Introduction to Asynchronous
Transfer Mode (ATM)
© N. Ganesan, Ph.D.
(Copyrights of sections as
acknowledged.)
Acknowledgment
• This presentation is based on ATM
related information provided on the
CISCO website, the IEC website and the
ATM Forum.
– Asynchronous Transfer Mode (ATM)
Switching
– ATM Fundamentals
– ATM Presentations (Good set of
PowerPoint presentations).
Chapter Objectives
Chapter Modules
ATM Defined
• “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)” –CISCO.
• It utilizes fixed length cells to carry the
information
Areas of Application
Infrastucture
Backbones
LAN
• LANsAsynchronous transfer mode (ATM) is a highperformance, cell-oriented switching and multiplexing
technology that utilizes fixed-length packets to carry different
types of traffic. ATM is a technology that will enable carriers to
capitalize on a number of revenue opportunities through
multiple ATM classes of services; high-speed local-area network
(LAN) interconnection; voice, video, and future multimedia
applications in business markets in the short term; and in
community and residential markets in the longer term.
• Infrastructure
– Backbones
– LAN
• Application
ATM Connections
Circuit Switching and Packet
Switching
• ATM is circuit switched because it
establishes virtual circuits for
communication
• At the same time, the virtual circuits are
established over packet switched
networks
• As such, it combines the benefits of
circuit switched and packet switched
technologies
ATM Usage and Bandwidth
• In theory, ATM can be deployed from
small LANs to very large WANs
– At present, it is used mostly on backbones,
but this may change in the future with
declining prices for ATM equipment
• ATM deployments can operate at
speeds starting in the Mbps range
scaling up to Gpbs range
– Speed wise, it is very scalable
ATM’s Efficiency
• It is an asynchronous technology and it
uses the links based on the need for
information to be transmitted
• ATM is based on fixed length cells and
the cells are small compared to many
other forms of transmission such as
frame relay etc.
ATM Cell Basics
• ATM carries information based on fixed
length cells
– Compare this to the other packet switching
technologies such as Frame Relay etc. where each
packet may be of a different length
• The length of each cell is 53 Bytes
– First 5 bytes are used as the cell header
– Next 48 bytes are used as the payload carrying the
data
ATM Cell Format
5 Bytes
Header
48 Bytes
Payload (Data)
Fixed Length Cell Advantage
• Delay or latency is significantly reduced
– ATM is therefore suited for voice and video
transmission
• Fixed length cells make it easier to switch
data across multiple networks
– ATM networks are built based on switches and
not routers
• Fixed length cell is similar to container based
road transportation
– Some parallels can be drawn with respect to the
advantage of fixed length transportation based on
the benefits of container transportation
ATM Cell Header Format
ATM Cell Header—UNI Format
ATM Cell Header—NNI Format
ATM Devices
• ATM networks are built around two
categories of devices
– ATM Switch
– ATM end-point
• An ATM switch can be connected to
either another ATM switch or and ATM
end-point
ATM End-Points
• Will contain and ATM end-point
adapter
• Examples of ATM end-points
–
–
–
–
–
Workstations
LAN switches
Routers
DSU/CSU Units
Video Coder-Decoders (CODECs)
Devices on Which ATM Has Been
Implemented
• PC, workstation, and server network interface
cards
• switched-Ethernet and token-ring workgroup
hubs
• workgroup and campus ATM switches
• ATM enterprise network switches
• ATM multiplexers
• ATM–edge switches
• ATM–backbone switches
Source: http://www.iec.org/online/tutorials/atm_fund/topic01.html?Next.x=33&Next.y=17
ATM Network Components
Source: CISCO
ATM Switches and Interfaces
• ATM switch supports two types of
interfaces
– User-Network Interface (UNI)
• Connects an ATM end-point to a switch
– Network-Network Interface (NNI)
• Connects two ATM switches
Public and Private Interfaces
• UNI and NNI can further be divided to
two types
• One is known as the private type and
the other is known as the public type
Private and Public UNI
• Private UNI
– Connects an ATM end-point to a private
ATM switch
• Public UNI
– Connects an ATM end-point or a private
ATM switch to a public ATM switch
Private and Public NNI
• Private NNI
– Connects two ATM switches within the
same private organization
• Public NNI
– Connects two ATM switches within the
same public organization
A third Type of Interface
• Known as Broadband Inter-Carrier
Interface (BICI)
– Connects two public switches from
different service providers
Public and Private Interfaces
Source: CISCO
ATM Standards
• UNI 2.0, 3.0, 4.0
• P-NNI
• LAN Emulation (LANE)
– For the use of ATM over Ethernet, Token
ring etc.
• Multi-protocol Over ATM
ATM Standards for Digital Lines
Area of Application Speed
Standard
T1
25 Mbps
Yes
T3/E3
OC-3
OC-12
155 Mbps
622 Mbps
2.4 Gbps
Yes
Y
Y
Check the ATM Forum for approved standards for
further information.
