Asynchronous Transfer Mode - Data Communications - Lecture Slides, Slides of Data Communication Systems and Computer Networks

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Chapter 11

Asynchronous Transfer Mode

Protocol Architecture

• Similarities between ATM and packet switching —Transfer of data in discrete chunks —Multiple logical connections over single physical interface
• In ATM flow on each logical connection is in fixed sized packets called cells
• Minimal error and flow control —Reduced overhead
• Data rates (physical layer) 25.6Mbps to 622.08Mbps

Reference Model Planes

• User plane —Provides for user information transfer
• Control plane —Call and connection control
• Management plane —Plane management - whole system functions —Layer management - Resources and parameters in protocol entities

ATM Logical Connections

• Virtual channel connections (VCC)
• Analogous to virtual circuit in X.
• Basic unit of switching
• Between two end users
• Full duplex
• Fixed size cells
• Data, user-network exchange (control) and network- network exchange (network management and routing)
• Virtual path connection (VPC) — Bundle of VCC with same end points

Advantages of Virtual Paths

• Simplified network architecture
• Increased network performance and reliability
• Reduced processing
• Short connection setup time
• Enhanced network services

Call

Establishment

Using VPs

VP/VC Characteristics

• Quality of service
• Switched and semi-permanent channel connections
• Call sequence integrity
• Traffic parameter negotiation and usage monitoring
• VPC only —Virtual channel identifier restriction within VPC

Control Signaling - VCC

• Done on separate connection
• Semi-permanent VCC
• Meta-signaling channel — Used as permanent control signal channel
• User to network signaling virtual channel — For control signaling — Used to set up VCCs to carry user data
• User to user signaling virtual channel — Within pre-established VPC — Used by two end users without network intervention to establish and release user to user VCC

ATM Cells

• Fixed size
• 5 octet header
• 48 octet information field
• Small cells reduce queuing delay for high priority cells
• Small cells can be switched more efficiently
• Easier to implement switching of small cells in hardware

ATM Cell Format

Generic Flow Control (GFC)

• Control traffic flow at user to network interface (UNI) to alleviate short term overload
• Two sets of procedures — Uncontrolled transmission — Controlled transmission
• Every connection either subject to flow control or not
• Subject to flow control — May be one group (A) default — May be two groups (A and B)
• Flow control is from subscriber to network — Controlled by network side

Single Group of Connections (1)

• Terminal equipment (TE) initializes two variables —TRANSMIT flag to 1 —GO_CNTR (credit counter) to 0
• If TRANSMIT=1 cells on uncontrolled connection may be sent any time
• If TRANSMIT=0 no cells may be sent (on controlled or uncontrolled connections)
• If HALT received, TRANSMIT set to 0 and remains until NO_HALT

Use of HALT

• To limit effective data rate on ATM
• Should be cyclic
• To reduce data rate by half, HALT issued to be in effect 50% of time
• Done on regular pattern over lifetime of connection

Two Queue Model

• Two counters —GO_CNTR_A, GO_VALUE_A,GO_CNTR_B, GO_VALUE_B