Cable Modem 6th Mar 2013

Azzam and Ransom pp 164 to 190

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ImportantThe syllabus for the unit tests and the final examination comprise of two parts1. The prescribed text books’ chapters. (the PPT clearly

mentions the text books and the page numbers )2. The PPT for each of the modules.

The PPT story line is generally as per the text book flow. However, in addition, certain additional topics which are relevant to the students, are also dealt with in the PPTs

Students are advised to read the text book and the PPTs and be thorough in BOTH

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ImportantThe students are advised to read the notes in the text area under each slide in all my presentations. It contains valuable informaation

Some of the abbreviations used CM ( Cable modem), US ( Up stream), UL (Uplink), DS (Down stream), DL (Downlink) , NW (Network)

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DOCSIS Block Diagram

Principal Function of the DOCSIS Cable Modem System Is to Transmit Internet Protocol (IP) Packets Transparently Between the Head end and the Subscriber Location.

The DOCSIS System Consists of:◦ Cable Modem Termination System (CMTS)

located at the headed◦ Cable Network◦ Cable Modem (CM) located at the Customer

Premise

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DOCSIS Model of a Basic Cable Modem system handling IP traffic

CableNetwork HFC

CustomerPremises

Equipment

Transparent IP Traffic Through the System

Cable Modem(CM)

CMTSNetwork Side

Interface

Wide-AreaNetwork

Cable Modem

TerminationSystem(CMTS) CM Customer Premises

Equipment Interface

DOCSIS Model of a Basic Cable Modem system handling IP traffic

Types of cable Modems (CM = Cable Modem)

Two types of cable modems

IEE 802.14 Based on ATM Based on IP Why ATM? High speed 655 Mbps Integrated services Lots of appliances Multiple QOS Cable modem centric DOCSIS 1.0, 2.0, 3.0 (data over cable service I/F specs) Developed by MCNS / SCTE  Why IP? IP is an engine driving high speed data Internet needs no explanation VOIP has already accepted IP as supreme

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(CM = Cable Modem)

Cable Modem Architecture

Transmit/UpstreamQPSK/QAM Modulator performs:

QPSK/QAM-16 modulation Reed-Solomon EncodingD/A ConversionUp-conversion to the selected frequency/channel (Baseband freq to RF frequency)

Receive/DownstreamRF Tuner

Converts TV Channel to a fixed lower frequency (6-40MHz)

QAM Demodulator performs:A/D conversionQAM-64/256 demodulationMPEG frame synchronizationError Correction (Reed-Solomon)MAC - Media Access Control Implemented partially in hardware and softwareData and Control Logic

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DOCSIS EvolutionDirection Downstream Upstream

Version DOCSIS EuroDOCSIS Both

1.x 42.88 (38) Mbit/s 55.62 (50) Mbit/s 10.24 (9) Mbit/s

2 42.88 (38) Mbit/s 55.62 (50) Mbit/s 30.72 (27) Mbit/s

3.0 4channel

+171.52 (+152) Mbit/s

+222.48 (+200) Mbit/s

+122.88 (+108) Mbit/s

3.0 8channel

+343.04 (+304) Mbit/s

+444.96 (+400) Mbit/s

+122.88 (+108) Mbit/s

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ATM centric vs IP centric

ATM centric vs IP centric ATM Centric IP centric PHY layer ITV J.83 PHY layer ITV J.83 IEEE 802.14 DOCSIS 1.0 MAC and higher layers are different. (SAR MAC & higher layers are different (Variable and IP over ATM layer) size IP Pkts based on ISO 8802.3 Security and Maintenance in Upper layers Security and Maintenance in upper layers are different are diffferent Regn is different Regn is different

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ATM and IP centric Modems convergence.

What is the confusion?ATM or IP?Fight of the centuryIs there room for two mighty technologies?Can IP provide high speed access?Will the services be ATM or IP centric?No easy answer eallyDAVIC is already aligning with ATM802.14 is aligning with IP

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Where it will end?ProbablyMSOs will play a big role in solving this

dilemmaMSO will establish a forum with cable

labs and vendorsAdopt IEEE 802.14Accelerate an Inter-operabilty processHijack the industry really………….

