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University of Ottawa
School of IT & Engineering
CSI 5171
Introduction to
Voice over ATM
Prepared by: Amjad Farran
ID number: 2700955
19/3/2002
Agenda
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ATM definition
ATM cells
ATM Network & Architecture
ATM Layering model
Adaptation Layers (AAL)
ATM Standards
Voice over ATM
Advantages and Difficulties
Asynchronous Transfer Mode
• Started as Broadband ISDN (B-ISDN)
• Multimedia and multi-service technology
• Connection-oriented (Virtual Circuit [VC])
• Packet-switched
• High speed transfer (622Mbps)
• Fixed-length cells
• Asynchronous: transports cells based on demand
ATM Cells are Fixed-Length
• This allow network cell delay to be predicted and controlled
• Allow fast switching (at the hardware level)
Header
(5 Bytes)
Information field (payload) (48 Bytes)
53 Bytes
• Header:
• To establish connection and manage data flow
• Information identifying a particular virtual circuit
• Payload:
• User information (voice, images, data, etc. in digital form)
• Could be used for signaling, or operations and maintenance
ATM network
ATM network
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UNI: User-Network Interface, is the standard technical
specification allowing ATM customer equipment from
various different manufacturers to communicate over a
network provided by another manufacturer.
NNI: Network-Node Interface, is the interface used
between nodes within the networks or between different
subnetworks.
INI: Inter-Network Interface, allows for intercommunication and clean operational and administrative
boundaries between interconnected ATM networks.
B-ICI: broadband inter-carrier interface. Connects two
public switches from different service providers
PNNI: private network-network interface. ATM topology
discovery and call establishment
ATM standards and ATM Forum
1. International Telecommunication Union
(ITU-T), where they have approved a packetbased multimedia communication systems
recommendation (H.323) in the end of the year
2000.
2. ATM Forum has approved specifications for
voice telephony over ATM, known as VTOA in
1997. It has put interoperability specifications to
address three applications for carrying voice over
ATM: desktop or LAN services, trunking or WAN
services, and finally mobile services.
ATM layering scheme
ATM layering scheme
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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 contains 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.
ATM layering scheme
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Physical layer—Analogous to the physical layer of the OSI
reference model, the ATM physical layer manages the mediumdependent transmission.
ATM layer—Combined with the ATM adaptation layer, the ATM
layer is roughly analogous to the data link layer of the OSI
reference model. The ATM layer is responsible for the
simultaneous sharing of virtual circuits over a physical link (cell
multiplexing) and passing cells through the ATM network (cell
relay). To do this, it uses the VPI and VCI information in the
header of each ATM cell.
ATM adaptation layer (AAL)—Combined with the ATM layer,
the AAL is roughly analogous to the data link layer of the OSI
model. The AAL is responsible for isolating higher-layer
protocols from the details of the ATM processes. The adaptation
layer prepares user data for conversion into cells and segments
the data into 48-byte cell payloads.
ATM adaptation layers
1.
AAL1 - Supports connection-oriented services that require constant
bit rates (CBR), such as voice and videoconferencing, and have
specific timing and delay requirements. Example are constant bit
rate services like DS1 or DS3 transport. CBR is transported by using
circuit emulation service (CES)
2.
AAL2 - Supports connection-oriented services that do not require
constant bit rates. In other words, variable bit rate (VBR)
applications like some voice or video schemes that do not have
constant data transmission speed. AAL2 uses 44 bytes of the cellpayload for user data and reserves 4 bytes for defining the AAL2
process.
ATM adaptation layers
3.
AAL3/4 - This AAL is intended for both connectionless and
connection oriented variable bit rate services (VBR). Originally two
distinct adaptation layers AAL3 and 4, they have been merged into
a single AAL. It is used to transmit switched multimegabit data
service
(SMDS)
packets
over
an
ATM
network.
4.
AAL5 - Supports both connection-oriented and connectionless data.
It is used to transfer most non-SMDS data, such as classical IP over.
It has similar services to what AAL3/4 has except that it does not
support multiplexing of traffic from different AAL connection
endpoints into a single ATM connection and it provides no error
detection capabilities over the SAR-PDU.
Voice over ATM
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IWF: Inter-Working Function. Converts the voice traffic
on the B-ISDN (ATM network) to voice traffic on the NISDN (narrowband telephony network).
CBR: Constant Bit Rate. An ATM service class providing
for the support of constant bit streams (e.g. Those used
by PBX)
VBR: Variable Bit Rate. Traffic containing bursts but
centered around an average bandwidth.
CCS: Common Channel Signaling. Voice signaling based
on use of a separate signaling channel, used by ISDN
PBXs
CAS: Channel Associated Signaling. Voice signaling based
on bits taken from voice timeslots used by many PBXs
Voice over ATM
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AAL1 is used for transporting voice over ATM on
conventional PBXs.
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ATM Forum has selected AAL1 for VTOA
It uses CBR
Connection-oriented
AAL5 can be used for voice over ATM
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Supports connectionless (like IP)
More suitable to non-ATM voice
Call Establishment
ATM Network
PBX
Caller
IWF
IWF
Remote
PBX
N-Setup
Proceeding
B-Setup
B-Setup
N-Setup
Proceeding
N-Connect
B-Connect
B-Connect
N-Connect
Connect Ack
Connect Ack
Call Termination
ATM Network
PBX
Caller
IWF
IWF
Remote
PBX
Release
Release Comp.
Release
Release
Release
Release Comp.
AAL1 Limitations
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Supports single user per PVC
Additional bandwidth is required to
reduce delay
Bandwidth is used even when there is
no traffic
Voice is always 64K or Nx64K
No partial payload
AAL2 Advantages
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Bandwidth efficiency
VBR ATM traffic class
Multiple user channels with varying
bandwidth on a single ATM connection
Supports idle voice channels
Best for low-rate, short, and variable
packets
VoATM evolvement
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VBR techniques for voice exploit
the inherently bursty nature of
voice communication, as there are
silence periods that can result in
increased efficiency.
Using AAL2 over switched virtual
circuits (SVC) rather than AAL1/PVC.
VoATM evolvement
Difficulties
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Delay
Expensive ATM equipment
Standardizing (AAL2/VBR)
Conclusion
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More work needs to be done in terms of
standardization and ATM product
development.
VoATM is feasible now within an
organization that uses ATM network and
technology already.
References
• Daniel Minoli and Emma Minoli, delivering voice over
frame relay and ATM, Wiley, 1998
• Martin Clark, ATM networks: principles and use, Wiley
Teubner, 1996
• Goncalves and Niles, IP multicasting: concepts and
applications, McGraw-hill, 1999
• ATM forum http://www.atmforum.com
• Performance technologies http://www.pt.com/tutorials/ss7
• International engineering consortium http://www.iec.org
• International telecommunication union
http://www.itu.int/home/index.html