ATM Switches

1. ATM Switches Cells Scalable QoS Perspective Virtual Circuits

2. ATM Switches • Asynchronous Transfer Mode • Basic Standards Set by ITU-T • Partner with ISO in OSI standards • ATM standards developed within OSI architecture • ATM Forum Sets Detailed Standards • Group of mostly ATM vendors • Moves quickly • Also tests for interoperability

3. ATM Switches • Has fixed-length frames are called cells • Always 48 octets of payload • Always 5 octets of header • So always 53 octets total • Fixed length gives predictability • Allows switches to process cells very rapidly using parallel circuitry Payload (48 octets) Header(5 octets) ATM Cell

4. ATM Switches • Small cell reduces latency (delay) at each switch • Some processing must wait for the entire frame arrives • Short frames finish arriving quickly • Critical for voice

5. ATM Switches • Highly Scalable • A few Mbps to a few gigabits per second • Once critical, but Ethernet speeds are now comparable • Very sophisticated • Quality of service (QoS)--delivery guarantees for maximum latency, exact timing between adjacent cells • Ethernet is only a best-effort service today

6. ATM Switches • Hardware is very expensive because of complexity • Retraining and ongoing labor are very expensive because of complexity • ATM has high overhead (extra characters) • 5 overhead octets for 48 data octets (10% overhead) • Actually even worse (see Module E)

7. ATM Switches • Unfortunately, very expensive • Has lost the desktop • It is usually cheaper to use high-capacity Ethernet switches with overprovisioning and perhaps priority, so that latency does not grow to the point where QoS is critical • In LANs, usually used only where service quality is critical, typically when voice is being carried

8. ATM QoS Categories • ATM Offers Varying Levels of QoS • Parameters • Peak cell rate (maximum burst speed) • Maximum burst size (bits per burst) • Sustainable cell rate (always allowed) • Cell Delay Variation Tolerance (CDVT): how exact cell-to-cell timing is; Critical for voice and video • Cell Loss Ratio: Losses during transmission

9. ATM QoS Categories • ATM Offers Varying Levels of QoS • For Voice and Video • ITU-T Class A • ATM Forum Service Category: Constant Bit Rate (CBR) • Low latency • Low Cell Delay Variation Tolerance • Strong guarantees for voice and video!

10. ATM QoS Categories • For IP and LAN Data • ITU-T Class D • Several ATM Forum Service Categories • Developed several categories over Time • Available bit rate (ABR) weak: send if capacity is available • Unspecified bit rate (UBR) weak: simpler than ABR, but can get almost no share of capacity • Guaranteed frame rate (GFR) gets roughly fair share of capacity during congestion

11. ATM QoS Categories • For IP and LAN Data • Several ATM Forum Service Categories • ABR, UBR, and even GFR give very low status to data transmission • Not even as good as Ethernet priority of service • Yet costs far more • So ATM QoS makes little sense if used entirely for data • Has other data transmission benefits, however

12. ATM QoS Categories • Other Categories • For Videoconferencing • May need momentary bandwidth increase if there is a burst of motion on the screen • Needs Low Cell Delay Variation Tolerance • ATM: Class B • ATM Forum Service Category: Variable Bit Rate-Real Time (VBR-RT) • Not widely used or implemented

13. ATM QoS Categories • Other Categories • For Connection-Oriented Data • ATM: Class C • ATM Forum Service Category: Variable Bit Rate-Not Real Time (VBR-NRT) • Most data not connection-oriented • Not widely used, implemented

14. ATM Switches: Virtual Circuits • Often Arranged in a Mesh • But all traffic between two stations still is consigned to a path called a virtual circuit that is set up before the first frame transmission ATM Cell Virtual Circuit

15. ATM Switches • Virtual Circuits Mean that there is Only a Single Possible Path between Any Two Stations • Virtual circuits simplify switch operation and so lower switch cost ATM Cell Virtual Circuit

16. ATM Switches • Permanent Virtual Circuits (PVCs) • Designed to operate for weeks, months, or years • Usually used between permanent sites in a corporation • Simplest and least expensive administratively because rarely changed • Most widely used form of virtual circuit

17. ATM Switches • Switched Virtual Circuits (SVCs) • Established just as communication between a pair of stations starts • More flexible than PVCs in what other stations a station can reach • Expensive because each setup costs money • Until recently, not used much because of complexity, added cost

18. ATM Switches • ATM Frame Header • Does NOT have a destination address field • Instead, has two fields that together contain a hierarchical virtual circuit number • Like a route number on a bus--names the route, not the destination Virtual Circuit Number ATM Header

19. ATM Switches • Hierarchical Virtual Circuit Number • Virtual Path Identifier • Higher-level number; Often specifies a site • Virtual Channel Identifier • Lower-level number; Often specifies a computer at a site Virtual Circuit Number ATM Header

20. ATM Switches • ATM Reliability • Virtual circuit reduces communication to a single path • If a switch or trunk line along the path fails, communication stops • But ATM switches also have addresses, which are used to set up a new virtual circuit fairly rapidly