Quality, availability and security

Transmitting data does not mean arriving on time

Talbahn wants to transmit conversations and measurements. From 1.3 you know a radio cell as a base station's coverage area. Besides coverage, the connection needs properties that suit the information.

A data packet is a bounded transmission unit containing payload and transport information. In simplified terms, it resembles an addressed shipment. A file or speech stream can be carried in many such units.

IP, the Internet Protocol, specifies rules for transmitting addressed packets. A protocol is a shared agreement on how technical participants exchange information. IP uses source and destination addresses and can also operate in a closed railway network. It is not a synonym for the public internet. IP alone guarantees neither the arrival of every packet nor a fixed transit time. [S19]

Keep three properties separate

The data rate describes an amount of data per unit of time. A high usable rate helps, for example, to transfer large measurement files on time. Comparisons must make clear whether theoretical capacity or the rate actually achieved is meant.

Latency is the delay between specified points. It can be measured between sending a message in the train application and receiving it in the workshop application. A small message can wait a long time even though the network carries a large amount of data per second overall. [S20]

Here, availability describes whether the required communication capability is usable under the specified conditions. For Lea, the service on Talstadt–Bergheim matters, not just a reachable server. A high data rate elsewhere does not compensate for a missing connection inside the tunnel.

In everyday language, data rate is often called “bandwidth”. In radio engineering, however, bandwidth means the width of a frequency range. It then describes how much frequency space a signal or transmission channel occupies and is a different quantity. We therefore use “data rate” for the amount of data per unit of time.

Two connections, two tasks

In this teaching example, connection A carries many measurement files per hour, but individual messages arrive late. Connection B carries less data but delivers the small message under consideration on time.

A may suit later file analysis, B the time-critical message in this example. The application determines which property is needed; one marketing figure does not describe overall suitability.

Comparing GSM-R and FRMCS

In circuit-switched communication, transmission capacity is assigned to a connection for its duration. The established transmission arrangement persists for that connection. In packet-switched communication, information travels as packets; data streams can share transmission resources. Packets from different exchanges are forwarded according to the intended rules. GSM-R also supports packet-switched data transmission. FRMCS is based on packet communication. “GSM-R cannot carry packets” would be wrong. [S02, S07]

Quality also needs protection and a usable service

Availability means the required function is usable under defined conditions. It depends on more than radio: incorrect role permission may prevent a conversation while packets are still transferred. Security protects against unauthorised access and manipulation. Safety concerns avoiding unacceptable risks to people and operations. The concepts are related but require different assessments.

Talbahn therefore asks four questions: does information arrive on time? Is the service usable when needed? Are access and changes adequately protected? What operational consequences follow a fault? Answering one well does not replace the others. Level 2 introduces technical procedures; here the essential outcome is a justified distinction. [S15; S25]

Check your understanding

A small message arrives late. The supplier points to the data rate. Which measurement is missing, which endpoints would you choose, and why does availability also matter?

Model answer

Latency between relevant points is missing, for example sending in the train application and receiving in the workshop application. The data rate does not answer this transit-time question. The exchange must also be usable on the intended route. The measurement boundary must be stated explicitly: reception does not automatically mean completed application processing.

Additional application

Why is a manipulated role permission not simply a data-rate problem?

Model answer

The change concerns security and can affect service availability and operations. More data rate does not restore correct permission.

What to remember

• IP can also be used in closed networks.
• Data rate, latency and availability answer different questions.
• The application's purpose determines the required performance.

Your Talbahn case file

For both pairs of communication partners, add questions about the three properties and name the endpoints.