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Docsis 20 getting to know
1. DOCSIS 2.0: Getting to Know the New Kid on the Block Tom Cloonan CTO- ARRIS BroadbandIntroductionWith the introduction of every new technology, vendors and their customers are typically filledwith many different emotions. Some are elated by the promise of the new technology, whileothers are unimpressed by the surrounding marketing hype. Some are confused by the proposedbenefits of the technology, while others are fearful of what it may imply about their pastdecisions and future directions. Many questions abound… - “Do I really need this new technology?” - “Will this technology really work as promised?” - “When will the technology really show up in products that can be deployed?” - “Is this technology compatible with the equipment that I purchased in the past?” - “Should I hold off on all future equipment purchases until this technology matures?” - “Will there be a technology following this new one that will obsolete the new one?”With all of this confusion, making decisions surrounding new technologies is, at best, achallenging task in today’s world.DOCSIS 2.0 is a new technology that seems to be suffering from all of the aforementionedconfusion. The DOCSIS 2.0 specification (first released in by CableLabs in December 2001)contains several changes to previous Cable Data specifications, and in the matter of a fewmonths, these changes have already created complications and confusions for many vendors,system operators, subscribers and investors.This paper will attempt to answer some of the confusing questions surrounding DOCSIS 2.0. It willtry to cut through the hype and identify some of the true benefits of DOCSIS 2.0, while un-maskingsome of the misconceptions surrounding the technology. The goal is to help those that are associatedwith the Cable Data space to make intelligent decisions and profitable plans as the Cable industrymoves forward in its quest to be THE provider of broadband services to the world.www.arrisi.com Page 1 of 36 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockBackground and HistoryThe introduction of Cable Data service to Cable Television subscribers is one of the most successfuldeployment stories in the history of the Internet. In a matter of a few years, Multiple System Operators(MSOs) transformed themselves from being providers of entertainment video into being the mostpopular providers of both entertainment video and affordable, broadband data services for most of theresidential subscribers in North America. Similar deployments outside of North America are allowingMSOs throughout the world to duplicate that accomplishment.Part of the success behind that accomplishment must be attributed to the establishment of standardsfor the delivery of Cable Data service, because the existence of standards allowed multiple vendors todevelop interoperable equipment, and the resulting competition and the creation of a volume chipmarket ultimately led to price reductions that were directly seen by the subscribers.The first standard (known as the Data-Over-Cable Service Interface Specification 1.0 or DOCSIS 1.0)was initiated by work in the Multimedia Cable Network System (MCNS) consortium in January 1996,and the Interim specifications became available in the spring of 1997. Preliminary versions of theDOCSIS 1.0 Physical (PHY) and Media Access Control (MAC) layer chipsets (which were beingdeveloped in parallel with the specifications) became available in the summer of 1997. Equipmentmanufacturers for both subscriber Cable Modems (CMs) and headend Cable Modem TerminationSystems (CMTSs) began designs using the DOCSIS 1.0 PHY and MAC layer chipsets in the sametimeframe. Interoperability testing occurred throughout 1997 and 1998, and actual CableLabsCertification Wave testing began in June 1998. The first DOCSIS 1.0 certifications (for CMs) andqualifications (for CMTSs) occurred in March 1999, ending a 2-year process from specificationavailability to certification/qualification of the resulting DOCSIS 1.0 equipment. Rapid deployment ofDOCSIS 1.0 equipment began in 1999, and the pace of deployment has increased ever since. (Note:The success of DOCSIS deployment sometimes masks the fact that the Cable industry has only beendelivering DOCSIS equipment to subscribers for about two-and-a-half years, i.e. as a technology, it isstill in its formative years and will continue to evolve and improve as technological advancementsbecome available.)The second standard (DOCSIS 1.1) was released as an Interim specification in March 1999. It isinteresting to note that the availability of the DOCSIS 1.1 specification coincides with the firstcertification/qualification of DOCSIS 1.0 equipment. In the early days of DOCSIS 1.1 equipmentdevelopment, many vendors (especially CMTS vendors) planned to add simple software upgrades totheir DOCSIS 1.0 equipment to create DOCSIS 1.1 equipment. However, implementing the DOCSIS1.1 features inside of legacy DOCSIS 1.0 hardware proved to be extremely challenging. In addition,the arrival of the DOCSIS 1.1 era also marked the beginnings of a new class of CMTS equipment thatcame to be known as “Carrier Class CMTSs” or “Next-gen CMTSs.” This new generation ofequipment introduced many novel features outside of the scope of the DOCSIS 1.1 specification, suchas scalability, reliability, enhanced observability, and wire-speed performance. Many of these featureswere being added in preparation for future telephony-over-IP services that would be offered over thehybrid fiber/coax (HFC) plant. As a result, many equipment vendors had to add many new elementsto their previous designs, so the development stage took longer than originally expected. In addition,www.arrisi.com Page 2 of 2 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the Blockmany vendors came to realize that the DOCSIS 1.1 specification itself was much more complicatedthan a simple point release on the DOCSIS 1.0 specification (as the 1.1 numbering would imply). As aresult, the development of DOCSIS 1.1 equipment and the development of corresponding testingprograms at CableLabs also took longer than originally expected. Interoperability testing occurredthroughout 2000 and 2001, and the first DOCSIS 1.1 certifications (for CMs) and qualifications (forCMTSs) occurred in September 2001, ending a two-and-a-half year process from DOCSIS 1.1specification availability to the certification/qualification of equipment. DOCSIS 1.1 equipmentdeployments began in the fourth quarter of 2001, but the deployment rates were initially hampered bya slowed economy and by the small number of vendors that had successfully achievedcertification/qualification for the very complicated DOCSIS 1.1 specification. However, thedeployment rates were beginning to show some acceleration in first half of 2002.The third standard (DOCSIS 2.0) was released as an Interim specification in December 2001. Thisrelease occurred three months after the first DOCSIS 1.1 equipment was certified/qualified. Lookinginto the future of DOCSIS 2.0 development is difficult at this point in time, but the current DOCSIS2.0 program plan at CableLabs calls for interoperability testing to occur in second quarter of 2002, andit calls for Certification Waves to begin in third quarter of 2002.  These ambitious plans may beachievable, but past experience indicates that other DOCSIS specifications required from two to two-and-a-half years between specification availability and the first certifications/qualifications ofequipment. Even if those typical schedules are compressed during the development of DOCSIS 2.0, itmay still take well into 2003 before a large number of equipment vendors are producing full DOCSIS2.0-certified/qualified equipment. Some equipment vendors may opt to deliver equipment withDOCSIS 2.0 features earlier than others, but to provide delivery on an expedited schedule, thedelivered equipment may have to initially sacrifice many of the “next-gen” features that are nowexpected by the MSOs. Other equipment vendors may opt to deliver subsets of the DOCSIS 2.0features (such as ATDMA only) in phased releases throughout the next two years.DOCSIS text text DOCSIS1.0 Spec 1.0 Cert DOCSIS text text DOCSIS 1.1 Spec 1.1 Cert DOCSIS DOCSIS text text text 2.0 Spec 2.0 Cert ? 