ZTE Corporation ZXG-OB06 Manuale Utente

 
ZXG10 OB06 ZTE  Integrated Outdoor GSM Base Station Technical Manual 

Confidential and Proprietary Information of ZTE CORPORATION 

The I frame structure of LapDm is shown in Figure 19. 

SAPI

N(S)

N(R)

Address

Control

Information

 

The I-frame in LapDm consists of the address field, control field and 

information field. 
The address field contains the SAPI. On the radio interface, LapDm 

supports two types of messages: signaling and short message service, 
distinguished by the SAPI. SAPI=0 represents the signaling link, and 
SAPI=3 represents the short message link. 
The maximum length of a LapDm frame on the TCH is 23 bytes, and 
21 bytes on the SACCH. The reason for this difference is that there are 

two special-purpose bytes in each SACCH block: Since the maximal 
length of the frame on the radio interface is of 21 or 23 bytes which 

cannot meet the need of most pieces of signaling, segmentation and 
regrouping are required to be defined in LapDm. Thus an “additional” 

bit is used to distinguish the last packet frame from other frames. 
Thanks to this mechanism, there will be no restriction to fix the packet 
length on the radio path, with the only exception when these messages 

must be transmitted on other interfaces, namely, 260 bytes mentioned 
in the radio interface specification. 
In the control field, N(S) represents the sending serial number and the 
I frame’s serial number currently sent by the sending end; N(R) 

represents the receiving serial number, the expected sending serial 
number of the next I frame. N(R) is used to predict the instruction 

from the receiving end. 

2.  UI frame operation 

The L3 message is sent in the no-serial-number frame mode, and 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.