ATM Switches Cells Scalable QoS Perspective Virtual Circuits

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

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

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

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

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)

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

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

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!

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

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

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

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

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 Virtual Circuit ATM Cell

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 Virtual Circuit ATM Cell

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

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

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

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

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