5. Asynchronous Transfer Mode (ATM)

Objectives of ChapterATM IntroductionATM TerminologyProtocol ArchitectureLogical ConnectionsATM cell structureService levels/categoriesATM Layers

IntroductionATM evolved from B-ISDN development effortsFrame Relay: high-speed WAN (1.5+ Mbps)ATM: very high speed WAN (155 Mbps and 622Mbps)

Connection-oriented packet-switched networkUsed in both WAN and LAN settingsCommonly transmitted over SONETother physical layers possible

ATM, like Frame Relay, was built on the assumption that the underlying physical media was reliable and flexible

Why faster than Frame Relay Faster than Frame Relay because :-

Minimal error and flow control capabilities thus reduces overhead per cell

More streamlined in nature

Use fixed-size cells simplifies the processing required at each ATM Node

History In 1988, CCITT (from ITU) issued the first two recommendations relating to B-ISDN:I.113 Vocabulary of Terms for Broadband Aspects of ISDNI.121 Broadband Aspects of ISDN

In 1911, the ATM Forum was created with the goal of accelerating the development of ATM standards

Specifications developed by ITU-T and ATM Forum

ITU-T is concerned with developing ATM Standard while, ATM Forum is interested in a broad range of ATM Application

Multiplexing Using Different Packet SizesMultiplexing Using Cells

ATM Multiplexing

"A transfer mode in which information is organized into cells; it is asynchronous in the sense that the recurrence of cells containing information from an individual user is not necessarily periodic".ATM definition

What is it really?Low-level network layerabove physical layer, below AAL (ATM adaptation layer)Single transport mechanism for different types of traffic (voice, data, video, etc.)Streamlined protocol, minimal error and flow control capabilitiesFixed packet size = ATM cellSimplified processing, management

ATM Features Cell are transmitted asynchronous (unlike SDH)A packet switched network, connection-oriented serviceLocal area, metro area, and wide area serviceCan support real-time traffic and non-real-time traffic (data arrives in order, low delay)Can support various levels of service (continuous, variable, available, and unspecified)Very fast (up to 622 Mbps)A complex technology and typically expensive

More What is ATM?Similarities between ATM and packet switchingTransfer of data in discrete chunksMultiple logical connections over single physical interface

In ATM flow on each logical connection is in fixed sized packets called cellsMinimal error and flow controlReduced overheadData rates - 25.6Mbps to 622.08Mbps (155.5 Mbps necessary for full-motion video)

Asynchronous Transfer Mode (ATM)

Overview of ATM NetworkATM is similar to IPa mesh network of routers (ATM switches)

Two types of links in ATMUNI (user network interface) connects switch to user deviceNNI (network network interface) connects two ATM switches;

ATM is connection-orientedUser must create a virtual circuit through the ATM network (using virtual circuit ID); Signals create circuit, maintain circuit, and dissolve circuit

Architecture of an ATM NetworkUNI: User-network interfaceNNI: Network-Network interface

ATM Protocol ArchitectureATM allows multiple logical connections to be multiplexed over a single physical interface.

In ATM , information flow on each logical connection is organized into Fixed-size packets called cellscell switching like packet switchingAlso know as Cell Relay

Protocol Layers2 primary protocol layers relate to ATM functions:

ATM Layer: Common layer providing packet transfers, logical connections (ATM)

ATM adaptation layer (AAL) Service dependent

ATM Conceptual Model Four Assumptions ATM network will be organized as a hierarchy.Users equipment connects to networks via a UNI (User-Network Interface).Connections between provided networks are made through NNI (Network-Network Interface).

ATM will be connection-oriented.A connection (an ATM channel) must be established before any cells are sent.Two levels of ATM connections:virtual path connectionsvirtual channel connections

ATM Conceptual Model Assumptions (cont.)Vast majority of ATM networks will run on optical fiber networks with extremely low error rates.

ATM must support low cost attachments.This decision lead to a significant decision to prohibit cell reordering in ATM networks. ATM switch design is more difficult.

