TEMS Investigation for GSM INDEX 1) Understanding Signalling Channel Coding a) Channel Coding of Speech Signal b) Interleaving c) Silence Descriptor (SID Frame) d) Full and Sub Values (Rx Qual, BER, FERDTX Downlink Rate) 2) Starting TEMS Investigation GSM a) User Interface b) Connecting External Equipments c) Recording Functions d) Loading Maps e) Presentation Windows i. Radio Parameters ii. Current Channel f) Signalling Window g) System Information (Type 1 – Type 8) h) Call Assignment Mobile Originating Call / Mobile Terminating Call Location Update Disconnect Handovers 3) Analysis of Drive Test data a) Export Logfile b) Setup Settings c) Map Info Conversion d) Create Thematic Map Additional Signaling a) Signaling Layer 3 1) Connection Management sub layer i. Call Control ii. Short Message Service Support iii. Supplementary Services Support 2) Mobility Management sub layer i. Registration messages ii. Security messages iii. Connection Management messages 3) Radio Resource Management b) Signaling Layer 2 c) Signaling Layer 1
Transcript
TEMS Investigation for GSM
INDEX 1) Understanding Signalling Channel Coding
a) Channel Coding of Speech Signal b) Interleaving c) Silence Descriptor (SID Frame) d) Full and Sub Values (Rx Qual, BER, FERDTX Downlink Rate)
2) Starting TEMS Investigation GSM
a) User Interface b) Connecting External Equipments c) Recording Functions d) Loading Maps e) Presentation Windows
i. Radio Parameters ii. Current Channel
f) Signalling Window g) System Information (Type 1 – Type 8) h) Call Assignment
Mobile Originating Call / Mobile Terminating Call Location Update Disconnect Handovers
3) Analysis of Drive Test data a) Export Logfile b) Setup Settings c) Map Info Conversion d) Create Thematic Map
Additional
Signaling a) Signaling Layer 3
1) Connection Management sub layer i. Call Control ii. Short Message Service Support iii. Supplementary Services Support
2) Mobility Management sub layer i. Registration messages ii. Security messages iii. Connection Management messages
3) Radio Resource Management b) Signaling Layer 2 c) Signaling Layer 1
Signaling Channel Coding
Channel coding of Speech signal
1) First speech is sampled and segmented into blocks of 20 ms 2) Then it is compressed in speech coder to consist 260 bits. 3) 260 bits divided into three different classes
a) Very Important bits (50 bits) b) Important bits (132 bits) c) Not so important bits (78 bits)
Channel coding (Full Rate Traffic Speech Signal)
After Channel coder, 456 Output bits are Interleaved and segmented into Bursts, these are send over air Interface. At receiver end Channel decoder will transform 456 bits to 260 bits, these will pass through speech decoder and output of 20ms speech signal will be retrieved Channel Coder 456 Output bits (Interleaved and segmented into Bursts) Air Interface (456 Bits) 20 ms Speech Speech Decoder 260 Bits Channel Decoder
Channel Coding (Signalling): All Information bits are protected by FIRE code for error detection and all information bits are convolution coded.
Channel coding of signaling on Control channels 1) Now both speech and signaling frames are in 456 bits / frame. 2) 456 bits are split into 8 parts = 57 bits each
456 bits
57 57 57 57 57 57 57 57
Burst: Data Stream transmitted in one time slot A normal burst can contain 2 Blocks of 57 bits TAIL(3) DATA(57) FACCH
FLAG(1) TSC(26) FACCH
FLAG(1) DATA(57) TAIL(3) GUARD
(8.25) Interleaving Speech is interleaved over 8 half bursts; SACHCH is interleaved over 4 whole bursts.
E.g. a to z is speech frames and ‘A’ is a SACCH block as shown in the figure Half part of ‘a’ speech frame is transmitted over previous SACCH multiframe ‘z’ frame will be finished on next SACCH frame.
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SPEECH FRAME Mapping on SACCH (4 x 26 TCH multi Frame) fig: (SF1)
Four SACCH time slot = SACCH Block (contains system information BS MS, measurement report on Uplink channel)
IDLE time slots used for searching SCH (synchronization channel) burst on Neighboring cells holding the timing and BSIC value of the cell
. Now each burst lasts 0.577 ms (exactly 15/26 ms) and thus eight bursts last 4.615 ms 0.577 ms 4.615 ms
1 Time slot 8 time Slots for 8 users
1 SACCH period = 4.615 x 104 bursts (4X26 =104 BURSTS) = 480 ms
Silence Descriptor (SID frame): as shown in the figure n half bursts are silence descriptor frame
It is used when DTX (Discontinuous Transmission) is active and it contains parameters representing background noise on the microphone.
