multimedia, the real-time traffic can be sent directly over
an ATM connection, with or without AAL processing, as
appropriate.  Corporate applications using the ATM technology
include:

  -  desk-to-desk videoconferencing
  -  multimedia conferencing
  -  multimedia messaging
  -  distance learning
  -  imaging tasks (including CAD/CAM)
  -  animation
  -  cooperative work (for example, joint document editing)
  -  desktop applications for enterprise networks

ATM is most likely to succeed if there is a LAN-based ATM
presence in the workplace and an established need for very high
bandwidth over the wide area; the wide area cost is reasonable;
wide area ATM service is a ubiquitous and uniform offering (a
lesson learned from narrowband ISDN); and there is a graceful
migration from other technologies and services to ATM.  

For more information
This bulletin summarizes NISTIR 5561, Asynchronous Transfer Mode
Procurement and Usage Guide, by Tim Boland, computer scientist,
Computer Systems Laboratory, NIST.  For a copy of this document,
contact the National Technical Information Service (NTIS) at
(703) 487-4650; the order number is PB95-174967 and the price is
$17.50.  The bulletin is also available online through NIST's WWW
server.  To access the CSL Home Page, use the URL
http://www.ncsl.nist.gov  From the CSL Home Page, click on
Publications, then click on Online NISTIRs.  Select NISTIR 5561
which is provided in WordPerfect and ASCII formats.