Concentrators The main function of a concentrator is to serve as a termination  point  for  cable  running  from  individual nodes in a network. The cable connects to the network or to another wire center. A concentrator may have multiple boards or boxes mounted on a rack. Each board is essentially a hub, a wiring center for a single network’s nodes. Such boards generally include light-emitting diodes (LEDs) to indicate the status of each port on the board. Hubs A hub is a box with a number of connectors to which multiple  nodes  (PCs)  are  attached.  It  serves  as  a common termination point that can relay signals along the appropriate paths. All hubs provide connectivity, and some even provide management capabilities. A hub usually   connects   nodes   that   have   a   common architecture.   Although   the   boundary   between concentrators and hubs is not always clear, hubs are generally simpler and cheaper than concentrators. Modems In module 2, we introduced you to modems and how   they   are   used   in   a   data   communications environment.  They  translate  data  from  digital  to  analog form at the sending end of the communications path and from analog to digital at the receiving end. From a conceptual  standpoint,  this  explanation  is  sufficient. However, if you are going to install a modem, you need MODEMS AT WORK.— Put simply, the object of a modem is to change the characteristics of a simple sine wave, referred to as a carrier signal. We know this carrier signal has several properties that can be altered to represent data. It has amplitude (height); it has frequency (a unit of time); and it has phase (a relative starting point). Modems are capable of altering one or more of these characteristics to represent data. The job a modem performs can be divided into two discrete   parts   or   phases   at   each   end   of   the communications link. At the sending end, it converts digital  bit  streams  (strings  of  0’s  and  1’s)  into  analog  sine waves.   This   is   the   encoding   process.   Another component   within   the   modem   then   changes (modulates) the analog signal so the data may be transmitted  simultaneously  with  other  data  and  voice traffic that has also been modulated. This process is basically reversed at the receiving end. There, the analog  signal  is  brought  back  to  its  basic  level (demodulated),   and   the   analog   sine   waves   are reconverted  (decoded)  back  into  their  corresponding  bit streams (see figure 2-3). CODECs.— In today’s  digital  communications lines, voice traffic is considered the outsider that digital data used to be to analog lines. Voice can enter the data communications  lines  only  after  being  encoded  into digital form. It then must be decoded to be audible again at the receiving end. The device used to perform the encoding and decoding functions is known as a codec. This is simply another black box conversion device that has always been in existence in a slightly different form to know some of the technical aspects of modems. as part of a modem. Figure  2-3.—Digital  data  as  it  is  encoded,  modulated,  transmitted,  demodulated,  and  decoded. 2-3