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TD-2 Protection Switching

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  • Jim Hebbeln
    The May 1955 Bell System Technical Journal (BSTJ) contains an article about automatically switching in the TD-2 microwave radio broadband protection channel
    Message 1 of 11 , Jul 22, 2004
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      The May 1955 Bell System Technical Journal (BSTJ) contains an article
      about automatically switching in the TD-2 microwave radio broadband
      protection channel when one of the five other working broadband
      channels failed - typically during a deep fade due to multipath
      reception (a 180-degree-out-of-phase mirage, if you will). This
      fading occurs mostly in the summer (July-August) during the wee
      morning hours around 2-5 AM when the atmosphere becomes more humid,
      quiet, and reflective. (I can scan and provide this manuscript, if
      someone can host it.)

      Quick aside: Is this why network television signed off at midnight
      until the next morning - because there were too many deep fades to
      provide a quality program signal? Although I suppose that time of
      night is when the radio techs performed routine maintenance and
      triode replacement.

      The BSTJ stated that either Space Diversity or Frequency Diversity
      could have been used to provide the protection path, but the Labs
      chose Frequency Diversity as more equipment was protected.
      Typically, 10-15 hops (200-300 miles) were contained in the Switching
      Section that was protected. The switching equipment was expensive,
      so they spread the protection over many hops.

      However, in recent years, most towers also had a third Space
      Diversity antenna installed. When did AT&T start adding the third
      anntenna, and did that make each hop capable of protection switching
      its own single hop? Was this costly? Were there other not-so-
      obvious benefits?
    • blitz
      ... Yes, that s a time it happens, and typically Spring in mountainous terrain as well, as valleys don t warm up as fast as hilltops, providing a pool of cold
      Message 2 of 11 , Jul 22, 2004
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        At 11:41 7/22/2004, you wrote:
        >The May 1955 Bell System Technical Journal (BSTJ) contains an article
        >about automatically switching in the TD-2 microwave radio broadband
        >protection channel when one of the five other working broadband
        >channels failed - typically during a deep fade due to multipath
        >reception (a 180-degree-out-of-phase mirage, if you will). This
        >fading occurs mostly in the summer (July-August) during the wee
        >morning hours around 2-5 AM when the atmosphere becomes more humid,
        >quiet, and reflective. (I can scan and provide this manuscript, if
        >someone can host it.)

        Yes, that's a time it happens, and typically Spring in mountainous terrain
        as well, as valleys don't warm up as fast as hilltops, providing a pool of
        cold air that acts as a reflector, providing that 180 deg shift.
        These don't last long, as soon as the sun pokes over the hilltops and
        begins illuminating the valleys, they dissipate quickly.


        >Quick aside: Is this why network television signed off at midnight
        >until the next morning - because there were too many deep fades to
        >provide a quality program signal? Although I suppose that time of
        >night is when the radio techs performed routine maintenance and
        >triode replacement.

        Think it had more to do with profitability. Why spend all that electricity
        $ on BIG tube space heaters when no one was watching, and more importantly
        no one was advertising. That all changed as lifestyles became more 24 hour
        oriented.


        >The BSTJ stated that either Space Diversity or Frequency Diversity
        >could have been used to provide the protection path, but the Labs
        >chose Frequency Diversity as more equipment was protected.
        >Typically, 10-15 hops (200-300 miles) were contained in the Switching
        >Section that was protected. The switching equipment was expensive,
        >so they spread the protection over many hops.

        Space diversity (SD) protects one hop. Each hop to it's neighbor needs the
        additional antenna, vs the frequency diversity (FD) which usually was on
        the same antennas. Theres pros and cons to both methods, and the costs of
        switching equipment came down in the later years.
        SD protects pretty well against multipath probs, FD better against
        equipment problems. (IMHO)
        Lets say a card feeding transmitter 1 starts giving fits and causes
        impairments, (not generating an alarm) space usually wont help as its at
        the common transmitter/rcvr rack, where switching entirely to another
        complete path should bypass the faulty card if the system is designed right.


