chapter 5
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Chapter 5. Asynchronous Transfer Mode (ATM). Introduction. ATM Protocol Architecture Logical connections ATM Cells Service categories ATM Adaptation Layer (AAL). ATM Protocol Architecture. Fixed-size packets called cells Streamlined: minimal error and flow control - PowerPoint PPT PresentationTRANSCRIPT
Chapter 5 Asynchronous Transfer Mode
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Chapter 5
Asynchronous Transfer Mode
(ATM)
Chapter 5 Asynchronous Transfer Mode
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Introduction
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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Figure 5.1
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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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Figure 5.2
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Advantages of Virtual Paths
Simplified network architecture Increased network performance and
reliability Reduced processing and short
connection setup time Enhanced network services
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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
Methods to request for a VC:– Semi-permanent VCCs– Request as needed
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ATM Cells
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
Generic flow control Virtual path identifier (VPI) Virtual channel identifier (VCI) Payload type Cell loss priority Header error control
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Figure 5.4
CLP = Cell Loss Priority bit
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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
For error detection In some cases, error correction of
single-bit errors in header
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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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Figure 5.8
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ATM Adaptation Layer (AAL)
Support non-ATM protocols– Maps to ATM cells
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
Circuit emulation (e.g., E-1/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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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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Figure 5.9
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Figure 5.10(CBR source)
(connectionless or connection-oriented)
(higher-layer connection-oriented services)
CBR Service
VBR Service
Connection-oriented Service
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AAL Type 1
Constant-bit-rate source SAR simply packs bits into cells and unpacks
them at destination Sequence Number (SN) used in
reassembling SAR PDUs Sequence Number Protection (SNP) provides
error detection/correction 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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Figure 5.13