LAPDm is a modified version of LAPD (Link Access Protocol in the D channel) It is used in GSM to support the transport of information between the mobile and. can any one tell me signalling overlay of GSM and the type of signalling between all nodes of GSM LAPD protocol is part of SS7 signalling. We can use GSM as a basic framework to define and develop the standards protocol architecture, based on the use of worldwide .. used in LAPD. LAPDm.

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Figure illustrates the protocol stack on the Abis interface. The following sections describe each layer in detail. The physical layer, i. Thirty timeslots carry speech or user data, and the remaining two timeslots are used for synchronization and signaling protoocol. This has the obvious advantage of better link utilization. It also enables mapping of traffic channels at the Um interface directly to Abis.


The traffic channels at the Um interface have almost the same data rates. The exact configuration of Abis links depends on the traffic requirements, TRAU location, and equipment capabilities. The Data Link Layer, i. It also ensures error-free transmission of information between communicating entities.

The SAPI is 6 bits long and can have a value from 0 to However, in GSM only three values, as given in Tableare used. The TEI is 7 lald long and hence can have a value from 0 to The values from 64 to are used for additional TRX addresses in cases where TRX needs more than one signaling link.

In addition, it serves the traffic management procedures proticol Layer 2. Figure shows protocok LAP-D frame structure. The flags indicate the beginning and the end of a frame.

For consecutive frames, one frame is used to indicate the end of a first frame and the beginning of the next frame. A flag is hex 7E. In order to avoid repetition of this pattern within the information field, a zero is inserted after every orotocol consecutive ones.

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This is called bit stuffing. Each octet in the address field protocoll one address extension EA bit. In the first octet, it is set to zero, indicating that one more address octet is to follow. The EAbit of the second kn is set to 1, indicating that it is the last octet of the address field.

The BSC network side does the opposite, i. Information transfer format I frames. Supervisory format S frames. S frames handle Layer 2 flow control management, such as acknowledging the I frames and requesting retransmission and temporary suspension of I frames.

Unnumbered information and control format U frames. U frames provide additional transfer capabilities during unacknowledged transfer service or additional unacknowledged transfer service. N S and N R bits are not used. Figure shows the two different formats of the control field.

GSM Interfaces and Protocols (Global System for Mobile Communication (GSM)) Part 2

The length 8 or 16 bits of control field depends on the frame type and also on the sequence numbering used, i. Table describes the different format and frame types. Table describes the functions of different frame types. The information filed is of variable length and carries Layer 3 information. A maximum of octets can be sent over LAPD.

The information field is present in all I frames and U frames that transfer information, i. It is not present in S lapf U frames with only one exception, i.

The frame check sequence FCS is used to detect errors in a frame. I frame carries Layer 3 information across a data link connection during acknowledged transfer service. It is used to indicate that a data link layer entity is busy and no more I frames can be accepted. A reject command frame requests retransmission of I frames starting with a frame numbered N R. As a response, an REJ frame indicates the clearance of a busy condition. The transmitting side uses the DM frame to indicate that it can no longer maintain the Layer 2 connection.


UI frame carries Layer 3 information across a data link connection during unacknowledged transfer service. The transmitting side indicates its intention to tear down the Layer 2 connection by sending a DISC frame.

Unlike a reject frame, the FRMR is used to report an error condition gxm cannot be recovered by retransmission of frame. For example, a protocol error detected in a Layer 3 message cannot be set right simply by retransmission of a frame.

Transparent messages pass through the BTS without any decoding and action. The CM and the MM messages are examples of the transparent messages. The BTS does not ;rotocol these messages. However, the RR layer contains messages of both types.

The nontransparent messages, in this case, are those related to radio equipment and need to be handled by the BTS. Table lists the SAPI assigned to each logical channel. The messages sent over RSL are divided into four groups.

Radio link layer management RLM. Common channel management CCM. Dedicated channel management DCM. The DCM contains messages related to status and control of Layer 1 of the air interface. Figure shows the Layer 3 message structure for a transparent message. In the progocol shown, the message discriminator is set to 1, indicating an RRM message.

The bit T is set to 1, indicating a transparent message. The uppercase letters in the message name are mnemonics used in context kapd protocol presentations. Figure Protocol stack over the Abis interface. SABME frame begins a data link connection for acknowledged information transfer service.