atm
TRANSCRIPT
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Asynchronous Transfer Mode
(ATM)
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Introduction Introduction
ATM Protocol ArchitectureLogical connectionsATM CellsService categoriesATM Adaptation Layer (AAL)
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ATM Protocol ArchitectureATM Protocol Architecture
Fixed-size packets called cellsStreamlined: minimal error and flow
control (high data rates)2 protocol layers relate to ATM functions:
– Common layer providing packet transfers– Service dependent ATM adaptation layer
(AAL)AAL maps other protocols to ATM
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ATM Layers
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Protocol Model has 3 planesProtocol Model has 3 planes
User: Provides for user information transfer, along with error and flow control.
Control: Performs call control and connection control functions.
Management: Performs management functions and coordination between all the planes.
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Architecture of an ATM Network
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Logical ConnectionsLogical Connections
VCC (Virtual Channel Connection): a logical connection analogous to virtual circuit in X.25
VPC (Virtual Path Connection): a bundle of VCCs with same endpoints
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Figure 5.2Figure 5.2
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TP, VPs, and VCs
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Example of VPs and VCs
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Connection Identifiers
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Figure 19-13
Routing with a Switch
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Virtual Connection Identifiers in UNIs and NNIs
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Advantages of Virtual PathsAdvantages of Virtual Paths
Simplified network architectureIncreased network performance and
reliabilityReduced processing and short connection
setup timeEnhanced network services
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Table 5.1Table 5.1
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VCC UsesVCC Uses
Between end usersBetween an end user and a network entityBetween 2 network entities
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Figure 5.3Figure 5.3
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VPC/VCC CharacteristicsVPC/VCC Characteristics
Quality of Service (QoS) : Cell loss ratioSwitched and semi-permanent virtual
channel connectionsCell sequence integrityTraffic parameter negotiation and usage
monitoring(VPC only) virtual channel identifier
restriction within a VPC
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Control SignalingControl Signaling
A mechanism to establish and release VPCs and VCCs
4 methods for VCCs:– Semi-permanent VCCs– Meta-signaling channel– User-to-network signaling virtual channel– User-to-user signaling virtual channel
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Control SignalingControl Signaling
3 methods for VPCs– Semi-permanent– Customer controlled– Network controlled
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Multiplexing Using Different Packet Sizes
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Multiplexing Using Cells
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ATM Multiplexing
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ATM CellsATM Cells
Fixed size5-octet header48-octet information fieldSmall cells reduce delay for high-priority
cellsFixed size facilitate switching in hardware
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Header FormatHeader Format
Generic flow controlVirtual path identifier (VPI)Virtual channel identifier (VCI)Payload typeCell loss priorityHeader error control
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Figure 5.4Figure 5.4
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Figure 19-28 ATM HeaderATM Header
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Virtual Connection Identifiers in UNIs and NNIs
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Generic Flow ControlGeneric Flow Control
Control traffic flow at user-network interface (UNI) to alleviate short-term overload conditions
When GFC enabled at UNI, 2 procedures used:– Uncontrolled transmission
– Controlled transmission Every connection either subject to flow control or not Flow control is from subscriber to network
– Controlled by network side
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Payload Type (PT)Payload Type (PT)
Indicates the type of the information in the information field.
3 bits.– 1st bit indicates user information.– 2nd bit indicates congestion.– 3rd bit indicates SDU (Service Data Unit).
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Payload Type (Cont…)Payload Type (Cont…)
PT Coding
Interpretation
000 User Data, congestion not experienced, SDU type = 0
001 User Data, congestion not experienced, SDU type = 1
010 User Data, congestion experienced, SDU type = 0
011 User Data, congestion experienced, SDU type = 1
100 OAM segment associated cell
101 OAM end to end associated cell
110 Resource management cell
111 Reserved for future function
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Cell Loss Priority (CLP)Cell Loss Priority (CLP)
Provides guidance to the network in the event of congestion.
A value ‘0’ indicates a cell of relatively higher priority.
A value ‘1’ indicates a cell of relatively lower priority.
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Header Error ControlHeader Error Control
8-bit field calculated based on remaining 32 bits of header
In some cases, error correction of single-bit errors in header
2 modes: – Error detection– Error correction
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Figure 5.5Figure 5.5
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Figure 5.6Figure 5.6
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Definition of TermsDefinition of Terms COS – Class of Service – sets a priority of data
delivery, based upon the class. Higher priority data get delivered before lower priority data (example – which should have higher priority – streaming video or email?)
