Virtual-Circuit Networks: Frame Relay and ATM
Two common WAN technologies use virtual-circuit switching. Frame Relay is a relatively high-speed protocol that can provide some services not available in other WAN technologies such as DSL, cable TV, and T lines. ATM, as a high-speed protocol, can be the superhighway of communication when it deploys physical layer carriers such as SONET.
Frame Relay is a virtual-circuit wide-area network that was designed in response to demands for a new type of WAN.
1. Prior to Frame Relay, some organizations were using a virtual-circuit switching network called X.25 that performed switching at the network layer.
Drawbacks of X.25:
1. X.25 has a low 64-kbps data rate. There was a need for higher data-rate WANs.
2. X.25 has extensive flow and error control at both the data link layer and the network layer.
3. Originally X.25 was designed for private use, not for the Internet.
4. Disappointed with X.25, some organizations started their own private WAN by leasing T-l or T-3 lines from public service providers.
a) If an organization has n branches spread over an area, it needs n(n - 1)/2 T-1 or T-3 lines. The organization pays for all these lines although it may use the lines only 10 percent of the time. This can be very costly:
b) The services provided by T-I and T-3 lines assume that the user has fixed-rate data all the time. For example, a T-l line is designed for a user who wants to use the line at a consistent 1.544 Mbps. This type of service is not suitable for the many users today that need to send bursty data.
In response to the above drawbacks, Frame Relay was designed.
a) Frame Relay operates at a higher speed (1.544 Mbps and recently 44.376 Mbps). This means that it can easily be used instead of a mesh of T-I or T-3 lines.
b)Frame Relay operates in just the physical and data link layers. This means it can easily be used as a backbone network to provide services to protocols that already have a network layer protocol, such as the Internet.
c) Frame Relay allows bursty data.
d) Frame Relay allows a frame size of 9000 bytes, which can accommodate all local area network frame sizes.
e) Frame Relay is less expensive than other traditional WANs.
f) Frame Relay has error detection at the data link layer only. There is no flow control or error control.
Frame Relay provides permanent virtual circuits and switched virtual circuits. The routers are used, to connect LANs and WANs in the Internet. In the figure, the Frame Relay WAN is used as one link in the global Internet.
Frame Relay is a virtual circuit network. A virtual circuit in Frame Relay is identified by a number called a data link connection identifier (DLCI). VCIs in Frame Relay are called
Each switch in a Frame Relay network has a table to route frames. The table matches an incoming port-DLCI combination with an outgoing port-DLCI combination. The only difference is that VCIs are replaced by DLCIs.
2. Frame Relay Layers
Frame Relay has only physical and data link layers.
No specific protocol is defined for the physical layer in Frame Relay. Instead, it is left to the implementer to use whatever is available. Frame Relay supports any of the protocols recognized by ANSI.
Data Link Layer
At the data link layer, Frame Relay uses a simple protocol that does not support flow or error control. It only has an error detection mechanism. The address field defines the DLCI as well as some bits used to control congestion.
The descriptions of the fields are as follows:
· Address (DLCI) field. The first 6 bits of the first byte makes up the first part of theDLCI. The second part of the DLCI uses the first 4 bits of the second byte. These bits are part of the l0-bit data link connection identifier defined by the standard.
· Command/response (CIR). The command/response (C/R) bit is provided to allow upperlayers to identify a frame as either a command or a response. It is not used by the Frame Relay protocol.
· Extended address (EA). The extended address (EA) bit indicates whether the currentbyte is the final byte of the address. An EA of 0 means that another address byte is to follow. An EA of 1 means that the current byte is the final one.
· Forward explicit congestion notification (FECN). The forward explicit congestionnotification (FECN) bit can be set by any switch to indicate that traffic is congested. This bit informs the destination that congestion has occurred.
· Backward explicit congestion notification (BECN). The backward explicit congestionnotification (BECN) bit is set to indicate a congestion problem in the network. This bit informs the sender that congestion has occurred.
· Discard eligibility (DE). The discard eligibility (DE) bit indicates the priority level ofthe frame. In emergency situations, switches may have to discard frames to relieve bottlenecks and keep the network from collapsing due to overload.