Cable Modem Technology & Services

Residential Internet and online usage has managed to grow tremendously despite maddeningly slow speeds available through existing dialup telephone modem connections, limited to 53 kbit/s or less. Touted as an interactive extravaganza, surfing the World Wide Web more typically offers narrowband users a click-and-wait experience. The frustration of narrowband online users has spurred demand for higher-speed connections.

In North America, cable companies have emerged as the leading providers of high-speed Internet access.

More than 105 million homes in North America are passed by broadband coaxial cable plant and more than 75 million homes are cable TV subscribers. With near-ubiquitous coverage, coaxial cable connections provide a powerful platform for providing residences and small business with high-speed data access. Cable operators have invested billions to upgrade one-way cable television systems into modern two-way networks to support advanced communications services. By mid-2004, cable operators had upgraded networks serving over 90 percent of North American cable homes to support broadband Internet access.

Cable TV Primer

Cable systems were originally designed to deliver broadcast television signals efficiently to subscribers' homes. To ensure that consumers could obtain cable service with the same TV sets they use to receive over-the-air broadcast TV signals, cable operators recreate a portion of the over-the-air radio frequency (RF) spectrum within a sealed coaxial cable line.

Traditional coaxial cable systems typically operate with 330 MHz or 450 MHz of capacity, whereas modern hybrid fiber/coax (HFC) systems are expanded to 750 MHz or more.

Logically, downstream video programming signals begin around 50 MHz, the equivalent of channel 2 for over-the-air television signals. The 5 to 42 MHz portion of the spectrum is usually reserved for upstream communications from subscribers' homes.

Each standard television channel occupies 6 MHz of RF spectrum. Thus a traditional cable system with 400 MHz of downstream bandwidth can carry the equivalent of 60 analog TV channels, and a modern HFC system with 700 MHz of downstream bandwidth has the capacity for some 110 channels.

Cable Modem Access Networks

To deliver data services over a cable network, one television channel (in the 50 to 750 MHz range) is typically allocated for downstream traffic to homes, and another channel (in the 5 to 42 MHz band) is used to carry upstream signals.

A headend cable modem termination system (CMTS) communicates through these channels with cable modems located in subscriber homes to create a virtual local area network (LAN) connection. Most cable modems are external devices that connect to a personal computer (PC) through a standard 10Base-T Ethernet card or Universal Serial Bus (USB) connection.

The cable modem access network operates at Layer 1 (physical) and Layer 2 (media access control/logical link control) of the Open System Interconnect (OSI) Reference Model. Thus, Layer 3 (network) protocols, such as IP traffic, can be seamlessly delivered over the cable modem platform to end users.

A single downstream 6 MHz television channel may support up to 27 Mbit/s of downstream data throughput from the cable headend using 64 QAM (quadrature amplitude modulation) transmission technology. Speeds can be boosted to 36 Mbit/s using 256 QAM. Upstream channels may deliver 500 kbit/s to 10 Mbit/s from homes using 16QAM or QPSK (quadrature phase shift key) modulation techniques, depending on the amount of spectrum allocated for service. Using 64 QAM, upstream speeds can be raised to 30 Mbit/s. This upstream and downstream bandwidth is shared by the active data subscribers connected to a given cable network segment, typically 500 to 2,000 homes on a modern HFC network.

See the following diagrams section for a visual overview of this architecture:

Figure 1: Cable Data Network Architecture

Figure 2: Home Environment

Figure 3: Distribution Hub

Figure 4: Business Environment

An individual cable modem subscriber may experience access speeds from 500 kbit/s to 3 Mbit/s or more, depending on the network architecture and traffic load – blazing performance compared to dialup alteratives. However, when surfing the Web, performance can be affected by Internet backbone congestion.

In addition to speed, cable modems offer another key benefit: constant connectivity. Because cable modems use connectionless technology, much as in an office LAN, a subscriber's PC is always online with the network. That means there's no need to dial-in to begin a session, so users do not have to worry about receiving busy signals. Additionally, going online does not tie up their telephone line.

A range of vendors are now offering cable modems, including Ambit Broadband , Arris Group Inc. (Nasdaq: ARRS), Cisco Systems Inc. (Nasdaq: CSCO), D-Link Systems Inc. , Linksys , Motorola Inc. (NYSE: MOT), Terayon Communication Systems Inc. , and Thomson Broadband .

Cable Internet Service Delivery

To get into the high-speed Internet services business, cable operators must do more than simply install cable modem gear. Rather, they have built a sophisticated end-to-end IP networking infrastructure in each community they serve that is robust enough to support tens of thousands of data subscribers. That includes items like Internet backbone connectivity, routers, servers, and network management tools, as well as security and billing systems.

Shared Network Platform Performance

Most cable modem systems rely on a shared access platform, much like an office LAN. Because cable modem subscribers share available bandwidth during their sessions, there are concerns that cable modem users will see poor performance as the number of subscribers increases on the network. Common sense dictates that 200 cable data subscribers sharing a 27-Mbit/s connection would each get only about 135 kbit/s of throughput, virtually the same speed as a 128-kbit/s ISDN connection – right? Not necessarily.

Unlike circuit-switched telephone networks where a caller is allocated a dedicated connection, cable modem users do not occupy a fixed amount of bandwidth during their online sessions. Instead, they share the network with other active users and use the network's resources only when they actually send or receive data in quick bursts. So instead of 200 cable online users each being allocated 135 kbit/s, they are able to grab all the bandwidth available during the millisecond they need to download their data packets – up to many megabits per second.

If congestion does begin to occur due to high usage, cable operators have the flexibility to add more bandwidth for data services. A cable operator can simply allocate an additional 6 MHz video channel for high-speed data, doubling the downstream bandwidth available to users. Another option for adding bandwidth is to subdivide the physical cable network by running fiber optic lines deeper into neighborhoods. This reduces the number of homes served by each network segment, and thus, increases the amount of bandwidth available to end users.

Cable Modem Service Availability

After years of technical trials, large cable operators finally began widespread deployments of cable modem services in late 1996. Cable Datacom News publisher Kinetic Strategies Inc. estimates that by mid-2004 cable operators were commercially offering high-speed Internet services to more than 100 million homes in North America and had attracted over 20 million paying subscribers.