3 Service of the physical layer

04.043GPPLayer 1 - General RequirementsRelease 1999TS

The physical layer supports transfer of bit streams on the radio medium according to the 3GPP Technical Specifications of the 05‑series. The scope of the 05‑series of 3GPP Technical Specifications is the definition of a framework for operation on the radio medium. The application of this framework on the radio medium results in a transmission service. General characteristics of the service obtained by applying the framework of the 05‑series at the operation on the radio medium are described in this clause.

3.1 Service Access Point

In the Reference Model for Open System Interconnection, Service Access Points (SAPs) of a layer are defined as gates through which services are offered to an adjacent higher layer (figure 3.1). Through a SAP the physical layer offers a service to the data link layer. The SAP is used both for the control of the service providing entity (in case this is the physical layer; commands related to the establishment and release of channels) and the transfer of data (in case of the physical layer; the transfer of bits). The physical layer service access points defined in the present document differ from the OSI physical layer Service Access Points; the layer 3 RR‑management instead of the data link layer controls the SAPs (establishment and release of channels).

Figure 3.1: Service Access Point principle

On the physical layer of the GSM system a SAP is defined between the physical layer and the data link layer for each control channel (figure 3.2). The characteristics of SAPs (channels) are listed in 3GPP TS 04.03.

Figure 3.2: SAPs between the physical layer and the data link layer in the MS

Moreover, on the physical layer of the GSM system a SAP is defined between the physical layer and the RLC/MAC layer for the packet data control channels and the packet data traffic channel (figure 3.2a). Multiplexing of these channels is controlled by the RLC/MAC layer, see 3GPP TS 04.60. The characteristics of channels are listed in 3GPP TS 04.03.

Figure 3.2a: SAP between the physical layer and the RLC/MAC layer in the MS

3.2 Service of the physical layer

* Access capabilities:

The physical layer offers a transmission service on a limited set of logical channels. The BS and MS access capabilities and the characteristics of logical channels (SAPs) are defined in 3GPP TS 04.03.

NOTE: Between 3GPP TS 04.03 and the 3GPP TS 05‑series there is a slight difference in terminology. The "channels" mentioned in 3GPP TS 04.03 are "logical channels" according to the 3GPP TS 05‑series (especially 3GPP TS 05.02). The "CCCH", a channel name commonly used in the 04‑series, covers the logical channels of the type RACH, PCH and AGCH. Similarly, the “PCCCH” covers the logical channels of the type PPCH, PAGCH and PRACH.

Logical channels are multiplexed on physical channels. Physical channels are the units scheduled on the radio medium. Some are reserved by the network for common use (e.g. a combination of CCCH and BCCH), others are assigned to dedicated connections with MSs (dedicated physical channels), or are assigned to a shared usage between MSs for packet switched data traffic (packet data physical channels). In time, the combination of logical channels used on an assigned physical channel may change. Allowed combinations of logical channels on a physical channel are defined in 3GPP TS 04.03. Data on SAPs of control channels is exchanged in discrete blocks with a size of 23 or 21 (SACCH) octets. Data on a SAP of packet data traffic channels is exchanged in discrete blocks with a size of 184, 271, 315 or 431 bits.

Synchronization between layer 1 and layer 2 (data link layer) is provided for piggy‑backing of RR (receive ready) frames, and the starting of timers (T200). See also 3GPP TS 04.06. Synchronization between the physical layer and the RLC/MAC layer is provided for the handling of timers, and the multiplexing of logical channels. See also 3GPP TS 04.60 .

* Error detection:

The physical layer offers an error protected transmission service, it includes error detection functions and to a lower level, error correction functions. Erroneous received frames are not offered to the data link layer or the RLC/MAC layer. The probability of one or more errors in a physical block transferred by the physical layer is defined in 3GPP TS 05.05. Due to not specified methods of quality detection, the probability of residual errors in transferred blocks may vary between implementations.

* Encryption:

Security related functions implemented at the physical layer are described in 3GPP TS 03.20.

An overview of the functions specified in the 05‑series which create the service of the physical layer can be found in 3GPP TS 05.01.

3.2.1 Specific services of the physical layer in the MS

The access capability service of the physical layer in the MS differs depending on the nature of the channel (traffic, packet data traffic or broadcast/common channels).

– Establishment of dedicated physical channels:

Establishment of dedicated physical channels on the physical layer is controlled by the radio resources management of layer 3 (3GPP TS 04.08). During operation on a dedicated physical channel, the physical layer measures the signals of neighbouring base stations and the signal quality of the used dedicated physical channel. Measurements are transferred to layer 3, measurement control information is offered by layer 3.

– Establishment of packet data physical channels :

Establishment of packet data physical channels on the physical layer is controlled by the radio resource management of layer 3. Packet access and the reservation of radio resource on packet data physical channels is controlled by the RLC/MAC layer in co-operation with layer 3 (3GPP TS 04.60 and 3GPP TS 04.08). During operation on packet data physical channels, the physical layer measures the signals of neighbouring base stations and the signal quality of the used packet data physical channel. Measurements are transferred to layer 3, measurement control information is offered by layer 3.

– cell/PLMN selection in idle mode or in packet mode:

In idle mode or in packet mode, the physical layer selects the best cell with its BCCH/CCCH in close co‑operation with layer 3, meeting requirements for PLMN selection specified in 3GPP TS 02.11. The idle mode procedures are not modelled within the present document. Examples of procedures for cell selection are described in 3GPP TS 05.08. The physical layer performs automatic crossover.