Paging system migration from one technology to another A system designer must show how either a new system or an old system will handle a “pure,” single format or mixed paging formats and mixed message types.
When paging systems migrate to a new technology, the system designer has to consider how to mix the new format with the current format. System specifications include system capacity (number of users), unused time and system loading. Variations include the use of all system transmitters at once, as in a simulcast system, or the selection of individual transmitters, as in a two-way paging system. A paging system could include only one type of paging message via one protocol; it could mix page types on single format; it could mix more than one format serving one type of page; or it could mix several types of pages to be carried on more than one format.
The paging formats (and transmission speeds in bits per second [bps]) include Golay sequential code (GSC) (300bps, 600bps), POCSAG (512bps, 1,200bps, 2,400bps), Flex (1,600bps, 3,200bps, 6,400bps) and European Radio Message System (ERMES) (6,250bps). These formats have different message types and lengths of tone-only, 10-digit numeric and 40-character alphanumeric. We assume that mixed paging formats are sharing one or several encoders within one paging terminal (paging switch) that feeds one RF transmitter. No assumptions are made regarding the features of traffic store-and-forward, priority schemes, users’ group calls or multiple messages calls. The designer has to know the specific variable operating characteristics (such as call user hour and number of offered users) for each format before making the calculations.
Paging formats * POCSAG — With respect to POCSAG, one preamble for every 30 batches (one event) is an assumed average. For 512bps, the preamble length is 1.125sec and batch length is 1.0625sec, including sync. One event of 30 batches takes 33 seconds (1.125 + 30 * 1.0625); i.e. in one hour, there are 109 events (3,600/33), which result in 52,320 (109 * [30 * 8 * 2]) code words/hour. For 1,200bps, preamble length is 0.48sec; batch length is 0.4533sec; one event is 14.08sec; 255 events result in 122,400 (255 * 480) code words/hour.
The “F. Sync” is a synchronization frame (takes one code word), and it precedes every batch. (See Figure 1 above left.) The code word of 32 bits may be used for address or message data. The difference between address and message code words is that there are 19 address bits in the address code word, starting with “0” (they start with “1” in the message code word), and bits 20 and 21 carry the source identifier (they are added to the 19 bits in message code words); 20 bits will be used for user message data. The last 11 bits on both code words (bit 22 to bit 32) are for even parity check. Messages must end on code word boundaries. The synchronization and the idle code words have a unique address code word.
Table 1 on page 72 summarizes the POCSAG paging code format. The warmup time usually is needed when two different formats are used to allow the encoder to combine two different formats or to switch from one to another. The warmup time precedes the first batch of the different format, and it has been ignored in calculating the batch length and number of events for a fully loaded POCSAG system. Table 2 on page 72 gives the required code words, and Table 3 on page 72 lists the number of pages per batch.
* GSC — In the standard GSC, the start code repeats for every batch (every 4.855 seconds). (See Figure 2 above.) In the extended GSC, the start code period repeats for as many as eight batches of start code and 16 messaging time slots, i.e., for as long as an 8 * 4.855 = 28.85-second interval. Each time slot serves a paging message. Messages end on time slot boundaries.
* Flex — As shown in Figure 3 on page 74, the first code word in block 0 (B0) is for system information only, i.e., 87 code words at 1,600 bps in each frame for user message data. The available speeds in Flex code are 1,600bps, 3,200bps and 6,400bps. The higher speeds of 3,200bps or 6,400bps are achieved by multiplexing two or four of the 1,600bps traffic subchannels (sometimes called phases) by interleaving 8, 16 or 32 blocks of 32 bits coded by Bose-Chadhuri-Hocquenghem (BCH) (32,21) coding interleaved at depth of eight. The available code words per frame are 87 at 1,600bps, 174 at 3,200bps and 348 at 6,400bps. The digital modulation used is 2-level frequency-shift keying (FSK) for 1,600bps and 3,200bps and 4-level FSK for 6,400bps.
The available frames/hour = 3,600/1.875 = 1,920 (independent of the speed).
A summary of the Flex coding structure appears in Table 4 on page 74. Table 5 on page 74 gives the required code words, and Table 6 on page 74 lists the number of pages per Flex frame.
The number of pages/Flex frame
= floor(frame length/message length) or = floor(frame code words/required code words/page)
The number of pages per frame is independent of the Call User Hour (CUH) of the page type, but it depends on the transmission speed.
When Flex format is mixed with any other paging format, a minimum of one frame should be transmitted every minute. The collapse value of 0,1,2,3,4 determines a number of 1, 2, 4, 8, or 16 active Flex frames per minute.
* ERMES — Figure 4 on page 75 describes the ERMES code format. There is only one signaling speed of 6,250bps in ERMES. The total number of batches/hour is = 60 * 5 * 16 = 4,800 batches of 750msec each. Table 7 on page 75 lists the paging types.