        >However, in recent years, most towers also had a third Space
        >Diversity antenna installed. When did AT&T start adding the third
        >anntenna, and did that make each hop capable of protection switching
        >its own single hop? Was this costly? Were there other not-so-
        >obvious benefits?

        The costs of antennas came down as more dishes went into service, and
        adding a per-hop SD was a big help in areas prone to weather induced
        multipath. All our stuff (sprint) went up originally with SD and FD was
        added later. I believe our rule was a 1 protection channel for 7 traffic
        channels.
      • Paul J Zawada
        ... From reading several Lenkurt items (articles from _The_Demodulator_ and such) I ve come to believe that SD was not really viewed as a reliable protection
        Message 3 of 11 , Jul 22, 2004
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          On Jul 22, 2004, at 12:40 PM, blitz wrote:

          > At 11:41 7/22/2004, jimhebbel@... wrote
          >
          >> The BSTJ stated that either Space Diversity or Frequency Diversity
          >> could have been used to provide the protection path, but the Labs
          >> chose Frequency Diversity as more equipment was protected.
          >> Typically, 10-15 hops (200-300 miles) were contained in the Switching
          >> Section that was protected. The switching equipment was expensive,
          >> so they spread the protection over many hops.
          >
          > Space diversity (SD) protects one hop. Each hop to it's neighbor needs
          > the
          > additional antenna, vs the frequency diversity (FD) which usually was
          > on
          > the same antennas. Theres pros and cons to both methods, and the costs
          > of
          > switching equipment came down in the later years.
          > SD protects pretty well against multipath probs, FD better against
          > equipment problems. (IMHO)
          > Lets say a card feeding transmitter 1 starts giving fits and causes
          > impairments, (not generating an alarm) space usually wont help as its
          > at
          > the common transmitter/rcvr rack, where switching entirely to another
          > complete path should bypass the faulty card if the system is designed
          > right.

          From reading several Lenkurt items (articles from _The_Demodulator_ and
          such) I've come to believe that SD was not really viewed as a reliable
          protection scheme by common carriers until industrial microwave users
          had proven so. The industrial microwave frequency pools were much
          smaller and did not allow for FD deployments. Industrial microwave
          users had SD as their only diversity option and functioned as the
          proving ground for that technology.

          >
          >> However, in recent years, most towers also had a third Space
          >> Diversity antenna installed. When did AT&T start adding the third
          >> anntenna, and did that make each hop capable of protection switching
          >> its own single hop? Was this costly? Were there other not-so-
          >> obvious benefits?
          >
          > The costs of antennas came down as more dishes went into service, and
          > adding a per-hop SD was a big help in areas prone to weather induced
          > multipath. All our stuff (sprint) went up originally with SD and FD was
          > added later. I believe our rule was a 1 protection channel for 7
          > traffic
          > channels.
          >

          There was also pressure from the FCC to be more spectrally efficient.
          As specialized common carriers stared to deploy their own networks in
          the early '70s, AT&T and WU could no longer justify keeping channels
          idle all over the place when folks like MCI wanted in on the business.

          --zawada
        • ozob99
          ... TV network feeds signed off(known as goodnight ) at various times from 11pm to 1am because that s what the networks contracted for;up until the 1970 s
          Message 4 of 11 , Jul 22, 2004
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            >
            > Quick aside: Is this why network television signed off at midnight
            > until the next morning - because there were too many deep fades to
            > provide a quality program signal? Although I suppose that time of
            > night is when the radio techs performed routine maintenance and
            > triode replacement.


            TV network feeds signed off(known as "goodnight") at various times
            from 11pm to 1am because that's what the networks contracted for;up
            until the 1970's there was little demand for (and availability of)
            network type programming from local stations after midnite as most of
            them went off the air;and those that stayed on found infomercials &
            old movies more profitable than network or syndicated shows;
            occasionally there was "overtime" ordered into the wee hours for
            special programming.

