Broadband Era: ADSL & Cable Modems

Welcome to the Broadband Era: ADSL & Cable Modems

Introduction

As the Internet spreads rapidly, a lot of people are talking about broadband data communication and WAN (wide area network) due to the fact that people are eager for a faster Internet connection for massive data transfers. The demand for a high-speed Internet connection results in the prosperity of the broadband communication technologies, such as ADSL and cable modem. In this article, we will introduce how broadband modems surpass the traditional modems and the development of broadband technologies.

The Demand for A Higher and Higher Networking Transfer Rate

One of the keys for the higher networking transfer rate demand comes from the ever-growing CPU clock rate. As the CPU clock rate grows, the overall computation capability for PC grows, and so does the demand of data quantity. Especially for applications such as multimedia, professional graphics design, high-fidelity video & audio, and 3D graphics, the demand of data quantity is relatively massive. We have already seen that many PC peripherals such as CDROM & DVDROM drives and AGP graphics cards are driven to run faster by fascinating and delicate multimedia applications. Even the LAN cards have a speed leap from 10Mbps to 100Mbps - a ten-time change.

It looks like everything is going prosperously, right? In fact, it is not. The PC industry was aware that the modem transfer rate has become the bottleneck of multimedia applications over the Internet. What is more terrifying is that the breakout technologies for this bottleneck and system transitions using this new technology are not under the PC companies' control. Instead, these are controlled by the telecommunication companies, which are running at a snail's pace compared with the PC companies.

For thirty years, most of the companies in the PC industry confine the scope of their research and development in a single computer, or, the most, a group of computers within an office. At that time, the data exchange with a remote computer is usually non-real-time. Spending minutes for transmitting a file over a 9,600bps modem is quite regular. As the Internet is widespread, users need a faster connection in order for the enjoyment of the Internet surf. They find that an Internet connection at 56Kbps is no longer satisfactory once they have experienced the 100Mbps connection in the office. Ever since, a modem with a transfer data rate at the level of "Mbps" has been in the dream of computer users.

The Emergence of Broadband Modems

There is no specific speed definition to distinguish a broadband modem from a traditional modem. Generally, the transfer speed of 1.5Mbps is used as the boundary between a broadband modem and a traditional modem. Two possibilities for the 1.5Mbps distinguishing boundary are:

The telecommunication backbones are usually T1 leased line with the speed of 1.544Mbps.
The MPEG-1 requires a minimum transfer rate of 1.5Mbps for compressed video and audio stream deliveries. Therefore, the minimum requirement for a broadband modem is the capability of handling digital video streams. In contrast, traditional modems cannot possibly afford such a high data rate.

To break through the transmission bottleneck and achieve a higher transfer rate at the level of "Mbps," the restrictions imposed by the design of the traditional telephone system should be overcome first. Twisted pair copper wire is the transmission medium adopted for traditional telephone exchange. The traditional modem's 56Kbps transmission rate is the extreme over the copper wire's 4KHz bandwidth. The ISDN exchanges have improved the transmission rate to 128Kbps by expending the bandwidth to 80KHz. Obviously, to make the transmission rate higher, the newly imposed technology should be able to expend the bandwidth further.

One important but frequently neglected factor is the "bandwidth usage efficiency," that is, the ratio of transmission rate (bps) and available bandwidth (Hz). Over the past twenty years, engineers for traditional modems have been puzzled over the improvement of the bandwidth usage efficiency. These efforts have made the transmission rate grown from 300bps to 56Kbps, and achieved the bandwidth usage efficiency of 56K/4K=14. The keys that cause this improvement are modulation and line coding technologies. In comparison to ISDN's 128K/80K=1.6 bandwidth usage efficiency, the transmission over the traditional telephone system is indeed fully explored.

