-1- 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 or 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 = Send Time The following sequence illustrates the data flow ( / is used to denote second boundaries) : / t /HHMMSS /HHMMSS /HHMMSS / 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 ends at the start of the next second. 3.2 Date Information will be transmitted in response to the received characters: d = Send Date and is sent in the following format: YYMMDD -2- 3.3 Controller Status information will be transmitted in response to the received characters s = 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 = Loopback hu = 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. -3- 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. -4- 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: 0345 010 313 (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. -5- 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