High Voltage Shareware/high1.zip/high1/DIR31/BBCTIME.ZIP/TIMEDATA.TXT

Text File | 1994-02-09 | 12KB | 301 lines

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TIMEDATA.TXT  Information about the BBC Dial-in Time Service     (9/02/1994)

1. Introduction

     This file gives details of the coded time information available
     from the BBC Radio Time Standard over the Public Switched Telephone
     Network (PSTN). Time Information is available in two formats on
     different British Telecom Callstream numbers:

          Coordinated Universal Time - UTC (notionally the same as Greenwich
          Mean Time) on 0891 516880 and,

          UK  Time-of-Day  -  TOD (adjusted  automatically  between  British
          Summer Time and GMT)  on 0891 516888


2. Host Equipment

     The BBC Radio Time Standard is based on a pair of GPS (Global
     Positioning System) Receivers working in a dual-redundant configuration
     together with an MSF Receiver. (MSF is a 60KHz Time Standard operated
     by the National Physical Laboratory.) Time data accurate to within 500
     uS is passed from the Time Standard to a pair of Leitch Clock System
     Controllers connected to PSTN modems for access by Callers.  One
     Controller operates on UTC with the other on TOD.


3. Format and Syntax of Data

     In the following examples <CR> or <cr> denotes the Carriage Return
     character. Data received from the Caller is shown in lower case (in
     practice either lower or upper case may be used) and data transmitted
     by the Controller in CAPITALS.

          3.1 Time Information will be sent for the three consecutive
          seconds following the receipt from a Caller of the ASCII
          characters:

               t <CR>                 =       Send Time

          The  following  sequence illustrates the data flow ( / is used to
          denote second boundaries) :

               / t<cr>    <CR>/HHMMSS    <CR>/HHMMSS    <CR>/HHMMSS    <CR>/

          Each  HHMMSS  field  refers to the time at the  start  of  the
          next second. The data transmitted by the Controller is timed so
          that the stop bit of each <CR> ends at the start of the next
          second.

          3.2 Date Information will be transmitted in response to the
          received characters:

               d <cr>                 =       Send Date

          and is sent in the following format:

               YYMMDD <CR>
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          3.3 Controller Status information will be transmitted in response
          to the received characters

               s <cr>                 =       Send Status

          and will consist of one of the following single characters:

               G                      =       System Good
               D                      =       Failure of Controller Internal
                                              Diagnostics
               T                      =       Controller  does  not   have
                                              correct Time

          3.4 Two further commands are available to callers:

               l  <cr>                =       Loopback
               hu <cr>                =       Hang Up

               The Loopback command will cause a Controller to echo the
               next  character received back to the  Caller.  This may be
               used by a Caller's equipment to calculate the round trip
               delay across the PSTN connection in order to apply a
               correction to the received time data.

               The Hang Up command will cause the Controller to drop the
               line immediately and terminate the call.


4. Accuracy of Time Information

     Leap Second corrections are made automatically by the Controllers at
     the appropriate times. The accuracy of Time Information transmitted by
     the Controllers will be maintained to within +/- 3mS (the duration of
     1 bit at a data rate of 300 Baud). The BBC cannot, however, guarantee
     the accuracy of Time Information received by callers for the  following
     reasons:

          4.1  The  delay in the PSTN connection may vary from call to
          call. However for calls originated in the UK, British Telecom
          claims  that, in general, the delays on the go and return paths of
          a single  connection will be equal and will not exceed  13mS in
          either direction.  This figure should also apply to calls routed
          via Mercury Communications circuits.

          4.2  The  delay  introduced by the modems at each end  of  the
          PSTN connection will vary from one type of modem to another.
          Although the delay in the modems at Broadcasting House is known
          (10 - 15 mS), the delay in Callers' modems cannot, in general, be
          quantified. Simple modems are likely to have a comparable delay to
          those used at Broadcasting House, whereas sophisticated modems
          capable of data compression and error correction may introduce
          delays of up to 40mS.


     Despite the above limitations, it is expected that many computer users
     will find the Service-as-received adequate for setting a computer clock
     to an accuracy of significantly better than one second.

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          4.3 Fortunately  there  is  a  means whereby  more  demanding
          applications can mitigate the effects of the above delays. By
          making the Calling equipment instigate a Loopback Test to measure
          the round trip delay on the connection, the delays in both the
          PSTN and the modems can be assessed. On the assumption that this
          overall delay is the sum of equal delays on both go and return
          paths, halving the round-trip delay will yield an offset which can
          be used to improve the accuracy of the received time information.