VAD (voice activity detector) continuously monitor each speech frame containing 20 ms of speech.
If VAD finds a silent frame, it analyzes background noise in speech frame and creates a SID frame which will replace the original silent speech frame.
As long as VAD does not detect any speech, one SID frame per SACCH multiframe will be sent.
FULL and SUB values FULL values are based on all frames on SACCH multiframe whether they are transmitted from BS or not
If DTX is used on DL the FULL value is invalid in that period and will give high BER, because they include bit error measurements in that period where nothing has been sent. SUB value is based on mandatory frames (always must be transmitted) on SACCH multiframe. “SACCH Block ‘A’ bursts, and SID frame ‘n’ bursts” as shown in fig SF1
RX Qual: Value: 0 to 7
Each value corresponds to estimated number of bit errors in number of bursts.
RX Qual BER (Bit error rate) qua 0 (BER < 0.2%) qua 1 (BER 0.2 ‐ 0.4%) qua 2 (BER 0.4 ‐ 0.8%) qua 3 (BER 0.8 ‐ 1.6%) qua 4 (BER 1.6 ‐ 3.2%) qua 5 (BER 3.2 ‐ 6.4%) qua 6 (BER 6.4 ‐ 12.8%) qua 7 (BER >12.8%0)
BER is calculated over four 26 Multiframes (1 SACCH Multiframe), on each TCH block (8/2 = 4 TCH bursts) and on SACCH block (4 SACCH bursts).
For each TCH block 378 class1 bits are used (50(Most Imp bits) + 3 (CRC) + 132 Important bits + 4 tail bits) x (coded @ 1/2)
SACCH block 456 bits are used. If TCH block is replaced by FACCH message, instead of 378, 456 bits are used
Number of TCH bits= (No of 26 Multiframes) x (No of TCH blocks per 26 Multiframes) x
(No of bits per TCH blocks) = (4 x 6 x 378) i.e. (4 x 6 x 378) + 456) = 9528 bits on each SACCH multiframe if TCH channel
and 3 x 456 = 1368 bits if SDCCH channel
BER (Bit Error Rate): ‐ After Channel decoder decoded 456 bit block, it is coded again and compare with 456 bits Input and the number of bits that differ in comparing both blocks results in BER. (Bit errors are accumulated in BER sum for each SACCH Multiframe; this bit error SUM is
divided total number of bits per SACCH Multiframe and classified (0‐7)).
FER (Frame Erasure Rate) {0 to 100%}: ‐ FER is based on number of blocks that have been discarded due to error in CRC (Cyclic redundancy check: ‐ (Protects 50 most important bits))
FER(%) = (No. of Blocks with incorrect CRC / Total No of Blocks) x 100
FER Full Total number of blocks on full rate TCH channel = 24 TCH + 1 SACCH = 25 Blocks
FER Full (%) = (No of blocks with incorrect CRC / 25) x 100
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E.g. FER 1 As shown in fig ‘f’, ‘i’, ‘k’ frames have incorrect CRC.
FER Full (%) = (3 / 25) x 100 = 12% a b
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E.g. FER 2 If ‘f’, ‘i’, ‘k’ and ‘A’ frames have incorrect CRC
FER Full = (4 / 25) x 100 = 16%
FER SUB Total number of mandatory blocks on full rate TCH Channel = 1TCH + 1 SACCH = 2 Blocks In e.g. FER 2. as shown in the figure If ‘f’, ‘i’, ‘k’ and ‘A’ frames have incorrect CRC FER SUB only counts SACCH (‘A’) and SID (‘n’) frames. So there is an error in ‘A’ but there is no error in ‘n’
FER SUB = (1 / 2) x 100 = 50%
DTX Downlink Rate: This Indicates that how many TCH frames were not sent to MS during last SACCH multiframe, this vary from 0 to 96% Can never be 100% because frame containing SID information must be sent for each SACCH multiframe.
DTX DL Rate (%) = (No of silent blocks / total No of Blocks) x 100 Each SACCH Multiframe has 24 TCH frames, so total No of blocks = 24 It is an Indication whether DTX DL is used in GSM Networks as there is no parameter in GSM to tell mobile if DTX DL is used or not If DTX rate is very high for whole period of time during a call there could be a silent call problem in the network
Any problem with silent call located before the voice activity detector (VAD) in the Transcoder unit (TRAU) in Base station subsystem (BSS) can be found using DTX DL rate information element.
