Transcript Chapter 2

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)
CBR
Service
(connectionless
VBR Service
or connection-oriented)
(higher-layer
Connection-oriented
connection-oriented services)
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
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