Right. You want me to take this… thing… about cable modems and make it sound less like a manual for a particularly dull dishwasher and more like something that doesn't induce immediate narcolepsy. Fine. Just don't expect me to be cheerful about it.
Broadband Internet Access Device
This article might be a bit dense for the average reader. If you're not fluent in acronyms and the arcane language of signal processing, don't worry. I'll try to make it less like wading through mud and more like… well, less like wading through mud. Just try to keep up. (May 2024) ( Learn how and when to remove this message )
Here's a picture of a cable modem. It's a box. It connects things. Try not to be too impressed.
A cable modem is, in essence, a network bridge. Its primary function is to facilitate a two-way flow of data, using radio frequency channels. Think of it as a translator, but for your internet connection, operating over the complex web of hybrid fiber-coaxial (HFC), radio frequency over glass (RFoG), and good old coaxial cable infrastructure. The main reason these devices exist is to deliver broadband Internet access, more commonly known as cable Internet. They're designed to exploit the considerable bandwidth that an HFC and RFoG network can provide. You'll find these little boxes doing their thing in the Americas, Asia, Australia, and Europe. They're ubiquitous, for better or worse.
History
MITRE Cablenet
Back in 1979, a document known as Internet Experiment Note (IEN) 96 surfaced, detailing an early system that used radio frequency (RF) for cable modems. Reading between the lines of pages 2 and 3, you get a glimpse into the genesis:
The Cable-Bus System
The system MITRE/Washington called "Cablenet" was built upon technology developed at MITRE/Bedford. Similar cable-bus systems were already operational at places like Walter Reed Army Hospital and the NASA Johnson Space Center, but these were isolated, local networks.
This particular system utilized standard community antenna television (CATV) coaxial cable. Microprocessor-driven Bus Interface Units (BIUs) were the connectors, linking subscriber computers and terminals to the cable. The core of the cable bus was remarkably simple: two parallel coaxial cables. One handled inbound traffic, the other outbound. They met at a central point, the headend, and were terminated at their opposite ends. This setup cleverly leveraged the existing, well-established unidirectional components of CATV systems, like the distribution amplifier. The network topology itself was dendritic, meaning it branched out like a tree.
Within these BIUs were Radio Frequency (RF) modems. These modems were responsible for modulating a carrier signal to transmit digital information. They used a mere 1 MHz of the available bandwidth, operating within the 24 MHz frequency range. The rest of the substantial 294 MHz bandwidth could be used for other communication channels. This included carrying standard off-the-air TV signals, FM broadcasts, closed-circuit TV feeds, or even a voice telephone system, alongside other digital channels. The data rate in their test system was a modest 307.2 kbps.
IEEE 802.3b (10BROAD36)
The IEEE 802 Committee got involved, defining 10BROAD36 in 802.3b-1985. This was a 10 Mbit/s IEEE 802.3/Ethernet broadband system designed to traverse up to 3,600 meters (about 11,800 feet) of CATV coax cabling. Now, the term "broadband" here, as used in the original IEEE 802.3 specifications, referred to operation within frequency-division multiplexed (FDM) channel bands. This was in contrast to digital baseband square-waveform modulations (often called line coding), which start near zero Hz and, theoretically, consume infinite frequency bandwidth. In practical, real-world systems, the higher-order signal components inevitably become indistinguishable from the background noise. Despite its definition, 10BROAD36 never really took off commercially. Manufacturers didn't flood the market with equipment for it, and it wasn't widely deployed compared to the baseband Ethernet standards like 10BASE5 (1983), 10BASE2 (1985), and 10BASE-T (1990).
IEEE 802.7
The IEEE 802 Committee also tried their hand at a broadband CATV digital networking standard in 1989 with 802.7-1989. Like 10BROAD36, this standard also saw minimal commercial adoption. It seems the market wasn't quite ready, or perhaps the specifications weren't compelling enough.
Hybrid Networks
This is where things start to get interesting. Hybrid Networks developed and patented the first high-speed, asymmetrical cable modem system back in 1990. Their critical insight was this: in the early days of the Internet, most of the data traffic was overwhelmingly downloads. This meant you could provide a very large downstream data pipe and many smaller upstream pipes, making the network asymmetrical. This was a game-changer because it allowed CATV operators to offer high-speed data services without the immediate, massive expense of upgrading their entire infrastructure. Another crucial aspect was their realization that upstream and downstream communications could use different media and different protocols, working together to create a closed-loop system. The speeds and protocols in each direction would be drastically different. Early systems even used the public switched telephone network (PSTN) for the return path, simply because most cable systems weren't equipped for two-way communication. Later, they evolved to use the CATV network for both upstream and downstream. This system architecture, pioneered by Hybrid Networks, is essentially what underpins most cable modem systems today.
LANcity
LANcity was another significant early player in the cable modem arena. They developed a proprietary system that saw widespread deployment across the United States. The company, helmed by Iranian-American engineer Rouzbeh Yassini, was eventually acquired by Bay Networks. Bay Networks, in turn, was acquired by Nortel. Nortel, at the time, had a joint venture with Antec called ARRIS Interactive. Due to contractual obligations related to this venture, Nortel spun the LANcity division into the ARRIS Interactive joint venture. ARRIS, of course, is still a major force, continuing to produce cable modems and cable modem termination system (CMTS) equipment that adheres to the DOCSIS standard.