1997 1998 1999 2000 2001 2002 2003 time Figure 1 - DOCSIS Time-lineThe time-lines for the various DOCSIS developments are shown in Figure 1.www.arrisi.com Page 3 of 3 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockFeature Evolutions Leading up to DOCSIS 2.0In order to understand the reasons for adding DOCSIS 2.0 features to the list of features alreadyprovided in the DOCSIS 1.0 and DOCSIS 1.1 specifications, it is important to understand the featuresthat already exist in the preceding specifications. Particular attention must be paid to the MAC andPHY layers in the preceding specifications. It is also important to understand the motives and goalsbehind each of the preceding specifications.Each of the DOCSIS specifications augmented the features of the previous specification with newfeature sets that provided more Cable Data functionality to the system operators and their subscribers.In this section, the primary features contained in each of the preceding DOCSIS specifications will beoutlined.DOCSIS 1.0 FeaturesThe DOCSIS 1.0 specification can be viewed as the starting point for all present and future CableData services. The principle focus of the DOCSIS 1.0 specification was to create a means oftransporting IP data bi-directionally across the existing HFC plant between subscriber devices and theInternet. The defined transport architecture had to be compatible with the existing video deliveryservices that were already resident on the HFC plant.The DOCSIS 1.0 specification can be viewed as the starting point for all present and future CableData services. It defines the fundamental mechanisms for moving data up and down the HFC plant,and it also defines the way that different sub-systems will interact with one another in a fully deployedCable Data network. The DOCSIS 1.0 specification defines basic vanilla, “Best Effort” data transportbetween the subscriber and the Internet. As a result, it represents the baseline Cable Data architecturefor the Cable industry. This baseline architecture is illustrated in Figure 2 with the two keycomponents (the CMTS and the Cable Modem) highlighted in yellow.www.arrisi.com Page 4 of 4 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the Block Telephony PSTN Gateway Video 1 Optical CMTS Dwnstream 50-860 Video 2 Transceivers RF MHz Headend Mod Downstream Combiner Backbone Router Network Data E/O Fiber Network Interface Node Upstream Upstream O/E Fiber Demod Pairs Data Splitter & 5-42 Filter MHz Bank HFC Local Coaxial Servers Operations Security & (DHCP, Support Access Distribution TFTP, System Controller Leg TOD, etc.) Distribution Hub or Headend Cable Modem CPE (PC) Figure 2 - Baseline Cable Data ArchitectureThe DOCSIS 1.0 specification was the successful mix of compromise proposals from several differentvendors, system operators and Internet Service Providers. The members of the MCNS consortiumdetermined the final content of the DOCSIS 1.0 specification. After much debate, the MCNSconsortium decided that the DOCSIS 1.0 specification should include means of providing all of thefollowing features: 1) Uniform and consistent service as seen by any subscriber 2) Open, non-proprietary operations that permit equipment from multiple vendors to interoperate 3) CMs with low power consumption (4-10 W) that could ultimately be sold in a retail market with no user-configurable parameters 4) Asymmetric transport of data with more downstream bandwidth than upstream bandwidth to match the asymmetric data flows of most Internet applications of the time (i.e. Web surfing) 5) Efficient downstream transport of data encapsulated in MPEG streams with 27-36 Mbps of total user bandwidth carried in a single 6 MHz-wide channel inside of the typical HFC downstream spectrum (88-860 MHz center frequencies) 6) Support for either 64QAM (30.341646 Mbps) and 256QAM (42.884296 Mbps) operation in the downstream channel 7) Flexible, robust upstream transport of data with 0.32-10.24 Mbps of total bandwidth carried in a single 0.2-3.2 MHz channel inside of the typical HFC upstream spectrum (5-42 MHz center frequencies) 8) Simple security measures that provide assurances of privacy for data transported over the shared HFC plantwww.arrisi.com Page 5 of 5 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the Block 9) Simple network management of equipment via the Simple Network Management Protocol (SNMP) 10) Remote software upgrades for improvementsWhile detailed analysis of all of the features in the DOCSIS 1.0 specification is beyond the scope ofthis paper, a glance at some details behind the upstream PHY and MAC layers will undoubtedly provebeneficial.The DOCSIS 1.0 upstream MAC assumes that Frequency-Division Multiplexing and Time-DivisionMultiple Access (FDM/TDMA) techniques will be used to coordinate the activity of all of theattached CMs on the shared upstream resource. This implies that multiple upstream channels willtypically be squeezed into the single 5-42 MHz upstream spectrum, but each CM will be assigned to asingle, particular upstream channel frequency (using FDM techniques). However, multiple CMs canbe assigned to the same upstream channel, so each CM that needs to transmit data up the HFC plantmust request bandwidth, and the CMTS will dynamically assign it a burst interval (time-slot) duringwhich it can send its data without interfering with the other CMs on that particular upstream channel.Thus, many CMs time-share the bandwidth on a single upstream channel (using TDMA techniques).The DOCSIS 1.0 PHY specification permitted the use of either 4-point Quadrature Phase ShiftKeying modulation (QPSK) or 16-point Quadrature Amplitude Modulation (16QAM) to encode thesignal bits into RF symbols for upstream transmission on the HFC plant. QPSK encodes two bitswithin each symbol, while the more efficient 16QAM encodes four bits in each symbol (Figure 3).The modulation format can be changed for each of the burst intervals. Q Q 0111 0101 1101 1111 01 11 0110 0100 1100 1110 I 0010 0000 1000 1010 I 00 10 0011 0001 1001 1011 (a) QPSK symbol mapping (b) 16QAM symbol mapping (2 bits/symbol) (4 bits/symbol) Figure 3 - Signal constellations for QPSK and 16QAMwww.arrisi.com Page 6 of 6 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockEach upstream channel connecting CMs to a CMTS must be centered in the 5-42 MHz portion ofthe HFC spectrum, but different channels will typically be set on different center frequencies. Inaddition, each channel must be assigned one of five different spectral widths. The assignedspectral width will determine the upstream symbol rate permitted on the particular channel, andthe symbol rate combined with the modulation format (QPSK or 16QAM) will determine theupstream bit-rate on the channel. Table 1 below indicates the various combinations of upstreamchannel parameters permitted by the DOCSIS 1.0 specification. Channel Width Symbol Rate Bit-rate for QPSK Bit-rate for 16QAM (kHz) (ksymbols/sec) (kbps) (kbps) 200 160 320 640 400 320 640 1,280 800 640 1,280 2,560 1,600 1,280 2,560 5,120 3,200 2,560 5,120 10,240 Table 1 - DOCSIS 1.0 Upstream Channel Parameters(Note: All upstream transmissions from a CM to a CMTS use a 25% Square Root Raised Cosine forSymbol Shaping which yields the symbol rate numbers shown in Table 1).The bit-rate associated with an upstream channel is closely tied to two fundamental characteristics ofthe channel: the symbol rate (R) and the modulation format. The modulation order (M) is defined asthe number of points in the signal constellation, and the channel bit-rate is therefore given byR*log2(M). Thus, an increase in the upstream channel bit-rate requires one or more of the followingchanges: 1) an increase in the upstream symbol rate R 2) an increase in the spectral efficiency (modulation order M) of the modulation format.Unfortunately, this increase in upstream channel bit-rate cannot be accomplished without paying aprice. The price is the resulting increase in required HFC plant signal-to-noise ratios that are requiredto produce acceptable bit-error-rate levels for higher symbol rates and higher-order modulationformats. For example, the points in the 16QAM signal constellation are more closely packed than thepoints in the QPSK signal constellation (see Figure 3). Thus, the bit-rate benefits of 16QAM do notcome for free, because 16QAM requires a higher signal-to-noise ratio for acceptable bit-error-ratelevels than QPSK. In a similar vein, higher symbol rates result in less time for symbol sampling, soerrors due to noise are also more likely to occur in high symbol-rate signals than in low symbol-ratesignals. In addition, higher symbol rates use a wider portion of the upstream spectrum. In addition tobeing a precious resource that may or may not be available, this wider portion of the spectrum alsopermits more noise power to be coupled into the channel (producing lower signal-to-noise ratios). Asa result, HFC plants with even moderate levels of noise may produce high bit error rates when using16QAM or