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Cable Modem operation

What is the objective?Transport high speed digital data in cableDeliver integrated services over cableShare access among multiple usersDeal with security and Privacy issues Tune to any of the 6/8 MHz band to receive ( m channels in

450 to 750 MHz)Tune also to a corresponding upstream channel to transmit ( n

channels in 5 to 42 MHz)◦ A CM needs to register first with HE◦ A dialog takes place between HE and CM for the registration

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Cable Modem / Head-end - Physical Topology

Head end Cable

Modem 1Cable

Modem 2Cable

Modem X

123

m

123

n

Down Stream 450 MHz to 750 MHz

Up Stream 5 MHz to 42 MHzFig 5.4 pp 167

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Down stream and Up stream

DS: CM tunes to a 6/8 MHz slot450 to 750 MHzReceives digital data signalsQAMUS: 5 to 42 MHzQPSK ( Better fighter against ingress noise in lower spectrums…….Amateur radios, Home appliances……….)QPSK is slower than QAM though

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DOCSIS Support Devices architecture

TOD: Time of Day; TFTP: Trivial File Transfer Protocol; DHCP: Dynamic Host Configuration Protocol 14

DHCP Server will provide IP addressing for the CM CPE.The Time of Day (TOD) Server provides universal timing for the CM.The Trivial File Transfer Protocol (TFTP) Server hosts the configuration file and firmware updates for the modem.

Cable Modem / Head end Layered Architecture

Others ATM LLC Others ATM LLC

Headend |MAC Cable Modem|MAC

UpstreamPHY

Downstream

PHYUpstream

PHYDownstrea

mPHY

Down Stream 450 MHz to 750 MHz

Up Stream 5 MHz to 42 MHz

MAC Laye

r

PHY Laye

r

ChannelFig 5.5 15

Cable Modem Speed Bandwidths :1.0 Mbps to 43 Mbps DS 1.5 Mbps US  US Access: Contention based Shared in a bus config with many users Handled by MAC of each modem BW for each user depends on number of users sharing

this bus and also their usage pattern If congestion is encountered, a smart CM will hop to

another freq under the instruction of HE This is known as freq agile CM

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Cable Modem LayersThree layers PHY Layer MAC Layer Upper layers PHY layer: CO AX cable mostly US and DS signals One DS transceiver and several Upstream

transceivers US transmission more complex

Why?CollisionsNoise IngressHow to deal with this?

Contention based access algorithms QPSK 17

Fundamental Layers of a Cable Modem

Others ** ATM LLC Other

s**

ATM LLC

* MAC/ATM Conv sublayerMAC access Arbitration

Up Stream 5 MHz to 42 MHz

MAC Laye

r

PHY Laye

r

Channel

* MAC/ATM Conv sublayerMAC access Arbitration

TC sublayer (DS only)PMD Sublayer

TC sublayer (DS Only)PMD Sublayer

Down Stream 450 MHz to 750 MHz

* MAC/ATM Convergence sub-layer is present in ATM centric modem only

** ATM native service for ATM centric modem only

18Fig 5.6

Cable Modem LayersPHY and MAC are embedded in the hardware

itself

S/W in modems just complement these layers

And give the modem its service personality e.g MAC/ATM convergence sublayer (SAR) will not be present in IP based CMs

These layers and sub-layers deal with specific dialog needed between HE and CM

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Complete (OSI) Protocol Stack of CM

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Sub Layers in PHYTwo sublayers (In PHY layer)1. PMD - Physical Medium

Dependent2. Transmission Convergence

◦ These layers provide appropriate interfacing for the attached transmission medium (cable, fiber, wireless……….)

◦ Some examples:Bit translational codingSynchronisationModulationE/O and O/ETransceivers

Table gives a good landscape

PMD SublayerModulationDemodulationSynchronisation

codes Error checks21

Up stream PMD Layer transmits in mini slots. Mini slots controlled by DOCSIS MAC One MAC frame may contain several contiguous mini slotsUp stream speed : 500 Kbps to 2.5 Mbps typ (Even 10 Mbps)

Down stream PMD layer : 7 MHz band in 91 to 857 Mbps rangeITUT -- J.83Down stream speed: 1 Mbps to 3 Mbps (even 27 Mbps)

TC Sublayer: To provide common receiving hardware at the CM for both video and dataTC sublayer receives MAC frames and produces a continuous stream of 188 byte MPEG pktsIt provides an opportunity for additional services over the physical-layer bitstream.

