Two-way broadband CATV-HFC networks: state-of-the-art and future trends

Ž .Computer Networks 31 1999 313–326

Two-way broadband CATV-HFC networks: state-of-the-art andfuture trends

Stephen Perkins, Alan Gatherer )

DSPS R&D Center, Texas Instruments, Dallas, TX 75265-5303, USA

Abstract

Cable modems allow two way digital data transmission between households and a central neighborhood server. In thisŽ .overview we briefly describe some of the common characteristics of the Hybrid Fiber Coax HFC operation as presented by

Ž .several standards bodies both completed standards and works in progress . This includes a description and comparison ofthe media access control and physical layer operations of several competing proposals. Our desire is to give a concise butcomplete picture of the cable modem problem and the proposed solutions. References are provided for readers interested infurther investigation. q 1999 Elsevier Science B.V. All rights reserved.

Keywords: CATV data networks; Cable modem; DOCSIS; HFC; Hybrid fiber coax; MAC; MCNS; QAM

1. Introduction

Cable modems allow transmission and receptionof digitally modulated signals over coaxial cables in

Ž .a cable television CATV network. CATV networkswere originally designed for the one-way broadcastof analog television signals but new modulationschemes and protocols are required to support bi-di-rectional communication.

To reduce the cost of equipment, cable companieswant mass produced interoperable systems. To thisend, the standardization of cable modem systems isoccurring in several places. At the time of writing,several proposals have been submitted to different

Žstandards organizations. The MCNS MultimediaCable Network Systems, a limited partnership of

) Corresponding author. Tel.: q1-972-997-5278; fax: q1-972-997-5693; e-mail: [email protected].

. Žcable companies submitted the DOCSIS Data-.Over-Cable Service Interface Specifications pro-

Žposal to the SCTE Society of Cable Telecommuni-.cations Engineers, an ANSI accredited organization

w x6,12,14 and the IEEE 802.14 working group iscurrently in ballot on a submission to the IEEE 802

w x w xcommittee 10,11 . Two other bodies, DAVIC 9ŽDigital Audio and Video Council, a consortium of

. w x Žmanufacturers and the ITU 13 InternationalTelecommunications Union, an international multi-

.governmental standards organization , have standard-Ž .ized physical layer PHY specifications. While there

are multiple standards being presented, the IEEE802.14 proposal and DOCSIS documents do incorpo-rate the PHYs presented by DAVIC. Finally, severalvendors are currently deploying proprietary solu-tions. In this tutorial, our primary goal is to describethe solutions converged on by the above standardsorganizations. When specific details are given, they

1389-1286r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-7552 98 00272-4

( )S. Perkins, A. GathererrComputer Networks 31 1999 313–326314

relate to the IEEE 802.14 proposal as it stands duringthe writing of this tutorial. One should note that802.14 proposal is in the balloting process and is notyet a standard. It should be considered a work inprogress. Where appropriate, the 802.14 approach iscompared to the DOCSIS approach.

This paper will view the cable plant system in twoways. Section 3 views the cable plant as a logicalcommunications medium. This aids in understandingthe operation of the communications protocols anddescribes how subscriber modems communicate overa CATV network. Section 4 views the cable plant asa pre-deployed network and presents a descriptionof the physical topology and the details of the cablemodem PHY operation. This aids in an understand-ing of the design rationale of the PHY and providesinsight into why the protocols are designed the waythey are. Section 2 gives an introduction to the basictopology of a cable network.

2. What is a cable television network?

A CATV system consists of a group of residencesand a single headend connected to a cable network.The ‘‘classical’’ cable system topology is shown inFig. 1. Users are connected to the headend through anetwork with a tree structure, where the leaves repre-sent the users and the headend corresponds to theroot. The headend is the name given to the cablecompany office which may receive its NTSC videosignals by land line or microwave. Such a network is

Fig. 1. Topology of a CATV network.

designed for transmission of analog NTSC videofrom the headend to the users. We shall call thistransmission direction the downstream direction,while the transmission direction from a user towardsthe headend shall be called the upstream direction.

Downstream communication, including analog TVtransmission occurs from 45 MHz up to 500 to 1000MHz depending on the quality of the system.

Ž .Quadrature Amplitude Modulation QAM is usuallyemployed for downstream transmission, using con-stellations with 64 to 256 symbols. A downstreamchannel occupies a bandwidth of about 6 MHz, andthe data rates can be as high as 30 to 40 Mbitrs.Upstream channel bandwidth, as presently standard-ized, varies from 160 kHz to 2.5 MHz with data ratefrom 320 kbps to 10 Mbps. QAM modulation with 4or 16 point constellations is used. Multiple access isachieved by time division, with each upstream usertaking a turn on the channel under control of theheadend. Upstream communication generally occursin the 5 to 40 MHz region where RF interferenceingressing onto the cable from external sources dom-inates the noise. The 5 to 40 MHz band can be verynoisy and has been called ‘‘the sewer’’ by some inthe cable business. So why transmit in this region atall? One reason is that this part of the spectrum isuseless for analog TV transmission due to noiseingress. Therefore cable operators can use it forupstream transmission, safe in the knowledge thatthey would never want to put a TV channel there. Itis also easier to design RF front ends for this bandthan for the other available regions above the analogTV band. The use of this region for upstream com-munications is adequate for the foreseeable future.

Recently many cable operators have been upgrad-Ž .ing their systems to a Hybrid Fiber Coax HFC

infrastructure where part of the trunk network isreplaced by a fiber optic link. HFC systems areeasier to maintain, reduce the need for amplifiersw x15 and have a larger bandwidth over the trunknetwork, allowing a single headend to serve a largerarea.