high symbol rates, so many system operators are forced to by-pass the bandwidth benefitsof 16QAM and high symbol rates and they are forced to use QPSK at lower symbol rates (1,280ksymbols/sec is common). As a result, many system operators are limited to raw bit-rates of (1.28Msymbols/sec)x(2 bits/symbol for QPSK)=2.56 Mbps on their upstream channels.www.arrisi.com Page 7 of 7 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockTDMA (Time-Division Multiple Access) technology permits multiple users (CMs) to share thebandwidth within an upstream channel by allowing each of the users to transmit by themselves withina unique burst interval (time slot). These burst intervals can be variable in length. There are twofundamental types of burst intervals that can be specified by the CMTS for upstream traffic transport-contention burst intervals and non-contention burst intervals. Defining the burst interval duration,defining the burst interval type (contention or non-contention), and defining the CM associated witheach non-contention burst interval are some of the fundamental tasks performed by the CMTSoperating in a TDMA mode of operation.Contention burst intervals define windows of time that can be shared by all CMs in a CSMA-likefashion. As would be expected, date corruption due to collisions can occur during contention burstintervals, because multiple CMs can be simultaneously transmitting in a single burst interval window.When this occurs, standard back-off and re-transmission techniques are employed.Non-contention burst intervals define a window of time during which a particular, pre-assigned CMcan transmit upstream data. Intelligent scheduling algorithms in the CMTS are used to assign a uniqueCM to a particular burst interval. The results of the scheduling algorithm are communicated to allCMs through MAPs that are periodically (once every 1-10 msec) injected into the downstreamchannels going from the CMTS to the CMs. CM #1 CM #2 Transmission Transmission (16QAM & (QPSK & 2.56 Msym/sec) 2.56 Msym/sec) Packet Packet Data Data Guard MAC Guard MAC Time Header Codeword Codeword Time Header Codeword Codeword Preamble Preamble Information FEC Parity Minislot Information FEC Parity Minislot (data) Bytes Padding (data) Bytes Padding time Figure 4 - Temporal sequencing of consecutive upstream burst intervalswww.arrisi.com Page 8 of 8 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockThe temporal sequencing of consecutive burst intervals is illustrated in Figure 4. Note that every burstinterval on a particular upstream channel must use the same symbol rate, but each burst interval canuse a different modulation format. The user data is transmitted within the codeword regions of thepacket data field.As noted in Figure 4, there are several required fields in the transmitted data stream that constituteoverhead. These include the guard time (used to separate the end of one CM’s transmission and thestart of another CM’s transmission), the preamble (used by the receiver to phase lock onto the arrivingdata from a new CM transmission), the MAC header (used to identify the source and/or the type of thedata transmission), the Forward-Error Correction (FEC) parity bytes (used to correct random errorsthat might occur due to noise within the packet data), and the Minislot padding (used to extend theburst interval to a Minislot boundary). Under normal operating conditions, the overhead can accountfor 5-30% of the channel bandwidth depending on the settings that are used.Typically, the required guard time period between two successive burst interval transmissions will bea function of symbol rate. Typical numbers might be eight symbol periods for 160 ksymbol/sec to 640ksymbol/sec rates, and sixteen symbol periods for 1.28 Msymbol/sec to 5.12 ksymbol/sec rates.The preamble field is programmable, and its length can range from zero to 128 bytes. The preamblefield period will be a function of symbol rate, modulation format, and channel noise levels. Typicalnumbers might be eight bytes for QPSK at 160 ksymbol/sec to 640 ksymbol/sec rates, twelve bytesfor QPSK at 1.28 Msymbol/sec to 5.12 ksymbol/sec rates, sixteen bytes for 16QAM at 160ksymbol/sec to 640 ksymbol/sec rates, and twenty-four bytes for 16QAM at 1.28 Msymbol/sec to5.12 ksymbol/sec rates.For data packets, the MAC header field will typically be a fixed length of six or eight bytes (althoughextended MAC headers of up to 246 bytes can also be used).In an upstream channel, errors can often occur that are due to burst noise. This type of noise typicallycorrupts a small numbers of bytes in the transmitted data block. In an attempt to combat these errors, aReed-Solomon Forward-Error Correction (RS-FEC) scheme can be used in the DOCSIS 1.0specification. This scheme segments the data into multiple codewords and adds correcting parity bytesrelated to the data within the codeword. The use of Forward-Error Correction introduces a bandwidthpenalty due to the overhead of the correcting parity bytes. The Forward-Error Correction algorithmused in DOCSIS 1.0 permits the correction of up to ten errored bytes within a codeword, and thenumber of parity bytes actually used for Forward-Error Correction is programmable.For example, assume that it is desired that the Forward-Error Correction algorithm be capable ofcorrecting T errored bytes within a codeword. Thus, 2T parity bytes must be added within eachcodeword. Assume that there are k information bytes stored within each codeword. Then for a datablock of length L bytes, there will be ceil(L/k) codewords created. The parity bytes are at the end ofeach codeword region. Typical values might be k=40 to 220 information bytes and T=4 to 10correctable bytes. Selection of these values must ensure a balance between the required errorcorrection given the channel noise and the resulting overhead of the additional Forward-ErrorCorrection parity bytes.www.arrisi.com Page 9 of 9 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockMinislot padding is added to extend burst intervals to minislot boundaries, and will often add anaverage of eight to sixteen bytes.It is important to realize that an upstream TDMA channel works by permitting multiple CMs to sharethe channel. CMs must take turns using the upstream burst intervals, and the CMTS is responsible forscheduling the CMs appropriately. At any moment in time, useful transmission is occurring if andonly if one and only one CM is transmitting in the upstream channel. If the channel supports a raw bit-rate of 2.56 Mbps, then a particular CM that is assigned the channel for a particular burst interval willbe the sole owner of that bandwidth and have access to all of the 2.56 Mbps transfer rate during theduration of that burst interval. For example, if a particular CM is assigned a 1 msec burst interval onthat 2.56 Mbps upstream channel, then the CM is capable of transferring up to (2.56 Mbps)x(1msec)/(8 bits/byte)=320 bytes in its burst interval (ignoring the overhead required for guard time,preamble, MAC Header, and Forward-Error Correction parity bytes). If on average, a particular CM isgranted ten percent of the burst intervals on a 2.56 Mbps upstream channel, then that CM will haveaccess to roughly 256 kbps of bandwidth in the upstream channel (ignoring the overhead required forguard time, preamble, MAC Header, and Forward-Error Correction parity bytes).DOCSIS 1.1 FeaturesFor all of its success, DOCSIS 1.0 was, for the most part, providing plain vanilla, Best-Effortbroadband data transport between subscribers and the Internet. As the forward-looking thinkersamong the DOCSIS architects began to visualize the future, they realized that there would be adefinite need for some improvements over the original DOCSIS 1.0 feature set. These needs led to theDOCSIS 1.1specification.DOCSIS 1.1 had several fundamental goals: 1) Add an 8-tap, symbol-spaced linear pre-equalizer to CMs to improve the upstream channel’s ability to correctly receive signals in the presence of micro-reflections and other HFC plant distortions 2) Remain backwards compatible with DOCSIS 1.0 3) Add Quality of Service (QoS) features to DOCSIS 1.0 to ensure that Voice over IP (i.e. PacketCable) could be provided in the near future 4) Add Quality of Service (QoS) features to DOCSIS 1.0 to ensure that tiered data services and other delay-sensitive applications could be supported in the near future 5) Add the ability to classify packets from one cable modem into different QoS service flows with different performance levels 6) Improve the HFC bandwidth efficiencies through the use of fragmentation, concatenation, and payload header suppression (especially for