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May 21, 1998
Chips and high-speed cable modems enable two-way communications
Stephen Kempainen, Technical Editor
New cable-modem specifications standardize high-speed, two-way
communications. Now, you can use highly integrated chips to customize consumer-friendly
modems for retail shelves.
Thanks to emerging standards, chip vendors' products can now accommodate
high-speed cable modems for two-way data communications. These products overcome the
barriers of using outdated cable-TV technology that once targeted only video broadcast.
Today's data communications on cable-TV networks require upstream signaling and increased
capacity for possible multitudes of data subscribers. A big step toward overcoming these
problems occurred with the emergence of the Data Over Cable Service Interface
Specification (DOCSIS).
Realizing a need for a specification to handle today's high-speed, two-way
communications, a group of leading cable-TV operators formed the Multimedia Cable Network
Systems (MCNS), which in turn introduced DOCSIS, specifying modem interfaces and
protocols. The completed first revision of DOCSIS meets the RF-interface, multicasting,
network-management, and mobility needs of today's applications. In turn, chip vendors can
integrate DOCSIS-compliant circuits into low-cost and efficient chips. Modem designers, in
turn, can use these chips to customize interoperable modems without sweating the
complexity of two-way RF communications.
Even though designers must produce standard modems, many opportunities are
available to differentiate modems. The features that make these modems popular with
consumers are ease of installation and ease of use. Features beneficial to
multiple-systems operators--cable-TV companies that operate multiple systems in different
geographic areas--are network management and multicasting.
Multicasting of premium programming, such as live concerts and stock
tickers, is attractive because the operator saves bandwidth by sending out a single copy
of the program to a multicast group address and allowing subscribers to join the multicast
group. The Internet Group Management Protocol (IGMP) from the Internet Engineering Task
Force (IETF) governs these multicast schemes. These schemes work well for controlled
intranets, such as cable networks, as long as the hardware and software for content
servers, routers, cable modems, and user PCs support IGMP.
Mobility is also important for both multiple-systems operators and
consumers; consumers should be able to take their modems to any cable operator's domain
and plug and play into the network. The National Cable Television Association estimates
that 47% of US cable networks will be capable of two-way communications by year's end.
This fact means that about 47% of the 60 million cable subscribers will be eligible for
megabit-per-second Internet access at the going rate--about $40/month.
You can use DOCSIS-compliant modems in cable systems worldwide because of
international standards recognition by the ITU (International Telecommunications Union).
DOCSIS reached widespread acceptance relatively quickly. Its acceptance grew out of MCNS'
impatience with the IEEE-802.14 data-over-cable work group. MCNS perceived a limited
market window for data over cable. It had to move quickly because
asynchronous-digital-subscriber-line (ADSL) technology for Internet access through
telephone lines might slam the window shut by dominating broadband access connections (Reference 1). Thinking the 802.14 work group's
dependence on asynchronous transfer mode (ATM) might delay the standard, MCNS and CableLabs quickly developed a data-over-cable standard
based on existing technology. They focused on transparently transmitting Internet Protocol
(IP) packets between the head-end and subscriber locations. Once DOCSIS was completed, the
Society of Cable Telecommunications Engineers adopted it
and promoted it as an update to ITU-T J.83 for data over cable. It appears that the ITU
will approve DOCSIS as an international standard this year.
 At the heart of DOCSIS is an
architecture with modems for the head and subscriber ends of a hybrid-fiber-coax (HFC) or
all-coax network. The architecture describes interfaces and communications protocols for a
scalable, bidirectional network (Figure 1). The
family of standards that describes these modems is available at CableLabs
Web site, www.cablemodem.com. Besides
modems, the DOCSIS family includes operation- and business-support specifications for
security, configuration, performance, fault, and accounting management. An optional
telephony return-path specification provides for networks in which the cable plant has
difficulty supporting upstream radio communications.
Developing the DOCSIS standard employed mainstream technology, such as
Ethernet, MPEG (Moving Pictures Experts Group), and the US Data Encryption Standard (DES),
whenever possible to quickly complete the standard. For example, DOCSIS uses 10-Mbit
Ethernet for connecting the subscriber modem to the end-user computer because this
technology was mature, cheap, and available. In keeping with this modus operandi, future
enhancements for USB computer connections will be available this year. Also on the
enhancement list is the availability of IEEE 1394 for modem-to-set-top-box connections.
