Data Communication & networking

This blog is for undergraduate/graduate students who require some basic information about their subjects or any other topic related to data communication.

Tuesday, 29 March 2011

Frame Relay

Frame Relay is a standardized wide area network technology that operates at the physical and logical link layers of OSI model. Frame relay originally designed for transport across Integrated Services Digital Network (ISDN) infrastructure, it may be used today in the context of many other network interfaces.
Frame relay is an example of a packet switched technology. Packet switched network enables end stations to dynamically share the network medium and the available bandwidth. 
Frame Relay is often described as a streamlined version of X.25, it is because frame relay typically operates over WAN facilities that offer more reliable connection services. Frame relay is strictly a layer 2 protocol suite, where as X.25 provides services at layer 3.
Some important characteristics of frame relay are,
1- It allows bursty data.
2- It allows the frame size 9000bytes, which can accumulate all LANs.
3- It is less expensive than other traditional WANs.
4- It has error detection only at data link layer, there is no any flow control and error control.
5- There is also a retransmission policy if frame is damaged.
6- 56 kbps, 64 kbps, 128 kbps, 256 kbps, 512 kbps and 1.5 Mbps.

Frame Relay Devices:
Devices attacted to a fram relay WAN are of two general categories.
1-Data terminal Equipment (DTE).
2-Data Circuit terminating Equipment (DCE).

DTEs generally are considered to be terminating equipment for a specific network and typically are located on the premises of a customer. They are owned by the customers. e.g. PC,routers, bridges.
DCEs are carrier owned internet working devices. The purpose of DCE equipment is to provide clocking and switching services in the network.

Frame Relay Virtual Circuits:
Frame Relay provides connection-oriented data link layer communications. This means that a defined communication exists between each pair of devices and that these connections are associated with a connection identifier (ID). This service is implemented by using a FR virtual circuit, which is a logical connection created between two DTE devices across a Frame Relay packet-switched network (PSN).
Virtual circuits provide a bidirectional communication path from one DTE device to another and are uniquely identified by a data-link connection identifier (DLCI). A virtual circuit can pass through any number of intermediate DCE devices (switches) located within the Frame Relay PSN.
Frame Relay virtual circuits fall into two categories: switched virtual circuits (SVCs) and permanent virtual circuits (PVCs).

Switched Virtual Circuits (SVCs)

Switched virtual circuits (SVCs) are temporary connections used in situations requiring only sporadic data transfer between DTE devices across the Frame Relay network. A communication session across an SVC consists of the following four operational states:
Call setup—The virtual circuit between two Frame Relay DTE devices is established.
Data transfer—Data is transmitted between the DTE devices over the virtual circuit.
Idle—The connection between DTE devices is still active, but no data is transferred. If an SVC remains in an idle state for a defined period of time, the call can be terminated.
Call termination—The virtual circuit between DTE devices is terminated.

Permanent Virtual Circuits (PVCs)

Permanent virtual circuits (PVCs) are permanently established connections that are used for frequent and consistent data transfers between DTE devices across the Frame Relay network. Communication across a PVC does not require the call setup and termination states that are used with SVCs. PVCs always operate in one of the following two operational states:
Data transfer—Data is transmitted between the DTE devices over the virtual circuit.
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.
DTE devices can begin transferring data whenever they are ready because the circuit is permanently established.

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