3. MAC overview

The logical topology of a cable modem network isshown in Fig. 2. The downstream path flows from

( )S. Perkins, A. GathererrComputer Networks 31 1999 313–326 315

Fig. 2. Logical topology of a cable modem network.

the headend to all stations and resides in a 6 MHzTV channel selected by the cable operator. Theheadend, which is owned and operated by the cablecompany, is the exclusive transmitter in the down-stream direction. Therefore, no downstream media

Ž .access control MAC mechanism is needed. Whilethe downstream channel is broadcast in nature, eachsubscriber unit is assigned an individual address thatallows it to filter out any data not addressed to it viaunicast, multicast, or broadcast transmission. Secu-rity and data encryption mechanisms are in place toprovide authentication and privacy and to help pre-vent theft of service and denial of service attacks.

The upstream paths flow from subscriber unitsŽ .also referred to as stations in this tutorial towardsthe headend. Each upstream channel is shared by anumber of stations and is divided in time into indi-vidually numbered allocation units called minislots.The shared nature of the upstream traffic requires aMAC to order transmissions.

The upstream channel is more complex than isshown in the diagram. Physical requirements of the

Ž .cable plant further discussed in Section 4 requireisolation of signals in the upstream direction. Thisisolation leads to a scenario where upstream trans-missions can only be heard by the headend and notby subscriber units. Thus, concurrent upstream sta-tion transmissions can collide, but individual stationsare unable to hear the transmissions of other stations.

ŽThis means that carrier sense MAC techniques e.g..CSMA will not work in a CATV environment.

3.1. MAC operation

The MAC is the entity that controls the allocationof bandwidth of the shared upstream media. Asmentioned earlier, only the upstream communicationneeds a MAC because the headend is the exclusive

transmitter in the downstream. However, down-stream transmissions are required for proper opera-tion of the MAC protocol. Specifically, MAC proto-col operation is split between individual subscriberunits and the headend controller. Most of thescheduling and processing burden is relegated to theheadend. During normal operation, the headend regu-larly sends control information in the downstreamchannel. Part of this control information consists ofmaps that describe the allocation of the shared up-stream bandwidth. Any station wishing to request anallocation must contend for access during periodsthat are specified in this map. If a request is success-ful, the headend will allocate upstream bandwidth forthe station in a future allocation map. If the requestresults in a collision, the headend will initiate acollision resolution process. Once a subscriber unitreceives a bandwidth allocation in the upstream map,it has the opportunity to piggyback new allocationrequests in its reserved upstream allocation. Piggy-backing allows stations to request more bandwidthwithout reentering the contention based request pro-cess. Fig. 3 illustrates the upstream mapping.

Upstream bandwidth is allocated in granular unitscalled minislots. Minislots are groups of eight octetsŽ .bytes and are typed and identified by a uniquenumber. Control of the minislot type and number is

Fig. 3. Upstream allocation.


provided by the headend. As described next, bothupstream timing and MAC scheduling are closelytied to the minislot number interpretation. The head-end and station must work together to interpret min-islot numbers.

3.1.1. MAC timing and the TC layerCable networks can have subscriber units as far as

Ž .25 miles 100 miles for DOCSIS from the headend.Typical distances are much less. Signal propagationtimes over such distances impose timing constraintson the MAC. The IEEE 802.14 specification statesthat the MAC must handle propagation delays as

Ž .large as 200 ms 800 ms for DOCSIS in eachdirection. Such delays cause problems in the up-stream since two widely separated units could starttransmission during consecutively numbered up-stream minislots yet have their data arrive at theheadend out of order or in a collision state. To helpsolve this situation, the MAC mandates that up-stream timing must be adjusted by each station insuch a manner that if two stations transmit during thesame minislot, both their transmissions will arrive atthe headend at the same instant. At startup, eachcable modem MAC determines its upstream timing

Žadjustment values through a ranging procedure as.further discussed in Section 3.1.5 .

To help manage the upstream timing, which hasphysical layer dependencies, a new sublayer is in-serted between the PHY and the MAC. This layer,shown in Fig. 4, is known as the Transmission

Ž .Convergence TC layer and helps hide PHY timingdependencies from the MAC. One job of the TC is tocoordinate with the MAC’s ranging procedure toacquire and maintain a lock on the upstream timing.

Fig. 4. Relation to OSI layering

A lock means that the TC knows the exact timingadjustment values needed to account for the propaga-tion delay of its upstream transmissions. Once a lockis achieved, the MAC can operate using the notion ofminislot numbers and the TC then handles the timingconsiderations necessary to deliver numbered minis-lots to the headend at the appropriate time.

3.1.2. Transport protocols and framingBoth the IEEE 802.14 and DOCSIS specifications

support the transport of multi-media traffic. How-ever, the foundations of the two specifications differ.The IEEE 802.14 MAC provides a fixed lengthtransport that is based on asynchronous transfer modeŽ .ATM . The DOCSIS MAC provides a variablelength transport that is geared towards efficientlydelivering IP traffic encapsulated in Ethernet frames.In both specifications, mechanisms are provided toencapsulate the respective fixed or variable length

Ž .protocol data units PDUs into the downstreamŽframing mechanism used on CATV systems typi-

w x.cally MPEG-2 9 .

3.1.2.1. IEEE 802.14. The basic PDU for this pro-posal is the ATM cell. IEEE 802.14 cells are actually54 bytes long while they remain inside the cablenetwork. The extra byte is MAC overhead and isremoved as cells leave the CATV MAC domain.Except for a few special cases, all PDUs, includingboth MAC layer management and data PDUs, areencapsulated via the ATM Adaptation Layer 5Ž .AAL5 . This allows for data payloads that spanmultiple ATM cells and provides an extra cyclicredundancy check to catch data transmission errors.

