ATM (Asynchronous Transfer Mode)

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Oct 23, 2013 (3 years and 8 months ago)


ATM (Asynchronous Transfer Mode)




1- Introduction and History of ATM.
2- General Information.
3- ATM Specifications.
4- ATM compared with current
5- Present and Future ATM
6- Appendix
7- Bibliography



Introduction and History of

Asynchronous transfer mode (ATM) was developed primarily from the need of networking equipment to
keep up with a large number of different services demanding different, and sometimes unknown
requirements. Video conferencing, High Definition TV (HDTV), high speed data transfer, video library and
home education are some of the many new services demanding high speed data processing for which
ATM was developed. Thanks to these facilities, the use of ATM network technology is increasing
constantly and will continue growing during the next years.

Before ATM, the networking world was dependant on specialized network services. The reliance on
specialized services causes many problems, including: Service Dependence, Inflexibility, and Inefficiency.
Each network was dependent on one specific type of service, and as technology increased it became
necessary for networks to be able to handle different types of service which the networks were not as
capable of (inflexibility). The internal resources of networks were not available to other networks, creating
an inefficiency of resources problem.

Although ATM emerged from the ex-CCITT's study groups (–see more information on the a Appendix-) in
the mid eighties, the new technology's roots lie in the Bell Labs research projects conducted in the late
sixties. Asynchronous Time Division Multiplexing, as ATM was first known, was designed as a way to
overcome the inefficiencies of classical TDM (Time Division Multiplexing - synchronous technique invented
during World War II to encrypt transatlantic radio conversation between Churchill and Roosevelt –see
more information on the a Appendix-).

The work of CCITT (–see more information on the a Appendix-) Study Group XVIII culminated in the core
set of recommendations covering public ATM and Broadband ISDN (B-ISDN) issued in 1990 (–see more
information on the a Appendix-).

In the autumn of 1991, the ATM Forum was created by a consortium of four computers and
telecommunications vendors to accelerate the development of ATM products and services. The number of
members of the Forum has increased dramatically in the last few years, and now has more than 750
members, including communications and computer industries, agencies, research organisations and

The ITU-T (formerly the CCITT) initiated work in 1993 on a series of recommendations on ATM equipment
functional operations and network management. This work was completed in November, 1995.
Although the Forum is not a standards body, the ITU-T recognises the ATM Forum as a credible working
group, and its specifications are passed up to its standards body for approval. The ATM Forum has
extended such standards for private network specific requirements, and has created entirely new


General Information

ATM Description
ATM like we know it, it is the evolution of the current and extended line RDSI, to support
higher speeds. It is a standard for transmission of information of several classes, such as
voice, video or also data. ATM networks are connection oriented. The information is sent
through the small, fixed-size cells.
ATM arose thanks to the organism ISDN (Integrated Digital Services Network) (–see more
information on the a Appendix-).
ATM forum is who controls the standards and it has extended its use so much to public nets
as private, and it has liberated its work in the following specifications:
1) User-to-Network Interface (UNI) 2.0
2) UNI 3.0 y UNI 3.1
3) Public-Network Node Interface (P-NNI)
4) LAN Emulation (Lane)
The ATM technology gives support to all the services of transmission of 64 Kbps, it is the
transmission speed of the current line RDSI, and at the same time it has to give a speed of
enough high transmission of 150 Mbps that are the requirements of the video as minimum.
Therefore the commutation device supports several transmission speeds and traffic
Basically ATM is of asynchronous nature, that is to say any information that is wanted to
send doesn't have to wait synchronize with any other one.

ATM SpecificationsATM Specifications

In the following lines we will try to explain the main characteristics of ATM.

As we say at the begging of this document, ATM is based on the efforts of the ITU-T
Broadband Integrated Services Digital Network (BISDN) standard. It was originally
conceived as a high-speed transfer technology for voice, video, and data over public
ATM is a cell-switching and multiplexing technology that combines the benefits of circuit
switching (guaranteed capacity and constant transmission delay) with those of packet
switching (flexibility and efficiency for intermittent traffic). It provides scalable bandwidth from
a few megabits per second (Mbps) to many gigabits per second (Gbps). Because of its
asynchronous nature, ATM is more efficient than synchronous technologies, such as time-
division multiplexing (TDM –see more information on the a Appendix-).