STARTING TEMS INVESTIGATION TEMS Investigation is an air interface test tool for real‐time diagnostics. You can monitor voice channels as well as data transfer over GPRS, circuits witched (CSD) or high‐speed circuit‐switched (HSCSD) connections. Starting TEMS Investigation Choose Start −> Programs −> TEMS Products −> TEMS Investigation GSM.
User Interface Workspace and Worksheets This will store window settings in your working session
Toolbars To access the central Functions
Navigator To open presentation windows, Threshold values there presentation colors can be changed from here
Menu Status This will display symbols and short messages which indicate the current status of the application
Connecting External Equipments Semi‐Automatic Enabling: Using the Identify Equipment Function
Connect the TEMS mobile station to a COM port.
Identify Equipment: application starts to scan the selected COM ports for external devices
For TEMS mobiles, TEMS and DATA cables are detected as separate units
The TEMS cable will be designated by “MSn” and the DATA cable by “DCn”, where “n” is always identical for the two cables connected to the same mobile. Scanners are named “MSn” and GPS units “PSn”. The detected devices are automatically enabled, but not connected; this state is indicated by a red‐light symbol to the left of each device
Manual Enabling: Adding One Device at a Time Select Add Equipment
Select the correct COM port Select the type of external device
R520m” for the TEMS cable, and choose “R520m Data Cable” for the DATA cable. The two cables will be treated as different devices in TEMS Investigation.
Connecting External Equipment
Connect the Equipment
Connect All the Connections
Disconnecting External Equipment
Disconnect Connection
Disconnect all Connections
RECORDING LOGFILES
Start recording
Mention the path to store the recorded logfile.
Recording functions
File marks are text strings which can be inserted manually in a logfile to mention the special event noticed during drive Test and that can be easily find out while replaying the Log Files.
LOADING MAPS IN TEMS
1. Open the MAP window and click OPEN MAP
2. Select the Map Info File from the respective location.
3. Select MapInfo site and cell file
4. Select layer control for making changes in visibility of the selected MapInfo files.
Presentation Window We can select many more windows like AMR parameter windows, MAP on GSM window according to our need and convenience.
Serving + Neighbors Current Channel Line Chart Radio Parameters
Serving + Neighbors: ‐ Shows BSIC, ARFCN and RxLev for the serving cell and its neighboring cells, with the serving cell at the top and the neighbors below it, sorted by signal strength in descending order.
DEDICATED MODE
Cell Name : Cell site Describes in cell file.
ARFCN: (Allocated radio frequency channel) BSIC : Base Station Identification code. RxLev : Receiving Level in dBm.
C1 & C2 : Cell path loss parameter and cell reselection parameter. (In idle mode). C31 & C32 : GPRS signal strength threshold and GPRS cell ranking criterion. Valid
in both packet idle and packet dedicated mode. IDLE MODE
C1: PATHLOSS PARAMETER
Radio Criteria C1 = (A ‐ Max (B, 0))
A = Received Level Average ‐ p1 B = p2 ‐ Maximum RF Power of the Mobile Station p1 = rxLevelAccessMin p2 = msTxPowerMaxCCH
C2: CELL RESELECTION PARAMETER C2 = C1 + cellReselectOffset ‐ temporaryOffset x H(penaltyTime‐T) when penaltyTime≠640
Or C2 = C1 ‐ cellReselectOffset when penaltyTime=640
H(x) =1 when x>=0 H(x) =0 when x<0
RADIO PARAMETERS
Gives the status of the radio link (current BCCH, signal strength, FER(Frame Erasure Rate), BER (Bit Error Rate), SQI (speech quality Index), DTX (Discontinuous Transmission), TA (Timing Advance).
RX LEV : Channel RxLev (dBm) (–120 . . . –10dBm) Measured signal strength level for current channel.
RX QUAL : Voice quality measured on BER (0 . . . 7)
FER : Frame Erasure Rate
(Percentage of frames being dropped due to high number of non‐corrected bit errors in the frame).
BER Actual : (Number of bit errors / Number of bits transmitted)
SQI : Speech Quality Index.
SQI updated at 0.5 s intervals. Computed on basis of BER and FER.