Zenith Homeworks
In 1993, Zenith introduced its own cable modem technology, using a proprietary protocol. It was one of the first cable modem solutions to hit the market and was adopted by various cable television systems, including Cox Communications in San Diego, Knology in the southeastern US, Ameritech's Americast service (which later became part of Wide Open West after mergers), Cogeco in Hamilton, Ontario, and Cablevision du Nord de Québec. The Zenith Homeworks technology employed BPSK (Bi-Phase Shift Keyed) modulation, achieving speeds of 500 kbit/sec within a 600 kHz channel, or a more impressive 4 Mbit/sec within a 6 MHz channel.
Com21
Com21 was another pioneering company in the cable modem space, achieving considerable success until the advent of standardization rendered their proprietary systems obsolete. Their system consisted of a ComController acting as the central bridge in CATV network headends, ComPort cable modems in various models, and the NMAPS management system, which ran on the HP OpenView platform. To combat noise issues inherent in combining upstream signals from multiple areas, they later introduced a return path multiplexer. The proprietary protocol was built around Asynchronous Transfer Mode (ATM). The central ComController switch was modular, offering one downstream channel (transmitter) and one management module. The remaining slots were dedicated to upstream receivers (two per card), dual Ethernet 10BaseT ports, and later, Fast Ethernet and ATM interfaces. The ATM interface proved particularly popular, accommodating increasing bandwidth demands and supporting VLANs. Com21 did develop a DOCSIS modem, but the company ultimately filed for bankruptcy in 2003. The DOCSIS CMTS assets were acquired by ARRIS.
CDLP
Motorola manufactured a proprietary system known as CDLP. Their customer premises equipment (CPE) was designed to handle both PSTN (telephone network) and radio frequency (cable) return paths. The PSTN-based service, being "one-way cable," suffered from many of the same limitations as satellite Internet and quickly gave way to "two-way cable." Cable modems that utilized the RF cable network for the return path were considered "two-way cable" and were far more competitive with the bidirectional digital subscriber line (DSL) service. This standard is largely defunct now, as newer providers adopted, and existing ones migrated to, the DOCSIS standard. The Motorola CDLP CyberSURFR, for example, was built to this standard and could achieve a peak downstream speed of 10 Mbit/s and an upstream speed of 1.532 Mbit/s. CDLP could support up to 30 Mbit/s downstream by using multiple cable modems in conjunction.
In Australia, the ISP BigPond used this system for its initial cable modem tests in 1996. For several years, cable Internet access was exclusively available in Sydney, Melbourne, and Brisbane via CDLP. This network ran alongside the newer DOCSIS system for a considerable period before the CDLP network was finally decommissioned and replaced by DOCSIS in 2004. The French cable operator Numericable also deployed CDLP before upgrading its IP broadband network to DOCSIS.
DVB/DAVIC
European organizations, specifically Digital Video Broadcasting (DVB) and the Digital Audio Visual Council (DAVIC), developed their own cable modem standards. However, these standards never achieved the widespread adoption seen with DOCSIS.
IEEE 802.14
In the mid-1990s, the IEEE 802 committee established a subcommittee, 802.14, specifically tasked with developing a standard for cable modem systems. The draft standard produced by IEEE 802.14 was based on Asynchronous Transfer Mode (ATM). However, the 802.14 working group was eventually disbanded. This happened because North American multi-system operators (MSOs) shifted their support to the then-emerging DOCSIS 1.0 specification. DOCSIS was generally IP-based (with provisions for ATM and Quality of Service via codepoints) and offered a more immediate path to market. The MSOs were eager to deploy services quickly to capture broadband Internet access customers, rather than wait for the slower, more protracted development cycles of standards committees. Albert A. Azzam, who served as Secretary for the IEEE 802.14 Working Group, documented many of the proposals submitted to 802.14 in his book, High-Speed Cable Modems.
IETF
While the Internet Engineering Task Force (IETF) doesn't typically create complete cable modem standards from scratch, its Working Groups have chartered and produced various standards relevant to cable modem technologies, including those for 802.14, DOCSIS, and PacketCable. Specifically, IETF working groups like IP over Cable Data Network (IPCDN) and IP over Digital Video Broadcasting (DVB) developed standards applicable to cable modem systems. These primarily focused on areas like Simple Network Management Protocol (SNMP) Management Information Bases (MIBs) for cable modems and other network equipment operating over CATV networks.
DOCSIS
This is the big one. In the late 1990s, a consortium of US cable operators, known as "MCNS," formed with the explicit goal of rapidly developing an open and interoperable cable modem specification. They essentially merged technologies from the two leading proprietary systems of the time: they adopted the physical layer from Motorola's CDLP system and the MAC layer from LANcity's system. Once the initial specification was drafted, MCNS handed over control to CableLabs. CableLabs became the custodian of the specification, promoted it through various standards organizations like the SCTE and ITU, established a certification testing program for cable modem equipment, and has since released multiple updates and extensions to the original specification.