ATM Cell Basic Format
ATM transfers information in fixed-size units called cells. Each cell is made up of 53 bytes.
The first 5 bytes contain the header information, and the remaining 48 contain the “payload”
(it means the user’s information).

Advantages of cells:
 switches and interfaces are easiest to implement
 host hardware data units are typically fixed size (e.g. pages)
Disadvantages of cells:
 what is the optimal cell payload size?

ATM Devices
An ATM net is composed of two classes of different devices, the “switch” and the
“endpoints”, as well called terminal points.
The first ones function is accept the incoming cell from an ATM endpoint or another ATM
switch. It then reads and updates the cell-header information and switches the cell to an
output interface toward its destination.
An endpoint contains an ATM network interface adapter, for example: workstations, routers,
digital service units (DSUs), LAN switches, and video coder-decoders (CODECs).

ATM Network Interfaces
An ATM network consists of a set of ATM switches interconnected by point to point ATM
interfaces. ATM switches support two primary types of interfaces:

 UNI (User-to-Network Interface): it connects ATM end systems (such as
hosts and routers) to an ATM switch.
 The NNI (Network-to-Network Interface): it connects two ATM switches.

By the other hand , UNI and NNI networks can be subdivided in public and private. A
private UNI connects an ATM endpoint and a private ATM switch. Its public counterpart
connects an ATM endpoint or private switch to a public switch. A private NNI connects
two ATM switches within the same private organization. A public one connects two
ATM switches within the same public organization.

ATM Cell-Header Format
As explained above, an ATM cell header can be one of two formats: UNI or the NNI. The
UNI header is used for communication between ATM endpoints and ATM switches in
private ATM networks. The NNI header is used for communication between ATM switches.
Unlike the UNI, the NNI header does not include the Generic Flow Control (GFC) field.
Additionally, the NNI header has a Virtual Path Identifier (VPI) field that occupies the first 12

ATM Cell-Header Fields
The following fields are used in ATM:
• Generic Flow Control (GFC)—Provides local functions, such as identifying multiple
stations that share a single ATM interface. This field is typically not used and is set to its
default value.
• Virtual Path Identifier (VPI)—In conjunction with the VCI, identifies the next destination of a
cell as it passes through a series of ATM switches on the way to its destination.
• Virtual Channel Identifier (VCI)—In conjunction with the VPI, identifies the next destination
of a cell as it passes through a series of ATM switches on the way to its destination.
• Payload Type (PT)—Indicates in the first bit whether the cell contains user data or control
data. If the cell contains user data, the second bit indicates congestion, and the third bit
indicates whether the cell is the last in a series of cells that represent a single AAL5 frame.
• Congestion Loss Priority (CLP)—Indicates whether the cell should be discarded if it
encounters extreme congestion as it moves through the network. If the CLP bit equals 1, the
cell should be discarded in preference to cells with the CLP bit equal to zero.
• Header Error Control (HEC)—Calculates checksum only on the header itself.

ATM Services
Three types of ATM services exist: permanent virtual circuits (PVC), switched virtual circuits
(SVC), and connectionless service.
A PVC allows direct connectivity between sites. PVC guarantees availability of a connection
and does not require call setup procedures between switches. Disadvantages of PVC
include static connectivity and manual setup.
An SVC is created and released dynamically and remains in use only as long as data is
being transferred. In this sense, it is similar to a telephone call. Dynamic call control requires
a signaling protocol between the ATM endpoint and the ATM switch. The advantages of
SVC include connection flexibility and call setup that can be handled automatically by a
networking device. Disadvantages include the extra time and overhead required to set up
the connection.