Protocol Model has 3 planesUser provides for user information transfer and associated controls (flow control, congestion control)Control performs call control and connection control functions (signaling)Management provides plane management and layer management and coordination functions Management planePlane managementManagement functions related to system as a whole; make sure the various planes coordinate their activities properlyLayer managementProvides operations, administration, and maintenance (OAM) services thru info packets that switches exchange to keep system running effectively

ATM Protocol Reference ModelVarious data rates (155.52 Mbps, 622.08 Mbps) over various physical media types (Fiber Optic, SONET, UTP, etc.)Framing, cell structure& Logical ConnectionsMap data to the ATM cellstructureVirtual Channel FunctionsVirtual Path Functions

Protocol ArchitecturePhysical planeDesigned to run over SONET but can also run over FDDI, T-1, and T-3ATM LayerDefines the cell format and how to respond to info found in the header. Also responsible for setting up and releasing connections, and performs congestion controlATM Adaptation Layer (AAL)Provides the interface between applications and the ATM layer

User Plane LayersUser informationUser information

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ATM Layers in End-Point Devices and Switches

ATM Logical Connections Two levels of ATM connections:Virtual Channel ConnectionsVirtual Path Connections

Indicated by two fields in the cell header: virtual path identifier VPI virtual channel identifier VCI

Logical ConnectionsVirtual Channel Connection (VCC):

Logical connections in ATM : a logical connection analogous to a virtual circuit in X.25, or Frame Relay data link connection

It is basic unit of switching in an ATM Network

Variable rate, Full-duplex flow between two end users

Used for user-network exchange(control signaling)

Used for network-network exchange (management/routing)

ATM Logical ConnectionsTwo types of virtual circuits

Permanent virtual circuit analogous to a leased telephone line

Switched virtual circuit created using a connection protocol based on ITU-T Q.2931

Virtual Path Connection (VPC): A bundle of VCCs with the same end points (not necessarily same end-users) andswitched along the same path

ATM Connection RelationshipsVirtual Channel: basic logical communications channelVirtual Path: groups of common virtual channelsPhysical Transmission Path: physical communications link

Virtual Path/Virtual Channel TerminologyVirtual Channel (VC) A generic term used to describe unidirectional transport of cells associated by a common unique identifier value

Virtual Channel Identifier (VCI) A unique numerical tag for a particular VC link

Virtual Channel LinkA means of unidirectional transport of cells between the point where a VCI is assigned and where it is translated or terminated

Virtual Channel Connection (VCC) A concatenation of VC links that extends between two connected ATM end-points

Virtual Path (VP) A generic term which describes unidirectional transfer of cells belong to VC that are associated with a common unique identifier

Virtual Path Identifier(VPI) Identifies a particular VP

Virtual Path LinkA group of VC links identified by a common identifier between the point where the identifier (VPI) is assigned and where it is translated or terminated

Virtual Path Connection (VPC) A concatenation of VP links that extends between ATM end-points where the VCIs are assigned and where they are translated or terminatedVirtual Path/Virtual Channel Terminology

Virtual Channel and Virtual Paths

Virtual ChannelsThe virtual channel (VC) is the fundamental unit of transport in a B-ISDN. Each ATM cell contains an explicit label in its header to identify the virtual channel.a Virtual Channel Identifier (VCI)a Virtual Path Identifier (VPI)A virtual channel (VC) is a communication channel that provides for the transport of ATM cells between two or more endpoints for information transfer. A Virtual Channel Identifier (VCI) identifiers a particular VC within a particular VP over a UNI or NNI.

Virtual Paths

A Virtual Path (VP) is a group of Virtual Channels that are carried on the same physical facility and share the same Virtual Path Identifier (VPI) value.

Aim: Control cost of network :By grouping connections sharing common paths through the network into a single unit. Thus network management action can be applied to a small number of groups of connections instead of a large number of individual connections.

Virtual Connection Identifiers in UNIs and NNIs

Connection IdentifiersThe VP boundaries are delimited by Virtual Path Terminators (VPT).AT VPTs, both VPI and VCI are processed. Between VPTs associated with the same VP, only the VPI values are processed (and translated) at ATM network elements. The VCI values are processed only at VPTs, and are not translated at intermediate ATM network elements.