            This did provide lots of time for routine maintenance.
            >
          • Charles Parkhurst
            This subject has brought back a lot of memories as a tech in Champaign,Il, in the late 1970 s. I got a callout around 1 A.M for a TD-2 alarm in the office.
            Message 5 of 11 , Jul 22, 2004
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              This subject has brought back a lot of memories as a tech in
              Champaign,Il, in the late 1970's.
              I got a callout around 1 A.M for a TD-2 alarm in the office. Upon
              investigation I found that up to 2 working channels plus the protect
              in a deep in and out fading condition. I notified the supervisor that
              there was nothing I could do about this situation and proceededed to
              close up and go home. It wasn't an hour later the same thing happened
              and she told me to go in anyway to verify that is was the same
              problem, it was.
              In the same general location, were the X-bar tandem switching eqpt.
              When the channels faded the switches started ratteling and it sounded
              like a bee hives plus other various alarms in their bays.
              Except for the late hours, I had a pretty nice paycheck that week.


              In coldwarcomms@yahoogroups.com, "ozob99" <ozob99@y...> wrote:
              >
              > >
              > > Quick aside: Is this why network television signed off at
              midnight
              > > until the next morning - because there were too many deep fades
              to
              > > provide a quality program signal? Although I suppose that time
              of
              > > night is when the radio techs performed routine maintenance and
              > > triode replacement.
              >
              >
              > TV network feeds signed off(known as "goodnight") at various times
              > from 11pm to 1am because that's what the networks contracted for;up
              > until the 1970's there was little demand for (and availability of)
              > network type programming from local stations after midnite as most
              of
              > them went off the air;and those that stayed on found infomercials &
              > old movies more profitable than network or syndicated shows;
              > occasionally there was "overtime" ordered into the wee hours for
              > special programming.
              >
              > This did provide lots of time for routine maintenance.
              > >
            • s92187
              ... recent years, most towers also had a third Space ... switching ... Here in the midwest only a few hops had space diversity for the earlier TD-2 routes.
              Message 6 of 11 , Jul 23, 2004
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                --- In coldwarcomms@yahoogroups.com, "Jim Hebbeln" <jimhebbel@y...>
                wrote:
                > The May 1955 Bell System Technical Journal (BSTJ) ... > However, in
                recent years, most towers also had a third Space
                > Diversity antenna installed. When did AT&T start adding the third
                > anntenna, and did that make each hop capable of protection
                switching
                > its own single hop? Was this costly? Were there other not-so-
                > obvious benefits?


                Here in the midwest only a few hops had space diversity for the
                earlier TD-2 routes. When AT&T started adding AR6A single sideband
                radios to existing routes around 1980, they added space diversity to
                most of the repeater sites on those routes. AR6A was sensitive to
                selective fading, and used one protection channel for seven working
                channels, so space diversity was used on a per hop basis in many
                places. DR-6 digital microwave radios, installed in the mid '80s,
                also had less tolerance for fading, so space diversity was added to
                most of the hops on those routes too.

                Most of the added antennas for space diversity were either an
                Andrew / Gabriel conical horn antenna, or shrouded parabolic dish
                antenna.

                Terry Michaels
                Tower Sites, Inc.
              • Jim Hebbeln
                To all, Thanks for the responses. Everyone had something of interest. While I tend to view these towers from, perhaps, a more romantic viewpoint - such as
                Message 7 of 11 , Jul 26, 2004
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                  To all,

                  Thanks for the responses. Everyone had something of interest. While I tend to view these towers from, perhaps, a more romantic viewpoint - such as we're the same age, or the geographic parallel to the Transcontinental Railroad over which the original microwave sentinals zigzagged their signals across the midwest/west - I also find the technology interesting.

                  I climbed a short 40-50' tower once on the north slope of Mt. Evans; it scared the hell out of me even though I was just barely above the trees. Terry Michael's pictures like those from Collins, IA, nearly make me sick from the heights. (Terry, you didn't climb the short mast did you?! Tell me you had a camera on the end of a pole when you took the picture looking down on the horns.)

                  My real interest over the years, though, has been switching the calls that went over these routes. (I worked #1, 1A, 2, 3, & 5 ESSs, and DMS-10 & DMS-100/SL-100 - where the ladders are only 11' high.)

                  The towers, though, tied it all together.