the smaller, jitter- intolerant VoIP packets) 7) Add SNMPv3 capabilities to guarantee secure network management 8) Add CM authentication to the security mechanisms to guard against theft of service 9) Add improvements to the key and encryption processes to provide improved privacy for data transported across the shared HFC 10) Add standardized methods for IP multicast supportwww.arrisi.com Page 10 of 10 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the Block 11) Add IP filtering capabilities to permit the establishment of subscriber-dependent firewalls 12) Augment the existing set of counts and statistics with new counts and MIBs that are useful for performance monitoring and billingWhile all of these goals are beneficial, most system operators have concentrated on the benefits ofQoS. Many clever QoS tools and features were added to the DOCSIS 1.1 specification that could beused to create and manage advanced data, voice, and video services that were simultaneously flowingacross the same CMTS, the same HFC plant, and the same CM. Most of these tools manipulate thesettings of a “service flow.” A service flow can be defined as a set of packets flowing through theCMTS or CM that share some unique, identifiable set of parameters (classifiers). Typically, all of thepackets within a service flow have a common source point and destination point within the Internet.Associated with each service flow is a service level definition defining how a packet within that par-ticular service flow should be treated by the network element. If an MSO provides these service leveldefinitions to their subscribers in the form of a service contract, the resulting contract is often definedas a Service Level Agreement. Proper use of QoS and Service Level Agreements is likely to generatemany new services and many new sources of revenue for system operators.For some operators, the first of these new revenue-generating services is likely to be Voice over IPtelephony service. The goal is to offer primary or secondary phone line service to their existing videosubscribers in a bundled package. CableLabs has developed the PacketCable specification to definethe architecture for Voice over IP telephony service in an MSO environment. This specificationdefines the manner in which IP-based telephone calls are set up and torn down over the HFC plant,and all of the approaches assume the existence of DOCSIS 1.1 QoS features within the network.Another new revenue-generating service is tiered data service. This approach to marketingacknowledges the fact that many existing cable TV subscribers are not willing to sign up for the dataservice at a high price. The theory is that a tiered data service with low-priority subscribers paying less(ex: $25 per month) and high-priority subscribers paying slightly more (ex: $40 per month) willproduce higher overall revenues than a single tier service offering at $40 per month. High-endsubscribers will still get their expected service levels, because their traffic is treated with higherprecedence than the low-end subscribers. Under most operating conditions, low-end subscribers willexperience much better throughput levels than they are currently experiencing with their dial-upservices. As a result, everybody is happy, and the MSO experiences higher revenues.Many MSOs expect to bundle all of the DOCSIS 1.1-enabled services (video, voice, and data) into asingle package deal that is priced to lure subscribers away from their existing telephony serviceproviders, DSL providers, and dial-up Internet Service Providers.www.arrisi.com Page 11 of 11 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockThe DOCSIS 2.0 SpecificationFrom the discussions above, it should be apparent that the DOCSIS 1.0 specification laid the basicfoundations for simple data transport across the HFC plant and concentrated primarily on thedefinition of a suitable set of PHY and MAC layer protocols to guarantee that data would beefficiently transmitted across the cable. The DOCSIS 1.1 specification shifted the focus of the work toQoS and other higher-level features that permitted advanced services to be mixed and offered over theHFC plant.In 2001, the architects of the DOCSIS 2.0 specification created a new specification in response to thegrowing demand for more upstream bandwidth on the cable. These demands emerged as peer-to-peerapplications (such as interactive gaming, MP3 file exchanges, voice over IP telephony, etc.) andbusiness-to-business applications (such as T1 replacements) began to appear on the HFC plant. Thesenew applications demanded a more symmetrical transport of data than the asymmetrical applications(i.e. Web surfing) that dominated the Internet in the past. In addition, upstream noise, HFC plantimpairments, and many legacy service offerings (such as previously-installed proprietary dataservices, set-top box transmissions, constant bit rate (CBR) Cable Telephony services, etc.) wereconsuming much of the available upstream bandwidth on the cable. (Note: Node splitting is onetechnique that can help circumvent these problems, because it effectively decreases the number ofsubscribers sharing an upstream spectrum. However, the costs associated with node splittingoftentimes make this approach undesirable.)To accommodate these new upstream bandwidth demands, the DOCSIS 2.0 specification shifted thefocus back to the PHY and MAC layer protocols in an attempt to augment the existing foundationwith some new and improved (and more complicated) modulation techniques. In particular, theDOCSIS 2.0 specification focused on changes to the upstream PHY and MAC layers. Beforeconsidering the actual benefits of these changes, it will prove beneficial to consider the original goalsbehind the DOCSIS 2.0 specification. These included: 1) Remain backwards compatible with DOCSIS 1.0 and DOCSIS 1.1 2) Provide the ability to support more symmetrical data transport 3) Increase the capacity in each upstream channel 4) Increase the spectral efficiency (bps/Hz) of the upstream spectrum 5) Provide for more noise immunity within the upstream channels 6) Correct any problems/oversights found in the DOCSIS 1.1 specificationFrom the above list, a few noticeable elements are excluded from the specification. First, DOCSIS 2.0does not provide any capacity enhancements for the downstream channel. (Note: Some chipsetvendors are providing proprietary solutions with advanced downstream improvements such as1024QAM embedded within their DOCSIS 2.0 chipsets.) In addition, DOCSIS 2.0 does not provideits capabilities with the promise of a simple software upgrade to DOCSIS 1.1 equipment. Theequipment requires new DOCSIS 2.0-capable hardware if the new features are to be enabled.www.arrisi.com Page 12 of 12 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockComparing the list of DOCSIS 2.0 goals with the goals listed for DOCSIS 1.0 and DOCSIS 1.1, it isthe opinion of the authors that the changes associated with DOCSIS 2.0 are less impacting than thechanges associated with DOCSIS 1.0 or DOCSIS 1.1. The delta between DOCSIS 1.0 and DOCSIS1.1 impacts a large amount of the operations and provisioning work carried out by MSOs, whereas thedelta between DOCSIS 1.1 and DOCSIS 2.0 basically affects the physical transport of bits on thecable. In fact, most of the changes for DOCSIS 2.0 will appear in the PHY and MAC layer chip-setsin the CM that place the modulated waveforms on the upstream cable and in the PHY and MAC layerchip-sets in the CMTS that extract the modulated waveforms from the upstream cable. Nevertheless,these DOCSIS 2.0 modulation changes do require new hardware and software (for CMs and CMTSs).In addition, the changes are likely to have an impact on the permitted subscriber services and theplanned HFC plant architectures in the future. As a result, it is advantageous to examine these changesin some level of detail.TDMA (as briefly described in an earlier section) is the catch all phrase defining the upstreammodulation technique used for DOCSIS 1.0 and DOCSIS 1.1. In DOCSIS 2.0, two new modulationtechniques have been proposed as improvements over the earlier TDMA scheme. These newmodulation techniques are known as Advanced Time-Division Multiple Access (ATDMA) andSynchronous Code-Division Multiple Access (SCDMA). DOCSIS 2.0 requires that the CMTS andthe CM support all three of these modulation techniques (TDMA, ATDMA, and SCDMA).It will be shown that the TDMA/ATDMA approaches are quite different from the SCDMA approach.In particular, one can draw some good analogies between these technologies and commoncommunication forums. For example, it will be shown that TDMA/ATDMA are