We live in a virtual world where most of us have network access in our homes -- the Internet -- which is a virtual...
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network. We access the virtual network with some form of TCP/IP connection, which is almost always a type of Ethernet.
That access gets to your home or office behind the scenes through the virtual network. For example, you might be using an asynchronous digital subscriber line (ADSL), dial-up or some other mechanism leading you into that "network cloud" where technologies such as synchronous optical network (SONET), synchronous digital hierarchy (SDH), wave division multiplexing (WDM), dense wave division multiplexing (DWDM), coarse wave division multiplexing (CWDM), optical carrier networks (OCx) and other technologies exist.
Why not just use TCP? Well, you probably are but TCP is a protocol that has to run on some sort of network transport. SONET is a network transport that carries TCP-based traffic. Ethernet, InfiniBand and Fibre Channel (FC) are all interfaces. Telephone carrier-based networks have been built for many years on SONET technology that typically employs optical fiber -- particularly for spanning great distances.
To summarize, optical fiber (maybe copper) is at the lowest level. The cabling then supports a network like SONET with Ethernet, FC or InfiniBand mapped directly onto it. You increase bandwidth using some form of multiplexing. On a SONET network, you can also allocate bandwidth into OCx, such as OC3, OC12 or other subdivisions, and those, in turn, can carry different traffic. While TCP (or TCP/IP) is your virtual network, you still need the physical network -- what's inside that "cloud" -- to support the protocols.
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