Last Mile Access, xDSL, Fibre and Baseband Modems

The Last Mile refers to the connection between a carrier's or service provider's central office or point-of-presence (PoP) and the customer site. This connection is normally accomplished over a pair of copper wires called a twisted pair. The system was originally designed for voice transmission only using analogue transmission technology on a single voice channel. Today, modems convert between analogue and digital signals for voice and data. Digital Subscriber Line (DSL) technology delivers digital signals directly over the Last Mile, and at a much higher bandwidth that is achieved for analogue voice.

DSL - digital subscriber line

"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 5.0 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

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, 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 bridge or router, with no need for an external one.

Intelligent modems

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 by remote control, activate diagnostics and receive real-time alerts on the system status. The management channel can operate in parallel with the data channel over the same link, wire or fibre. Fibre optic technologyDeregulation and demands for increased bandwidth have reduced the price of fibre optic cabling and installation. Fibre 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. Fibre optic modems are usually smaller and lighter and consume less power than copper modems.

Distance and bandwidth

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

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

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

Advantages of RAD's fibre optic products

RAD's fibre optic products offer:

• a variety of models for all applications
• a choice of fibre type, light source and connectors
• non-powered operation
• optical redundancy
• diagnostic capabilities
• Wave Division Multiplexing (WDM), which uses different wavelengths over the same fibre in order to reduce the amount of fibre required.

Point-to-point wireless links

In many cases, connecting businesses over a point-to-point, license-exempt wireless connection can be an alternative to a fibre optic connection.

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

Sequence spread spectrum technology (W-CDMA) for wideband digital radio communications enables wireless transmission of multiple services such as legacy TDM traffic together with Ethernet/IP-based next generation services including Internet, multimedia and video.

AirMux, RAD's first wireless solution, is a point-to-point multiplexer that aggregates fractional E1/T1 and Ethernet traffic over a 2.6 Mbps full duplex wireless link, extending data/voice transmission up to 16 km (10 miles).

Broadband rate and media converters and repeaters

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 cost-effective 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.