Advantages of Virtual PathsSimplified network architecture allows separation of functionality into individual logical connections (virtual channel) and related groups of logical connections( virtual path)Increased network performance and reliability. - Network deals with fewer entitiesReduced processing and short connection setup time complex setup tasks are in virtual paths, simplifies setup of new virtual channels over existing virtual path- no call processing is required at transit nodesEnhanced network services supports user-specified closed groups/networks of VC bundles

Call Establishment with Virtual PathsVirtual path control mechanism including calculating routes, allocatingCapacity, and storing connection State information

Virtual Channel Connection UsesCell sequence integrity is maintained by VCCExchange between end usersCarry end to end user datacontrol signaling Exchange between an end user and a network entityUsed for user-to-network control signalingExchange between 2 network entitiestraffic managementrouting functions

VPC/VCC Characteristics1-4 for both VPC and VCC ; 5 VPC only Quality of Service (QoS): deals with parameters such as loss ratio and cell delay variation Switched and semi-permanent virtual channel connectionsSwitched VCC : on demand connection, which requires call control signaling for setup and tearing downSemi-permanent VCC : is of long duration and is set up by configuration or network management action Cell sequence integrityTraffic parameter negotiation and usage monitoringaverage rate, peak rate, burstiness peak duration, etc.5) (For VPC only) virtual channel identifier restriction within a VPC: Reserved for network use

ATM SignalingXXXXXXXXXPrivate UNIPublic UNINNIPrivate NNIPrivate ATM networkPublic UNIB-ICIPublic UNIPublic ATM network APublic ATM network BQ-2931Q-2931PNNIPNNIPNNI

Control SignalingA mechanism to establish and release VPCs and VCCs The exchange of information involved in this process is referred to as control signaling

4 methods for VCCs (as per ITU-T Rec. I.150):

Semi-permanent VCC: no control signaling required

Meta-signaling channel: permanent channel, low data rate channel for setting up signaling channels

User-to-network signaling virtual channel: set up between user and network to carry user data

User-to-user signaling virtual channel: set up between users within a VPC, allowing users to set up and tear down VCCs, without network intervention

Control Signaling: exchange of information

3 methods for VPCs

Semi-permanent: No control signaling required. It is by prior agreement.Customer controlled: Customer uses a signaling VCC to request VPC from the networkNetwork controlled: Network establishes VPC for its own control and signaling use. The path may be network to network, user to network or user to user.

ATM CellFixed size5-octet header48-octet information fieldWhy so small? Small cells reduce queuing delay for high priority cells (essential for low delay)Small cells can be switched more efficiently(essential for very high data rates)Easier to implement switching of small cells in hardwareFixed-size makes programming more easy

ATM Cell Format

Cell FormatUser-Network Interface (UNI)

host-to-switch format GFC: Generic Flow Control (still being defined)VCI: Virtual Circuit IdentifierVPI: Virtual Path IdentifierType: management, congestion control, AAL5 CLP: Cell Loss Priority HEC: Header Error Check (CRC-8)

Network-Network Interface (NNI)

switch-to-switch formatGFC becomes part of VPI field

Header FormatGeneric Flow ControlControls flow of data from user device into the ATM network onlyUsed to control traffic flow for different qualities of service at user-network interface (UNI) to alleviate/ facilitate short-term overload conditionsNote: not employed in network core (N-N Interface)

When GFC is enabled at the UNI, two procedures are used:Uncontrolled transmission: not subject to flow control; network simply enables or disables sending of cells like X-ON/X-OFF flow controlControlled transmission: flow control constraintsnetwork provides info to user regarding how many cells it can send like a credit mechanism for flow control

Header FormatVirtual path identifier (VPI)Constitutes a routing field for the network 8 bit at U-N-I and 12 bit at N-N-I: to support for an expanded no of VPCs internal to the networkVirtual channel identifier (VCI)Used for routing to and from the end user A 16-bit channel ID. Together, VPI and VCI identify a logical connection

Payload type 3 bitsIdentifies cell as user data or network management cell, presence of congestion, Service Data Unit (SDU) type)Provide inband control information Header Format

Header FormatCell loss priority Provide guidance to the network in the event of congestion 0 value : Cell of high priorityShould not be discarded unless no alternate ; 1 value: Cell of low prioritySubjected to discard within the networkCan assign to any cell who violet the agreement of traffic parameters Header error control Used for error control

Header FormatGeneric Flow Control

Used to control traffic flow for different qualities of service at user-network interface (UNI) to alleviate/ facilitate short-term overload conditionsNote: not employed in network core (N-N Interface)

When GFC is enabled at the UNI, two procedures are used:Uncontrolled transmission: not subject to flow controlControlled transmission: flow control constraints (using GFC mechanism) are in force

Generic Flow Control (GFC) Field Coding

Header Error Control8-bit field - calculated based on the other 32 bits in the headerCRC based on x8 + x2 + x + 1 -> generator is 100000111error detection

In some cases, error correction of single-bit errors in header2 modes: Error detectionError correction

HEC Operation at ReceiverBased on recognition of fact that bit errors occur in bursts.

