Intro' to the TCP/IP Protocol Suite.
Internets, while abstracting away the complexities of multiple physical
layers, still have to transmit their data over a physical network. It is
therefore worth considering the physical side of networking briefly.
Modem
Modems provide a simple point-to-point connection between two computers,
over a normal voice band phone line. Modern modems are full-duplex so
conceptually they provide two parallel circuits, one Tx->Rx, and the other
Rx<-Tx.
-------------- -------- -------- --------------
| | --| Tx |-- --| Rx |-- | |
| | / -------- \ / -------- \ | |
| Computer 1 |-< >--- Phone Line ---< >-| Computer 2 |
| | \ -------- / \ -------- / | |
| | --| Rx |-- --| Tx |-- | |
-------------- -------- -------- --------------
The more modern modems (>1.44Kbps) don't operate like this, they use
various trellis lattices and other schemes to improve bit rate over the
3.6KHz usable bandwidth in an average voice phone like.
Trivia: Most modern modems no longer have a baud rate, as they no
longer transmit bauds
Ethernet
Ethernet is a broadcast network technology. All nodes on an ethernet
network broadcast data frames to all other nodes on the network. Each node
has a 6 octet ethernet address, which is associated with each Ethernet NIC.
Each frame contains an ethernet address in the frame header, and NIC's can
be instructed to only pass frames containing certain addresses to the
network software (in this case a tcp/ip stack). Some of the ethernet
address space has been designated as broadcast/multicast addresses.
Broadcast addresses are important to internet protocols, and network
technologies which don't provide them are often forced to emulate a
broadcast. In the case of ethernet an address of all one's is a network
wide broadcast (note this means physical network, not internetwork).
Ethernet addresses are normally written as 6 hexidecimal numbers seperated
by colons. ie My personal ethernet card has the address 08:00:00:04:50:60.
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Andrae Muys