Frame Relay Protocols Overview
Before going ahead with Frame Relay protocol, and
its operation, we discuss virtual circuits. Remember that a circuit provides
connection between end nodes by means of an electrical connection. In data
circuits, the term virtual circuit is also used in similar sense. A virtual
circuit provides a logical connection between end nodes for the flow of
information. There are two types of virtual circuits:
Permanent Virtual Circuit (PVC): PVC is a permanent connection between
the end nodes (DTEs) within a Frame Relay network. The virtual circuit is always
available irrespective of whether any data is being transmitted or not. This
type of connection (PVC) is used when it is required to consistently transfer
data between the end nodes. A PVC can have two operational states as given
Data transfer state: Data is transmitted between the end nodes over the
Idle state: No data is transferred between the end nodes. Note that PVC
does not terminate the virtual circuit even when there is no data being
transferred between the end nodes.
Switched Virtual Circuit(SVC): A switched virtual circuits (SVC)
provide temporary connection between end nodes (DTEs) across a Frame Relay
network. An SVC communication session has four states:
Call setup: The virtual circuit between two Frame Relay end nodes is
Data transfer: Data is transmitted between the end nodes over the virtual
Idle: The connection between end nodes is still active, but no data is
transferred. An SVC call is terminated after a certain period of idle time
Call termination: The virtual circuit between end nodes is terminated.
If there is some more data to be transmitted at a later time, an SVC is
negotiated again. SVCs are advantageous when you have burst traffic, and you
don't want to block the network bandwidth for a given virtual circuit 24hours a
Unlike SVC, there is no call setup, and call termination procedures in PVC.
This results in simple link management procedures, and more efficient data
Frame Relay Protocol: FR is an HDLC protocol based network. We have discussed
HDLC in earlier sections, and the HDLC frame is given below. Other protocols
that use HDLC frames include SDLC, Frame Relay, and X.25. They primarily differ
in how the address and control bits in HDLC frame are used.
The different fields are explained below with respect to Frame Relay:
Flag (both opening and closing flags): 8 bits (01111110
or 7E hex)
Address (Also known as Frame Relay Header): It is a 16-bit field as given
Data Link Connection Identifier (DLCI): The DLCI is 10-bit wide. DLCI
identifies the virtual connection between the end node (a DTE device) and the
switch (a DCE device).
C/R: The C/R bit says whether the frame is a command or response.
Forward Explicit Congestion Notification (FECN): This is a single-bit
field that can be set to either 0 or 1 by a switch. Normally, FECN is zero. A
value of 1 indicates network congestion in the direction of source to
destination, known as Forward Explicit Congestion Notification.
Backward Explicit Congestion Notification (BECN): This is a single-bit
field that can be set to either 0 or 1 by a switch in the FR network. Normally,
BECN is zero. A value of 1 indicates that the FR network has experienced
congestion in the direction of destination to source.
By using FECN and
BECN, upper layer protocols can control the communication
for efficient utilization of FR network.
Discard Eligibility (DE): This is set by the DTE device to indicate
that the marked frame may be discarded in the event of network congestion.
Discard Eligible frames are discarded first before removing frames that do not
have DE bit set, in the event of network congestion.
Note that all
FECN, BECN, and DE enable FR network congestion control by
regulating the communication, and prioritizing traffic.
Extended Address (EA): The eighth bit of each byte of the Address
field (header) is used to indicate the EA. If the EA value is 1, then the
current byte is determined to be the last octet of the DLCI.
Data: This field contains encapsulated upper-layer protocol data. It
has variable length up to 16,000 octets.
FCS (Frame Check Sequence) or CRC (Cyclic Redundancy Code): It is either
16 bits, or 32 bits wide. Frame Check Sequence is used to verify the data
integrity. If the FCS fails, the frame is discarded.