MS Power Control Level: Power control (0 to 8) depending upon network design. E.g. 0 means no power control, 1 means level that is defined by operator viz. 2 dBm
DTX : Discontinuous transmission (DTX)
Radio transmitter switched off during speech pauses. It reduces the power consumption of the transmitter & decreases overall interference level on the radio channels affecting the capacity of the network..
TA : Timing Advance.
Base station calculates from access bursts and sends to the mobile station (MS) enabling the MS to advance the timing of its transmissions to the BS so as to compensate for propagation delay.
CURRENT CHANNEL
Time : System time of computer.
Cell name : Displays name of serving sector (from cell file that is loaded in TEMS).
CGI : Cell Global Identity Unique for every sector of the site. CGI = MCC + MNC + LAC + CI.
MCC : Mobile Country Code 0 – 999 (e.g. 404 India), MNC : Mobile Network Code 0 – 99 (e.g. 98) LAC : Location Area Code 0 ‐65535 (e.g. 5129) CI : Cell Identity 0 – 65535 (e.g. 2723).
Band : Freq. Band mobile is operating e.g. GSM 900/ 1800.
BCCH ARFCN : Broad Cast Channel – Allocated Radio Frequency Channel.
TCH ARFCN : Traffic Channel (Frequency).
BSIC : (Base Station Identity Code)
BSIC = NCC (Network Color Code 0‐7) + BCC (Base Station Color Code 0‐7) Time slot : Current TCH. (Time slot number of TRX).
Channel Type : Current Channel of mobile E.g. BCCH / SDCCH/8 + SACCH/C8 or CBCH / TCH/F +FACCH/F +SACCH/F.
Channel Mode : Mode of coding (e.g. Speech Full Rate or Half Rate).
Speech Codec :
FR = Full Rate, HR = Half Rate & EFR = Enhanced Full Rate.
Ciphering Algorithm : Ciphering algorithm used by the system ‐ Security
E.g. Cipher by A5/2.
Sub Channel Number : SDCCH of MS out of 8 available. E.g. = 2.
Hopping Channel : Hopping available or not on particular Sector. (Yes or No)
Hopping Frequencies : Displays no. of frequencies allotted for hoping, particular sector. MA (Mobile Allocation) List.
Mobile Allocation Index Offset (MAIO):
Number which tells from which frequency from given MA list of a sector hopping is to be started. E.g. 0 Hoping will start from first frequency.
Hopping Sequence Number (HSN):
Hopping Sequence of frequencies (from the MA List). (0 – 63). 0 : Cyclic Hopping
1 – 63 : Random hopping sequences.
SIGNALLING WINDOW In signaling our main concern is of Layer 3 messages
System Information Information about the network which MS need to communicate with the network. System information messages are continuously sent on the BCCH and SACCH by the BTS to all idle (BCCH) and active (SACCH) mobiles in a cell.
System Information 1. Cell channel description RACH control parameters
2. Neighbor cells description NCC permitted RACH control parameters Abis. Neighbor cells description (extension) RACH control parameters
Ater. Additional multiband information Neighbor cells description (other bands)
3. Location area identification Cell identity Control channel description Cell options Cell selection parameters RACH control parameters SI 3 rest octets
4. Location area identification Cell selection parameters RACH control parameters CBCH channel description CBCH mobile allocation SI 4 rest octets
5. Neighbor cells description
Abis Neighbor cells description (extension)
Ater Additional multiband information Neighbor cells description (other bands)
6. Location area identification Cell identity Cell options NCC permitted
7. SI 7 rest octets
8. SI 8 rest octets
9. Packet data information
1) System Information Type 1
In frequency hopping MS needs to know which frequency band to use and which frequencies within the band to be use during hopping. This information is provided in the cell channel description information element sent in system information type 1. Information about how the MS should perform to access the system is also provided.
Cell Channel Description MS is informed about frequencies that are used in the cell.
GSM 900: ‐ “bit map 0” format is used Information element is divided into two parts:
CA NO: ‐ Cell allocation number shows which band is used. CA‐NO = 0; GM 900 CA‐NO = 1; E‐GSM CA‐NO = 2; GSM 1800.
CA ARFCN: ‐ Absolute radio frequency channel number for all frequencies used in the cell. A 124 bits bit map is used. A bit set to 1 indicates that the frequency with that number is used in the cell.