similar to thecommunication forum used in conference presentations. In particular, each speaker takes control ofthe podium for a specific period of time, and they must speak rapidly to communicate theirinformation before relinquishing the podium to the next speaker (who must repeat the process).SCDMA, on the other hand, is similar to a party where many conversations are occurring in parallel,but the speaker and listener of each conversation are only “tuned in” to their information exchange,and the other conversations merely create background noise. To make the analogy even more correct,assume that each of the many conversations at the party is being spoken in a different language so thatthe other conversations are not even understood by the two people in a particular conversation.Whether employing ATDMA or SCDMA, most of the changes specified by the DOCSIS 2.0specification can be sub-divided into two fundamental areas of concern: upstream noise mitigationimprovements and upstream bandwidth improvements. The following sections will explore the detailsof ATDMA and SCDMA, and they will also highlight the noise mitigation improvements and theupstream bandwidth improvements provided by each of the technologies.www.arrisi.com Page 13 of 13 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockCharacteristics of Advanced Time-Division Multiple Access (ATDMA)In general, the DOCSIS 2.0 ATDMA scheme is very similar to the DOCSIS 1.X TDMA scheme, butATDMA includes some important enhancements.As their names would imply, both TDMA and ATDMA are Time-Division Multiple Accesstechnologies that permit multiple users (CMs) to share the bandwidth within an upstream channel byallowing each of the users to transmit by themselves within a unique burst interval (time slot). This iswhy TDMA and ATDMA transmissions use the bandwidth in a manner similar to the way in whichspeakers share the podium in a conference: one speaker at a time.In both the TDMA and the ATMDA case, burst intervals can be variable in length, and bothcontention burst intervals and non-contention burst intervals can be specified by the CMTS forupstream traffic transport. The temporal sequencing of consecutive burst intervals illustrated inFigure 4 can be applied to the ATDMA space as well as the TDMA space.Let us now explore the improvements enabled by the ATDMA approach specified within DOCSIS2.0.Upstream Bandwidth Improvements Provided by ATDMAThe ATDMA specification provides many new techniques that will enable MSOs to operate upstreamchannels with higher throughputs (assuming the noise mitigation techniques described below willpermit the higher throughput operation in the presence of the channel noise). Several mechanismswere added to the ATDMA specification to permit this improved operation: 1) Three new upstream channel modulation formats were added by ATDMA. These include 8- point QAM (8QAM), 32-point QAM (32QAM), and 64-point QAM (64QAM). The modulation format can be changed for each of the burst intervals. 2) A higher symbol rate of 5.12 Msymbol/sec was added by ATDMA. 3) The preamble can be transmitted with higher power to permit synchronization to occur more rapidly. This may permit the use of shorter preambles, which will eliminate some of the overhead associated with ATDMA transmission. This results in more bandwidth available for the transmission of user traffic. 4) The maximum preamble length was increased to 1536 bits to aid in channel synchronization when using the higher 6.4 MHz channels. (Note: DOCSIS 1.X limited the preamble length to 1024 bits).www.arrisi.com Page 14 of 14 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the Block Channel Symbol Rate Symbol Rate Bit-rate Bit-rate Bit-rate Bit-rate Width R R for for for for (kHz) (ksymbols/sec) (ksymbols/sec) 8QAM 16QAM 32QAM 64QAM (kbps) (kbps) (kbps) (kbps) 200 160 320 480 640 800 960 400 320 640 960 1,280 1,600 1,920 800 640 1,280 1,920 2,560 3,200 3,840 1,600 1,280 2,560 3,840 5,120 6,400 7,680 3,200 2,560 5,120 7,680 10,240 12,800 15,360 6,400 5,120 10,240 15,360 20,480 25,600 30,720 Table 2 - ATDMA Upstream Channel ParametersTable 2 indicates the various combinations of upstream channel parameters permitted by the ATDMAspecification.Upstream Noise Mitigation Improvements Provided by ATDMAThe ATDMA specification provides many new techniques that will enable MSOs to operate upstreamchannels within noisy environments. These include: 1) The maximum number of Reed-Solomon Forward-Error Correction parity bytes within each codeword was increased so that up to sixteen errored bytes could be corrected. This increases the overall immunity of the transmission to lengthy burst errors at the expense of added overhead for the parity bytes. (Note: DOCSIS 1.X only permitted up to ten errored bytes to be corrected.) 2) The number of taps within the CM-based linear pre-equalizer was increased so that up to 24 symbol-spaced taps could be programmed. As a result, micro-reflections with even more delay can be suppressed. In addition, the shorter symbol spacings that result from the use of 5.12 Msymbol/sec channels in DOCSIS 2.0 can also be more readily accommodated. In addition, most vendors have greatly enhanced their algorithms for pre-equalizer training. (Note: DOCSIS 1.X only permitted up to eight symbol-spaced taps to be programmed.) 3) Vendor-specific (proprietary) ingress noise cancellation techniques based on advanced digital signal processing have also been added to the CMTS receivers by all of the DOCSIS 2.0 chipset vendors. These techniques identify common noise types found in the upstream channel and attempt to intelligently filter them out of the ATDMA data stream.www.arrisi.com Page 15 of 15 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the Block 4) The bytes within multiple codewords are interleaved (mixed) so that bytes from a single codeword are no longer transmitted consecutively within the upstream channel. As a result, a burst error whose duration spans multiple bytes will corrupt only a few bytes from each of the codewords instead of corrupting many bytes from a single codeword. This increases the overall immunity of the transmission to lengthy burst errors, because the Reed-Solomon Forward-Error Correction techniques can easily correct errors if the number of errored bytes per codeword is kept to a minimum. (Note: DOCSIS 1.X transmitted the bytes for a single codeword consecutively, so a lengthy burst error could corrupt many bytes within a single codeword and make it difficult for the Forward-Error Correction techniques to correct all of the byte errors).In general, all of these noise mitigation techniques should aid the upstream transmission ofsignals in the presence of many different types of channel noise, including Additive WhiteGaussian Noise (AWGN), impulse noise, burst noise, and micro-reflections.www.arrisi.com Page 16 of 16 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockCharacteristics of Synchronous Code-Division Multiple Access(SCDMA)Whereas the DOCSIS 2.0 ATDMA scheme and the DOCSIS 1.X TDMA scheme are somewhatsimilar in basic structure and operation, the DOCSIS 2.0 Synchronous Code-Division Multiple Access(SCDMA) scheme is a member of an entirely different genre of transmission techniques. As statedearlier, SCDMA uses the bandwidth in a manner similar to the way in which people at a party canhave many different parallel conversations in different languages and not interfere with one another.SCDMA transmission has been described as a “spread-spectrum” technology or a “spread-time”technology. Neither of these terms truly describes the clever set of tricks that are used in SCDMA. Letus attempt to describe how SCDMA actually operates (at a high level).As a starting point, consider a baseline 3.2 MHz-wide upstream channel that is capable of transportinga DOCSIS 1.X TDMA signal using 16QAM. Using TDMA technology, a single cable modem cantransmit in the upstream channel in a given burst interval. The transmission produces a sequentialstream of 16QAM symbols, and each symbol has a period of 390.625 nsec. The permitted symbol ratein the channel is 2.56 Msymbol/sec, and the resulting bit-rate (with four bits per symbol) is 10.24Mbps.Now assume that we want to use the same 3.2 MHz-wide channel to transmit a stream of 16QAMsymbols using SCDMA transmission instead of TDMA transmission. From a high-level point-of-view, SCDMA modifies the original symbol-stream using two clever tricks. The first trick is known assymbol spreading. Symbol spreading requires that each symbol be stretched (or spread) in time by afactor of 128, so a single spread symbol would have a period of (390.625 