Any network that encompasses a large geographic area is referred to as a WAN or Wide Area Network. Many large businesses and government agencies use WANs to keep their employees and citizens connected as well as provide a quick and effective way to send and receive information. WANs often connect multiple smaller networks, such as local area networks (LANs) or metro area networks (MANs).
The world’s most popular WAN is the Internet. Some segments of the Internet, like VPN-based extranets, are also WANs in themselves. Finally, many WANs are corporate or research networks that utilize leased lines.
WANs generally utilize different and much more expensive networking equipment than do LANs. Key technologies often found in WANs include SONET, Frame Relay, and ATM.

SONET – Synchronous Optical Network
SONET is a physical layer network technology designed to carry large volumes of traffic over relatively long distances on fiber optic cabling. SONET was originally designed by the American National Standards Institute (ANSI) for the USA public telephone network in the mid-1980s.
SONET possesses several characteristics that make it appealing on the Internet today:
• SONET defines clear interoperability standards between different vendors’ products
• SONET can carry nearly any higher-level protocol (including IP), and
• SONET includes built-in support for ease of management and maintenance.
Generally speaking, SONET performs at very high speeds. At the base signalling level called “STS-1,” SONET supports 51.84 Mbps. The next level of SONET signalling, STS-3, supports triple the bandwidth, or 155.52 Mbps. Higher levels of SONET signalling increase the bandwidth in successive multiples of four, up to approximately 40 Gbps!
The speed and cost of SONET make the technology competitive with alternatives like ATM and Gigabit Ethernet.

Frame Relay
Frame relay is a telecommunication service designed for cost-efficient data transmission for intermittent traffic between local area networks (LANs) and between end-points in a wide area network (WAN). Frame relay puts data in a variable-size unit called a frame and leaves any necessary error correction (retransmission of data) up to the end-points, which speeds up overall data transmission. For most services, the network provides a permanent virtual circuit (PVC), which means that the customer sees a continous, dedicated connection without having to pay for a full-time leased line, while the service provider figures out the route each frame travels to its destination and can charge based on usage. An enterprise can select a level of service quality – prioritizing some frames and making others less important. Frame relay is offered by a number of service providers, including AT&T. Frame relay is provided on fractional T-1 or full T-carrier system carriers. Frame relay complements and provides a mid-range service between ISDN, which offers bandwidth at 128 Kbps, and Asynchronous Transfer Mode (ATM), which operates in somewhat similar fashion to frame relay but at speeds from 155.520 Mbps or 622.080 Mbps.
Frame relay is based on the older X.25 packet-switching technology which was designed for transmitting analog data such as voice conversations. Unlike X.25 which was designed for analog signals, frame relay is a fast packet technology, which means that the protocol does not attempt to correct errors. When an error is detected in a frame, it is simply “dropped.” (thrown away). The end points are responsible for detecting and retransmitting dropped frames. (However, the incidence of error in digital networks is extraordinarily small relative to analog networks.)
Frame relay is often used to connect local area networks with major backbones as well as on public wide area networks and also in private network environments with leased lines over T-1 lines. It requires a dedicated connection during the transmission period. It’s not ideally suited for voice or video transmission, which requires a steady flow of transmissions. However, under certain circumstances, it is used for voice and video transmission.
Frame relay relays packets at the data link layer of the Open Systems Interconnection (OSI) model rather than at the Network layer. A frame can incorporate packets from different protocols such as Ethernet and X.25. It is variable in size and can be as large as a thousand bytes or more.

ATM – Asynchronous Transfer Mode
ATM is a high-speed networking standard designed to support both voice and data communications. ATM is normally utilized by Internet service providers on their private long-distance networks. ATM operates at the data link layer (Layer 2 in the OSI model) over either fiber or twisted-pair cable.
ATM differs from more common data link technologies like Ethernet in several ways. For example, ATM utilizes no routing. Hardware devices known as ATM switches establish point-to-point connections between endpoints and data flows directly from source to destination. Additionally, instead of using variable-length packets as Ethernet does, ATM utilizes fixed-sized cells. ATM cells are 53 bytes in length, that includes 48 bytes of data and five (5) bytes of header information.
The performance of ATM is often expressed in the form of OC (Optical Carrier) levels, written as “OC-xxx.” Performance levels as high as 10 Gbps (OC-192) are technically feasible with ATM. More common performance levels for ATM are 155 Mbps (OC-3) and 622 Mbps (OC-12).
ATM technology is designed to improve utilization and quality of service (QoS) on high-traffic networks. Without routing and with fixed-size cells, networks can much more easily manage bandwidth under ATM than under Ethernet, for example. The high cost of ATM relative to Ethernet is one factor that has limited its adoption to “backbone” and other high-performance, specialized networks.