frame relay by erdem yilmaz. what is frame relay? high-performance wan protocol operates at the...
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FRAME RELAY
by
Erdem YILMAZ
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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
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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
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Frame Relay Devices
data terminal equipment (DTE) terminating equipment for a specific network typically are located on the premises of a
customerExamples: terminals, personal computers,
routers, and bridges
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Frame Relay Devices
data circuit-terminating equipment (DCE)carrier-owned internetworking devices to provide clocking and switching services in a
networkactually transmit data through the WAN
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Frame Relay Devices
Figure 1 Frame Relay Devices
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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
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Frame Relay Virtual Circuits
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
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Frame Relay Virtual Circuits
Permanent Virtual Circuits (PVCs)used for frequent and consistent data
transfers between DTE devices across the Frame Relay network
Data Transfer Idle
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Congestion Control Mechanism
Forward-explicit congestion notification (FECN)
Backward-explicit congestion notification (BECN)
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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
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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
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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
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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
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Frame Relay Network Implementation consists of a number of DTE devices
connected to remote ports on multiplexer equipment via traditional point-to-point services
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Frame Relay Network Implementation
Figure 2 A simple Frame Relay network connects various devices to different services over a WAN.
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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
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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
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Frame Relay Frames
Figure 3 Frame Relay Frame
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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)
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Frame Relay Frames
The address area (2 bytes)10 bits represents the actual circuit identifier6 bits of fields related to congestion
management
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Frame Relay Frame Formats
Standard Frame Relay Frame LMI Frame Format
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Standard Frame Relay Frame
FlagsDelimits the beginning and end of the frameThe value of this field is always the same (7E
or 01111110)
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Standard Frame Relay Frame
Address – contains the following informationDLCI: 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
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Standard Frame Relay Frame
AddressExtended 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
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Standard Frame Relay Frame
AddressCongestion Control: consists of the three bits;
FECN, BECN, and DE bits
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Standard Frame Relay Frame
Data – Contains encapsulated upper-layer dataserves to transport the higher-layer protocol
packet (PDU) through a Frame Relay network
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Standard Frame Relay Frame
Frame Check Sequence Ensures the integrity of transmitted data
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LMI Frame Format
Figure 4 Nine fields comprise the Frame Relay that conforms to the LMI format
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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
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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
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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
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Thanks For Listening
Erdem YILMAZ