2. Modem, Network and Cable Modem
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. For
further explanation of the various acronyms please see the other sections of this tutoral
or refer to www.whatis.com (lots of
short concise explanations of especially the network terms).
| 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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