ATM Switches Cells Scalable Qo S Perspective Virtual
ATM Switches Cells Scalable Qo. S 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 ATM Cell Payload (48 octets) Header (5 octets)
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 (Qo. S)--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 Qo. S is critical – In LANs, usually used only where service quality is critical, typically when voice is being carried
ATM Qo. S Categories • ATM Offers Varying Levels of Qo. S • 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 Qo. S Categories • ATM Offers Varying Levels of Qo. S • 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 Qo. S 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 Qo. S 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 Qo. S makes little sense if used entirely for data – Has other data transmission benefits, however
ATM Qo. S 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 Qo. S 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 ATM Cell Virtual Circuit
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
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 ATM Header Virtual Circuit Number
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 ATM Header Virtual Circuit Number
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
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