THE INTERACTIVE NETWORK DESIGN MANUAL Building a Frame Relay Network The Essential Guide to Frame Relay Technology In this section we'll focus on the architectural elements that are truly relevant to designing and maintaining a frame relay network using a public carrier service. Frame relay is first and foremost an interface , a method of multiplexing traffic to be submitted to a WAN. Carriers build frame relay networks using switches from vendors such as Cascade Communications, Cisco Systems/StrataCom, Northern Telecom, Newbridge Networks or Bay Networks. As a customer, your devices see only the switch interface, and are blind to the inner workings of the carrier network, which may be built on very high-speed technologies such as T1, T3, Sonet and/or ATM. As of this writing, all major carrier networks implement permanent virtual circuits (PVCs). These circuits are establi shed via contract with the carrier and typically are build on a flat-rate basis. Although switched virtual circuits (SVCs) have standards support and are provided by the major frame relay backbone switch vendors, they have not been widely implemented in customer equipment or carrier networks. Your carrier programs its frame relay switches to allow your traffic to pass through the network. These are the essential parameters you must understand: Access rate is the rate at which your access circuits join the frame relay network. As discussed earlier , these are typically at 56 Kbps, T1 (1.536 Mbps) or Fractional T1 (a multiple of 56 Kbps or 64 Kbps). It is not possible to send data at higher than access rate. Port speed is the rate at which the port on the frame relay switch is clocked. It is not possible to send data at higher than port speed. Committed information rate (CIR) is the amount of data that the network will receive from the access circuit. All frames received at this level will be passed. Committed burst size (Bc) is the maximum amount of data that the network will transfer in a burst, defined over a (short) interval. All frames received at this level will be passed. Excess burst size (Be) is the amount of data above Bc that the network will try to deliver. Frames submitted at this level may be marked as "Discard Eligible," indicating that they may be dropped if there is not capacity in the cloud. A Data-Link Connection Identifier (DLCI) identifies a virtual circuit to your equipment. This is an end point for a PVC. Based on the standard, this number doesn't describe anything more than that, and has no significance beyond the single link. In practice, however, many designers assign some significance to it: It often represents either a site or a circuit. Oversubscription An instance where the su m of CIRs from multiple PVC's to a given location are higher than the port or access channel rate. What do you pay for? Port speed is typically the most costly parameter to increase, though access rates can jump dramatically if new local loops are involved, such as a move from 56 Kbps to T1. Individual PVC fees are next costly. Once a PVC is established, the additional cost to increase CIR is typically small and can be done in small (4 Kbps) increments. Monitoring Congestion and Discard Eligible Bits It's important to know that the carrier's backbone is shared by many users and possibly multiple services. To keep you (and everybody else) from sending more data than the network can hold, frames sent above your contracted rate may be marked as Discard Eligible (DE). DE bits are set by the carrier network, not your equipment. If your equipment receives DE-marked frames, this indicates that data sent at this rate in the future may get dropped. This may be an early indicator of traffic rates that you didn't plan for in the design of your frame relay WAN. Frame relay equipment notices congestion when it sees frames marked with the Forward Error Correction Notification (FECN) and Backward Error Correction (BECN) bits. These merely indicate an overload within the carrier network, and are only of value in monitoring the carrier's health. You might expect your equipment to notify end stations to stop sending data to keep additional frames from being discarded or hitting a congested network. In practice, however, this doesn't happen: Most routers, bridges and frame relay access devices (FRADs) do nothing when these bits get set. Instead, they expect higher layer protocols, such as TCP/IP, to know how to react implicitly to the packet loss. The Broadcast Problem Broadcasts over PVC-based networks such as frame relay create special problems. If the broadcast must go to multiple remote sites through PVCs as signed to a single access channel, your equipment is forced to pump it out over each DLCI in turn. As noted elsewhere, you'll want to minimize the use of broadcasts whenever you can. Next Updated October 16, 1996 Print This Page E-mail this URL