DOCSIS describes the cable modem at the subscriber location and the
cable-modem termination system (CMTS) at the head end or distribution hub. DOCSIS also
specifies the RF interface between the two modems, describes the interface between the
customer-premise equipment (CPE) and the cable modem, and describes the interface between
the Internet network and the CMTS. Both modems contain cable-system-specific media-access
control (MAC) for managing the RF link. The MAC specification for the cable modem targets
the lowest cost for subscriber modems, and the CMTS includes the complex circuitry to
manage sending and receiving multiple-megabit-per-second data streams. DOCSIS leaves
defining the remainder of the modem specification to equipment designers who can customize
their modems by making trade-offs between features and cost.
Enormous bandwidth
Cable networks boast enormous bandwidth because of well-designed cable
plants and modems. By optimizing cable modems for the HFC topology, DOCSIS takes advantage
of this bandwidth (see box "Hybrid-fiber coax
and upstream-modulation impairments").
The HFC network broadcasts downstream to all nodes. The upstream subsplit,
which allows bidirectional traffic on one cable by dividing the frequency spectrum,
requires end nodes to negotiate bandwidth reservations: Upstream signals use 5 to 42 MHz,
or 65 MHz for the Digital Audio Visual Council (DAVIC),
and downstream broadcasts use from 50 MHz to the upper frequency limit--typically 300 to
860 MHz, depending on the system. Each subscriber modem acquires only the portion of the
downstream broadcast addressed to it. These downstream transmissions achieve 27 Mbps on
each 6-MHz video channel when using 64 quadrature amplitude modulation (QAM). QAM packs
multiple bits into one symbol period; thus, 64 QAM refers to the 64 signals per symbol.
More than 115 video channels can exist in a 750-MHz cable system, and you can calculate
the aggregate bandwidth. Also, the data rate can increase to 40 Mbps on each channel if
the head end, subscriber mo-dems, and cable plant can support 256 QAM. (For a more
in-depth discussion of the potential bandwidth of cable systems, check out www.cablelabs.com/ModemPerf.pdf.)
Along with the enormous downstream bandwidth comes a significant upstream
bandwidth in the HFC network. DOCSIS uses frequency-division multiple access to divide the
5- to 42-MHz upstream frequency band into 200-kHz- to 3.2-MHz-wide carriers. Different
carrier widths accommodate different data-rate requirements (Table
1). Each of these carrier channels also has TDMA slots, or minislots, so
multiple cable modems can simultaneously use one channel. A cable modem that wants to
transmit upstream negotiates with the CMTS for minislots in the cable modem's assigned
frequency carrier.
This HFC cable-system bandwidth lets operators start with a few downstream
channels assigned to data and then allocate more video channels for data as cable-modem
subscribers and traffic increase. Cable operators plan to expand data capacity without
losing video channels by using another borrowed technology, MPEG. DOCSIS incorporates the
MPEG-2 transport technology used in digital-TV standards to pack as many as six video
streams into the 6-MHz analog NTSC channel. Each 6-MHz channel uses MPEG-2 to multiplex
compressed digital video and even data streams into the same channel.
Negotiating upstream paths
The HFC system has plenty of shared bandwidth in both directions. Cable
modems can easily snoop--listen to all traffic in--the broadcast for data addressed to
them. The difficult technical question is: How do potentially thousands of cable modems
avoid transmitting over each other in the upstream direction? DOCSIS derived a solution
for sharing the upstream bandwidth among thousands of nodes. The DOCSIS upstream
transmissions use a hybrid MAC developed for cable TV's shared network.
The hybrid MAC benefits from
carrier-sense-multiple-access/collision-detection (CSMA/CD) and Token Ring MACs. CSMA/CD
is the Ethernet media-access protocol that allows nodes on a shared network to negotiate
for transmission. CSMA/CD's best feature is the zero latency that results when a
transmitting node senses the network is idle and just starts to transmit. However, when
nodes sense that the network is in use, they wait before trying to transmit again. If two
nodes collide--be-cause both sense an idle interval and begin to transmit
simultaneously--they back off and retransmit after a random time interval. This scheme
causes latency, efficiency, and fairness to suffer when many nodes want to transmit. Token
Ring works best when many nodes want to transmit. The Token Ring protocol passes a
transmit token around the ring, which gives each node a fair chance at transmission. The
node with the token transmits in the next time slot, thereby allowing fair and efficient
access to a congested network. However, when the network is idle, efficiency suffers,
because a node must wait until the token comes back around before the node can transmit.
