The
Beauty of
Asynchronous
Transfer Mode
Thao
Mong Le
Note:
This Asynchronous Transfer Mode (ATM) series column
discusses the aspects of an ATM network and may be used as a guide to build
an ATM data and computer communications network, suitable for both technical
and non-technical people who have little technical background in the ATM
communications field. The readers will benefit from reading this column
by better understanding the concepts and fundamentals of ATM communications
network architectures, protocols, and devices. This column was not intended
to cover all ATM network architectures and its communications equipment,
or to delve into the design of ATM network elements itself.
Everyone can
agree that more work needs to be done in completing the ATM technology.
But there is uncertainty about what we should do over the next few years?,
whether to look on ATM for interim and target dates. Is there a sufficient
set of defined standards and products for your organization to proceed
with full scale operational use of ATM or is it prudent to wait for further
standardization and more complete products before changing to ATM?
ooo
Abstract:
A number
of changes are occurring in the computing, data communications, and telecommunications
fields. As computing systems become increasingly powerful and versatile,
data communications and telecommunications networks must evolve to support
the performance of computing systems. Probably the best cell-base switching
and multiplexing technology used to support the development of these types
of high-performance communications networks is the Asynchronous Transfer
Mode (ATM). ATM combines a user�s data, voice, and video into fixed length
cells, and multiplexes it into a single bit stream transmitted across a
physical medium. ATM supports a wide range of high-performance data communications
and telecommunications services for local, national and international communities.
In addition to providing a technical overview of ATM in terms that are
understandable to the readers, this paper will discuss the driving forces
behind ATM, and various ATM applications that make the "future"
telecommunications come to the world.
INTRODUCTION
The Asynchronous
Transfer Mode (ATM) is one of today�s most attractive communications technologies.
It is an emerging technology that uses a very flexible method to provide
a high bandwidth, as well as a very high transmission rate for a communications
or telecommunications network. ATM is a method of communication which can
be used as the basis for both high-speed local area network (LAN) and wide
area network (WAN) technologies to support the performance of advanced
computing systems. Technically, ATM is a packet-like switching and multiplexing
technique that relays traffic via an address contained within the packet
for broadband signals, and performs two simple functions: path determination
which will be done mainly in software, and cell-switching done in hardware.
ATM provides
a tool that allows network designers to solve many pressing network problems
- from internetworking to electronic publishing, electronic fund transfer,
and financial industry reports, from videoconferencing to medical image,
more affordable health care, to expanding telecommuting, work at home,
and many, many more.
The flexibility
bandwidth of ATM allows various types of services (e.g., data, voice ,
video, and image ) with differing requirements to map onto a single network.
With ATM, separate networks for data, voice, video and image will not be
required.
ATM is promised
to be fast, reliable, and a cost effective bandwidth for meeting both the
current and future communications and telecommunications needs.
THE DRIVING
FORCES BEHIND ATM
Today, telecommunications
companies around the world are building digital transmission facilities
implemented various modern switches to meet the current and future needs
of their business clients. Business users are concerned with maximizing
their investment in computing and data communications equipment. Users
want a network where any one user can connect with any type of interfaces
and protocols. This evolution in technology is the primary motivating factor
to develop ATM. There are several forces driving the move to this ATM technology.
Among them are:
· ATM
is chosen as an underlying router-based network since ATM can work in a
hierarchical network that provides a wide range of telecommunications services
for local, national and international communities.
· ATM
is chosen since it is the only international standard designed to accommodate
the simultaneous transmission of data, voice, video, and image at a very
high speed over a single network. This international standard allowing
interoperability of all types of information, regardless of the "end-system".
Telecommunications companies may minimize the loss when investing in the
ATM technology.
· ATM
meets the growing telecommunications demand, as business becomes more geographically
diperse, for domestic and international digital communications service
for data, voice, video and image. ATM provides a network that can integrate
all these types of communication into one universal network.
· ATM
provides network a "unlimited" bandwidth with scalability. Scalability
is the ability to increase the amount of bandwidth without changing the
information structure. Today�s telecommunications networks are not constructed
to provide such features. Packet data services, for example, are typically
limited to transmission rates of about 56 kbps, and circuit switched services
are typically limited to speed of about 45 Mbps; as the demand for the
rapid transfer of information increases dramatically, these speeds are
no longer adequate and other transmission technology must be found.
· ATM
provides an effective communication protocol for LAN-LAN, LAN-WAN and WAN-WAN
interconnection. It has added to users� needs for connectivity expand from
the local to national, and finally international connectivity. As ATM continues
to be deployed, the line between LAN and WAN will blur to form an ATM-
based global network.
· The
ATM protocol allows implementation of many advanced network applications
that would be difficult to use with current network technologies. This
protocol allows network designers to increase network simplicity, flexibility,
and control at an economical price.