                  Watching the sun set on Buckhorn Mountain,
                  Jim Hebbeln
                  Fort Collins

                  s92187 <tmichaels@...> wrote:
                  --- In coldwarcomms@yahoogroups.com, "Jim Hebbeln" <jimhebbel@y...>
                  wrote:
                  > The May 1955 Bell System Technical Journal (BSTJ) ... > However, in
                  recent years, most towers also had a third Space
                  > Diversity antenna installed. When did AT&T start adding the third
                  > anntenna, and did that make each hop capable of protection
                  switching
                  > its own single hop? Was this costly? Were there other not-so-
                  > obvious benefits?


                  Here in the midwest only a few hops had space diversity for the
                  earlier TD-2 routes. When AT&T started adding AR6A single sideband
                  radios to existing routes around 1980, they added space diversity to
                  most of the repeater sites on those routes. AR6A was sensitive to
                  selective fading, and used one protection channel for seven working
                  channels, so space diversity was used on a per hop basis in many
                  places. DR-6 digital microwave radios, installed in the mid '80s,
                  also had less tolerance for fading, so space diversity was added to
                  most of the hops on those routes too.

                  Most of the added antennas for space diversity were either an
                  Andrew / Gabriel conical horn antenna, or shrouded parabolic dish
                  antenna.

                  Terry Michaels
                  Tower Sites, Inc.


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                • s92187
                  ... those from Collins, IA, nearly make me sick from the heights. (Terry, you didn t climb the short mast did you?! Tell me you had a camera on the end of a
                  Message 8 of 11 , Jul 28, 2004
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                    --- In coldwarcomms@yahoogroups.com, Jim Hebbeln <jimhebbel@y...>
                    wrote:
                    > To all,
                    >
                    > Thanks for the responses. <snip> Terry Michael's pictures like
                    those from Collins, IA, nearly make me sick from the heights.
                    (Terry, you didn't climb the short mast did you?! Tell me you had a
                    camera on the end of a pole when you took the picture looking down on
                    the horns.) <snip>
                    >> Jim Hebbeln
                    > Fort Collins


                    Hi Jim:

                    I climbed up and took the photo. I own 9 former AT&T sites, I've made
                    it a point of climbing to the highest accessible spot at each one.
                    They all have an approximately 13 foot tall mast on the top platform
                    that typically supports a beacon or strobe and a lightning rod, that
                    is usually the highest part of the structure. It's actually easy to
                    climb, and there are three horizontal metal rings at the top of the
                    mast that encircle you for fall protection.

                    Terry
                  • Jim Hebbeln
                    ... a ... on ... made ... platform ... that ... to ... Terry, I would much rather hold barehanded onto a buss bar at 48 volts with 2000 amps flowing through it
                    Message 9 of 11 , Jul 29, 2004
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                      --- In coldwarcomms@yahoogroups.com, "s92187" <tmichaels@t...> wrote:
                      > --- In coldwarcomms@yahoogroups.com, Jim Hebbeln <jimhebbel@y...>
                      > wrote:
                      > > To all,
                      > >
                      > > Thanks for the responses. <snip> Terry Michael's pictures like
                      > those from Collins, IA, nearly make me sick from the heights.
                      > (Terry, you didn't climb the short mast did you?! Tell me you had
                      a
                      > camera on the end of a pole when you took the picture looking down
                      on
                      > the horns.) <snip>
                      > >> Jim Hebbeln
                      > > Fort Collins
                      >
                      >
                      > Hi Jim:
                      >
                      > I climbed up and took the photo. I own 9 former AT&T sites, I've
                      made
                      > it a point of climbing to the highest accessible spot at each one.
                      > They all have an approximately 13 foot tall mast on the top
                      platform
                      > that typically supports a beacon or strobe and a lightning rod,
                      that
                      > is usually the highest part of the structure. It's actually easy
                      to
                      > climb, and there are three horizontal metal rings at the top of the
                      > mast that encircle you for fall protection.
                      >
                      > Terry

                      Terry,

                      I would much rather hold barehanded onto a buss bar at 48 volts with
                      2000 amps flowing through it (nothing happens at that voltage, at
                      least with dry hands) than ever consider your feat.

