Transcript ppt
CDA 6505 Network Architecture
and Client/Server Computing
Lecture 5
Asynchronous Transfer Mode
(ATM)
by
Zornitza Genova Prodanoff
Lect1..ppt - 01/06/05
Outline
• ATM Protocol Architecture
• Logical connections
• ATM Cells
• Service categories
• ATM Adaptation Layer (AAL)
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ATM Protocol Architecture
• Fixed-size packets called cells
• Streamlined: minimal error and flow control
• 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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Protocol Model has 3 planes
• User
• Control
• management
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Logical 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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Advantages of Virtual Paths
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Simplified network architecture
Increased network performance and reliability
Reduced processing and short connection setup time
Enhanced network services
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VCC Uses
• Between end users
• Between an end user and a network entity
• Between 2 network entities
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VPC/VCC Characteristics
• Quality of Service (QoS)
• Switched and semi-permanent virtual channel
connections
• Cell sequence integrity
• Traffic parameter negotiation and usage monitoring
• (VPC only) virtual channel identifier restriction within
a VPC
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Control 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 Signaling
• 3 methods for VPCs
– Semi-permanent
– Customer controlled
– Network controlled
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ATM Cells
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Fixed size
5-octet header
48-octet information field
Small cells reduce delay for high-priority cells
Fixed size facilitate switching in hardware
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Header Format
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Generic flow control
Virtual path identifier (VPI)
Virtual channel identifier (VCI)
Payload type
Cell loss priority
Header error control
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Generic 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
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Header Error Control
• 8-bit field calculated based on remaining 32 bits of
header
• error detection
• in some cases, error correction of single-bit errors in
header
• 2 modes:
– error detection
– Error correction
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Service 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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ATM Adaptation Layer (ATM)
• 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 ATM
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Circuit emulation (e.g., T-1 synchronous TDM circuits)
VBR voice and video
General data services
IP over ATM
Multiprotocol encapsulation over ATM (MPOA)
LAN emulation (LANE)
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Applications of AAL and ATM
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AAL Protocols
• AAL layer has 2 sublayers:
– Convergence Sublayer (CS)
• Supports specific applications using AAL
– Segmentation and Reassembly Layer (SAR)
• Packages data from CS into cells and unpacks at
other end
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AAL Type 1
• Constant-bit-rate source
• 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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AAL Type 3/4
• May be connectionless or connection oriented
• May be message mode or streaming mode
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AAL 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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