IEC 62056-42:2002 pdf download
IEC 62056-42:2002 pdf download.Electricity metering – Data exchange for meter reading, tariff and load control -Part 42: Physical layer services and procedures for connection-oriented asynchronous data exchange.
6 Protocol specification
6.1 Physical layer protocol data unit (PHPDU)
The PHPDU is specified to be one byte. For transmission purposes however this data byte may be extended (error detection / correction) or modified (bit-stuffing) by the modem device, depending on the modulation scheme used. See also explanation to Figure A.4 — Data exchange between the calling and called stations.
6.2 Transmission order and characteristics
The PHSDU byte — the data parameter of the PH-DATA services — shall be completed with one start bit and one stop bit before transmission. The resulting frame shall be transmitted starting with the start bit, followed by the least significant bit first, with the least significant bit identified as bit 0 and the most significant bit as bit 7.
All characteristics of the physical medium and the signal(s) used on this medium are not in the scope of this international standard.
6.3 Physical layer operation — description of the procedures
6.3.1 General
The physical layer — together with the physical media — is a shared resource for the higher protocol layers. Multiple higher layer connections/associations can be modelled as different instances of the protocol stack, which need to share the resources of the physical layer.
For this reason, the physical connection manager application process manages the physical connection establishment and release — see 6.3.2 and 6.3.5. Any application process wishing to use the COSEM protocol shall check the connection state of the physical layer before requesting a connection. If the physical layer is in non-connected state, it shall request the physical connection manager to establish the connection. If the application layer (see IEC 62056-53) invokes an COSEM-OPEN.request service and the corresponding physical connection is not established, the COSEM-OPEN.request will be locally confirmed with error = NO_PHYSICAL_CONNECTION (see in detail in IEC 62056-53).
Once the physical connection is established, the physical layer is ready to transmit data.
An optional identification service — as described in 6.3.3 — is available. This enables the client to identify the protocol stack implemented in the server.
After the identification procedure is completed — or if it is not used — the upper protocol layers and the applications can exchange data — see 6.3.4. The user of the PH-DATA services is the next protocol layer above the physical layer.
A physical disconnection may be requested by the physical connection manager (either on the server or the client side), or may occur in an unsolicited manner (e.g. the phone exchange cuts the line). While physical disconnection management is the exclusive responsibility of the physical connection manager, indication of an unsolicited disconnection (PH-ABORT.indication) is sent both to the next protocol layer and to the physical connection manager. See 6.3.5.
6.3.2 Setting up a physical connection
Both the client and the server device can act as a calling device, initiating a physical connection to a remote device, which is the called device.
The execution of the PH-CONNECT.request service depends on the physical connection type and on the modem used.
In Annex A, an example is given as to how this is performed in the case when intelligent Hayes modems are used through the PSTN.
In other cases, all the operations required — dialling, handling eventual error messages (busy, etc… ), negotiating the line modulation/baud-rate parameters, etc. — might be executed by the physical layer itself.
In order to allow using a wide variety of physical connection types, this international standard does not specify how the execution of the PH-CONNECT.request should be done.
At the called device side, when the physical connection initiation is detected, the connection needs to be managed: negotiated and accepted or refused. These actions — similarly for the execution of the PH-CONNECT.request service — depend on the physical connection type and on the modem used, and might be done in an autonomous manner or by the physical layer itself. The specification of these actions is not within the scope of this standard.
When the physical layers of the Calling and Called device complete establishing (or not establishing) the required physical connection, they inform the service user entity about the result, using the PH-CONNECT.confirm (calling side) and the PH-CONNECT.indication (called side) service primitives. In this COSEM profile, the service user of these service primitives is exclusively the physical connection manager process.
