Networking and Telecom Glossary

ATM (Asynchronous Transfer Mode)
ATM is a dedicated-connection switching technology that organizes digital data into 53-byte cell units and transmits them over a physical medium using digital signal technology. Individually, a cell is processed asynchronously relative to other related cells and is queued before being multiplexed over the transmission path. Because ATM is designed for easy implementation in hardware (rather than software), faster processing and switch speeds are possible. The prespecified bit rates are either 155.520 Mbps or 622.080 Mbps. Speeds on ATM networks can reach 10 Gbps. ATM is a key component of broadband ISDN (BISDN).

DSL (Digital Subscriber Line)
DSL is a technology for bringing high-bandwidth information to homes and small businesses over ordinary copper telephone lines. xDSL refers to different variations of DSL, such as ADSL, HDSL, and RADSL. Provided that you're situated close enough to a telephone company central office offering DSL service, you may be able to receive data at rates up to 6.1 megabits (millions of bits) per second (of a theoretical 8.448 megabits per second), enabling continuous transmission of motion video, audio, and 3-D animation. More typically, individual connections will provide from 1.544 Mbps to 512 Kbps downstream and about 128 Kbps upstream. A DSL line can carry both data and voice signals; the data portion of the line is continuously connected.

Frame relay
Frame relay is a telecommunication service designed for cost-efficient data transmission for intermittent traffic between LANs and between end-points in a 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.

Since the network (in most cases) provides a permanent virtual circuit, the customer sees a continous, dedicated connection without paying for a full-time leased line. An enterprise can select a level of service quality, prioritizing some frames over others. Provided on fractional T-1 or full T-carrier system carriers. Frame relay complements and provides a mid-range service between ISDN (at 128 Kbps) and ATM, which operates in somewhat similar fashion to frame relay but at speeds from 155.520 Mbps or 622.080 Mbps.

T1 and T3
The T-carrier system, introduced by the Bell System in the U.S. in the 1960s, was the first successful system to support digitized voice transmission. The original transmission rate (1.544 Mbps) in the T1 line is in common use today in ISP connections to the Internet, as are T-3 lines, which provide 44.736 Mbps. Another commonly installed service is a fractional T1, which is the rental of some portion of the 24 channels in a T-1 line, with the other channels going unused.

The T-carrier system is entirely digital, using pulse code modulation and time-division multiplexing (TDM). The system uses four wires and provides duplex capability (two wires for receiving and two for sending at the same time). The T1 digital stream consists of 24 64-Kbps channels that are multiplexed. (The standardized 64 Kbps channel is based on the bandwidth required for a voice conversation.) The four wires were originally a pair of twisted pair copper wires, but can now also include coaxial cable, optical fiber, digital microwave, and other media. A number of variations on the number and use of channels are possible.

In the T1 system, voice signals are sampled 8,000 times a second, with each sample digitized into an 8-bit word. With 24 channels being digitized at the same time, a 192-bit frame (24 channels each with an 8-bit word) is thus being transmitted 8,000 times a second. Each frame is separated from the next by a single bit, making a 193-bit block. The 192 bit frame multiplied by 8,000 and the additional 8,000 framing bits make up the T1's 1.544 Mbps data rate. The signaling bits are the least significant bits per frame.

Local and wide area networks (LANs and WANs)
Local area networks (LANs) enable users to move and share information across multiple devices — most notably servers and clients — using common protocols to regulate the flow of data. The busier the network gets, however, the more likely it becomes that two computers will transmit data simultaneously, causing a collision. The Ethernet protocol seeks to avoid this by checking the network for conflicting transmissions before sending data. Collision detection systems identify collisions and resend data as necessary — which can ultimately impose a serious burden on the network. In token-ring networks, a special packet of data (the token) is passed around the network; a given computer must hold the token before being allowed to transmit.

LANs often are linked into wide area networks (WANs), which can include networks across a building or on the other side of the world.

VoIP (Voice over IP) or IP telephony
Voice over IP — voice communications delivered via the Internet Protocol — sends voice information digitally in discrete packets rather than using the dedicated circuit-switched protocols of the public switched telephone network (PSTN). This bypasses the toll structure of conventional phone service and can result in substantially improved ROI. In addition, because it uses digital information sent over data networks, VoIP offers the same efficiencies, conveniences, and opportunities for integration as other IP-based digital technologies — such as email, directory services, and the web.