nsec)*(128)=50 usec. Thepermitted symbol rate for this single SCDMA symbol stream in the channel is 20 ksymbol/sec, andthe resulting bit-rate (with four bits per symbol) is 80 kbps (which is 1/128th of the bit-rate for theTDMA symbol stream carried in the same 3.2 MHz-wide channel).The longer symbol period in SCDMA transmission is known as the “spreading interval.” UsingFourier Analysis techniques, it can be shown that these spread symbols consume only 1/128th of thespectral bandwidth that is used by the shorter-period TDMA symbols (see Figure 5(a) and Figure5(b)).www.arrisi.com Page 17 of 17 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the Block y(t) 390.625 nsec Y(f) signal F 0 50 time (usec) 0 5 freq (MHz) (a) TDMA signal y(t) 50 usec Y(f) spread F signal 0 50 time (usec) 0 50 freq (kHz) (b) SCDMA spread signal y(t) 390.625 nsec chip Y(f) F 0 50 time (usec) 0 5 freq (MHz) (c) SCDMA-encoded signal Figure 5 - Signals and Fourier transforms of signalsThe second clever trick used by SCDMA is to re-use the entire spectrum within the channel (since thespectral bandwidth of the spread symbol is only 1/128th of the spectrum in the original TDMA signal).SCDMA accomplishes this by multiplying each spread symbol by a spreading code containing aunique string of 128 code symbols. Each code symbol must be assigned either a +1 value or a –1value, and these code symbols are typically called “chips.” Since 128 code symbols (or chips) mustfill the entire 50 usec spreading interval, the chip duration is (50 usec)/128 =390.625 nsec. It isimportant to note that the SCDMA chip duration is identical to the period of the symbols used in theoriginal TDMA case, so the resulting SCDMA chip rate is identical to the original TDMA symbolrate. Fourier Analysis techniques can be employed to show that the bandwidth utilized by the streamof SCDMA-encoded spread symbols is practically the same as the bandwidth utilized by the originalTDMA symbol stream (see Figure 5(a) and Figure 5(c)).The receiver (de-spreader) for an SCDMA transmission system is actually a matched filter orcorrelator that correlates the received data stream with the spreading code associated with thedesired transmitter.There are many different spreading codes (combinations of +1 and –1 chips) that can bespecified. If a particular sequence of +1 and –1 chips associated with one spreading code areused to fill the elements of a column vector x and if a different sequence of +1 and –1 chipsassociated with a second spreading code are used to fill the elements of a second column vectory, then employing a simple concept from linear algebra, the two vectors are said to be orthogonalwww.arrisi.com Page 18 of 18 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the Blockif the inner product of the two vectors is zero (xTy=0). The selection of orthogonal spreadingcodes provides a major benefit, because symbol streams from many different sources (cablemodems) can be SCDMA-encoded using different spreading codes and they can then becombined on the same upstream channel. It can be shown that any one of the source symbolstreams can be recovered from the resulting aggregated mix of symbol streams if the spreadingcodes from each source are orthogonal to one another. The receiver at the CMTS can “tune” to asymbol stream from a particular source using the unique spreading code associated with thatparticular source. The recovery of a particular symbol stream from the aggregated mix of symbolstreams is accomplished by multiplying the aggregated mix of symbol streams by the uniquespreading code associated with the source and summing the terms to produce a weighted versionof the desired symbol stream at the receiver. This process is known as “de-spreading” thetransmitted symbol stream, and it essentially calculates the inner product of the combinedstreams (Ax+By) and the spreading code for the desired source. For example, x would be used tode-spread symbols from source #1. The result of this inner product calculation is (Ax+By)Tx =AxTx+ByTx = AxTx+0 = A*|x|2, which is a weighted version of the original spread symbol A.All of these operations are illustrated in Figure 6. Source A [-A +A] #1 X Spreading code Modulator xT=[-1 +1] + Spreading code Modulator yT=[-1 -1] Channel X Source #2 B [-B -B] [-A-B +A-B] -1 +1 =2A [-A-B +A-B] 2A Demod Correlator Source #1 Output spreading code xT=[-1 +1] Figure 6 - Typical SCDMA channel with two sourceswww.arrisi.com Page 19 of 19 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockIt should be apparent that de-spreading works only if x and y are orthogonal. If the spreading codesfrom different transmitters are not phase aligned, then orthogonality between the spreading codes issacrificed, and the recovery of the original spread symbols becomes prone to errors. Thus, the transmitclocks within the SCDMA sources (cable modems) must maintain a high degree of accuracy tomaintain adequate chip-level phase alignment and modulator carrier phase alignment. Thisnecessitates synchronous operation within SCDMA transmitters. The required SCDMA cable modemranging accuracy must be approximately +/- 0.01 of the nominal symbol period, which ensures thatthe spreading codes from different cable modems remain fairly well synchronized.In DOCSIS 2.0 SCDMA operation, up to 128 simultaneous symbol streams (with differentspreading codes) can be driven by many different cable modems. A burst from a single cablemodem may be transmitted on two or more spreading codes within a frame, so up to 64 cablemodems could be simultaneously transmitting in a frame. The CMTS mapping algorithmcontrols which combinations of cable modems are transmitting on a frame-by-frame basis. Thismapping algorithm is responsible for changing the number of spreading codes assigned to eachcable modem, and it can also change the frame size, so the algorithm can maintain tight controlover the amount of bandwidth assigned to each cable modem (see Figure 7). The intelligencewithin the mapping algorithm will ultimately determine the efficiency and fairness of theSCDMA transport scheme. burst interval burst interval burst interval burst interval w/ 16QAM & w/ QPSK & w/ QPSK & w/ 8QAM & 2.56 Msym/sec 2.56 Msym/sec 2.56 Msym/sec 2.56 Msym/sec burst interval burst interval burst interval burst interval w/ 8QAM & w/ 32QAM & w/ 64QAM & w/ 8QAM & 2.56 Msym/sec 2.56 Msym/sec 2.56 Msym/sec 2.56 Msym/sec frame 1 frame 2 frame 3 code 127 burst interval w/ 64QAM & CM #4 CM #4 CM #4 2.56 Msym/sec code 96 code 95 CM #3 CM #3 code 64 code 63 CM #2 CM #2 code 32 code 31 CM #1 CM #1 code 0 time Figure 7 - Example of multiple CMs using different spreading codeswww.arrisi.com Page 20 of 20 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockUpstream Bandwidth Improvements Provided by SCDMASCDMA provides many new techniques that will enable MSOs to operate upstream channelswith higher throughputs.  SCDMA can use all of the bandwidth improvement techniquesdefined for ATDMA. In addition, SCDMA may permit shorter preambles due to the use ofsynchronous transmission. SCDMA also offers another modulation format known as 128-pointQAM or 128QAM. This format can only be enabled when a particular noise mitigation techniqueknown as Trellis-Coded Modulation (TCM) is being used. 128QAM with TCM provides thesame bit-rate performance as64QAM without TCM.Channel Symbol Rate R Bit-rate for Bit-rate Bit-rate Bit-rate Bit-rate Width (ksymbols/sec) QPSK for 8QAM for for for (kHz) (kbps) (kbps) 16QAM 32QAM 64QAM (kbps) (kbps) (kbps) 1,600 1,280 2,560 3,840 5,120 6,400 7,680 3,200 2,560 5,120 7,680 10,240 12,800 15,360 6,400 5,120 10,240 15,360 20,480 25,600 30,720 Table 3- SCDMA Upstream Channel Parameters (without TCM enabled)Table 3 indicates the various combinations of upstream channel parameters permitted by the SCDMAspecification when Trellis-Coded Modulation (TCM) is not enabled. Table 4 indicates the variouscombinations of upstream channel parameters permitted by the SCDMA specification when TrellisCoded Modulation is enabled. It should be noted that the three lowest symbol rates (160ksymbols/sec, 320 ksymbols/sec, and 640 ksymbols/sec) that are permitted by TDMA and ATDMAoperation are not permitted by SCDMA operation. Note also that with TCM enabled (Table 4), one ofthe bits in each symbol is borrowed for use as the TCM code bit.Channel Symbol Rate R Bit-rate Bit-rate Bit-rate Bit-rate Bit-rate Bit-rate Width (ksymbols/sec) for for for for for for (kHz) QPSK 8QAM 16QAM 32QAM 64QAM 128QAM (kbps) (kbps) (kbps) (kbps) (kbps) (kbps) 1,600 1,280 1,280 2,560 3,840 5,120 6,400 7,680 3,200 2,560 2,560 5,120 7,680 10,240 12,800 15,360 6,400 5,120 5,120 