RFDECT Tutorial: Basics, Specifications, Frame Structure, and Protocol Stack
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This tutorial covers DECT (Digital Enhanced Cordless Telecommunications) system basics, specifications, frame structure, and protocol stack. DECT was developed in 1993 to provide service to both domestic and corporate users. It supports both circuit-switched (CS) voice and data services.
It’s similar to the GSM cellular system, but the key difference lies in cell size: GSM cells are larger, while DECT cells are smaller. It can also interface with PBXs (Private Branch Exchanges).
DECT System Basics
DECT stands for Digital Enhanced Cordless Telecommunications. The concept of cordless telephony was originally introduced to provide mobility within a home or office from a main telephone terminal using a handset. The handset and the main base terminal are connected via an analog wireless link.
DECT-based systems typically cover distances between 30 and 100 meters. It operates in the 1.88 GHz to 1.9 GHz radio frequency band, providing a bandwidth of about 20 MHz. The access technology for resource allocation is TDD/TDMA/FDMA.
DECT achieves a data transmission rate of about 1.152 Mbps.
DECT System Specifications or Features
Here are the key features and specifications of the DECT system:
| Specification or Feature | DECT system support |
|---|---|
| RF Carrier frequency | 1.88 to 1.9GHz |
| Access | TDD/TDMA/FDMA |
| Cell radius | 25 to 100 meters |
| Channel Spacing | 1.728 MHz |
| No. of carriers | 10 |
| No. of channels per carrier | 12 |
| Speech codec | ADPCM with 32kbps speech rate |
| Modulation techniques | Gaussian, FSK, 4PSK, 8PSK, 16-QAM, 64-QAM |
| Bit rate | 32 Kbps |
| Time slots | 2 x 12 (upstream, downstream) |
| Channel Allocation Method | Dynamic |
| Traffic density | 10000 Erlangs/Km2 |
DECT Frame Structure
The DECT frame duration is about 10ms. It consists of 24 time slots. 12 time slots are allocated for the base terminal to handset direction, and 12 time slots are allocated for the handset to base terminal direction. Each time slot has a duration of 0.417ms.
One Time slot = preamble (16 bits) + sync (16 bits) + A field (64 bits) + B field (320 bits) + X field (4 bits) + Guard bits (60 bits)
A field = Header (8 bits) + Data (40 bits) + CRC (16 bits) B field = Data (64 bits) + CRC (16 bits) + Data + CRC + Data + CRC + Data + CRC
The sync field is used by the receiver to synchronize with the start of the frame. The A field carries control or management signals, and the B field carries user data/information. The figure above illustrates the DECT frame structure.
DECT Protocol Stack
The protocol stack of DECT, as depicted in the figure, is used both at the Mobile Unit and Base Station Unit. It consists of the Physical layer, MAC (Medium Access Control), Data link control layer, and different services.
Services include CC (Call Control), Supplementary services, Connectionless message service, Connection-oriented message service, and MM (Mobility Management).
The DECT MAC layer controls the physical layer (Layer 1). It also provides connection-oriented, connectionless, and broadcast services to the upper layers in the DECT protocol stack. It also provides encryption functionality using the Standard Cipher protocol.
DECT ULE (Ultra Low Energy)
DECT ULE is the latest standard upgrade of the legacy DECT standard described above. It supports many additional features not available/supported by the original DECT standard. It offers lower power consumption and delivers longer range compared to DECT. It supports moderate data rates and lower latency-based applications.
