X.25 Devices - Digital Subscriber Line (xDSL) - WAN Basics

X.25 Devices

X.25 networks implement the internationally accepted ITU-T standard governing the operation of packet switching networks. Transmission links are used only when needed. X.25 was designed almost 20 years ago when network link quality was relatively unstable. It performs error checking along each hop from source node to destination node. The bandwidth is typically between 9.6Kbps and 64Kbps. X.25 is widely available in many parts of the world including North America, Europe, and Asia. There is a large installed base of X.25 devices.

Digital Subscriber Line (xDSL)

 - DSL is a pair of “modems” on each end of a copper wire pair
 - DSL converts ordinary phone lines into high-speed data conduits
 - Like dial, cable, wireless, and T1, DSL by itself is a transmission technology, not a complete    solution
 - End-users don’t “buy” DSL, they “buy” services, such as high-speed Internet access, intranet, leased line, voice,    VPN, and video on demand
 - Service is limited to certain geographical areas

Digital subscriber line (DSL) technology is a high-speed service that, like ISDN, operates over ordinary twisted-pair copper wires supplying phone service to businesses and homes in most areas. DSL is often more expensive than ISDN in markets where it is offered today.
Using special modems and dedicated equipment in the phone company's switching office, DSL offers faster data transmission than either analog modems or ISDN service, plus-in most cases-simultaneous voice communications over the same lines. This means you don't need to add lines to supercharge your data access speeds. And since DSL devotes a separate channel to voice service, phone calls are unaffected by data transmissions.

DSL Modem Technology

DSL has several flavors. ADSL delivers asymmetrical data rates (for example, data moves faster on the way to your PC than it does on the way out to Internet). Other DSL technologies deliver symmetrical data (same speeds traveling in and out of your PC).
The type of service available to you will depend on the carriers operating in your area. Because DSL works over the existing telephone infrastructure, it should be easy to deploy over a wide area in a relatively short time. As a result, the pursuit of market share and new customers is spawning competition between traditional phone companies and a new breed of firms called competitive local exchange carriers (CLECs).

Asynchronous Transfer Mode (ATM)

ATM is short for Asynchronous Transfer Mode, and it is a technology capable of transferring voice, video and data through private and public networks. It uses VLSI technology to segment data at high speeds into units called cells. Basically it carves up Ethernet or Token ring packets and creates cells out of them.

Each cell contains 5 bites of header information, 48 bites of payload for 53 bites total in every cell. Each cell contains identifiers that specify the data stream to which they belong. ATM is capable of T3 speeds, E3 speeds in Europe as well as Fiber speed, like Sonet which is asynchronous optical networking speeds of OC-1 and up. ATM technology is primarily used in enterprise backbones or in WAN links.