The Abis interface connects the Base Station Controller (BSC) and the Base Transceiver Station (BTS) in a GSM network. It transfers synchronization information, signaling between the BSC and BTS, and traffic such as encoded speech. The Link Access Procedure on the D-channel (LAPD) protocol is used to provide acknowledged and unacknowledged signaling over the Abis interface using frames that contain flags, frame check sequences, commands, addresses, and information fields. Key information carried over Abis includes synchronization, signaling between the Transceiver Handler and Transceiver Controller, encoded speech using various coding schemes, and in-band signaling between the BSC and BTS components.

1. ABIS-INTERFACE
Lecturer : Saut Sopheakk
Subject : Mobile Telecom
Group : 7
Group Members : NHEM Katanhou
MENG Kimleng

2. DESCRIPTION
I. Introduction
II. Information transferred over the Abis interface
① synchronization information
② TRH-TRXC signaling
③ Traffic
④ In-band signaling
⑤ DXU and TRI signaling
III. LAPD
IV. LAPD Frame structure
V. Abbreviation

3. I. INTRODUCTION
 Nodes In BSS
• BTS (Base Transceiver Station) : The equipment that handles the radio traffic in one
cell.
• BSC (Base Station Controller): Control all the radios related function in the system.
 Abis is the name at the interface connecting the BSC and BTS nodes in the BSS.

4. II. INFORMATION TRANSFERRED OVER THE ABIS INTERFACE:
1. In the BTS there is a synchronization function
needed for air time slot synchronization.
2. TRH-TRXC signaling: refers to all signaling
between the BSC synchronization information
and BTS.
TRH: Transceiver handler, which resides in the BSC, is a functional
concept consist of application hardware and software.
TRH: Terminate the physical connection to the BTS. A TRH handles
eight physical connection or devices. Seven out of device is
used for supervision and testing.
TRXC: Transceiver controller is the control part of transceiver.
 One time slot on the Abis link is needed to carry the TRH-TRXC signaling for

5.  Speech processing functions consist of:
 speech transcoding
 Discontinue transmission (DTX) and voice activity Detection
(VAD)
 Comfort noise functions
 Speech frame substitution
 Speech transcoding: reduces the bits rate from 64kbps to 13kbps.
 Full rate Speech Coder (FR)
• Contains 260 bits for every 20 ms
• Coding scheme: Linear predictive coding-long term prediction-regular pulse
excitation (LPC-LTP-RPE).
 LPC&LTP
• Class Ia: bits are protected with error correction code and error detection
code
• class Ib: bits are protection with an error correction only
 RPE : Class-II bits are unprotected
3.
Traffic

6.  Enhanced full rate coder and half rate coder
 EFR speech encoders: speech connections with a quality
matching that of the fixed network can be offered
 HF speech encoders: the numbers of TCHs for a given number
of transceivers will double compared to FR case
 Discontinuous Transmission (DTX) and Voice Activity Detection
(VAD)
• DTX: is a mode of Operation where the transmissions are
switched on for frames containing useful information only
• VAD: determines whether a specific block of 20ms from the
speech coder contains speech or not
3. Traffic

7.  Comfort Noise Insertion
• generates background noise parameters to be used with DTX
operations.
• When Transmission is on, the background noise is transmitted with the
speech
• Comfort noise: is reduced in the system by insertion of an artificial for the
noise contrast effect
• Background noise: the parameters needed for generations of the comfort
noise
• SID-frames (silence Descriptor): the frames containing the estimated
background noise
 Lost speech frame substitution and Muting:
 In the receiver, Frames maybe lost due to transmission error or frame stealing
by FACCH. (Fast Associated control channel)
 Lost speech frame: is substituted by a predicted frame based on the previous
frame.
 Insertion of silent frame: will seriously decrease the speech quality
 Muting technique: must be used to indicate to the listener that the
3. Traffic

8.  The in-band information can be:
 Frame type : indicating whether the frame contains speech, data, O&M or if
it idle
 Channel type: which can be full rate, enhance full rate or half rate
 Bad Frame Indicator(BFI) and SID: are two identities that are used only in
the uplink direction from the channel decoder in the BTS to the Speech
decoder in the BSC.
o The binary BFI flag indicates whether the speech frame is considered to
contain meaningful information or not.
o BFI=1 indicates that bit errors have been detected among the class Ia
bits, and that this burst is to be substituted with the previous 20 ms
speech frame.
o SID: indicate if the channel decoder has detected the SID code word in
the frame
o The SID code word consists of 95 bits which are all zero
4. In-band Signaling
In GSM on Abis interface in-band signaling is used between the remote
transcoder Handler which controls the RTC (Remote Transcoder) from the BTS.