10,240 15,360 20,480 25,600 30,720 Table 4- SCDMA Upstream Channel Parameters (with TCM enabled)www.arrisi.com Page 21 of 21 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockUpstream Noise Mitigation Improvements Provided by SCDMASCDMA provides many new techniques that will enable MSOs to operate upstream channels withinnoisy environments. SCDMA can use all of the noise mitigation techniques defined for ATDMA. Inaddition, SCDMA offers several other noise mitigation techniques to help provide even morerobustness in the presence of impulse noise and burst noise. These other tricks are directly related tothe symbol spreading function, the symbol de-spreading function, and the Trellis-Coded Modulationfunction that are unique to the SCDMA specification.SCDMA symbol spreading can lead to improved noise immunity for specific types of burst noise,because burst noise is typically found to exist within a short duration of time. As an example, considera short noise burst that spans five symbols in the TDMA system of Figure 5(a). In the SDCMAsystem of Figure 5(c), that same short noise burst will exist within a single spread symbol, corruptingonly five chips within that single spread symbol. As a result, the short noise burst will potentiallycorrupt five symbols in the TDMA system, but it will potentially corrupt only one spread symbol inthe SCDMA system. Simple error correction schemes can be used to permit the single spread symbolto be corrected in the SCDMA system, whereas slightly more complex error correction schemes mustbe used to permit the five symbols to be corrected in the TDMA system.SCDMA symbol de-spreading can lead to improved noise immunity for specific types of impulsenoise (whose bandwidth is narrow and band-limited). It can be shown that the de-spreading functioneffectively spreads the spectrum of impulse noise across a broader spectral range, which effectivelyattenuates the amplitude of the noise within the frequency range of the recovered symbol stream. Thiscreates a higher effective signal-to-noise ratio for the SCDMA signals (when compared to theircounterpart TDMA or ATDMA signals), which leads to lower symbol error rates. This effect isknown as the “processing gain” of SCDMA, and the gain can be shown to be proportional to thenumber of chips used in the spreading code. Trellis-Coded Modulation (TCM) is another technique added to the SCDMA solution that leads toimproved noise immunity. TCM combines the functions of coding and modulation to transmitinformation with lower overall error probabilities. It uses a non-obvious approach in which thenumber of points within a signal constellation is doubled to make room for coding bits that caneffectively be used for error correction. The doubling of points within a signal constellation would atfirst appear to produce higher error rates for a given signal-to-noise ratio, because the points in thesignal constellation are more closely packed. However, the use of a carefully selected bit-level codingscheme permits many receiver bit errors to be corrected, and the overall impact of error correctionover-rides the negative impact of a more densely packed signal constellation.www.arrisi.com Page 22 of 22 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockCombining DOCSIS 1.0, DOCSIS 1.1, DOCSIS 2.0, TDMA, ATDMA,and SCDMAThe DOCSIS 2.0 specification recognizes the need for backwards compatibility, coexistence, andinteroperability between the different types of equipment that may be used on a single HFC plant. Thevarious combinations of CMTS and cable modem functionalities are illustrated in Figure 8, and theresulting modes of operation for each of these combinations is also shown. CMTS 1.0 1.1 2.0 CM/CMTS pair CM/CMTS pair CM/CMTS pair 1.0 operates in operates in operates in 1.0 mode only 1.0 mode only 1.0 mode only CM CM/CMTS pair CM/CMTS pair CM/CMTS pair 1.1 operates in operates in operates in 1.0 mode only Either 1.0 mode or Either 1.0 mode or 1.1 mode 1.1 mode CM/CMTS pair CM/CMTS pair CM/CMTS pair 2.0 operates in operates in operates in 1.0 mode only Either 1.0 mode or Either 1.0 mode, 1.1 mode 1.1 mode or 2.0 mode Figure 8 - CMTS/cable modem interoperabilityWhile the DOCSIS 2.0 specification provides a simple mechanism for allowing DOCSIS 1.0,DOCSIS 1.1, and DOCSIS 2.0 cable modems and CMTSs to interoperate with one another, theco-existence of TDMA, ATDMA, and SCDMA is slightly more complicated. One seriouscomplication in DOCSIS 2.0 arises from the fact that a CMTS operating with TDMA andATDMA cable modems must specify time references when granting burst intervals to the cablemodems (see Figure 4), and the transmitting cable modem consumes the entire upstream channelduring its burst interval, precluding the use of channel sharing during any particular burstinterval. On the other hand, a CMTS operating with SCDMA cable modems must specify bothwww.arrisi.com Page 23 of 23 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the Blocktime references and spreading codes when granting burst intervals to cable modems (see Figure7), and each transmitting cable modem can share the upstream channel with other cable modemsduring a frame. As a result of these differences, TDMA/ATDMA cable modems must not betransmitting during frames when SCDMA cable modems are transmitting, and SCDMA cablemodems must not be transmitting during burst intervals when TDMA/ATDMA cable modemsare transmitting. To simplify the coordination of the different types of transmission schemeswhen TDMA/ATDMA cable modems must share a single physical upstream channel withSCDMA cable modems, the DOCSIS 2.0 specification added a new concept known as a “logicalchannel.”A single physical upstream channel can be sub-divided into multiple logical channels- one forTDMA/ATDMA and another for SCDMA. Each logical channel is centered on the same centerfrequency, but each is essentially independent of the other, because each logical channel has itsown set of MAPs and Upstream Channel Descriptors (UCDs). An example of time-interleavedTDMA/ATDMA frames and SCDMA frames from two different logical channels is illustrated inFigure 9. The MAP scheduler within the CMTS is responsible for distributing idle periods toguarantee that the two logical channels do not overlap in time. frame 1 frame 2 frame 3 frame 4 frame 5 Logical Channel burst burst idle burst burst idle burst A interval interval interval interval interval (TDMA/ ATDMA) burst interval burst Logical interval Channel idle burst idle idle B interval (SCDMA) burst burst interval interval time Figure 9 - Timing of two logical channels within a physical channelwww.arrisi.com Page 24 of 24 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockIt should be noted that mixing a high-bandwidth logical channel with a low-bandwidth logical channelinside of the same physical channel results in inefficient utilization of the upstream spectrum. Forexample, consider the mix of a 6.4 MHz DOCSIS 2.0 channel with a 3.2 MHz DOCSIS 1.X channel.Whenever the low-bandwidth 3.2 MHz channel is transmitting, half of the 6.4 MHz spectrum isunused (Figure 10). This problem may force system operators to carefully design their HFC plants forfuture mixes of DOCSIS 1.0, DOCSIS 1.1, and DOCSIS 2.0 cable modems. 6.4 MHz 3.2 MHz channel channel centered @ centered @ 25 MHz 25 MHz Upstream Spectrum freq (MHz) 20 25 30 wasted spectrum for the 3.2 MHz channel ! Figure 10 - Potential inefficient bandwidth utilizationwww.arrisi.com Page 25 of 25 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockAnswering The Tough Questions About DOCSIS 2.0 DeploymentsNow that the details of DOCSIS 1.0, DOCSIS 1.1, and DOCSIS 2.0 are well understood, howdoes a system operator actually use all of these capabilities to deliver advanced data, voice, andmultimedia services to their subscribers? The answer to this question (and others) is dependenton the details of the particular system, but an attempt can be made to answer some of thesequestions in a general sense.How Much Upstream Bandwidth Can You Really Get With DOCSIS 2.0?DOCSIS 2.0 offers new 6.4 MHz upstream channels that can run with 64QAM, and each of theseupstream channels can provide a system operator with 30 Mbps of upstream bandwidth. While this istrue on paper, there may be some issues that will be encountered if a system operator attempts todeploy this capability.First, consider an example DOCSIS 2.0 CMTS 2x8 blade that supports two downstreamchannels and eight upstream channels. In an example HFC architecture, these two downstreamchannels might be distributed across eight fiber nodes, and each fiber node would send returndata on one of the eight