ATM Virtual Connections
ATM networks are fundamentally connection oriented, which means that a virtual channel
(VC) must be set up across the ATM network before to any data transfer. (A virtual channel
is similar to a virtual circuit.)
Two types of ATM connections exist: virtual paths, which are identified by virtual path
identifiers, and virtual channels, which are identified by the combination of a VPI and a
virtual channel identifier (VCI).
A virtual path is a bundle of virtual channels, all of which are switched transparently across
the ATM network on the basis of the common VPI.

ATM Switching Operations
The basic operation of an ATM switch is easy: The cell is received across a link on a known
VCI or VPI value. The switch looks up the connection value in a local translation table to
determine the outgoing port (or ports) of the connection and the new VPI/VCI value of the
connection on that link. The switch then retransmits the cell on that outgoing link with the
appropriate connection identifiers. Because all VCIs and VPIs have only local significance
across a particular link, these values are remapped, as necessary, at each switch.

ATM Reference Model
ATM functionality corresponds to the physical layer and part of the data link layer of the OSI
reference model.
The ATM reference model is composed of the following planes:
• Control—This plane is responsible for generating and managing signaling requests.
• User— This plane is responsible for managing the transfer of data.
• Management— This plane is formed by two components:
— Layer management manages layer-specific functions, such as the detection of
failures and protocol problems.
— Plane management manages and coordinates functions related to the
complete system.

The ATM reference model is composed of the following ATM layers:

The ATM Physical Layer
The ATM physical layer has four functions: bits are converted into cells, the transmission
and receipt of bits on the physical medium are controlled, ATM cell boundaries are tracked,
and cells are packaged into the appropriate types of frames for the physical medium.
The ATM physical layer is divided into two parts:
 The physical medium-dependent (PMD) sublayer: It synchronizes
transmission and reception by sending and receiving a continuous flow of bits
and it specifies the physical media for the physical medium used, including
connector types and cable.
 The transmission-convergence (TC) sublayer: it has four functions: Cell
delineation, header error-control (HEC) sequence generation and verification,
cell-rate decoupling, and transmission-frame adaptation.

The ATM Layer
ATM layer does the following functions:
 Generic Flow Control (GFC): May be used for contention resolution for
multiple access at the UNI.
 Cell Header: Assigns the VPI and VCI fields values to each cell.
 VCI/VPI translation: Does the cells translations between VPIs and VCIs.
 Cell Multiplex: ATM layer multiplexes and demultiplexes the different cell flow
to give to ATM Physical Layer only one cell flow.

ATM Adaptation Layers: AAL1
AAL1, is a connection-oriented service, this layer can execute circuit-emulation applications,
such as voice and video conferencing. AAL1 requires timing synchronization between the
source and destination so we could say that AAL1 depends on a medium that supports

ATM Adaptation Layers: AAL2
AAL2 provides bandwidth-efficient transmission of low-rate, short and variable packets, it is
ideal for applications sensitive to delays. It supports VBR and CBR. AAL2 does not define a
PDU for the CS sublayer. PDU SAR format consists on a Sequence Number, Information
Type, Length Indicator and Cyclic Redundancy Check.

ATM Adaptation Layers: AAL3/4
AAL3/4 supports both connection-oriented and connectionless data. It was designed for
network service providers and is closely aligned with Switched Multimegabit Data Service
(SMDS). AAL3/4 is used to transmit SMDS packets over an ATM network. AAL3/4 prepares
a cell for transmission in four steps.
An AAL 3/4 SAR PDU header consists of Type, Sequence Number, and Multiplexing
Identifier fields. Type fields identify whether a cell is the beginning, continuation, or end of a
message. Sequence number fields identify the order in which cells should be reassembled.
The Multiplexing Identifier field determines which cells from different traffic sources are
interleaved on the same virtual circuit connection (VCC) so that the correct cells are
reassembled at the destination.

