Cable Modem Theory

by: Rolf V. Østergaard

Modem Cable Networks - Page 2

 

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Modem

A modem connection is about 50 kbit/s, and is used point-to-point. The distance is virtually unlimited, including multiple satellite hops etc.

Ethernet

An ethernet (LAN) connection is 10 Mbit/s or 100 Mbit/s, and is used to connect many computers that can all "talk" directly to each other. Normally they will all talk with a few servers and printers, but the network is all-to-all. The distance is normally limited to below 1 km.

Cable Modem

A Cable Modem connection is something in-between. The speed is typically 3-50 Mbit/s and the distance can be 100 km or even more. The Cable Modem Termination System (CMTS) can talk to all the Cable Modems (CM's), but the Cable Modems can only talk to the CMTS. If two Cable Modems need to talk to each other, the CMTS will have to relay the messages.

The OSI layer stackup for a DOCSIS Cable Modem looks like this.

OSI   DOCSIS
Higher Layers   Applications DOCSIS
Control
Messages
Transport Layer   TCP/UDP
Network Layer   IP
Data Link Layer   IEEE 802.2
Physical Layer   Upstream Downstream
TDMA (mini-slots)
5 - 42(65) MHz
QPSK/16-QAM		 TDM (MPEG)
42(65) - 850 MHz
64/256-QAM
ITU-T J.83 Annex B(A)

Items in parenthesis refer to EuroDOCSIS, which is a version of DOCSIS with a modified physical layer targeted at the more DVB centric European market.

External box cable modems with ethernet interface normally acts as either MAC-layer bridges (low-end models) or as routers (high-end SOHO models).

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