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•
Flag: Flag is the beginning and the end token of a frame. It is an 8-bit
of data starting and ending with 0s and containing six consecutive 1s
in between.
UI-frame Operation
The L3 message is sent in the no-serial-number frame mode. The receiver
is not required to send the received confirmation after receiving the UI-
frame. This operation mode does not provide flow control or error
recovering mechanism.
Figure 17 shows the UI-frame structure.
F
1 7 – L A P D U I F
I G U R E
Address
SAPI
TEI
UI-frame structure consists of the following fields:
•
Address field: It contains SAPI and TEI, which has the same function
as in I-frame Multi-frame Operation.
•
Control field: In the control field P represents the query bit; if this bit is
set to 1, it means to require the response frame from the peer entity.
•
Information field
LAPDm Protocol
In GSM, LAPDm is a data link protocol for signaling transmission between
MS and ZXG10-OB06 (V1.0). It uses the Dm channel to transmit
messages for entities of Layer 3 through the radio interface. LAPDm is
based on LAPD, with some simplification and modification.
In the ZXG10-OB06 (V1.0), LAPDm implements the following functions:
•
Provides point-to-point data-link connection and multiple services for
the upper layer. It identifies data-link connections by DLCIs in the
frames. The DLCI only contains SAPI, and indicates the service that is
accessed.
•
Provides identification of diversified frame types.
•
Supports transparent transmission of L3 message between L3 entities.
•
Provides sequence-control, to maintain the sequence of frames
connected through data link.
S
R A M E
T R U C T U R E
Control Information
000
P
0011
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Chapter 3 - Interfaces and Protocols
37
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