5. Communications Details

     Callers will require a PSTN modem which can operate at 300 Baud Full
     Duplex (CCITT Recommendation V21). The data format is:

          1 Start Bit,  8  Data  Bits,  No Parity, 1 Stop Bit.

     Error Correction and Data Compression should be disabled.

     The Controllers operate a data inactivity timeout. If no data has been
     received from a Caller over a period of 15 seconds, the current time is
     transmitted and the call is then terminated. If the Caller's equipment
     uses software to dial, make the connection and synchronise its
     real-time clock to within one second, the duration of the PSTN call
     should typically be less than 25 seconds (of which upto 10 seconds may
     be needed for the modems to go On-line after answering).  A slightly
     longer call involving a Loopback test (and possibly an iterative
     routine) will be required if Callers wish to achieve a higher degree of
     accuracy.

     Callers will be charged in accordance with the prevailing tariff for
     the British Telecom Callstream Service. Callstream is a Premium Rate
     service and, at the date of issue, calls are charged at the national
     rate of 48 pence per minute (peak/standard rate) and 36 pence per
     minute (cheap rate) inclusive of VAT. Calls of less than 60 seconds
     duration will be charged proportionately. It is understood that, as
     from March 31 1994, these rates will increase to 49 and 39 pence per
     minute respectively.


6. Applications

     The main application for the Service is likely to be the
     synchronisation of real-time clocks on computers and other electronic
     equipment. In this context it should be noted that:

          6.1 Although typical figures are given in 4 above for PSTN and
          modem delays, the accuracy to  which  the  Caller's real-time
          clock can be synchronised  will depend upon the processor type,
          speed and the software technique used.

          6.2 IBM PCs, XTs, ATs and compatibles cannot normally resolve
          time to better than 55 mS and their inbuilt real-time clocks are
          likely to drift by this amount over a few hours.




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7. Further Information

     Any queries arising from the operation of the Service should be
     directed to:

               BBC Engineering Information
               White City
               201 Wood Lane
               London W12 7TS

               Telephone: [phone]  (charged at local call rate)
                      or: 081 752 5040

     Engineering  Information  will also hold details of software  available
     for automatically synchronising IBM compatible PCs.


8. The Legal Stuff follows:

     General Conditions attaching to the BBC Dial-in Time Service


8.1. Nature of Service

     The Service comprises the provision of coded time information over the
     Public Switched Telephone Network (PSTN) in the format detailed above.
     It should be noted that the Service is a Pilot Scheme only and that the
     BBC reserves the right at its absolute discretion to discontinue the
     Service at any time after the first year of operation following the
     commencement date on the first of December 1993 if the operation of the
     Service becomes incompatible with the BBC's Public Service
     Broadcasting role or its policies.


8.2. Quality of Service

     The BBC will use reasonable endeavours to ensure that time
     information supplied by its equipment will be maintained at all times
     to the degree of accuracy detailed in 4 above. However the Service is
     not fault-free, and the BBC accepts no responsibility for any loss or
     damage incurred through a failure to maintain the accuracy of
     information supplied.

     It should be noted that the accuracy of time information delivered to
     Callers is dependent on delays in the PSTN and in Callers' modem
     equipment which are beyond the control of the BBC.


8.3. Availability of Service

     Subject to the Conditions applicable to the BT Callstream facility the
     Service will be available to Callers 24 hours a day, 365 days a year.
     However the BBC reserves the right to suspend the Service for periods
     not exceeding 24 hours for the purposes of maintenance or repair.




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8.4. Force Majeure

     The Service may be suspended in the event of fire, flood, civil
     commotion, act of God or dictate of Government or other Civil Body.


8.5. Charge for Service

     The Service is provided at the cost of the telephone call which  will
     be charged to the Caller in accordance with the prevailing tariff for
     the British Telecom Callstream Service. It should be noted that
     Callstream is a Premium Rate service, and that, at the date of issue,
     calls are charged at the national rate of 48 pence per minute
     (peak/standard rate) and 36 pence per minute (cheap rate) inclusive of
     VAT. These rates are due to increase to 49 and 39 pence per minute
     respectively from March 31 1994.


8.6. Notice of Termination of Service

     Notice of termination of the Service will be published in the
     Noticeboard feature in Radio Times (or elsewhere in Radio Times should
     the Noticeboard feature be discontinued) not less than 28 days prior to
     such termination.


8.7. Disclaimers

          8.7.1 The BBC accepts no liability with regard to the use of
          software for the purposes of accessing and utilising data provided
          under the Service.

          8.7.2 The BBC accepts no responsibility for any loss or damage
          howsoever caused arising from the provision of or failure to
          provide time information under the Service.






















timedata 09/02/94