49. 4. 1 Dect protocol architecture

The DECT protocol (ETSI. 1992b) is based on the principles of the OSI model. The complete common interface relevant to public access applications is defined in terms of the three lowest layers modified to account for the specific requirements of radio transmission and in-call handover. The structure is shown in Figure 49.9 where four DECT layers are identified. The OSI layers are indicated for reference. The Physical Layer (PHL) has the task of modulation and demodulation, acquire bit and burst synchronisation, control synchronisation and independent burst collision detection and measure the received signal strengths.

The Medium Access Control Layer (MAC) performs two main functions:

1. It selects the radio channels and then establishes and releases the communication link.

2. It multiplexes and demultiplexes all information into burst packages.

These two functions are used to create three services; a broadcast service, a connection orientated service (e. g. for telephony) and a connectionless service (e. g. for packet-like transmission).

The broadcast service is always transmitted from every base in a reserved data field (A field) on at least one physical channel. This beacon transmission allows portable parts (PP) to quickly identify and lock on to any suitable base (fixed part, FP) without requiring PP transmissions. The Data Link Control Layer (DLC) is largely responsible for providing very reliable data links to the network layer. This layer has two operational planes. The C-plane is con­cerned mainly with the fully error controlled transmission of inter­nal control and signalling information. The U-plane offers a similar function in support of the specific requirements of the services being conveyed. For example the transparent unprotected service used for speech transmission.

Finally the Network Layer (NWK) is the main signalling layer of the protocol. It functions by exchanging messages between peer entities in support of, for example, establishment, maintenance and release of calls. Many additional messages support a range of independent capabilities and services. One group contains the necessary procedures that support cordless mobility which includes FP and PP authentication and location registration.

The lower layer management entity is concerned with procedures that involve more than one layer and yet are often only of local significance. Consequently they are only defined in general terms.

49. 4. 2 Intel-working Units (iwu)

The transmission of information to end users beyond the DECT link requires additional protocols that are outside the DECT specifica­tion. Thus to interface a DECT link with, say, a GSM fixed part will require an appropriate IWU to establish proper unambiguous mess­age transfer and in the process influence the service standard to be offered. Clearly the IWU concept will play a very important role in the full exploitation of the DECT specification.

49. 4. 3 Spectrum resource

Throughout Europe the band 1880MHz to 1900MHz has been set aside for use by the DECT system. This 20MHz of spectrum must however be used efficiently and flexibly if it is to meet the require­ments of high capacity business systems. It is the main purpose of the Physical Layer to bring this about by ensuring that adequate capacity radio channels are created in a manner that permits high orders of radio channel re-use. The DECT spectrum resource has been distributed in space, frequency and time.

Spatial distribution is brought about because DECT supports the use of the well-known concept of cellular radio channel re-use. In this process the area to be served is covered by a number of base stations, each of which provides radio coverage over a limited radius. This radius is typically of the order 20-50 metres depending partly on the construction nature of the building being served but more likely on the density of telecommunications traffic to be catered for. In this latter case each small cell is able to offer a certain number of radio channels and the smaller the cell the shorter the distance away the same channels can he re-used with acceptable co-channel interference ratio. This is the classical cellular frequency re-use concept that gives very high orders of spectrum efficiency, expressed in terms of Erlangs per MHz per sq. km.

Frequency distribution is achieved by segmenting the available band into ten carrier frequencies from 1881. 792MHz to 1897. 344MHz and separated by 1728 kHz.

Time distribution has been achieved by employing time division multiple access (TDMA) coupled with time division duplex trans­mission (TDD) to provide two-way communication on the same carrier frequency.

Exercise 1 Learn the following words and word combinations

1	to interface	согласовывать, состыковывать
2	back-end interface	интерфейс (для связи) с базами данных; внутренний интерфейс
3	baseband interface	сопряжение по частоте модулирующего сигнала
4	bit-serial interface	последовательный интерфейс; последовательный стык
5	front-end interface	входное устройство сопряжения; внешний интерфейс
6	PHL (the physical layer)	физический уровень
7	MAC (the medium access control layer)	Промежуточный уровень управления доступом
8	NWK (the network layer)	уровень сетевой иерархии
9	spatial (space) distribution	пространственное распределение
10	frequency distribution	частотное распределение
11	time division	распределение по времени; временное распределение
12	TDMA (time division multiple access)	многостанционный доступ с временным разделением каналов
13	multi-carrier TDMA	мультичастотный многостанционный доступ с временным разделением каналов
14	common spectrum	совместная спектральная плотность
15	radio access	радиосвязь с абонентами
16	bearer	1 носитель 2 Однонаправленный канал, передача данных
17	collision detection	обнаружение столкновений (в сети)

Exercise 2 Read the text

Exercise 3 Give the Russian equivalents for the following English ones:

1	around the mid 1980’s
2	a digital cordless telecommunication standard
3	multi-carrier time division multiple access
4	eventually
5	the coexistence interface specification
6	ISDN-based applications
7	to conserve spectrum for other users
8	the GSM digital cellular fixed network
9	public intelligent telecommunication networks
10	to multiplex and demultiplex all information
11	to provide very reliable data links
12	in support of
13	the transparent unprotected service
14	to meet the requirements of
15	the density of telecommunication traffic
16	the smaller the cell the shorter the distance
17	the available band
18	in this latter case

Exercise 4 Answer the following questions:

1	What led to DECT?
2	What levels of standardisation does DECT have?
3	What do you know about the application areas of DECT?
4	How can you prove that DECT has the capability to become an access technology that is integrated with the network?
5	What can you tell about the DECT protocol architecture, its four layers and their functions?
6	Why do IWUs play a very important role in the full exploitation of the DECT specification?
7	How has the DECT spectrum resource been distributed in space, frequency and time?
8	Does the DECT system meet the requirements of high capacity business systems?

Part IV (49.4.4 – 49.4.8)