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Transcript
Asynchronous Transfer Mode (ATM) and QoS COMP680E by M. Hamdi 1 ATM Era : Multiservice Networks Departure from Service Specialization bulk data video Multiservice Network voice interactive data COMP680E by M. Hamdi 2 Why ATM Did Not Make it the Way it was Initially Envisioned Advantages of ATM Disadvantages of ATM • Single network optimized for everything (Data, phone, TV) • Large overhead for packets • Same technology for WAN, MAN, LAN (Seamless integration) • QoS oriented and highspeed oriented • Fast Hardware • Tremendous amount of research has been done Commercial Factors • Millions of networks already installed • QoS is a bit complicated • Lack of applications from the applications • Expensive at the LAN point of view and network (where it really matters) management point of • No strong business view incentive for QoS (even • Not that great from web up to now) browsing (which is one • Can achieve similar killer application) speed with an IP router as compared to an ATM switch (May be) COMP680E by M. Hamdi 3 What is ATM? ATM is packet switching! • Switched or permanent connections • Traffic type independent (voice, data, interactive video) • Fixed length packet - 53 bytes (cell) header payload Fixed length packet = cell COMP680E by M. Hamdi 4 ATM Cell Relay: The Underlying Technology Cell Features Benefit Small Low latency to support real-time services like audio and video (What is an appropriate size?) Fixed Length Fast hardware switching and scalability Standardized Usable in all networks (LAN and WAN) Voice Data Cells Video COMP680E by M. Hamdi 5 Without Short Cells Router I n p u t p o r t s O u t p u t Voice packet (50) Data packet (2000) p o r t s A voice packet waits behind a large data packet COMP680E by M. Hamdi 6 With Short Cells Router I n p u t p o r t s O u t p u t Data #40 (50) Data #2 (50) Voice (50) Data #1 (50) p o r t s Voice packet to be transmitted after Data #1 •Voice packet can go immediately after data packet #1 •Waiting for voice is reduced significantly COMP680E by M. Hamdi 7 Virtual Paths & Virtual Channels A Virtual Path (VP) describes the semi-permanent route between two end points. A Virtual Channel (VC) describes a cell transmission channel inside a virtual path VCs VP VCs VP Physical Transmission Link VP VCs VP VCs • Unique on a link-by-link basis • Virtual channels are contained within virtual paths • Interpreted at each switch to: • determine output link • determine outgoing VPI/VCI • Two-level structure: • allows “trunking” of virtual channels as one virtual path • virtual path can be switched • both used to route cells through network COMP680E by M. Hamdi 8 Connection Identifiers COMP680E by M. Hamdi 9 ATM switch routing Virtual Paths ATM Switch ATM Switch ATM Switch ATM Switch ATM Switch Virtual Circuits COMP680E by M. Hamdi 10 ATM Switches Input Output Port VPI/VCI Port 45 VPI/VCI 1 29 2 45 2 45 1 29 1 64 3 29 3 29 1 64 29 64 2 1 3 29 • ATM switches translate VPI/VCI values • VPI/VCI value unique only per interface— eg: locally significant and may be re-used elsewhere in network COMP680E by M. Hamdi 11 ATM Switching Connections (routes) set up by software • Routing (path through multiple-switch network) and resource allocation is performed once per connection by switch control CPU Cells are switched by hardware • Hardware (table lookup + switching fabric) switches each incoming cell to appropriate output port • Once a connection is established, cells are not touched by software COMP680E by M. Hamdi 12 VP and VC Switch • Two types of ATM switch • VP switch does not look at VCIs, switching is based on VPIs only • VCI does not change when passing through a VP switch; VPI may change • VC switch looks at both VPI and VCI • VCI (as well as VPI) may change when passing through a VC switch COMP680E by M. Hamdi 13 Routing with a VP Switch COMP680E by M. Hamdi 14 A Conceptual View of a VP Switch COMP680E by M. Hamdi 15 Routing with a VC Switch COMP680E by M. Hamdi 16 A Conceptual View of a VC Switch COMP680E by M. Hamdi 17 ATM Protocol Stack Upper Layers ATM Adaptation Layer ATM Layer Physical Layer COMP680E by M. Hamdi 18 ATM Architecture Application Upper Layer Protocols Presentation Session ATM Adaptation Layer Transport Network ATM Layer Data Link Transmission-convergence physical medium dependent Physical COMP680E by M. Hamdi 19 Adaptation Layers: Service Classes COMP680E by M. Hamdi 20 Service Classes and Capacity of Network COMP680E by M. Hamdi 21 QUEUES and PRIORITY CBR Traffic Priority 2 Priority 3 Output ABR Traffic Classifier VBR Traffic Priority 1 Priority 4 UBR Traffic COMP680E by M. Hamdi 22 ATM Adaptation Layer: Summary Class ATM Adaptation Layer (AAL) A Service Categories AAL1 AAL2 Connection Timing Mode Concern Yes • Bandwidth and throughput guaranteed • Good for voice and video Connection(Variable) Oriented Yes • Best effort bandwidth and throughput • Good for live video, multimedia, LAN-to-LAN ConnectionOriented No • Best effort with congestion feedback • Reliable delivery of bursty traffic if latency okay Connectionless No • No guarantee • For SMDS/LAN VBR ATM Layer VBR-RT and VBR-NRT C AAL5 ABR (Available) D AAL3/4 Application Examples ConnectionOriented CBR (Constant) B Physical Layer Bit Rate UBR (Unspecified) COMP680E by M. Hamdi 23 QUALITY OF SERVICE – Max CDT, Mean CTD, CDV, CLR, CER, SECBR, CMR CBR rt-VBR nrt-VBR ABR UBR PCR Yes Yes Yes Yes Yes SCR N/A Yes Yes N/A N/A MBS N/A Yes Yes N/A N/A MCR No No No Yes No CDVT(PCR) Yes Yes Yes Yes Yes CDVT(SCR) N/A Yes Yes N/A N/A COMP680E by M. Hamdi 24 Application Requirements Bandwidth • Peak Cell Rate (PCR) • Sustained Cell Rate (SCR) • Minimum Cell Rate (MCR) Delay • Cell Transfer Delay (CTD) • Cell Delay Variation (CDV) Reliability Cost ($ or Admin) • Cell Loss Ratio (CLR) • Link Weighting COMP680E by M. Hamdi 25