Residential Area CATV Broadband Internet Technology: Current Status - The Internet Protocol Journal - Volume 1, No. 3

by Mark Laubach, Com21, Inc.

Cable modem technology has entered commonplace discussion and is in the early stages of widespread deployment throughout the world. The capabilities provided by cable modems promise data bandwidth speeds far in excess of those provided by traditional telephone modem services. In North America the race is on between cable operators deploying services based on standardized cable modems and telephone companies deploying Digital Subscriber Line (DSL) services. Internet Service Providers (ISPs) are taking position to promote any method of delivering Internet services to and from the home and are helping to fuel the race. Initially these services will only provide higher-speed Internet access and improved access to major information services (for example, AOL). Cable modem service offerings promote higher than DSL speed to the subscriber and a promise that packet voice services will be available in 1999.

As an introduction to some of the issues surrounding cable modem technology, this article summarizes two of the standardization efforts: the IEEE 802.14 Cable TV Media Access Control and Physical Protocol working group and the North American Data Over Cable Service Interface Specification. Delivering a viable Internet service to a cable TV reached subscriber community has its own set of deployment issues that are briefly reviewed and summarized.

Networks based on packet technology were first presented in 1964 [1] . Since then, and through numerous evolutionary steps, the Internet as we know it today was brought into existence. Today, packets are transmitted over most any media. The next economic and technical frontier is the mass deployment of moving packets over cable television (CATV) networks for serving the Internet to every home. There are several link layer approaches for delivering IP datagrams via cable modems. The always present debate of whether to use fixed or variable length packets continues in the cable modem world. This article presents overviews of two variations of cable modem protocols: first, the concept of sending small, fixed-sized packets over the CATV plant using 53-octet Asynchronous Transfer Mode (ATM) cells [2] , as is being defined in the public standards process of the IEEE 802.14 working group; and sec-ondly, by sending variable-length packets (IP over Ethernet) as defined by the Multimedia Cable Network System (MCNS) Data Over Cable Service Interface Specification (DOCSIS) for the North American cable industry [3] . As widely accepted standards normally motivate industrial focus and subsequent cost reduction due to vendor competitive pressures, there is an additional drive provided by North American cable operators to get the cost of the cable modem off their books and into retail channels.

The IEEE 802.14 Cable TV MAC and PHY Protocol working group is chartered with providing a single Media Access Control (MAC) and multiple physical sublayer (PHY) standard for cable TV networks. The efforts of 802.14 must support IEEE 802 layer services (including Ethernet) and must also be ATM compatible.

The DOCSIS specifications are managed by CableLabs on behalf of its cable television system operator members. The project was initiated by an organization called Multimedia Cable Network System (MCNS) Partners, L. P., which consists of Comcast Cable Communications, Cox Communications, Tele-Communications, Inc., and Time Warner Cable. In addition to MCNS, Rogers Cablesystems Limited, MediaOne, and CableLabs have all contributed to the DOCSIS documents, as have several networking and telecommunications vendors. DOCSIS documents describe the internal and external network interfaces for a system that allows bidirectional transfer of IP traffic, between the cable system head-end and customer premises, over a cable television system [4] . The customer network interface in common use today is Ethernet 10BaseT. There is a mandate for a 10 Mbps Ethernet interface in the home. Subscriber access equipment can be a personal computer, X-Terminal, or any such device that supports the TCP/IP protocol suite. Future home interfaces from the cable modem will include the Universal Serial Bus (USB) and IEEE 1394 (FireWire).

IP Over CATV System Challenges
From an IP perspective, a CATV system almost appears to be another data link layer. However, experience gained thus far has demonstrated that the marriage of IP over CATV radio frequency (RF) channels is not as straightforward as IP over any other high-speed serial point-to-point link.

In the CATV space, the downstream channels in a cable plant (cable head-end to subscribers) is a point-to-multipoint channel. This does have very similar characteristics to transmitting over an Ethernet segment where one transmitter is being listened to by many receivers. The major difference is that baseband modulation has been replaced by a more densely modulated RF carrier with very sophisticated adaptive signal processing and forward error correction (FEC).