ATM Adaptation Layers: AAL5
AAL5 is the primary AAL for data and supports both connection-oriented and connectionless
data. It is used to transfer most non-SMDS data, such as classical IP over ATM and LAN
Emulation (LANE). AAL5 also is known as the simple and efficient adaptation layer (SEAL)
because the SAR sublayer simply accepts the CS-PDU and segments it into 48-octet SAR-
PDUs without adding any additional fields.

ATM Connections
ATM supports two types of connections: point-to-point and point-to-multipoint.
Point-to-point connects two ATM end systems and can be unidirectional (one-way
communication) or bidirectional (two-way communication). Point-to-multipoint connects a
single-source end system (known as the root node) to multiple destination end systems
(known as leaves). Such connections are unidirectional only. Root nodes can transmit to
leaves, but leaves cannot transmit to the root or each other on the same connection. Cell
replication is done within the ATM network by the ATM switches where the connection splits
into two or more branches.
It would be desirable in ATM networks to have bidirectional multipoint-to-multipoint

ATM and Multicasting
ATM requires some form of multicast capability. AAL5 (which is the most common AAL for
data) currently does not support interleaving packets, so it does not support multicasting.
If a leaf node transmitted a packet onto an AAL5 connection, the packet can get intermixed
with other packets and be improperly reassembled. Three methods have been proposed for
solving this problem: VP multicasting, multicast server, and overlaid point-to-multipoint
connection. Under the first solution, a multipoint -to-multipoint VP links all nodes in the
multicast group, and each node is given a unique VCI value within the VP. Interleaved
packets therefore can be identified by the unique VCI value of the source.
A multicast server is another potential solution to the problem of multicasting over an ATM
network. All nodes wanting to transmit onto a multicast group set up a point -to-point
connection with an external device known as a multicast server. The multicast server, in
turn, is connected to all nodes wanting to receive the multicast packets through a point -to-
multipoint connection. The multicast server receives packets across the point -to-point
connections and then retransmits them across the point -to-multipoint connection—but only
after ensuring that the packets are serialized (that is, one packet is fully transmitted prior to
the next being sent). In this way, cell interleaving is not allowed.

An overlaid point-to-multipoint connection is the third potential solution to the problem of
multicasting over an ATM network. All nodes in the multicast group establish a point -to-
multipoint connection with each other node in the group and, in turn, become leaves in the
equivalent connections of all other nodes. Consequently, all nodes can both transmit to and
receive from all other nodes. This solution requires each node to maintain a connection for
each transmitting member of the group, whereas the multicast-server mechanism requires
only two connections.

ATM Signaling and Connection Establishment
When an ATM device wants to establish a connection with another ATM device, it sends a
signaling-request packet to its directly connected ATM switch. This request contains the
ATM address of the desired ATM endpoint, as well as any QoS parameters required for the
connection. ATM signaling protocols vary by the type of ATM link, which can be either UNI
signals or NNI signals. UNI is used between an ATM end system and ATM switch across
ATM UNI, and NNI is used across NNI links.

The ATM Connection-Establishment Process
ATM signaling uses the one-pass method of connection setup that is used in all modern
telecommunication networks, such as the telephone network. An ATM connection setup
proceeds in the following manner. First, the source end system sends a connection-
signaling request. The connection request is propagated through the network. As a result,
connections are set up through the network. The connection request reaches the final
destination, which either accepts or rejects the connection request.

LAN Emulation (LANE)
LANE is a standard defined by the ATM Forum that gives to stations attached via ATM the
same capabilities they normally obtain from LANs, such as Ethernet and Token Ring. As the
name suggests, the function of the LANE protocol is to emulate a LAN on an ATM network.
Specifically, the LANE protocol defines mechanisms for emulating either an IEEE 802.3
Ethernet or an 802.5 Token Ring LAN. The current LANE protocol does not define a
separate encapsulation for FDDI (–see more information on the a Appendix-). (FDDI
packets must be mapped into either Ethernet or Token Ring emulated LANs [ELANs] by
using existing translational bridging techniques.)
The LANE protocol defines a service interface for higher-layer (that is, network layer)
protocols that is identical to that of existing LANs. Data sent across the ATM network is
encapsulated in the appropriate LAN MAC packet format. Simply put, the LANE protocols
make an ATM network look and behave like an Ethernet or Token Ring LAN—althrought
operating much faster than an actual Ethernet or Token Ring LAN network.

