Introduction to Introduction to Voice over ATMVoice over ATMIntroduction to Introduction to Voice over ATMVoice over ATM

University of OttawaUniversity of Ottawa

School of IT & EngineeringSchool of IT & Engineering

CSI 5171CSI 5171

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 Advantages and Difficulties

• 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

Asynchronous Transfer Mode

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

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

ATM Cells are Fixed-Length

ATM network

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

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

2.2. ATM ForumATM 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 standards and ATM Forum

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 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 layering scheme

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 cell-payload 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.

ATM adaptation layers

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 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

IWFIWF IWFIWFPBXCaller

ATM NetworkATM Network

RemotePBX

N-Setup

B-Setup

B-SetupN-Setup

N-ConnectB-Connect

B-ConnectN-Connect

Proceeding

Proceeding

Connect Ack

Connect Ack

Call Termination

IWFIWF IWFIWFPBXCaller

ATM NetworkATM Network

RemotePBX

Release

Release

ReleaseRelease

Release Comp.

Release Comp.

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

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

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

Delay Expensive ATM equipment Standardizing (AAL2/VBR)

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.

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

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