Data link layer

Data Link Layers

A more detailed description of each link layer can be found in Appendix.

The data link layer transports data between two “boxes”. It’s not addressed as there is only one box at the other end (except when a RS485 bus is used). Most of the data link layers offered has reliable transmission, i.e. either a message is delivered error-free, or the system reports a device error.

Note that all link layers are capable both to connect local devices, and exchanges in AlphaNet! The “payload” messages going between applications have traditional headers and trailers added by link layers. This means that the byte stream observed with a data monitor on the physical wires in principle looks like this:

 <Network header> <Application message> 

ISO 1745 Link Layer

This is an old multi-drop protocol also used e.g. by the wireless pager protocol ESPA 4.4.4 A master polls one device at a time, and messages are transmitted one at a time with checksum and timer supervision, and possibly one or more re-transmissions.

The AlphaCom have two alternative physical connections:

• Point-to-point, typically short range via RS232 to a single external device (RIO or PC).
• Multidrop, up to 1 km on RS485 to 1 to 10 devices (RIO).

The ISO protocol uses a number of reserved control characters, so the message payload is always ASCII HEX. The polled, half-duplex nature of the protocol plus the HEX coding makes it fairly slow. On point-to-point the exchange polls for input messages only once per second!

Stentofon Data Link Layer

This is a AlphaCom proprietary protocol with so-called sliding window. It’s used both internally in a multi-module exchange (called Inter Card Cage Link, ICCL), and between exchanges in AlphaNet.

Sliding window means that the transmitting box can send up to 7 messages before receiving acknowledge to the first one. If a message is lost, the lost message plus all following it in the window must be retransmitted.

The messages are normally binary coded (but if you send HEX to the exchange, it will also answer back in HEX). The full-duplex sliding-window operation means that the application can send messages very rapidly in both directions.

Note: It’s allowed for an external device to hook up to this link layer using window size 1, i.e. send only one message to the exchange and wait for response. OK messages are ACKed, else error handling can simply be timeout and re-transmission.

Stentofon Simple Link Layer

There is always someone who wants to do things simple. Binary data, timing and re-transmissions are awkward to handle in a simple near-sequential PC program. Also, testing the exchange simply by typing data commands on a terminal can be very useful.

The solution is the “simple” link layer. It’s ASCI HEX, optional checksum only, and there is a number of user friendly shortcuts which means that just the most important fields need to be typed. You can even define yourself as a device type which is not pinged (else you have to type pong every 40 seconds…)

This link layer is medium speed, as it’s full duplex, and there is no acknowledge to wait for, but quite a lot of characters to send.

It’s possible to build a fairly reliable link with this protocol also. Your PC can attach a checksum to messages to the exchange, and messages from the exchange always have checksum (but the only thing it can be used to is to ignore faulty messages). A lot of the data commands to the exchange has functional acknowledge, i.e. explicit messages reporting success or failure of the attempted operation, and these can be used to re-transmit on a the application level.

Stentofon Multidrop Master/Slave

The frequent AlphaPro user will notice that it’s possible to select Multidrop master/slave as link layer. It’s a 4-wire RS422/485 low-trafic bus concept developed for an engineering project. The audio network must still be point-to-point. If you have a special application which could use such a concept, please contact Product Support for Stentofon.