1. 1 EE 400 Asynchronous Transfer Mode (ATM) Abdullah AL-Harthi

2. 2 Objective  Understand ATM technology, concepts and cell structure.

3. 3 Outline: n Introduction. n ATM Layer. n ATM Cell. n ATM Service Categories. n ATM Adaptation Layer (AAL). n Conclusion.

4. 4 Introduction n ATM :Multi-speed network environment that provides a variety of complex network services. n Can carry voice, data, video separately or simultaneously. n Fixed-lenth packets (cells). n Can be used in LANs, or WANs. n Minimal error and flow control capabilities. n Connection-oriented virtual channel. n Allows multiple logical connections to be multiplexed.

5. 5 Protocol Architecture

6. 6 ATM Layer n Connection-oriented without acknowledgements. n Two possible interfaces: - UNI : User-Network Interface: Boundary between an ATM network and host. - UNI : User-Network Interface: Boundary between an ATM network and host. - NNI : Network-Network Interface: Between two ATM switches.

7. 7 UNI/NNI Interface

8. 8 ATM Cells n Fixed size – 53 bytes. n 5 octet header. n 48 octet information field. n Small cells reduce queuing delay for high priority cells. n Small cells can be switched more efficiently. n Easier to implement switching of small cells in hardware.

9. 9 Header Format n Generic flow control –Only at user to network interface –Controls flow only at this point n Virtual path identifier - a routing field for the network - a routing field for the network -8 bits at user-network interface -12 bits at the network-network interface n Virtual channel identifier - user for routing between end users - user for routing between end users n Payload type - indicates the type of information in the information field - indicates the type of information in the information field –e.g. user info or network management

10. 10 Header Format n Cell loss priority -used to provide guidance to the network in the event of congestion. -used to provide guidance to the network in the event of congestion. value 0: cell of high priority, should not be discarded unless no other alternative is available value 0: cell of high priority, should not be discarded unless no other alternative is available value 1: cell is subject to discard within the network value 1: cell is subject to discard within the network n Header error control -used for both error control and synchronization -used for both error control and synchronization

11. 11 ATM Cell Format

12. 12 Virtual Connections n Virtual Channel Connection (VCC) Full duplex virtual circuit with logical connection between source and destination – can be PVC or SVC Full duplex virtual circuit with logical connection between source and destination – can be PVC or SVC n Virtual Path Connection (VPC) Semi-permanent (or customer controlled or network controlled) connection that provides a logical collection of virtual channels that have the same endpoint Semi-permanent (or customer controlled or network controlled) connection that provides a logical collection of virtual channels that have the same endpoint

13. 13 ATM Service Categories n Real time –Constant bit rate (CBR) –Real time variable bit rate (rt-VBR) n Non-real time –Non-real time variable bit rate (nrt-VBR) –Available bit rate (ABR) –Unspecified bit rate (UBR) –Guaranteed frame rate (GFR)

14. 14 Real Time Services n If want to avoid or decrease variation of delay (jitter), use CBR or rt-VBR n CBR Fixed data rate continuously available n Commonly used for uncompressed audio and video –Video conferencing –Interactive audio n rt-VBR Best for time sensitive applications –Tightly constrained delay and delay variation n rt-VBR applications transmit at a rate that varies with time –e.g. compressed video – Produces varying sized image frames –Original (uncompressed) frame rate constant (isochronous) –So compressed data rate varies

15. 15 Non-Real Time n Intended for applications with bursty traffic and limited constraints on delay and delay variation. n Greater flexibility, greater use of statistical multiplexing.

16. 16 nrt-VBR n May be able to characterize expected traffic flow n Improve QoS in loss and delay

17. 17 UBR n Unused capacity of CBR and VBR traffic made available to UBR n For application that can tolerate some cell loss or variable delays –e.g. TCP based traffic

18. 18 ABR n Application specifies peak cell rate (PCR) it will use and minimum cell rate (MCR) it requires n Resources allocated to give at least MCR n Spare capacity shared among ABR and UBR sources n e.g. LAN interconnection

19. 19 ATM Adaptation Layer (AAL) n Maps higher-layer information into ATM cells to be transported over an ATM network n Collects information from ATM cells for delivery to higher layers  AAL-1: for CBR traffic  AAL-2: for VBR type traffic  AAL-3: connection-oriented VBR  AAL-4: connection-less service  AAL-5: most popular for data. Also called seal(simple and efficient adaptation layer)

20. 20 Conclusion n Minimal error and flow control n Fixed size simplified processing at each ATM node. n Small cells reduce queuing delay n Minimal addressing info on each cell n Efficient traffic management