Frame Relay

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Frame Relay
Chapter 6
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Frame Relay

• Evolved from X.25
• Backbone network operates in physical and data
  link layers
• Advantages
• Less expensive than other tradition WANs (Figure
  1)
• Allows bursty data (Figure 2)
• Less overhead than X.25 due to improved
  transmission media (Figure 3)
• Supports speed from 1.544 Mbps to 44.376Mbps
• Allows frame size of 9000 bytes (cover all LAN
  frames)
• Congestion management

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Figure 1
Frame Relay versus Pure Mesh T-Line Network
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Figure 2
Fixed-Rate versus Bursty Data
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Figure 3
X.25 Traffic
Frame Relay Traffic
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Disadvantages

• Speed not higher than B-ISDN (600Mbps)
• Allows variable-length frames gt varying delays
  for different users
• Not suitable for sending delay sensitive data
  such as real-time voice or video

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Figure 4
Frame Relay Network
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Virtual Circuits in Frame Relay

• A Data Link Connection Identifier (DLCI) is used
  to identify a virtual circuit
• When a VC is established, a DTE is given a DLCI
  to send frames to the remote DTE.
• DLCI has local significance
• Provides two types of connection PVC and SVC

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Figure 5
DLCIs
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Permanent / Switched Virtual Circuit

• PVC
• PVC connection is established and stay permanent
  between two DTEs by the network provider.
• SVC
• A new VC connection should be established each
  time a DTE wants to make a connection with
  another DTE.

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Figure 6
PVC DLCIs
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Figure 7
SVC Setup and Release
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Figure 8
SVC DLCIs
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Figure 9
DLCIs Inside a Network
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Figure 10
Frame Relay Switch
Switch table matches an incoming port-DLCI with
an outgoing port-DLCI
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Frame Relay Layers

• Physical Layer
• No specific protocol is defined
• It supports any of the protocols recognized by
  ANSI
• Data Link Layer
• Uses a simplified version of HDLC (core LAPF)
• No control field, some bits are used to control
  congestion and traffic

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Figure 11
Frame Relay Layers
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Figure 12
Comparing Layers in Frame Relay and X.25
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Figure 13
Frame Relay Frame
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Congestion Control in Frame Relay

• Congestion may occur if users send data into
  network at a rate greater than that allowed by
  network resources
• It deceases throughput and increase delay
• Frame Relay does not use flow or error control
  (provided by upper layer protocols)

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Congestion Avoidance in Frame Relay(1)

• Backward Explicit Congestion Notification (BECN)
• Set the BECN bit to warn the Sender of congestion
  in the network
• The switch can use response frames from the
  receiver or else a defined connection (DLCI1023)
  to send special frames to the sender. The sender
  can respond to this warning by reducing the data
  rate.

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Congestion Avoidance in Frame Relay (2)

• Forward Explicit Congestion Notification (FECN)
• Set the FECN bit to warn the Receiver of
  congestion in the network
• In response, the receiver can delay the
  acknowledgement, thus forcing the sender to slow
  down.
• Discard Eligibility (DE)
• Set by the sender or any switch in the network.
• When DE1, the network can discard this frame if
  there is congestion.

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Figure 14
FECN
BECN
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Figure 16
Four Cases of Congestion
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Flow Control by Proportional Backdrop
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Quality of Service (QoS) in Frame Relay

• Attributes to control traffic
• Access Rate (bits per sec)
• Bandwidth of the channel connecting the user to
  the network
• Committed Burst Size (Bc)
• Max. no. of bits in a predefined period of time
  that the network is committed to transfer without
  discarding any frame or setting the DE bit.
• Committed Information Rate (CIR)
• Similar to Bc except that it defines an average
  rate in bits per sec.
• Excess Burst Size (Be)
• Max. no. of bits in excess of Bc that a user can
  send during a predefined period of time if there
  is no congestion.

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Figure 17
Relationship between Traffic Control Attributes
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Figure 18
User Rate in Relation to Bc and Bc Be
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Figure 19
Three Address Formats
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Figure 20
Frame Relay Assembler / Disassembler (FRAD)
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Local Management Information (LMI)

• A keepalive mechanism to check if data are
  flowing
• A multicast mechanism to allow a local DTE to
  send frames to more than one remote DTE
• A mechanism to allow a DTE to check the status of
  a DCE
• LMI DLCI (1023)

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Example Frame Relay Network
T1/E1
56/64Kbps
Trunks
56/64Kbps
56/64Kbps

• Physical topology
• Switches
• Access links
• Trunks
• CSU/DSU

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Summary
Definitions DLCI Data Link
Connection Identifier CIR Committed Information
Rate Bc Committed burst in bits Be Excess
Burst in bits FECN Forward Explicit Congestion
Notify BECN Backward Explicit Congestion
Notify DE Discard Eligible
Frame Relay Switch Maps DLCIs to form a
PVC Controls each PVCs CIR, Bc, Be Congestion
Notification FECN, BECN Provides Accounting and
Monitoring
FRAD
SDLC
DLCI 31
Router
DLCI 21
DLCI 32
DLCI 22
S 0
DLCI 23
DLCI 33
Router