The hybrid MAC uses the best of both negotiation schemes. Each node that
wants to transmit upstream can transmit in reserved minislots when network congestion
occurs or can use a collision and back-off scheme when the network is uncongested. DOCSIS
also provides for the end node to reserve minislot groups for block file transfers.
To keep data streams orderly, the hybrid-MAC protocol uses a service ID
(SID). The SID identifies a cable modem to the CMTS for upstream bandwidth allocation and
enables the CMTS to guarantee bandwidth and delay parameters so multimedia streams can
have quality of service. To identify data streams, the CMTS assigns one unique SID or more
to a cable modem during registration. Each SID corresponds to a class of service--latency
and throughput--requested by the cable modem. The most basic cable modem uses one SID for
best-effort IP service. More complex modems use SIDs to support multiple "data
flows." Protocols such as the ReSerVation Protocol (RSVP) introduce data flows, which
allow IP traffic to meet time-constraint requirements for streaming-video applications,
such as videoconferencing and multimedia.
Now, consider the problems associated with the plug-and-play nature of the
cable modem. When a cable modem comes online in a network, it must first synchronize with
the head end by detecting and interpreting broadcast timing information. Then, the cable
modem scans the downstream signals for the upstream channel-map messages that the CMTS
periodically broadcasts. The map messages include the upstream negotiation parameters.
Using the map, the cable modem begins a message-passing scheme that lets the head end and
cable modem find out each other's capability for data rates, IP addresses, and
upstream-transmission requirements. Because the upstream-transmission parameters adjust to
changing line conditions, such as incoming noise, the head end periodically updates the
upstream bandwidth availability map to each subscriber node. Through this procedure, the
cable modem knows which frequency channel and which minislots to use for upstream
transmission.
Another complex part of the up-stream-transmission system is the timing
synchronization of multiple cable modems participating in TDMA transmissions from
disparate locations. The transmitted packets from cable modems can have miles of delay
difference to the head end but still must arrive at the CMTS up-stream demodulator at
precisely the assigned mini-slot. A messaging scheme, using synchronization and ranging
(acquiring the correct timing offset to align to mini-slot boundaries) messages,
accomplishes this task. As the modem comes online, it receives the global timing reference
distributed by the CMTS. The cable modem calculates the delay between itself and the CMTS
and adjusts its clock accordingly. The cable modem then sends a ranging message to the
CMTS with the delay value that makes the cable modem appear to be right next to the CMTS.
The cable-modem registration then completes through more message exchanges with the CMTS.
Complex MAC in the CMTS
Most of the complexity involved with configuration, timing, and up-stream
negotiation resides in the CMTS because of a conscious decision by the DOCSIS architects
to keep down the cost of the cable modem. The CMTS is usually a line card in the cable
head end equipment. Other components in the head end are MPEG-2 digital-video components,
analog-video components, and HFC optical-transport equipment. The CMTS line card in-cludes
the downstream QAM and the upstream burst demodulator.
The principal function of the CMTS is forwarding IP packets (Reference 2). The CMTS MAC moves packets between
the network-side interface and the RF interfaces and between the upstream RF interface and
the downstream interface. MAC forwarding may use the layer-2 bridging, or it may use
layer-3 routing protocols from the Open Systems Interconnect network model. The CMTS must
also support spanning-tree protocols to reconcile any loop paths that may exist in the
network. Because cable modems transmit to only the CMTS, the shortest path between nodes
in the same upstream trunk forwards packets from the upstream to the downstream RF
interface. DOCSIS calls this function the "MAC forwarder function," and it
provides connectivity for nodes in the same MAC domain.
The CMTS MAC must control many functions with many algorithm trade-offs.
