Frame Relay uses leased lines?

I'm confused about what, exactly, is and what is not a "leased line".

My understood definition of a "leased line" is a dedicated serial connectection that is leased from a carrier. Leased lines use HDLC or PPP as encapsulation types. Subscribers of leased lines do not share bandwidth, and data always travels the same path from one router to another..

What I'm confused about is whether or not Frame Relay is also a type of leased line. The reason for my confusion is that on page 391 of Interconnecting Cisco Network Devices, Part 1 (Cisco Press), the book states "Most Frame Relay connections are PVCs rather than SVCs. The connection to the network edge is often a leased line, but dialup connections are available from some providers using ISDN or xDSL lines."

I understand that Frame Relay runs on a packet-switched network and that it uses either Cisco's or IETF encapsulation. These points are obviously different from the previously-mentioned dedicated serial connection. However, this quote from the book makes it sound like Frame Relay links are also a kind of leased line.

I guess part of the ambiguity is that "leased line" refers to a kind of physical connection, whereas Frame Relay is a kind of data transmission method. People seem to throw around the term "leased line" rather loosely to mean any line that has been leased from a carrier.

So are "leased lines" only the name for dedicated serial links, or can a "leased line" also be the name for a type of packet-switched serial link?

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Comments

EdTheLad

A leased line is a permanent e2e circuit.A leased line can be a dedicated physical line or it can be a logical assignment.You could have a dedicated serial link, dedicated E1, a timeslot within an E1, a FR pvc, ATM pvc. The idea is that this line will always be available and there is no contention with anyone else.

katachi

EdTheLad wrote: »

You could have a dedicated serial link, dedicated E1, a timeslot within an E1, a FR pvc, ATM pvc. The idea is that this line will always be available and there is no contention with anyone else.

Forgive my ignorance here, but am I correct in assuming that yes, you could have something technically known as a "Frame Relay leased line PVC packet-switched circuit"?

For some reason, I feel unsteady about the issue of what-is-what as far as basic WAN topologies. This likely means that I don't fully understand the issue, so I'm hesitant to continue my (ICND1) studies until I understand. Perhaps I am thinking too much at this stage in my education?

tim100

katachi wrote: »

Forgive my ignorance here, but am I correct in assuming that yes, you could have something technically known as a "Frame Relay leased line PVC packet-switched circuit"?

For some reason, I feel unsteady about the issue of what-is-what as far as basic WAN topologies. This likely means that I don't fully understand the issue, so I'm hesitant to continue my (ICND1) studies until I understand. Perhaps I am thinking too much at this stage in my education?

The leased line is your connection to the provider's frame-relay switch. You are leasing bandwidth. The frame-relay cloud is the connection between frame-relay switches and this is where the packet switching takes place based on DLCIs. The pvc (permanent virtual circuit) is the logical connection between endpoints. Each endpoint is assigned a DLCI (Data Link Connection Identifier). Let's say endpoint 1 is assigned DLCI 102 and endpoint 2 is assigned DLCI 201 in a point-to-point configuration. One router configured with DLCI 102 is connected to a frame-relay switch. The other router configured with DLCI 201 is connected to another frame-relay switch. The logical connecton would be DLCI 102 <---> DLCI 201 so when a packet goes out DLCI 102 it is switched through the frame-relay cloud over to DLCI 201. So basically your routers are logically connected point-to-point in this particular example topology.

katachi

So let me see if I have got this correct:

A "leased line" could refer to one of two things:

a physical endpoint-to-endpoint connection linking a DCE to another DCE, such as a dedicated synchronous serial link utilizing either HDLC or PPP
the physical connection linking a DCE to a frame relay switch (which then utilizes a virtual circuit to send frames)

Most literature refers to "leased line" in its strictest sense as defined by #1.

Frame Relay, while utilizing a line leased from the provider (and therefore might technically be called a 'leased line'), is not considered to be a type of "leased line" as the term is generally known.

Is this correct?

katachi

Thank you very much for clearing that up. I'm beginning to understand now.

Is something like a dedicated T1 considered to be a serial link, or are they two entirely different mediums? When books say "dedicated serial link", I tend to think of the serial cable that links two routers in a lab environment (which I assume is not done in a real world WAN topology).

Is DCE a single device such as a CSU/DSU, or is it a general name for the group of equipment at the CO? Or is it both? In the case of frame relay, I was under the assumption that a CSU/DSU was used (thus it was a DCE) and that the frame relay switch was closer to the cloud, so the link would look like:

DTE---DCE---(frame relay cloud containing frame relay switches)---DCE---DTE

Is this incorrect?