In the downstream, ATM AAL5 cells are packedinside the payload of an MPEG-2 transport. From areceivers perspective, this downstream MPEG-2transport is viewed as an octet stream from whichAAL5 encoded MAC PDUs are delineated and re-

Ž .assembled more on this in Section 3.1.5 . In theupstream, ATM AAL5 cells are split into minislotsized units. Since 802.14 PDUs are 54 bytes inlength, an integral number of minislots are requiredto successfully transmit a single PDU to the headend.

3.1.2.2. DOCSIS. DOCSIS MAC PDUs are alsoencapsulated in the payload of an MPEG-2 stream.MAC PDUs can start anywhere in the MPEG-2


payload and can span MPEG-2 frame boundaries.Unlike 802.14, DOCSIS does not specify an ATMframe format. However, to allow for future modifica-tions, the DOCSIS standard does provide a mecha-nism for the MAC to determine if a frame encapsu-lates ATM cells. This allows an existing MAC toignore ATM data and remain interoperable withpossible future encapsulations. In the upstream, MACPDUs are packed into minislots. Since upstreamframes are not required to have a size that is amultiple of the upstream minislot size, the MACmust provide any padding that is necessary.

3.1.3. AddressingAll subscriber units are assigned a unique 48 bit

IEEE 802 MAC address. However, this address isnot the primary means of addressing a station and is

Žonly used during the registration process see Section.3.1.5 . During registration, subscriber units are as-

signed one or more 14 bit local identifiers. Theseidentifiers describe a mapping between the sub-scriber unit and the headend controller and are onlyvalid inside a cable modem MAC domain. The firstidentifier assigned to a station is the primary localidentifier. It is used for MAC administration, secu-rity, and PHY control. Other local identifiers may beassigned to support station management, class ofservice, and multicast groups. Table 1 lists the set of

Žreserved well known local identifiers as described.by the IEEE 802.14 specification .

Unlike IEEE 802 MAC addresses, no bit leveldifferentiation exists in the 14 bit local identifiers.Unicast and multicast identifiers are assigned by theheadend from the same 14 bit address space. Multi-cast identifiers are also associated with 48 bit IEEE802 multicast identifiers. When a new multicast groupis formed, the headend allocates a new local identi-fier to associate with that group. Stations may jointhe group by making requests to the headend. This

Table 1Well known local identifiers

Value Meaning

0=00 NULL0=3FFE Registration multicast address0=3FFE Broadcast

process will cause the station to recognize the multi-cast identifier and will also allow that station toreceive the encryption keys that allow it to decodethe information in the multicast stream. Stations mayleave the multicast group through similar procedures.Once a station leaves the multicast group, the head-end will cease sending the multicast encryption keysto that unit.

3.1.4. Bandwidth allocation and contention resolu-tion

In both DOCSIS and IEEE 802.14, stations thatwant to transmit data must make an allocation re-quest to the headend. Allocation requests are shortminislot-sized messages that contain one of the sta-tion’s local identifiers and a count of the number ofminislots needed. If a station has already been allo-

Žcated upstream bandwidth due to a previous re-.quest , and is waiting for the transmit time to arrive,

that station may piggyback new allocation requestswhen the transmission time does arrive. However,stations that do not have a pending allocation mustmake their requests in the contention based alloca-

Ž .tion request minislots as shown in black in Fig. 3 .Allocation request mini-slots are groups of mini-

slots in which any subscriber unit can make a band-width request. If only one station makes a request inany given minislot, the headend will receive thatrequest and respond with an allocation pending noti-fication message. This message is unicast back to thestation and will be followed by an allocation for thatstation in a future upstream allocation map. If, how-ever, multiple stations select the same allocationrequest minislot, a collision occurs.

The IEEE 802.14 specification uses a sophisti-cated tree-splitting based collision resolution policy.This policy supports FIFO like admission controland priority based differentiated quality of service.The DOCSIS specification requires the subscriberunit to follow a truncated binary exponential backoffalgorithm to access the minislots in the group. Inboth specifications, the headend dynamically con-trols the parameters necessary for the algorithms.

3.1.4.1. IEEE 802.14 collision resolution. An 802.14network uses an n-ary tree algorithm for collisionresolution. To support the n-ary tree algorithm,


groups of 802.14 allocation request minislots havean additional parameter associated with them called

Ž .the request queue RQ value. Stations making theirinitial contention requests can only contend in allo-cation request groups having an RQ value of zero.When a collision occurs the headend will allocate anew group of n allocation request minislots havinga different RQ value. These minislots are reservedfor the units that were involved in the collision. Theheadend then broadcasts a notification that associatesthe minislot number containing the collision with thenew RQ value.

If a station sees this association and determinesthat it sent its request in the minislot that containedthe collision, it will contend again using requestminislots having the new RQ value. Collisions thatoccur during the retries are handled the same way. Inthis manner, collisions have priority over new ac-cesses and can be quickly resolved.

The collision resolution policy has several otheruseful parameters. The first is called the Admission

Ž .Time Boundary ATB and its use supports FIFObased contention access rights. The ATB is a minis-lot number that is dynamically generated by theheadend and sent to all stations. This minislot num-ber is used to divide those stations wishing to con-tend for access into ‘‘transmitting’’ and ‘‘non-trans-mitting’’ groups. The mechanism is simple. Eachstation keeps track of the minislot when its initialrequest was generated. This request has not yet beensent since the station is waiting to contend for ac-cess. When appropriate allocation request minislotsarrive, each station checks its request generation timeagainst the ATB. If the request generation time isless than or equal to the ATB, the stations maycontend during the current contention period. Other-wise, the station must defer. Proper selection of theATB by the headend can lead to various admissioncontrol policies.

