Opimizing of transfer methods and networks for industrial data transmission.

Seminsky, Jaroslav ; Koenig, Steffen

1. INTRODUCTION

During the last few years a quick rising demand for multimedia and data transmissions between widespread components of systems like e.g. metering points, data loggers, machining tool interfaces and their corresponding systems over short and long distances can be observed. The (IP-based) internet and its state-of-the-art broadband subscriber connections like cable modem (CMS) or Digital Subscriber Line SDSL/ADSL service seem to be the most appropriate medium to provide the necessary connection.

The mentioned equipment also often uses virtual high layer peer-to-peer connections through the Integrated Services Digital Network ISDN or, by use of industrial heavy duty modems, dialup POTS or analogue leased line connections.

This paper deals with the impact factors and necessary facts that have to be considered to find the optimum link technology to serve a specific system's demands.

2. DATA TRANSFER DEMANDS

Data transfer in industrial environments has always introduced demands towards the transfer network that were of less importance for office-like services like telephony, internet access or file transfer.

Installations like data loggers, point-of-sale (POS) credit card terminals, automatic meter reading or remote service for elevators have in common, that the amount of data to transfer is relatively small. In most cases data transmission is triggered by coincident occurrences.

For example a remote readable meter may be polled on demand by the data collecting instance, while an elevator emergency or a credit card transaction occurs--from the system's point of view--coincidently but nevertheless has to be processed immediately.

Up to now most systems use modem or ISDN-based dialup connections for data transfer. Each dialup link requires its own dedicated and cost intensive POTS subscriber line.

The growing availability of low-cost broadband IP-based network accesses like DSL though fuels the demand to migrate the mentioned applications towards IP-based network solutions. During the last few years a constant trend towards technologies, especially real-time applications, like e.g. audio--and video connections and multi subscriber conferences, can be observed.

Tab.1 summarizes a selection of the most common services and their characteristics.

Note that "security demands" only refers to applications where persons or goods of high value are in danger if a necessary data connection cannot be established immediately.

3. NETWORK STRUCTURES

The demands of the mentioned data endpoints are up to now served by different network structures:

* The public telephone network, with POTS lines

* Switching over Integrated Services Digital Network ISDN Public Exchange

* Permanent Virtual Connections PVC over Public Network Structures like links accordant to IEEE standard X.31

* Leased Lines, that are switched permanently

* Use of Permanent Virtual Connections over Public IP-based routable Network structures, e.g. Internet links.

Links over POTS telephone lines always require a modem and are known to be very reliable; the operating range of a POTS line is usually up to more than 15 km from the next Private or Public exchange without the need of active components for signal regeneration and there are only minimum requirements towards the quality of cabling. In combination with the high availability, this technology is up to now mostly used for remote meter reading e.g. for electricity suppliers.

Disadvantages are the relatively high basic charges for telephone lines. Each metering point requires an exclusive line; line sharing is only possible where multiple meters are concentrated in the same location. Furthermore the achievable transfer rates are, compared to those within IP broadband networks, rather low: Up to 56 Kbit/s are available by use of IEEE standard V.92.

Metering data blocks though are usually of a maximum size of less than 600 Bit per transmission; so the limitation to 56 Kbit/s does not necessarily mean a reduction in service quality.

An even more apparent problem is the tremendous demand for lines if e.g. remote electricity metering shall be introduced for an entire metropolitan site; the huge number of metering points yields another solution. This is where our research starts out.

4. TRANSPORTATION PROTOCOLS

The mentioned limitations to dialup lines must be faced by use of IP-based network structures.

The Integrated Services Digital Networks ISDN has been a significant step into that direction. Nevertheless ISDN is from the far point of view due to be replaced by follow up technologies. ISDN though offers both line-oriented switching, e.g. for voice service or other real-time data traffic, and packet-oriented services, which will still be subject to discussion.

In the ETSI standardized European variant of ISDN, it offers 64 Kbit/s exclusive basic (B-) channels, that can be bundled to nx64 Kbit/s transparent links. A maximum Bit Error Rate BER of [less than or equal to] [10.sup.-7] is guaranteed (Koenig, 2008).

To face the many situations where small amounts of data must be transmitted in coincident intervals, like at point-of-sale card terminals or alarm equipment in machine tools, the X.31 service through ISDN D-channel is an appropriate solution.

