Bearer Services

The basic services of a GSM network are the foundation for data transmission, i.e. a basic service provides the fundamental technical facilities at the end terminal interface (reference point R) to transport user payloads. The basic services are called transport services [7] or bearer services in ISDN - and therefore in GSM, too. The GSM bearer services offer asynchronous or synchronous data transport capabilities with circuit-switched or packet-switched data rates of 300-9600 bit/s, with a 13 kbit/s bearer service for voice.

Bearer services carry only the coding- and application-independent information transport between the user-network interfaces (Figure 4.1); they represent the services of Layers 1, 2, and 3 of the OSI Reference Model. The operators of the Terminal Equipment (TE) can use these services and employ arbitrary higher-level protocols, but they are responsible for the compatibility of the protocols used in the terminal equipment, quite in contrast to teleservices, where the protocols in the terminal equipment are also standardized [7]. An overview of the most important bearer services is given in Table 4.2. Each bearer service has its own number, e.g. BS26 is the bearer service for circuit-switched asynchronous data transfer at 9600 bit/s.

Besides the asynchronous and synchronous circuit-switched data services (BS21-BS34), packet-switched data services are also provided. These packet services are realized either as asynchronous access to a Packet Assembler/Disassembler (PAD), BS41-BS46, or as direct synchronous Packet Access, BS51-BS53.

The bearer services for GSM data transfer are offered in two fundamentally different modes (Table 4.2): transparent (T) and nontransparent (NT). In the transparent mode, there is a circuit-switched connection between the mobile terminal (TE) and the inter-working module in the MSC, from where the connection to other networks is handled. This connection is protected by Forward Error Correction (FEC). The most important common characteristics for all transparent services are constant bit rate, constant transport delay, and residual bit error ratio dependent on the current channel conditions. The nontransparent mode activates a special Layer 2 protocol for the additional protection of the data transfer, the Radio Link Protocol (RLP) which is specially adapted to the GSM radio channel. This protocol terminates in the mobile station and in the MSC. It uses ARQ procedures to request retransmission of blocks with residual errors which could not be corrected by forward error correction.

This gives a much more significant reduction in the residual error rate. In essence an error-free information transport is achieved, hence it is approximately independent of the

Table 4.2: GSM bearer services (excerpt)a

Service

Structure

BS no.

Bit rates (in bit/s)

Mode

Transmission

Data

Asynch

21

300

Tor NT

UDI or3.1 kHz

22

1200

Tor NT

UDI or3.1 kHz

23

1200/75

T or NT

UDI or3.1 kHz

24

2400

T or NT

UDI or3.1 kHz

25

4800

T or NT

UDI or3.1 kHz

26

9600

T or NT

UDI or3.1 kHz

Data

Synch

31

1200

T

UDI or3.1 kHz

32

2400

T or NT

UDI or3.1 kHz

33

4800

T or NT

UDI or3.1 kHz

34

9600

T or NT

UDI or3.1 kHz

PAD

Asynch

41

300

T or NT

UDI

42

1200

T or NT

UDI

43

1200/75

T or NT

UDI

44

2400

T or NT

UDI

45

4800

T or NT

UDI

46

9600

T or NT

UDI

Packet

Synch

51

2400

NT

UDI

52

4800

NT

UDI

53

9600

NT

UDI

Altern. speech/data

61

13000 or 9600

Speech followed by data

81

13000 or 9600

a T/NT, transparent/non-transparent; UDI, unrestricted digital information; PAD, packet assembler/disassembler; asynch/synch, asynchronous/synchronous a T/NT, transparent/non-transparent; UDI, unrestricted digital information; PAD, packet assembler/disassembler; asynch/synch, asynchronous/synchronous momentary channel conditions. However, with changing error behavior of the radio channel, the frequency of block repetitions also varies, and thus the average transfer delay and the net bit rate of the data service vary too. Activation of the nontransparent data service is especially interesting for rapidly moving mobile stations or for cases of bad radio conditions, where high fading rates and deep fading/shadow holes occur. In such situations, a meaningful transport of user data with transparent mode can become impossible. At the expense of a net data rate decrease, the nontransparent mode then still allows a reliable data transport.

The GSM bearer services 21-53 are further categorized into Unrestricted Digital Information (UDI) and 3.1 kHz (Table 4.2). The services differ mainly in the way in which they are handled outside of the PLMN, i.e. the kind of interworking function that needs to be activated. The UDI service category corresponds to the UDI of ISDN and supplies a channel for the unrestricted transfer of digital information. The data transfer is unrestricted in the sense that no bit patterns are reserved or explicitly excluded from transmission. The

3.1 kHz category is used to activate in the MSC an interworking function for 3.1 kHz audio and to select a modem. Within the GSM PLMN (from user-network access to the interworking function), the data are still transferred as UDI. The designation "3.1 kHz" rather refers to the fact that the transfer outside of the PLMN uses a service "3.1 kHz Audio." This service is offered by conventional PSTN as well as by ISDN networks. For transfer with this service, the data has to be converted in the IWF of the MSC with a modem to an audio signal with a bandwidth of 3.1 kHz.

Further important GSM bearer services contain voice (telephone) service (BS61 to BS81), which can be (multiple times) changed during a call at the request of the user to a data service (alternate speech/data). Another alternative is that the user at first establishes a voice connection and then changes to a data connection, which cannot be changed back to voice (speech followed by data).

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Responses

  • hyiab
    What are different bearer services?
    3 years ago
  • thomas stanford
    What is non transparent and transparent barrier services in mobile communication?
    12 months ago

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