ERMES is a Europe-wide high speed paging system with a user data rate of 6250 bps, in comparison to POCSAG, which has a maximum rate of 2400 bps. ERMES radio data may be transmitted using frequency or time multiplexing, or both. All transfer modes do however utilize the same modulation format on the same frequency. ERMES is used in several European countries, whereas Asian countries tend use FLEX as standard, which is a technically comparable Motorola system.

Radio Link

ERMES employs a radio link transfer protocol conforming to the ETSI pr ETS 300 133-4 standard (ETS 300 133-1 to ETS 300 133-7). Transmissions are within the range from 169.4125 MHz to 169.8125 MHz all over Europe. Channel spacing is 25 kHz. The nominal frequencies and the channel numbering are defined as:

fn = 169.425 + n * 0.025 MHz

n = Channel number (0...15)

ERMES transmitter allocations follow the CEPT T/R 25-07, annex 1 recommendation.

Modulation

ERMES modulation is 4-PAM/FM. The four frequency pulse-amplitude modulation carries two bits (dibit) per frequency step. In addition to coherent phase keying ERMES also utilizes pre-modulation pulse shaping. To decrease bit error rate, data is encoded using the Gray code.

The nominal frequencies are:

The ERMES Protocol

A sequence of 60 seconds is partitioned into 60 cycles. The sequences are synchronized to UTC. The cycles have duration of exactly one minute, and synchronize the various ERMES networks (transmitters). In this way the receivers will only receive one or more cycles and thus power consumption is substantially reduced.

Each cycle is subdivided into five subsequences of 12 seconds each. In order to maintain synchronism between networks the subsequence number (command SSN = 0) is transmitted preceding every UTC minute marker.

A subsequence may also have duration of less than 12 seconds. The remaining time is used for transmitter switching.

Each subsequences is further divided into 16 batches designated A to P. Thus the pagers are divided into 16 groups. The transfer mode (tone call only, numerical call, alphanumerical call) is controlled by the position of the batch number.

The receiver addressing only takes place within the appropriate batch. After decoding its address the receiver will wait on the same frequency for data. Data may be transmitted within the same batch, within another subsequence batch or within subsequent subsequences.

Each batch is subdivided into four parts: synchronization, system information, address and text.

Protocol Structure

System Information

Network and system information is transmitted within the system subdivision of a batch. The system information is divided into two parts, System Information (SI) and Supplementary System Information (SSI).

Depending on the value of the SSI flag the Supplementary System Information (SSI) carries information on zone, local time and date. Another option displays day of week, month of year and year.

Data

ERMES transmits data in fixed-length frames of 36 bits. A frame may carry an additional data field and the text data.

Message Frame (MHEAD)

The Variable Information Field (VIF) has two main options depending of the status bit ALL = 0 or ALL = 1.

Variable Information Field Without Supplementary Information

Variable Information Field With Supplementary Information

The ETS 300 133-4 standard has a very fine grained subdivision of the VIF and this enables ERMES to be used for a wide range of applications.

ERMES and other pager systems are losing importance in Europe due to the surge in GSM usage, and several networks have been closed down.