ATM Services
• There are basically three types of ATM
services
– Permanent Virtual Circuits (PVC)
– Switched Virtual Circuits (SVC)
– Connectionless Service
• Note: SVC is the most widely used
service
ATM Virtual Connections
• Two types of connections exist
• One is known as the Virtual Path (VP)
• The other is known as the Virtual
Circuit (VC)
Virtual Connection Composition
• There can be a number of virtual paths
along a physical connection and there
can be a number of virtual circuits
within a virtual path
Virtual Circuits and Paths
Source: CISCO
Transmission Path (Physical Media)
• Physical medium standards to carry ATM
include the following:
– Synchronous Digital Hierarchy/Synchronous
Optical Network (SDH/SONET)
– DS3/E3
– Over Multi-Mode Fiber (MMF) at 155 Mbps
– Over Shielded Twisted Pair (STP) at 155 Mbps
ISO-OSI and ATM Models
LAN Emulation (LANE)
• Defined by the ATM forum to emulate
popular LAN protocols such as
Ethernet and Token Ring
• In other words, “the LANE protocol
makes an ATM network look and
behave like either the Ethernet or the
Token Ring network” – CISCO
• An emulated LAN is known as ELAN
Multi-Protocol Over ATM (MPOA)
• Enables the transmission of data
between ELANs without having to
continuously go through certain routers
• After the initial flow of data through
routers known as Multi-Protocol
Servers (MPS), it would be directed
along the path of ATM switches (See
next slide and CISCO documentation)
Data Flow Between ELANs with and
without MPOA
Without
MPOA
With
MPOA
ATM Quality of Service (QoS)
• ATM’s advantage over competing
technologies is that it is able to offer a
specified QOS
• For example, ATM providers can
guarantee to their customers that the
end-to-end latency will not exceed a
specified level
QoS Offered by ATM
• Constant Bit Rate (CBR)
• Variable Bit Rate (VBR)
• Unspecified Bit Rate (UBR)
• Available Bit Rate (ABR)
Constant Bit Rate (CBR)
• Highest grade service (Grade A)
• A constant bandwidth is available for
the virtual channel
• Suitable for voice-over-IP (Transfer
fixed rate uncompressed video), and
other traffic that requires steady,
guaranteed throughput
Variable Bit Rate (VBR)
• Second in the level of service (Grade B)
• It is divide into to categories
– Variable Bit Rate Non-Real Time (VBRNRT)
– Variable Bit Rate Real Time (VBR-RT)
Variable Bit Rate Non-Real Time
VBR-NRT
• Provides bandwidth only as needed
• Traffic that is not very sensitive to celldelay or latency is handled by this
service
• An example use would be to send
multimedia e-mail
Variable Bit Rate Real Time VBR-RT
• Again, this is similar to VBR-RT in
providing bandwidth as needed
• But VBR-RT is designed for
applications where cell-delay or latency
cannot be accommodated
Sample Applications for VBR-RT
• Voice with speech activity detection
– Telephony
• Compressed interactive video
– Video conferencing
Available Bit Rate (ABR)
• Third in the level of service (Grade C)
• Bandwidth is adjusted based on the
amount of traffic in the network
• Suitable for data traffic such as file
transfer and Email that are basically
not time sensitive
Unspecified Bit Rate (UBR)
• Fourth in the level of service (Grade D)
• Dose not offer any fixed values for the
traffic parameters
QoS Enforcement
• Traffic contract
• Traffic shaping
• Traffic policing
• (See CISCO document for more details)
Competing Technologies
• Frame Relay competes at the backbone
– Eventually, ATM will be the choice for
backbone technology
• Gigabit Ethernet
– ATM’s competition on the desktop area
and LANs will be challenged effectively by
high speed and proven Ethernet
technologies
End of Module
Practical ATM Networks
© N. Ganesan, Ph.D.
Chapter Objectives
ATM Interfaces
ATM
ATM and other technologies.
Source: CISCO
ATM Network Building Block
• The main building block of ATM
networks is the ATM switch
–
–
–
–
Workgroup switch
Campus switch
Enterprise switch
Multi-service access switch
ATM Network Hierarchy
Source: CISCO
Work Group Switch
• These switches will have Ethernet and
ATM ports
• Used for connecting Ethernet networks
to ATM switches
• Note: At the desktop level Ethernet still
remains as the leading network
technology
Campus Switch
• Used for designing small-scale ATM
backbones
• Can be used for connecting various
campus backbone technologies
– FDDI
– Gigabit Ethernet etc.
Move from Traditional to New
Backbones
Enterprise ATM Switch
• Used in building large enterprise
backbones
• Can be used for integrating different
services and technologies on the
backbone
Multi-Service Access Switch
• A common ATM infrastructure can be used in
a public network to support disparate
services
– Frame relay
– LAN interconnection
– Public ATM services
• Their use usually extends beyond a private
network to a public network
• Used by service providers to connect MANs
and WANs for example
ATM Interface Types
• User-to-Network Interface (UNI)
• Network-to-Network Interface (NNI)
Interface Types in a Network
Source: CISCO
ATM Interfaces
• LAN interfaces
• Backbone interfaces
ATM to LAN Interface
Source: http://www.cisco.com/univercd/cc/td/doc/cisintwk/idg4/nd2008.htm#28807
LAN Emulation Issues
• Emulate an Ethernet LANs
• Map traditional LAN addresses to ATM
addresses
ATM to WAN Interface
Source: CISCO
DXI – Data Exchange Interface
HSSI – High Speed Serial Interface
DSU – Data Service Unit
Load Balancing with ATM Switches
Source: CISCO
Summary
• ATM networks are basically built around ATM
switches
• ATM interfaces can be used for integrating disparate
WAN technologies
• At the desktop LAN, ATM can be used for emulating
the traditional Ethernet environment for example
– But, in general, the Ethernet technology is very popular in
terms of ease of use and speed
– Therefore, ATM at the LAN front is likely to be used for
connecting Ethernet LANs to the backbone if the backbone is
based on ATM
• For the backbone and large public backbones, ATM is
bound to become the technology of choice
References
• At CISCO
– http://www.cisco.com/univercd/cc/td/d
oc/cisintwk/idg4/nd2008.htm
– http://www.cisco.com/univercd/cc/td/d
oc/cisintwk/idg4/nd2012.htm
• ATM Forum
– http://www.atmforum.org/