Frame Relay

1
Frame Relay

• X.25
• Provides extensive error checking and flow
  control
• station-to-station checking at the data link
  layer
• Error checking from source to receiver at network
  layer
• It consumes network bandwidth and requires
  storage at each station

2
Frame Relay

• No error checking and acknowledgment at the data
  link layer
• All error checking is left to the protocols at
  the network and transport layers
• operates at only the physical and data link layers

3
Frame Relay Layers

• Eliminates all of the network layer functions and
  a portion of conventional data link layer
  functions.
• Physical Layers no specific protocol is defined
  for the physical layer. It is left to the
  implementers.

4
Frame Relay Layers

• Data Link Layer
• employ a simplified version of HDLC - HDLC fields
  for extensive error checking and flow control
  not needed.
• Addressing and control fields combined into one
  field.

5
Frame Relay Layers

• Frame fields
• Addressing(DLCI) fields two parts (6bits,
  4bits). A part of the 16-bit data link connection
  identifier.
• Command/Response(C/R) allows upper layers to
  identify a frame as either a command or a
  response.
• Extended address (EA)EA0 another address byte
  follows, EA1 the current byte is the final one.
• Forward explicit congestion notification(FECN)
  indicate that traffic is congested in the
  direction in which the frame is traveling.
• Backward explicit congestion notification (BECN)
• Discard eligibility (DE) priority level of the
  frame

6
Frame Relay Operation

• Transmission is based on permanent virtual
  circuit
• DLCI identifies a permanent virtual circuit that
  is set up when the system is put in place
• Replay
• Routing information is included in the
  destination information.
• The path form point A to point D always passes
  through the same node.
• The functions of routing and switching can be
  handled by the data link layer.
• Frame relay (frame switching) occurs at the date
  link layer where the transmission unit is the
  frame
• Packet switching occurs at the network layer
  where the transmission unit is the packet

7
Frame Relay Addresses
8
Frame Relay Operation

• Switching
• Two operations of a switch
• checks a frame for errors using the FCS field if
  an error, discard it
• compares the DLCI to an entry in a switch table
  and find an outgoing port for the PVC identified
  by the DLCI

9
Frame Relay Operation

• Congestion Control
• does not solve the problem, but does provide ways
  to lessen the probability of its occurrence
• A switch in a PVC warns its downstream switches
  and destination by turning on the FECN bit
• The receiver, in turn, set BECN to warn upstream
  switches and the sender that the link is
  congested and to send frames more slowly.
• This option can not be used unless the channel is
  either full- or half-duplex and the receiver is
  sending its own data or acknowledgments to the
  sender

10
Frame Relay Implementation

• The most likely implementation
• Used as a WAN backbone to connect a number of
  LANs using T-1 links
• Frame relay assembler/disassembler(FRADs)
  assembles and disassembles packets coming from
  other protocols to allow them to be carried by
  frame relay frames