What is frame relay, in which layer it comes?
Frame relay is a packet switching technology. It will operate in the data link layer. Because the protocol assumes it is functioning over a private connection, Frame Relay does not monitor whether the frame is error-free. A Frame Relay node can start switching traffic out onto a new line as soon as it has read the first two bytes of addressing information at the beginning of the frame. This lets a frame pass through several switches and arrive at its destination with only a few bytes’ delay. Because the delays are relatively small, network latency is not much different from leased line connections, but costs are lower because the network is shared though the service provider. Frame Relay packets are routed through one or more virtual circuits known as DLCIs (Data Link Connection Identifiers). These are permanent virtual circuits that the service provider sets at subscription time. At this time the ISP also sets the CIR (Committed Information Rate), which specifies the amount of guaranteed bandwidth. If you send data frames faster than this rate, the network will flag the excess frames with a DE (Discard Eligibility) bit. The marked frames may still get to their destination, but they may also be discarded if the network is congested. When a company purchases a Frame Relay service, it can specify its desired CIR level. The higher the level set, the higher the cost. However, companies often set the CIR level to low or zero, thus saving money but also setting themselves up for potential data loss. On the other hand, because data is sent over a private line, it is less vulnerable to eavesdropping or alteration by malicious individuals.