Faster, Cheaper, BetterYou Can t Go Wrong with Frame Relay

Faster, Cheaper, BetterYou Can't Go Wrong with Frame Relay

These days, sites that use Frame Relay hugely outnumber sites that use leased lines. The reasons for Frame Relay's popularity vary, but it boils down to some basics: For less cash up front and less per month, you can send more data. Hmmm…that seems like a no brainer to me, and most people agree. But it is useful to consider why Frame Relay is faster, cheaper, and better than using leased lines. That will give me a good reason to talk about a few other important things relating to the technology.

You Can Still Use Serial Links, but It Will Cost You Up Front

Point-to-point WANs tend to require more hardware than does the equivalent network built with Frame Relay. To see why, consider what the telco does to create three leased circuits, as shown in Figure 15-6.

Figure 15-6. Three Leased Lines to Connect Three Routers

It takes twice as much hardware and twice as many cables to accomplish the network in Figure 15-6, as compared to using Frame Relay. In the figure, R1 requires two serial interfaces and two separate CSU/DSUs. Also, the telco has to install two cables between the local CO and the office building in Atlanta where R1 resides. Likewise, R2 needs two CSU/DSUs, and it needs two cables run to the telco; the same goes for R3. With the three-site Frame Relay network in Figure 15-4, you only needed one cable from each router to the telco, one serial interface, and one CSU/DSU at each router. Even in this small network, you already need more hardware and more cables.

Now think about what happens when the company grows to 10 sites or 100 sites. For each point-to-point line, R1 will need a separate physical serial interface and a separate CSU/DSU, and the telco will need to run another 4-wire cable from the Atlanta CO to Fredsco's Atlanta headquarters. That requires a lot of extra money for router and CSU/DSU hardware on Fredsco's part, and the telco has to run many more cables. Running those cables costs money, and, of course, Fredsco gets to pay for that.

In short, using leased lines will end up requiring more hardware and costing more cash up front than using Frame Relay.

Get Your Free Bandwidth Here! Free Bits!

So, you can save a little (or a lot) of money based on how much hardware you need to buy when using Frame Relay. Now imagine that Fred, the president of Fredsco, has a network with two sites and a single WAN link with a speed of 128 Kbps. The Frame Relay company's salesman gets a bonus if he signs up a new customer today, so he stops by to see Fred. "You know, we can install Frame Relay instead of your 128 Kbps point-to-point WAN link. For the same price, we can guarantee that you can send at least 128 Kbps between those two sites using Frame Relay. But most of the time, you'll be able to send double that256 Kbpsfor no extra cash! And after you sign, I'll buy lunch."

Although the salesman's promise sounds suspicious, Frame Relay really would provide equal or higher speeds at the same price, just as he claims. Frame Relay protocols actually define the technical details behind the salesman's claims. To see how that works, look at Figure 15-7, which shows three routers using Frame Relay.

Figure 15-7. Typical Frame Relay Network with Three Sites

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.