Effect of Error at Cell Header

Impact of Random Bits on HEC Performance

ATM Service CategoriesReal-time serviceConstant bit rate (CBR)Real-time variable bit rate (rt-VBR)Non-real-time serviceNon-real-time variable bit rate (nrt-VBR)Available bit rate (ABR)Unspecified bit rate (UBR)Guaranteed frame rate (GFR)

Constant bit rate (CBR)Used by application that require a continuous fixed data rateTight upper bound on delayCan support uncompressed audio and videoEg: Video conferencingInteractive audio (telephony) A/V distribution and retrieval (television, distance learning , video on demand)

Tightly controlled by Peak Cell Rate (PCR), Cell Transfer Delay (CTD), and Cell Delay Variation (CDV)

Real-time variable bit rate (rt-VBR)Time sensitive applicationTightly constrained delay and delay variationrt-VBR applications transmit at a rate that varies with time wile CBR doesnt

Examples include bursty voice and videoCan statistically multiplex connectionsMore flexible than CBRParameters include Peak Cell Rate, Sustainable Cell Rate, and Maximum Burst Size

Non-real-time variable bit rate (nrt-VBR)Intended for bursty traffic with no tight constraints on delay and delay variationParameters include Peak Cell Rate, Sustainable Cell Rate, Maximum Burst Size, Cell Loss Ratio, Cell Transfer Delay

Examples include airline reservations, banking transactions

Available bit rate (ABR)Application specifies Peak Cell Rate (PCR) and Minimum Cell Rate (MCR)No performance guarantees other than minimum cell rate (MCR), but low loss expectedNo bound on traffic user can sendUsers are able to shape their traffic to varying network conditionsResources allocated to give at least MCRSpare capacity shared among all ABR sources

Examples include LAN interconnection and basic critical data transfer systems such as banking, defense information (flying standby)

Unspecified bit rate (UBR)

For application that can tolerate some cell loss or variable delays (non-critical apps)Cells forwarded on FIFO basisDo not specify traffic related service guaranteesExamples include text/data/image transfer, messaging, remote terminalsBest effort service (wear your parachute)

Guaranteed frame rate (GFR)Recent addition to support IP backbone sub networks.Provide better services than UBR for frame-based traffic, including IP and Ethernet. To optimize the handling of frame based traffic that passes from a LAN through a router onto an ATM backbone network. Allow user to reserve capacity for each GFR VC. User is guaranteed that minimum capacity will be supported

ATM Bit Rate Service Levels

ATM Bit Rate Service Levels comparison With ABR it is possible to make use of added buffering in the routersFor Bursty Data: ABR > GFR > VBR > UBR > CBR

ClassDescriptionExampleCBRConstant Bit RateT1 circuitRT-VBRReal Time Variable Bit RateReal-time videoconferencingNRT-VBRNon-real-time Variable Bit RateMultimedia email

ABRAvailable Bit RateBrowsing the WebUBRUnspecified Bit RateBackground file transfer

ATM Adaptation LayerNeed of an adaptation layer to support information transfer protocol not based on ATM.Essentially the translation layer between ATM layer and other layers, such as PCM(pulse code modulation ) and IP:PCM (voice)Assemble bits into cellsRe-assemble into constant flowIPMap IP packets onto ATM cellsFragment IP packetsUse LAPF over ATM to retain all IP infrastructure

ATM Adaptation Layer (AAL)Support higher-level protocols and/or native applicationse.g., PCM voice, LAPF, IPAAL ServicesHandle transmission errorsSegmentation and re-assemblyTo enable larger blocks of data to be carried in the information field of ATM cellsHandle lost and misinserted cells (cells routed the wrong way)Perform flow control and timing control

