Last Mile Access, xDSL, Fiber, Baseband Modems & Wireless

"xDSL" is a generic name for a group of technologies that pass high speed digital transmissions over twisted copper wire pairs, where DSL stands for "digital subscriber line." Included are standard technologies such as HDSL, SHDSL, ADSL, VDSL, and proprietary DSL-based technologies such as IDSL and MSDSL.

 

The new SHDSL standard enables symmetrical transmission at rates of up to 2.3 Mbps over 2-wires or 4.6 Mbps over 4-wires, to a range of 5 km (3.1 miles). The HDSL standard is focused on 2 Mbps symmetrical transmission up to 4.8 km (3 miles) using existing 2- or 4-wire copper cables. IDSL (based on ISDN technology) is for symmetrical transmission at rates up to 128 kbps over 2-wire. MSDSL enables the transmission rate to adapt to the DTE rate, thereby providing increased range over 2- or 4-wire at rates between 64 kbps to 2.3 Mbps. Symmetrical VDSL enables transmission at data rates of up to 6, 13 or 26 Mbps for short ranges.

Baseband modems, also known as short haul or short range modems, connect computers, bridges, routers and other digital communications equipment over relatively short distances, such as inside buildings and campuses or within cities. The modems overcome limitations of digital interfaces and can provide LAN extension when connected to bridges and routers. Many of RAD's baseband modems support LAN extension using a built-in remote bridge or router, with no need for an external one.

Intelligent modems (RAD's "i" series) are baseband modems with management capabilities. These modems include a management channel that enables the user at the central site to configure the unit at the customer premises, remotely activate diagnostics, and receive real-time alerts on the system status. The management channel can operate in parallel to the data channel over the same wire or fiber link.

Deregulation and demands for increased bandwidth have reduced the price of fiber optic cabling and installation. Fiber is generally superior to copper wires because it provides higher bandwidth over longer distances, is immune to electrical interference and power surges, and provides data security. Fiber optic modems are usually smaller and lighter and consume less power than copper modems.

Distance and bandwidth are determined by fiber type and light source.

 

Fiber type - There are two types of fiber optic strands: multimode and single mode. Multimode fiber allows propagation of light at various paths, resulting in high attenuation, while single mode fiber allows a single transmission path, which results in lower attenuation and higher speeds.

 

Light source - Signal attenuation in fiber optics also depends on wavelength. Low attenuation occurs at three different wavelengths: 850 nm, 1310 nm and 1550 nm.

License-exempt wireless links provide high bandwidth connectivity over long distances, at a fraction of the cost and time required to deploy fiber optic cables. Businesses can also avoid the cost of leased line service by connecting remote campus locations over wireless links.

 

The Airmux is RAD's wireless point-to-point multiplexer that aggregates E1/T1 circuits and Ethernet traffic over wireless links, extending data/voice transmission for up to tens of kilometers.

Today, many high speed devices, such as ATM and Gigabit Ethernet switches, routers and SDH/SONET add-drop multiplexers (ADMs), support a limited number of interfaces and rates, which makes it difficult for them to interoperate with the wide range of devices on the market. In addition, the price of high speed (e.g., 622 Mbps) ports in these devices requires a costeffective conversion solution.

 

Rate and media converters are the "missing link" for carriers, system integrators and end users. Such devices can convert between a wide range of media and act as repeaters. Rate converters enable connectivity between high speed SDH/SONET, ATM and Gigabit Ethernet devices.

Miniature short range modems offer the same communications functions as ACpowered modems but with fewer features, such as limited diagnostics capability and no BER tests. Usually they operate over shorter distances at lower rates (normally up to 19.2 kbps). However, certain modems can operate at data rates up to 115.2 kbps in async transmission and up to 128 kbps in sync transmission.