ABIS-interface

I hope it will be useful and a little helpful.It is good for who are searching in GSM, especially on Abis interface, because it is not easy to find the kind of this reference ,in my experience. However, i look forward to hearing from your feedback and improving ideas.

1. Abis -Interface Lecturer : Saut Sopheakk Subject : Mobile Telecom Group ​​​​​​ ​​​​​​ : 7 Group Members : NHEM Katanhou MENG Kimleng

2. Description Introduction Information transferred over the Abis interface synchronization information TRH-TRXC signaling Traffic In-band signaling DXU and TRI signaling LAPD LAPD Frame structure 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: 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 each transceiver , if no LAPD.

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 transmission has been interrupted 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. The binary BFI flag indicates whether the speech frame is considered to contain meaningful information or not. 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. SID: indicate if the channel decoder has detected the SID code word in the frame The SID code word consists of 95 bits which are all zero 4. In-band Signaling I n 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: Acknowledge: information transfer, where each signal is acknowledged when the frame has reached the destination successfully 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 TEI : Terminal Endpoint Identifier-the signaling links over the Abis interface are addressed to the different physical entities by the TEI 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) Traffic management TRX signaling 2.Operation maintenance link (OML) Serves for maintenance related info and transmission of traffic statistics. 3. Layer 2 management link (L2ML) 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 Command Address 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): Provide the error checking for the frame If error is found frame will be retransmitted Command: has two modes, acknowledged and unacknowledged C/R: This bit indicates whether it is command or response 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. 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 A ccess P rocedure on the D- channel 12- TEI : Terminal Endpoint Identifier-the signaling link 13- RSL: Radio S ignaling Link 14- OML: Operation M aintenance L ink 15- L2ML: Layer 2 Management L ink 16- FCS: Frame Check sequence 17- SID: S ystem I dentification N umber 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