Darren continues his series on the Public Switched Telephone Network with an explanation of Time Division Multiplexing, while showing off his artistic side on the white board.

Time Division Multiplexing Explanation

3 Comments

  • Dredd
    Reply

    I worked in Telecoms for 15 years, all over the world. I worked with a lot of G.703 systems – and boy did I hate T1.

    On T1 links, you can have AMI or B8ZS, which are different ways to handle repeating zeroes. Voice coding (mu-code) is set up in such as way so it never codes to 8 zeroes, so substitution on AMI or B8ZS never really kicks in until you start sending data instead of voice.

    I was in Taiwan one time, which uses T1. There were whole sequences of T1 links which *should* all have been set consistently to AMI or B8ZS, but nobody really knew if they were or not.

    Until one day we started connecting some of the DS0 channels to 64-kbit SS7 signalling channels, which can send strings of 8 or more 0’s in the data stream.

    Guess what happened. When an AMI link saw the B8ZS substitution (or maybe vice versa – this was years ago) the link decided there was an error and dropped out, indicating a fault. A whole T1 with 24 channels was killed by a single channel of data.

    The fix was to invert the SS7 signalling data stream. Originally it was designed never to repeat more than 6 1’s, and could have lots of 0’s, by inverting the data stream you were guaranteed never to have more than 6 0’s in a row, so substitution never kicked in.

    Never mind all the different verisons – Frame, Super Frame, Extended Super Frame, ack – give me E1 any time!

  • jwolf
    Reply

    Darren,

    I loved the talk on TDMA. Back in the day I did some RF engineering for a now defunct cell carrier with the initials VS. Then TDMA and CDMA ruled the airwaves and we were putting in one of the first GSM networks in the region. Although most people won’t be touching a MUX now days it’s a good overview for the younger kids who don’t understand some of the tech underlying the old (and even new) systems. Keep it up!

    WD

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