A second useful parameter is the Contention Res-Ž .olution Priority Profile CRPP . This is an 8-bit

vector that accompanies each group of allocationrequest minislots. All stations on the network areassociated with one of 8 priorities. Each priority isthen represented by one of the bits in the CRPP.Stations wishing to contend in the current allocationrequest group must check their priority against theproper bit in the CRPP associated with this group. If

their bit is not set, then they must defer their con-tention.

3.1.4.2. DOCSIS collision resolution. The truncatedbinary exponential backoff algorithm requires twoparameters for operation. The first is the initialback-off window and the second is the maximumback-off window. These values are sent as part of thebandwidth allocation map. To begin a request, thesubscriber unit sets its window to the size of theinitial back-off window. It then chooses a randomvalue that is within the window range. Once a valueis chosen, the station must let that many allocationrequest minislots pass before it makes its request.

If a subscriber unit has made a request but doesnot receive a response before a timeout value, itassumes a collision. In such a situation, the unitincreases its window size by a factor of 2, as long asthat size is less than the maximum back-off windowsize. It then retries the request using the new windowvalue. This process continues for a maximum ofsixteen tries. After sixteen tries, the station discardsthe PDU and continues. One should note that theheadend can make this collision resolution processlook like that of Ethernet by setting the initial back-off window to a size of 0 and the maximum back-offwindow to a size of 1024.

3.1.5. Registration and entry proceduresAs mentioned previously, cable modem networks

require precise clock synchronization between theheadend and all subscriber units. This synchroniza-tion is what guarantees that all transmissions startedin a given upstream minislot arrive concurrently atthe headend. Since the distance between a specificmodem and the headend is initially unknown, eachmodem must go through ranging and power controlprocedures during startup. Ongoing adjustments arethen maintained throughout the period a subscriberunit remains active.

Fig. 5 shows the steps involved in ranging anIEEE 802.14 modem. After the PHY has acquired alock on the downstream channel, the MACrTC layermust acquire cell delineation. This is the process offinding the boundaries of PDUs given an octet stream

w xand follows ITU-T Recommendation I.432 5 . Oncecell delineation is acquired, the MAC and TC begin


Fig. 5. Registration steps.

to listen on the channel for downstream timestamps.Obtaining timestamps gives the modem a lock intime with respect to the numbered upstream minis-lots. At this point, the modem joins a specific well

Ž .known registration multicast group see Table 1and waits for an invitation to join the network. Theseinvitations, periodically broadcast by the headend,instruct the station about the available upstreamchannels and the next ranging window in which theunit can contend for the right to join. The ranging

Ž .window shown as ‘‘Station Register’’ in Fig. 3consists of a large group of contiguous minislots inwhich unranged stations can make initial registrationrequests. Since new stations are unranged, the size ofthe window must accommodate the largest propaga-tion delay expected.

When a new station sends a registration request, itfirst does so at minimal power. If the station is closeto the headend, the request may be received, but if itis far away, upstream attenuation will keep the mes-sage from arriving with sufficient power. If therequest is received, the headend will respond to thestation on the downstream. If no response is found,the station will move to the next upstream channeland try again. After trying all available upstreamchannels at minimal power, the modem will bumppower up a notch and cycle through again.

At some point, the headend will receive the regis-tration request. It will compare the actual arrival time

of the request against the minislot start time of theranging window. The headend then makes an initialestimate of the propagation delay and feeds thisinformation, along with power control information,

Žback to the registering unit performed with a broad-cast message that contains the stations IEEE 802MAC address since there is no local identifier yet

.assigned . These steps are repeated until power andtiming are within tolerable bounds. When complete,the headend assigns that unit a local identifier andstarts the authentication and privacy procedures. Thisis possibly followed by other registration proceduresŽ .e.g. DHCP for DOCSIS . At this point, the sub-scriber modem can fully communicate on the net-work without interfering with other stations.

3.1.6. Steady state operationSteady state operation is attained once a station

has completed the registration process, authenticateditself, and negotiated its initial session keys. Incom-ing PDUs must be delineated and removed from thedownstream MPEG-2 frames. During this delineationprocess, PDUs may have their header checksumverified and will have their destination addresschecked. If a PDU is destined for the local station, itis forwarded to the protocol stack. PDUs that arerecognized as MAC management PDUs are for-warded to the MAC handler interface.

Ž .Outgoing data is segmented if necessary , framed,and queued. The arrival of outgoing data starts theupstream bandwidth allocation request procedures.The queued PDUs are sent when a data allocation isreceived and the time of that allocation arrives. Allupstream transmissions are multipoint-to-point. Theymust go through the headend which acts as an IEEE802.1 bridge or a router.

3.2. Security

Security considerations play a key role in cablemodem networks. The topic of security deserves anarticle in itself and is only briefly described here.The security models of competing specifications dif-fer but rely on the same set of principles. They areall designed to protect against theft of service anddenial of service caused by alteration of subscriberunit security components, interception and replay of


the data stream, cloning, masquerading of valid units,and misuse of upstream bandwidth. The securitysystems provide:Ø Identification and authentication of the sub-

scriber: both protocols use the concept of a secretcookie for authentication. How that cookie isoriginally supplied to the station is part of secu-rity policy. Both the IEEE 802.14 and DOCSISproposals suggests that MAC address be given tothe headend via some sideband channel. Further,the DOCSIS specification outlines the capability

Ž .to use X.509v3 digitally signed certificates gen-erated by the manufacturer. These certificates areused to authenticate the validity of the subscriberdevice.

