What is a Frame?
The transport, network, data link, and physical layers of the OSI Model each have a different protocol data unit (PDU):
Layer 4: Transport layer = Segment
Layer 3: Network layer = Packet
Layer 2: Data link layer = Frame
Layer 1: Physical layer = Bits
While network layer devices like routers transport packets, devices in the data link layer use frames. It’s mostly a semantic difference. A frame, however, has both a header and trailer. A packet only has a header.
How Does Frame Relay Work?
Frame relay is typically used to transfer data between geographically separated LANs or across WANs. Frame relays can also be useful in home lab environments. CBT Nuggets trainer Jeremy Cioara recounts while studying for his CCIE R&S, he was spending too much time recabling his lab.
“I had to re-cable every time I wanted a new topology. So instead, I took all of my routers and connected them to a single frame relay switch in a rack of equipment. And then, just by re-configuring that frame relay switch, I could say which routers were connected to what without actually physically moving cables around.”
Switched Virtual Circuits & Permanent Virtual Circuits: What’s the Difference?
In the same way, companies use frame relays to move data among remote sites or across a WAN. Rather than using a full-time leased line between remote sites, frame-relay devices create one of two types of connections: Switched virtual circuits (SVC) or permanent virtual connection (PVC).
Frame relay devices create SVCs when data needs to be transferred and then close those connections when they aren’t in use. Permanent virtual connections (PVC) keep the connection between two locations open all the time. PVCs are much more common.
What are Data Link Connection Identifiers?
These dedicated connections link endpoints, which are usually either routers configured as frame relay switches or provided by a service provider. In either case, the endpoints are labeled with unique identifiers called Data Link Connection Identifiers (DCLI). Frames are then transferred between two static points.
Frame relay is bizarre because you don’t go directly from a source router to a destination router. You use a Permanent Virtual Connection (PVC) to send data. On either side, there will be a Data Link Connection Identifier (DLCI) as the source and destination. In this example, the source will be DLCI 102, and the destination will be DLCI 201.
Recap: When Router One wants to send to Router Two, it’s going to send data from DLCI 102, fly through the air, and then come out on DLCI 201. It’s kind of like leaving out of gates of an airport. When Router Two sends data back, the data is going into 201, flying through the air, and come out on 102.