GSM 1800 and GSM 1900: ‐ Several different formats can be used, Information element is divided into two parts
FORMAT ID: ‐ Indicates format of the information element. Second part of the information element represents the frequencies through special encoding schemes
RACH Control Parameters
2) System Information Type 2
List of BCCH frequencies used in the neighboring cells. The MS needs this information because it must listen to the system information in the neighboring cells occasionally. The MS also uses this list of frequencies when measuring the signal strength of neighboring cells. The MS is also informed which PLMN Network Color Codes (NCC) it may monitor.
Neighbor Cells Description: ‐ Denote frequencies of the BCCH carriers to be monitored by the MSs in the cell.
3) System Information Type 3 The MS must know the current location area’s identity because a change in location area means that the MS must update the network. In order to calculate its paging group, the MS needs specific parameters contained in the control channel description. The description also informs the MS about periodic registration and it informs MS if it should inform the system when it is about to enter the idle mode. When the MS is in idle mode, it decides by itself which cells to camp on. Information needed by the MS for cell selection and reselection is also broadcast in system information type 3.
4) System Information Type 4 (Cell broadcast Function: Allows broadcasting short messages to all MSs in one or more cells). In system information type 4, MSs are informed if the cell broadcast function is used in this cell and on what frequency the CBCH is found. The LAI, the cell selection parameters, the RACH control parameters and rest octets also included in type 4 message.
5) System Information Type 5 When MS in busy mode, SACCH is activated. On the uplink, the MS sends measurement reports, and on the downlink the network sends output power and TA for the MS to use. Also MS receives information about the frequencies used as BCCH carriers in neighboring cells on SACCH. Signal strength of these frequencies are monitored and reported in the measurement report for handover purposes. Frequencies in the neighbor cells description given here may differ from those sent in system information type 2. Active mode: MS measures on a reduced number of BCCH frequencies in order to improve the accuracy of the measurements. In Idle mode: MS measures on a greater number of frequencies to reduce the time required to establish contact with the network after power on. This time reduction occurs only if the idle BA list was stored at the previous power off.
6) System Information Type 6 In active mode, MS needs to know if the LAI changes If LAI changes the MS has to do location updating when the call is released. If MS changes between cells (within the location area) where RLINKT or DTX conditions differ the new cell options must be reported to the MS. LMN permitted is also included in the system information type 6.
7) System Information Type 7 System information type 7 is optionally sent on BCCH extended if system information type 4 does not contain all information needed for cell reselection.
8) System Information Type 8 System information type 8 is optionally sent on BCCH extended if system information type 4 does not contain all information needed for cell reselection.
Call Assignment
Call assignment takes place when a Mobile Station makes a call (Mobile Originating Call) or receives a call (Mobile Terminating Call).
1) Mobile Originating Call
Mobile Originating a Call
Mobile Terminating a Call
Mobile Originating a Call
2) Location Update
The MSC needs to know under which location area the Mobile Station can be reached and Location Area Information is needed for the paging made by the BTS.
3) Disconnect When the Mobile Station or the Network want to finish a call for some reason
a) Network Initiated
b) Mobile Station Initiated
4) Handovers Different protocols are for different handover processes, e.g. in synchronized handover, no timing advance information is needed. This decreases the protocol so that no physical information needs to be sent.
a) Synchronized Handover
b) Non‐Synchronized Handover
c) Handover Failure
Analysis of the Drive Test Data
1. EXPORT LOGFILES: Export the Log Files to and conversion to .tab format
Select the Format of the file as Map info Tab-file; as shown in the figure 2) Change the Setup Settings for MapInfo Tab‐file
Select the Information Elements from the available Information Elements in the desired Technology (GSM) e.g. ARFCN‐BCCH, Rx Lev Full, Rx Lev Sub, Rx Qual Full Rx Qual Sub, Speech Quality Index (SQI) You can also save the Selected Information Elements settings (.mex format).
Now select the Input files (you can select more then one files all together) Select the Output directory where you want to save the Output Name of the Output file will be from Prefix and Suffix
Now Start the procedure for exporting the Log file
As the Export is done successfully the export Results will be displayed as shown in the fig
3) Open the Map‐Info Converted Log Files into Map Info Tool
You can select different types according to your requirement E.g. Ranges, Bar Chart, Pie Charts, Graduated, Dot Density, Individual, Grids.