Ø Confidentiality of user information through en-cryption: session keys are used to encryptrde-crypt the payload of all unicast and multicastPDUs. Initial key values are exchanged during theregistration process and are then continually up-dated according to security policy. Each MACPDU has an encryption key identifier that aids inswitching between security keys. This allows keysto be negotiated and switched ‘‘on-the-fly’’. Eachmulticast sessions has a separate set of keys.

Ø Clone detection: This can be implemented inseveral ways. The IEEE 802.14 specificationmaintains counters in the headend and subscriberunits and increments these counters under certaincircumstances. The counters are included duringsecurity transactions. If the headend receives acounter with an unexpected value, it is likely thata cloned station exists.

Ø Anonymity: The registration procedures are de-signed such that the 48 bit IEEE MAC addressand a station’s local identifier are never senttogether in the clear. This helps to provideanonymity by hiding distance measures that canbe derived from monitoring the ranging process.

3.2.1. 802.14 securityThe IEEE 802.14 specification allows for two

modes of key exchange. The quick key exchangeŽ . Ž .QKE and main key exchange MKE . The firstuses a less computationally expensive hashing algo-rithm and relies on the secrecy of the station’scookie. The latter uses a more computationally ex-pensive Diffie–Hellman exchange that provides per-

fect forward secrecy. Perfect forward secrecy meansthat if an attacker knows all the parameters stored inthe non-volatile memory of a station, and is also ableto monitor both sides of the MKE key exchange, thatattacker will be unable to determine the new sessionkey.

Stations have the choice of selecting several oftheir security parameters from lists generated by theheadend. As part of station registration, the stationselects from a list of 4 encryption algorithms. Cur-rently, DES and Triple-DES are the only ones de-fined. The station also has the ability to choose the

Žlength of the encryption key 40 or 56 bit for DES.and 112 or 168 bit for triple-DES and the Diffie–

ŽHellman modulus length 512, 768, and 1024 bit.modulus . The full security specification as well as

an informative security policy annex can be found inw xthe IEEE 802.14 specification 10 .

3.2.2. DOCSIS securityThe DOCSIS security model can manifest in one

of two forms. The first is a full security specificationwhile the second is a simplified version called Base-line PriÕacy. Both provide privacy through data en-cryption. The encryption algorithm is based on a 56bit DES cipher block chaining algorithm. RSA pub-lic key encryption is used to provide authenticationthrough the use of initial keys stored in the sub-scriber unit. These keys are generated and digitallysigned by the manufacturer. If one chooses to useBaseline PriÕacy, the authentication capabilities areremoved. In the absence of authentication, the sys-tem can still ensure that a subscriber unit can onlyreceive information that it is authorized to access.However, Baseline Privacy is not able to detectclones or to protect against some of the more sophis-ticated attacks. More information on the DOCSIS

w xsecurity model is provided in 1–4 .

4. Physical layer

The physical layer consists of two basic parts; theresidence modem and the central office modem.Both downstream and upstream transmission relies

Ž .heavily on Time Division Multiple Access TDMAtechniques. For downstream there is only one trans-mitter so TDMA in this case means transmitting only


one user’s data for a period of time and then switch-ing over to transmit another user’s data. From aphysical layer point of view this is transparent as thephysical layer is unaware who the data belongs to.This is equally true at the receiver, where the physi-cal layer will demodulate the whole bitstream and letthe protocol layer decide what part of the databelongs to that user. In the upstream the TDMAsystem consists of each user transmitting at a physi-cally different time so the transmissions do not over-lap in time. In this case the physical layer is aware ofthe TDMA nature of the system which is manifestingitself in burst transmissions of data of periods speci-fied by the protocol layer. Advanced upstream trans-mission techniques will be described in this sectionthat do not use TDMA, but the systems being cur-rently standardized and deployed are predominantlyTDMA based.

4.1. Residence modem architecture

At the time of writing several companies arealready selling cable modem chips, information on

w xwhich can be found in 26–30 . In Fig. 6 we show ablock level diagram of one way of implementing themodem. Though it may differ from other designs andis a simplified representation, it illustrates the prob-lems that have to be solved.

4.1.1. ReceiÕerEven though there is a wide variety of cable

network systems with widely differing performancew x16,17 there is enough commonality to require thefollowing general tasks to be performed at the re-ceiver:

4.1.1.1. Timing recoÕery. Timing frequency errorsare due to differences in modem clock frequencies.Usually, off the shelf crystals are used to generate

Fig. 6. Modem block diagram.

the same sampling frequency in the transmitter andreceiver. Crystals that are guaranteed to be within

Ž .100 parts per million ppm of each other are readilyavailable.

