01 Interface&Signalingnew

Interface and Signaling SSMC Training Center

Network optimization SSMC Training Center 2003 SSMC TI

1

Interface and Signaling

Contents 1 Air Interface - Um 2 1.1 Layer 1 - Um 5 1.2 Layer 2 - Um 45 1.3 Layer 3 - Um 53 2 Interface Abis 71 3 A-Interface/Signaling System CCS7 104 4 Signaling Sequences 148 4.1 Complete Call Sequences 149 4.2 Message Flow of Basic Circuit Switched BSS Procedures 153 5 TEMS Investigation 176 6 K1205 Protocol Tester 196

SSMC Training Center Air Interface - Um

1 Air Interface - Um

SSMC Training Center Network optimization 2003 SSMC TI

2

Air Interface - Um SSMC Training Center

The MS is linked to the fixed PLMN structure via a radio link. The air or radio interface Um describes the radio link function. The MS/BSS interface must ensure: z z

z

use of the same standard interface by the MS and terminal equipment (TE) use of MSs from different manufacturers in the whole system area of the GSM network connection with terminal equipment using the same identifiers and codes independent of the respective location of the unit

The transmission of speech, data and signaling is carried out on the air interface Um via radio channels (RFCs). The RFCs form layer 1 of the GSM system air interface. Layer 1 (Um) is described in GSM-Rec. 04.04.


3


Layer 3

CC SS SMS

CM MM RR

Layer 2

Layer 1

Logik

Physik

Fig. 1 Layer 1 - 3 of the air interface Um

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1.1 Layer 1 - Um Speech and data transmission on the air interface Um is carried out via the physical channels. A physical channel is defined by a specific carrier pair (RFC = Radio Frequency Channel) in the UL and DL and the number of the time slot in the TDMA frame. Layer 1 (physical Layer: GSM 04.04) is a physical bi-directional point-to-point connection in multiframe mode. Layer 1 communicates with layer 3 directly according to channel management and measurement control. The physical layer will offer layer 2 appropriate channels by usage of the following functions: z Burst transmission z Error correction and -detection z Supervision of RSS Link Control Furthermore the layer 1 protocol defines the mobile station's search for a suitable BCCH and the seizure of DCCH through the MS (after allocation by the base station)


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RFC174

RFC1

RFC2

7

6

5

4

3

2

1

RFC3

0

TDMA

UL

FDMA

7

6

5

4

3

2

1

RFC3

0

7

6

5

4

3

2

1

RFC2

0

RFC1

RFC174

TDMA

FDMA

DL

Fig. 2 Physical channel in the FDMA and TDMA frame

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1.1.1 The TDMA Frame

A TDMA frame is divided into 8 time slots (= TS). The whole TDMA frame lasts 4.615 ms, an individual time slot 0.577 ms.

A physical channel is assigned exactly one time slot TS in the TDMA frame. Each subscriber receives a time slot and sends all 8 time slots1 once. Transmission is not allowed outside the allocated time slots TS (i.e. after the TS has expired) so other physical channels are not exposed to interference.

1With full rate transmission; with half rate transmission every 16 time slots; HSCSD and GPRS are not yet taken into consideration


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TDMA-frame

4,615 ms

0 1 2 3 4 5 6 7

0,577 ms

0 1 26 7

Fig. 3 Assignment / repeat of a TDMA frame

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1.1.2 Burst / Burst Types Sending the information in the individual time slot of 0.577 ms with a permanently defined bit sequence is called a burst. The burst is realized by the MS by switching on, transmitting briefly and switching off the transmitter again.


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Time structure of the time frame / bit sequence A time slot is defined at a time frame of 577s (to be precise: 576 12/13 s 576.923 s). This time frame is divided into 156.25 bit. This means an individual bit has duration of 3.6923 s. The 156.25 bits are used as follows: z 142 bits for information transmission, z 3 bits each as tail bits (TB) for edge limitation of the time slots. They are also

used as protection zones if a neighboring channel happens to interfere with the first or lasts bits.

z 8.25 bits as a guard period (GP) (exception: the GP for access burst is 68.25 bits long) for collecting variable run or reception times (determined by the distance BTSE MS).


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Burst

. . . 7 0 1 2 3 4 5 6 7 0 1 . . .

TB3 142 ? Information

Burst

. . . 7 0 1 2 3 4 5 6 7 0 1 . . .

TB3 142 ? Information

TB3

GP8,25



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Fig. 4 Breaking down a time slot into bits


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Realization of information transmission in the burst As a rule 142 bits of information transmission are realized in a burst presented as "1" or "0" as indicated below. They are in the time middle of the burst transmission in the "useful part". The two 3 tail bits (TB) on the edge of the information section provided as security zones are realized as constant transmission of "0". No information is transmitted in the guard period (GP). There are 5 different types of burst z Normal burst z Frequency correction burst z Synchronization burst z Access burst z Dummy burst Each of these bursts has a different configuration and has a particular purpose.

The normal burst It contains z 2 x 3 bits as tail bits (TB); z 2 x 57 encrypted data bits which carry the actual information z 2 x 1 bit as a "stealing flag" which tell the receiver that data transmission is being

interrupted briefly and signaling data is being transmitted instead of useful data (or vice versa).

z 26 bits for synchronizing and problem detection (training sequence), which allow both the BS and the MS to synchronize themselves to a burst and allocate the data bits exactly. Distorted or incomplete received signals can thus be reconstructed.

z 8.25 bits guard period (GP) for collecting run times and reception times (determined by the distance BTSE MS).