Short explanation
Today's telecommunication networks are characterized by specialization. This means that
for every individual telecommunication service at least one network exists that transports this
service. Each of these networks was specially designed for that specific service and is often
not at all applicable to transporting another service.
The networks of today are very specialized and suffer from a large number of
Each network is only capable of transporting one specific service and most of current
technologies are not scalable. Many of the current technologies based on frames use
permanent connections the whole time.
Advances in audio, video and speech coding and compression algorithms and progress in
VLSI technology influence the bit rate generated by a certain service and thus change the
service requirements for the network. A specialized network has great difficulties in adapting
to new services requirements. The internal available resources are used inefficiently,
resources which are available in one network cannot be made available to other networks.
The most important parameter is the emergence of a large number of communication
services with different, sometimes yet unknown requirements. In this information age,
customers are requesting an ever increasing number of new services. The most famous
communication services to appear are HDTV(High Definition TV), video conferencing, high
speed data transfer, videophony, video library, home education and video on demand. This
large length of requirements introduces the need for one universal network which is flexible
enough to provide all of these services in the same way. Two other parameters are the fast
evolution of the semi-conductor and optical technology and the evolution in system concept
ideas - the shift of superfluous transport functions to the edge of the network. ATM is a
network capable of transporting all types of services that will be able to adapt itself to new
needs. Efficient in the use of its available resources. All available resources can be shared
between all services, such that an optimal statistical sharing of the resources can be
obtained. Since only one network needs to be designed, manufactured and maintained the
overall costs of the design, manufacturing, operations and maintenance will be lower. Major
telecom interest surely will make it the preferred wide area digital interconnect in a few
ATM can assign wide of band low demand where and when it is necessary through virtual
circuits. This dynamic assignment provides the administrators of LAN of a powerful
administration capacity and ATM will allow the integration of LANs and WANs, allowing that
the same types of data are used in any place. also bill with the advantage of being easy to
administer. ATM also provides legacy LAN emulation.
Therefore we could conclude on the advantages of ATM that is a simple, very fast, switching
and routing process based on the virtual channel identifier (VCI) in the cell address. Within
the network, no processing occurs above the cell level, thus simplifying and increasing the
speed of message handling. The simple, fast message handling can be used to create high

speed, self-routing switches, that can grow in both size and speed to meet future
But ATM also has disadvantages it has more than enough current technologies,
It is flexible to efficiency’s expense, at present, for any one application it is usually possible to
find a more optimized technology than ATM and cost, although it will decrease with
time.New customer premises hardware and software are required . Competition from other
technologies -100 Mbps FDDI (–see more information on the a Appendix-), 100 Mbps
Ethernet and fast ethernet . Presently the applications that can benefit from ATM such as
multimedia are rare.Tthe wait, with all the promise of ATM’s capabilities many details are still
in the standards process.
But today ATM didn’t has real standards yet. No low-latency switches, in practice, as
telecom doesn't mind 10 microsecond latencies. Serious inefficiencies and latencies arise
when an intermediate switch drops a packet due to transient overload if the packet is part of
a block of digital data. No low-latency interfaces, all have software driver overheads. No
support for caching data to reduce apparent latency.


Present and Future
of ATM

Nowadays, ATM has achieved world-wide acceptance. One of the reasons is that ATM
technology influences, and will influence in the next future, many segments of our society,
touching the consumer (interactive multimedia applications and games, etc.), public-service
(videoconferencing, etc.) and commercial markets (telemedicine, etc.). But there are also
other benefits:

It provides a single network for all traffic types, improving efficiency and manageability.
It enables new applications, due to its high speed and the integration of traffic types.