The Era of ADSL Has Arrived

By taking the aforementioned improving directions, the ADSL technologies have made the available bandwidth over the twisted pair copper wire to be 1.1MHz, and the transmission rate to be 16.5Mbps, i.e., a bandwidth usage efficiency of 15. However, the commercialized ADSL can only provide a maximum transmission rate of 8.8Mbps since the copper wires are with different qualities in different countries. The bandwidth usage efficiency of ADSL is unprecedented high, thanks to the line coding method - DMT (discrete multi tone).

The ADSL technology has been developed for more than ten years. In 1989, Bellcore proposed the concept of ADSL. However, this proposal did not come to the market at that time because of the complexity and high cost of the technology. For video applications (e.g., video on demand) that require a high bandwidth, ADSL still cannot be applied because of the restriction of the telecommunication environment.

In 1993, due to the invention of DMT line code and the growth of the Internet, the ADSL technology was re-born. In 1994, the ADSL Forum committed itself to standardization and commercialization of ADSL. In 1995, ANSI (American National Standards Institute) adopted ADSL as T1.413, issue 1. Afterwards, in 1996, some prototype ADSL machines were running in ANSI for some experiments.

In the meanwhile, the PC industry again faced the bandwidth demand challenge for the Internet. Not waiting any longer for the ADSL experiments, Intel, Compaq, IBM, etc. formed UAWG (Universal ADSL Working Group) to commercialize ADSL.

To accelerate the commercialization, some features of the original ADSL were either reduced or simplified. The following are two of the major changes:

Lower the transmission rate to 1.5Mbps for lower system costs and prolong the transmission distance.
Remove the splitters that split the high-frequency spectrum and the low-frequency spectrum, which are used by ADSL modems and telephone sets, respectively. Instead, a software approach to avoid interference is taken. This simplifies the installation complexity.

In 1998, ITU (International Telecommunication Union) adopted two ADSL standards: G.992.1 (a.k.a. G.dmt) and G.992.2 (a.k.a. G.lite). The latter is the aforementioned 1.5Mbps version of ADSL. Meanwhile, ANSI updated the T1.413 to issue 2. Because so many standards coexist, ITU defined G.994 (a.k.a. G.hs) for the handshaking among those standards. At this point, the task that remained is the interoperability among the products from different ADSL manufacturers.

The Interoperability Among the Products From Different ADSL Manufacturers

For tests of the interoperability among the products from different ADSL manufacturers, New Hampshire University established an interoperability laboratory (a.k.a. NHUIOL) for this purpose. The majority of the original telephone exchange companies (e.g., Alcatel, Cisco, Notel, Ericsson, NEC, and Nokia) manufacture ADSL exchanges (a.k.a. DSLAM, which is referred to as digital subscriber line access multiplexer) also. And, many of the original manufacturers for traditional modems, networking devices, and computers also devoted themselves to the production of client ADSL modems.

Similar to traditional modems, ADSL modems is also designed to have internal and external models. The internal ADSL modem is usually based on the PCI bus for one-person use only. It usually has a cheaper price and therefore a reduced set of functions. The external ADSL modem usually has an Ethernet interface that can be connected to the computer side. The Ethernet interface can be connected to a hub and forms a network gateway.

ATM (asynchronous transfer mode) is noticeable. ATM is a high-speed link layer protocol, which is applicable to a network at a speed ranging from several Mbps to hundreds Mbps. ATM is often regarded as the future star in the field of networking. Before the ADSL technology, fiber is the only medium that can carry such a high data rate. However, fiber to home is still impractical at this moment. Therefore, ADSL becomes the best candidate to cooperate with ATM protocol and forms a high-speed WAN.

Recently, Mocrosoft has been very aggressive in the ADSL/ATM software development. In Windows2000, NDIS 5.0 (network driver interface specification) is used to communicate with the ATM driver to drive the ADSL modem. NDIS can also talk upwards to Windows Socket 2.0 through TCP/IP, or makes a phone call by the use of TAPI.

