Virtual Circuit in Frame Relay

Virtual Circuit in Frame Relay

The operation of Virtual Circuit in Frame Relay and the bandwidth control mechanisms are explained in this article.

What is Virtual Circuit?

The connection through a Frame Relay network between two DTEs is a VC. The circuits are virtual because there is no direct electrical connection from end to end.
The connection is logical, and the data is transferred from end to end without a direct electrical circuit. With VCs, Frame Relay shares bandwidth among multiple users, and any individual site can communicate with any other individual site without using several dedicated physical lines.
There are two ways to establish VC:

• Switched virtual circuits (SVC) : they are established dynamically by sending signaling messages to the network (CALL SETUP, DATA TRANSFER, IDLE, CALL TERMINATION).
• Permanent virtual circuits (PVC) : the service provider preconfigures them and, once established, they only work in IDLE and DATA TRANSFER modes. Note that, in some publications, PVCs are called “private VCs”.

Note : PVCs are implemented more frequently than SVCs.

VCs provide a two-way communication path from one device to another. VCs are identified by DLCIs, as shown in Image 1. Typically, the Frame Relay service provider assigns DLCI values .

Multiple Virtual Circuits Frame Relay

SEVERAL VC

Frame Relay is multiplexed statistically, which means that it only transmits one frame at a time, but many logical connections can coexist on a single physical line.
The Frame Relay access device (FRAD) or the router connected to the Frame Relay network can have several VCs that connect it to the various terminals. Several VCs on a single physical line are distinguished because each VC has its own DLCI. Remember that the DLCI has only local importance and may be different at each end of a VC.
Below is an example of two VCs on a single access line, each with its own DLCI, that connect to a router (R1).

This capability usually reduces the complexity of the network and the equipment required to connect several devices, which makes it a very cost-effective replacement for a mesh of access lines. With this configuration, each terminal needs only a single line and access interface.

COST BENEFITS OF HAVING SEVERAL VCS

With Frame Relay, customers pay for the bandwidth they use. In effect, they pay for a Frame Relay port. When the client increases the number of ports, it pays for more bandwidth, but does not pay for more equipment , because the ports are virtual. There are no changes in the physical infrastructure. Compare this to the acquisition of more bandwidth through dedicated lines.

FRAME RELAY ENCAPSULATION

Frame Relay takes data packets from a network layer protocol, such as IPv4 or IPv6, encapsulates them as the data portion of a Frame Relay frame and then passes the frame to the physical layer for cable delivery.
To understand how this works, it is convenient to understand how it relates to the lower levels of the OSI model. Frame Relay encapsulates data for transport and drops it to the physical layer for delivery:

• First, Frame Relay accepts a packet from a network layer protocol, such as IPv4.
• It then wraps it with an address field that contains the DLCI and a checksum value.
• Indicator fields are added to indicate the beginning and end of the frame.
• The indicator fields mark the beginning and end of the plot, and are always the same.
• The indicators are represented as the hexadecimal number 7E or as the binary number 01111110.
• Once the package is encapsulated, Frame Relay passes the frame to the physical layer for transport.

FRAME RELAY HEADER

The CPE router encapsulates each layer 3 packet within a header and a Frame Relay trailer before sending it through the VC. The header and trailer are defined in the bearer services specification for the link access procedure for Frame Relay (LAPF), ITU Q.922-A.

ADDRESS FIELD

As shown in Image 4, the Frame Relay header (address field) contains:

• DLCI : The 10-bit DLCI is one of the most important fields in the Frame Relay header. This value represents the virtual connection between the DTE device and the switch.
• C / R : is the bit that follows the most important DLCI byte in the address field. The C / R bit is not currently defined.
• Extended Address (EA) : If the value of the EA field is 1, it is determined that the current byte is the last octet of the DLCI. The eighth bit of each byte in the Address field indicates the EA.
• Congestion control : consists of 3 bits of Frame Relay congestion notification. They are specifically referred to as "explicit forward congestion notification bit" (FECN), "explicit backward congestion notification bit" (BECN) and "eligible discard bit".

Typically, the physical layer is EIA / TIA-232, 449 or 530, V.35 or X.21. The Frame Relay frame is a subset of the HDLC frame type; therefore, it is delimited with indicator fields.
The 1-byte indicator uses bit pattern 01111110. The FCS determines if errors occurred in the Layer 2 address field during transmission.
The sending node calculates the FCS before transmission, and the result is inserted in the FCS field. At the far end, a second value of FCS is calculated and compared to the FCS in the frame. If the results are the same, the plot is processed. If there is a difference, the plot is discarded.
Frame Relay does not notify the source when a frame is discarded. Error control is reserved for the upper layers of the OSI model.