From: "James Egan" <
[email protected]>
|
| Perhaps you can explain something for me, Dave, since I'm not familiar
| with cable modems and, like Gaby, I thought a cable modem was similar
| to a dial-up modem?
|
| Let's take as an example a standalone PC connected via a USB cable
| modem to an ISP and compare it to an external (dialup) modem connected
| to a PC via a serial cable.
|
| My understanding is that in both cases, the PC's (virtual) network
| adapter (ie the dialup adaptor or the virtual adaptor created by the
| USB cable modem driver) receives the external ip address allocated by
| the ISP. In other words, the PC receives the external ip address not
| the modem. And the modem doesn't have it's own ip address since it's
| just there to modulate/demodulate the signals?
|
| Jim.
James:
With a DUN modem you have a limited bandwidth. The POTS line is designed for audio
comminications and not high fidelity as well. It is designed such that the common areas of
human speech can pass through a twisted pair transmission line. Therefore a DUN modem uses
FSK or the later ITU standardized protocol v.34[
http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci213936,00.html ] to allow a
digital signal to be transposed upon that transmission line and the modem will modulate and
demodulate the signals accordingly. It is upto the coputers at both end to use the Point to
Point Protocol (PPP) to assign an IP address and pass TCP/IP communications along the now
created path.
With Cable and DSL modems the modems (which is not an appropriate name) already are using
network protocols to pass along the transmision line.
In a cable modem the standard is DOCSIS (Data Over Cable Service Interface Specification)
use DHCP to assign an IP address on the PC. The devide acts as a Bridge of two dissimalar
networks to to allow TCP/IP to be used. In this case Cable and Ethernet.
In a DSL modem protocols such as; G.DMT, G.Lite, G.Test, G.AOM or G.HS allow didgital
signaling at high frequencies to be superimposed upon the POTS twisted pair transmission
line. The signaling is based upon , Discrete Multitone Modulation (DMT, Carrierless
Amplitude Modulation (CAP) or Quadtarture Amplitude Modulation (QAM) techniques. What is
applied will determioned if the DSL signal is or is NOT POTS compatible. That is whether it
can be used in conjunction with a audio telephone line or with a "dry pair" telephone line
where to telephone conversations will be used. What signaling will be used will also be
based upon if the DSL is Asyncnronous (different upload and download speeds -- ADSL) or
Synchronous (same upload and download speeds -- SDSL). My verizon DSL is Asyncronous DSL
with 1.5Mb/s download and 768Mb/s upload speeds. The modem bridges the this network with
Ethernet. DSL will either use DHCP to assign an IP address or us the Point to Point
Protocol over Ethernet (PPPoE) and a DHCP like protocol called Internet Protocol Control
Protocol (IPCP) to assign a an IP address. Note that PPPoE is a Tunneling protocol and
while standard Ethernet uses a 1500 byte packet for a Maximum Transmission Unit (MTU) the
overhead of PPPoE will consume 8 bytes of said 1500 bytes so the MTU is 1492 for for DSL
usiing PPPoE.
The important concept here is that in DUN modems networking protocols are NOT used between
the two points while with DSL and Cable there are network protocols and the so called modem
bridges the the two dissimalr network cabling types.
A good way to example this is that Cable and DSL can't directly use IPX/SPX since it the
wrong network protocol while TCP/IP is the right network protocol. Using a DUN modem you
can use IPX/SPX directly. The only way to pass IPX/SPX over a DSL or Cable network would be
to use an IPTunnel which creates a tunnel where IPX/SPX can be passed inside a TCP/IP
packet. Novell's IPTunnel uses TCP port 213. Thus a remote PC only using IPX/SPX as the
network transport can uses a DUN modem to connect directly to a Novell Server. You could
not use Cable or DSL networks without loading both TCP/IP and IPX/SPX on the remote PC.
IPX/SPX to access the Novell server and TCP/IP to allow IPX/SPX to be tunneled within TCP
port 213.