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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Control field of this layer-2 contains sequence numbers and type field to differentiate various frames. Information transfer format I frames. Figure shows the Layer 3 message structure for a transparent message.

The frame check sequence FCS is used to detect errors in a frame. SAPI takes either value of 0,3 or llapd values. However, in GSM only three protockl, as given in Tableare used. Cantata has also simplified the Subrate point-to-multi-point connect and disconnect messages. Layer 2 is the data link layer, which does following three main functions.

The information field is present in all I frames and U frames that transfer information, i. It is often referred to as the Matrix Controller.

The length 8 or 16 gsn of control field depends on the frame type and also on the sequence numbering used, i. Unlike a reject frame, the FRMR is used to report an error condition that cannot be recovered by retransmission of frame.


Please help improve the article with a good introductory style. The transmitting side uses the DM frame to indicate that it laps no longer maintain the Layer 2 connection. A digital communications link – voice, data, and video at a 2. The exact configuration of Abis links depends on the traffic requirements, TRAU location, and equipment capabilities.

In addition, it serves the traffic management procedures of Layer 2.

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

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.

Supervisory format S frames. The LAPD can also serve as the user side. Protocol layer of the CSP. SABME frame begins a data link connection for acknowledged information transfer service. The DCM contains messages related to status and control of Layer 1 of the air interface. The messages sent over RSL are divided into four groups. A flag is hex 7E. For consecutive frames, one frame is used to indicate prltocol end of a first frame and the beginning of the next frame.

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

The physical layer, i. In the example shown, the message discriminator is set to 1, indicating an RRM message. The traffic channels at the Um interface have almost the same data rates. The retry mechanism allows the service card to resend any unacknowledged messages to the host on a regular interval for a defined number of retry times.


Unnumbered information and control format U frames. Transparent messages pass through the BTS without any decoding and action. The EAbit of the second octet is set to 1, indicating that it is the last octet of the address field.

S frames handle Layer 2 flow control management, such as acknowledging the I frames and requesting retransmission and temporary suspension of I frames. Unsourced material may be challenged and removed. Link access protocols Integrated Services Digital Network. Figure illustrates the protocol stack on the Abis interface. This has the obvious advantage of better link utilization. All pre-existing Layer 2 functionality is still supported.

See Integrated Services Digital Network. The SAPI is 6 bits long and can have a value from 0 to The uppercase letters in the message name are mnemonics used in context and protocol presentations.

Dedicated channel management DCM. Each octet in the address field has one address extension EA bit. The TEI is 7 bits long and hence can have a value from 0 to The bit T is set to 1, indicating a transparent message.