In the upstream direction (subscriber cable modems transmitting towards the head-end) the environment is many transmitters and one receiver. This introduces the need for precise scheduling of packet transmissions to achieve high utilization and precise power control so as to not overdrive the receiver or other amplifier electronics in the cable system. Since the upstream direction is like a single receiver with many antennas, the channels are much much more susceptible to interfering noise products [5, 6] . In the cable industry, we generally call this ingress noise. As ingress noise is an inherent part of CATV plants, the observable impact is an unfortunate rise in the average noise floor in the upstream channel. To overcome this noise jungle, upstream modulation is not as dense as in the downstream and we have to use more effective FEC as used in the downstream. There is a further complication that there are many upstream "ports" on a fully deployed Hybrid Fiber-Coaxial (HFC) plant that requires matching head-end equipment ports for high-speed data [7] .

To further the rub on the upstream channel use, the arcane regulations of the FCC from back in the mid 1980s mandated that upstream frequency spectrum be reserved on all cable plants, regardless of whether it was actually used. This was typically the 5—42 MHz region, leaving above 50 MHz for downstream transmissions. (Note that there are other regions available for upstream, but the overwhelming majority of cable plants only use 5—42 MHz.) This leaves precious little spectral bandwidth for upstream communications.

The existing environment for high-speed data protocols therefore provides for relatively clean bandwidth in the downstream direction, allowing for higher-speed data rate channels, while in the upstream, individual channels are of lesser data rate. However, multiple upstream channels can be used per downstream channel to get effective symmetric aggregate bandwidth. Typically, we speak of cable modem systems as providing asymmetric services (higher downstream data rate than upstream). Note though that this asymmetry closely matches what we expect initially for residential high-speed data services. That is, many more subscribers at home pulling things off the Internet via web services, than pushing data back in.

Modern modulation techniques provide for a range of data carrying capability ("baud rate"). A low order modulation rate called Quadrature Phase Shift Keying (QPSK) provides for two data bits per symbol encoding. Quadrature Amplitude Modulation (QAM) provides a lower order modulation of 16 QAM (four bits per symbol) through higher order rates of 64 QAM (six bits per symbol) and 254 QAM (eight bits per symbol). Low order modulations are more robust in higher average noise environments. Higher order modulations are least robust. Therefore, high order modulations are suitable for downstream channels due to the low noise performance, while the order of upstream channel modulation is heavily effected by noise. Typically, cable modem systems will see QPSK used for upstream channels. When the plant is very clean, noise-wise, 16 QAM may be used. One additional challenge is that the speed of RF signals in fiber and coaxial cable is much less than the speed of light. For system deployments to be effective, the cable modem protocols must support cable modems out to a wire distance of 50 miles (80 km).

At these distances the round trip propagation delay will be on the order of 800 microseconds; which is several times the length of time it takes to transmit a 64-byte packet on the upstream channel. The IEEE and DOCSIS cable modem protocols have been engineered to overcome these propagation delays in order to increase channel utilization; that is demand-based scheduling of a slotted upstream channel coupled with precise station ranging and timing.

Another challenge is in using an IP-over Ethernet approach to providing a reliable public switched packet service to an abundance of subscribers. Traditional Ethernet networking has always relied on all the Ethernet stations being within the same administrative walls with all users sharing the same common interests. Not so with metropolitan area public access networks. Data communications must now be encrypted such that the privacy of user communications is not invaded by promiscuous neighbors. In addition, users are paying for access in this cable modem world, and any abusive behavior of users must be contained so as to not affect other users. This calls for sophisticated fairness scheduling in the head-end systems and the use of comprehensive cryptological and packet filtering techniques. It is all very complicated both to create, and to manage. Each standard has its own approach for dealing with these issues.

Where IP over CATV appeared to be fundamentally similar to Ethernet when the industry first started out, in reality it is not. High-speed cable data networking, as demonstrated by the work output from various standards activities, is fundamentally a new approach to what at first appeared to be similar old problems. It's not ALOHA anymore [8] , nor is it your grandfather's Ethernet [9, 10] .

IEEE 802.14 Cable TV MAC and PHY Protocol Working Group Let's briefly examine the first comprehensive standard activity created to address the current emerging world of high-speed cable data systems. In November 1994, the IEEE 802.14 CATV MAC and PHY Protocol working group met for the first time as an approved project within the 802 standards committee. Previous work had been done in 1993 through 1994 in the 802.catv study group in preparation for formal IEEE 802 project approval. The Project Authorization Request (PAR) charter of the group specifies that it will standardize a single MAC layer protocol and multiple PHY layer protocols for two-way HFC networks. Consistent with the IEEE LAN/MAN 802 Reference Model [11] , 802.14 is producing a solution that supports the 802 protocol stack while at the same time supporting ATM in an ATM-compatible manner.

