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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. ATM definition ATM cells ATM Network & Architecture ATM Layering model Adaptation Layers (AAL) ATM Standards Voice over ATM

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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



  • ATM definition

  • ATM cells

  • ATM Network & Architecture

  • ATM Layering model

  • Adaptation Layers (AAL)

  • ATM Standards

  • Voice over ATM

  • Advantages and Difficulties

Asynchronous transfer mode
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
ATM Cells are Fixed-Length

Header (5 Bytes)

Information field (payload) (48 Bytes)

  • 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

Atm network1
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

Atm standards and atm forum
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.

Atm layering scheme1
ATM layering scheme

  • 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 scheme2
ATM layering scheme

  • Physical layer—Analogous to the physical layer of the OSI reference model, the ATM physical layer manages the medium-dependent 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
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.

Atm adaptation layers1
ATM adaptation layers

  • 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.

  • 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
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

Voice over atm1
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

Call establishment
Call Establishment

ATM Network















Connect Ack



Connect Ack

Call termination
Call Termination

ATM Network









Release Comp.



Release Comp.

Aal1 limitations
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

Aal2 advantages
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

Voatm evolvement
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.


  • Delay

  • Expensive ATM equipment

  • Standardizing (AAL2/VBR)


  • 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.


  • 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

  • Performance technologies

  • International engineering consortium

  • International telecommunication union