DOCSIS - The Broadband Cable Modem Service (IEEE 802.16)

This report was written along with three other people for a University Unit Advanced Networking (ADNET) which talks about the history of cable modems, how they work, and future uses.

Cable Modems came into existence as one answer to the issue regarding the lack of bandwidth and speed available to home users. Home users traditionally used the analogue telephone system to transmit and send data across the Internet. The fastest they could connect on a standard connection was limited by the poor infrastructure (wiring etc) of a system designed to support voice communication not computer data transmission. The 56k dialup modems were the fastest achievable on such setups.

Due to the boom in cable Television, brought about in part by the deregulation of the cable television stations but the 1984 Cable Act (allowing the stations to broadcast popular sports programs etc), it is estimated that by the end of the 80?s 53 million households were cable subscribers. With the infrastructure of cable already in place and cable of high speed communications via the coaxial cabling and fibre optical cables (far superior to the analogue telephone system) it is of no surprise that the Cable Modems, allowing the home user to connect to the Internet via their cable connections, proved popular and a viable success economically. The new Cable Modems allowed connections far in excess of that available on the old 56k dialups.

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Contents

1. Concept
1.1 Background
1.2 Other alternatives
1.2.1 ADSL (Asymmetric Digital Subscriber Line)
1.2.2 VDSL (Very high bit-rate DSL)
1.2.3 ISDN (Integrated Services Digital Network)
1.3 Cable Modem Model
2. Architecture - Physical Layer
2.1 Technology
2.2 Inside the Cable Modem
2.2.1 Tuner
2.2.2 Demodulator
2.2.3 Burst Modulator
2.2.4 MAC
2.2.5 Interface
2.2.6 CPU
2.3 Downstream
2.4 Upstream
2.4.1 Reserved Slots
2.4.2 Contention Slots
2.4.3 Ranging Slots
2.5 Modulation
2.5.1 Quadratic Phase Shift Keying
2.5.2 Quadratic Amplitude Modulation
2.6 Error Control
2.6.1 FEC (Forward Error Correction) RS - Reed-Solomon
2.6.2 CRC (Cyclic Redundancy Check)
2.6.3 Interleaving
3. Architecture ? Protocols
3.1 Access Methods
3.2 Routing
3.2.1 Routing Block Diagram
3.2.2 CMTS
3.3 Management
3.3.1 Upstream Channel Descriptor
3.3.2 DHCP (Dynamic Host Control Protocol) Address Assignment
3.3.3 TFTP (Trivial File Transfer Protocol) Configuration Assignment
3.4 Encapsulation
3.5 Frame Formats
3.5.1 DOCSIS MAC Header
3.5.2 Downstream Frames
3.5.3 Upstream Frames
3.5.4 Frame Life
4. Security
4.1 Authentication
4.2 QoS (Quality of Service)
4.2.1 Packet Priorities
4.2.2 CMTS Control
4.2.3 Unsolicited Grants
4.2.4 Packet Fragmentation
4.2.5 Payload Header Suppression
5. Conclusion
5.1 Future Uses for Cable Modems
5.2 VoIP (Voice over IP)
5.3 End Conclusion (The Comparison to Other Broadband Technologies)
Bibliography
Glossary of Terms
Appendix A - Packet Headers
IP Header
TCP Header
Appendix B - OSI Layers