These functions, such as timing synchronization, dynamic host configuration, cable-modem
registration, and operational parameter transfers, must comply with the DOCSIS standard,
yet there is room for product differentiation in performance and scalability. So far, only
Broadcom offers its version of CMTS MAC in a
commercially available chip, the BCM3210. The chip controls head-end functions with four
key message types (Table 2, pg 68). Messages sent
downstream both to cable-modem MACs and to the CMTS upstream demodulator are the upstream
bandwidth-allocation map (MAP) and the upstream channel description. These messages
contain information on upstream channel configuration and minislot allocation. The CMTS
MAC appends the bandwidth- and ranging-request messages received from the cable modems at
the upstream demodulator. The messages need additional information, such as time stamps,
upstream channel identification, ranging information, and power offsets, before forwarding
to the processor for system management. In addition to controlling upstream scheduling,
the external processor controls the IP packet bridging and the configuration for the MAC
forwarding domain.
 A CMTS reference
design demonstrates its functionality on a line card (Figures 2
and 3). The BCM93210 DOCSIS-compliant CompactPCI line card has four upstream
demodulators and a downstream modulator in addition to the MAC device.  The card provides 64 and 256 QAM
downstream and quadrature phase-shift keying (QPSK) or 16 QAM upstream. The board uses a
Pentium processor for Transmission Control Protocol/IP (TCP/IP) bridging applications
running on QNX RTOS and supplies a full graphical
interface. The 100BaseTX Ethernet connects to the head-end backbone.
Cable modems for the masses
Although simpler than the CMTS, cable modems have their own complexities.
DOCSIS specifies a flexible cable modem for connection to CPE. The modem can be a
stand-alone type for use with a PC, or it can be the front end to a set-top box for TVs in
sports bars or for home-entertainment centers. Business applications may require a
combination modem-Ethernet hub in a SOHO (small office, home office). The number of CPE
addresses that a cable modem can handle is vendor-dependent. In all cases, the cable modem
forwards IP packets using layer-2 transparent bridging.
The cable modem must include a CPU and associated memory to provide an
internal IP host for running the TCP/IP stack. This provision puts cable operators at ease
because it offers a barrier from the CPE. DOCSIS also provides for management functions
and software downloads carried by the IP to the modem, thus requiring the IP host inside
the modem. This function also allows for Simple Network Management Protocol communications
to deliver and collect the network-management data from the cable modem for a complete
managed network.
Cable modems also require the downstream demodulator that supports 64 QAM
with the option for 16 and 256 QAM; an upstream RF-modulator return, telephone return, or
both; and the MAC. Chip vendors partition these functions in different ways; the goal of
some vendors is complete integration. Broadcom and Rockwell have three chips in their chip sets--one
for each function. Libit offers two chips: the
modulator and demodulator integrated into one chip and a MAC. Analog Devices and Stanford
Telecom offer the modulator and demodulator in one chip but do not yet offer any MACs.
Interoperability requires chip vendors to adhere to standard modulation
and demodulation, which leaves little room for differentiation. However, some features
stand out among vendor offerings. Analog Devices and Libit together developed the AD6201 and LBT4030, which
integrate the downstream demodulator and upstream modulator on one chip. The chip uses a
reference-clock input with on-chip circuitry to derive all other clocks. The design
supports only North American forward-error-correction algorithms to make the chip as cheap
as possible.
In addition to North American forward error correction, data-over-cable
systems use three approaches to forward error correction. These approaches are the basis
for the four annexes of the ITU-T J.83 recommendation for digital multiprogram systems for
TV, sound, and data services for cable distribution. Because regional cable-TV operators
independently develop and provisionally implement cable-distribution systems with four
types of forward-error-correction algorithms, the ITU simply standardized all four
submittals, creating annexes A, B, C, and D. The European-dominated DAVIC uses Annex A forward-error-correction algorithms for
its data-over-cable specifications; North American cable operators support Annex B.
Japanese cable operators primarily use Annex C, and no cable-modem chip vendors support
Annex D.
Stanford Telecom's Stel-2176 also integrates modulation and demodulation,
but it supports annex A, B, and C forward error correction with one programmable device.
The chip works in designs targeting Europe, Asia, and North America. In addition,
Stel-2176 offers an acquisition range of ±200-kHz offset for the downstream carrier
input. It also boasts an acquisition time of less than 100 msec. Stanford Telecom offers a complete evaluation kit for the
STEL-2176 for $3200, which includes software, a board, and a graphical user interface for
a PC.