The general 802.14 requirements include:

Communications support for all coaxial and hybrid fiber-coaxial cable TV network tree and branch topologies. (See Figure 1)

Support of symmetrical and asymmetrical rates

Support of Operation, Administrations, and Maintenance (OAM) functions

Support of one-way delays on the order of 400 microseconds (round-trip delays to 800 microseconds)

Support of a large number of users

Support for moving data from an originating subnetwork to a destination subnetwork, which may be the same or a different one

Figure 1: CATV Tree and Branch Network

(Click on image to enlarge.)

The working group completed a first-release revision of a functional requirements document back in 1995 [12] , which detailed the 802.14 cable topology model; defined key assumptions, constraints, and parameters; defined key performance metrics and criteria for the selection of multiple PHY protocols and a MAC protocol; and defined the support of Quality-of-Service (QoS) parameters. The working group's work plan called for the close of formal proposals in November 1995, with the recommended protocol defined in July 1996. Seventeen MAC protocol proposals were submitted to the working group. Needless to say, it took awhile for the working group to sort through all the issues and opinions. After much consideration, debate, and wrangling of both solutions and personalities, IEEE 802.14 stabilized on a working group draft in September 1998. This working group draft is now being submitted through the IEEE 802 standard approval process.

The 802.14 MAC and PHY specification includes:

Definition and operational specifications for cable system Head-End Controller and cable modem Stations. (See Figure 2)

Support of both connectionless and connection-oriented services

Support of a formal QoS for connections; support for dynamically allocated bandwidth for different types of traffic, including Constant Bit Rate (CBR), Variable Bit Rate (VBR), and Available Bit Rate (ABR)

Support for unicast, multicast, and broadcast services; interoperability with ATM

Predictable low-average access delay without sacrificing network throughput

Fair arbitration for shared access to the network within any level of service

Downstream channel support for 64 QAM or 256 QAM modulation

Compatibility for both international and North American downstream digital video standards

Upstream channel support for QPSK or 16 QAM modulation

Figure 2: IEEE 802.14 General Model

(Click on image to enlarge.)

The selection of ATM cells as the data link layer protocol data unit for IEEE 802.14 networks has the advantage that it provides a suitable integrated multiplexing platform capable of supporting a mix of guaranteed (predictive) traffic flows with best-effort (reactive) traffic flows. See Figure 3. Cable operators can deploy IEEE 802.14 based ATM systems as part of an evolutionary path to a fully integrated multimedia bearer service offering. A residential ATM bearer service easily supports Internet access to the home via the Classical IP over ATM standards of the Internet Engineering Task Force [13] or by providing an IP over Ethernet adaptation overlay service [14]. The development of QoS scheduling support in the Head-End Controller is left for vendors to implement [15, 16, 17].

Figure 3: IEEE 802.14 ATM Protocol Model

(Click on image to enlarge.)

IEEE 802.14 Status
At the time of this writing, the IEEE 802.14 working group just finalized a working group draft suitable to introduction into the IEEE standards process. The entire IEEE process takes about a year from acceptance of the working group letter ballot to producing a published standard.

The DOCSIS project is an activity of major cable companies and selected vendors to rapidly develop, on behalf of the North American cable industry, the necessary set of communications and operations support interface specifications for cable modems and associated equipment. The activity was triggered by John Malone in December 1995, in response to competition, vendor postures, and unfortunate lack of progress in the public standards process (that is, IEEE 802.14). The target for the specification was to produce a residential, "low-cost," off-the-shelf, Internet access service, with wide-scale vendor interoperability for base functions with sufficient hooks and room for vendor differentiation.

MCNS specifications are intended to be non-vendor specific, allowing cross-manufacturer compatibility for high-speed data communications services over two-way HFC cable television systems. MCNS met its specification release deadline and published versions of the DOCSIS Radio Frequency (RF) Interface Specification V1.0. The first draft specification was published in December 1996. The latest specification was published in July 1998 [3] . The DOCSIS RFI protocol is based on the original LANCity symmetric 10 Mbps protocol, evolved to an asymmetric system, with multiple upstream and high-speed down-stream (for example, 30 Mbps) channel support.