                      But I do appreciate your comments about the three rings. I always
                      thought they were some type of mobile radio antenna for communication
                      to the site mtce trucks out on the road. (I better go back to
                      switching, eh? [I run Canadian Nortel C.O. equipment.])

                      Now pardon me, I have to go puke...

                      Jim Hebbeln
                    • Matthew S. Smith
                      ... Terry (or anyone): do any of your sites have guy wires to the mast? Mine shows evidence of having had them in the past (several pieces of rusty guy wires
                      Message 10 of 11 , Jul 29, 2004
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                        s92187 wrote:

                        > I climbed up and took the photo. I own 9 former AT&T sites, I've made
                        > it a point of climbing to the highest accessible spot at each one.
                        > They all have an approximately 13 foot tall mast on the top platform
                        > that typically supports a beacon or strobe and a lightning rod, that
                        > is usually the highest part of the structure. It's actually easy to
                        > climb, and there are three horizontal metal rings at the top of the
                        > mast that encircle you for fall protection.

                        Terry (or anyone): do any of your sites have guy wires to the mast? Mine
                        shows evidence of having had them in the past (several pieces of rusty
                        guy wires with turnbuckles scattered around the top platform and a few
                        places where they were attached to the top platform grating, and the
                        mast itself), but none still attached at both ends. I've since removed
                        the scraps, as they were discoloring the grating, as well as a
                        nuisance/hazard to step over/around.

                        I've climbed the mast on mine, but must admit that I didn't really enjoy
                        the experience. You have to climb about as high as you can get to see
                        over the horns (cool view, but made me a bit queasy), and the mast feels
                        wobbly when you do that. Seems securely bolted down, but feels like it's
                        twisting -- at least more than you'd expect for a chunk of steel i-beam
                        that big. Being on the top platform doesn't bother me any more, but at
                        the top of the mast does. Maybe it's just the thought that if it were to
                        tip over, I wouldn't hit the platform, but continue on to the ground
                        (125' below the platform).

                        Matt
                      • s92187
                        ... Mine ... rusty ... few ... the ... removed ... enjoy ... see ... feels ... it s ... beam ... at ... were to ... ground ... Hi Matt: I have not seen
                        Message 11 of 11 , Jul 29, 2004
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                          --- In coldwarcomms@yahoogroups.com, "Matthew S. Smith" <matt@j...>
                          wrote:
                          > s92187 wrote:
                          >
                          > > I climbed up <snip>

                          > Terry (or anyone): do any of your sites have guy wires to the mast?
                          Mine
                          > shows evidence of having had them in the past (several pieces of
                          rusty
                          > guy wires with turnbuckles scattered around the top platform and a
                          few
                          > places where they were attached to the top platform grating, and
                          the
                          > mast itself), but none still attached at both ends. I've since
                          removed
                          > the scraps, as they were discoloring the grating, as well as a
                          > nuisance/hazard to step over/around.
                          >
                          > I've climbed the mast on mine, but must admit that I didn't really
                          enjoy
                          > the experience. You have to climb about as high as you can get to
                          see
                          > over the horns (cool view, but made me a bit queasy), and the mast
                          feels
                          > wobbly when you do that. Seems securely bolted down, but feels like
                          it's
                          > twisting -- at least more than you'd expect for a chunk of steel i-
                          beam
                          > that big. Being on the top platform doesn't bother me any more, but
                          at
                          > the top of the mast does. Maybe it's just the thought that if it
                          were to
                          > tip over, I wouldn't hit the platform, but continue on to the
                          ground
                          > (125' below the platform).
                          >
                          > Matt


                          Hi Matt:

                          I have not seen evidence of guy wires supporting the beacon mast at
                          any of our sites, however at least one site has two steel angles
                          running from the top of the mast down at about a 45 degree angle to
                          the platform to brace it, the two angles are placed at right angles
                          to each other. This is a mast that has a DB-420 antenna mounted at
                          the top.

                          Terry
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