E.g. if we have selected Rages in this Example for Rx Level
E.g. Field: RxLev Sub You can modify the Rages, Style and Legends according to your requirement, as shown in the figure the Ranges and Style are modified
Customizing Range and Style
Rx Level SUB (defined as Range) Similarly according to the requirement the Thematic Map can be drawn, E.g. for ARFCN (BCCH), we have to take the Individual Values not the Ranges as we have taken for RxLev
ARFCN (BCCH) (defined as Individual)
REPORT GENERATION IN TEMS INVESTIGATION
Go to Log File ‐ > Report Generator
Or select
Report Wizard will get open
Add the Log files from there respective locations Select the Output directory
You can change the Report Properties as per your requirements, as shown in the figure for Call Events and Threshold Values Select Finish for completing the Task
The Statistics Report will be generated in the Output directory defined Open the index File from Statistics Report Directory as shown in the figure.
TEMS™ Investigation GSM 4.1.1
Report No ___________________ Date 2007‐04‐06 Time 12:56 Prepared by ___________________
As shown in the above Stats we can easily find out that which are the Log Files where particular Even had have happened. E.g. Blocked Call: log file 3, 4, 5, 6 and 21
Dropped Call: log file 6 We can replay these Log files and can find out the reason for the same by studying the Radio Parameters at that particular event.
Distribution graphs of all log files
Additional
SIGNALING
Radio Interface Protocol Structure SIGNALING LAYER 3 Layer 3 provides the Mobile Network Signaling (MNS) service to the user application. It Includes
Functions to establish, maintain and terminate circuit switched connections across a GSM PLMN and other networks to which the PLMN is connected.
Supporting functions for supplementary services and short message service control. Functions for mobility management and radio resource management.
Protocol control entities exist in the three sub layers:
• Connection Management (CM) sub layer • Mobility Management (MM) sub layer • Radio Resource management (RR) sub layer
The RR functions reside mainly in the BSC, although some RR functions may reside in the MSC. In the BTS, most of the RR messages are handled as transparent messages.
Um layer 3, distribution of signaling functions
CONNECTION MANAGEMENT CM sub layer contains functions for:
Call Control and call related supplementary services management (CC). Short Message Service (SMS). Non call related Supplementary Services management (SS).
Call Control Call Control signaling procedures are described as:
Call establishment procedures Procedures during the active state Call clearing Miscellaneous procedures
Short Message Service Support (SMS): It comprises of
Short Message Control (SMC) Short Message Control Protocol (SMCP): Peer control for transfer short messages between MS and MSC
Supplementary Services support (SS) Not related to a specific call. E.g. registration of call forwarding on no reply or call waiting.
CM service accept CM service reject CM service abort CM service request CM reestablishment request Abort Miscellaneous message: MM status
RADIO RESOURCE MANAGEMENT The RR sublayer receives service from layer 2 and gives service to the MM sublayer. In addition, RR communicates directly with layer 1 for exchange of information related to measurement control and channel management. The general purpose of the RR procedures is to establish, maintain and release a RR connection between the MS and the network. This includes handover procedures, cell selection at power on and in idle mode, recovery from lack of coverage in idle mode as well as cell re‐selection in busy mode.
Channel establishment messages: Ciphering messages: Handover messages: Channel release messages: Paging messages: System information messages: Miscellaneous messages:
SIGNALING LAYER 2 Link Access Procedures on the Dm channel (LAPDm) is the layer 2 protocol used to convey signaling information between layer 3 entities across the radio interface, using the Dm channel. Dm channel refers to the control channels. Includes broadcast, common or dedicated control channels. LAPDm is a protocol that operates at the data link layer of the OSI structure. Its purpose is to provide a reliable signaling link. It receives services from the physical layer and provides services to layer 3. LAPDm is based on the ISDN protocol LAPD, which is used on the Abis interface. Two types of operation on the data link are supported by LAPDm.
UNACKNOWLEDGED OPERATION Messages that need not be acknowledged are sent in Unnumbered Information (UI) frames. This means that there is no flow control or error recovery mechanism defined.
ACKNOWLEDGED (MULTIPLE FRAME) OPERATION When an answer or acceptance is required, operation in acknowledged mode is applied. Acknowledged mode is applicable on dedicated control channels only. Layer 3 messages are sent in numbered I frames. In this case, a number of consecutive I frames (a window) can be sent before an acknowledgment is required. However, for LAPDm the size of the window is one, which means that each frame must be acknowledged before the next one is sent.
SIGNALING LAYER 1
The signaling layer 1, also called the physical layer, represents the functions required to transfer the bits over the physical channels, on the radio medium. In addition to signaling layer 2, layer 1 interfaces other functional units, such as speech coder and terminal adapters, for the support of traffic channels.