available upstream channels. There exists a maximum bandwidth of ~40Mbps (256QAM) on each downstream channel, and the total downstream bandwidth emanatingfrom a 2x8 CMTS blade would be approximately 2*40Mbps=80 Mbps. A DOCSIS 2.0 2x8blade would be able to support eight upstream channels, and each upstream channel couldtheoretically support ~30 Mbps (5.12 Msymbols/sec at 64QAM). As a result, the total theoreticalupstream bandwidth moving up the cable into a 2x8 blade could be 8*30Mbps=240 Mbps (threetimes more bandwidth than the 80 Mbps of downstream bandwidth).However, from a practical standpoint, a deployment that experiences three times more upstreambandwidth than downstream bandwidth is highly unlikely. In particular, it is well known that DOCSIStraffic has been predominantly asymmetrical (with more downstream bandwidth than upstreambandwidth) in the past due to the nature of Web-surfing traffic. The architects of the DOCSIS 2.0specification realized that there is a move toward more symmetrical traffic flows in the future with theadvent of interactive gaming and servers located in homes. However, it seems unrealistic to assumethat traffic patterns will reverse their nature and produce an asymmetry in the opposite direction (withsubstantially more upstream bandwidth than downstream bandwidth). It can probably be argued thattraffic patterns will more likely approach symmetry (downstream bandwidth equal to upstreambandwidth) in the foreseeable future. If this is the case, it is unlikely that subscriber applications willbe driving much more than approximately 80 Mbps upstream to the 2x8 blade (since the downstreambandwidth on a 2x8 blade is limited to 80 Mbps). Thus, it seems unlikely that MSOs will beubiquitously deploying DOCSIS 2.0 upstream channels operating at 30 Mbps in the near future.(Note: The use of 30 Mbps upstream channels seems more likely if the HFC architectures evolve tothe point where a single 40 Mbps downstream channel is directed to one and only one fiber node.With symmetrical traffic in this futuristic scenario, the use of a single 30 Mbps upstream channel fromthe fiber node becomes justified).www.arrisi.com Page 26 of 26 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockIn addition, many real-world HFC plants will not even be able to capitalize on the 30 MbpsDOCSIS 2.0 upstream capability because of several possible HFC plant impediments. Forexample, some system operators may not have 6.4 Mhz of contiguous, low-noise bandwidthavailable on the upstream cable in which to place a 30 Mbps upstream channel.Another potential HFC plant impediment may result from the fact that the required signal-to-noise (SNR) ratios for very high-speed DOCSIS 2.0 operation may be difficult to ensure onmany upstream cables. For example, assume a bit error rate of 10-8 is desired, and assume thatthe operator decides to use Reed-Solomon FEC codewords of length 255 with T=16. Therequired signal-to-noise ratios (according to one chip vendor) for various DOCSIS 2.0transmission schemes are illustrated in Table 5.  These relatively high signal-to-noise ratiorequirements can be very difficult to guarantee in a live HFC plant. Technology Modulation Channel Symbol Rate Raw Data Minimum Format BW (MHz) (Msym/sec) Rate (Mbps) Signal-to- Noise Ratio ATDMA 16QAM 3.2 2.56 10.24 15.75 SCDMA 32QAM(TCM) 3.2 2.56 10.24 14.1 ATDMA 64QAM 6.4 5.12 30.72 22 SCDMA 128QAM(TCM) 6.4 5.12 30.72 20.5 Table 5 - Required signal-to-noise ratios for DOCSIS 2.0 transmissionThus, the ultimate upstream bandwidth that will be used in a particular HFC plant will bepredominantly determined by two factors: the signal-to-noise ratio of the upstream channels andthe demanded upstream-to-downstream bandwidth ratios. In many plants, the signal-to-noiseratios and the demanded upstream-to-downstream bandwidth ratios may preclude the near-termuse of the higher bandwidth channels offered by DOCSIS 2.0.How Well Do The Noise Mitigation Techniques Work in DOCSIS 2.0?Since noise on the upstream channel may greatly limit the applicability of the higher bit-ratetransmission techniques offered by DOCSIS 2.0, one may want to explore the use of the various noisemitigation techniques supplied by DOCSIS 2.0 in an attempt to circumvent this problem. Manydisagreements exist between different camps regarding the relative merits of the different noisemitigation techniques associated with ATDMA and SCDMA. A lot of the debate is centered on howone models the noise in a live HFC plant. For example, what is the “typical” duration of burst noise?What is its spectral content? Every researcher has a different set of noise models, and the bit-error-rateresults seem to vary depending on the details of the noise model being used. However, all of the noisemitigation techniques are likely to yield some level of benefit, and the actual benefits realized by aparticular system operator will be highly dependent on the type and amplitude of the noise found intheir plant.www.arrisi.com Page 27 of 27 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
DOCSIS 2.0: Getting to Know the New Kid on the BlockIn determining the relative merits of a particular noise mitigation technique, it is important torecall that many of the techniques can have detrimental effects on the user bandwidth and delayin the upstream channel. For example, the overhead associated with longer preambles and FECparity bytes must always be considered. The delay associated with symbol spreading andinterleaving must also be considered. These are often viewed as hidden costs associated with theadvanced technologies.Nevertheless, for a particular HFC plant that was limited (by noise) to use lower symbol rates orless efficient modulation formats with DOCSIS 1.X, the noise mitigation techniques of DOCSIS2.0 may very well provide the improvements needed to upgrade to the next higher symbol rate orto a slightly more efficient modulation format. The addition of many new modulation formatswithin DOCSIS 2.0 increases the likelihood that a higher-efficiency modulation format can beutilized.Are There Any Other Hidden Costs Associated with DOCSIS 2.0?One limitation of DOCSIS 2.0 is often overlooked when considering mixes of existing DOCSIS 1.Xequipment with DOCSIS 2.0 equipment. DOCSIS 1.0 and DOCSIS 1.1 cable modems cannot beoperated on one of the new 5.12 Msymbol/sec (6.4 MHz) channels, and this may produce a hiddencost for DOCSIS 2.0 deployment. If DOCSIS 1.X and DOCSIS 2.0 cable modems are to placed onthe same physical channel, system operators must either limit the physical channel bandwidth to the3.2 MHz limit imposed by DOCSIS 1.X or they must endure the inefficient use of bandwidthresulting from the mix on a 6.4 MHz channel (see Figure 10). Thus, the mix of DOCSIS 1.0 andDOCSIS 1.1 cable modems with 5.12 Msymbol/sec DOCSIS 2.0 cable modems on a single HFCcoaxial distribution leg may force system operators to think about using at least two physical upstreamchannels to be established between the cable modems and the CMTS. One of these physical channelswould operate at a symbol rate of 2.56 Msymbol/sec or less, and it would be able to transport data forthe DOCSIS 1.0 and DOCSIS 1.1 cable modems. The other physical channel would operate at thehigher symbol rate of 5.12 Msymbol/sec, and it would be able to transport the higher bandwidth datafor the DOCSIS 2.0 cable modems. This two-channel approach separates the cable modems intoseparate physical channels and decreases the gain produced by statistical multiplexing. It can also leadto inefficient utilization of the upstream channel spectrum and the CMTS receivers. To accommodatethis requirement within a DOCSIS 1.0/DOCSIS 1.1/DOCSIS 2.0 mixed environment, systemoperators must ensure that their upstream spectra will support adequate bandwidth and that theirCMTS will support an adequate number of upstream receivers in their DOCSIS 2.0 blades.Another hidden cost associated with DOCSIS 2.0 results from the fact that a mix of TDMA,ATDMA, and SCDMA cable modems on the same physical upstream channel will require thechannel to be divided into two logical upstream channels (one for TDMA/ATDMA operationand one for SCDMA operation). This requirement for two logical channels separates the cablemodems into separate groupings and decreases the gain produced by statistical multiplexing ofmany cable modems on a single channel.www.arrisi.com Page 28 of 28 Version 2.0 October 15, 2002 From North America, Call Toll Free: 1-866-36-ARRIS • Outside of North America: +1-678-473-2000 All contents are Copyright © 2002 ARRIS International, Inc. All rights reserved.
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