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:
1byte 2 bytes Variable 2 bytes 1 byte
Flags Address Data FCS Flags
" 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:
6 7 8 12 13 14 15 16 bit
DLCI C/R E DLCI FECN BECN DE EA
" 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 variable-
length 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:
1byte 2 bytes 1 byte 1 byte 1 byte 1 byte
Flags LMI DLCI I-Indicator Protocol Dis Call Ref M-Type
Information Elements (Variable) FCS Flags
" 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.