4.1.1.2. Equalization. An equalizer is a filter thatmitigates the effect of signal distortion introduced bythe channel. Long trunk lines have amplifiers thatboost the signal and also perform a ‘‘detilting’’

Žoperation which removes the rolloff a few tenths of.a dB every 6 MHz of the coax preceding the

amplifier. Trunk amplifier are typically placed everyw x1000–2000 feet 15 . Therefore the only significant

rolloff is due to the last few hundred feet after thecoax enters the neighborhood. This last section ofcable is generally called drop cable and is a differenttype from the trunk cable with typical losses found

w xin 8 . There is approximately 0.5 dB drop across a 6MHz channel. The quality of the analog signal isheavily dependent on reflections in the network.These occur if a cable is not properly terminated. Inanalog TV, the reflection can cause the ‘‘ghosting’’effect on the image that those of us who haveexperienced bad cable and terrestrial reception arefamiliar with. In the frequency domain ghostingcauses a slow ripple in the signal amplitude. Assum-ing the CATV system quality is such that ghosting

w xdoes not occur 7 , analysis of worst case conditionsleads to the conclusion that over 6 MHz, a halfperiod of a sinusoidal ripple with amplitude of about4.5 dB can occur. This is not a particularly challeng-ing channel to equalize and in the downstream equal-izers of 20 to 30 taps are common. The equalizer

Žwill also track AM hum due to the AC power.supply to the amplifiers in the network which causes

60 Hz amplitude modulation of the signal. From thew xstudy in 18 the depth of modulation appears to be

less than 5%.

4.1.1.3. Frequency correction. Frequency errors aredue to differences in the transmitter and receiver RF

w xsections of the modems. In the DAVIC standard 9the stability of the transmitter modulation frequencyis specified at "20 ppm at the highest modulationfrequency. Selecting 750 MHz as the highest fre-quency we get a frequency error of plusrminus 15kHz. Assuming there is the same accuracy in thereceiver RF section then the digital portion of the


demodulator will observe a frequency error of up toplusrminus 30 kHz. For a 256 QAM constellationwith 5 MHz symbol rate this leads to a rotation of21.68 every symbol period. Hence the frequencyerror is quite large and good carrier frequency track-ing is required. The signal center frequency will alsovary with time due to FM hum. FM hum is causedby the power supply in the cable modem itself. Thecenter frequency of the modulated signal varies at120 Hz with a maximum frequency deviation of up

w xto several kHz 19 .

4.1.1.4. Coding. Additive noise present in CATVnetworks can take the form of random noise or burstnoise. Random noise is due to thermal effects, inter-ference from other sources and laser clipping. Burstnoise is, as would be expected from its name, arandomly occurring, finite duration burst of noisethat tends to wipe out the signal for a short period oftime producing a large group of errors close together.

The effect of burst noise can be combatted by theuse of interleaving. This technique is part of theproposed cable modem standards which all specifysome form of convolutional interleaving. Essentially,a convolutional interleaver groups the input data intolength N blocks, block i having elements to b , toi,0

b and then re-sorts the data into output blocksi ,Ny1

by delaying the elements by different amounts.Specifically, output block i will have elementsb ,b ,b down to b . The deinter-i,0 iy1,1 iy2,2 iy Nq1, Ny1

leaver at the receiver will re-sort the data back intoits original order by grouping the received data intolength N blocks, block i having elements to c toi,0

c , and then delaying the data so that the outputi ,Ny1

block will have elements c , c ,iy Nq1,0 iy Nq2,1

c , down to c . The overall effect is toiy Nq3.2 i ,Ny1

delay the data by Ny1 everywhere. After deinter-leaving, a noise burst will be spread over a widerange of symbols, as shown in Fig. 7. Generally theburst noise will be spread so thinly that for themaximum expected burst length, only samples p ofthe burst noise appear in each block of the errorcorrecting decoder which is capable of correcting atleast p errors per block. Hence a single, large uncor-rectable group of errors has been converted into alarge number of small groups of errors, with eachgroup correctable by the decoder. The source ofburst noise in the downstream is unknown, possibly

Fig. 7. The effect of the deinterleaver on burst noise.

being due to leakage from the upstream RF into thedownstream RF in the user’s modem. Channel mea-surements reveal that bursts of 10’s of microsecondsare possible and all of the proposed standards arespecified to correct bursts of at least 25 ms.

There are essentially only two coding schemesproposed for downstream transmission. We describethem using their ITU J.83 designations of Annex AŽ . Žfor the ‘‘DAVIC’’ standard and Annex B for the

.‘‘MCNS’’ standard . Both Annex A and Annex Bare similar in that they use a subset of 16, 64 and256 QAM constellations with no spectral shapingand a symbol rate of a little over 5 MHz:

w xØ The Annex A solution 9,13 allows the user toŽ .send either 256 QAM 8 bits per symbol , 64

Ž . ŽQAM 6 bits per symbol or 16 QAM 4 bits per.symbol at 5 Msymbolsrs rate. It uses Reed–

w xSolomon coding 20 followed by interleaving.Ž .The Reed–Solomon R–S Coder adds 16 parity

bytes to each 188 byte packet so that up to 8 byteerrors can be corrected in every length 204 block.These are then passed into a convolutional inter-leaver. The combination of R–S coding and con-volutional interleaving is optimal for burst error

w xcorrection 21 and therefore Annex A is opti-w xmized for burst errors. From the analysis in 21

for 256 QAM the Annex A R–S decoder cancorrect bursts of length 1632 bytes or 0.33 ms.For the 64 and 16 QAM case the R–S decodercan correct 96 bytes. For 64 QAM this is 128symbols or 25.6 ms. For 16 QAM it is 192

Žsymbols or 38.4 ms. To achieve ‘‘flawless’’ less. Žthan one corruption a day video transmission a

y12 w x.bit error rate of 1=10 9 annex A requiresan input symbol error to the decoder of 1.59=

y3 Ž10 . For TCPrIP transmission bit error rate


y8 w x.better than 1=10 22 Annex A requires aninput symbol error rate to the decoder of 4.4=

