2 Replies Latest reply: Mar 28, 2019 11:52 PM by Dennis RSS

    Chips, Symbols and Bits

    Dennis

      My understanding is that 1 Data Bit, when encoded using Barker 11,  is “spread” (encoded) into 11 digits, individually called Chips. And this group of 11 Chips is collectively referred to as a Symbol. But reading through various publications, I’m left confused and not so sure.

      For example from Cisco’s WIFUND – Implementing Cisco Wireless Network Fundamentals – Student Guide it states…
      The Barker code allows you to send a chip of 11 digits (or symbols) instead of the value itself…

      While in the book CCNA Wireless 640-722 Official Cert Guide it states…
      Each of the new coded bits is called a chip. The complete group of chips representing a data bit is called a symbol.”

      These two statements seem to contradict each other or am I missing something obvious here? The first one states that a Chip contains 11 digits, while the latter seems to state that the individual digits are called Chips and together are called a Symbol.

      However, it seems a Symbol can also be associated with Modulation, in particular Phase Modulation i.e. changes in the phase of the carrier wave can represent more than one “bit” of information. But does that bit represent a Data Bit or a Chip? As this is another point of confusion for me.

      From CCNA Wireless 640-722 Official Cert Guide
      It states in relating to DQPSK “The main difference is that the data bits are taken pairwise (after scrambling), and encoded so that two-bit pairs map to 11-chip symbols

      From the book Scalable VoIP Mobility
      It states “This time, chips are taken two at a time and modulated onto the carrier signal by using DQPSK

       

      So, one book states, in relation to DQPSK modulation that 2 Data Bits are taken and the other that 2 Chips are taken. How can this be?

       

      I know these questions may seem somewhat irrelevant, but it seems understanding Chips and Symbols are fundamental to understanding wireless / RF communications.

       

      Any comments would be welcome.

       

      Dennis

        • 1. Re: Chips, Symbols and Bits
          phil morgan

          Ahh welcome to the world of wireless.

           

          I am often confused by the explanations people try to use.

           

          Honestly I would ignore everything to do with DSSS in the real world, and just remember that one bit is spread into 11. This actually changes as you go from 1-2-5.5-11, but you get the idea. The End. Review it for the exam, then forget it.

           

          A symbol is the fundamental building block of OFDM, and takes 4us. (3.6 if you have SGI turned on). It has a quiet period (Guard Interval) then bursts data on all the subcarriers for a short time (symbol time - Guard Interval).

           

          That's it, now you know what a symbol really is. Ignore all the stuff where people try to explain DSSS and CCK spreading as a symbol.

           

          My opinion is to turn off "b" anyway, so there is no DSSS. (Now that should start some comments coming in).

          • 2. Re: Chips, Symbols and Bits
            Dennis

            Thanks for the reply, Phil.

             

            As you said, they’re not going to go in-depth in to DSSS in the exam – but it’s more that I’d like to know why Symbols and Chips are mentioned interchangeably, and inconsistently between publications. 

            I’ve looked through the following books to find a definitely or unifying explanation, but unfortunately they seems to, at some stage or another, contradict each other.

            CCNA Wireless 640-722 Official Cert Guide

            CCNA Wireless Official Exam Certification

            Scalable VoIP Mobility

            CCNA Wireless 200-355

            RF Engineering for Wireless Networks

            802.11 Wireless Networks: The Definitively Guide, 2nd Edition