The CTS radio interface retains the following GSM features:

• Transmit/receive on pairs of frequencies (Frequency Division Duplicating - FDD)
• Dame time-oriented subdivision of the physical channel into time slots, multiframes, superframes and hyperframes (Time Division Multiple Access - TDMA)
• Same modulation and channel coding schemes.
• Same radio packet formats and sizes. Modifications only affect the logical channels and they are multiplexed onto the physical channel.

"Beacon Frequency" Channel concept
This replaces the conventional GSM signalling channel used by GSM mobiles to register with a GSM cell. It transmits a minimum of logical information discontinuously but regularly (one frame every 26 seconds). Mobile can this register to determine the identity of the base station and accessibility status.

Adaptive Frequency Allocation Algorithm (AFA)
This helps solve the choice of frequency least exposed to interference.

Since the base station and mobiles registered with it regularly measure the radio environment of the CTS, the AFA can output a list of frequencies classified by interference level.

The base station then uses the least loaded frequency for the beacon frequency channel.

Total Frequency Hopping Algorithm
This used to make CTS more robust against interference on all physical channels except the beacon frequency channel.

Radio Interface Modes
There are three modes :
• Standby: the fixed part is powered up but has no terminal connected
• Idle: the fixed has at least one terminal connected
• Dedicated: Corresponds to a traffic call between a terminal and a fixed part.

Multiframe Structures
The GSM traffic multiframe, with 26 frames, is used for communication in dedicated mode. The non-connected modes (idle and standby) use a 52-frame multiframe, timed by the regular transmission of the beacon frequency channel every 26 frames.

A traffic channel can therefore be set up by the base station in parallel with the beacon frequency channel during "unused frames when in connected mode. When in non-connected mode, the radio interface is quiet and generates little interference.

More:
CTS Introduction
General CTS Technical Overview
CTS Security
CTS Protocols
CTS system Architecture
Cellular technologies

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* Much of this information was sourced from the CTS MoU