Up stream and down Stream Upstream Mini PDUs can be concatenated for ATM centric cells and IP

centric Pkts Hunting and sliding techniques identify delineation

boundaries When CM needs to send, it requests HE an allocation by

sending a request PDU in a mini PDU If contention is successful HE allocates the requested US

bandwidth thro a GRANT message and inform CM thro DS The allotted slots will depend on BW requested The CM now sends its PDUs in the allotted slots Collisions are resolved thro contention algorithms  Downstream CM receives a stream of PDUs Mgement PDUs address a particular CM with its unique

identifiers Each CM filters its incoming PDUs based on the identifier CM obtains IP address thro DHCP

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breakdown of data for both upstream and downstream data.

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Role of the Sublayers of the Physical layer (Cable Modem Perspective)

Sublayers of the Physical layer (Cable Modem Perspective)

MPEG 188 bytes per ITU-H.222.0MPEG Sync and RecoveryMPEG PID (Pkt ID) Mux and DemuxDilineation of PDUs and ScramblingControls: Ranging, Power, Sync

TC

PHYSICAL LAYERDown Stream Upstream  

  Preamble Generator

PMD

Scrambler ScramblerReed Solomon Encoder Reed Solomon

Encoder  Pulse Shaping

RF, QAM modulator per ITU J.83

QPSK or 16 QAM Modulator

26Fig 5.7

TC ( Transmission Covergence ) sublayerDown Stream

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₠ For DS, TC sublayer refines the data and assembles it into a frame

₠ TC establishes synchronisation₠ TC identifies MPEG frame boundaries₠ TC achieves Sync by sliding window hunting.₠ Keeps shifting the sliding window till it finds the CRC bits₠ Reconfirms 5 times CRC in successive frames₠ To declare out of sync, 9 consecutive frames shd miss CRC check₠ Once MPEG frame boundary is established, TC extracts CM pkt

data from MPEG payload. The format is shown in figure

(pp174)

MAC layer MAC Layer

Like in a LANMost challenging to specifyComplex becos of shared mediumComplex becos of QOS guarantee to each userMAC devices a mechanism to

Provide random accessResolve contentionPreserve QOS for every userMinimise jitter delay for voice and videoAllot CBR bandwidth

What is special about Mac in Cable Modem? CM MAC does all the above

Plus more challengesBandwidth intensive Multimedia applns / servicesMAC operates in a Public environmentUser expectations are high

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Frame Control (8 bits) MAC Parameter (8 bits)Length (16 bits)

Extended Header (0-240 bytes)?

Header Check Sequence (16 bits)

Data

DOCSIS MAC Header Frame format

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FC Control Type field (2 bits)00: Packet PDU MAC Header01: ATM PDU MAC Header10: Reserved PDU MAC Header11: Reserved MAC Specific Header

Parameter field whose use is dependent on FC:

The length of the MAC frame. The length is defined to be the sum of the number of bytes in the extended header (if present) and the number of bytes following the HCS field.

(EHDR) is used to implement data link security and can be for additional functions in future releases. I

HCS field is a 16-bit CRC that ensures the integrity of the MAC Header, even in a collision environment

Cable Modem Modulation formats

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Down Stream frame

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The payload portion takes the Ethernet frame and breaks it into segments of 184 bytes.

Some bits in the Header identify the payload as being the start of the Ethernet frame, within the middle of the Ethernet frame, or the end of the Ethernet frame.

Reserved

The upstream frame format

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Example: Ethernet frame is put within a MAC frame

The upstream frame is variable length and it uses an FEC to help decrease errors in transmission.

