Cable Modems share the cable TV line coming in your home. It provides an asymmetrical transfer rate with the upstream being significantly lower then the downstream. For example, upstream 2 Mbps and downstream 30 Mbps.
The downstream speed depends on number of users using the cable system in the area as the line is shared with other subscribers on cable line.
There are three competing standards:
Originally there were security issues as you could see others through Microsoft's Network Neighborhood. Now that has been addresses and each cable modem connection is encrypted.
You can get very fast transfers through the high download speeds, faster then ADSL. The cost is very competitive and is usually bundled with your cable TV. The best part is that your home is prewired making it easy to install.
DOCSIS Protocol Stack
Data Over Cable Service Interface Specifications (DOCSIS) is the standard in North America for running data over the cable service.
DOCSIS and the OSI Model
Data Link Encryption uses the Baseline Privacy Interface (BPI) because of the shared medium. It uses a public/private key exchange to encrypt data between cable modem and Cable Modem Termination System (CMTS).
Transmission Convergence layer: MPEG-2 is used on the Transmission Convergence layer. All data, voice, audio and Internet is encoded in MPEG-2 frames. This is used for the downstream communications only.
Media Access Control layer (MAC) - uses Hybrid Fiber Coax (HFC) plant which is similar to Ethernet. It is used only for upstream communications. It is used for determining "when to transmit data" and "for how long" on the CMTS (Cable Modem Termination System).
PHY - Physical Layer uses QPSK or QAM-16 for uUpstream modulation and QAM 64 or QAM 256 for downstream modulation.
Cable Frequency Plan
The Hybrid Fiber Coax (HFC) plant operates on NTSC (National Television Standards Committee) with 6 MHz per channel in the frequency bandwidth of 48 to 870 MHz which gives approximately 137 channels.
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