4. NETWORK COMPONENTS

Once an appropriate LAN infrastructure (such as structured wiring for example) is in place, the LAN still needs the connection of terminals, printers, PC’s and servers to create the network. The following paragraphs outlines some of the devices that might be seen ona typical LAN.

PC Cards

Most PC systems are equipped with an asynchronous communications port which allows data to be sent and received at low speed (up to 115Kbps.). This is quite adequate for text based systems, but nowadays with the advent of the Graphical User Interface (GUI) the bulk of data is in the form of file transfers, and a connection via a low speed serial port is no longer adequate. Usually a corporate user will require large amounts of data to be transferred either to a laser printer or to or from a file server both of which require a much faster operation, and this means connecting those users to a LAN.

To achieve this, a card needs to be added to the PC system which then allows the PC to connect directly to the LAN. This offers the advantage that the card is sitting on the main communications path of the computer, (which operates at high speed), so data can be transferred directly to disk or memory without disabling the computer for extended periods of time.

Intelligent PC cards

In the earlier days of PC LANs the processing power of the PC was such that the PC NICs (Network Interface Cards) drained a significant amount of resources from the PC. Thus activity not directly related to an individuals PC (such as broadcast messages operating in the background) would affect that users PC’s performance.

A number of communications vendors (such as Case Communications) developed intelligent cards with their own processors and memory which could handle these background tasks without impeding on the PC’s performance. However as PC’s grew ever more powerful the need for such sophisticated cards, became a thing of the past, today the majority, if not all NIC cards use the main PC procesor to handle LAN activity.

It should be noted that not all computers need cards. Apple Macintosh machines, for instance have all the necessary hardware to communicate wth other Local Talk devices built in, so that all that is required is a relatively cheap network adapter.

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Terminal Servers

For customers with traditional terminals (such as a DEC VT100 for example), which communicate via their serial port, its necessary to use a Terminal Server in order to allow these devices to talk to devices on the LAN. Within TCP/IP terminal emulation is handled by Telnet, and thus Async terminals connected to a terminal server would tend to look like PC’s running Telnet sessions to the IP Host.

Host Servers

Mini-Computers were originally designed to have a number of terminals connected each having one port permanently assigned to the computer. The advent of LANs has changed the need and while some computers are still designed to operate in this manner, additional capabilities have been incorporated for LAN connections.

The most sophisticated computers in terms of LAN connections simply link directly to the network and can handle many terminals working across the network at the same time.

Resources

Network resources are any devices which can be shared by more than one user. These may be computers, file servers, gateways or printers.

File Servers

The disadvantage of LANs were that, unlike a central computer, users could not share information easily. The file server was designed to overcome this by offering a central disk drive that any authorised user could access, both to send and receive files. This meant that information could be shared without having to pass floppy disks around.

As file servers became more sophisticated, it became possible to access information on and update it while another user was also looking but unable to update. Similarly some users could be restricted to viewing but would not be able to change or copy files. File servers typically offer sophisticated resource management and accounting facilities, and can provide network managers with powerful administrative tools. In addition the use of RAID and dual power supplies allow file servers to offer high levels of resilience and redundancy.

Printers

The most typical application of early LANs was the sharing of expensive high quality printers. These were very often laser printers which require a great deal more information per page than earlier ‘Dot matrix’ printers which received one byte of data and produced one character.

Typically a network can support several printers, each of which is given a different name or address on the network. This is then used to identify the information being sent to the printer.

File servers can also act as spooling devices for printers. This eliminates the need for users to contend for printers and therefore the need to wait for an available printer during busy periods.

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