The traditional computer communications environment in the 1960s and early 1970s centred around a host computer, for example a mainframe.
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In this centralized computing environment unintelligent terminals used low-speed access lines to communicate with the centralized host.
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IBM's computers with Systems Network Architecture (SNA) networks using multidrop lines, and X.25 public data networks, are typical examples of this type of networking environment.
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Accessing resources, running programs, and copying files are relatively straightforward tasks on a single computer.
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The computer identifies the requesting user and the desired destination device or program and coordinates access between them.
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In this scenario, the single computer is the master of all resources and thus can easily manage and coordinate them.
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Coordinating resources becomes much more complex in a network of even only two computers.
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Transferring information requires tasks such as

• addressing
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• error detection
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• error correction
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• synchronization
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• transmission coordination
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The introduction of the PC revolutionized traditional communications and computer networks.
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As businesses realized the flexibility and power of these devices, their use increased.
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Initially PCs were standalone devices, each requiring its own resources such as disk space and printers.
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Local area networks (LANs) evolved to connect PCs together and thus allow the sharing of such expensive devices.
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Early LANs were isolated but organizations quickly realized the strategic importance of interconnecting them.
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These internetworks provided the basis for enterprise-wide applications such as e-mail and file transfer.
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In the 1970s and 1980s minicomputers and shared Wide Area Networks (WANs) evolved.
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Minicomputers were often located away from the central data center.
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Their processing power allowed for the emergence of distributed data processing.
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The Digital Equipment Corporation (DEC) VAX systems and DECnet networking are typical of this era.
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In general, however, applications remained separate and independent, and different communications protocols were developed.
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Today's networks are a mixture of old and new technologies.
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IBM networks operate in parallel with the newer LAN interconnected networks, electronic commerce, and messaging systems.
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Organizations have used local networks, public data networks, leased lines, and high speed mainframe channels on an opportunistic basis, with little regard for overall integration and consistency.
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Moving applications from central hosts to distributed servers has generated new networking requirements and changing traffic patterns.
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The approach to computer communications in most organizations is changing rapidly in response to new technologies, evolving business requirements, and the need for "instant" knowledge transfer.
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To meet these requirements, the internetwork must be flexible, scalable, and adaptable to suit any organizational level.
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Internetworks tie LANs and WANs, computer systems, software, and related devices together to form the corporate communications infrastructure.
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An internetwork moves information anywhere within a corporation and to external suppliers and customers.
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By serving as the organization's ^Rinformation highway^r, the internetwork has become a key strategic asset and a competitive advantage.
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An internetwork is sometimes referred to as an internet.

This should not be confused with the Internet itself.
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Today's sophisticated users are placing more and more demands on networks.
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Networks have to deal with

• more graphics and imaging
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• larger files and programs
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• client-server computing
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• bursty network traffic
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Users nowadays are demanding

• more bandwidth
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• bandwidth on demand
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• low delays
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• the integration of voice, data, 
  and video
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Future global internetworks will provide even greater bandwidth for new emerging applications.
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Many of these applications have multimedia requirements of high definition imaging, full motion video, and digitized audio.
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Modern network administrators demand the following features from internetworks:

• connectivity
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• reliable performance
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• management control
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The internetwork must be able to connect many separate, and usually different, networks in order to serve the organization depending on it.
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It must be able to do this regardless of the range of media attachments, transmission speeds, and other technical details.
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The internetwork must be reliable.
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The organization depends on internetwork tools such as

• the operator interface
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• the ability to distribute network 
  software updates
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• utilities to log and monitor 
  performance
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• the functions to secure access to 
  resources
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An organisation must be able to effectively manage its internetwork.
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The administrators need to be able to control how the critical resources, such as color printers, are allocated.
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They also need to be able to straightforwardly perform troubleshooting tasks.
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Expanding internetworks demand flexible administrators.
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Expansion and consolidation efforts may mean overcoming physical or geographic boundaries.
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LANs are designed to operate within a limited geographic area.
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This could be within a building or floor of a building.
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LANs connect multiple desktop devices (usually PCs).
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These devices can access high-bandwidth media via the LAN.
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Workstations connected to LANs generally have full-time access to local services, such as printers.
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When an organisation operates a LAN it purchases whatever media and connections are used in the LAN.
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The organisation can privately control the LAN as it chooses.
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Devices you could find on a LAN include

• repeaters
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• bridges
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• hubs
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• Ethernet switches
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• routers
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• gateways
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• Asynchronous Transfer 
  Mode (ATM) switches
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Repeaters regenerate and propagate signals from one network segment to another.
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They do not change or analyze the address or data in any way - they simply pass the data on.
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Bridges are intelligent devices used to connect different LANs together.
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In addition to regenerating and propagating a signal, as a repeater does, a bridge forwards packets of data based on MAC addresses.
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Bridges can also be used to filter traffic.
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They can determine the source and destination involved in the transfer of packets.
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They read the specific physical address of a packet on one network segment and then decide to filter out the packet or forward it to another network segment.
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Using hubs allows you to concentrate LAN connections.
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You can then connect the devices to the hub using twisted pair copper media.
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Ethernet switches offer full-duplex dedicated bandwidth to LAN segments or desktops.
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Routers are concerned with the routing of packets across a network.
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They can perform all the functions of a bridge as well as much more complex tasks.
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Routers open up the data packet and make routing decisions based on the contents of the packet.
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Gateways link networks that have different protocols.
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They can change an entire protocol stack into another.
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They do this by using protocol conversion and routing services.
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Gateways examine the entire packet, including the data portion, in order to translate incompatible protocols.
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For example they can convert from ATM cells to Frame Relay frames.
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ATM switches provide high-speed cell switching.
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They use a cell relay technology which combines the advantages of conventional circuit- and packet-based systems.
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WANs operate beyond the LAN's geographic scope.
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They usually use the services of local telecommunications carriers such as Regional Bell Operating Companies (RBOCs), Sprint, and MCI.
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Organisations pay the carrier or service provider of their choice for connections used in the WAN.
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The carriers are usually regulated by tariffs.
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WANs allow access over serial interfaces (usually a LAN-to-WAN device) operating at lower speeds.
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WANs rarely shut down.
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However since an organisation must pay for services used it may restrict access for certain workstations.
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Also, all WAN services are not available in all locations.
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Like LANs, WAN devices include:

• routers
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• ATM switches
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Other WAN devices include:

• X.25 or Frame Relay switches
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• modems, Channel Service 
  Units/Data Service Units (CSU/DSU),
  Terminal Adapters/Network 
  Termination 1 (TA/NT1) devices
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• communications servers
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• multiplexors
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X.25 or Frame Relay switches are used to connect to public data communications services.
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Modems are used to interface voice-grade services.
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CSU/DSU devices interface T1/E1 services.
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TA/NT1 devices interface Integrated Services Digital Network (ISDN) services.
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Communication servers concentrate dial-in and dial-out user communications.
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Multiplexors allow a WAN facility to be shared among several demand channels.
