The Similarities and Differences of ATM and Frame Relay

ATM was created to use much-higher-bandwidth interfaces, at consistent data rates. From the begi ATM protocol was designed to be able to support voice, data, and video traffic, commonly referred multiservice traffic. This is accomplished by the use of fixed-length ATM cells. ATM switches provid<

core, similar to the core provided by Frame Relay switches that provides CPE devices, such as rout circuits using virtual paths and virtual channels. In fact, when you lease a Frame Relay circuit from provider, they will, most likely, provision your Frame Relay circuit on an ATM switch, such as a Cisi switch. When you document your network, you will show routers connected to a Frame Relay clouc shown in Figure 4-1, because your service provider will probably not provide detailed network infoi their network. The Frame Relay traffic is encapsulated in ATM cells, sent through the ATM core netw traffic, and translated back into Frame Relay at the edge ATM switch, as shown in Figure 4-2. Beca is primarily focused on routing and switching technologies, ATM switching is not covered in any det

Figure 4-1. Customer Perception of Frame Relay Networks

Figure 4-1. Customer Perception of Frame Relay Networks

Figure 4-2. Frame Relay over ATM Networks

What makes the two technologies so similar is the fact that they both use virtual circuits to provide of service. Frame Relay virtual circuits are identified using locally significant Data-Link Connection . (DLCIs). Generally speaking, Frame Relay circuits are provisioned with certain levels of service; a < Information Rate (CIR), which dictates the guaranteed access rate. It is possible to order less-expe effort Frame Relay service. With Zero CIR service, the switch only forwards Frame Relay traffic dur no congestion; the term is not necessarily referring to the connection between your routers. At son congestion within your service provider's network may affect traffic that you cannot see from your Frame Relay also supports bursting, or the transmission of excess frames during periods of low util Frame Relay traffic can be controlled, or shaped, using the Sustained Burst Rate (Bc) and Excess B to perform traffic shaping on the edge router.

With Frame Relay, low-priority traffic is marked for discard eligibility using the Discard Eligible (DE^ Frame Relay switch receives a frame containing a DE = 1 bit during a period of congestion, it consid frame low priority and it is discarded. Unfortunately, in most cases, when the DE bit is not changec default value, and the network is congested, all frames leaving the router are considered discard el the Frame Relay switches in a congested network path may indiscriminately drop any of those fram Frame Relay is a connectionless protocol, it relies on upper-layer protocols, such as TCP, to retrans from those lost frames.

Frame Relay networks also have a QoS congestion notification system. This system uses forward e. congestion notification (FECN) and backward explicit congestion notification (BECN) frames to notif downstream neig hbors on congested network paths. Because the use of the FECN and BECN frames explicitly be configured throughout the network on customer and service provider equipment, howe congestion notification is not configured, it does not offer much value. When devices are not config upon congestion notificati on frames, the only benefit they provide is a historical reference of netwo by means of thN Frame Relay counters. So, during periods of congestion, Frame Relay networks th. configured to use traffic shaping and congestion notification may prove to be very unreliable.

ATM was designed to support many of the same technologies that were originally created for Fram' networks. When Frame Relay was originally designed, most of the QoS features were left for vendo implementation, so the use of these features depends on the Frame Relay hardware/software vend Relay implementation, and the service provider's Frame Relay network design and configuration. B a newer technology and it was designed after the technical community had experience with older X Frame Relay technologies, however, ATM networks inherently support QoS by use of the ATM Adop (AAL) types and ATM classes of service, shown in Table 4-5.