Draw and explain frame Format of frame relay and explain Address fields, how it provides Congestion Central and quality of service
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Frame Relay:

  • It is a high performance Wide Area Network protocol which operates at the physical and data link layers of the OSI model. Frame Relay was designed for Integrated Services Digital Network (ISDN) interfaces. Frame relay is based on the old X.25 packet-switching technology.
  • Frame Relay enables you to expand the WAN by adding less hardware to it. It is designed with the concept of a Commited Information Rate (CIR).
  • Frame relay is a form of packet-switching service designed for cost-efficient data transmission for intermittent traffic between local area networks (LANs) and between endpoints in wide area networks (WANs). Frame relay is used to connect LANs, WANs and private network environments with major backbones with leased T-1 lines. It requires a dedicated connection during the transmission period and is not ideal for voice or video, which require a steady flow of transmissions. A frame can incorporate packets from different protocols such as Ethernet and X.25
  • The end points are responsible for detecting and retransmitting dropped frames.

Frame Relay frames:

  • It is similar to the HDLC frame, but the control field is missing. The address field defines the DLCI as well as some bits used to control congestion and traffic.
  • Three primary components make up the frame relay's frame, the header and the address area, the user-data portion, and the frame check sequence.
  • The fields of the Frame relay frame are:

| Flags (8) | Address (16) | Information | FCS (16) | Flags (8) | | --- | --- | --- | --- | --- | * Flags: It indicates the beginning and end of the frame. The value of this field is always the same and is represented either as the hexadecimal number 7E or as the binary number 01111110.

The fields of the address field are:

  | DLCI (6) | C / R  (1) | EA (1) | DLCI(4) | FECN (1) | BECN  (1) | DE(1) | EA (1) | | --- | --- | --- | --- | --- | --- | --- | --- |

  • Address (DLCI) field: It is 6 bits from the first byte and 4 bits from the second byte of the address field make up the 10 bit DLCI.
  • Command/Response (C/R): The C/R bit is provided to allow the up per layers to identify a frame as either a command or response frame. It is not used by the Frame Relay protocol.
  • Extended Address (EA): This bit indicates whether the current byte is the final byte of the address. An EA of 0 means that another address is to follow. EA of 1 means that the current byte is the final one.
  • Forward explicit congestion notification (FECN): This bit can be set by any switch to indicate that traffic is congested in the direction that the frame is traveling. This bit informs the destination that congestion has occurred.
  • Backward explicit conges tion notification (BECN): This bit can be set by any switch to indicate that traffic is congested in the direction opposite to the one in which the frame is traveling. This bit informs the sender that congestion has occurred.
  • Discard eligibility (DE): The DE bit indicates the priority level of the frame. In emergency situations, switches may have to discard frames to relieve the bottlenecks. The network may discard a frame whose DE bit is set. This bit may be set either by the sender of the frames or by any switch in the network..
  • Extended Addresses: The Frame relay address may be extended from the original 2-byte address to a 3- or 4- byte address. The EA field defines the number of bytes. In the 3- and 4- byte address formats the bit before the last bit is set to 0.
  • FRAD: To handle frames arriving from other protocols Frame Relay uses a device called a Frame Relay assembler / disassembler (FRAD). FRAD can be implemented as a separate device or as a part of a switch
  • VOFR: Frame Relay networks offer an option called Voice over Frame Relay that sends voice through the network. Voice is digitized using PCM, compressed and sent as data frames over the network. The quality of voice is not as good as voice over a circuit-switched network.
  • LMI: Local Management Information (LMI) is a protocol added recently to the Frame Relay protocol to provide management functions. LMI can provide:
  1. A keep-alive mechanism to check if data are flowing.
  2. A multicast mechanism to allow a local end system to send data to more than one remote end system.
  3. A mechanism to allow an end system to check the status of a switch.

How it provides Congestion Central and quality of service:

  • Frame relay provides a minimal service, primarily a way to determine the start and end of each frame, and detection of transmission errors.
  • If a bad frame is received, the frame relay service simply discards it.
  • It is up to the user to discover that a frame is missing and take the necessary action to recover.
  • Unlike the X.25, frame relay does not provide acknowledgements or normal flow control. It does have a bit in the header.
  • However which one end of a connection can set to indicate to the other end that problems exist. The use of this bit is up to the users.
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