The Rise of Cable Modem

At this point, please get back to the technology side and think: what else can be improved besides the bandwidth and bandwidth usage efficiency? Can we give up the twisted pair copper wire and use another transmission medium? Is there another transmission medium that can provide more bandwidth and a longer transmission distance? Is the new medium widespread enough?

The twisted pair copper wire network is quite dense because of the telecommunication construction over the past hundreds of years. Besides, coaxial cable network of the cable TV system, fiber network, power supply network, and wireless communication have been considered as the replacement as well. Fiber is without doubt the best candidate but not widespread enough. Power supply network and wireless cannot combine both a longer transmission distance and a higher transfer rate. Therefore, cable modems that operate on the coaxial cable network of the cable TV system become the only competitor of ADSL modems.

Some countries have the same density for their cable network as telecommunication network. At the time that the copper wire operated at the bandwidth of 200KHz, the coaxial cable already has a surprising bandwidth of 500MHz for video transmission. Everywhere in the Internet is full of business change. Of course, the cable TV companies will not miss any chance to get involved in the Internet business. Similar to ADSL's skip of the frequency range of the telephone service to ensure the backward compatibility, cable modem also uses the non-occupied channels for data service.

QAM (quadrature amplitude modulation) is the modulation technology for the cable modem system. Depending on the quality of the channel, each 6MHz-wide NTSC channel can provide a data channel at the rate ranging from 30Mbps to 40Mbps. Within the currently used 550MHz to 860MHz frequency range that the cable modem can use, a coaxial cable is capable of providing a rate of 1530Mbps or 2040Mbps for data transfer. In contrast to an ADSL modem that uses a dedicated channel for the connection with a DSLAM modem, several client modems can be connected to one single head-end modem in a cable network. The shortcoming for the cable network is the network complexity and data security. On the opposite side, the cable network has the benefits of better flexibility and resource sharing.

The cable modem technology has been developed for more than ten years. At the prime, there was no standard specification for cable modem interoperation. Therefore, manufacturers had to produce both head-end and client modems by themselves in order for the interoperability. In 1994, a team was formed to construct the IEEE802.14 standard. However, the developing progress of IEEE802.14 was very slow. In 1996, DOCSIS (data-over-cable service interface specifications) was proposed by MCNS (Multimedia Cable Network System Partners LTD, a subsidiary unit from CableLabs), which was formed by the cable TV companies. After the release of DOCSIS, cable modem was finally standardized. CableLabs also conducted a series of interoperability tests for cable modems. This reveals CableLabs' determination of commercializing cable modems and makes the market share of cable modems higher than ADSL modems. In DOCSIS 1.1, VoIP (voice over IP) is enclosed. This announces that cable can not only be used for the carry of data and TV signals, but also for telephone services. This threatens the telecommunication companies greatly.

Being different from ADSL's ATM, cable modem's DOCSIS uses MPEG-2 packets for Ethernet packet enclosure. This is a preparation step for the future integration of the cable modem and the set-top box. With Ethernet packets enclosed, there is no need for a LANE (LAN emulation) layer for the TCP/IP connection.

The Competition of Worldwide Manufacturers

At this moment when the ADSL modem and the cable modem are still competing, the manufacturers are not willing to give up on any of them. Therefore, the manufacturers have invested on both technologies evenly. The leading DSP company - TI, and the traditional modem chip vendor - Conexant, both have R&D teams for ADSL and cable modem chips. The networking giant - Cisco, has taken the leading position in providing ADSL DSLAMs and cable modem head-ends. In Taiwan, modem manufacturers and networking device vendors have already devoted themselves to the manufacturing of both ADSL and cable modems. All the above drives the broadband era to arrive faster.

ASUSTeK Computer, Inc. deployed a new R&D team for the development of broadband modems in 1999. Inheriting the nature of high quality and superior technology, ASUSTeK unveiled the broadband modem products - ADSL & cable modems to the public in late 1999. We hope that all the efforts will accelerate the coming of the broadband era and make people enjoy the high-speed Internet surfing.