Networking basics: How does Frame Relay work?

What is Frame Relay?
The short definition of frame relay is: It is a cost-effective method to connect Local Area Networks or to transport data between endpoints in Wide Area Networks (WANs).
What is a frame?
Each layer of the OSI Model has a different protocol data units (PDU):
Layer 4: Transport layer = Segment
Layer 3: Network layer = Packet
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Start trainingLayer 2 Data link layer = Frame
Layer 1: Physical layer = Bits
Network layer devices, such as routers, transport packets. Devices in the data link layer use frames. It is mostly semantic. A frame has both a trailer and a header. A packet has only a header.
How does Frame Relay work?
Frame relay is used to transfer data across WANs and between geographically distant LANs. Frame relays are also useful in home lab environments. SPOTO trainer Jeremy Cioara relates how he spent too much time recabling his lab while studying for his CCIE R&S.
“I had to re-cable each time I wanted a different topology. Instead, I connected all my routers to a single frame relay switch within a rack of equipment. By simply re-configuring the frame relay switch, I was able to tell which routers were connected to which without having to physically move cables around.
What is the difference between permanent and switched virtual circuits?
Frame relays are used by companies to transfer data between remote sites or across a WAN. Frame-relay devices create two types of connections, one permanent virtual connection (PVC) and one switched virtual circuit (SVC).
Frame relay devices create SVCs for data transfers and close them when not in use. Permanent virtual connections (PVC), which keep the connection between two places open at all times, are called permanent virtual connections. PVCs are more common.
What are Data Link Connection Identifiers (DLCI)?
These dedicated connections link endpoints. They are usually routers configured as frame relay switch routers or provided by a service providers. Endpoints are identified with unique identifiers called Data Link Connection Identifiers, (DCLI). Frames are then transferred to two static points.
Frame relay is a strange method of sending data. It doesn’t connect directly from a source router, but it does connect to a destination router. To send data, you use a Permanent Virtual Connection. Each side will have a Data Link Connection Identifier, (DLCI), as the source and destination. In this example, the source is DLCI 102 and the destination is DLCI 201.
Recap: Router One will send data to Router Two from DLCI 102. It will then fly through the air and return to Router One on DLCI 201. It’s almost like leaving an airport’s gates. Router Two sends data back to Router Two, which means that the data is going into Router 201, flying through air, and coming out on 102.
How to configure a frame relay switch on Cisco IOS
To configure a frame relay switch using Cisco IOS, first determine your topology.
In this case, we will use a topology that:
PVC A will link DLCI 102 to DLCI 201. R2 can send data to R1 by using PVC A. It will send data to DLCI 201.
PVC B will link DLCI 103 to DLCI 301. R1 will send data to R2 when it wants to. It will send the data to the destination of DLCI 102.

These Cisco IOS commands will allow you to change the R2 to R1 settings of a PVC.
# Frame-relay route
< input dlci to be switched >
# Frame-relay route 101?
< interface outgoing interface for pvc switching >
# Frame-relay router interface 102
# frame-relay router interface 102 interface s0/1
< output dlci to use when switching >
# frame-relay router interface 102 interface s0/1201
You will need to connect each endpoint in each direction. Update the final command to configure PVC from R2 R1.

Networking basics: How does Frame Relay work?
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