It operates packet oriented and does not require an exclusive line; data packets are sent through the Data (D)channel, which is included in every subscriber access. This is the only exception, where the D-channel transports payload data; it usually is reserved for connection control data exclusively.

5. MERGING TOWARDS IP-NETWORKS

Investigations here have shown, that, considering the specific demands towards reliability, data bandwidth, cost effectiveness etc. most of the mentioned applications can be migrated to IP-based networks if the protocols are mapped to corresponding IP suitable instruction sets (Schulzrinne et al., 2006). Various systems and set-ups have been tested and classified towards their specific characteristics concerning real-time requirement, size and shape of transfer data, transportation security aspects, communication principle (polling or transmit on demand).

The survey revealed the major existing problem, that, although many other sources already have specified IP-suitable communication standards, like H.323 (ITU 2008a), its derivates H.245.0 (ITU 2006) and H.225.0 (ITU 2008b), there is still a lack in knowledge about reliability issues. Hence users are still reluctant to implicate new standard's equipment.

A sophisticated system must allow migrating single or small-defined groups of data points towards the new technology. For a reasonable period of time hybrid structures with old and new protocols simultaneous in use must be possible; this approach is known as "soft migration". It seems to be the most reliable and cost effective way to migrate existing structures and systems.

6. IP-STANDARD H.323 FOR MAPPING

A main question of this survey is: How does an IP-based link behave under varying load conditions? How much will third party traffic affect link latency and throughput? What issues have to be considered to ensure reliable link performance?

[FIGURE 1 OMITTED]

E.g. a connection according to ITU-T standard H.323 shall be established and replace a connection through ISDN. Standard H.323 is a framework and thus can be subdivided into the signaling standards H.225.0 and H.245; these standards specify the connection related signaling.

H.225.0 uses the messages specified by ITU-T standard Q.931, which is also used for ISDN D-channel and signaling protocol SS7 for digital public exchanges. H.225.0 encapsulates the SS7 messages into a TCP connection through IP. Nevertheless hybrid connections are easy to realize, because no protocol mapping is necessary within gateways to ISDN-based telephone systems. It generally is necessary to set up virtual channels, or connections respectively, prior to transferring real-time data through routing networks like the IP-based Internet.

Fig.1 outlines the signaling steps that have to be performed for a connection between the two involved multimedia terminals A and B.

When using H.323, different codecs can be implemented. The most common ones for audio coding are those according to ITU-T G.711, G.723, G.726 and G.729. Minimum mandatory audio codec is ITU-T G.711, additional codecs, according to (IETF 2006), are optional.

7. CONCLUSIONS AND OUTLOOK

Our surveys have revealed, that there is a rising demand for the discussed data equipment to enable soft migration. As investigations have shown, the Internet and its various IP-based transmission protocols offer transmission capabilities and enable the applications for migration to IP technology. Future research shall lead to datagram algorithms with improved real time behaviour. Furthermore design requirements for reliable heavy duty IP-network interfaces shall be developed.

8. REFERENCES

Koenig, S. (2008). Telekommunikationsdienste II. Paketorientierte Protokolle., H.323. FH Friedberg, Germany

Schulzrinne H.; Casner S.; Frederick R. & Jacobson V. (1996). RTP: A Transport Protocol for Real-Time Applications: RFC1889

*** IETF Internet Engineering Task Force (2006). An Offer/Answer Model with the Session Description Protocol (SDP), RFC 3264, New York, USA

*** ITU-T Recommendation H.245 (5/2006). Control protocol for multimedia communication. Bern, Switzerland

*** ITU-T Recommendation H.323 (2008). Packet-Based multimedia communications systems. Bern, Switzerland

*** ITU-T Recommendation H.225.0 (2008). Call signaling protocols and media stream packetization for packet based multimedia communications Systems. Bern, Switzerland

Tab. 1. Applications and their data transmission demands

Service Amount of Regular/ Security Actual
 Data per Coincid. Demands Network
 transmission

Meter 56...600 Byte REG NO POTS,
reading ISDN

Data logging <100 Kbyte REG NO POTS,
 ISDN

POS- <1 Kbyte CO NO POTS,
Terminal ISDN,
 X.31

Elevator 64k stream CO YES Leased
alarm Link,
 ISDN,
 IP-Net

Controlling Variable CO YES Wired
and Service, stream link,
Actuation BUS, IP-
 Net

Fire alarm <1 Kbyte CO YES Leased
systems link