9. 5. DXU and TRI signaling: is a unit responsible for
distributing the information coming from the BSC to the
correction TRU/TRX. It is controlled from the BSC via
signaling over the Abis interface.
• The combination of BFI and SID determines whether or
not this frame is good and whether or not it contains
speech.
• In the downlink direction, there is a speech flag (SP):
indicates whether the burst contains speech or is idle.
 SID = 2 means more than 93 zeroes
 SID = 1 means between 93 and 79 zeroes
 SID = 0 means less than 79 zeroes
4. In-band Signaling

10. II. LAPD (LAYER2)
 All signaling message on the Abis interface use the link access procedure on
the D-channel (LAPD protocol)
 LAPD provide two kind of signaling:
o Acknowledge: information transfer, where each signal is acknowledged
when the frame has reached the destination successfully
o Unacknowledged: information transfer with no guarantee that the
information is successfully delivered
 LAPD is a layer2 protocol that operates at the data link layer of the OSI
architecture
 LAPD link handling: is a basic function to provide data link on the 64 kbps
physical connections between BSC and BTS. Links are provided for operation &
maintenance of the links for O&M of the BTS equipment and for transmission
of Layer 3 Abis messages.

11.  Each Physical connection can support a number of data links (logical connection). On
each physical connection each data link is identified by a unique TEI/SAPI
o TEI : Terminal Endpoint Identifier-the signaling links over the Abis interface are
addressed to the different physical entities by the TEI
o SAPI : Service Access Point Identifier-address the different functional entities within
one physical entity.
 LAPD has three sub signaling channels.
1.Radio signaling link (RSL)
o Traffic management
o TRX signaling
2.Operation maintenance link (OML)
o Serves for maintenance related info and transmission of traffic statistics.
3. Layer 2 management link (L2ML)
o management of the different signaling on the same time slot.
 SAPI = 0 for RSL
 SAPI = 62 for OML
 SAPI = 63 for L2ML

12. IV.LAPD FRAME STRUCTURE
Flag Fcs Info Length Comman
d
Addre
ss
Flag
 Flag: delimits a frame and has a pattern of 01111110. To avoid a flag
occurring inside a frame, a technique called bit stuffing is used. This technique
in the transmitter insert a ‘’0’’ after five consecutive “1”. At the receiver
removes the extra “0”.
 FCS (Frame Check sequence):
o Provide the error checking for the frame
o If error is found frame will be retransmitted
 Command: has two modes, acknowledged and unacknowledged
o C/R: This bit indicates whether it is command or response
o In acknowledged mode has N(s) and N (R)
 N (S): is a sequence number of frame sent.
 N (R): is the sequence number of frame expected to receive next.
o In unacknowledged mode contains only fixed bits.

13. V. ABBREVIATION
1- BTS : Base Transceiver Station
2- BSC : Base Station Controller
3- FR : Full rate Speech Coder
4- EFR: Enhanced full rate coder
5- HF : Half rate coder
6- DTX: Discontinuous Transmission
7- VAD: Voice Activity Detection
8- FACCH: Fast Associated control channel
9- BFI :Bad Frame Indicator
10- SP : Speech flag
11- LAPD :Link Access Procedure on the D-channel
12- TEI : Terminal Endpoint Identifier-the signaling link
13- RSL: Radio Signaling Link
14- OML: Operation Maintenance Link
15- L2ML: Layer 2 Management Link
16- FCS: Frame Check sequence
17- SID: System Identification Number
20, LPC-LTP-RPE (Linear predictive coding-long term prediction-regular pulse excitation )
18- RTC: Remote Transcoder
19- TRU/TRX: Transceiver Radio Unit/Transmission and Reception Unit