The Message length in seconds = 3,600/181,500 for tone = 3,600/103,72 for numeric = 3,600/36,300 for alphanumeric
System loading * User Hour — The User Hour for each paging type, format and transmission speed is the number of pages that could be delivered in one hour when CUH = 1. The User Hour for each paging type, format and transmission speed is different because of the difference in the total number of available code words per hour and the required number of code words for each page type under each format. For example, the total available code words per hour for POCSAG 512 format is 52,320 code words [(3,600/33) * 30 * 16). The required code words for tone, 10-digit numeric and 40-character alphanumeric pages are one, three and 15 code words respectively. In Flex format, the required code words are two, four and 17 respectively. The required number of code words for a page type will not change by transmission speed as the total available code words do.
The User Hour = (total available code words/hr)/required code words or = Total system capacity * CUH
The average time needed for one page message = 3,600/the total user hour. See Tables 8 and 9 on page 76.
* System capacity — Here are three system operating conditions under which system capacity differs: non-mixed system. mixed page types using the same format. mixed format types using the same page types.
As a rule of thumb, if the call rate of the user of a page type is x calls per hour “CUH,” then the system capacity (number of users served) = the User Hour of that page type/x. Another way to define the system capacity is to divide the total available code words by the product of the required code word for a page type and its corresponding call user hour (CUH).
Non-mixed system. Table 10 on page 77 reflects a single-format, single-page-type (tone-only at .15 CUH, 10-digit numeric at .25 CUH or 40-character alphanumeric at .3 CUH) and 100% system efficiency.
The system capacity numbers in the table match the published numbers claimed by each standard authority body.
Mixed page types system. Table 11 on page 77 reflects system capacity when different page types are mixed using one encoder of one format. The table shows only one example with certain ratios of each page type (10% tone-only at CUH 0.15, 70% 10-digit numeric at 0.25 CUH and 20% 40-character alphanumeric at 0.3 CUH)
The total system capacity in each format = floor(3,600/[(SIGMA) (message time in milliseconds * CUH * page type traffic ratio)/1,000)]).
The page type user = page type percentage * system capacity.
These same formulas are used for any other mixing ratios.
The total system capacity numbers for POCSAG format in the mixed-page-types system capacity table (Table 6) match published numbers for that particular test case of mixed traffic ratios as released by Motorola in April 1994. The same formulas are used for the other formats as well.
Mixed format systems. This information is intended to help you to visualize how the system will migrate from one format, such as POCSAG, to Flex or ERMES. Mixing different formats will be based on the frame length (not the message length), and the minimum number of inserted frames is one per minute. Normally, at the beginning of migration, the old format is predominately loading the system with a higher percentage of traffic. Certain assumptions must be made by the designer for the Flex collapse value parameter before analysis.
The designer has to allow about 100msec of unused time for the nominal encoder delay to switch from one format to another or to combine two formats in a transmission frame in the paging terminal. Because the insertion of Flex code format will be based on frame lengths (of 1,875 msec) within the POCSAG event lengths (no insertion within the batches) or within the ERMES sequences lengths, the encoder delay will fall within the already unused time (as a warmup time). As long as the encoder places pages in the expected frames, the order of the Flex frames does not matter.
Mixed format using same page types. By way of example, consider a system using POCSAG and Flex with the following properties: POCSAG 512 Warmup period time (programmable) = 100msec to 400msec when needed. Number of POCSAG users = 100,000. Number of Flex users = 10,000 (10% of the POCSAG users). Flex collapse value = 1 (for a period of two frames per minute, the Flex pager will look for a page). CUH = 0.15 for tone-only. CUH = 0.25 for 10-digit numeric. CUH = 0.30 for 40-character alphanumeric.
Assuming only one page type is used at a scenario, find the system utilization factor, the lost time percentage and the system capacity for each scenario for each page type. (See Tables 12, 13 and 14 on page 78.)
The formulas for mixed page formats (same page types) used in deriving values in the tables are as follows: Number of batches or frames/min. is determined from the collapse value look-up-table (Table 12). Number of POCSAG preambles/min. = ceiling(number of maximum batches per minute/30), the number of batches/frame preceded with one preamble. POCSAG pages time/min. = number of batches per minute * batch time (1.0625 at 512bps) + number preambles * preamble time (1.125). Flex pages time/min. = number of frames per min. * frame time (1.875sec). ERMES pages time/min. = number of batches per min. * 0.75sec. Unused time percentage = 100 * [(60 – [POCSAG pages time + Flex pages time])/60]. Warm-up time/minute = 100 msec if the unused time is less than 100 msec. Number of traffic batches or frames = (number of users * CUH)/(number of pages per batch or frame). Air time for Flex traffic in minutes = (number of traffic frames/minimum Frames per minute) according to the collapse value table. Maximum number of POCSAG mixed batches = ceiling of Flex airtime minutes * maximum POCSAG batches per minute (derived from collapse value look-up table). Overflow POCSAG time in minutes = [(traffic batches – maximum batches)/30] * (33/60), assuming 30 batches per event (of 33sec each event). Total airtime in minutes = ceiling(Flex traffic needed time + overflow POCSAG time + warmup time.) System utilization = (total air time in minutes/60) * 100. System capacity = (100 * number of offered users per system utilization percentage).
The same principles apply for mixing ERMES and POCSAG formats.
The technology does not migrate in a short time. The system designer has to help to present a business case that shows that the current system serving the current formats is not going to be thrown away or replaced on a cutover date and time. He also has to show how many users will be served by both the new format and the old format. Any format used might serve any page type or mixed page types.