CCNA Exam: Frame Relay Overview

In preparation of your CCNA exam, we want to make sure we cover the various concepts that we could see on your Cisco CCNA exam. So to assist you, below we will discuss one of the more difficult CCNA concepts; Frame Relay. As you progress through your CCNA exam studies, I am sure with repetition you will find this topic becomes easier. So even though it may be a difficult concept and confusing at first, keep at it as no one said getting your Cisco certification would be easy!

Frame Relay: WAN Protocol for Internetworking

Frame Relay is a WAN protocol for LAN internetworking which operates at the physical and data link layer to provide fast and efficient method of transmitting information from a user device to another across multiple switches and routers.

Frame Relay is based on packet-switched technologies similar to x.25, which enables end stations to dynamically share the network medium and the available bandwidth. It employs the following two packet techniques: a) Variable-length packets and b) Statistical multiplexing. It does not guarantee data integrity and discard packets when there is network congestion. In reality, it still delivers data with high reliability.

The Frame Relay frame is transmitted to its destination through virtual circuits, which are logical paths from an originating point in the network to a destination point. Virtual circuits provide bidirectional communication paths from one terminal device to another and are uniquely identified by a data-link connection identifier (DLCI). A number of virtual circuits can be multiplexed into a single physical circuit for transmission across the network. This capability often can reduce the equipment and network complexity required to connect multiple terminal devices. A virtual circuit can pass through any number of intermediate switches located within the Frame Relay packet switched network.

There are permanent virtual circuits (PVCs) or switched virtual circuits (SVCs). PVCs are set up administratively by the network manager for a dedicated point-to-point connection; SVCs are set up on a call-by-call basis using the same signaling for ISDN set up.

There are two flavors of Frame Relay, one is based on the Q.922 LAPF, which is the popular and standard deployment, and the other conforms to LMI specifications, which is less used.

Due to its bandwidth efficiency and high reliability, Frame Relay offers an attractive alternative to both dedicated lines and X.25 networks for the inter-connecting of LANs through switches and routers.

Protocol Structure – Frame Relay: WAN Protocol for Internetworking

The Frame Relay (LAPF Q.922 based) frame structure is as follows:

• Flags– Delimits 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.
• Address– Contains the following information:

• DLCI – Datalink Connection Identifier field represents the address of the frame and corresponds to a PVC.
• C/R– Designates whether the frame is a command or response.
• EA– Extended Address field signifies up to two additional bytes in the Frame Relay header, thus greatly expanding the number of possible addresses.
• FECN– Forward Explicit Congestion Notification (see ECN below).
• BECN– Backward Explicit Congestion Notification (see ECN below).
• DE- Discard Eligibility.
  o Data- Contains encapsulated upper-layer data. Each frame in this variablelength field includes a user data or payload field that will vary in length up to 16,000 octets. This field serves to transport the higher-layer protocol packet (PDU) through a Frame Relay network.
  o Frame Check Sequence- Ensures the integrity of transmitted data. This value is computed by the source device and verified by the receiver to ensure integrity of transmission.

Frame Relay frames that conform to the LMI specifications consist of the fields as follows:

• Flag– Delimits the beginning and end of the frame.
• LMI DLCI– Identifies the frame as an LMI frame instead of a basic Frame Relay frame. The LMI-specific DLCI value defined in the LMI consortium specification is DLCI = 1023.
• Unnumbered Information Indicator– Sets the poll/final bit to zero.
• Protocol Discriminator– Always contains a value indicating that the frame is an LMI frame.
• Call Reference– Always contains zeros. This field currently is not used for any purpose.
• Message Type – Labels the frame as one of the following message types:
• Status-inquiry message– Allows a user device to inquire about the status of the network.
• Status message– Responds to status-inquiry messages. Status messages include keepalives and PVC status messages.
• Information Elements– Contains a variable number of individual information elements (IEs). IEs consist of the following fields:
• IE Identifier– Uniquely identifies the IE.
• IE Length– Indicates the length of the IE.
• Data– Consists of 1 or more bytes containing encapsulated upper-layer data.
• Frame Check Sequence (FCS)– Ensures the integrity of transmitted data.