The MCNS system model is very similar to the IEEE 802.14 general model and includes many interfaces to a cable modem system, as shown in Figure 4. The goal of the DOCSIS project is to produce specifications for the CATV RF interfaces, including behavior of the Cable Modem Termination System (CMTS) and Cable Modem (CM) with respect to delivery of the residential IP over Ethernet service.

Figure 4: Data-Over-Cable RFI Reference Architecture Cable Modem

(Click on image to enlarge.)

The DOCSIS RFI system is asymmetric, with one to several downstream channels operating asymmetrically with one to several upstream channels. Specific features of MCNS DOCSIS RFI Version 1.0 include:

Switched Ethernet service for Internet transport via a variable length MAC packet protocol

Best-effort service

Downstream data channel rates from 20 Mbps (16 QAM) to 40 Mbps (256 QAM) with a typical configuration of 30 Mbps (64 QAM) in 6 MHz channels

Compatibility for North American downstream digital video standards. (See article starting on page 27.)

Downstream data channel rates selected from 320 Kbps (QPSK) through 10.24 Mbps (16 QAM). Channel spectral widths from 200 KHz to 3.2 MHz

Software flexibility: ability to download new software to change/ update CM behavior

Many filters and features for controlling packet flow and classification

Comprehensive MIB specifications for control of the cable modem and cable modem termination system

A single large LAN segment

Due to the time-to-market push for DOCSIS RFI V1.0 interoperable modems, little to no attention was been given for QoS needs however, vendors will likely include some QoS support in their offerings. (Upstream packet fragmentation was removed from the December 1996 draft release.)

CMs and the CMTSs have basically the same protocol stack: downstream and upstream PHY, the DOCSIS RFI MAC, Ethernet and an Ethernet switching layer with substantial filtering, IP/Address Resolution Protocol (ARP), User Datagram Protocol (UDP), and Simple Network Management Protocol/Dynamic Host Configuration Protocol/ Trivial File Transfer Protocol (SNMP/DHCP/TFTP).

The DOCSIS RFI includes upstream and downstream optional packet encryption using the Data Encryption Standard (DES) to provide link privacy. RSA public key exchange is used between the CM and CMTS.

CableLabs is actively driving multiple vendor interoperability with the goal of having "silicon interoperability" as soon as possible for DOC-SIS "certified" CMs and CMTSs. CableLabs runs a variety of test and certification laboratories in their facility. Numerous vendors are participating. It was the expectation to have many cable modem vendors certified by the cable industry major trade show, the Western Cable Show, in December, 1998. However, as interoperability does take time to work out, the process is taking longer than expected. There will likely be some certified vendors by December 1998, with many more in first quarter 1999. It is now expected that the first widespread deployments of DOCSIS cable modems will start in late first quarter 1999.

The DOCSIS project is currently updating the RFI Version 1.0 specification to include better support for bandwidth management and QoS support. The changes being studied include support for multiple Service Identifiers (SIDs), filters to perform the classification of IP packets to different SIDs for differentiated services (QoS), and the signaling support for dynamic SID creations and deletion. A scheme for packet fragmentation will be included which will give substantially better support for managing jitter for delay sensitive traffic, such as packet voice. The primary motivation for adding these extensions to DOCSIS RFI V1.0 is to provide for better support of packet voice and video over DOCSIS IP services. A major focus of the North American cable industry is to support "near toll quality" voice and video services via DOCSIS systems. The cable industry effort writing specification for packet voice and video is called PacketCable [18] . It is expected that the DOCSIS RFI V1.1 and initial PacketCable specifications will appear in December 1998.

DOCSIS RFI Version 1.0 was adopted by the Society of Cable Television Engineers (SCTE) Data Standards Subcommittee in July 1997 as the North American residential cable modem system standard. Substantial work is in progress in the IETF IP over Cable Data Networks (ipcdn) working group to standardize the DOCSIS MIBs [19, 20] and to standardize IP over DOCSIS [21].

