Asynchronous Transfer Mode (ATM) is a network technology that allows different types of digital data such as voice, video, and data to be transmitted over the same physical medium. It uses fixed-length cells known as ATM cells that are 53 bytes long, consisting of a 5-byte header and 48-byte payload. ATM provides guaranteed bandwidth and quality of service for real-time, interactive applications through connection-oriented virtual circuits identified by a virtual path/channel identifier in each cell header. ATM supports both connection-oriented and connectionless services and can accommodate both constant and variable bit rate traffic.

1. Asynchronous Transfer Mode
(ATM)
Mathews Vergis
TEL 660
Winter trimester 2005 - 2006

2. Topics to be discussed:
• Need for network convergence
• Introduction to ATM
• ATM Interfaces and Service categories
• Basic ATM Concepts and Operation
• ATM Cell Structure and Addressing
• ATM Layers

3. Popular misconceptions even among
Engineers
• The phasor was invented by Captain Kirk of
Star Trek
• Armature reaction is a chemical reaction
• ATM stands for Automated Teller Machines

4. Need for Network
Convergence
• PSTN sometimes used as a data network
backbone – but since it is circuit switched
(voice optimized) not very WAN efficient
• Delay sensitive traffic such as voice not
possible on data networks since there is no
guarantee of QoS

5. Types of Traffic and demand on a
communication channel
Voice
• Its generation is asynchronous (a speaker may speak
anytime)
• Its transmission must be synchronous (once the
message starts, it must flow continuously as it is
spoken)
• The bandwidth required for a voice conversation in
digital communication is relatively small and constant
(64K)
• The signals may contain a high degree of error and
the information can still be retrieved correctly

6. Types of Traffic and demand on a
communication channel
Video
• The generation is synchronous (continuous)
• Its transmission is synchronous. The
bandwidth required is variable and it could
range from under 64 Kbps to several Mbps in
the same session.
• Error control should be tight - otherwise the
wrong information on the monitor may trigger
severe wrongful actions

7. Types of Traffic and demand on a
communication channel
Data
• Its generation could be either asynchronous (text) or
synchronous (telemetry)
• Its transmission in general can be asynchronous (data
typically can wait patiently in buffers)
• The information is extremely error-sensitive, so
extreme caution must be exercised in transmission
and error control must be very tight.

8. How can we combine voice , data
and video on the same link?
• Fixed and relatively short packets
• Delays associated with each packet are going
to be short and fixed – predictable
transmission
• If Voice and Video can be given priority
handling – then mixing is possible without any
diminishing in quality

9. Introduction
• ATM – Asynchronous Transfer Mode
• It is a high speed, connection – oriented
switching and multiplexing technology
capable of transmitting voice, video and data
and interconnecting LAN’s
• ATM is asynchronous because information
streams can be sent independently without the
need of a common clock

10. History of ATM
• Developed in the early 80’s as a switching technology
for Broadband Integrated Services Digital Network
• Anchorage Accord in 1996 declares availability of
specs required to implement a multi-service ATM
network

11. Market Segments of ATM

12. Features and Benefits of ATM
• Convergence of Voice , Video and Data on
one network
• High speed switching at hardware level
• Bandwidth on demand
• Predefined and guaranteed QoS and CoS
• Superior Management features
• Scalability in network size and speed
• Ease of integration with other technologies

13. ATM Applications

14. ATM Fast Packet Standards and
Services
• Handles traffic through fast
– packet switching
technique
• Must be able to handle both
circuit and packet switching
• Must also be able to
accommodate the different
bit rates – variable (packet
switching) and constant
(circuit switching)
• Uses Cell relay technology

15. Important terms relevant to ATM
• Quality of Service (QoS) :A broadly used term that refers to
the performance attributes of an end-to-end connection. A QoS
definition for data would address attributes such as error rates,
lost packet rates, throughput, and delay
• Class of Service (CoS) :It is a way of managing traffic in a
network by grouping similar types of traffic together and
treating each type as a class with its own level of service
priority
• Fast Packet Switching :A packet switching technique that
increases the throughput by eliminating overhead. Overhead
reduction is accomplished by allocating flow control and error
correction functions to either the user applications or the
network nodes that interface with the user. Cell relay is an
implementation of this.

16. Cells and Cell relay
• A Cell is a formatted packet that uses a fixed length
data unit
• Cell relay is the process of moving these cells
through switching elements
• Fixed size cells can be switched at a very high speed
and add predictability to data transmissions
• Variable length frames produce unpredictable
patterns and performances as the buffer time cannot
be determined
• Cell tax – overhead imposed by ATM cells which can
cut into amount of data that can be transferred

17. ATM Interfaces
• ATM is a connection User to Network Interface (UNI) :
oriented technique designed Connection existing between the
user equipment and ATM
to transport both connection equipment.
and connection-less services Network to Network Interface
• Operations at the boundary (NNI) : Connection via which
of a network are connection traffic travels between ATM
oriented devices in the same network.
Intercarrier Interface (ICI) : Used
• Within the network the to send traffic across intermediate
operation is connectionless networks
Data Exchange Interface (DXI) :
Used to transmit packets rather
than cells to the ATM interface
when non – ATM equipment is
used

19. ATM Service Categories
• Allow for traffic to be buffered and queued for later
transmission
• Can permit loose timing and asynchronous operations between
sender and receiver

20. Fundamental ATM Operations
Concept
• A virtual or logical connection is established
• ATM forms a packet of fixed length – 53 octets ( 5
octet header and 48 octet information field )
• Cells are placed in a queue, on reaching ATM switch
• Cells are then multiplexed asynchronously with other
cells for transmission
• Switch adapts the incoming bit rate to match the
transmit channel bit rate
• Switch inserts dummy cells to meet the aggregate bit
stream rate of 155.52 Mbps