Supported Application typesFour AAL protocols defined:AAL 1: CBR traffic, e.g. circuit emulation (T-1 over ATM), voice over ATM, real-time videoAAL 2: rt-VBR traffic, e.g. MPEG voice and videoAAL 3/4: nrt-VBR traffic, e.g. general data service (not really used by anyone)AAL 5 (successor to AAL 3/4): e.g. nrt-VBR: voice on demand; nrt-VBR: frame relay, ATM; UBR: IP over ATM

ATM Adaptation Layer (AAL)

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AAL Types

AAL ProtocolsAAL layer has 2 sublayers:

Convergence Sublayer (CS)Supports specific applications/protocols using AALUsers attach via the Service Access Point (like a port number) which is address of application. Common part (CPCS) and application service-specific part (SSCS)It is service dependent

Segmentation and Reassembly Sublayer (SAR)Packages data from CS into ATM cells and unpacks at other end

AAL Protocols and PDUs

AAL Protocol and ServicesBasis for classification:requirement for a timing relationship between source and destinationrequirement for a constant bit rate data flowconnection or connectionless transfer

AAL Service ClassificationTiming between source and destinationClass A Class B Class C Class DRequired Not requiredBit Rate Constant VariableConnection Mode Connection oriented ConnectionlessCircuitEmulationPacketizedvoice/videoConnectionOrientedDataDatagramAttributeAAL1 AAL2AAL 5AAL 4AAL 3

Applications of AAL and ATMCircuit emulation (e.g., T-1 synchronous TDM circuits)VBR voice and videoGeneral data servicesIP over ATMMultiprotocol encapsulation over ATM (MPOA)LAN emulation (LANE)

Segmentation and Reassembly PDUs

Segmentation and Reassembly ATM Adaptation Layer (AAL)AAL 1 and 2 designed for applications that need guaranteed rate (e.g., voice, video)AAL 3/4 designed for packet dataAAL 5 is an alternative standard for packet dataAALATMAALATM

AAL Type 1AAL 1 is the interface between a real-time uncompressed byte stream and ATMGot to be fast!AAL 1 takes 46 or 47 bytes of data and puts a one or two byte header on frontConstant-bit-rate sourceNo convergence sublayer, only SAR sublayerSAR simply packs bits into cells and unpacks them at destination.No CS PDU structure is defined since CS sublayer primarily for clocking and synchronization

AAL Type 1

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Figure 19-23

AAL1

AAL Type 2

Intended for use with applications with variable bit-rate service on multiple channels (multiplexing), or low bit rate, short-frame traffic

Header SN IT 47 Octets Payload LI CRCSAR PDUATM PDUSN: Sequence numberIT: Information Type:BOM,COM,EOM,SSMLength Indicator

Intended for variable bit rate applications that generate bursty data and demand low loss

Originally, connectionless (AAL4) or connection (AAL3) oriented, now combined into single format (AAL 3/4)

Provides comprehensive sequencing and error control mechanisms

AAL Type 3/4

AAL 3/4

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Figure 19-25

AAL3/4

AAL 3/4 Example (message mode)

AAL Type 5AAL 5 packets can be very large up to 65,535 byte payloadAAL 5 not designed for real-time trafficSAR sublayer takes the potentially large convergence sublayer packets and breaks them into 48 byte chunks, ready for the ATM layerSAR sublayer also adds a 32-bit CRC at the end of the packet, which is applied to the entire packet

AAL Type 5Streamlined transport for connection oriented protocolsReduce protocol processing overheadReduce transmission overheadEnsure adaptability to existing transport protocolsprimary function is segmentation and reassembly of higher-level PDUs

AAL Type 5

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Figure 19-26

AAL5

AAL Type 5

AAL Protocol Descriptions

References ATM Forum (http://www.atmforum.com/) White papers describe basic concepts of ATM

Review Questions1. Discuss main functions of ATM?2. Discuss the layers of ATM?3. How VCC is relate to VPC?4. Discuss the layout of a cell?5. What are the different service categories and when might each be used?6. What is the function of the AAL?7. What are the sublayers of AAL?

**5. ATM5. ATM *5. ATM5. ATM *Slides with details for these?*5. ATM5. ATM *