Asynchronous Transfer Mode (ATM) and QoS

ATM Era : Multiservice Networks Departure from Service Specialization bulk data video Multiservice Network voice interactive data

Why ATM Did Not Make it the Way it was Initially Envisioned Advantages of ATM Disadvantages of ATM Commercial Factors Single network optimized for everything (Data, phone, TV) Same technology for WAN, MAN, LAN (Seamless integration) QoS oriented and high-speed oriented Fast Hardware Tremendous amount of research has been done Large overhead for packets QoS is a bit complicated from the applications point of view and network management point of view Not that great from web browsing (which is one killer application) Millions of networks already installed Lack of applications Expensive at the LAN (where it really matters) No strong business incentive for QoS (even up to now) Can achieve similar speed with an IP router as compared to an ATM switch (May be)

Fixed length packet = cell 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 Temorary or permenant connections connection oriented or connectionless oriented Traffic type independent - media independent - rate independent Fixed length - easily implementable in silicon - predictiable Typical LAN packets are several Kbytes

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) Cells Voice Data Video SMALL - other data doesn’t get stuck behind it Fixed Length - predictable delays

Without Short Cells A voice packet waits behind a large data packet

With Short Cells Voice packet can go immediately after data packet #1 Waiting for voice is reduced significantly

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 Physical Transmission Link VCs VP 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 Physical transmission link = cable/media (copper or fiber) switched virtial channels within permanent virtual path Key point: customers can have SVCs even though service providers (carriers) have only PVP services. Often referred to as SVC tunnelling. Also, PVP’s can provide some firewalling capabilities VCs associated to VPs are globally switched VPs mostly between switches reduce reroute time - upon failures

Connection Identifiers

ATM switch routing Virtual Paths ATM ATM Switch Switch ATM Switch ATM Circuits

ATM Switches ATM switches translate VPI/VCI values Input Output 45 Port VPI/VCI Port VPI/VCI 1 29 2 45 2 29 64 2 45 1 29 1 1 64 3 29 3 3 29 1 64 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

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 • Resource allocation is by connection, not packet • Better performance and price - with hardware switching • Scalability Hardware Switching = fast ATM = scalable

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

Routing with a VP Switch

A Conceptual View of a VP Switch

Routing with a VC Switch

A Conceptual View of a VC Switch

ATM Protocol Stack Upper Layers ATM Adaptation Layer ATM Layer Reference Model Analogous to Link and Physical layer of OSI, but w/ enhancement • connection oriented • integral routing Transmission • Cell muxing • Cell headering generation • UPI/VCI translation Physical Layer

ATM Architecture Application Upper Layer Protocols Presentation Session ATM Adaptation Layer Transport Network Data Link ATM Layer Transmission-convergence physical medium dependent Physical

Adaptation Layers: Service Classes

Service Classes and Capacity of Network

QUEUES and PRIORITY Classifier Output CBR Traffic Priority 1 VBR Traffic Priority 2 Classifier Output ABR Traffic Priority 3 Priority 4 UBR Traffic

ATM Adaptation Layer: Summary Class Service Categories Bit Rate Connection Mode Timing Concern Application Examples ATM Adaptation Layer (AAL) A AAL1 CBR (Constant) Connection- Oriented Yes Bandwidth and throughput guaranteed Good for voice and video B AAL2 VBR (Variable) VBR-RT and VBR-NRT Connection- Oriented Yes Best effort bandwidth and throughput Good for live video, multimedia, LAN-to-LAN ATM Layer C AAL5 ABR (Available) Connection- Oriented No Best effort with congestion feedback Reliable delivery of bursty traffic if latency okay Physical Layer D AAL3/4 UBR (Un-specified) Connection- less No No guarantee For SMDS/LAN

QUALITY OF SERVICE Max CDT, Mean CTD, CDV, CLR, CER, SECBR, CMR

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 Cell Loss Ratio (CLR) Cost ($ or Admin) Link Weighting