Introduction to Voice over ATM

1. University of Ottawa School of IT & Engineering CSI 5171 Introduction to Voice over ATM Prepared by: Amjad Farran ID number: 2700955 19/3/2002

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

3. 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

4. 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

5. ATM network

6. 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

7. 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.

8. ATM layering scheme

9. 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.

10. 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.

11. 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.

12. 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.

13. 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

14. 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

15. Call Establishment ATM Network PBX Caller IWF IWF Remote PBX N-Setup B-Setup Proceeding B-Setup N-Setup Proceeding N-Connect B-Connect Connect Ack B-Connect N-Connect Connect Ack

16. Call Termination ATM Network PBX Caller IWF IWF Remote PBX Release Release Release Comp. Release Release Release Comp.

17. 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

18. 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

19. 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.

20. VoATM evolvement

21. Difficulties • Delay • Expensive ATM equipment • Standardizing (AAL2/VBR)

22. 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.

23. 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