21. ATM Virtual paths and
Channels
ATM Virtual Circuit Terminology
• Virtual Channel (VC) – provides
a fixed pathway or route between
2 points. Setup across an ATM
network whenever data transfer
begins.
• Virtual Path (VP) – groups of
VC’s used to tell a switch how to
forward an ATM cell through an
ATM network
• Virtual Path Identifier (VPI) – in
the ATM header used to identify
route established in the ATM
Switch
• Virtual Channel Identifier (VCI)
- in the ATM header used to
identify a channel within a VP

22. ATM Virtual paths (cont’d)
• Transmission path – physical
media transporting the cells
• Virtual Channel Connection
(VCC) – connection from source
end user VCI to destination user
VCI
• Virtual path Connection (VPC) –
connection from source end user
VPI to the destination end user
VPI
• Operation : Cell is received
across a link on a known VPI or
VCI value - their values are
remapped as necessary as all
VCI’s and VPI’s are only
significant to the local link

23. ATM VCI and VPI Swapping
• Logical ID Swapping : This is the technique by which the Logical ID of
one link is changed to another one as the cell passes through it. ATM
switch changes the cell header VPI/VCI fields to reflect a new VPI and
VCI for the outgoing cell. This can be done in one of 2 ways : VP is
predefined in the switch or VP is set up dynamically when cell reaches
the switch.
• Forwarding process depends on 2 lookup tables within the switch:
VP table – record of VP’s on each link
VC table – maintain the output VP and VC to send the cell
• Connection Admission Control (CAC) - procedure used to decide if a
request for an ATM connection can be accepted based on the attributes
of both the requested connection and the existing connections

24. VC and VP Swapping

25. ATM Cell Structure

26. ATM Cell Structure

27. ATM Addressing
• Uses addressing similar to
numerical addressing for
telephone numbers
• Uses E.164 addresses for public
ATM (B- ISDN) networks
• Extended ATM addressing to
include private networks –
Overlay model – ATM layer maps
network layer (IP) addresses to
ATM addresses. Address format
uses OSI network service access
point (NSAP) addresses
• NSAP address – providing the
logical point between the network
and transport layers of the OSI
model – the location of this point
is identified by network service
provider

28. ATM Address formats
• Currently 4 formats used for ATM networks
• The fields are divided into 2 sections: network and user
• Network prefixes : fields as needed by the network side of the
UNI
• User prefixes : Fields as needed by the user side of the UNI

29. ATM address formats (cont’d)

30. ATM Routing Domains and Areas
ATM address – 20 byte string that has the following
fields :
• Country code
• Administrative authority
• Routing domain
• Area identifier
• End system identifier
• NSAP
• International code
• ISDN telephone number

31. ATM Routing Domains and Areas
• Authority and Format Identifier (AFI): This identifier is
part of the network level address header. Value for 1st AFI field
can be : DCC (hex 39),E.164 (hex 45), ICD (hex 47)
• Routing domains : used for traffic management and allocating
bandwidth capacity. Defined in the lookup tables in the
switch.
• End System Identifier (ESI) : identifies an end system
( computer or LAN) within an area
• Selector Field not used by the ATM network
• Purpose of the ATM address format is identify ATM devices
in an ATM network

32. ATM Routing Domains and Areas

33. ATM Layers
• ATM architecture uses a logical reference
model to describe its functions
• ATM functions correspond to physical layer
and part of the Data Link layer of the OSI
model
• On its own ATM has function at layers 1and 2
of the OSI model, but today TCP/IP is routed
over ATM networks which means it can also
function at layers 3 and 4 of the OSI model.

34. ATM reference model
• The ATM reference model constructed as 3 planes
which span all the layers :
• Control – generates and manages the signaling requests
• User – manages the transfer of data
• Management – contains 2 parts:
– Layer management : manages layer specific functions
– Plane management : manages and co – ordinates functions related to
the whole system

35. ATM and OSI model

36. ATM Adaptation layer
• Has 2 sublayers:
– Convergence Sublayer ( CS )
• Determines the Class of service (CoS) for the incoming
traffic
• Provides a specific AAL service at an AAL network
service access point (NSAP)
– Segmentation and Reassembly Sublayer (SAR)
• Segments higher – level user data into 48 – byte cells
plus necessary overhead at the sending node and
reassembles cells at the receiving node

37. AAL Types and Class of Service
(CoS)
• Depending on data type, the AAL protocol provides 5 AAL
types to accommodate a particular service class
• AAL 5 is the most popular AAL type
– For IP, LAN frames , signaling messages, frame relay , video

38. ATM Layer
• Performs the framing , multiplexing /
demultiplexing of cells and also does the
switching
• Generates cell headers on transmitting node –
based on information from higher layers
• Generic flow control
• VCI /VPI translation
• Extracts cell headers on a receiving node and
passes cell payload to higher layers

39. Physical Layer
• Transports ATM cells on a communications channel
and defines mechanical specifications ( connectors
etc.)
• 2 sublayers:
– Transmission Convergence sublayer
• Maps cells into the physical layer frame format on transmit and
delineates ATM cells in the received bit stream
• Generates HEC on transmit
• Generates idle cells for cell rate decoupling or speed matching
– Physical medium sublayer
• Medium dependent function like bit transfer, bit alignment

40. References:
• http://www.techfest.com/networking/atm/atm.htm
• http://www.dit.upm.es/snh/arhelp/glossaries/atmf/gloss-a.html
• http://www.rhyshaden.com/atm.htm
• http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/at
m.htm
• Trivedi, Carol, “Wide Area Networks”; EMCParadigm 2004