Cable Networksindigoo.comContents1. Cable network architecture2. Relevant cable network standards3. DOCSIS Peter R. Egli 20152/15Rev. 3.00

indigoo.comCable Networks1. Cable Network architecture (1/3)Traditional cable network architecture:In traditional cable networks, TV signals received in regional head ends in dish antennas aredistributed to local head ends (LHE). LHEs distribute TV signals to residential customers viacoaxial cables.Distribution coaxialcable with amplifiersand tapsLHELHEcTrunksplitterTapLHECoaxialtrunkcable Peter R. Egli 2015RegionalHead End(RHE)LHETapcFiberring forCATV HELHE Local Head EndRHE Regional Head EndCATV CAble TeleVision3/15Rev. 3.00

indigoo.comCable Networks1. Cable Network architecture (2/3)Cable network architecture for TV and data service:Cable networks upgraded for combined TV and data service (Internet access) employ a hybridfiber coax (HFC) cabling architecture.Trunk coaxial cable of legacy cable networks are replaced by fibers that terminate in fiber nodes.Homes are connected to fiber nodes through coaxial cables fitted with bidirectional amplifiers.BidirectionalamplifierCoaxialfeeder cableUpgraded LHE with fiber node, CMTSand servers for Internet service.TapFNFiberringCATVCMTSCoaxialdrop r)FN Peter R. Egli 2015DHCPServerTFTPServerDNSServersyslogServerCATV Cable TVCMTS Cable Modem Termination SystemHFC Hybrid Fiber CoaxNSI Network System InterfaceNSIInternetCM Cable ModemFN Fiber NodeLHE Local Head EndHP High Pass4/15Rev. 3.00

Cable Networksindigoo.com1. Cable Network architecture (3/3)Limitations of traditional cable networks for data service:Traditional cable networks were targeted at TV distribution (broadcast TV signals to customer).As cable networks were one-way only (signals only from cable HE to customer), they wereunsuited for 2-way data service. In earlier days the upstream for data service (from customer toHE) was realized with dial-up phone lines but this proved not a viable solution. In order to offertwo-way data service on a cable network, additional infrastructure was required.Cable networks tree-branch topology:Cable networks have a tree-and-branch topology. The coax cables are simply branched forhooking up subscribers (homes). This means that a cable network is a shared medium (allsignals are always present on the entire network). Amplifiers need to be installed at regularintervals (distances) to regenerate the signals. Up to some 35 amplifiers can be cascaded thusaffording wide geographical coverage.Cable network upgrade for data service:In the course of upgrading traditional TV-distribution networks to two-way data networks,digital signaling, two-way amplifiers on coax cables and fiber trunks with greatly increasedcapacity were introduced. This resulted in a combined fiber-coax infra-structure (HFC - HybridFiber Coax). Fibers run between LHEs and fiber nodes. Coax cables (feeder cables) connect toindividual homes.Two-way cable networks provide an ‘extended Ethernet’ network that can span up to 100 miles. Peter R. Egli 20155/15Rev. 3.00

indigoo.comCable Networks2. Relevant cable network standards (1/2)Most of the relevant cable network standards were developed and published by CableLabs Inc. ( addresses physical and data link layer operation while PacketCable defines the upper layersfor providing additional services like IP and VoIP.ITU-T and ETSI have adopted CableLabs DOCSIS standards and published these under theirrespective nomenclature.Upper layersOSI L5-7 (application layer)OSI L4Transport layerOSI L3Network layerOSI L2Data link layerOSI L1Physical layer Peter R. Egli T J.112/J.122/J.222ETSI TS 102 6396/15Rev. 3.00

