Frame Relay

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Frame Relay ATM

• CS4323

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Virtual Circuit Background

• In the beginning there was X.25
• Then came Frame Relay and ATM

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Virtual Circuit Background X.25

• X.25
• 1970's technology
• Implemented at the network layer
• 64 kbps
• Implements extensive error correction and flow
  control.

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Virtual Circuit Background Frame Relay

• Frame Relay
• Late 1980's 1990's technology
• Operates at rates ranging from 1.544 Mbps (DS-1
  T1) to 44.376 Mbps (DS-3 T3).
• Operates at the physical and network link layer
  so it can interface with existing Network layer
  protocols.
• Error detection at the data link layer. No flow
  control or error correction control (less
  overhead)
• Allows for digital voice communication (VOFR)

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Frame Relay

• ATT is the world's largest provider of Frame
  Relay service.

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Frame Relay Virtual Circuits

• Each switch in a frame relay network has a
  forwarding table.
• Virtual circuit identifiers in frame relay are
  dubbed as data link connection identifiers
  (DLCI).

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Figure 8.13 Source-to-destination data transfer
in a virtual-circuit network
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Figure 18.1 Frame Relay network
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Figure 18.2 Frame Relay layers
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Figure 18.3 Frame Relay frame
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Frame Relay Frame

• The flag at the start of the frame has bits
• 01111110
• 16 bit Address information field
• Data field (up to 9000 bytes)
• FCS error detection using CRC.

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Address Field

• The address field contains sub-fields that allow
  for congestion control (quality of service)
• QoS allows a frame relay switch to drop some
  packets if there is data congestion.
• DE is the discard eligibility field. If this bit
  is set, the switch can discard the frame during
  congestion.

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ATM Asynchronous Transfer Mode

• The combination of ATM and SONET allows
  high-speed interconnection of all the world's
  networks.
• ATM is a virtual switching technology
• SONET is a transmission technology

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ATM

• Designed in the 1980's
• Designed to take advantage of the bandwidth of
  optical fiber transmission media.
• Many of the protocol functions are implemented in
  hardware (not software) to insure the best
  possible performance.
• 1.54Mbps to 155 Mbps user copper, or 622 Mbps
  using optical fiber.

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Background Frame Size vs Performance

• Each layer of a protocol adds header information.
• The ratio of data to packet size diminishes as
  more protocol layers are added.
• Increasing the data payload leads to
  inefficiencies in data transmission
• Small packets are padded
• Large packets may need retransmission wasting
  bandwidth
• Large packets delay other packets

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Frame Size vs Multiplexing

• A low priority large frame can delay higher
  priority small frames if the large frame arrives
  before the small frames.

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Figure 18.6 Multiplexing using different frame
sizes
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Solution Cell Networking

• The single uniform size cell makes transmission
  of data more predictable.
• No frame size issues exist when multiplexing same
  size cells.
• Small cells arriving at rapid speed can appear to
  be nearly continuous.
• Voice video

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Figure 18.7 Multiplexing using cells
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ATM Multiplexing

• ATM uses asynchronous time division multiplexing.

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Figure 18.8 ATM multiplexing
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ATM Multiplexing

• A time slot in an asynchronous ATM multiplexed
  link will be empty only if there is no data from
  any of the input links.

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ATM Architecture

• Cell switched architecture.
• UNI user to network interfaces
• NNI network to network interfaces

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Figure 18.9 Architecture of an ATM network
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Figure 8.13 Source-to-destination data transfer
in a virtual-circuit network
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ATM Cell Format

• All cells are 53 bytes
• 5 byte header
• 48 byte data payload

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Figure 18.14 An ATM cell
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Figure 18.11 Example of VPs and VCs
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VCI, VPI

• The switches connected to user interfaces and
  network interfaces rely on both the virtual
  circuit identifier and virtual path identifier
• The NNI relies on the VPI for forwarding.
• The UNI relies on both the VCI VPI

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ATM Cell Header Format

• The 5 byte header contains VPI and VCI
  information.
• Both the VPI and VCI are used to forward
  information to its destination.

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Figure 18.13 Virtual connection identifiers in
UNIs and NNIs
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Figure 18.12 Connection identifiers
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ATM Virtual Switching

• The routing tables have six fields
• Input
• Interface number
• VPI
• VCI
• Output
• Interface number
• VPI
• VCI

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Figure 18.15 Routing with a switch
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ATM Layers

• ATM is implemented in the physical and data link
  layers.
• Any physical layer carrier can carry ATM cells
  (wired, optical, wireless).

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Figure 18.17 ATM layers in endpoint devices and
switches
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The ATM Layer

• The ATM layer provides
• Routing
• Switching
• Multiplexing
• Flow control (quality of service)
• Error detection and correction

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ATM's AAL Application Adaptation Layer

• There are four variations of AAL to deal with
  different data types.
• AAL-n is designed to accept
• AAL1 handles data streams of audio (voice) and
  video by allocating the streams into cells.
• AAL-5 handles other data communications like
  Internet traffic.

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Commercial ATM

• ATM is used in DSL and B-ISDN services (Digital
  Subscriber Lines and Broadband Integrated
  Services Digital Network)
• ATM works with any transport medium and ATM
  streams are multiplexed into SONET streams.