QOS – Quality of Service – involves establishing certain parameters for a specific transmission – e.g. amount of bandwidth required for a given priority data transmission, max. amount of latency tolerated, etc
Both are required to deliver real-time voice and video traffic
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Service CategoriesService Categories
Real-time service– Constant bit rate (CBR)– Real-time variable bit rate (rt-VBR)
Non-real-time service– Non-real-time variable bit rate (nrt-VBR)– Available bit rate (ABR)– Unspecified bit rate (UBR)– Guaranteed frame rate (GFR)
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Real Time ServicesReal Time Services If want to avoid or decrease variation of delay (jitter), use CBR or rt-VBR CBR Fixed data rate continuously available Commonly used for uncompressed audio and video
– Video conferencing– Interactive audio– A/V distribution and retrieval
rt-VBR Best for time sensitive applications– Tightly constrained delay and delay variation
rt-VBR applications transmit at a rate that varies with time– e.g. compressed video – Produces varying sized image frames– Original (uncompressed) frame rate constant – So compressed data rate varies
Can statistically multiplex connections
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Non-Real Time Non-Real Time
Intended for applications with bursty traffic and limited constraints on delay and delay variation
Greater flexibility, greater use of multiplexing
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nrt-VBRnrt-VBR
May be able to characterize expected traffic flow
Improve QoS in loss and delayEnd system specifies:
– Peak cell rate – Sustainable or average rate – Measure of how bursty traffic is
e.g. Airline reservations, banking transactions
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Unspecified Bit Rate (UBR)Unspecified Bit Rate (UBR)
Unused capacity of CBR and VBR traffic made available to UBR
For application that can tolerate some cell loss or variable delays– e.g. TCP based traffic
Cells forwarded on FIFO basisBest efforts serviceText/Data/image transfer, messagingRemote Terminal (telecommuting)
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Available Bit Rate (ABR)Available Bit Rate (ABR)
Application specifies peak cell rate (PCR) it will use and minimum cell rate (MCR) it requires
Resources allocated to give at least MCRSpare capacity shared among ABR and
UBR sourcese.g. LAN interconnection
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Guaranteed Frame Rate (GFR)Guaranteed Frame Rate (GFR)
Designed to support IP backbone sub networks
Purpose: optimize handling of frame based traffic passing from LAN through router to ATM backbone– Used by enterprise, carrier and ISP networks– Consolidation and extension of IP over WAN
UNI establishes hand shaking between NIC and switch
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Figure 5.8Figure 5.8
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ATM Layers
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ATM Layers in End-Point Devices and Switches
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Functions of the LayersFunctions of the Layers
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ATM Adaptation Layer (AAL)ATM Adaptation Layer (AAL)
Support non-ATM protocols– e.g., PCM voice, LAPF
AAL Services– Handle transmission errors– Segmentation/reassembly (SAR)– Handle lost and misinserted cell conditions– Flow control and timing control
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Applications of AAL and ATMApplications of AAL and ATM
Circuit emulation (e.g., T-1 synchronous TDM circuits)
VBR voice and videoGeneral data servicesIP over ATMMultiprotocol encapsulation over ATM
(MPOA) (IPX, AppleTalk, DECNET)LAN emulation (LANE)
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AAL ProtocolsAAL Protocols
AAL layer has 2 sublayers:– Convergence Sublayer (CS)
Supports specific applications using AALservice dependant
– Segmentation and Reassembly Layer (SAR)Packages data from CS into cells and unpacks at
other end
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AAL Types
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Figure 5.9Figure 5.9
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Figure 5.10Figure 5.10
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AAL Type 1AAL Type 1
Constant-bit-rate source: Circuit Emulation
SAR simply packs bits into cells and unpacks them at destination
One-octet header contains 3-bit SC field to provide an 8-cell frame structure
No CS PDU since CS sublayer primarily for clocking and synchronization
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Figure 19-23
AAL1
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AAL 2AAL 2
Variable bit rate source: MPEG Video.Analog Applications such as audio and
video.Timing information but not CBR.Not clearly defined.
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AAL Type 3/4AAL Type 3/4
May be connectionless or connection oriented
May be message mode or streaming mode
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Figure 19-25 AAL3/4
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Figure 5.11Figure 5.11
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Figure 5.12Figure 5.12
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AAL Type 5AAL Type 5
Streamlined transport for connection oriented protocols– Reduce protocol processing overhead– Reduce transmission overhead– Ensure adaptability to existing transport
protocols
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Figure 19-26
AAL5
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Figure 5.13Figure 5.13
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ATM Layer
Defines the cell format and connection setup.