10y3.Ø The Annex B solution implements 256 QAM, 64

QAM but not 16 QAM. It uses a concatenatedŽ .code R–S encoder followed by a trellis coder

and an interleaver to achieve a higher coding gainthan Annex A. The R–S coder in Annex B uses

Ž .symbols in GF 128 or 7 bit symbols. The R–Scode used has block size 128 symbols and cor-rects up to 3 errored symbols per block. Betweenthe R–S code and trellis code is an interleaver.This is done because the trellis code exhibits errorpropagation and hence the errors tend to come outof the trellis decoder in bursts. The interleavercan be used to separate the errors due to errorpropagation as well as error bursts due to thechannel. In Annex B there is a fixed mode for 64QAM to be backward compatible with a previousversion and a variable mode for both 64 QAMand 256 QAM. For the fixed mode the maximumlength of a correctable burst is about 92 ms. Thisis about 0.28 the burst error protection capabilityof Annex A, and not all of this will be used onchannel burst errors as some of the burst errorseen by the R–S decoder will be due to errorpropagation in the trellis. For the variable mode,the best burst protection is 358 ms for 64 QAMand 268 ms for 256 QAM. This is 0.8 the burstprotection of Annex A for 256 QAM and about14 times the burst protection of Annex A for 64QAM. However, these results are got at the costof enormous latency, on the order of 10’s ofmilliseconds, which is much larger than allowed

w xby the MAC layer 22 . Analysis and simulationof the trellis code by the authors has shown thepower normalized gain of this code to be 4.07 dB.The famous Wei 4D and 8D codes have codinggains of 4.66 dB and 5.41 dB respectively. TheAnnex B code is therefore a good but not out-standing code.To compare Annex A and Annex B we take the

waterfall diagrams for the two R–S codes and movethe Annex B code up by 4 dB. The result for 64QAM is shown in Fig. 8. We also plot for conve-nience the error probability of an uncoded QAMsymbol and note that the results for Annex A and Bare only accurate when the uncoded probability is

Fig. 8. Performance of Annex A and Annex B codes.

Ž .small less than 0.01 because of the approximationsused in generating the curves. Note that in the regionthat cable modems are most often required to oper-ate, output error probabilities between 10 – 8 and10 – 12, the Annex B code is overall about 2 dB betterthan the Annex A code.

4.1.2. TransmitterAll the proposed standards define the transmitter

in the residence to be a simple QPSK or 16 QAMmodulator. Such systems are described in many stan-dard textbooks and will not be redescribed here.Coding for upstream will be described in Section4.2. However there are some features that are impor-tant to the cable modem system:

4.1.2.1. Power control. While travelling upstream asignal may encounter several splitters and taps thatare part of the downstream distribution tree. In orderto prevent ghosts in the downstream analog signalŽ .due to reflections travelling upstream the splittersand taps will significantly attenuate upstream signals.As the number of splitters and taps traversed willvary depending on the position of the modem in thenetwork, the upstream signal can experience a largerange of attenuation. For this reason the modulatormust have a large dynamically controllable powerrange. Variations of 50 dB are recommended.

4.1.2.2. Loop timing and frequency control. As willbe described further in the next section, the symboltiming and the carrier frequency for upstream trans-mission are often derived from the downstream sig-


nal. This simplifies the carrier recovery and timingrecovery in the central office receiver and also syn-chronizes the carrier and timing frequencies of thedata transmitted by the residences. This is importantto ensure good separation of the upstream signals inboth frequency and time.

4.1.2.3. Pre-equalization. In the upstream, equaliza-tion may be needed to counteract the effect of thechannel. However, the headend will not be able toequalize the signal received from a user who initially

Žrequests bandwidth in contention mode as explained.in Section 3.1.4 since the user is unknown to the

headend. Therefore it seems advisable that the userpre-equalize its signal before sending it to the head-end. The penalty to be paid is some power increasein the transmitted signal, but with a large powervariation allowed in the transmitted signal this shouldnot represent a significant drawback. Simulations

w xdescribed in 31 predict that no equalization will berequired for QPSK and that the number of equalizertaps required for a Decision Feedback Equalizerimplementation in a 16 QAM system is only acouple for the feedforward filter and a couple for thefeedback filter.

4.2. Central office modem architecture

The ingress noise in upstream transmission has itssource in shortwave broadcast radio, Citizens’ BandŽ .CB and Ham radio transmissions. Though the cableis shielded the noise ingresses at points where thecable shielding is damaged, at unshielded connec-tions, and at open connections in residences. Thenoise at the headend receiver is large due to the‘‘noise funneling’’ effect. Because system access inthe upstream direction is multipoint to point, all thenoise generated at the residences is funneled back upto the headend and adds at the receiver. So, eventhough the headend is listening to only one user’ssignal at any point in time, it is experiencing all theusers’ noise. Noise is characterized by its powerspectrum, which is a measure of the power of thenoise at each frequency. For examples of upstream

w xnoise spectrums see 17,23 . In this section we de-scribe the upstream PHY devised by MCNS andadopted by IEEE 802.14. It is a simple implementa-tion when understood within the context of the cable

modem system. The headend will first monitor theupstream channel to determine the areas in the spec-

Ž .trum where the signal to noise ratio SNR is highenough for transmission. One way to do this is tomeasure the noise power spectrum and then comparethis to the available signal power. The headend thenspecifies different channels in terms of their band-width and SNR using a set of allowed symbol ratesin a way that fills the high SNR spectrum as fully aspossible so maximizing the total available bit rate.For each channel the headend specifies the constella-tion and the error protection used, called the PHYprofile for the channel, so that data can be reliablytransmitted on this channel. Given this is done, thePHY layer can be described for a single channel inwhich there is no significant noise ingress.

