Chapter 18' VirtualCircuit Networks: Frame Relay and ATM

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Chapter 18. Virtual-Circuit NetworksFrame Relay
and ATM

• 18.1 Frame Relay
• 18.2 ATM
• 18.3 ATM LANs

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Wide area network and switching methods
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Circuit switching

• Create a real circuit (dedicated line) between
  source and destination
• Physical layer technology

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Packet Switching Datagram Approach

• Mostly used in the network layer
• Routing (selecting the best route for a packet)
  is performed at each router

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Packet Switching Virtual Circuit Approach

• Packets (frames) are switched along a
  pre-determined path from source to destination
• Virtual circuit network has two addresses
• Global address which is unique in the WAN
• Virtual circuit identifier which is actually used
  for data transfer
• VCI has switch scope it is used between two
  switches
• Each switch can use its own unique set of VCIs

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VCI Phases

• Two approaches for the VC setup
• Permanent virtual circuit (PVC)
• Switched virtual circuit (SVC) setup, data
  transfer, teardown

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Data Transfer Phase

• All switches need to have a table entry for the
  virtual circuit

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Data Transfer using VCI
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SVC Setup Request and Acknowledgment
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Frame Relay

• Frame Relay is a virtual circuit wide area
  network
• VCIs in Frame Relay are called DLCIs(Data Link
  Connection Identifier)

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Frame Relay Features

• Frame relay operates at a higher speed. It can
  easily be used instead of a mesh of T-1 or T-3
  lines (1.544 Mbps or 44.376 Mbps)
• Frame relay operates just the physical and data
  link layers. It is good as a backbone to provide
  services to protocols that already have a network
  layer protocol, such as Internet
• It allows bursty data
• It allows a frame size of 9000 bytes
  accommodating all LAN frame sizes
• It is less expensive than other traditional WANs
• It has error detection at the data link layer
  only. There is no flow control pr error control
• X.25 ? Leased Lines ? Frame Relay

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Frame Relay vs. T-line Network
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Frame Relay vs. X.25 Network
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Frame Relay Layers

• Frame relay operates only at the physical and
  data link layers

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Comparing Layers X.25 Frame Relay
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Frame Relay Frame
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Congestion Control

• Frame relay requires congestion control, because
• Frame Relay does not have a network layer
• No flow control at the data link layer
• Frame Relay allows the user to transmit bursty
  data
• Congestion avoidance
• Two bits in the frame are used
• BECN(Backward Explicit Congestion Notification)
• FECN(Forward Explicit Congestion Notification)
• Discard eligibility(DE)
• Priority level of the frame for traffic control
• Discarding frame to avoid the congestion or
  collapsing

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BECN
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FECN
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Four Cases of Congestion
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Extended Address Three Address Formats
FRAD
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ATM

• Asynchronous Transfer Mode
• ATM is the cell relay protocol designed by ATM
  forum and adopted by ITU-T
• ATM uses asynchronous TDM
• Cells are transmitted along virtual circuits
• Design Goals
• Large bandwidth and less susceptible to noise
  degradation
• Interface with existing systems without lowering
  their effectiveness
• Inexpensive implementation
• Support the existing telecommunications
  hierarchies
• Connection-oriented to ensure accurate and
  predictable delivery
• Many functions are hardware implementable

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Multiplexing using Cells

• The variety of packet sizes makes traffic
  unpredictable
• A cell network uses the cell as the basic unit of
  data exchange
• A cell is defined as a small, fixed sized block
  of information
• Cells are interleaved so that non suffers a long
  delay
• A cell network can handle real-time transmissions
• Network operation is more efficient and cheaper

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Synchronous vs. Asynchronous TDM
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ATM Architecture

• UNI user-to-network interface
• NNI network-to-network interface

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Virtual Connection

• Connection between two endpoints is accomplished
  through
• Transmission path (TP)
• Virtual path (VP)
• Virtual circuit (VC)
• A virtual connection is defined by a pair of
  numbers VPI and VCI

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VPI and VCI Hierarchical Switching
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Identifiers and Cells
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VP Switch and VPC Switch
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ATM Layers
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ATM Layer and Headers
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Application Adaptation Layer (AAL)

• Convert data from upper-layer into 48-byte data
  units for the ATM cells
• AAL1 constant bit rate (CBR) video and voice
• AAL2 variable bit rate (VBR) stream ?
  low-bit-rate traffic an short-frame
• traffic such as audio (ex
  mobile phone)
• AAL3/4 connection-oriented/connectionless data
• AAL5 SEAL (Simple and Efficient Adaptation
  Layer)
• No sequencing and error
  control mechanisms

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AAL1
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AAL2
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AAL3/4
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AAL5
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ATM LAN

• ATM is mainly a wide-area network (WAN ATM)
  however, the technology can be adapted to
  local-area networks (ATM LANs). The high data
  rate of the technology has attracted the
  attention of designers who are looking for
  greater and greater speeds in LANs.

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Pure and Legacy ATM LAN
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Mixed Architecture ATM LAN
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LAN Emulation (LANE)

• Connectionless versus connection-oriented
• Physical addresses versus virtual-circuit
  identifiers
• Multicasting and broadcasting delivery
• Interoperability
• Client/Server model in a LANE
• LANE Configuration Server (LECS), LANE Server
  (LES), LANE Client (LEC)
• Broadcast/Unknown Server (BUS)

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Mixed Architecture Using LANE