Explain frame format of Frame relay
1 Answer


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:

  • The Frame relay Frame formats are: Standard Frame Relay Frame and LMI frame format.

| Flags (8) | Address (16) | Data (Variable) | FCS (16) | Flags (8) | | --- | --- | --- | --- | --- | Flags: It indicates the beginning and end of the frame. It is made of the header and address area, user portion and Frame Check Sequence(FCS).

  • Address: The address are is 2 bytes of which 10 bits represent the actual circuit id and 6 bits relate to congestion management.
  • Flags: It delmits the beginning and end of the frame and the value is always same.
  • Standard Frame Relay Frame:
  • Address: It contains of DLCI, extended address and congestion control.
  • Data: It contains encapsulated upper-layer data.

Frame Check Sequence (FCS): It ensures the integrity of transmitted data.

LMI Frame format:    


| Flag  (1) | LMI DLCI  (2) | Unnumbered Information Indicator (1) | Protocol discriminator  (1) | Call reference (1) | Message type  (1) | Information elements (Variable) | FCS  (2) | Flag (1) | | --- | --- | --- | --- | --- | --- | --- | --- | --- | * Flag: It delimits the beginning and end of the frame. * LMI DLCI: It identifies the frame as an LMI frame instead of a basic frame relay frame. * Unnumbered information indicator: Sets the poll or final bit to 0. * Protocol discriminator: It contains a value which indicates that the frame is a LMI frame. * Call reference: It always contains zeroes. * Message type: The status inquiry message allows a user device to inquire about the status of the network. The status message include keep alives and PVC status messages. * Information elements: It contains a variable number of individual information elements. * Frame Check Sequence: It ensures the integrity of transmitted data.

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