The Frame Relay provider has two speed settings that let the salesman make the claims he made to Fred. First, to guarantee the least amount of bandwidth between the two sites, the provider would set the committed information rate (CIR) for the PVC. Remember: A PVC is the closest thing in Frame Relay networks to a leased circuit, so the Frame Relay provider commits to a minimum amount of traffic on that PVC. Inside the Frame Relay network, hidden from us, is a large network that can forward the frames. The sophisticated equipment watches the volume of traffic for each PVC. Those devices know to do everything possible to ensure that for that PVC, over time, the PVC gets to send at least the CIR's worth of bits.
Because the CIR is the speed of a logical or virtual thing, it does not define the physical transmission rate (clock rate) at which the bits are transmitted. CIR is more of a legal contract, with the equipment carrying out the contract, as configured by the Frame Relay provider. When a router actually sends the bits, it sends them at the physical transmission speed (clock rate) of the access link. That speed is referred to as the access rate. In the example in Figure 15-7, the access rate of each access link has been set to 256 Kbps.
Now, let me pull together a few conclusions from these facts about speed:
-
Routers always physically send data at the physical transmission speed of the access links, called the access rate.
-
The Frame Relay provider knows that if the access rate is larger than the CIR of a PVC, the router can send more bits than the CIR on that single PVC.
-
The Frame Relay provider commits that its network will support at least CIR's worth of bits per second.
-
When a router sends more than the CIR worth of bits on a PVC (and it will), if the provider's internal network isn't currently overloaded, it will send the traffic anyway, giving the customer more than he paid for!
That last point in the list is what makes the salesman's suspicious claims actually become true. In reality, many Frame Relay providers engineer their Frame Relay networks so that the network almost never gets too busy to forward the excess traffic. In spite of my tongue-in-cheek pretend sales pitches, it really is great for the Frame Relay provider when the salesman can say something like the following to close a deal, and have it be true: "Hey Fred, since you're my buddy now, let me tell you, we never throw away traffic you send over the CIR for a PVC. You'll really get that 256 Kbps, and you'll just be paying for 128. Just don't tell my boss that I let you in on our secret." Wink wink, nudge nudge, sale made, and it's time for the golf course.
It might seem that no one would use leased lines any more now that Frame Relay is available. As it turns out, in some remote rural sites, there might not be a Frame Relay service available. Some companies simply don't bother changing an existing leased line to Frame Relay. Also, a Frame Relay network might delay packets a small fraction of a second, causing slightly more delay as compared to a leased line. However, you can do things on the routers to minimize the impact of that slight sub-second delay.
Seriously, from a business perspective, Frame Relay is easy to choose over leased circuits. It's like free moneyit's cheaper, and you get at least as much bandwidth (and typically more) than you would with WAN links. Knowing that, it's no surprise that Frame Relay is popular.
Sections
Comments (0 posted)
Syndication