For modem designers that would rather not design a MAC ASIC, Broadcom, Libit, and
Rockwell offer chips compliant with the DOCSIS
MAC version 1.0 standard. Future versions of the MAC will include enhancements that
fine-tune the quality of service and isochronous features. But, for version 1.0, the
cable-modem MAC controls the IP traffic to and from the CPE, forward-error-correction,
MPEG-transport, and RF-interface protocols.
All three cable-modem MAC chips help control the RF interface by
processing all the messages to and from the head end. These messages concern timing,
registration, upstream-communication parameters, upstream negotiations, and data privacy.
All these MACs provide for either the RF or the telephony upstream path to the CMTS.
The cable modem processing the downstream data includes the MPEG-transport
control and stream demultiplexing. For cable data, the MPEG-2 transport packets
encapsulate the variable-length IP packets into their 188-byte format--a 4-byte header and
184-byte payload. The receiving cable modem then demultiplexes the MPEG-2 packets into
video streams and data streams for delivery to the TV display or to a computer.
 Besides these
required functions, the MAC chips differentiate themselves in the programming interface
and in data transfer to the CPE. The Rockwell
HM8416 has an I/O register and separate upstream and downstream data FIFOs that are all
32-bit-wide interfaces with additional control and status pins for fast synchronous data
transfers (Figure 4). The Broadcom BCM3220 has DMA support for two downstream
queues and one upstream queue. The Libit LBT4030 positions itself in
the business and SOHO category by boasting that it supports 16 CPE devices and multiple
SIDs. This CPE side of all MACs will see further differentiation as USB and Ethernet cores
become the next step in integrating more functionality into the MAC.
For data privacy, all the devices include a DES engine for encryption and
decryption specified by the baseline-privacy function. The DOCSIS Baseline Privacy
Interface (BPI) is a document describing data privacy across the shared RF network.
Besides data privacy, cable networks have security concerns with theft of service. BPI
does not directly address theft by unauthorized users, but BPI does help prevent such
theft, because modems must register to participate in cable service, and this registration
deters unknown or unauthorized modems from involvement in this service. Baseline privacy
uses the US DES with a 56-bit key. The modems use the cryptographic algorithm from RSA Data Security (www.rsa.com)
to transfer the DES key between the CMTS and the cable modem. Another function of the SID
for upstream bandwidth allocation is that the SID also identifies security associations
for both the upstream and downstream data flows.
Beyond BPI, DOCSIS specifies optional full-security specifications
involving plug-in smart cards, or "removable security modules." The cards use
the PC Card (formerly, PCMCIA) interface and provide user authentication.
After examining how complex DOCSIS can be, you can understand why
interoperability is a primary concern for all modem designs. To achieve this
interoperability, CableLabs has been conducting
interoperability testing since late 1997. Chip vendors, modem OEMs, and cable operators
are participating in the testing to have their designs blessed. The results have proved
that the standard is workable and that performance is as good as expected.
These tests also shake down the specification for ambiguities and
inconsistencies. When the testing finds problems with the specification, en-
gineering-change orders go out for review by all participants in the DOCSIS community.
When the community agrees on the appropriate changes, CableLabs
posts them on its Web site. More than 30 of these change notices have emerged so far. By
joining the DOCSIS vendor list, you too can access the Web site.
Future enhancements to the DOCSIS standard will deal with CPE interfaces,
isochronous data traffic, and quality-of-service details. The work of the IEEE 802.14 work group dealing with ATM could
add the attachments for quality of service and bandwidth on demand that future
applications may require. But, as the interoperability testing proves, the standard is
ready for cable operators to deploy data-over-cable services to consumers and businesses
that need the high bandwidth and services that a cable-TV network offers.
Kempainen, Stephen, "ADSL: the end of the wait
for home Internet?" EDN, Oct 10, 1996, pg 52.
Hernandez-Valencia, Enrique J, "Architectures for broadband
residential IP services over CATV networks," IEEE Network, January/February 1997, pg
36.
Wright, Maury, "Delivering digital video,"
EDN, March 14, 1996, pg 38.
Schweber, Bill, "Line drivers and
receivers push signals through cable's reality," EDN, Aug 1, 1996, pg
44.
Kempainen, Stephen, "Set-top-box chip sets
evolve for digital TV," EDN, April 9, 1998, pg 97.
Acknowledgment
Thanks to Bob Cruickshank of CableLabs
for help in understanding a complex modem system.
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