From: Ravi Kalakota <kalakota@uhura.cc.rochester.edu>

Connectivity allows applications to transparently communicate with other 
programs or processes, regardless of their location.  The key element of 
connectivity is the network operating system (NOS). NOS provides 
services such as routing, distribution, messaging, file and print, and 
network management services.  NOS rely on communication protocols to 
provide specific services. The protocols are divided into three groups: 
media, transport and client-server protocols. Media protocols determine 
the type of physical connections used on a network (some examples of 
media protocols are Ethernet, Token  Ring, Fiber Distributed Data 
Interface (FDDI), coaxial and twisted-pair). A transport protocol provides 
the mechanism to move packets of data from client to server (some 
examples of transport protocols are Novell's IPX/SPX, Apple's AppleTalk, 
Transmission Control Protocol/ Internet Protocol (TCP/IP), Open Systems 
Interconnection (OSI) and Government Open Systems Interconnection 
Profile(GOSIP)).  Once the physical connection has been established and 
transport protocols chosen, a client-server protocol is required before the 
user can access the network services. A client-server protocol dictates the 
manner in which clients request  information and services from a server 
and also how the server replies to that request (some examples of client-
server protocols are NetBIOS, RPC, Advanced Program-to-Program 
Communication (APPC), Named Pipes, Sockets, Transport Level Interface 
(TLI) and Sequenced Packet Exchange (SPX)).

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