· ATM
is chosen since it provides primarily a connection-oriented service, but
it can support a connectionless service. Moreover, ATM uses virtual calls
to transport user information in the closest approximation to true bandwidth-on-demand
service at both a constant bit rate (CBR) and variable bit rate (VBR).
AN TECHNICAL
OVERVIEW OF ATM
* Fast Packet
Switching
Unlike most
current networks, which use circuit or packet switching, ATM uses fast
packet switching. By definition, fast packet switching is a statistical
switching mechanism, also know as asynchronous time division, allowing
its network to provide a large amount of bandwidth, high throughput with
a very small delaying time.
Fast packet
switching consists of two techniques, frame relay and cell relay, that
allow its network to simultaneously transfer a variety of types of traffic
with high quality and reliability. Frame relay operates with a variable
length frame, normally in the range of 64 octets to 1024 octets. Cell relay
uses a fixed length frame consisting of 53 octets. Frame relay and cell
relay enable their networks to provide end-to-end error-checking and minimize
flow control overhead. These allow a reduction of delaying time during
the transfer of information. ATM employs cell relay.
* Standards
The ATM standards
are in development within the American National Standards Institute (ANSI)
and the International Telecommunications Union - Telecommunications (ITU-T)
as transport protocols for use in an ATM-based network, such as the Broadband
Integrated Service Digital Network (B-ISDN). Although much of the initial
interest and early research on ATM was done by the public telecommunication
providers and ITU, the private network providers driven by the benefits
of ATM became interested in the development and standardization of this
new technology. This interest from the private network providers led to
the establishment of the ATM Forum, whose initial objective was to ensure
interoperability between private and public ATM implementations. This forum
includes almost all major players in the telecommunications area, and it
is essentially supplanting the ITU as the major driver of private and public
ATM networking standards.
Today, the
specification of the ATM cell structure is in place and implementable.
The ITU I.361 standard defines the physical make-up of ATM cells, but the
development of the higher-layer specifications is still underway. The ITU
I.121 standard provides the initial references to ATM as the target solution
for providing B-ISDN support. Further partial standards described more
details of ATM are as follows:
I.150 - ATM
protocol, I.15X - General aspects of B-ISDN, I.2XX - Service principles,
and I.413 - Details of the User-Network Interface.
Remaining
open ATM issues are being addressed rapidly within an industry consortium
as the ATM Forum. The ATM Forum developed the user-network interface specification,
which assists vendors in developing interoperable ATM products. The initial
payload data rate envisioned for ATM is 44.736 Mbps, transported via the
carrier existing T-3 (44.736 Mbps) networks. The ATM Forum also worked
on the development of interface transmission specifications of 155 Mbps
and other interfaces for ATM networks.
* ATM Transmission
Facility
ATM use SONET
(Synchronous Optical Network) as a digital transmission facility. SONET
is a standardized hierarchical transport scheme designed to address the
need for communications and telecommunications networks that desire transmission
rates higher than 44.736 Mbps (i.e., T3 or DS-3). It was originally developed
for the public telephone network to improve overall network performance
and management and provide a needed bandwidth to the customer.
ATM use SONET
over the public telephone network to convert synchronous transport signals
(STS) (i.e., electrical signals) to optical carrier (OC) signals over a
fiber optic network. SONET's basic building block is a 125 micro-second
frame. The frame can be broken down into overhead and user information
or payload. The STS-n electrical signal generated by SONET transport equipment
is converted directly into an optical signal with no further processing
or added overhead, therefore the OC-n are identical with the STS-n rates.
The hierarchy is made up of a basic transport tributary called Optical
Carrier level 1 or OC-1 running at a rate of 51. 84 Mbps. Higher level
OC can be generated by multiplexing of OC-1. In this hierarchical structure,
standard rates of each OC transports are OC-1 (51.84 Mbps), OC-3 (155.52
Mbps), OC-9 (466.56 Mbps), OC-12 (622.08 Mbps), OC-24 (1,244.16 Mbps),
OC-36 (1,866.24 Mbps), and OC 48 (2.488.32 Mbps). Currently, the rates
below OC-3 have been regionally optimized, and the rates between OC-3 and
OC-12 have been adopted for international use.
The ATM Forum
originally was recommending SONET as the primary transmission vehicle for
ATM, but with the premature SONET, many existing ATM products use T1 (1.544
Mbps) or T3 as its physical layer.
* Interfaces
An ATM network
is modeled as a set of end-systems (e.g., workstations) and a set of intermediate
nodes (e.g., ATM switches), all linked by a set of point-to-point ATM links.
Two major types of interfaces in ATM networks are the User-to-Network Interface
(UNI) and the Network-to-Network Interface (NNI). The use of these terms
has been broadened to include both ATM private and ATM public networks,
as illustrated in Figure 2. While the ITU defines standards for NNI, the
ATM Forum developed standards for public and private UNI. The major differences
between these types of interfaces are administrative and signaling functions.
