| FAQ |
  |
| Networking : Tricky Ones : Frame-Relay vs. Leased-Line Connections |
    |
| Question: | Kindly clarify these doubts of mine.
1. What is the difference between frame-relay connection and a leased line connection? 2. How many DLCIs can an interface on a router support or in other words how many PVCs can an interface of a router support? |
| Answer: | Frame-relay and leased-lines are both similar and very different! First, the answer is always “It Depends.” It depends on whether you’re talking physical layer or data- link layer.
From the physical layer, they’re similar. For example (depending on which part of the world you’re in), you can run either of them over a T-1 line or 64k circuit. The actual physical bit-movement relies on some other mechanism. When we move into layer 2 though, things are different. A leased line physically means that a circuit goes between two places (doesn’t it always?), or more importantly, physically between two of your offices as an example! The layer 2 encapsulation can be HDLC (default) or PPP most commonly. This is just a way to get the bits into frames and start to structure our world a bit. From a logical standpoint, we’re connected point-to-point, so we always know who is going to be on the other end, and it won’t dynamically change since we’re tied to a physical circuit. A frame-relay circuit, on the other hand is designed to work between many multiple points all at the same time in a shared arrangement. It does this by talking to a central location (frame-switch) that moves things along the appropriate direction. Virtual Circuits (permanent, PVC, or dynamic/switched, SVC) are set up and all run within the same physical circuit. This gives us a much more robust, and definitely less expensive way of connecting multiple offices together. So I hope that answers the first part of your question adequately before we move into the second part. Looking into the format of a frame on a frame-relay circuit gives us some insight to this. First though, let’s talk about the link working a little more. I mentioned that the end devices (DTE/router) had to talk to a central location (DCE) to manage things. This is all accomplished via LMI (local management interface), which describes the types of control messages between the frame router and the frame switch to delineate and carry traffic. Now, onto the DLCI information. DLCI, or Data Link Connection Identifier, is a number assigned to each virtual circuit so that everyone can keep things straight about what data goes where. So onto the frame-relay frame format. The address field consists of two bytes of information broken up as follows: 5 bits DLCI (high-order bits) 1 bit C/R Command/Response bit 1 bit EA Address field extension (future use) 5 bits DLCI (low-order bits) 1 bit FECN Forward Explicit Congestion Notification 1 bit BECN Backward Explicit Congestion Notification 1 bit DE Discard Eligible 1 bit EA Address field extension (future use) Now, check my math, but I think that’s all 16 bits! The DLCI all together is 10 bits. And here’s the time for advanced math! 10 bits, or 2 to the 10th power is what? (I hope it’s not early in the morning when you’re reading this!) The answer is 1,024. There are 1,024 possible values in that field. So, theoretically, you could expect 1,024 possible separate virtual circuits to be used on any given physical link. Reality, however, is a little different. And I’ll refer back to the “it depends” answer! First, it depends on which LMI protocol you are using. On Cisco routers, you have three choices: * LMI (Cisco, default) based on FRF1.1 * ANSI (T1.617, Annex D) * Q933A (ITU Q.933A, Annex A) Cisco/LMI specification uses DLCI 1023 for signaling. ANSI and Q933A both use DLCI 0 for signaling. So right off there are some differences. It also varies in what DLCIs have been reserved for various network functions. Zero through 16 are always reserved. Some LMI types reserve from 1007 to 1023. Others reserve from 993 to 1023. So it will depend on which LMI type you are using as well. Minimum is from 16 to 993 though, giving you about 977 or so possible virtual circuits, which should be plenty to keep you busy! And, as if this weren’t enough, you have the router to contend with! Different routers have different limitations for total number of interfaces supported. It used to be a hard limit in the IDB (Interface Descriptor Block) of 300 interfaces per router. This has been changed to focus more on processor and RAM capabilities of different routers. So there should be plenty there to digest about frame relay! If you’re looking for more information, I suggest checking out the Frame Relay forum at frforum.com or specifically some of their training materials at www.frforum.com/basicguide/toc.html. |
| last updated 01.03.2003 | |
| 5 user questions (display) | |
user rating:  comments on ratings | |
| [previous FAQ] [next FAQ] | |
| List FAQ in this category | List categories | List all FAQ | List programs |
|