Introduction%20to%20Voice%20over%20ATM

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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
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Agenda

• ATM definition
• ATM cells
• ATM Network Architecture
• ATM Layering model
• Adaptation Layers (AAL)
• ATM Standards
• Voice over ATM
• Advantages and Difficulties

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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

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ATM Cells are Fixed-Length

• This allow network cell delay to be predicted
  and controlled
• Allow fast switching (at the hardware level)

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

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ATM network
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ATM network

• 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
  inter-communication 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

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ATM standards and ATM Forum

• International Telecommunication Union (ITU-T),
  where they have approved a packet-based
  multimedia communication systems recommendation
  (H.323) in the end of the year 2000.
• 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.

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ATM layering scheme
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ATM layering scheme

• ControlThis plane is responsible for generating
  and managing signaling requests.
• UserThis plane is responsible for managing the
  transfer of data.
• ManagementThis 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.

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ATM layering scheme

• Physical layerAnalogous to the physical layer
  of the OSI reference model, the ATM physical
  layer manages the medium-dependent transmission.
• ATM layerCombined 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.

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ATM adaptation layers

• 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)
• 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 cell-payload for user data and
  reserves 4 bytes for defining the AAL2 process.

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ATM adaptation layers

1. 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.
2. 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.

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Voice over ATM

• IWF Inter-Working Function. Converts the voice
  traffic on the B-ISDN (ATM network) to voice
  traffic on the N-ISDN (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

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Voice over ATM

• AAL1 is used for transporting voice over ATM on
  conventional PBXs.
• ATM Forum has selected AAL1 for VTOA
• It uses CBR
• Connection-oriented
• AAL5 can be used for voice over ATM
• Supports connectionless (like IP)
• More suitable to non-ATM voice

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Call Establishment
ATM Network
IWF
IWF
PBX Caller
Remote PBX
N-Setup
B-Setup
Proceeding
B-Setup
N-Setup
Proceeding
N-Connect
B-Connect
Connect Ack
B-Connect
N-Connect
Connect Ack
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Call Termination
ATM Network
IWF
IWF
PBX Caller
Remote PBX
Release
Release
Release Comp.
Release
Release
Release Comp.
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AAL1 Limitations

• 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

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AAL2 Advantages

• 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

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VoATM evolvement

• 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.

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VoATM evolvement
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Difficulties

• Delay
• Expensive ATM equipment
• Standardizing (AAL2/VBR)

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Conclusion

• 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.

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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/tutoria
  ls/ss7
• International engineering consortium
  http//www.iec.org
• International telecommunication union
  http//www.itu.int/home/index.html