An IP over Cable Modem Example
This section presents a brief overview of a hypothetical IP over HFC system. It is meant to be an informative example to illustrate the application of the IP technology and some of the issues that surround provision of the service over a residential cable TV network. Moving IP datagrams in and out of the home over the cable plant is the important issue. The specific technology and protocols used by the cable modem vendor are important only in their ability to provide required IP service support.

For this example, consider a system that has the following design goals and requirements:

One-to-many service will be supported in the downstream direction; that is, many cable modems are reachable via the downstream channel

Many-to-one service will be supported in the upstream direction; that is, the upstream channel bandwidth will be shared. There may be up to several upstream channels

The protocol used between the Head-End Controller and the headends is not significant as long as it meets the needs of the IP service

The head-end owns the upstream bandwidth and allocates resources to cable modems

IP over Ethernet 10BaseT is the required interface in the home

IP over Ethernet or IP over ATM is the required interface at the head-end

This example will rely on the DOCSIS RFI information presented previously in this article. The CMTS can transmit packets to any cable modem on the channel in any order or rate appropriate to the scheduling information it has and controls. The CMTS also participates in the IP multicast group membership (Internet Group Management Protocol [IGMP]) and IP Resource Reservation Protocol (RVSP) and makes changes in the cable modem resource assignments and allocations as needed. The home cable modem is permitted to use only the upstream channel under direction of the CMTS. Guaranteed and best-effort bandwidth allocations are dynamically assignable by the CMTS. It is assumed that the cable modem protocol has a bandwidth request facility that allows a CM to ask the CMTS for bandwidth. The function of the bandwidth management process is to sort these requests for service and give fair access to the requesting cable modems.

The method for implementation of an Ethernet and 802.3 bridging function over DOCSIS essentially permits the RF channels to act as a serial connection between a half-bridge function in each cable modem with a master in the CMTS. Figure 5 illustrates the protocol stack for this solution. The system presents an Ethernet-like segment to the cable operator. It is well-known how to put together such segments to construct larger internetworks.

Cable modems provide demarcation between the Internet Service Provider’s network and each home network. To help the Internet Service Provider offer fair access service to its residential customers, the cable modem will require sufficient dynamic functionality for multilayer protocol filtering and various forms of rate management (see Figure 6). The goal of this filter is to create a defense perimeter at the first point of entry to the cable network; this perimeter will protect the upstream channel from being saturated or abused by misbehaving home networks. Some examples of this filtering functionality include, but are not limited to:

Filtering on Ethertype for permitting only certain protocols to pass upstream; for example, IP and ARP only

Filtering on IP source or destination address to permit/deny access from the home network

IP and Ethernet broadcast rate limiting; that is, keep any home network broadcast storms confined to the home network

IP Multicast group address filtering; that is, explicitly permit participation of the home network in an IP multicast group

Figure 6: Internet Services via Cable Modem Deployment Model

(Click on image to enlarge.)

It should be noted that these filtering functions are under consideration by numerous cable modem manufacturers, and they are being discussed in the IETF ipcdn working group.

A brief overview of IP over cable TV networks has been presented. From an engineering and deployment viewpoint, making the Internet move over cable modems is deceptively straightforward. Many issues are beyond the scope of this article: address allocation methods, backend network design, configuration services, server placement, home customer support services, installation, firewalls, and troubleshooting.

This article has presented an overview of the work in progress of the IEEE 802.14 Cable TV MAC and PHY Protocol Standards working group and the MCNS DOCSIS effort. Initial review of these works is positive; indications are that data over HFC systems are viable. The IEEE 802.14 effort began as a study group in late 1993 and has yet to produce a standard. The MCNS DOCSIS process started in early 1996, moved rapidly, and has produced an accepted international standard specification for North American cable operators for residential cable modem service. The IEEE 802.14 standard appears to be destined for some international use and in systems where ATM over CATV is preferred by cable operators.

The cable network environment will provide a very usable and scaleable bandwidth platform for delivering Internet services to and from the home [22]. A hypothetical example was provided that illustrates a general equipment deployment model. Actual deployment of Internet to the home will occur in many areas of North America in 1998 with increasing and substantial deployment in 1999.

For More Information
Information on the IEEE’s 802.14 working group can be found on the World Wide Web at:

Information the Internet Engineering Task Force’s IP over Cable Data Networks working group can be found at:

Information on the North American MCNS DOCSIS effort can be found at:

Information on the North American PacketCable effort can be found at:

[1] Baran, Paul, "On Distributed Communication Networks." IEEE Transactions on Communication Systems, Vol. CS-12, pp. 1–9, March 1964.