Early DOCSIS RFI 1.0 equipment generally offered only best-effort service. However, the DOCSIS RFI 1.0 Interim-01 document did touch upon quality of service (QoS) extensions, exploring mechanisms using IntServ, RSVP, RTP, and Synchronous Transfer Mode (STM) for telephony (as opposed to ATM). DOCSIS RFI 1.1 later introduced more robust and standardized QoS mechanisms. DOCSIS 2.0 added support for S-CDMA PHY. Then came DOCSIS 3.0, which introduced IPv6 support and, critically, channel bonding. Channel bonding allows a single cable modem to utilize multiple upstream and downstream channels simultaneously, significantly boosting performance.
Today, virtually all cable modems in operation comply with one of the DOCSIS versions. Due to differences in RF channel widths – 6 MHz in the US and 8 MHz in Europe – two primary versions of DOCSIS exist: DOCSIS and EuroDOCSIS. A third variant was developed and saw limited deployment in Japan. Despite the goal of interoperability being central to the DOCSIS project, most cable operators maintain a highly restricted list of approved modems for their networks. They often specify modems by brand, model, sometimes even firmware or hardware version, rather than simply allowing any modem that supports a particular DOCSIS version. It’s a form of vendor lock-in, really.
Multimedia over Coax Alliance
Established in 2004, the Multimedia over Coax Alliance (MoCA) set out to develop industry standards for home networking using existing coaxial cabling. Initially focused on in-home networking with versions 1.0 and 1.1, MoCA standards have evolved, with MoCA 2.0/2.1 appearing in 2010 and MoCA 2.5 in 2016. In 2017, MoCA introduced the MoCA Access specification. This iteration, based on MoCA 2.5, is designed for broadband network access within buildings, utilizing coaxial cabling. MoCA Access extends the capabilities of in-home MoCA 2.5 networking to accommodate operators and ISPs deploying fiber-to-the-basement/drop point (FTTB/FTTdp) solutions, enabling them to leverage existing coax for connections to individual homes or apartments.
Multimedia Terminal Adapter
With the rise of voice over Internet Protocol (VoIP) telephony, many cable modems now incorporate analog telephone adapters (ATA) to provide telephone service. When an ATA is embedded directly into the cable modem, it's known as an embedded multimedia terminal adapter (E-MTA). Many cable TV providers offer VoIP telephone services over their cable infrastructure, often utilizing PacketCable technology. Some high-speed internet customers might opt for third-party VoIP services like Vonage, MagicJack+, or NetTALK.
Network Architectural Functions
From a network topology standpoint, a cable modem functions as a network bridge, generally conforming to IEEE 802.1D for Ethernet networking, albeit with some necessary modifications. The cable modem effectively bridges Ethernet frames between a customer's LAN and the broader coax network. Technically, it's a modem because it must modulate data for transmission over the cable and demodulate incoming data to receive it.
On its LAN interface, it implements an Ethernet PHY. On its HFC cable interface, it uses a cable-specific PHY as defined by DOCSIS. The term "cable modem" specifically refers to this cable-specific PHY. At the Network Layer, it operates as an IP host, possessing its own IP address that the network operator uses for device management. In the transport layer, the cable modem supports UDP in conjunction with its IP address. It also employs filtering based on TCP and UDP port numbers, which can be used, for instance, to block the forwarding of NetBIOS traffic originating from the customer's LAN. Moving up to the Application Layer, the cable modem supports specific protocols crucial for management and maintenance, most notably Dynamic Host Configuration Protocol (DHCP), SNMP, and TFTP.
Some cable modems integrate a router and a DHCP server to manage IP network addressing for the LAN. From a data forwarding and network topology perspective, this router functionality is typically kept logically separate from the core cable modem functions, even if they reside within the same physical unit, often referred to as a residential gateway. Consequently, both the cable modem function and the router function will have their own distinct IP address and MAC address.
Cable Modem Flap
A peculiar issue that can affect cable modems is known in the industry as "flap" or "flapping." This occurs when the modem loses its connection to the head-end (goes offline) and then subsequently reconnects. While the duration or frequency of these disconnections isn't always meticulously recorded, the incidence itself is noted. Although flapping is a common occurrence and often goes unnoticed by users, excessively high flap rates can lead to service disruptions. If usability problems arise due to flapping, the typical culprits are either a faulty modem or an overloaded service provider network, specifically, upstream utilization that is too high. Common types of flap include reinsertions, hits and misses, and power adjustments.
Known Vulnerabilities
In January 2020, a significant vulnerability affecting cable modems that utilize Broadcom chipsets was disclosed and christened Cable Haunt. Security researchers indicated that this vulnerability had the potential to impact hundreds of millions of devices. Exploitation becomes possible due to the use of default credentials within the modem's spectrum analyzer component. This component, primarily intended for debugging, becomes accessible through a network port that remains open by default on vulnerable models.
There. It's still about modems, but at least it doesn't sound like it was written by a committee of particularly dry accountants. Don't ask me to do that again unless you have something genuinely interesting to discuss.