As ATM is not based on a specific type of physical transport, it is compatible with currently
deployed physical networks (It can be transported over twisted pair, coaxial and fiber optics).
Because ATM is evolving into a standard technology for local, campus/backbone and public
and private wide area services, it will simplify network management by using the same
technology for all levels of the network.
ATM has been designed to be scaleable and flexible in geographic distance, number of
users and access and trunk bandwidths.
ATM is protocol independent.
ATM guarantees quality of service.
To take profit of these advantages, more than 20 national carriers have committed to
deploying ATM in Europe, according to the ATM Forum. The National companies are
focusing their activities on multimedia services, using broadband networks based on ATM
technology. There are some very important projects being developed in this field, such as
the ACTS projects GINA, JAMES or NICE. JAMES, a pan-European ATM project, carried
by 18 European Telecommunications operators representing 17 countries is a trans-
European platform for the experimentation of high-speed networks that will give support to
some 100 projects financed by the European Commission. Moreover, companies are
currently deploying ATM switches in some 13 countries, including Australia, France,
Germany, Japan, Netherlands, UK and US.
But the deployment of ATM is also important in other regions. Last year, in a Business
Research Group study that included 100 members from companies, universities and public
sector organisations, 29 per cent reported they are implementing or will implement ATM in
the near future, while 26 per cent are evaluating implementation. By 1999, ATM is expected
to outplace all other technologies and grow to 33 %.

Multimedia will be one of the key applications to use ATM. Multimedia workstations exist
today but the benefits brought by ATM will be in connecting these workstations. In the near
future ATM is intended to be used as a backbone for other existing services like frame relay.
An entire network of ATM components exclusively is unlikely. ATM is expected to improve
LAN/Client -Server architectures and LAN interconnection. ATM will provide the resources
to ease the network demands caused by the growing number of users that need to connect
to a LAN, and with applications requiring more bandwidth.
Many of the details that are necessary to provide ATM benefits are still in the standards
process. As of 1994, the ATM Forum was working on the interface between private
networks, details of LAN emulation, and MPEG video over ATM. The IETF (–see more
information on the a Appendix-) was working on IP over ATM and routing over large clouds.
The wide range of voice services offered by telephone companies today will not be provided
by ATM for some time.
ATM is currently the focus of the communications industry but much further down the road,
ATM will likely be replaced by another technology such as Wavelength Division Multiplexing


TDM: With TDM, each user is assigned to a time slot, and no other station can send in that
time slot. If a station has a lot of data to send, it can send only when its time slot comes up,
even if all other time slots are empty. If, however, a station has nothing to transmit when its
time slot comes up, the time slot is sent empty and is wasted. Because ATM is
asynchronous, time slots are available on demand with information identifying the source of
the transmission contained in the header of each ATM cell.

IETF: (Internet Engineering Task Force) : is a loosely self-organized group of people who
make technical and other contributions to the engineering and evolution of the Internet and
its technologies. It is the principal body engaged in the development of new Internet
standard specifications.

FDDI: FDDI (Fiber Distributed Data Interface) has found its niche as a reliable, high-speed
backbone for mission critical and high traffic networks. It can transport data at a rate of 100
megabits per second, and can support up to 500 stations on a single network. FDDI was
designed to run through fiber cables, transmitting light pulses to convey information between
stations, but it can also run on copper using electrical signals.

CCITT: Comité Consultatif International Téléphonique et Télégraphique, an organization
that sets international communications standards. CCITT, now known as ITU (the parent
organization) has defined many important standards for data communications, including the

ISDN: Which stands for Integrated Services Digital Network, is a system of digital phone
connections which has been available for over a decade. This system allows data to be
transmitted simultaneously across the world using end-to-end digital connectivity.


ATM Switching: Chapter 20 of an ATM standard book in pdf format.

ATM / B-IDSN Tutorial Notes by James P. G. Sterbenz

Optimized Engineering Technical Compendium

The OSI reference model

ATM Reference

CISCO - ATM Internetworking (White paper released by CISCO in 1995).

Radcom Academy Protocols