By definition,
the UNI represents a regulatory boundary between an end-device and an ATM
switch, public or private. The NNI refers to an interface between two public
ATM switches, which exchange information such as routing tables that are
unrelated to a particular circuit set-up. For links between private ATM
switches and between different public switches, switch-to-switch links,
also known as Inter-Switching-System- Interfaces (SSSI), are used. The
ATM Forum proposed the use of the same signaling protocols across all types
of interfaces, which will greatly facilitate implementations. * ATM Related
Functional Layers
In terms of
the Open Systems Interconnection (OSI) 7-layer Reference Mode, ATM operates
roughly between Layer 1 (Physical layer) and Layer 3 (Network layer). Figure
1 shows the ATM related functional layers. The lowest layer is the physical
layer, which is capable for transmitting ATM cells over various media,
such as fiber optic cable, electrical wire, and satellite. The ATM Physical
Layer, which consists of two sub-layers: the Physical medium Dependent
(PMD) sublayer, and the Transmission Convergence (TC) sublayer. The PMD
sublayer deals with electrical interfaces, such as bit timing and the physical
medium (e.g., cables, connectors). The TC sublayer performs a convergence
function that receives a bit stream from the PMD, and extracts cells to
pass to the ATM Layer. The ATM Physical Layer uses SONET as a transport
medium facility to scale to the very high speeds required for ATM.
Higher
Layers
ATM
Adaptation Layer (AAL)
ATM
Layer
Physical
Layer (optical, electrical, etc)
Figure
3: The Functional ATM Layers Model
Next, the
layer above the Physical Layer is the ATM Layer , which performs the basic
operations of ATM, such as multiplexing, demultiplexing, cell relaying,
delay handling, and cell loss priority processing.
On the top
of the ATM Layer is the ATM Adaptation Layer (AAL). This layer categorizes
five types of protocol layers: 1,2,3,4, and 5. The AAL allows the ATM network
to provide bandwidth savings by taking advantage of the bursty nature of
packet services. The main functions of the AAL include cell assembly and
disassembly, handling of loss cell conditions, cells recovery, mapping
of control signal into ATM cell stream, dividing information blocks up
into ATM cells, and rate adaptation. The AAL provides a service to the
higher layers that correspond to the four classes of traffic as defined
below.
Class A: CBR,
connection oriented, synchronous traffic (e.g., uncompressed voice or video)
Class B: VBR,
connection oriented, synchronous traffic (e.g., compressed voice and video)
each of which was designed to optimally carry one of the four above classes
of traffic.
Class C: VBR,
connection oriented, asynchronous traffic (e.g., X.25, Frame Relay services,
etc.)
Class D: Connectionless
packet data (e.g., LAN traffic, SMDS, etc.)
Finally, the
Higher Layers, whose protocols are defined across the ATM layers to support
signaling and control. The main functions of the Higher Layers are to perform
transfer of connectionless network service (CLNS) and connection-oriented
network service (CONS) data and video/voice communications at high speeds
over the ATM network.
* ATM Cells
ATM is an
emerging technology that involves a high speed protocol to move cells.
Each ATM cell is 53 octets (bytes) long, which is divided into two fields.
The header is 5 octets consisting of four octets of routing/switching and
status information and one byte of header error correcting code. The payload
is the remaining 48 octets used to carry the user�s information (e.g.,
data, voice, video, and image). The use of a small cell offers ATM two
main advantages. First, the length of queuing time used to process in an
ATM-based network is very short and manageable since cells from different
kind of traffic sources (e.g., data, voice, video, and image) can be interleaved,
Second, the use of small fixed size cells allows the ATM-based network
to be switchable efficiently.
The ATM cell
header field is used by the UNI, NNI, and switches to route the cells.
This header field holds the information on dividual cell routing and connection
type. The organization of the header will vary slightly depending on whether
it contains information related to NNI or UNI. Three principal functions
of the header field are virtual channel identification, error detection
on the header, and unassigned cell indication. The other main functions
performed by the header field are error correction on the header, cell
loss detection, cell sequence numbering, terminal identifier, virtual path
identification, and line equipment identification. In operation, header
values are assigned to each section of a connection when required, and
release when no longer required. The connection identified by the headers
remains unchanged during the lifetime of a call.
* ATM Switch
ATM connections
are most often implemented via an ATM switch, which performs the statistical
multiplexing and virtual connection management functions. An ATM switch
is capable of handling both types of virtual circuits: switched and permanent.
For a switched virtual circuit (SCV), when an ATM switch receives a connection
request signal, it examines the destination request, and verifies the sufficiency
of memory, port, and bandwidth of the network before allowing a connection.
