ATM / OCx


	ATM (asynchronous transfer mode) is a switched, connection-oriented networking technology that provides high-speed connections using cell-based fast-packet communications. It can support speeds from 1.544 Mbps to 10 GBps and is completely scalable. Dedicated parallel media connections allow ATM to handle simultaneous conversations, eliminating data bottlenecks that exist with ethernet, token ring, and FDDI. It can be run on any media, though fiber optics is usually used.

	When data is transferred using ATM, a switched virtual circuit (SVC) is established. Information is transmitted in fixed-length cells, then reassembled into data packets once received. ATM relies on reliable digital lines to ensure data integrity; it does not use error correction. Multiple transmissions can occur for easy, reliable dialogues. ATM can be installed into an existing network without the need to upgrade an entire LAN.

	ATM networks contain two types of devices: endstations and switches. Along with the devices comes two interfaces for ATM networking: user-network interface (UNI) and network-network interface (NNI). The user-network interface conects endstations to switches; the network-network interface connects switches to switches. LANs are able to communicate with ATM networks so long as the router used can understand the ATM interface.

	There are three types of modes that can be used. Synchronous transfer mode (STM) utilizes time-division multiplexing. This helps to make sure there is a dedicated amount of bandwidth at all times. Packet transfer mode (PTM) utilizes packet switching, where the packes are of different sizes to provide flexible bandwidth. Asynchronous transfer mode (ATM) was designed to transport data flexibly, too. however, ATM has been optimized for voice, video, and computer data. It allows for switching data units through networks, much like X.25 and frame relay.

	ATM uses a fixed data packet size called a cell. The cells are 53 bytes in size. 5 bytes are used for the header. The remaining 48 bytes are for data. Virtual circuits and virtual paths are used to help establish the link between two ATM units.

	OC stands for optical connection (using fiber optics). This type of Internet connection is usually used by major ISPs and those that literally own the Internet backbone. OC-1 runs at 51.8 MBps. OC-3 is three OC-1s tied together for a total bandwidth of 155 MBps. An OC-12 runs at 622 MBps. Microsoft supposedly has two of them.

	Currently, several major ISPs and other major networking organizations are implementing OC-48 cabling, which can handle 2.4 GBps. Russ Haynal's ISP Page provides links to the maps of some major companies and ISP providers. Unfortunately, some of the links are broken. But others (such as Sprint) provide a map of their WAN network. You should be able to find a link to a company's network infrastructure by going to their home page.

	MCI Worldcom (UU Net) currently has a dedicated OC-192 fiber backbone stretching from Houston, through Atlanta, to D.C., then New York. From there, it goes to Chicago, Sacramento, and Los Angeles. Their OC-48 infrastructure has become quite defined, with many multiple fiber loops. UU Net also has several OC-48 lines going to Europe. The Asian connections are slower, but there are still several OC-3 lines laid.

	A couple of new technologies allow for more data to be forced through the same level of fiber. Wave division multiplexing (WDM) can increase the capacity of a particular fiber by 16. Dense wave division multiplexing can increase the capacity of fiber by up to 10,000. These technologies are currently more prevalent in the digital video field.