indigoo.comCable Networks2. Relevant cable network standards al layer &data link layerData Over Cable Service Interface Specification.Version 1.0 through 3.0 based on 6MHz TV channels (8MHz inEurope).Version 3.1 (October 2013) based on 20kHz-50kHz OFDM subcarriers bonded inside a 200MHz spectrum.All DOCSIS versions are downwards compatible.EuroDOCSISCable EuropePhysical layer &data link layer,EuropeanspecialtiesFrequency allocations for European CATV systems based on8MHz channels affording more digital bandwidth per channel.Activities of Cable Europe were transferred to CableLabs in2013.ES 102 639ETSISame asEuroDOCSISEuroDOCSIS adopted by ETSI.J.112, J.122, J.222ITU-TPhysical layer &data link layerDOCSIS adopted by ITU-T.J.112 DOCSIS 1.0. J.122 DOCSIS 2.0. J.222 DOCSIS 3.0.DOCSISPacketCableCableLabsLayer 3 – 7PacketCable builds on top of DOCSIS and addresses OSIlayers 3 (IP) through 7 for the provisioning of Internet accessand real-time multimedia services like VoIP.EuroPacketCableCable Europe /CableLabsSame asPacketCableOnly minor differences to PacketCable Peter R. Egli 20157/15Rev. 3.00

indigoo.comCable Networks3. DOCSIS (1/8)Cable network protocol stack:User traffic is typically bridged on the cable modem (CM).The application layer protocols DHCP, TFTP and SNMP on the CM are used for configurationand management of the cable modem.The CMTS may employ bridging or IP forwarding.ClientCMTSCMUpper layers(applicationlayer)DHCP / TFTP / SNMP / SyslogDHCP / SNMPTCP / UDPUDPUDPIPv4 / IPv6IPv4 / IPv6IPv4 / IPv6802.2 LLC802.2 LLC802.3 MAC802.3 MACEthernet PhyEthernet PhyBridging802.2 LLCLink sec.802.2 LLCLink sec.Cable MACCable MACCable PhyCable PhyForwardingData linkPhyInternetCMCI (CPE interface) Peter R. Egli 2015Cable network RFICMTS NSI8/15Rev. 3.00

indigoo.comCable Networks3. DOCSIS (2/8)Cable network physical layer (1/2):In DOCSIS 1.0 and 2.0, downstream and upstream transmission use TDM access (TDMA).The frequency spectrum is divided into timeslots and temporarily assigned to modems.This dynamic timeslot allocation is managed by the CMTS.DOCSIS 3.0 uses TDMA or S-CDMA (Synchronous CDMA).DOCSIS 1.0 and 2.0 provided a single 6 or 8 MHz channel per customer. DOCSIS 3.0 introducedchannel bonding that allows combining multiple 6 or 8MHz channels for a single customer.Upstream transmission uses lower frequency bands which are noisier by nature. Thus noiseon the medium is aggregated in the upstream direction which requires far less bandwidth thanupstream (asymmetric traffic pattern of residential users).dBNoisy band (noise aggregated in upstream direction)6MHz (US) / 8MHz (EU) downstreamchannels( to customer)2MHz upstream ( to HE)5MHz Peter R. Egli 201542MHz54MHzf[Hz]865MHz9/15Rev. 3.00

indigoo.comCable Networks3. DOCSIS (3/8)Cable network physical layer (2/2):Downstream modulation and bandwiths:Channel bandwidth /standardQAM64 modulationQAM256 modulation6MHz (DOCSIS 1.0, 2.0)31.2MBit/s41.6MBit/s8MHz (EuroDOCSIS 1.0, 2.0)41.4MBit/s55.2MBit/s6MHz (DOCSIS 3.0)m*31.2MBit/sm*41.6MBit/s8MHz (EuroDOCSIS 3.0)m*41.4MBit/sm*55.2MBit/sUpstream modulation and bandwidths:One 2MHz QAM16 or QPSK upstream channel yields about 0.5-10Mbit/s.Upstream and downstream bandwidth is typically shared by 500-2000 subscribers. With cableequipment becoming cheaper the number of customers per LHE is falling.DirectionModulationStandardsUpstreamQPSK, 16QAMDOCSIS 1.x.UpstreamQPSK, 8QAM, 16QAM,32QAM, 64QAM, 128QAMDOCSIS 2.0, 3.0.Upstream4096QAMDOCSIS 3.1. Peter R. Egli 201510/15Rev. 3.00

Cable Networksindigoo.com3. DOCSIS (4/8)Cable MAC (Media Access Control):Cable networks use TDMA (Time Division Multiple Access) as MAC protocol (CM and CMTSare synchronized). The synchronization is necessary since cable networks span large distancesand thus a simple collision detection algorithm like Ethernet MAC cannot be used.Upstream TDMA MAC:The upstream channel is divided into mini-slots (time division).There are 3 types of time slots:a. Ranging (used to synchronize CM and CMTS for TDMA operation)b. Contention (used by CM to send Request frames).Contention slots are accessed by CM with a truncated binary backoffalgorithm similar to Ethernet.c. Reserved (for ordinary data transmission)Before transmitting the CM requests a channel by sending a „Req“ frame indicating theamount of data to be transmitted; the CMTS responds with a „Grant“ frame.The CMTS runs a Bandwidth allocation algorithm (fair share among CMs, maybe prioritizingcertain classes of service, e.g. voice).For this purpose, the CMTS sends an upstream bandwidth allocation map message to theCMs indicating when which CM may transmit data (1 entry per CM indicates the allowedtransmit duration). Peter R. Egli 201511/15Rev. 3.00

indigoo.comCable Networks3. DOCSIS (5/8)Cable MAC frame format:MAC header:DOCSIS defines a number of frame types all of which use the following frame format.11-220-24020, 18-1522FCMAC-PARMLEN / SIDEHDRHCSPayloadUser data frame:End user traffic is encapsulated in user data frames.The payload carries a complete 802.3 MAC frame.11FCMAC-PARM2LEN2HCSSAFC Frame ControlLEN LengthEHDR Extended MAC HeaderDA Destination Address Peter R. Egli 20150, 18-1522 bytes payload802.3 Ethernet frame with user dataDAType/LenUser DataFCSMAC-PARM MAC ParameterSID Service IdentifierHCS Header Check SumSA Source Address12/15Rev. 3.00

indigoo.comCable Networks3. DOCSIS (6/8)Link layer security:Since cable networks are shared networks (logical bus), link layer encryption is required toachieve ‚wire equivalent privacy‘ (privacy equivalent to dedicated wires for each customer).DOCSIS security has the following goals:a. Data privacy across cable network (HFC)b. Protection of cable MAC from unauthorized accessDOCSIS main security features:Feature128 bit AES traffic encryption (MAC frame payload only, header remainsunencrypted)Source IP address verificationCertificate revocationKey management and exchange protected by digital certificates and 3DES forkey exchange between CM and CMTS Peter R. Egli 201513/15Rev. 3.00

Cable Networksindigoo.com6. DOCSIS (7/8)Boot sequence (1/2):1. Scanning:The CM scans for available downstream channels for the synchronization with the CMTS(CMTS sends synchronization broadcast frames as a timing reference).2. Receive upstream parameters:The CM receives the upstream parameters and informs the CMTS of its presence.3. Ranging (frame synchronization CM with CMTS):a. 3 consecutive time slots are reserved for ranging.b. The CM sends in the second of the three time slots (1st and 3rd timeslots are gapsto ensure that ranging burst does not collide with data in other time slots).c. The CMTS measures CMs transmission and instructs CM to adjust its internal clock.Ranging is necessary due to physical distance between the CM and the CMTS (delay, varyingattenuation levels).d. The CMTS informs the CM of upstream and downstream frequencies. Peter R. Egli 201514/15Rev. 3.00

Cable Networksindigoo.com6. DOCSIS (8/8)Boot sequence (2/2):4. The CM acquires an IP address by DHCP and a TFTP server address along with the name ofthe configuration file.5. The CM downloads its configuration from the TFTP server.6. The CMTS checks that the CM received the configuration from a legitimate TFTP server.7. Link security startup with the exchange of keys for encryption.8. The CM registers with the CMTS.9. The CMTS assigns a SID (Service Identifier) to the CM and authorizes the CM to send trafficonto the network. Peter R. Egli 201515/15Rev. 3.00

Cable Networks 1. Cable Network architecture (2/3) Cable network architecture for TV and data service: Cable networks upgraded for combined TV and data service (Internet access) employ a hybrid fiber coax (HFC) cabling architecture. Trunk coaxial cable of legacy cable networks are replaced by fibers that terminate in fiber nodes.