An ATM permanent virtual circuit (PVC) switch has dedicated path through
the switches for information transfer, therefore it is capable for handling
a larger volume of information and allowing faster transferring than the
use of an ATM SCV switch.
ATM APPLICATIONS
Business users
are concerned with maximizing their investment in computing and data communications
equipment. Users want a network where any one user can connect with any
type of interfaces and protocols. This is the expectation that an ATM network
is setting today. ATM provides a better application integration and speeds
up application development. ATM offers a number of advanced data, voice,
video and image applications with different performance, quality, and business
requirements. For example, ATM can support television, and video telephony
services ranging from 2 Mbps to 600 Mbps. ATM video conferencing will extend
to applications in different locations to be able to communicate with each
other. The ATM network will also be used for TV distribution services to
be provided on the same transmission equipment for all voice, data, and
video traffic. Some consumer and commercial service applications requiring
the use of ATM are listed below.
· Multipoint
videoconferencing service, video-on-demand, and on-line video libraries
· Telecommuting
(work at home)
· Home
entertainment/shopping service · E-mail and data/audio/voice Internet
service
· Interactive
multimedia
· Medical
imaging
· Distributed
data access
· Distance
learning
· Videoconferencing
· LAN,
LAN-WAN, and WAN-WAN interconnection.
Today, industry,
government, and community leaders began to recognize innovation and excellence
in use of ATM. Some ATM applications areas, whose successful stories can
be found at "https://www.atm.at-work.com/success_folder/Successatm.html"
are listed below.
· Business
(e.g., GTE Telephone Operations National Network Monitoring and Support
Network, The Virtual Corporation, Agile Cable Production Service for The
Information Highway);
· Department
Store (e.g. JC Penney);
· Education
(e.g., Oregon ATM Network, Technical University of Delft, Gigaswitch/ ATM
at La Trobe University, Shared Remote Teleclasses on the North Carolina
Information Highway, St. Louis Public Schools District Facility LAN Infrastructure
Model, NYNet)
· Energy
(e.g., Westinghouse, Amoco);
· Entertainment
(e.g., World Cup USA 1994 Tournament, Cinema of the Future);
· Financial
Services (e.g., Lufkin, Donaldson, Jenrette);
· Government
(e.g., Commonwealth Telecommunications Network NAVSEA, Sandia National
Laboratories Supercomputer Consolidation Network)
· Health
& Science (e.g., USC-ABC's Advanced Telemedicine Network, Penn Allegheny
Health, ATM Testbed Network for Health Care in Neuro-Imaging, Congressional
Healthcare Demonstration; Department of Surgery Grand Rounds, I-SNET Pilot
Telemedicine Service, NIIT's National Telemedicine Demonstration, Teleradiology/Telemedicine
Demonstration at the Radiological, Society of North American Conference);
· Research
and Development (e.g., Monterey Bay Aquarium Research Institute, NASA,
Cablevision Systems Corporation, Concurrent Technologies Corp.)
CONCLUSION
ATM is a technology
that is being applied to support a variety of advanced applications. An
ATM-based network, such as B-ISDN, can support very high speed interfaces
for various classes of traffic - data, voice, video and image - in the
one transmission and switching fabric. An ATM network can provide flexible
bandwidth allocation and simple routing using connection oriented technology.
ATM allows for the integration of networks improving efficiency and manageability.
ATM brings
the digital network to the world. ATM offers dramatic increases in the
speeds of data transfer, real-time videoconferencing, video-on-demand,
and remote broacasting and audio transfer. ATM is well suited for emerging
multimedia applications, such as interactive data, voice, video and imaging,
suitably applicable for entertainment, telecommuting, home shopping, video
on demand, global virtual collaboration, international product design,
worldwide banking, video conferencing, distance learning, high speed image
visualization, and tele-medicine. ATM will allow its network to bring increased
capabilities, reduces costs, and improves productivity to organizations
both large and small. Moreover, ATM using wireless capability and satellite
links is under very serious consideration. ATM has a potential to be a
center for interconnecting different types of networks to build the National
Information Infrastructure.
REFERENCES
[1] Andrew
S. Tannenbaum, "Computer Networks", Prentice -Hall, Inc., 1981
[2] William
Stalling, "ISDN and Broadband ISDN", Macmillian, 1992
[3] David
E. McDysan, et .... "ATM", McGraw-Hill, Inc., 1995
[4] Craig
Partridge, "Gigabit Networking", Addison-Wiseley, 1994
[5] Thao M.
Le, "A Guide to ISDN and B-ISDN", GSA, 1994
[6] Thao M.
Le, "ISDN and B-ISDN: Communications network Needs & Technologies",
VACETS Technical Magazine, Volume 1, February 1996
Thao Mong Le
[email protected]
For
discussion on this column, join [email protected]
Copyright
© 1994 - 1998 by VACETS and Thao M. Le
:
