ATM - A Technology Perspective

Madge Networks
Cat no. LNC - 142568 Rev. A
Technology White Paper
ATM – A Technology
Perspective

ATM - A Technology Perspective Madge Networks
Summary
As businesses build their enterprise networks for
the 21st century, Asynchronous Transfer Mode
(ATM) is their technology of choice. ATM’s high
bandwidth — from 25 to 622Mbps — offers network
managers both a broad highway on which to carry all
of their Data, Voice, and Video traffic and a unified
LAN/WAN internetworking model. But ATM is a
complex, often misunderstood technology. This
White Paper, which explains ATM concepts and
terms, is a tutorial designed to help IS professionals
understand ATM technology and products. Armed
with this knowledge, they can better plan the role
ATM will play in the enterprise network.
ATM offers users a high bandwidth network,
able to carry an arbitrary mix of data, voice and
video traffic, and provides a unified LAN/WAN
networking model. ATM can deliver on these
promises because it is based on the connection-
oriented transfer of data in small, fixed-sized cells,
which are individually switched across virtual
circuits. An ATM data transfer involves three stages,
as follows:
1. Call Set-Up
Point-to-Point connections called “Virtual
Circuits” are first established between sender
and receiver. This is done by the ATM
network at the sender’s request. The sender
uses a process called signaling to request the
network to establish these connections, and to
specify connection attributes, such as
requesting the desired bandwidth.
2. Data Transfer
After the connection to the receiver is
established, the data to be sent is put into
fixed-sized cells which are sent over the
connection.
3. Call Termination
After the data transfer is complete, the sender
again uses signaling to tell the network the
connection is no longer needed. The network
then releases the “Virtual Circuit.”
The document describes in detail the operation
of ATM networks covering such topics as:
• Virtual Circuit set-up and usage
• ATM Adaptation Layers (AALs) which take
care of arranging the data into cells and assigning
Classes of Service to connections
• ATM cell structure
• ATM congestion control mechanism
• ATM LAN Emulation, which enables existing
LAN-based stations to communicate over ATM
backbones.
• ATM application examples
The appendices to this White Paper explain the
ATM reference Model (the ATM “cube”), the ATM
Standards and Specifications including the
organizations involved in defining them, and a brief
overview of ATM switch architectures.
Documentation Convention
The document has been compiled with both
detailed explanations of the subjects covered and
summaries and overviews of these subjects and their
coverage in this document.
Detailed explanations are presented in regular
text, with supporting material being included in the
Appendices.
Summaries of subjects that have been addressed
are shown by using a shaded background.
May 19, 1997 No. WP 142568 ATM — A Technology Perspective
W H I T E P A P E R
T E C H N O L O G Y

Page 2 ATM - A Technology Perspective Madge Networks
Introduction
This document on ATM (Asynchronous
Transfer Mode) is intended to provide an
introduction to ATM, its concepts and terms, and
help you understand the technology and the products.
This enables the user to plan the role ATM will play
in their networks, and put the LANNET ATM
offering in perspective.
Why ATM?The ATM Promise
First, let us examine the performance advantages
of ATM, to see what advantages it provides its users
and exactly why it merits so much attention.
The Need for ATM
Demand for high speed networks is being driven
by a number of factors, all of which fuel the need for
greater bandwidth. These include:
• Distributed processing
• Client server applications
• Bandwidth consumers  Graphics based
applications such as animation, and medical
applications such as MRI require extremely large
bandwidths
• Multimedia applications which combine data
with voice and video
ATM can deliver the high-bandwidth necessary
for the widespread deployment of these applications.
ATM also offers service integration, that is
voice, data and video. With ATM network managers
can offer a fully integrated service to the desktop
providing voice, video and data communication
services.
Another feature of ATM that attracts a lot of
attention is the issue of quality of service, a service
parameter which is essential when handling voice and
video traffic, as these are sensitive to both latency
and delay variations.
ATM utilizes digital signaling and optical fiber
for transmission, providing high speed
communications with an extremely low error rate.
As a result, minimal error checking and no error
correction is done. If an error is detected the packet
is discarded, leaving the higher layer end-to-end
protocols to handle errors and packet retransmission.
The ATM Promise
The services and features which will be provided
by ATM networks, such as increased flexibility,
service integration and a growth path, together with
the standards development activities of the ATM
Forum, have made ATM the focus of attention in the
enterprise network market. These services and
features include:
High bandwidth, low latency
ATM can offer extremely high bandwidths with
very low latency. Because it is on cell switching
technology, ATM provides a service which combines
the best features of circuit switching and packet
switching.
Wide range of services — data, voice and video
ATM does have the capability to handle data,
voice and video, but the standards to support these
services, and the associated end user equipment are
currently being developed.
Single architecture for both LANs and WANs
ATM promises a single architecture for both
Local Area Networks and Wide Area Networks.
This is the long term goal but in the transition to that
goal which will take a number of years, network
managers must map out a smooth migration path
from today’s existing shared and switched LAN and
WAN environments.
Scalable architecture
ATM provides a scaleable architecture, which
means that the same technology can be used to
provide a 25 Mbps service to the desktop or a 622
Mbps carrier service. Today, the underlying optical
fiber technology is being tested at speeds up to 4.8
Gbps.
Switched technology, allows QoS and better
resource allocation
The switched technology allows for the Quality
of Service provided by the connection to be
negotiated when the service is established. In
addition, network resources are allocated on a
connection basis, and thus can be better allocated
and managed. Quality of Service will be covered in
more detail later.

Protocol independent
Since ATM operates at the lower two layers of
the OSI model (the Physical and Data Link layers) it
is protocol independent.
Simplified configuration and management
ATM provides simplified configuration and
management as compared to router based networks.
A simplified network architecture using the concept
of Virtual LAN’s will be the model of the future.
Routers, rather than being replaced, will be
reassigned to provide routing between the Virtual
LANs and sub-ATM speed WAN internetworking.
Improved security
Improved security is provided by the fact that
switched, rather than shared, connections are utilized
and through the added functionality of Virtual LANs
and Virtual Routing. Virtual LANs and Virtual
Routing will not be addressed and are the subject of
other white papers, available from LANNET.
ATM Overview
Before delving into ATM and explaining it in
greater detail it may be useful to present an
overview, an “ATM in a Nutshell” description.
Data transfer in ATM networks works as
follows:
Call Set-Up
Point-to-Point connections called “Virtual
Circuits” are first established between sender and
receiver. This is done by the ATM network at the
sender’s request. The sender uses a process process
called signaling to request the network to establish
these connections and to specify connection
attributes, such as requesting the desired bandwidth.
Data Transfer
After the connection to the receiver is
established, the data to be sent is put into fixed-sized
cells which are sent over the connection.
Call Termination
After the transfer is complete, the sender again
uses signaling to tell the network the connection is no
longer needed. The network then releases the
“Virtual Circuit.”
ATM to Telephone Network Comparison
In order to clarify how ATM operates, let us
compare ATM data transfer with data transfer using
a standard modem over the public telephone network
Table 1: ATM to Telephone Network Comparison
Telephone Network ATM Network
Call Set-Up
Modem uses dialing tones to ask the network to set up a
connection to the sender and to specify the connection’s
desired attributes, such as transfer speed, error
correction protocol to use, etc.
The Network sets up a sender-to-receiver circuit
composed of a sequence of links between telephone
exchanges
Call Set-Up
ATM Client uses signaling to ask the ATM network
to set up connection to the sender and specify the
connection’s desired attributes, such as Bandwidth
required, Quality of Service desired, etc.
The Network sets up a sender-to-receiver Virtual
Circuit composed of a sequence of links between
ATM switches
Data Transfer
The data is put in a form of a stream of bits, which are
transformed by the modem into analog signals and sent
over the circuit.
The network transfers the data across the circuit from
exchange to exchange one bit at a time.
Data Transfer
The data is put in a form of a stream of fixed-sized
cells, which are transformed by the ATM client into
digital signals and sent over the Virtual Circuit.
The network switches the data along the Virtual
Circuit from switch to switch, one cell at a time.
Call Termination
The modem hangs up, telling the network the connection
is no longer needed.
The network releases the circuit created for this call.
Call Termination
The ATM client uses signaling to tell the network the
Virtual Circuit is no longer needed.
The network releases the Virtual Circuit created for
this call.

ATM  A Tutorial Introduction
We will describe ATM network operations by
following application data to be transmitted from the
sending application through the stages of setting up
the ATM connection, segmenting the data into ATM
cells, transmitting the cells over physical links, and
switching the cells at each ATM switch along the
transmission path.
The conceptual model that will be followed is
described in the Figure 1.
Figure 1: The ATM Model Layers
As already mentioned, an ATM conversation has
three stages: call set-up, data transfer, and call
termination. These will be reviewed in sequence,
explaining ATM as each stage is addressed.
Call Set-Up
The first step in any ATM conversation is to
establish a connection between the participants which
carries the traffic and supports the necessary
characteristics. This is done by using signaling to
request the ATM network to set-up a connection and
negotiate the connection’s attributes.
The ATM Connection Types — PVCs and SVCs
ATM connections are point-to-point connections
called Virtual Circuits. There are two types of
connections that can be set up between sender and
receiver: Permanent Virtual Circuits and Switched
Virtual Circuits.
Permanent Virtual Circuit (PVC)
A Permanent Virtual Circuit, or PVC, is a circuit
which exists permanently in a network. It is often
referred to as a nailed up circuit by
telecommunication carriers. Since the circuit is
permanently available, there is no call setup
procedure, and the Quality of Service parameters are
defined when the service is leased from the carrier.
PVCs are analogous to leased data lines.
Figure 2: Permanent Virtual Circuit
Switched Virtual Circuit (SVC)
The alternative to a PVC is a SVC, or Switched
Virtual Circuit. With a SVC the circuit exists only
for the duration of a call. This type of service is
analogous to that provided during an ordinary
telephone call or when using a switched 56 Kbps
circuit. Signaling procedures are defined and used to
establish, maintain and release a call.
There is a delay in establishing an SVC, but this
delay is minimal — like ISDN services, being in the
order of tens of milli-seconds. The Quality of
Service parameters required of the SVC are
negotiated with the network at the time the call is
established.
Figure 3: Switched Virtual Circuit
Voice
Video
Connectionless
Services
Connection Oriented
Services
User
Data
CBR
AAL
VBR/ABR
Connectionless
AAL
VBR/ABR
Connection
Oriented AAL
Segmentation and Reassembly
ATM Adaptation Layer
Convergence
Sublayer
SAR
Sublayer
ATM Layer
Physical Layer
(SONET/SDH)
SVC
PVC

The concept of establishing a connection which
supports specific traffic characteristics will now be
addressed by explaining ATM Classes of Service
and ATM Quality of Service.
ATM Service Classes and Quality of Service
Different applications need different kinds of
network services. For example, electronic mail may
be sent over a store-and-forward network, where
each stop along the path can store the message for
minutes or even hours. On the other hand, video
conferencing requires a network that will carry the
data in real-time, with a very small delay and no
delay variations.
ATM Service Classes
Because ATM can support various types of
traffic such as voice, video and data, it requires
network managers to understand the concept of
classes of traffic. Four fundamental classes of traffic
are supported by ATM; class A, B, C and D. Each
class of traffic must have an associated ATM
Adaptation Layer (AAL) to be supported by ATM.
The AAL’s will be covered later in further detail.
Table 2 summarizes the ATM service types.
(See below)
Consider the type of service each of these
classes provide, and the applications that are likely
to require these types of services:
• Class A provides a Circuit Emulation type
service, such as that required by video or voice
types of traffic.
• Class B provides a variable bit rate voice or
video service, such as that required by video
conferencing or compressed voice or video
applications.
• Class C provides a connection-orientated data
service — analogous to the service provided by
X.25 or Frame Relay switched virtual circuits.
• Class D provides a connectionless data transfer
service, such as the services commonly available
in today’s LANs and WANs.
ATM’s cell relay technology can support all four
of these classes of service. The classes of service
that can be provided by Frame Relay, X.25 and
SMDS are shown for comparison.
Quality of Service (QoS)
The classes of service described above are only
qualitative attributes. To make them useful we need
to introduce some quantitative measures for these
services. It is very well to demand a “connection-
oriented constant bit rate” service (Class A), but we
still need to specify some quantities — what is the
desired bit rate? While 14,400 bps may be sufficient
for an Internet connection, we need 128 Kbps for a
video conference call.
To completely specify a desired service, we
need to not only specify the service types, but also
the Quality of Service (QoS) required.
These QoS transmission parameters can be
grouped into three main attributes: Throughput,
Delay and Accuracy. Table 3 shows the Quality of
Service parameters that have been defined for the
different classes of types of traffic. Class D types of
traffic are not connection-oriented and therefore do
not have an associated QoS. (See table next page)
Table 2: ATM Classes of Service
Attribute Class A Class B Class C Class D
Connection mode Connection-oriented Connectionless
Bit rate Constant Variable
Timing relationship between
source and destination
Required Not required
Examples Circuit emulation Variable bit rate
voice or video
Connection-
oriented data
Connectionless data
transfer
ATM Adaptation 1 2 3 4
Layer (AAL) 5
Service Class can be
supported by
ATM ATM ATM
Frame Relay
X.25
ATM
Frame Relay
X.25
SMDS

Where : CBR stands for Constant Bit Rate traffic type.
VBR stands for Variable Bit Rate traffic type.
ABR stands for Available Bit Rate traffic type. See ABR
in the “Congestion Control” section.
ATM Signaling
We have seen that ATM can provide various
classes of services, and provide them at different
levels of quality of service. In order to use these
services, the sender’s application has to have a way
to ask the ATM network for the particular service
type and QoS desired. This is done through
signaling.
The function of ATM signaling is to establish,
maintain and release ATM connections. When a
circuit is established, the traffic characteristics or
Quality of Service parameters for the connection are
negotiated through signaling; after the transmission is
complete, the circuit is released using signaling. A
special AAL, called the Signaling ATM Adaptation
Layer (SAAL), is used for signaling. The ATM
signaling standard is called Q.2931 and it defines
how an ATM client can talk to the network, to
negotiate call set-up and QoS parameters, etc.
Signaling is achieved by sending ATM cells
addressed to the network containing the requests
from the network, rather than to a destination as in
the transfer of data.
This is called the ATM Network Interface.
ATM Network Interfaces
ATM actually uses two types of network
interfaces: a User to Network Interface (UNI) and a
Network to Network Interface (NNI).
User to Network Interface (UNI)
The User to Network Interface (UNI) defines the
interface between the user and the network. There
are two types of UNI interfaces: a private UNI, used
when an ATM user connects to an ATM switch on
the same corporate network, and a public UNI, used
when an ATM user or network connects to a public
service provider’s ATM Network.
Figure 4: User to Network Interface (UNI)
Network to Network Interface (NNI)
The NNI, or Network to Network Interface,
defines the interface between two ATM switches in a
public ATM network. Figure 5 shows the UNI and
NNI interface points in a multi-location ATM
network.
The NNI specification addresses the routing of
SVCs across multi-vendor ATM switches while still
meeting QoS criteria.
Table 3: Quality of Service Parameters
Quality of Service Traffic Type
Attribute Parameter Class A
CBR
Class B
VBR
Class C
ABR
Throughput Peak Rate
Committed Information Rate
Committed Burst Size
Excess Burst Size
X X
X
X
X
X
X
Delay Mean Transit Delay
Maximum Transit Delay X X
X
Accuracy Rate of Loss
Bit Error Rate
X
X
X
X
X
Public
UNI
Public
ATM
Network
Private
UNI

Figure 5: Network to Network Interface
If the network had been a private ATM network
created using a number of ATM switches, then the
UNI and NNI interfaces would be classified as
private interfaces. The private NNI or P-NNI
interfaces occurring between the ATM switches.
Summary of ATM Call Set-Up Considerations
ATM call set-up is a complex procedure,
involving many issues. In ATM networks, the
connection is rigorously defined when it is first
established; the data transfer through this connection
is thus simplified.
To recap the call set-up process:
Using appropriate signaling (UNI or NNI) the
ATM client negotiates with the ATM network to
establish an ATM connection to the destination (a
permanent or switched virtual circuit) of the desired
service type (constant or variable bit rate,
connection-oriented or connectionless, etc.) and with
the required quality of service attributes.
ATM Data Transfer
Once the ATM connection is established, data
can be sent over it by segmenting it into ATM cells
and transferring the cells along the Virtual Circuit
making up the connection.
The ATM model actually sees Virtual Circuits
as being composed of Virtual Paths and Virtual
Channels. Virtual Paths and Virtual Channels are the
mechanism used to transport and route cells in ATM.
Virtual Paths and Virtual Channels
A Virtual Channel (VC) is a logical
transmission path which is used to transport cells
between two end points. Each end point uses a
Virtual Channel Identifier (VCI) to identify the
transmission path.
A Virtual Path (VP) is a group of Virtual
Channels that share a common transmission path and
have the same Virtual Path Identifier (VPI).
Figure 6: Virtual Paths and Virtual Channels
An ATM sender-to-receiver connection or
Virtual Circuit, is a particular Virtual Channel inside
a particular Virtual Path.
To better understand the relationship between
Virtual Circuits, Virtual Paths, Virtual Channels and
their identifiers (VPI/VCI’s), let us return to the
telephone network analogy. To transfer data between
two particular users we need to set up a point-to-
point connection, called a Virtual Circuit, between
them.
If we look at all the telephone numbers in a
particular area, we will see that they all start with a
common prefix — their area code. If we are
transferring data from San Jose to Boston, it is useful
to transfer in bulk all the traffic from the San Jose
area code intended for users in the Boston area code.
For this stage, we don’t need to know for which
particular user in Boston the traffic is intended — it
is sufficient to look at the area-code part of the
destination telephone number. This is analogous to
the notion of a Virtual Path in ATM.
Once the bulk-transmitted traffic reaches the
Boston Exchange (switch), the area code is no longer
useful (all intended recipients have the same area
code) and the traffic is switched according to the
local component only — analogous to the Virtual
Channels In ATM.
So we can view the complete Virtual Circuit,
identified by the full telephone number, as being
composed of two parts — a part where data is
NNI
NNI
UNI
UNI
UNI
Public
ATM
Network
NNI
VPVC
VPVC
VP Transmission
Path
VP
VP
VP VC
VC
VC

transferred in bulk using just a common prefix of the
circuit identifier, and a local part using just the
unique part of the Virtual Circuit identifier. In ATM
the common prefix is called the VPI — Virtual Path
Identifier, while the remainder is called the VCI —
Virtual Channel Identifier. We will study ATM
switching in more detail in a later section; but first,
let’s examine the ATM cell structure in greater detail
and see how the VPI and VCI are specified in each
ATM cell.
The ATM Cell Structure
All ATM data transfer is done in the form of
fixed-sized cells, consisting of 53 bytes. Of these 53
bytes, 48 bytes are the Cell Payload, into which the
user data to be transferred is placed, and 5 bytes are
the Cell Header.
Figure 7: ATM Cell Structure
The structure of the cell payload depends on the
ATM Adaptation Layer (AAL) used, and is
discussed in the next section.
The Cell Header carries the information required
to allow the network to switch it along the
appropriate Virtual Circuit. The cell header is
slightly different for a User-to-Network Interface
(UNI) and for a Network-to-Network Interface (NNI)
as shown in Figures 8 and 9.
GFC — Generic Flow Control
The Generic Flow Control field is only used to
provide flow control over the UNI interface. No flow
control information is carried across the network and
the GFC field does not occur in a NNI interface. In
the NNI interface these bits are used to extend the
VPI field.
Figure 8: UNI Cell Header Structure
Figure 9: NNI Cell Header Structure
VPI/VCI — Virtual Path Identifier and Virtual
Channel Identifier
The VPI and VCI fields are used to route the cell
through the ATM network, together making up a
complete identification of the ATM connection, or
Virtual Circuit.
PT — Payload Type
The Payload Type field is a three bit field that
indicates whether the cell contains user or
management information. This field is also used to
provide network congestion notification.
CLP — Cell Loss Priority
The Cell Loss Priority bit, if set, indicates to the
network that in the event of congestion occurring, the
cell may be discarded.
HEC — Header Error Control
The Header Error Control field is only used to
detect errors occurring in cell headers; no error
checking is done on the payload or data field. Cells
received with header errors are discarded. Higher-
Byte
GFC VPI
VPI VCI
1
2
3
4
5
6~53 PAYLOAD (48 bytes)
8 7 6 5 4 3 2 1
VCI
VCI PT CLP
HEC
Byte
VPI
VPI VCI
1
2
3
4
5
6~53 PAYLOAD (48 bytes)
8 7 6 5 4 3 2 1
VCI
VCI PT CLP
HEC
HEC
Payload
5 bytes 48 bytes
Header
VPI VCIGFC PT CLP

layer protocols are responsible for initiating the
retransmission of lost cells.
Setting the Cell’s Contents — The ATM
Adaptation Layer
Now that we have an ATM connection to
transfer data across, we need to put the user’s data in
the payload of a sequence of ATM cells. This task
is carried out by the ATM Adaptation Layer.
The ATM Adaptation Layer is composed of two
sub-layers. The Convergence Sublayer (CS)
determines the way User data is to be segmented into
cells (according to the type of service required),
while the Segmentation And Reassembly (SAR)
sublayer carries out the actual segmentation of user
data into ATM cells and reconstruction of user data
from incoming ATM cells.
AAL Types: The Convergence Sublayer and the
ATM cell Payload structure
ATM provides different AAL types to support
the different classes of service. This means that
ATM will pack user data differently for each type of
service provided. For example, it is clear that if we
need a timing relationship between sender and
receiver, each cell has to carry with it some sort of
time-stamp. The appropriate AAL (AAL 1 in this
case) will define the format and location of the time
stamp in the ATM cell’s payload part, and the format
and location of the user’s data (e.g. the video data).
The CS sublayer also ensures that cells arrive in
the correct sequence by placing sequence numbers in
the payload of the cells. The following drawings
show some examples of how the cell’s payload
contents is set by different AAL types; however a
complete discussion of the AAL layers is beyond the
scope of this document.
Figure 10: Cell Payload Structure Examples
Where: SN: Sequence number
SNP: Sequence number protection
IT: Information type
MID: Message indicator
LI: Length indicator
CRC: Cyclic redundancy check
SAR — The Segmentation And Reassembly
Sublayer
After the different AAL types have determined
the format of the ATM cells payload, the data must
be segmented into outgoing cells according to the
appropriate payload structure. For incoming cells,
the process is reversed; the AAL analyzes the
structure of their payload, and extracts the incoming
user data. These tasks are handled by the SAR
sublayer of the AAL.
The ATM Layer
At this stage we have a stream of ATM cells to
be transported across the network. These cells will
be transmitted using a particular physical method
which will be discussed next. The crucial point is
that the ATM network needs to quickly and
efficiently switch the cells at each switch along the
Virtual Circuit that the cells are to follow. This is
done by the ATM layer.
The Heart of ATM — Cell Switching
ATM switching is done cell by cell, according to
the information in the cell’s header. An ATM switch
may use just the VPI part of the cell’s header to
decide how to forward the cell (Virtual Path
Switching), or just the VCI part (Virtual Channel
Switching), or both. In general, the switching works
as follows:
Figure 11: ATM Switching — General Description
SN IT MID User Data LI CRC AAL type 3/4
2 bits 4 bits 10 bits 44 bytes 6 bits 10 bits Payload structure
SN SNP Pointer User Data AAL type 1
4 bits 4 bits (optional) 46 or 47 bytes Payload structure
8 bits
a
b
c
d
w
x
y
z
Routing
Table
VPI=6, VCI=17
VPI=8, VCI=35
VPI/VCI In Port In VPI/VCI OutPort Out
6,17 b z 8,35
11,3 d w 19,5
ATM Switch

The figure above shows a general depiction of the
ATM switching mechanism. The ATM switch
receives a cell on a particular incoming port, and the
cell is marked (using the VPI/VCI fields of the
header) as belonging to a particular Virtual Circuit.
The switch then examines its routing table, from
which it finds out :
a. on which outgoing port to forward the cell
b. to what values the VPI/VCI need to be set on the
outgoing cell.
However, just as in our telephone call switching
example, it is useful to switch ATM cells “in bulk”
using only the VPI field in the cell header.
Virtual Path switching routes all cells with the
same VPI, irrespective of their VCI, to the same
output port. For example, virtual path 40 is switched
to virtual path 32 carrying unchanged the associated
virtual channels, that is 21 and 45.
Figure 12: Virtual Path Switching
Once cells “arrive at the general location,”
switching is then done using the “local” part only —
the VCI field.
Figure 13: Virtual Channel Switching
Virtual Channel Switching switches each
virtual channel independently. For example, in the
figure below, virtual channels 31 and 19 are switched
to different virtual paths.
Most ATM switches perform VPI/VCI
translation, that is a data packet entering an ATM
switch with a specific VPI/VCI will have a different
VPI/VCI on leaving the switch because it has been
switched from one transmission path to another. For
example, an incoming cell with a virtual channel
identifier of 28 and a virtual path identifier of 31, is
switched to the output port with a virtual channel
identifier of 16 and a virtual path identifier of 14.
Figure 14: Combined VPI/VCI Switching
ATM Addressing & Management
ATM Addressing
Every ATM device connected to an ATM
network is identified using the OSI Network Service
Access Point (NSAP) address format. When an
ATM device connects to the network for the first
time it will register its MAC address with an Address
Registration service provided by the network and
will then be assigned an ATM NSAP address which
is based on the MAC address.
The Address Registration procedure with the ATM
network is conducted via the Interim Local
Management Interface, ILMI.
ATM Management
The Interim Local Management Interface
(ILMI) is part of the ATM Forum UNI
specification. The ILMI utilizes SNMP-based
management to provide status, configuration and
control information for an ATM Interface.
VP=40 VP=40VP Switching
VC=52
VC=17
VC=45
VC=21
ATM Switch
VP=18
VC=11
VC=32
VP=32
VC=45
VC=21
VP=40 VP=32
VP=17
VP=31 VP=20
VP=14
VC=45
VC=60
VC=11
VC=21
VC=16
VC=18
VC=45
VC=28
VC=31
VC=21
VC=19
VC=23
ATM Switch
VP=17
VP=31 VP=20
VP=14
VC=60
VC=11
VC=16
VC=18
VC=28
VC=31
VC=19
VC=23
VC Switching
ATM Switch

The ATM MIB defined for the UNI interface has
seven groups. Of these groups, the ATM Layer
Statistics group is optional, the other six groups are
required. The required groups address the Physical
Layer, ATM Layer, Virtual Path Connection, Virtual
Channel Connection, Network Prefix and Address.
It is expected that individual ATM equipment
vendors will offer private MIB extensions to support
proprietary features.
Figure 15: ATM UNI ILMI MIB
Congestion Control
As we have seen, an ATM client can ask for a
particular Quality of Service for each particular
connection initiated. Since many data transfers have
a variable demand for resources, it is most
economical to ask for some average level of service,
with a margin of safety, rather than the maximum
service available. If, at a particular moment, too
many users flood the network with above-average
demands, the network may become congested. Thus,
the network must be able to cope with congested
conditions, both by taking immediate steps (usually
discarding cells) and by propagating the knowledge
that congestion occurred back to the clients so that
they can reduce their resource consumption levels, or
re-route their traffic away from the congested
devices.
A full coverage of congestion control is beyond
the scope of this document, but we’ll take a brief
look at the general mechanism of Available Bit Rate
(ABR) service, which is the protocol used for
congestion control.
ABR works by adding to the stream of user-data
cells special cells, called OAM (Operation,
Administration and Maintenance) cells that are
generated by the ATM devices. These cells travel
along the same Virtual Circuit as the user’s data. If
any ATM device along the route starts to be
congested, it marks these cells by setting a bit called
Explicit Forward Congestion Indication (EFCI).
All ATM devices from that point on will look at this
bit, and will note that congestion exists along this
circuit. When the OAM cells reach the destination,
the EFCI notification is sent back along the same
circuit, until it gets back to the origin. In this manner
all ATM devices along the Virtual Circuit learn
about the congestion, and can take appropriate
action.
When using an ABR service, the sender device
uses the CLP bit in the cell headers to mark cells to
be discarded first if congestion occurs. Cells will be
discarded as necessary until the sender device
receives notification of the congestion via the receipt
of OAM cells. Once an OAM cell indicating
congestion is received, the sender will reduce the
rate of cell transmissions, and will continue reducing
the rate until returning OAM cells indicate the new
rate no longer causes congestion.
The Physical Layer
How do we arrive at an ATM transmission speed
of 155 Mbps?
To conclude our explanation of data transfer in
ATM networks, we will examine the physical
transmission of ATM cells from switch to switch.
The physical layer defines the electrical
properties of the carrier signals, such as voltage and
frequency, and the physical properties of the media,
such as fiber and connector type.
ATM cells may, in principle, be transmitted over
any physical medium, but in most cases, the physical
layer used will consist of a Synchronous Optical
Network (SONET) network over fiber optic links.
The transmission convergence sub-layer
provides the adaptation of the physical layers for the
transfer of ATM cells. Examples are SONET or
SDH framing, and Header Error Control generation
and verification.
Synchronous Optical Network or SONET,
defines a fiber optic transmission system offering
services as Optical Channels from OC-1 at 51 Mbps
to OC-96 at 4.8 Gbps.
Synchronous Transport System or STS in North
America, and Synchronous Digital Hierarchy or SDH
ATM UNI ILMI MIB
Physical
Port
ATM
Layer
Virtual
Path
Connection
Virtual
Channel
Connection
ATM
Statistics
Network
Prefix
Address

in Europe, are the electrical signaling specifications
for SONET.
The Physical Layer Interfaces included in the
ATM Forum UNI specification are shown below,
including the recently adopted 25 Mbps standard
proposed by IBM.
Private & Public UNI
• 34 Mbps - E3
• 45 Mbps - DS3/T3
• 51 Mbps - SONET STS-1/OC-1
• 139 Mbps - E4
• 155 Mbps - SONET STS-3c/OC-3c
Private UNI
• 25 Mbps - UTP
• 100 Mbps - Multimode Fiber (TAXI)
• 155 Mbps - Multimode Fiber
The interfaces for private ATM networks do not
have to meet the physical framing and timing
requirements of public ATM networks. The 100
Mbps private UNI, commonly known as the TAXI
interface, allows users to migrate from FDDI to
ATM using existing fiber optic cable and network
equipment.
As ATM technology is developed, new speeds
and interfaces will come into the market; probably
the first to appear will be a SONET OC-12, 622
Mbps interface.
Where do these speeds come from?
The SONET/SDH standard defines a way to
send bits over fiber optic links in blocks called
frames, defined as having a duration of exactly 125
uS. Obviously the more bits we are able to transmit
within this duration, the higher the bandwidth of the
transmission. SONET and SDH define several
possibilities for the this number, and so several
possible bandwidths are supported.
The time slot of 125 uS exists for technical and
historical reasons, if we send a single bit within each
time slot, the resulting bandwidth would be 8 Kbps,
which is the amount required to carry a single
telephone-quality channel.
If we transmit, within that time slot, 6480 bits
(this number is the product of complex technical
considerations), and as there are 8000 such time
slots in each second, we get the following bandwidth:
6480 Bits/Slot x 8000 Slots/Second
= 51,840,000 bits per second = 51.84 Mbps.
This is called an OC-1 or STS-1 frame and is
the basic unit of SONET transmissions, with all other
speeds being multiples of this unit.
If we transmit 3 STS-1 frames in each time-slot,
this will result in sending:
51.84 Mbps x 3 = 155,520,000 bits per second
= 155 Mbps.
This is called an OC-3 or STS-3 transmission
unit, and is the first level of transmission for SDH
networks.
It should now be clear that OC-12 transmission
level means sending 12 STS-1 frames in each time
slot, with a resulting bandwidth of:
51.84 x 12 = 622 Mbps.
Call Termination
Once the data transfer is concluded, the ATM
client again uses signaling (ATM cells built
according to the SAAL) addressed to the network
(via a standard, pre-defined Virtual Channel
Identifier), to tell the network the conversation is
complete, and that the virtual circuit providing the
connection can be released.
This is done by removing the appropriate
VPI/VCI’s from all the forwarding tables of all the
switches along the now defunct Virtual Circuit and
freeing up any resources reserved for it by the
switches.
ATM Data Transfer Summary
In the preceding paragraphs we have seen how
ATM data transfer is carried out in three stages:
• Call set-up — the sender uses signaling to set
up a channel with the appropriate attributes to
the destination
• Data Transfer — the data to be sent is packed
into ATM cells in a form determined by the
AAL type used; these cells are then switched
along the ATM connection using the VPI/VCI
fields in the cell header
• Call termination — signaling is again used to
tell the network the connection may be released.

ATM Network Applications/Services
As previously mentioned, the eventual goal of
ATM is to provide an end-to-end unified network for
all clients. However, most established networks in
use today rely on existing LAN/WAN equipment and
protocols. If we want to be able to introduce ATM
networks in an evolutionary way, without scrapping
the existing network and replacing it completely, we
need a way to integrate today’s networks with ATM.
LAN Emulation (LANE)
Purpose & Overview of LAN Emulation
The purpose of LAN emulation is to allow
existing LAN clients to send data over ATM
backbones, and to communicate with ATM-based
resources, without requiring any change in the LAN-
based clients.
The LAN emulation service essentially hides the
ATM network from the LAN clients by emulating
LAN behavior, so that LAN-based devices can
continue to use LAN based protocols.
Some of the issues LAN emulation has to
address are:
The broadcast nature of LANs vs. the point-to-
point nature of ATM
LANs assume that a packet sent over the LAN
will be seen by all stations belonging to that LAN.
This allows a station to generate a Broadcast by
sending a single packet, with an “all” destination
address. In ATM networks, all connections are
point-to-point, so that a packet generated by a station
will only get to a single receiver. To reconcile these
different models, the LAN emulation service has to
generate copies of the packet to be broadcast, and
send these copies to each member of the LAN being
emulated.
Different addressing schemes
Since we want LAN-based stations to keep
working without changes, it follows that they must be
able to use LAN type addresses to identify
destinations. Since ATM networks use ATM
addresses to identify stations, and VPI/VCI tags to
identify connections, the ATM LAN Emulation
service will have to provide for address translation
and resolution between the different forms.
The LAN Emulation Service
LAN Emulation is a Layer 2 service and is
independent of upper layer protocols, supporting
both routable and non-routable protocols.
The LAN Emulation User to Network Interface
(version 1.0) or LUNI, as defined by the ATM
Forum, specifies how Ethernet or Token Ring
attached workstations connect to their counterparts
over an ATM network. It also defines how ATM
attached servers communicate with devices on
existing Ethernet or Token Ring LANs.
FDDI must be converted to either Ethernet or
Token Ring to be transported over ATM.
Emulated LANs do not mix media; a router is
required to provide this level of connectivity. For
example, an Ethernet client cannot communicate to a
Token Ring server, this connectivity must be
provided by a router. LUNI supports multiple
Emulated LANs on the same physical LAN, but
routers are required to provide connectivity between
Emulated LANs.
LAN Emulation allows existing LAN protocols,
such as IP, IPX, AppleTalk, etc., to run over ATM
networks without requiring any changes to the
applications.
LAN Emulation has a number of components, the
LAN Emulation Client or LEC, the LAN Emulation
Configuration Server or LECS, the LAN Emulation
Server or LES, and the Broadcast and Unknown
Server or BUS.
An emulated LAN provides the functionality of a
single Ethernet segment or Token Ring.

Figure 16: ATM LAN Emulation
LAN Emulation supports Unicast, Multicast and
Unknown traffic types through the mechanisms
provided by the LEC, LECS, LES and BUS.
The main function of the LAN Emulation Client
is that of address resolution, that is mapping MAC
addresses to ATM addresses. The LEC interfaces to
the ATM network over a LAN Emulation UNI
(LUNI).
The LAN Emulation Configuration Server
provides a LEC with configuration information, and
the address of the LES. There is one LECS for all
emulated LAN’s, but each emulated LAN has its
own LES and BUS.
The LAN Emulation Server is responsible for
registering and resolving MAC addresses to ATM
addresses.
The Broadcast and Unknown Server handles the
function of broadcasting and multicasting over the
ATM network.
The activity on an Emulated LAN can be
summarized as follows:
Initialization
In order to establish an ATM connection, a LEC
must find the ATM address of the LES so it can join
the Emulated LAN. The LEC uses the ILMI (Interim
Local Management Interface) to try to get the address
of the configuration server from the ATM switch to
which it is attached. The LEC then attempts to set up
a connection to the configuration server to get the
LES ATM address. If the LEC fails to locate the
configuration server it reverts to ILMI to try to locate
another configuration server or to see if the address
of the LES is listed.
If this fails, the LEC uses a “well known ATM
address”, that is an agreed upon address for the
configuration server that will be specified in LUNI
and used on every ATM network. Additional
mechanisms are also defined in LUNI to address the
situation where these methods fail.
Configuration
The LEC tells the configuration server its ATM
address, MAC address, type of LAN supported,
maximum frame size which will be accepted, and
LAN Emulation
Client
LES
BUS
LEC
LECS
Edge
Adapter
LEC
Broadcast and Unknown
Server
Edge
Adapter
LEC
LAN Emulation
Configuration
Server LAN Emulation
Server
LAN Emulation
Client

optionally the name of the emulated LAN it wants to
join. The configuration server then tells the LEC the
type of Emulated LAN it is about to join and the
maximum frame size allowed.
Joining
When the LEC has all of the above information it
joins the emulated LAN. This is done by first
creating a bi-directional connection to the LES. It
then sends a JOIN REQUEST to the LES, containing
its ATM address, LAN type, maximum frame size,
and proxy indication. The LEC provides this
information in case it has not successfully reached a
configuration server.
The LES sends back a JOIN RESPONSE, either
registering the client with the emulated LAN or
refusing its request. If a LEC request is refused it
must terminate the connection to the LES.
Registration and BUS Initialization
When the LEC registers with the LES, it requests
the ATM address of the BUS, that is the ATM
address to be used for broadcasts. The LEC then
sets up a data connection with the BUS which adds
the LEC to its broadcast table.
Data Transfer
When the LEC receives a LAN packet, it checks
to see if it has the ATM address of the target and
whether the target address is a broadcast address. If
the packet is identified as a broadcast or multicast
packet the packet is sent out to the BUS for
distribution. If it is not a broadcast packet the LEC
checks to see if it knows the ATM address of the
target, then it checks to see if it has a virtual
connection established with the target, if it does it
sends the packet., otherwise it uses UNI signaling to
set up a connection with the target.
If the LEC does not know the ATM address of
the target, it issues a LE-ARP (LAN Emulation
Address Resolution Protocol) packet to the LES
requesting it. While it is waiting for a response it
sends the packet to the BUS which broadcasts it over
the emulated LAN. After the LES responds with the
ATM address, the LEC sets up a connection with the
target. If the LES does not respond with an address,
the LEC continues to broadcast the packets by
directing them to the BUS.
IP Over ATM — RFC 1577
Purpose and Overview of IP over ATM
There are many networks using IP as the network
layer protocol, and they use a variety of transport
methods — Ethernet, Token-Ring, FDDI, Dial-Up
lines, etc. In each case, the IP implementation is
aware of the transport mechanism used, and uses this
knowledge to provide an efficient service.
If we view ATM as just another Transport
mechanism for IP to run over, we can customize the
IP standard for transport over ATM to provide an
efficient implementation.
In particular, if IP is customized for ATM
transport, then it can use ATM addresses for the
link-level addresses.
This is best understood by looking at the
operation of the IP Address Resolution Protocol
(ARP) on a traditional Ethernet LAN and on an ATM
network.
An IP station wishing to send to a destination
when only the destination’s IP address is known,
needs to find out the destination’s link-level address
to be used in the link-level packets.
Ethernet
In an Ethernet LAN, the sender generates an
ARP broadcast essentially asking “will the
station whose IP address is X please identify
itself?”. This broadcast is received by ALL LAN
stations, and the intended destination responds
with an ARP response containing its link-level
(MAC) address. The sender then uses this
address to send the data.
IP over ATM
The sender sends a packet to an ARP server
(not broadcast) asking for the destination’s link-
level address. The server returns the
destination’s ATM address, which is used to set
up the sender-to-receiver connection
RFC 1577 “Classic IP over ATM”
Classic IP and ARP over ATM is defined in
RFC 1577 issued by the IETF (Internet Engineering
Task Force). This specification addresses the
transportation of IP over ATM, using AAL5
encapsulation for IP and ATM ARP.

RFC 1577 introduces the concept of a Logical IP
Subnetwork or LIS, where the LIS members are those
devices attached to the ATM network. Network
members outside of the LIS are accessed via a
router.
The IP address resolution depends on whether a
SVC or PVC is used. In the case of a SVC, the IP
address is mapped to an ATM address; with a PVC,
the IP address is mapped to a Virtual Channel.
The LAN Emulation Alternative
It must be understood that it is possible to run IP
over ATM networks using the LAN emulation
service. Since, as we have seen, LAN emulation
hides the ATM network from the LAN clients,
allowing them to continue using the existing
applications and protocols, IP networks can run over
ATM using their current IP implementation.
IP network managers must decide whether to use
LAN emulation or a modified IP implementation.
WAN/ATM Connection
Routers
In the last two years, most routers have been
enhanced to support ATM by implementing the DXI,
or Data Exchange Interface in the router, providing
an interface to an ATM DSU (Data Service Unit).
The ATM DSU implements the functions of the
Adaptation, ATM and Physical layers. ATM
interfaces for routers are now also available,
eliminating the need for the ATM DSU.
Frame Relay to ATM
Frame Relay has become widely deployed as the
service of choice for WAN connectivity and both
Network, and Service internetworking specifications
have been developed by the Frame Relay Forum.
Network Internetworking
Network Internetworking allows two Frame
Relay networks to communicate over an ATM
backbone, the ATM network acting as the
transport for the Frame Relay data.
Service Internetworking
Service Internetworking enables a Frame
Relay network to communication with an ATM
network, thereby allowing users on one network
to access resources on the other.
SMDS to ATM
Network Internetworking has also be
implemented for SMDS, allowing two SMDS
networks to communicate over an ATM backbone.
ATM is clearly becoming the transport medium
of the future, with integration interfaces being built
between ATM and other current wide area network
architectures.
ATM Applications
The challenge faced in implementing ATM
networks is to deploy ATM without disrupting
existing network operations, while protecting ones
investment in existing routers and hubs. Ideally one
would like to take advantage of the bandwidths
offered by ATM without having to change Network
Interface Cards (NICs), cabling, network operating
systems, or having to retrain end users.
The ATM implementation needs to integrate the
LAN environment into that of ATM, providing LAN
to LAN connectivity over ATM, and LAN to ATM
internetworking. The internetworking of LAN and
ATM networks allows resources on one network to
be accessed by clients on the other.
Point-to-Point Inter-hub Connection
Two Hubs
Figure 17 shows the deployment of ATM as the
high speed communication link between two hubs
supporting switched Ethernet clients. A LAN to
ATM switch module is installed in each hub and
ATM is being used as the transport media for data
between the hubs.
LET36 LET36
Switched
Ethernet
ATM Link
Switched
Ethernet

Figure 17: Point-to-Point Inter-hub Connection Two Hubs
Point-to-Point Inter-hub Connection
Multiple Hubs
The ATM backbone can be extended to
additional hubs, as shown in Figure 18. A second
ATM backbone could also be established between
the hubs to create a fault tolerant environment.
Figure 18: Point-to-Point Inter-hub Connection Multiple Hubs
Native ATM Connection
Figure 19 shows the integration of switched LAN
and native ATM resources. The switched LAN
clients are attached to a hub which is connected to
the ATM switch using an LAN to ATM switch
module in the hub. The server, which is fitted with an
ATM NIC card, is also attached to the ATM switch.
Through LAN Emulation Client services running in
the LAN to ATM switch module and the associated
LAN emulation server functions running in the ATM
switch, the LAN clients are able to communicate
with the ATM attached server.
Figure 19: Native ATM Connection
ATM Backbone using ATM Switches
Figure 20 shows a campus environment utilizing
multiple ATM switches and hubs to provide
broadband communication services to clients over a
high speed ATM backbone. Wide area network
connectivity to other sites in the network is achieved
by an ATM WAN link to one of the ATM switches.
Figure 20: Sample Application: ATM Backbone Using ATM
Switches
LE T 3 6
LE T 3 6LE T 3 6
Shared Ethernet to ATM
Switched FDDI to ATM
Switched
Ethernet/Token Ring
to ATM

Appendix A
The ATM Reference Model
The ATM reference model is a familiar — it
appears in many articles on ATM — but it is not
well understood as it is a complex three dimensional
model. The position of the classes of service and
their associated ATM Adaptation Layers are shown
in this model.
The ATM reference model consists of three planes:
User, Control and Management.
• The User plane is the interface to the upper
layers of the user applications.
• The Control plane is required to support
Switched Virtual Circuits. The Control plane
handles call control, including call set-up,
management and release.
• The Management plane provides the ability to
exchange information between the User and
Control planes. It also provides layer specific
management coordination functions related to the
complete system.
It should be noted that the ATM standards only
address the Adaptation, ATM and Physical layers of
the model. These layers map to the Physical and
Data Link layers of the OSI reference model.
Figure 21: ATM Reference Model
Physical Layer
The physical layer defines the electrical
properties of the carrier signals, such as voltage and
frequency, and the physical properties of the media,
such as fiber and connector type.
The transmission convergence sub-layer
provides the adaptation of the physical layers for the
transfer of ATM cells; for example, SONET or SDH
framing, and Header Error Control generation and
verification.
Synchronous Optical Network, or SONET,
defines a fiber optic transmission system offering
services as Optical Channels from OC-1 at 51 Mbps
to OC-96 at 4.8 Gbps.
Synchronous Transport System or STS in North
America, or Synchronous Digital Hierarchy or SDH
in Europe, are the electrical signaling specifications
for SONET.
The Physical Layer Interfaces included in the
ATM Forum UNI specification are shown, the
recently adopted 25 Mbps standard proposed by
IBM is included.
Private & Public UNI
• 34 Mbps - E3
• 45 Mbps - DS3/T3
• 51 Mbps - SONET STS-1/OC-1
• 139 Mbps - E4
• 155 Mbps - SONET STS-3c/OC-3c
ATM
Standard
Signaling
& Control
Class A
Constant Bit
Rate Circuit
Emulation
Class B
Variable Bit
Rate
Voice/Video
Class C
Connection
Orientated
Services
Class D
Connection-
less
Services
AAL 1 AAL 2
AAL 3 AAL 4
AAL 5
Adaptation Layer
ATM Layer
Physical Layer
Management Plane
Control
Plane
User Plane

Private UNI
• 25 Mbps - UTP
• 100 Mbps - Multimode Fiber (TAXI)
• 155 Mbps - Multimode Fiber
The interfaces for private ATM networks do not
have to meet the physical framing and timing
requirements of public ATM networks. The 100
Mbps private UNI, commonly known as the TAXI
interface, allows users to migrate from FDDI to
ATM using existing fiber optic cable and network
equipment.
New speeds and interfaces can be expected for
ATM as technology develops, and a SONET OC-12,
622 Mbps interface will certainly be added.
ATM Layer
The function of the ATM layer is switching. It
provides the interface between the Physical and
ATM Adaptation layers and is independent of the
physical medium implemented in the physical layer.
The ATM layer bases its operation on the cell
header.
Flow control provides a mechanism to control
traffic flow at the UNI to eliminate congestion.
The ATM layer handles the cell header construction
and verification, cell routing, and cell multiplexing
and demultiplexing. Cell multiplexing and
demultiplexing combines cells from several
applications into a single channel.
The ATM layer provides the translation between
the Application Service Access Point, or SAP, and
the Virtual Path and Virtual Channel Indicators, that
is VPI and VCI.
Cell routing is based on a two-level addressing
structure, VPI and VCI. A VPI/VCI pair being
assigned to each application
ATM Adaptation Layer
The AAL (ATM Adaptation Layer), interfaces
the higher layer protocols to the ATM layer. The
AAL has two sub layers:
The convergence sublayer prepares the user data
for the Segmentation and Reassembly sublayer. The
user data fields can range from one to 9,000 bytes.
The convergence sublayer is service dependent and
uses different AAL’s to support the different classes
of service.
AAL1 provides a CBR (Constant Bit Rate)
service for circuit emulation. AAL1 has a 47 byte
payload, one byte being used for clocking and
sequencing. AAL1 cannot emulate a T1 (1.544
Mbps) circuit over an ATM T1 service because of
the ATM cell overhead and the AAL1 overhead of 1
byte per cell.
AAL2 is the place holder for variable bit rate
video transmission and is still undefined by the
international standards organizations.
AAL3 and 4 were combined when it was realized
that a single protocol could be used to support both
connection-oriented data services and connectionless
datagrams. AAL3/4 is an inefficient AAL having a 4
byte protocol overhead.
AAL5 or SEAL, Simple and Efficient Adaptation
Layer, provides more limited functions — for example,
error detection but not recovery — but has lower
processing and bandwidth requirements. The ATM
Forum is developing a specification for carrying CBR
MPEG2 bit stream data using AAL5.
The ATM OAM, Operational Administration
and Maintenance services, use their own AAL known
as SAAL (Signaling ATM Adaptation Layer). SAAL
is also used for the Local Management Interface —
LMI, Interim Local Management Interface — ILMI,
Quality of Service and congestion management
traffic.
The Segmentation and Reassembly (SAR)
sublayer protocol segments the protocol data units,
or PDU’s, into 48 byte cells and reassembles
received cells into packets.

Appendix B
ATM Standards and Specifications,
and the Defining Organizations
There are a number of organizations involved in
the standards related to ATM, including:
ITU-T (CCITT)
ITU-T, a division of the International
Telecommunications Union, which has taken over the
responsibilities of the CCITT, sets worldwide
standards. These are essential when it comes to the
interworking of the carriers in different countries.
Standards related to ATM which have been
established by the ITU-T, are covered in their I and
Q series of standards. These include:
• I.150 - B-ISDN ATM Functional Characteristics
• I.361 - B-ISDN ATM Layer Specification
• Q.2931- B-ISDN User-Network Interface Layer
3 Protocol Signaling
ANSI and ETSI
The standards set by bodies such as ANSI
(American National Standards Institute) and ETSI
(European Telecommunication Standards Institute)
are regional standards (North America and Europe
respectively), that are compatible with those of the
ITU-T.
The International and National standards are
influenced by the specifications developed by
organizations such as Bellcore, the research arm of
the seven US Regional Bell Operating Companies,
the IETF (Internet Engineering Task Force) and the
ATM Forum.
Organizations such as the ATM Forum and IETF
do not set standards, they develop specifications or
recommendations which they submit to the standards
organizations for consideration for inclusion in future
standards.
IETF
The IETF produces specifications are known as
RFC’s, or Request For Comments.
The ATM RFC’s include:
• RFC 1483 - Multiprotocol Encapsulation over
ATM AAL 5
• RFC 1577 - Classical IP and ARP over ATM.
• RFC 1695 - the AToM MIB for SNMP
management.
ATM Forum
The mission of the ATM Forum is to
“Accelerate the use of ATM products and services,
through the rapid convergence of interoperability
specifications, promotion of industry cooperation and
other activities.”
The Forum has three classes of membership,
Principal (200), Auditing (350) and User (122)
members. The Principal members participate in the
development of specifications and vote on proposals.
The Forum has three committees which address
different aspects of their operations, Technical,
Marketing and Education. The Technical Committee
has a number of Work Groups who are developing
specifications for different aspects of ATM.
The ATM Forum does not set standards, it
develops specifications which are used as the basis
for standards set by bodies such as the ITU-T and
ANSI.
The ATM Forum has developed and released a
number of specifications, and is currently working on
many more. These specifications which are
developed by ATM Forum Work Groups, include:
• UNI 3.0 - User-to-Network Interface
• UNI 3.1 - User-to-Network Interface
• P-NNI - Private Network-to-Network Interface
• IISP - Interim Interswitch Signaling Protocol
• Interim Local Management Interface
• LANE - LAN Emulation 1.0
Special Interest Groups
Special Interest Groups, such as the Desktop
ATM25 Alliance, are formed by companies who are
interested in promoting a particular technology
platform, (in this case IBM’s 25 Mbps ATM
offering). This group has been successful in getting
the ATM Forum to accept their offering, which has
been it added to the ATM Forum UNI specification.

Appendix C
ATM Switch Architectures
To evaluate ATM switches two components of
the architecture must be analyzed; the switching
fabric, or cell transport mechanism, and the switch
control.
An ATM switch consists of a device which has a
number of input and output ports, a switch fabric and
a management and control processor. The switch
fabric is the mechanism that routes cells from input
to output ports. There are two major types of
switching fabric, time division and space division
There are two types of switch control
architecture, distributed switch control which has a
CPU in each switch and centralized switch control
which utilizes a single computer control workstation.
Time Division Switching
The diagram shows a time division switching
fabric which has 16 155 Mbps, OC-3 ports.
Figure 22: ATM Time Division Switch
Switch fabric
The switch fabric is termed non-blocking if the
throughput of the switch fabric is greater than or
equal to the sum of the throughput of the ports. The
switch fabric in this switch shown is 2.5 Gbps and is
therefore classified as non-blocking.
Latency
Time division switching provides predictable
low latency, which is required for supporting video,
voice and multimedia applications. Latency in this
type of switch fabric can be less than 10 uS.
Multicasting support:
Multicasting is supported without cell copying.
This feature is important in video applications, such
as interactive training.
Interface speeds:
Multiple ATM speeds can be supported within a
single switch fabric. To support a 622 Mbps
interface in the shown switch, the OC-3 multiplexer
is removed, providing a 622 Mbps interface to the
switch fabric.
Scalability:
This type of switch fabric can be scaled, or
increased in size, by using non-blocking space
division switch fabric to interconnect the time
division switches. However it will be found that
large switches are not required for campus
environments.
Space Division Switching
Space Division Switching is also known as
multistage matrix switching. An example of a sixteen
port space division switch is shown in the diagram.
This type of switch fabric is known as the Banyan
switch fabric.
Figure 23: ATM Space Division Switch
2.5 Gbps Switch
Fabric
4 x 155 Mbps
(OC-3) Interface
622 Mbps
(OC-12) Interface

Switch fabric
Blocking is possible in this switch fabric; so to
reduce, but not eliminate, the possibility of cell
collisions and congestion, the following features may
be employed:
• Buffers at each switching element
• Path randomizing at each switching element
• Increase core switching speed
Space division switching has a theoretical
throughput of 58% of the maximum switch capacity.
Latency
Space division switch fabrics have variable
latency due to path routing and buffering. The
minimum latency is typically in the excess of 20 uS.
Multicasting support
Multicasting support requires cell copying,
which can result in congestion and the possibility of
blocking.
Interface speeds
Upgrades are complex as the switch fabric,
latency and buffering is affected.
Scalability
Space division switch fabrics are not scalable as
their complexity and cost grows dramatically as
number of ports increase.
An interesting application of this architecture is
IBM’s Prizma chip, which supports 16 input ports
and 16 output ports using space division
multiplexing. Each port can operate at 400 Mbps,
allowing a net throughput of 6.4 Gbps.
Switch Control Architecture
The switch control architecture of an ATM
Switch is the element that controls call setup, call
tear-down and switch monitoring.
Comparing the two switch control architectures,
the following features should be considered:
Connection setup
The distributed architecture is faster as
resources are allocated locally. With centralized
control the setup time is dependent on the central
processor work load.
Fault tolerance
The centralized control architecture provides a
single point of failure.
Scalability
The centralized architecture growth is limited by
the central control processor, whereas the distributed
architecture adds processing power with each switch
added to the network.

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