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Frame Relay

Frame Relay is a packet-switched Layer 2 WAN protocol that provides no error recovery and focuses on speed. Higher layer protocols carried by Frame Relay, such as TCP/IP, can be used to provide reliability.

Frame Relay multiplexes multiple logical connections over a single physical connection to create Virtual Circuits; this shared bandwidth model is an alternative to dedicated circuits such as T1s. A PVC (Permanent Virtual Circuit) is always connected, analogous to a real dedicated circuit like a T1. A Switched Virtual Circuit (SVC) sets up each “call,” transfers data, and terminates the connection after an idle timeout. Frame Relay is addressed locally via Data Link Connection Identifiers (DLCI, pronounced “delsee”).

X.25

X.25 is an older packet-switched WAN protocol. X.25 provided a cost-effective way to transmit data over long distances in the 1970s through the early 1990s, when the most common other option was a direct call via analog modem. X.25’s popularity has faded as the Internet has become ubiquitous.

The global packet-switched X.25 network is separate from the global IP-based Internet. X.25 performs error correction that can add latency on long links. It can carry other protocols such as TCP/IP, but since TCP provides its own reliability, there is no need to take the extra performance hit by also providing reliability at the X.25 layer. Other protocols such as frame relay are usually used to carry TCP/IP.

ATM

Asynchronous Transfer Mode (ATM) is a WAN technology that uses fixed length cells. ATM cells are 53 bytes long, with a 5-byte header and 48-byte data portion.

ATM allows reliable network throughput compared to Ethernet. The answer to “How many Ethernet frames can I send per second” is “It depends.” Normal Ethernet frames can range in size from under 100 bytes to over 1500 bytes. In contrast, all ATM cells are 53 bytes.

SMDS (Switched Multimegabit Data Service) is older and similar to ATM, also using 53-byte cells.

MPLS

Multiprotocol Label Switching (MPLS) provides a way to forward WAN data via labels, via a shared MPLS cloud network. This allows MPLS networks to carry many types of network traffic, including ATM, Frame relay, IP, and others. Decisions are based on labels, and not encapsulated header data (such as an IP header). MPLS can carry voice and data and be used to simplify WAN routing. Assume 12 offices connect to a data center. If T1s were used, the data center would require 12 T1 circuits (one to each office); with MPLS, the data center and each office would require a single connection to connect to the MPLS cloud.