TETRA

Description

Advantages

• The much lower frequency used gives longer range, which in turn permits very high levels of geographic coverage with a smaller number of transmitters, thus cutting infrastructure costs.
• During a voice call, the communications are not interrupted when moving to another network site. This is a unique feature, which dPMR networks typically provide, that allows a number of fall-back modes such as the ability for a base station to process local calls. So called 'mission critical' networks can be built with TETRA where all aspects are fail-safe/multiple-redundant.
• In the absence of a network, mobiles/portables can use 'direct mode' whereby they share channels directly.
• Gateway mode - where a single mobile with connection to the network can act as a relay for other nearby mobiles that are out of range of the infrastructure. A dedicated transponder system isn't required in order to achieve this functionality, unlike with analogue radio systems.
• TETRA also provides a point-to-point function that traditional analogue emergency services radio systems did not provide. This enables users to have a one-to-one trunked 'radio' link between sets without the need for the direct involvement of a control room operator/dispatcher.
• Unlike cellular technologies, which connect one subscriber to one other subscriber, TETRA is built to do one-to-one, one-to-many and many-to-many. These operational modes are directly relevant to the public safety and professional users.
• Security TETRA supports terminal registration, authentication, air-interface encryption and end-to-end encryption.
• Rapid deployment network solutions are available for disaster relief and temporary capacity provision.
• Network solutions are available in both reliable circuit-switched architectures and flat, IP architectures with soft switches.

Disadvantages

• Security issues have been identified, although mitigations were quickly introduced in the TETRA security standard. There was an intentional weakening of the TEA1 cipher to meet export control criteria, allowing a break within a minute on consumer hardware.
• Requires a linear amplifier to meet the stringent RF specifications that allow it to exist alongside other radio services.
• Data transfer is slow by modern standards.

Technical details

Radio aspects

Radio frequencies

Air interface encryption

Security vulnerabilities of the air interface encryption

• The Air Interface Encryption keystream generator is vulnerable to decryption oracle attacks due to the use of publicly-broadcast network time—keystream reuse can be triggered for acknowledged unicast communication when using static keys. This is not applicable for group communication. Mitigations for this were introduced into version 4 of the TETRA standard.
• TEA1 contains a reduction step that effectively downgrades the cryptographic strength from 80 to 32 bits implemented to allow export under the Wassenaar convention, allowing anyone to break the cipher and subsequently decrypt the signal in as little as one minute using a modern-day consumer laptop. "There's no other way in which this can function than that this is an intentional backdoor," "This constitutes a full break of the cipher, allowing for interception or manipulation of radio traffic", according to the news report posted on ComputerWeekly. The reduction in strength of TEA1 seems to be known in intelligence circles and is referred to in the famous 2006 Wikileaks dump of US diplomatic communications.
• AIE contains no authentication for the ciphertext, making malleability attacks possible.
• The cryptographic anonymization scheme is weak and can be partially reversed to track users. A revised identity encription scheme was introduced in version 4 of the TETRA standard in mitigation.
• The authentication algorithm theoretically allowed attackers to set the derived cipher key to 0 but not to decrypt traffic. Although this attack remains theoretical, mitigations were introduced in version 4 of the TETRA security standard.

Cell selection

Cell re-selection (or hand-over) in images

Initial cell selection

• Refer to EN 300 392 2 16.3.1 Activation and control of underlying MLE service
• Note 18.5.12 Minimum RX access level

Cell improvable

Cell usable

• The MS subscriber class is supported on the neighbour cell but not on the serving cell.
• The neighbour cell is a priority cell and the serving cell is not.
• The neighbour cell supports a service that is not supported by the serving cell and the MS requires that service to be available.
• The cell service level indicates that the neighbour cell is less loaded than the serving cell.

Cell relinquishable (abandonable)

Radio down-link failure

Man-machine interface (MMI)

Virtual MMI for terminals

• Allow any given agent while in exercise, to exploit any given radio terminal without materiel constraint.
• Provide specific transportation application software to the end-user agents.

1. Service acquisition
2. Status SDS
3. End-user parameters

TETRA Enhanced Data Service (TEDS)

• JSR-118;
• Mobile Information Device Profile, JSR-37;
• Wireless Messaging API, JSR120;
• Connected Limited Device Configuration, JSR-139; and
• Technology for the Wireless Industry, JTWI-185.

Comparison to Project 25

• TETRA: It is optimized for high population density areas, with spectral efficiency. It is suitable for high population density areas and supports full duplex voice, data and messaging; but, it is generally unavailable for simulcast, VHF band - however particular vendors have introduced Simulcast and VHF into their TETRA platform..
• P25: it is optimized for wider area coverage with low population density, and support for simulcast. however, it is limited to data support.

Professional usage

Amateur radio usage

• In most countries, encryption cannot be used.
• Most older terminals don't cover the Region 1 amateur radio frequency range natively, and must be modified via software with a possible impact on RF performance.