frame relay
TRANSCRIPT
Frame relay
Frame relay
“Frame relay is a high performance WAN protocol that operates at physical and data link layer of the OSI reference model.”
X.25Prior to Frame Relay, some organizations were using a virtual-circuit switching network called X.25 that performed switching at the network layer.
Why it was replaced?
Extensive flow and error control X.25 has its own network layer Fixed-rate data all the time
Why Do we need Frame relay? More effcient network use. Faster data transfer.
Technology used✖ FR is an example of a packet-switched
technology.
✖ Packet-switched networks enable end stations to dynamically share the network medium and the available bandwidth
Frame relay cloud
frame relay dvices
Two categories: Data terminal equipment (DTE) Data communication equipment (DCE, also
called data circuit-terminating equipment)
frame relay dvices
Data terminal equipment (DTE)
o terminating equipment for a specific network
o typically are located on the premises of a customer
o Examples: terminals, personal computers, routers, and bridges
frame relay dvices
Data circuit-terminating equipment (DCE)o Carrier-owned internetworking
deviceso To provide clocking and switching
services in a networko Actually transmit data through the
WAN
Frame relay devices
Frame Relay Virtual Circuits
Virtual Circuits (VCs)The logical connection through the frame relay network between two DTEs is called a virtual circuit(VC).The term “virtual” here means that the two DTEs are not connected directly but through a network.
Virtual Circuit provides bidirectional communication from DTE device to another and are uniquely idendified by a
Data Link Connection Identifier (DLCI). A number of VCs can be multiplexed
into a single Physical circuit for transmission across the network.
Types of VCsThere are two types of VCs.
I. Switched Virtual Circuits (SVCs)II. Parmanent Virtual Circuits (VCs)
Switched Virtual Circuits (SVCs)
These are temporary , short connections used in situations where sporadic data transfer between DTE devices across the Frame Relay Network .
An SV consists of following Four operational States.
• Call Setup - The virtual circuit between two Frame Relay DTE devices is established.
• Data Transfer - Data is Transmitted between two DTE s over the virtual circuit.
• Idle – The connection between DTE devices is still active , but not data is transferred. If an SVC remains in an idle state for a defined period of time , the call can terminated.
• Call Termination – The virtual circuit between DTE devices is terminated.
Parmenent Virtual Circuit (PVC)These are permanently established connections that are use for frequent and consistent data transfers between DTE devices across the Frame Relay Network.
It 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 circuits.
• 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.
Have two drawbacks• First they are Costly because two
parties pay for the connection.• second, a connection is created from
one source to one sinsle destination. But if a source needs connections with several destinations its needs a PVC for each connection.
PVCs Drawbacks
Frame Relay SwitchingThe switches in the Frame Relay are supposed to route packets from sender to receiver. For this purpose each switch has table. The table matches an incoming port- DLCI combination with an outgoing port-DLCI combination. The packet forwarding is illustrated with the help of a diagram
FRAME RELAY
LAYERS
Frame relay is a layer-2 protocol. It operates only at PHYSICAL LAYER
and DATA-LINK LAYER i.e it eliminates all layer-3 processing.
Physical LayerNo specific protocol is defined for physical layer in FR. Instead it is left to the implementer . FR supports any of the protocols recognized by ANSI ( ANSI T1.618, ANSI T1.617, ANSI LMI,FR NNI PVC).
FRAME RELAY LAYERSData-Link LayerAt data-link layer, FR uses a simple protocol that does not support flow or error control.It has only an error detection mechanism.
FR FRAME FORMAT
Frame header is altered slightly to contain the DATA LINK CONNECTION
IDENTIFIER(DLCI) and congestion bits.
FRAME RELAY FRAME FORMAT
FRAME RELAY FRAME FIELDS
✖Flags✖Frame Relay Header✖Information/ Data✖Frame Checking Sequence(FCS)
FLAGS
Indicates the beginning and ending of frame..
FR HEADER
It further consist of six fields.Contains the destination address as DLCI and congestion control mechanism.
DATA
Contains encapsulated data for upper layers.Variable length of frame can be upto 16000 octets.FCS
Frame Check Sequence ensures the intregrity of transmitted data.computed by sending device and verifid by receiving one.
FRAME RELAY HEADER
Frame relay header is altered slightly to contain the DATA LINK CONNECTION IDENTIFIER(DLCI) and congestion bits.It is further consist of six fields that are important to discuss.
i. DLCIii. C/Riii. EAiv. FECNv. BECNvi. DE
DLCIDLCI—The 10-bit DLCI is the essence of the Frame Relay header. This value represents the virtual connection between the DTE device and the switch. Each virtual connection that is multiplexed onto the physical channel will be represented by a unique DLCI. The DLCI values have local significance only, which means that they are unique only to the physical channel on which they reside. Therefore, devices at opposite ends of a connection can use different DLCI values to refer to the same virtual connection.
Headquarter can communicate to both
branches now.There is one DLCI of 23 representing a connection from
HQ to branch 1. There is one DLCI of 51 from HQ to branch 2.
Hello!HQ IP address (9.9.9.9)
Branch 1 IP address(1.1.1.1)Branch 2 IP address(2.2.2.2)
HQ will need to map branch 1 IP address to DLCI 23 and map branch 2 IP address to DLCI
51.After that it can encapsulate a FR frame with an appropriate DLCI number and send it to the
destination.
Command/response
The command/response bit in the header is provided to allow the upper layers to identify a frame
as either a command or rsponse. DL-layer starts communication by
initiating a command to upper layers which is followed by
response from upper layers.
EXTENDED ADDRESS(EA)
EA is the extended DLCI address (23 bits address). 10 bits DLCI field only permits DLCI values
between 16 and 1007. But with EA, DLCI suuports values
between 16 and 8388607. Basically it was the solution of
range of addresses.
The points will be explained later
BECNFECN DE
Thanks!Any questions?
DO NOT CONFUSE YOURSELF!!!!!!
Congestion-control
mechanisms
FRAME RELAY EMPLOYS CONGESTION NOTIFICATION
SYSTEM
Frame relay uses:
Forward-Explicit Congestion Notification (FECN) Backward-Explicit Congestion Notification (BECN)
These Bits Are Used TO Inform
The Terminal Equipments About The
Congestion In The Network.Based On This Information ,
higher osi layers decide
the flow control and take actions
depending upon the warning.
DISCARD ELIGIBILITY (DE)
BIT
Discard Eligibility is nothing but a way to recognize the lesser important traffic over the Network.
BIGGER QUESTIONHow would the network know
that there is a congestion before dropping the lesser
important traffic off the network???
SOLUTION: FORWARD-EXPLICIT
CONGESTION NOTIFICATION (FECN) bit
BACKWARD-EXPLICIT CONGESTION NOTIFICATION (BECN) bit
FECN-Mechanism• A part of address field in the Frame Relay
frame header.• It works when a terminal euipment sends
Frame Relay frames into a network.• In case of Congested Network , DCE(switches)
set the values of frames’FECN bit to 1.• When it reaches to its destination, DTE device
knows about the congestion that took place.• If the condition is severe , a flow control is
initiated by the higher layers.
BECN-MECHANISM
• A part of address field in the Frame Relay frame header.
• It works when DCE devices set the value of the BECN bit to 1 in the frames travelling in the opposite direction of the frames with their FECN bit set.
• This informs the DTE devices that opposite direction neywork is congested.