The MAC layer of the modem will instruct thePHY to send a burst of data starting at a certain time.The PHY layer transmits the data on a headendspecified channel using either QPSK or 16 QAMdepending on a predefined PHY profile set up by theheadend for the channel. So that different users’signals do not overlap in time there is a guard bandbetween user transmissions. The guard band is usedto let the user ramp up into transmission and rampdown after a transmission as any sudden change insignal energy from a user will cause energy tosplatter into other channels. Specifically, in theMCNS specification the guard time is specified be-tween 5 and 255 symbols.

At the start of a burst there will be a preamble toallow the receiver in the headend to lock to the burst.The preamble size depends on how accurate the apriori estimates of the burst parameters are in theheadend. If, for example the system has accurate

Žloop timing where the user uses the timing obtained.from its receiver for its transmitter then the headend

will have a very good estimate of the samplingfrequency and will only have to learn the samplingphase. Similarly, it is possible for the headend re-ceiver to store estimates of the user’s frequencyoffset obtained from previous bursts transmitted fromthat user. Then the headend will have an accurateestimate of the carrier frequency too. Another possi-ble approach is for the headend to transmit to theuser the carrier error observed during a burst so thaton the next burst the user can adjust its carrierfrequency to match the frequency desired by the


headend receiver. Typically the more informationlearned a priori the shorter the preamble will be. Ingeneral it is desirable to have a preamble that isvariable in length and in MCNS the preamble isbetween 0 and 1024 bits. The headend defines thepreamble.

The SNR of the signal varies significantly de-pending on the channel used. Therefore the amountof error correction required will vary. In MCNS R–Scoding is used for error correction and the size of theR–S code block as well as the number of errors thatcan be corrected in a block are variable and definedby the headend.

5. Advanced PHY proposals

Most of the present standards activity in PHY isdirected towards improvements in the upstream. Theproposals all try to avoid having to dynamicallyplace channels in between areas of large noise powerspectral density and instead have high bandwidthfixed channel which can somehow cope with thenoise.

Synchronous Code Division Multiple AccessŽSCDMA, a variant on the well known wireless

.communication technique has been proposed byw xTerayon 32 . A set of orthogonal length 256 se-Ž .quences ‘‘codes’’ , one for each 64 kbps ‘‘user’’,

are used to ensure that data transmitted at the sametime in the same bandwidth can nevertheless beseparated. In the process the bandwidth of the signalis increased from near 10 kHz to about 6 MHz.When one user is extracted at the receiver the effec-tive noise power is the total noise power in the 6MHz channel divided by 256. Hence a large peak inthe noise power spectral density in the 6 MHzchannel is shared equally among the users and hasless effect on each user. If there are no peaks in thenoise power spectral density the use of code divisionwill not produce any gains over the usual frequencydivision of channels. If a user requires more than 64kbps it will have to use several sequences. Becausethe channel causes the sequences to lose orthogonal-ity, the receiver will have to carefully equalize thechannel. Terayon claim this technique can achieve10 Mbps in a 6 MHz channel. This is about the sameas a QPSK signal if the QPSK signal could span the

whole bandwidth. Because of the noise peaks in thepower spectral density, which must be avoided, mul-tiple QPSK signals will be required and the totalbandwidth would be significantly lower.

Ž . w xIn Synchronized Discrete Multitone SDMT 25Ž . w xand Discrete Wavelet Multitone DWMT 24 a

high bandwidth channel is divided into a set ofsmaller channels by frequency division. Whereverthere is low SNR the system simply chooses not touse the low SNR channels in that region. This issimilar to what is done in the present upstream PHY

Žexcept the channels are very small a few tens of.kHz and are generated and received in groups using

the fast Fourier transform or discrete wavelet trans-form. One significant advantage of SDMT is there isno need to use an equalizer because repetition of thelast few symbols of every data block makes theeffect of the channel look like element by elementmultiplication of the channel frequency response withthe data. There is a small penalty to pay in loss ofdata bandwidth.

6. Concluding remarks

This document presents a general overview of theoperation of HFC cable networks. Due to spaceconstraints, it is not possible to provide details onevery area of each system presented. However, read-ers interested in a more thorough investigation can

w xstart by reading 36 and follow with papers relatingw xto broad-band access systems 33–35 . These sources

reference many other excellent articles. Online infor-mation specific to the 802.14 proposal can be found

w xat 10,37 and that relating to the DOCSIS proposalw xcan be found at 38 .

References

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w x3 Data over Cable Service Interface Specifications, BaselinePrivacy Interface Specification, SP–BPI–I01–970922,MCNS, September 22, 1997.

w x4 RSA Data Security, http:rrwww.rsa.comr.w x5 B-ISDN User-Network Interface – Physical Layer Specifica-

tion, ITU-T Recommendation I.432, March 1993.


w x6 A. Gatherer, Splitter and Drop Tap Measurements, IEEEProject 802.14 Cable TV Protocol Working Group, Docu-ment a IEEE 802.14-96r84, March 1996.

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http:rrwww.cablenet.orgrscter97.1.pdf.w x11 J.W. Eng, J.F. Mollenauer, IEEE project 802.14: standards

for digital convergence, IEEE Commnications Magazine 33Ž . Ž .5 1995 20–23.

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mission over HFC links, SPIE, vol. 2609, pp. 168–175.w x25 J.A.C. Bingham, Synchronized Discrete Multitone: a band-

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IEEE 802.14-96r096, March 1996.w x32 S. Rakib, Synchronous-CDMA: the solution for high-speed

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