(EHDR) is used to implement data link security and can be for additional functions in future releases. I

Power-up SequenceWhen you turn the power on for the CM, it goes through a sequence of steps before the modem can start sending and receiving data as follows…Initialization The CM scans all the TV channels looking for data. It is told

which upstream channel to use.Ranging The CM adjust its transmit timing ands adjusts its transmit

power levels.Establish IP connectivity Temporary IP address are assigned for the modem (not the

computer)Receives the current time of day The modem receives configuration information.Registration CMTS sets up access to network, assigns IDs, slotsBaseline Privacy Establish the encryption and decryption keys. 33

Registration process: CM listens to DS channels seeking entry to register

itself Finds the strongest channel in the DS and CM reads

the sub system freq allocation layout Finds the corresponding US channel and sends a

request in it with a registration request message. Headend acks this request CM goes thro authentication, ranging and initialisation

routines legitimizing itself with the network Starts providing data + pictures to the user

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CMTS and Cable Modem Startup

Provision modem in the Cable Router (operator configured or automatically provisioned)

Install modem at subscriber premise (cable and power)

HFC

CMTS

MODEM

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Monitor for SYNC Message Periodically transmitted by CMTS SYNC message contains a time stamp CM to synchronize its time-based reference clock so that its

transmission on the upstream will fall into the correct mini-slots

CMTS

SYNC Message

MODEMHFC

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DHCP Discover - IP ConnectivityCM sends a broadcast DHCP request via the CMTS to the DHCP Server DHCP server returns:

◦ IP address and Subnet Mask◦ CM configuration file name and

IP address of TFTP server◦ UTC time offset to establish

local time◦ TOD Server IP address

CMTSDHCP-RSP

DHCP-REQ

Server

MODEMLAN/WAN HFC

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The modem will then request DHCP information (DHCP Discover).

DCHP server will respond with a DHCP Offer containing the IP address for the modem,, the address of the Time of Day server, TFTP server address and the file name of the modems configuration file.

CM responds to the offer with a DHCP Request. DHCP server responds with a DHCP ACK. CMTS snoops the DHCP exchange between CM

and DHCP server to learn IP address of the CM.

Time of DayWhat is the role of TOD server in CM? CM sends a request to the ToD Server ToD Server responds: GMT TOD is needed for time stamping events and key management by the

security system. The time retrieved from the TOD server (UTC) will be used to create

the current local time

CMTSToD-RSP

ToD-REQ

Server

MODEMHFC

LAN/WAN

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Cable Modem Registration Sequence.

Variable Constellation Multitone (VCMT)

Multicarrier Modulation Approach 7 MHs band is divided into discrete set of tones VCMT is based on transmitting multiple harmonically related

tones in the assigned freq channel The tones are modulated with the QAM constellation (from

QPSK to 256 QAM) VCMT uses Pilot tones to synchronise with HE receiver When HE requests CM can hop to a different freq channel

within that 7 MHs band The intention is to avoid noisy sub bands and transmit in sub

bands where C/N is high Similar to Discrete Multitone (DMT) in DSL

Adv: Evades Noise by hopping to a better freqCompatible with DOCSIS 1.0 or 802.14 MAC

Disadv: By hopping unused spectrum is wastedRunning away from the problemMore complex to implement

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Advanced FA-TDMAFA-TDMA uses a single carrierDynamic changing constellation sizeReed Solomon FECTrellis coded modulationsFEC, Symbol rate change and Interleaving provides

robustness against Impulse and Burst noiseConstellation formats: QPSK, 8 , 16 , 32 , 64 , 128 QAM HE monitors the channel noise and adjusts the constellation

sizeAdv: Single carrier and therefore less complex to implementProven TechnologyCompatible with DOCSIS 1.0 or 802.14 MAC

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S-CDMA technology IS 95 CDMA Spread spectrum Asynchronous CDMA produces self generating noise Why? Users of Async CDMA are not aligned with each other thus

losing orthoganality of codes Therefore Sync CDMA is better suited In S-CDMA, all CMs have their codes synchronised to each

other Supports CBR, ABR, VBR HE manages BW allocation and BWAdv: Less noisy than Async CDMA

Guaranteed data rates – CBR, ABR, VBRSpreading makes C/N less stringentSpreading protects against impulse/burst

noiseFwd looking technology

Disadv Sync requirement is stringentImplementation is complex

What is the final HI_PHY selected by DOCSIS/IEEE 802.14? A combination of TDMA and S-CDMA

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