Frame Relay - 1

And now, back to our program. Even though I'm running a bit short on time this morning, I wanted to drop a few quick thoughts about Frame Relay down before I lose them. Frame Relay is a popular WAN technology because it allows high data transfer rates without having to have the cost of dedicated point-to-point lines for each connection. You can have several FR links on a single interface, reducing cost yet providing a decent throughput. It is the IT manager's dream!

FR utilizes Virtual Circuits, which are logical links through a service provider's network cloud. While the path through "the cloud" may be varied and use several different devices, from the router's perspective it is a direct connection to the other end of the virtual circuit. While a packet may go through several devices between the two ends of the virtual circuit, it still sees it as one hop. Virtual circuits define which devices communicate.

Some of the tpolgies for FR are hub-and-spoke, partial mesh and full mesh. These are exactly like the "networking 101" definitions, so I won't bother to explain them. All that's required to know is that as you move from the hub-and-spoke topology to a full mesh, you increase reliability and cost.

Virtual circuits are either Permanent or Switched. A Permanent Virtual Circuit (PVC) is up all the time adn provides instantaneous data transfer. A Switched Virtual Circuit (SVC) provides data transfer on-demand. These are less common today, and an example would be an ISDN line that connects when there is data to be sent to the other end and torn down when it is inactive.

LMI (Local Management Interface) is the language two devices speak that communicate the state of virtual circuits to a router. Newer versions of IOS are able to automagically determine what LMI is in use, but in older versions it is necessary to enter it manually.

DLCI's (Direct Link Connection Identifier) are the numbers that identify where traffic is destined to reach. These are in essence used as the hardware address to identify the reciving end's network interface, because serial interfaces have no MAC address. They are locally significant only. In other words, if I know that to reach my location in Texas I send frames out virtual circuit with DLCI 100, the Texas router does not need t know what DLCI I send to. It only knows what number returns to me, and could be completely different. A great analogy that I read is one of flight numbers. A flight from Minnesota to Texas needs only be known by the Minnesotan getting on the plane. When he arrives in Texas, he is there and the flight number is unimportant now. When he wants to return to Minnesota, chances are he'll get on a plane with a completely different flight number. As long as the sending terminal knows which plane to put him on, he arrives at his destination. That's how DLCI's work. They answer the question "to get to location X, I send data to circuit Y."

Of importance to network managers is the difference between Local Access Rate and Committed Information Rate. Local access rate is another term for line speed, and is the total bandwidth of an interface. Committed Information Rate (CIR) is the service provider's guarantee for throughput, and as their lines become more heavily subscribed, they will provide at minimum this amount. An interface's local access rate will determine the realistic bandwidth available, and the CIR of all circuits cannot exceed that amount or your line is "over subscribed." When a line is over subscribed, it will allocate bandwidth to all circuits, but perhaps not meet the CIR for each. It's then time to check your SLA and monitor your throughput so you are getting all you pay for.

To keep traffic from backin up too much, service providers use BECN (Backward Explicit Congestion Notifiers) to modify the headers of traffic returning to a router to tell it to slow down its transmission. These are sent back to a transmitting router if there is a large mismatch between the Local Access Rate and the CIR, causing congestion in the line. The converse of BECN is the Forward Explicit Congestion Notifier (FECN), which is a signal from the sending router to the receiver which prompts the receiver to send a BECN in the case where there is no return traffic acknowledgement (such as in a UDP stream).

Lastly, if you are sending traffic above and beyond your CIR, a service provider may mark some of your traffic as "Discard Eligible." Marked as such, most packets still arrive safely. But if the line gets too congested, these will be the first to drop.

Tomorrow, more Frame Relay. It's a complex topic, but once you understand the terms and ideas behind it, the configuration is really quite simple. More on that later.