frame relay

FRAME RELAY
by
V.Jothi lakshmi

What is Frame Relay?
 high-performance WAN protocol
 operates at the physical and data link
layers
 Originally designed for use across ISDN
interfaces
 An example of packet-switched
technology
 described as a streamlined version of
X.25

Frame Relay vs. X.25
 Frame Relay is a Layer 2 protocol suite,
X.25 provides services at Layer 3
 Frame Relay offers higher performance
and greater transmission efficiency than
X.25

Frame Relay Devices
 data terminal equipment (DTE)
 data circuit-terminating equipment (DCE)
Examples: terminals, personal computers,
routers, and bridges

Frame Relay Devices
Figure 1 Frame Relay Devices

Frame Relay Virtual Circuits
 provides connection-oriented data link
layer communication
 a logical connection between two data
terminal equipment across a Frame Relay
packet-switched network
 provide a bi-directional communications
path from one DTE device to another

 Switched virtual circuits (SVCs)
temporary connections requires sporadic data
transfer between DTE devices across the
Frame Relay network
 Call Setup
 Data Transfer
 Idle
 Call Termination

 Permanent Virtual Circuits (PVCs)
used for frequent and consistent data
transfers between DTE devices across the
Frame Relay network
 Data Transfer
 Idle

Congestion Control Mechanism
 Forward-explicit congestion notification
(FECN)
 Backward-explicit congestion notification
(BECN)

Forward-explicit congestion
notification (FECN)
 initiated when a DTE device sends Frame
Relay frames into the network
 When the frames reach the destination
DTE device, the frame experienced
congestion in the path from source to
destination
 flow-control may be initiated, or the
indication may be ignored

Backward-explicit congestion
notification (BECN)
 DCE devices set the value of the BECN bit
to 1 in frames traveling in the opposite
direction, informs the receiving DTE
device that a particular path through the
network is congested
 flow-control may be initiated, or the
indication may be ignored

Frame Relay Discard Eligibility
(DE)
 (DE) bit is used to indicate that a frame
has lower importance than other frames
 When the network becomes congested,
DCE devices will discard frames with the
DE bit

Frame Relay Error Checking
 common error-checking mechanism
known as the cyclic redundancy check
(CRC)
 CRC compares two calculated values to
determine whether errors occurred during
the transmission

Frame Relay Network
Implementation
 consists of a number of DTE devices
connected to remote ports on multiplexer
equipment via traditional point-to-point
services

Frame Relay Network
Implementation
Figure 2 A simple Frame Relay network connects various
devices to different services over a WAN.

Public Carrier-Provided Networks
 Frame Relay switching equipment is
located in the central offices of a
telecommunications carrier
 The DCE equipment also is owned by the
telecommunications provider
 The majority of today’s Frame Relay
networks are public carrier-provided
networks

Private Enterprise Networks
 the administration and maintenance of the
network are the responsibilities of the
enterprise
 All the equipment, including the switching
equipment, is owned by the customer

Frame Relay Frames
Figure 3 Frame Relay Frame

Frame Relay Frames
 Flags indicate the beginning and end of
the frame
 Three primary components make up the
Frame Relay frame
the header and address area
the user-data portion
the frame-check sequence (FCS)

Frame Relay Frames
 The address area (2 bytes)
10 bits represents the actual circuit identifier
6 bits of fields related to congestion
management

Frame Relay Frame Formats
 Standard Frame Relay Frame
 LMI Frame Format

Standard Frame Relay Frame
 Flags
Delimits the beginning and end of the frame
The value of this field is always the same (7E
or 01111110)

 Address – contains the following
information
DLCI: The 10-bit DLCI is the essence of the
Frame Relay header, values have local
significance only, devices at opposite ends
can use different DLCI values for the same
virtual connection

 Address
Extended Address (EA): used to indicate
whether the byte in which the EA value is 1 is
the last addressing field, the eighth bit of each
byte of the Address field is used to indicate
the EA

 Address
Congestion Control: consists of the three bits;
FECN, BECN, and DE bits

 Data – Contains encapsulated upper-layer
data
serves to transport the higher-layer protocol
packet (PDU) through a Frame Relay network

 Frame Check Sequence
Ensures the integrity of transmitted data

LMI Frame Format
Figure 4 Nine fields comprise the Frame Relay that
conforms to the LMI format

LMI Frame Format
 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
 Unnumbered Information Indicator - Sets
the poll/final bit to zero

LMI Frame Format
 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
 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 keep-alives and
PVC status messages

LMI Frame Format
 Information Elements—Contains a variable
number of individual information elements (IEs)
 IE Identifier: Uniquely identifies the IE
 IE Length: Indicates the length of the IE
 Data: Consists of one or more bytes containing
encapsulated upper-layer data
 Frame Check Sequence (FCS) - Ensures the
integrity of transmitted data

frame relay

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