Cable Network Crib Notes

These are my ‘crib notes’ that I’ve made to serve as a last minute refresher. Please forgive the grammer / spelling as I did not develop these notes with publishing in mind


*** Theory ***

  • DOCSIS – Standard governing how cable operators reserve bandwidth for data transfers. When modem boots up it finds a DOCSIS channel (scans for RF for QAM lock). CMTS sends 3 messages (MAP, UCD, SYNC) to modem. It then requests IP from DHCP Server. Modem gets config file via TFTP (address given by DHCP). Modem then register with CMTS and negotiates QoS etc
  • ADSL – Up to 8MB DL and 1MB UL. Limited to 18,000 feet limitation. Can use phone via POTS Splitter.
    • Coding methods
      • CAP – Single Carrier Method – Divides phone line into three separate channels. (V, Upstream, Downstream) – Been replaced by DMT
      • G.Lite – one of two multicarrier methods “splitterless ADSL”. Limited to 1.5MBPS DL and 512 KBPS UL = slow
      • DMT – The 2nd multicarrier method – Uses 256 channels to carry data
    • HDSL – Same UP/DL rate (Symmetric). Can’t use the phone
    • HDSL2 – Allows for VOIP
    • RADSL – UL/DL are adjusted dynamically
    • Satellite – Very slow.  DL 500K and UL 50K (On a clear day!)
      • Problems
        • Attenuation – Signal gets weak
        • Impedance Mismatch – Bad splice or corrosion
        • Cross talk (Inside)
        • AM Radio (Outside)
      • ATM – Uses DSLAM Switches (has DSL card) for data transport.
        • PPPoE vs PPPoA – Key difference is oA uses routing and oE uses bridging
        • PPPoE (RFC 2516) – Typically uses Chap.  Host devices uses discovery to get MAC of PPPoE Server. This creates SESSION_ID.
          • Interface setups
            • Connection to DSLAM – No IP address need and dial pool number (needed) which binds a dialler interface to an Ethernet one.
            • Dialler
              • Ip mtu 1492 – Reduce from 1500 to allow for PPPoE headers
              • Ip address negotiated – Allows for DHCP address to be given
              • Ip nat outside (if using Nat)
          • Default route should be dialler interface
          • Use dialler interface when using NAT inside for PAT.
      • PPPoA – If encapsulation is running under PVC, you are running PPPoA
        • Interface Setups
          • Connecting to DSLAM (ATM 0/0)
            • No ip address
            • Dsl operating-mode auto / Auto negotiate modulation with downstream router
            • Pvc 100/120 / Like DLCI
            • Pppoe-client-dialer-pool-number-1
      • RFC 1483/2684 Bridging – Easy to setup. Multiprotocol. Single user environment. Uses lots of broadcasts, not scalable, can be attacked.