CABLE MODEMS
For cable operators, home networking is an opportunity to increase the use of broadband data and video services. Historically, cable companies were in the business of offering traditional TV services to their customer base. These companies are moving into the telecommunications sector to offer a variety of services, including high-speed Internet access, free e-mail accounts, hosting of e-commerce sites, and delivering advanced IP-based applications. If you want your home network to interface with these services, you need to buy or rent a home gateway device called a cable modem. The number of North American cable modem subscribers surged to 1.8 million in 1999 and is expected to double by the end of 2000, according to Cahners In-Stat Group. The high-tech market research firm also forecasts that there will be more than 9.5 million broadband cable data subscribers worldwide by the end of 2002. In-Stat's research also found:
More than 110 million homes in North America are passed by a broadband coaxial cable line and more than 77 million of those homes currently subscribe to cable TV services.
Cable broadband service revenues will increase from $1 billion to almost $4 billion by the end of 2002.
From a technical perspective, a cable modem is a digital modem that uses a coaxial cable connection. Cable modems operate over two-way hybrid fiber and coaxial lines. Some cable operators that have not upgraded their networks to provide full bidirectional services use their hybrid fiber coaxial (HFC) network for high-speed downstream data transfer and use the local telephone network for return path data. A standard cable modem will have two connections; one port is connected to the TV outlet on the wall and the other to the subscriber's PC. The cable modem will then communicate over the cable network to a device called a CMTS (Cable Modem Termination System). The speed of the cable modem depends on traffic levels and the overall network architecture. Theoretically speaking, cable modems are capable of receiving and processing multimedia content at 30 Mbps, literally hundreds of times faster than a normal telephone connection to the Internet. However, in reality subscribers can expect to download information at between 0.5 and 1.5 Mbps because the bandwidth is shared by a number of home networks throughout the neighborhood. In regard to frequency ranges, operators define a portion of the frequency spectrum to carry the data. In general, the downstream path (head-end to home network) will lie between 50 MHz and 750 MHz. The frequency range can vary from network to network and from country to country. The frequency range 5 to 42 MHz is used when transmitting information from the in-home network to the head-end. Cable modems are primarily used for receiving and transmitting TCP/IP traffic including multimedia content and Web access data. A major benefit of a cable modem home gateway is the fact that the connection is always open, so you never have to go through the slow procedure of establishing a dial-up connection. In some cases, this can take up to a minute, which is very frustrating.
When using a cable modem as a home gateway, a couple of installation issues need to be considered. First and foremost, it is well documented that the in-home coaxial wiring system is the major source of electromagnetic interference. To reduce noise levels, it may be necessary to install a new piece of coaxial cable. Also of concern to home network users is the presence of various terminals such as TV sets, VCRs, and radios. Because these devices can generate interference, which could affect the performance of a home gateway, it may be necessary to identify ways of isolating the cable modem from other electrical devices in the house. Today, the provisioning of a cable modem service in the home may require a technician to install the new hardware and configure the TCP/IP protocol stack. The service provider will also need to configure the servers that run the applications (Web hosting, chat, e-mail, etc.). This whole process is expensive and very time consuming. Consequently, the strategy for most service providers is to develop cable modem technology to allow users to provision the service themselves. Once a cable modem is connected to the HFC network, users would be presented with a registration Web site where they can select or confirm specific service plans. The elimination of the need for a cable technician is unlikely to happen in the short term. However, as people become more familiar with home networking technologies and manufacturers begin selling computers with internal cable modems, the need to send a technician to a home will decrease. The security of cable modems is also an issue for users of home networks. An intruder could, in theory, tap into the coaxial cable in a HFC network and capture packets of data that have been transmitted by devices on the network. So a cable modem used to transmit confidential information needs to make life difficult for the "cable hackers" by using encryption technologies.
There are many cable modem brands. The most common are from Com21, Motorola, Bay Networks, RCA, Cisco, Toshiba, 3Com, and Terayon. For more information on cable modem manufacturers, visit http://www.catv.org.
How a Cable Modem Works
Cable modems are different from standard modems that perform a "handshake"—an agreement on how to transmit data—with a compatible modem outside of the provider's network. Cable modems need to handshake with modems located at the cable operator site. Complex signalling, use of frequencies, and authentication need to be agreed on between the two modems. Modems located at the operator's site provide the following functionality:
Equalization to compensate for signal distortion
Address filtering so that the modem only accepts messages intended for the correct recipient
Transmitting and receiving functions
Automatic power adjustments to compensate for power fluctuations
Adjustments in amplitude (signal strength or wave height) due to temperature changes
Figure 9.1. Architecture of a typical cable modem
Modulation of the signal (i.e., analog to digital conversions and vice versa)
Compensation for delays caused by variable distances
From Figure 9.1, we can see that the tuner in the cable modem receives a digital signal from the network and isolates a particular channel that contains the Internet data. It then converts the signal from RF levels back to baseband.
The baseband output signal from the tuner is then forwarded to a demodulator. The function of the demodulator is to sample the signal and convert it to a digital bitstream. The bit-stream contains video, audio, and IP data. Once the bitstream has been recovered, it is forwarded to the forward error correction unit and checked for problems. The signal is then passed to the control unit.
The control unit comprises the following subsystems:
Boot loader to ensure that software updates are completed in a secure manner.
Read-only memory to store firmware and Media Access Control addresses.
RAM used as a temporary storage area for data flowing between the processor and the various hardware components.
A processor to manage all devices in the control unit and execute program instructions. Note that the cable modem uses a 32-bit bus to carry information between the various control unit components.
From the control unit, data is passed on to your home network using one of the following high-speed data port interfaces:
Universal Serial Bus
ATM
IEEE 1394
10 Base-T
In regard to sending data from devices on your in-home network to the Internet, the signal is again processed by the control unit. It then needs to be error-checked and processed in compliance with international standards.
The data then needs to be converted into a quadrature phase-shift keying (QPSK) format using a modulator. Once the signal has been modulated, it is passed to the transmitter component and interfaced with the HFC network using the diplex. The diplex unit has the main task of separating upstream and downstream informational flows.
Bridging and Routing
In addition to simply passing data between your home network and the operator's HFC network, a cable modem gateway device needs to also be capable of bridging and routing information.
In the case where a cable modem behaves as a bridge between an in-home system and the outside broadband world, it must ensure that local traffic is not transmitted on to the broadband network. To support bridging functionality, the cable modem needs to learn the MAC addresses of all the devices that are connected to its high-speed data port interface. Once the modem knows the MAC addresses of all the devices, traffic can be filtered according to location. For instance, if a file is sent from your desktop computer to a printer, the cable modem will prevent this file from been broadcast to the broadband network. Security conscious users may want to configure their modems to behave like routers. The primary functions of a gateway router are as follows:
Relaying Dynamic Host Configuration Protocol (DHCP) messages between both networks, if the DHCP server on the broadband network is assigning addresses to hosts on a LAN.
Making decisions of where to direct TCP/IP traffic.
Restricting access from all other users connected to the cable operator's broadband network.
Standards
If consumers purchase cable modems from retailers, they need to be certain that the modem is compatible with their provider's modem. To this end, the cable industry has set standards for cable modems.
If you live in North America, most of the cable modems will be compliant with a standard called Data Over Cable Service Interface Specification (DOCSIS). If you live in Europe, Asia, or Australia, you will more than likely be purchasing a gateway device that is compliant with a specification called EuroModem.
Let's take a closer look at technical features of both of these standards.
DOCSIS
In the United States, a consortium called MCNS (Multimedia Cable Network System) has developed a set of technical documents for bidirectional communications over a cable network called DOCSIS (Data Over Cable Service Interface Specification). This group comprises the four leading cable television operators: Comcast Cable Communications Inc.; AT&T; Cox Communications Tele-Communications, Inc.; and Time Warner Cable. These companies have partnered with Rogers Cablesystems Limited, Continental Cablevision, and Cable Television Laboratories, Inc. to implement the project. A number of large networking companies have built products based on the DOCSIS standard that are commercially deployed around the world. The main focus of the DOCSIS standard is on offering Internet services to PC users. Today, most new cable modem deployments throughout the world are DOCSIS-compliant networks.
EuroModem
The EuroModem specification was developed to allow cable operators to deliver Internet access and timing-critical services, such as voice telephony and video conferencing services, to their subscribers. The EuroModem specification describes a cable modem solution that fits very well into European cable networks.
Members of an organization called EuroCableLabs have developed the EuroModem specification. The cable operators who are involved in the consortium are from various European countries. The consortium has defined two different types of modems: Class A modems are capable of transmitting data at very high speeds in a downstream direction (maximum of 50.8 Mbps) and 3 Mbps in the upstream direction. They are capable of accessing the Internet at high speeds and support a number of security technologies. Class B is the second type of modem being considered by the group. It extends the functionality of class A devices through the support of time-critical services such as video conferencing and telephony. For a more detailed description of the EuroModem home gateway, we suggest that you consult the EuroCableLabs Internet draft repository site: http://ifn03.ifn.ing.tu-bs.de/ecl/EuroModem.html.
Cable Modem Service Providers
There are several popular cable modem providers, but the big three are:
Excite@Home
Chello
Road Runner
Excite@Home
Excite@Home is a fast Internet service for home networks. With Excite@Home, users can access the Internet at speeds up to 100 times faster than a 28.8 Kbps telephone modem. Excite@Home leverages the high-bandwidth capabilities of cable TV lines, as well as a hierarchical, distributed network architecture with proprietary caching and replication technologies to overcome many of the bottlenecks that plague typical Internet connections today. Excite@Home has affiliate partnerships with 16 leading cable companies in the United States and Canada, including AT&T Broadband and Internet Services, Bresnan Communications, Cablevision Systems, Century Communications, Charter Communications, Cogeco Cable, Comcast, Cox Communications, Garden State Cable, Insight Communications, InterMedia Partners, Jones Intercable, Midcontinent Cable, Prime Cable, Rogers Cablesystems, Shaw Communications, Suburban, and Videon CableSystems—access to over 60 million households. The massive demand for the Excite@Home service has not been limited to the North American market. The company's international division has been creating and marketing Excite@Home services around the globe. At the time of writing, Excite@Home had become the largest broadband provider in the world with the announcement that they had topped a million customers.
Chello
Chello is Europe's first and leading broadband Internet service provider and is commercially active in six countries. It has the largest reach in Europe, delivering next-generation, always-on interactive services and broadband content to consumers via a state-of-the-art network—without huge phone bills. It also provides a turnkey service to international cable network operators. Chello is an operating company of United Pan-Europe Communications (UPC), headquartered in Amsterdam. Chello has developed its own IP backbone, called Aorta, with nodes in Amsterdam, Brussels, California, Chile, Frankfurt, London, Miami, New York, Oslo, Paris, Stockholm, and Vienna, and satellite links to Australia and New Zealand. To maintain its position as Europe's fastest-growing and largest broadband network, Chello has established strategic alliances with the following companies: Inktomi, Terayon, Sun Microsystems, Microsoft, Cisco Systems, GTS Carrier Services, and Nortel Networks. At the time of going to press, Chello had announced that 106,500 subscribers had signed up for their high-speed data offerings.
Road Runner
Road Runner is currently Excite@Home's biggest rival. Road Runner is a high-speed online service delivered to the PC over the cable television infrastructure. By employing the latest advances in Web technology and leveraging the speed made possible by cable's HFC network, Road Runner integrates multimedia programming with communication tools and personalized services such as e-mail, chat listings, and personal home pages—all within a broadband environment. Road Runner is a joint venture among affiliates of Time Warner, MediaOne, Microsoft, Compaq, and Advance/Newhouse. The Road Runner IP infrastructure is an end-to-end client-server-enabling network. It transports multimedia applications to the PC based on TCP/IP and related technologies. The foundation of this infrastructure lies in the existing cable television network, upgraded with fiber optics. The networking model assigns significant operating and technical responsibilities to the technical staff. The company owns, operates, and maintains the regional network and all of the IP infrastructure beyond the distribution hub, while the cable operator maintains and operates the HFC plant and the cable modem. This structure creates a hierarchy that separates functions and keeps protocols isolated, providing a manageable architecture that will facilitate the integration of ever-evolving new technologies into Road-Runner-based systems.