[2] ATM Forum, "ATM User-Network Interface Signaling 4.0," Specification number af-sig-0061.000,, July, 1996.

[3] MCNS, "Data-Over-Cable Service Interface Specification—Radio Frequency Interface." SP-RFI-I02-981008,, July, 1998.

[4] MCNS,, main page, April 1998.

[5] Kim, Albert. "Two-Way Plant Characterization." Technical Session 23, National Cable Television Association Show and Conference, Dallas, Texas, May 9, 1995.

[6] Chelehemal, M., Prodan, R., et al., "Field Evaluation of Reverse-Band Channel Impairments." Society of Cable Telecommunications Engineers, Emerging Technologies Conference, San Francisco, California, January 9– 12, 1996.

[7] Laubach, Mark, "Avoiding Gridlock on the Data Infobahn: Port Mismatches Pose Challenges." CED Magazine, March 1998.

[8] Abramson, Norman, "Development of the ALOHANET." IEEE Transactions on Information Theory, Vol. IT-31, pp. 119–123, March 1985

[9] XEROX, "The Ethernet, A Local Area Network: Data Link Layer and Physical Layer Specification." X3T51/80-50, Xerox Corporation, Stamford, Connecticut, October 1980.

[10] IEEE, "Carrier Sense Multiple Access with Collision Detection (CSMA/ CD) Access Method and Physical Layer Specifications." Standard 802.3- 1985 (ISO DIS 8802/3), IEEE, New York, ISBN 0-471-82749-5, 1985.

[11] IEEE, "IEEE Standards for Local Area Networks: Logical Link Control, ANSI/IEEE Std 802.2-1985." Fifth printing, February 1988.

[12] IEEE 802.14 Working Group, "Cable-TV Functional Requirements and Evaluation Criteria." Work in progress, IEEE 802.14/94-002R2, IEEE 802 Committee, February 1995.

[13] Laubach, Mark. "Classical IP and ARP over ATM." RFC 1577, January 1994.

[14] Laubach, Mark, "Logical IP Subnetworks over IEEE 802.14 Services." Work in progress, draft-ietf-ipcdn-ipover-802d14-01.txt, November 1997.

[15] Laubach, Mark, "Serving Up Quality of Service." CED Magazine, April 1997.

[16] Laubach, Mark, "Deploying ATM Residential Broadband Networks." NCTA Cable 96 Conference, Los Angeles, California, April 30, 1996.

[17] Nichols, Kathleen, and Laubach, Mark, "On Quality of Service in an ATM-based HFC Architecture." IEEE ATM Workshop 96, San Francisco, California, August 27, 1996.

[18] PacketCable, "What is PacketCable?", April 1998.

[19] Roeck, Guenter, "Cable Device Management Information Base for MCNS compliant Cable Modems and Cable Modem Termination Systems." Work in progress, draft-ietf-ipcdn-cable-device-mib-05.txt, October 1998.

[20] Roeck, Guenter, "Radio Frequency (RF) Interface Management Information Base for MCNS compliant RF interfaces." Work in progress, draft-ietf-ipcdn-rf-interface-mib-05.txt, October 1998.

[21] White, Gerry, "Logical IP Subnetworks over MCNS Data Link Services." Work in progress, draft-ietf-ipcdn-ip-over-mcns-00.txt, August 1997.

[22] Lucien Rhodes, "The Race for More Bandwidth." (Interview with Milo Medin of @Home), Wired Magazine, Vol. 4.01, January 1996

Internet Drafts are works in progress and can be retrieved from:

MARK LAUBACH holds a B.E.E. and M.Sc. from the University of Delaware. He is Vice President and Chief Technical Officer at Com21, Inc. in Milpitas, California, and is responsible for the end-to-end systems architecture and ATM over HFC protocol specification of the Com21 product family. Prior to Com21, he was with the Hewlett-Packard Company for 14.5 years. Laubach is a member of the IETF, and is past chair of the IP over ATM working group. He is the author of RFC 1577, "Classical IP and ARP over ATM." He regularly attends IETF, IEEE, and SCTE working group meetings. He is a Senior member of the IEEE and a member of the SCTE. E-mail: