Until recently, the available methods for transmitting and receiving data at high capacity were equally high cost. Large companies used microwave and satellite networks to span long distances, and rented expensive T1 and E1 lines for dedicated telephone and data communication between facilities. .

Needless to say, these technologies were out of reach for small and medium business enterprises (SMEs). 

Today, superior data capacities can be delivered at a relatively low cost using Asymmetric Digital Subscriber Line (ADSL) technology. Using existing telephone lines, virtually and telephone subscriber in the country can now qualify for high speed data access. 

Extending the Capacity of Copper  

Modern telephone networks start at the consumer end with twisted-pair copper cable that runs to a switching centre or exchange, where it connects to the telephone network. The backbone that connects these exchanges is mainly fibre optic cable, which provides high-capacity, high-quality transmission. 

The consumer-end copper wire was designed to carry voice or modem signals within the frequency band of 0Hz to 3.4kHz at speeds up to 56,000 bits per second (56 Kbps). This narrow-band, low frequency enables the wire to carry transmissions up to 18,000 feet (5,500 meters) to an exchange without attenuation or degradation of sound quality. 

Achieving higher data transmission speeds means utilizing a much broader  range of frequencies, which is referred to as broadband communications. Each range of frequencies can act as a separate channel on the same wire, so the more frequencies you use the more data the wire can carry. Unfortunately, high frequencies transmitted over copper wire lose energy or attenuate rapidly, and encounter interference or crosstalk, so the data doesn't travel far enough to even reach the exchange. 

Optical fibre is capable for far greater bandwidth and greater transmission speeds at greater distances without attenuation, giving it the capacity to carry much higher volumes of data than copper wire. However, optical fibre comes at a high cost, and extending it to reach every business or household is prohibitively expansive, as cable operators often find in rollouts. 

The question is then how to increase the capacity of copper cable to support high-speed broadband data such as video conferencing, multi-media, high-speed internet access and interactive services. 

The answer is ADSL 

ADSL Technology 

ADSL is an important variation of the DSL family of technologies. When both a modem and a POTS (Plain Old Telephone Line) splitter are used, ADSL provides both normal telephone services and high-speed digital transmission on an existing telephone line, as shown in the following diagram (Detailed ADSL Configuration).



 

This allows a single existing twisted-pair copper wire to carry:

(1)   normal telephone communications on the 0 to 3.4kHz range,

(2)   data upload from the consumer in the 30kHz to 138kHz range, and

(3)   data download to the consumer at up to 1104kHz

The reason ADSL succeeds where previous technology failed is because it takes advantage of a phenomenon that was observed in crosstalk interference of signals between copper telephone wires. 

When a signal comes into an exchange, it has greater crosstalk that the signal going out of the exchange. This is because the copper wires are combined into larger bundles as they get closer to the exchange and signals transmitted at the same frequency can "leak"� from one line to another. This crosstalk has little or no effect on normal telephone communications at low frequencies, but is one of the primary factors in limiting transmissions at higher frequencies. 

Because there is more crosstalk on signals coming into an exchange, SDSL minimises its impact by transmitting from the consumer in the low frequencies of 30kHz to 138kHz. Since the lower frequencies have less attenuation, the signal is still strong when it reaches the noisy crosstalk within the exchange. 

Conversely, the signal coming out of the exchange has very little crosstalk, which means the ADSL transceiver can send data from the exchange at high frequencies and the signal will still be strong enough to reach the consumer at the other end. 

The difference in carrying capacity between these frequency ranges gives rise to the term asymmetric. Because the signal coming from the consumer uses a narrow range of frequencies, it has less data capacity than the broad range of high-frequency signals coming in. This is shown below (Symmetric versus Asymmetric):



 System Configuration 

At the consumer end, a remote ADSL Transceiver Unit (ATU-R or modem as it has become called) is placed at the consumer's site and is configured as needed to support voice, data and video. If the location is a high-rise building with multiple offices and apartments, or a campus with various data needs, the ATU-R can be equipped with additional functionality such as bridging, routing or multiplexing. 

At the exchange end, a Digital Subscriber Line Access Multiplexer (DSALM) is installed. A single DSLAM can handle and route traffic from multiple ATU-R installations, keeping the cost low because it is shared among all service users. 

The existing telecommunications network then carries the data to the destination, such as a branch office, again going through a DSLAM and ATU-R at the receiving end. This is depicted in the figure below (Point-to-Point ADSL Configuration):

Benefits of Broadband Service 

ADSL unlocks the capacity of existing copper wire infrastructure enabling broadband service on the same line as the telephone without interfering with the telephone signal. There is no need to rewire offices, high-rises, campuses or dormitories because the telephone infrastructure is already in place. 

The NEXTEP Broad band service allows SMEs to download data a speeds up to 6.144 megabits per second (Mbps) compared to the standard modem speed of 56Kbps, and upload data at speeds up to 640Kbps. 

NEXTEP Broadband offers a cost-effective platform for SMEs to build wide-area networks (WANs), provide video conference links between offices, support high-speed email and data transfer, as well as high-speed internet access. 

ADSL technology offers benefits that exceed any other access technology currently available, such as ISDN and T1/E1 lines.

•  Cost effectiveness - Equipment installation and usage costs are far lower
• Speed - DSL provides the fastest data transfer for applications that require intensive resources, such as broadcast-quality bi-directional video conferencing.
• Ease of use - Internet and email access is instantaneous wit no dialing or modem connections. The continual broadband access will support streaming applications such as multicasting for businesses and education.
• Reliability - NEXTEP Broadband is a solely owned division of NEC Australia, one the pioneers of ADSL technologies Australia and a supplier of ADSL equipment to Telstra.
• Competitive - ADSL technology is available today at highly competitive rates.

In summary, NEXTEP Broadband ADSL services provide the opportunity for small and medium-size businesses, TAFEs, universities, government departments, research institutions, providers of internet and datacasting services, high-rise developers and body corporates to establish high-speed dedicated digital networks. 

(This article was written by NEC/Nextep, May 2001)