Frame Relay

Frame Relay
 Frame Relay is a high-performance
WAN protocol that operates at the
physical and data link layers of the OSI
reference model.
 Frame Relay originally was designed for
use across Integrated Services Digital
Network (ISDN) interfaces
 Uses packet switching technique.
Frame Relay 1

Packet Switching
 Frame Relay is based on packet-switched
technology.
 Packet switching enables the end stations
to dynamically share network media and
bandwidth. So that it provides two type of
bandwidth CIR (Committed interface Rate),
EIR(Extended interface Rate).
 The following two techniques are used in
packet-switching technology:
Variable-length packets
Statistical multiplexing
Frame Relay 2

Frame Relay Devices
 Devices attached to a Frame Relay WAN fall into
the following two general categories:
 Data terminal equipment (DTE)
 Data circuit-terminating equipment (DCE)
 Examples of DTE devices are terminals, personal
computers, routers, and bridges.
 DCEs are carrier-owned internetworking devices.
The purpose of DCE equipment is to provide
clocking and switching services in a network,
which are the devices that actually transmit data
through the WAN. In most cases, these are packet
switches
Frame Relay 3

Frame Relay Virtual Circuits
 Frame Relay provides connection-oriented data link layer
communication.
 This service is implemented by using a Frame Relay virtual
circuit, which is a logical connection created between two data
terminal equipment (DTE) devices across a Frame Relay
packet-switched network (PSN).
 Virtual circuits provide a bidirectional communication path from
one DTE 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.
 A virtual circuit can pass through any number of intermediate
DCE devices (switches) located within the Frame Relay PSN.
 Frame Relay virtual circuits fall into two categories:
 switched virtual circuits (SVCs)
 permanent virtual circuits (PVCs).
Frame Relay 4

Switched Virtual Circuits
 Switched virtual circuits (SVCs) are
temporary connections used in situations
requiring only sporadic data transfer
between DTE devices across the Frame
Relay network.
○ Call setup—The virtual circuit between two Frame
Relay DTE devices is established.
○ Data transfer—Data is transmitted between the DTE
devices over the virtual circuit.
○ Idle—The connection between DTE devices is still
active, but no data is transferred. If an SVC remains in
an idle state for a defined period of time, the call can be
terminated.
○ Call termination—The virtual circuit between DTE
devices is terminated.
Frame Relay 5

Permanent Virtual Circuits
 Permanent virtual circuits (PVCs) are permanently
established connections that are used for frequent and
consistent data transfers between DTE devices across
the Frame Relay network.
 Communication across a PVC does not require the call
setup and termination states that are used with SVCs.
○ Data transfer—Data is transmitted between the DTE devices
over the virtual circuit.
○ Idle—The connection between DTE devices is active, but no
data is transferred. Unlike SVCs, PVCs will not be terminated
under any circumstances when in an idle state.
 DTE devices can begin transferring data whenever they
are ready because the circuit is permanently established.
Frame Relay 6

Data-Link Connection
Identifier
 Frame Relay virtual circuits are identified
by data-link connection identifiers (DLCIs).
DLCI values typically are assigned by the
Frame Relay service provider (for example,
the telephone company).
 Frame Relay DLCIs have local
significance, which means that their values
are unique in the LAN, but not necessarily
in the Frame Relay WAN
Frame Relay 7

Special DLCI numbers
 DLCI 0 (zero) and 1023 are reserved for
management
 DLCI 1 to 15 and 1008 to 1022 have
been reserved for future use.
 DLCI 992 to 1007 are reserved for layer
2 management of frame relay bearer
service.
 DLCI numbers 16 to 991 are available
for subscribers for each user frame relay
network
Frame Relay 8

Data-Link Connection Identifier
A Single Frame Relay Virtual Circuit Can Be Assigned Different DLCIs on Each End of a VC
Frame Relay 9

Congestion-Control Mechanisms
 Frame Relay reduces network overhead by implementing
simple congestion-notification mechanisms rather than
explicit, per-virtual-circuit flow control.
 Frame Relay implements two congestion-notification
mechanisms:
 Forward-explicit congestion notification (FECN)
 Backward-explicit congestion notification (BECN)
 FECN and BECN each is controlled by a single bit
contained in the Frame Relay frame header.
 The Frame Relay frame header also contains a Discard
Eligibility (DE) bit, which is used to identify less important
traffic that can be dropped during periods of congestion.
Frame Relay 10

Congestion-Control
Mechanisms
 The FECN bit is part of the Address field in the
Frame Relay frame header.
 The FECN mechanism is initiated when a DTE
device sends Frame Relay frames into the network.
Frame Relay 11
If the network is congested, DCE devices (switches) set the
value of the frames' FECN bit to 1. When the frames reach the
destination DTE device, the Address field (with the FECN bit
set) indicates that the frame experienced congestion in the path
from source to destination. The DTE device can relay this
information to a higher-layer protocol for processing.
Depending on the implementation, flow control may be
initiated, or the indication may be ignored.
If the network is congested, DCE devices (switches) set the
value of the frames' FECN bit to 1. When the frames reach the
destination DTE device, the Address field (with the FECN bit
set) indicates that the frame experienced congestion in the path
from source to destination. The DTE device can relay this
information to a higher-layer protocol for processing.
Depending on the implementation, flow control may be
initiated, or the indication may be ignored.

Congestion-Control
Mechanisms
 The BECN bit is part of the Address field in the
Frame Relay frame header.
 DCE devices set the value of the BECN bit to 1 in
frames traveling in the opposite direction of frames
with their FECN bit set.
 This informs the receiving DTE device that a
particular path through the network is congested.
 The DTE device then can relay this information to
a higher-layer protocol for processing. Depending
on the implementation, flow-control may be
initiated, or the indication may be ignored
Frame Relay 12

Frame Relay Discard
Eligibility
 The DE bit is part of the Address field in the Frame
Relay frame header.
 The Discard Eligibility (DE) bit is used to indicate
that a frame has lower importance than other
frames.
 DTE devices can set the value of the DE bit of a
frame to 1 to indicate that the frame has lower
importance than other frames.
 When the network becomes congested, DCE
devices will discard frames with the DE bit set.
Frame Relay 13

Frame Relay Error
Checking
 Frame Relay uses a common error-
checking mechanism known as the cyclic
redundancy check (CRC).
 The CRC compares two calculated values
to determine whether errors occurred
during the transmission from source to
destination.
 Frame Relay reduces network overhead by
implementing error checking rather than
error correction.
Frame Relay 14

Frame Relay
Local Management
Interface
 In 1990 Cisco, Digital Equipment Corporation (DEC), Northern
Telecom, and StrataCom formed a consortium to focus on its
development. They produced a protocol that provided additional
capabilities for complex inter-networking environments. These
Frame Relay extensions are referred to as the Local Management
Interface (LMI).
 The LMI global addressing extension gives Frame Relay (DLCI)
values global rather than local significance. DLCI values become
DTE addresses that are unique in the Frame Relay WAN.
 LMI virtual circuit status messages provide communication and
synchronization between Frame Relay DTE and DCE devices.
These messages are used to periodically report on the status of
PVCs, which prevents data from being sent into black holes (that is,
over PVCs that no longer exist).
Frame Relay 15

Frame Relay Frame Formats
 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, and the frame check sequence (FCS).
 The address area, which is 2 bytes in length, is comprised of 10 bits
representing the actual circuit identifier and 6 bits of fields related to
congestion management. This identifier commonly is referred to as the
data-link connection identifier (DLCI).
Frame Relay 16

Frame Relay

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