Layer 2 Signaling

The LAPDm protocol is the data link protocol for signaling channels at the air interface. It is similar to HDLC. It provides two operational modes:

• Unacknowledged operation

• Acknowledged operation

In the unacknowledged operation mode, data is transmitted in UI frames (unnumbered information) without acknowledgement; there is no flow control or L2 error correction. This operational mode is allowed for all signaling channels, except for the RACH which is accessed in multiple access mode without reservation or protection.

The acknowledged operation mode provides protected data service. Data is transmitted in I frames (information) with positive acknowledgement. Error protection through retransmission (ARQ) and flow control are specified and activated in this mode. This mode is only used on DCCH channels.

In LAPDm, the Connection End Points (CEPs) of L2 connections are labeled with Data Link Connection Identifiers (DLCIs), which consist of two elements:

• The Layer 2 Service Access Point Identifier (SAPI) is transmitted in the header of the L2 protocol frame.

• The physical channel identifier on which the L2 connection is or will be established, is the real Layer 2 Connection End Point Identifier (CEPI). The CEPI is locally administered and not communicated to the L2 peer entity. (The terminology of the GSM standard is somewhat inconsistent in this case - what is really meant is the respective logical channel. The physical channels from the viewpoint of LAPDm are the logical channels of GSM, rather than the physical channels defined by frequency/time slot/ hopping sequence.)

When a Layer 3 message is transmitted, the sending entity chooses the appropriate SAP and CEP. When the service data unit SDU is handed over at the SAP, the chosen CEP is given to the L2 entity. Conversely, when receiving an L2 frame, the appropriate L2-CEPI can be determined from the physical/logical channel identity and the SAPI in the frame header.

Table 7.2: Logical channels, operational modes and Layer 2 SAPIs

Logical channel


SACCH assoc. with SDCCH SACCH assoc. with TCH




Unacknowledged and acknowledged



Unacknowledged and acknowledged

Unacknowledged and acknowledged

Unacknowledged and acknowledged

Unacknowledged and acknowledged

Sdcch And Tch
Figure 7.19: Sample configuration of the MS data link layer Specific SAPI values are reserved for the certain functions:

In the control plane, these two SAPI values serve to separate signaling messages from packet-oriented user data (short messages). Further functions needing a new SAPI value can be defined in future versions of the GSM standard.

An LAPDm entity is established for each of the pertinent physical/logical channels. For some of the channel/SAPI combinations only a subset of the LAPDm protocol is needed (e.g. unacknowledged operation), and some channel/SAPI combinations are not supported (Table 7.2). These LAPDm entities perform the Data Link procedure, i.e. the functions of the L2 peer-to-peer communication as well as the service primitives between adjacent layers. Segmentation and reassembly of Layer 3 messages is also included.

Further Layer 2 procedures are the Distribution Procedure and the Random Access (RA) procedure. The distribution procedure is needed if multiple SAPs are associated with one physical/logical channel. It performs the distribution of the L2 frames received on one channel to the respective data link procedure, or the priority-controlled multiplexing of L2 frames from multiple SAPs onto one channel. The random access procedure is used on the random access channel (RACH); it deals with the random controlled retransmission of random access bursts, but it does not perform any error protection on the unidirectional RACH.

Layer Signaling
Figure 7.20: LAPDm frame formats

For certain aspects of RR, the protocol logic of Layer 3 has to have direct access to the services of Layer 1. Especially, this is needed for functions of Radio Subsystem Link Control, i.e. for channel measurement, transmitter power control, and timing advance.

A possible link layer configuration of an MS is shown in Figure 7.19. The base station has a similar configuration with one PCH + AGCH, SDCCH and SACCH/FACCH for each active mobile station.

Figure 7.2O shows the different types of protocol data frames used for communication between L2 peer entities in MS and BTS. Frame formats A and B are used on the

SACCH, FACCH and SDCCH channels, depending upon whether the frame has an information field (Type B) or not (Type A). For unacknowledged operation (BCCH, PCH, AGCH), format types Abis and Bbis are used on channels with SAPI = 0. The Abis format is used when there is no information to be transmitted on the respective logical channel.

In contrast to HDLC, LAPDm frames have no flag to designate beginning and end of a frame, rather the delineation of frames is done as in RLP at the link level (see Section 7.2.3) through the fixed-length block structure of Layer 1. The maximum number of octets N201 per information field depends on the type of logical channel (Table 7.3). The end of the information field is given by a Length Indicator, a value of less than N201 indicates that the frame has to be supplemented with fill bits to the full length. In the case of an SACCH channel, for example, this yields a fixed-length LAPDm packet of 21 octets. Combined with the fields for transmitter power control and timing advance, an SACCH block of Layer 1 is thus 23 octets long.

The address field may have a variable length, however; for use on control channels it consists of exactly one octet. Besides other fields, this octet contains an SAPI (3 bits) and the Command/Response (C/R) flag known from HDLC. In LAPDm, the coding of the control field with sending and receiving sequence numbers and the state diagram describing the protocol procedures are almost identical to HDLC [31]. Some additional parameters are required at the service interface to Layer 3; for example, a parameter CEP designating the desired logical channel. Furthermore, the LAPDm protocol has some simplifications or peculiarities with regard to HDLC:

• The sending window size is restricted to k = 1.

• The protocol entities should be implemented in such a way that the state RECEIVER BUSY is never reached. Thus RNR packets can be safely ignored. The HDLC polling procedure for state inquiry of the partner station need not be implemented in LAPDm.

• Connections to SAPI = 0 are always initiated by the mobile station.

In addition, the repetition timer T200 and the maximum number of allowed repetitions N200 have been adapted to the special needs of the mobile channel. In particular, they have their own value determined by the type of logical channel.

Table 7.3: Logical channels and the maximum length of the LAPDm information field

Logical channel



18 octets


20 octets


22 octets

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