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Chapter 5 Communication Equipment

Physical Equipment and devices used to provide an interface between the computers and terminals on a network.  Two functions:
 1. Combine data signals before they are transmitted over the communication network
 2. Convert the data to an acceptable format for transmission

Multiplexer (mux) – device that receives the input signals from several devices, compresses them into a single stream of data, and transmits it over a single communication line.  Four types:

1. Frequency Division Multiplexing (FDM) – dividing the bandwidth of the entire communication line into narrower frequency bandwidths so that each terminal on the line can use one section to transmit data.

2.  Time Division Multiplexing (TDM) – divides the transmission line into time segments.

3. Statistical Time Division Multiplexing (SDM) – time slots are assigned to devices on the network, if the  device doesn’t have anything to send, the next device takes over.

4. Wavelength Division Multiplexing (WDM) – use of fiber-optic cable; each signal is assigned a particular wavelength on an optical fiber communications line.
 

Multiplexer Configurations:

1. simple configuration
2.  inverse multiplexer – connect high-speed communication lines; An inverse multiplexer takes data from the high-speed lines, separate the data from those lines, and place in on lower-speed lines.
3. casading multiplexer – use multiplexers to connect communication lines to another multiplexer

Concentrator – combines data from several terminals onto a single communication line.  Used singly.  Some processing and data storage.  Concentrator is placed in a communication network between a group of terminals and the host computer.  Accepts data from the terminals, does some validation of the data, stores the data for a short time, and then sends the data to the host computer.

Front-End Processor – Used at the host computer.  Performs all the functions of a concentrator, as well as take over many of the network management tasks of the host computer so it can be dedicated to processing data.  FEP is a computer that handles polling, error checking and correction, and more P. 5-11.  Types :
1. nonprogrammable FEP – permanently programmed, or “hard wired” to handle certain communication functions.
2.  programmable FEP – has an operating system and a communications program that can be changed and reloaded to reflect changes in the network config.

Controllers – a scaled-down version of a FEP.  Can connect up to 32 terminals to a single communication line and transmit data to an FEP connected at the receiving end.  Types:
1. Clustor controller – designed specifically for IBM mainframes
2. Remote controller – located at a site some distance from the host computer; simultaneous data transmission in both directions is allowed between the FEP and the remote controller
3. Local controller – located near the host computer.  Connected directly to the FEP
 

Protocol – a rule that defines how two devices will talk to each other.  Two or more devices are using different protocols, then communication is established through the use of a protocol converter.  English – French
1. hardware – designed for data conversion between two specific types of devices
2. software – software package is installed on the host computer to allow most types of terminals to use the computer.
3. PC – adapter boards that perform protocol conversion, which makes the PC look like a particular type of terminal
4. gateway – LAN, used to connect dissimilar network by providing a conversion from one network protocol to another
 

Data/Network Security equipment
Encryption – changing data in such a way that it looks scrambled to someone without the proper decoding procedure or device

Call back units – block unauthorized users from the network
 

Diagnostic equipment
Line monitors – monitor the actual data being transmitted over the line; examine the flow of data to see where errors are occurring;

Breakout box – checks the voltage levels of a communication line to protect against errors caused by low voltage levels; also used to check for signals on a specific line or to isolate and test an individual circuit without affecting the rest of the network

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