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TB Information S Training Sequence S Information TB GP

148 bit = 546,12 s (+8,25 bit = 30,44 s )

3 57 1 26 1 57 3 8,25 bit

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Fig. 5 Normal burst

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13


Frequency correction burst The frequency correction burst is transmitted from the BTS for frequency synchronization. The bit sequence of the fixed bits corresponds with an unmodulated carrier, i.e. a pure sine wave, so the MS can synchronize itself to the preset frequency. The repeat of frequency correction bursts is also known as the frequency correction channel (FCCH). The frequency correction burst consists of: z 2 x 3 bit tail bits (TB); z 142 bit as fixed bits (sine wave2) for frequency synchronization; z 8.25 bit guard period (GP);

2To be more precise: the fixed bits (142 x signal 0) lead via this modulation, to a sinusoidal signal for this period with a frequency being 67,7 kHz above the carrier central frequency


14


TB fixed Bits TB GP

3 142 3 8,25 bits

Fig. 6 Frequency correction burst

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15


Synchronization burst The MS receives the first information on the BS from the synchronization burst allowing it to synchronize time on the base station. The burst contains a long training sequence, the TDMA number and the identity code of the BS, BSIC3. The TDMA frame number is required as one of the parameters for encryption. The synchronization burst is transmitted together with the frequency correction burst in the TDMA time slot zero. The repeat of synchronization bursts is called synchronization channel. The synchronization burst consists of: z 2 x 3 bit tail bits (TB); z 2 x 39 bit which contain the TDMA frame number and the identity code of the BS

(BSIC); z 64 bit training sequence for time synchronization and fault detection; z 8.25 bit guard period (GP);

3BSIC ( Base transceiver Station Identity Code): Identity code of the BTS allowing the MS to distinguish between different BTS; it consists of 6 characters: 3 characters for the NCC (Network Color Code = PLMN identity) and 3 characters for the BCC (Base Color Code) which allows different RFCs with the same frequency in neighboring clusters to be distinguished.


16


TB Information Training Sequence Information TB GP

3 39 64 39 3 8,25 bits

Fig. 6 Synchronization burst

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17


Access burst The access burst is used for the MS contact setup with the BTSE. It is characterized by a long protection time (GP = 68.25 bits or 252 s), which takes the signal run time4 from MS to BTSE into consideration. The MS does not know the distance to the BTSE first of all and therefore does not know how the transmission has to be staggered (moved forward). The access burst consists of: z 8 + 3 bit tail bits (TB); z 41 bit synchronization sequence; z 36 bit information bits; z 68.25 bit guard period (GP);

Dummy burst The dummy burst is sometimes sent as padding if there is no other information. It does not contain any information but has the same format as the normal burst.

4Note: The length of the access burst is decisive for the maximum cell size of a GSM900 cell. When a contact is setup with an MS, the 68.25 bits with a duration of 252 s are sufficient as a security distance for 3 x 108 m/s x 252 s = 75.6 km. The cell radius must therefore be less than 37.8 km taking the way BTSE MS BTSE into consideration


18


TB Synchro.- Sequence Information TB GP

8 41 36 3 68,25 bits

Fig. 8 Access burst


19


GMSK - Gaussian Minimum Shift Keying The information to be transmitted consisting of a sequence of binary data (bit sequence) must be modulated on an information carrier with a specific rate (bit rate). The receiver must then be able to remove them from this carrier. To obtain the best possible bandwidth utilization taking the minimum requirements for transmission quality into consideration, a frequency modulation method was chosen for the GSM system. It is known as GMSK (Gaussian Minimum Shift Keying). In principle the bandwidth of a radio channel (RFC) of 200 kHz varies here around the center of the bandwidth (carrier frequency ft) with a maximum frequency displacement f = 67.7 kHz5 .

MSK - Minimum Shift Keying MSK is a frequency shifting modulation process, which involves the information in the instantaneous frequency of the HF signal. This process stands out because of the continuous phase6 processing of the modulation signal resulting in excellent bandwidth utilization. The binary signal is modulated7 on the carrier using a modulation index = f/fmod = 0.5. The instantaneous frequency of the HF signal changes with the applied modulation data. When there is a "1", the carrier frequency ft is increased by f, when there is a "0" decreased by f. With a modulation index of 0.5 f corresponds with the half modulation frequency fmod. With MSK the phase angle of the carrier is changed linearly and continually during the bit duration T. It is changed by +90 for a logical "1" at the modulator input and by -90 for a "0". The frequency of the HF signal can be seen in the context of the phase relationship. It is also obtained from the trajectory of the phase path or the phase path is obtained from the integral of the frequency path.

f can also be calculated from bit duration T and the change in the phase relationship (). f = (/t) / 2; with = /2 (90) and t = T = 3.6923 s hence: f = 1/(4T) = 1/(4*3.6923s) = 67.7 kHz

5 f = 1/(4T) applies; T = duration of a bit = 3.6923 s 6 i.e. there is no phase jittering 7 f = carrier deviation; fmod = modulation frequency; fmod < 1/2 bit rate fbit


20


MSK ModulationPhase response

Binary

signal

Frequency

response

Phase

+180

+90

t

-90

-180

1

0

f f

f

f

f

Fig. 9 Frequency and phase response with the MSK modulation process

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21


GMSK Reducing the bandwidth required for MSK can be achieved by prefiltering the data signal. The "increments" of the data (0 1) and thus also frequencies or unsteadiness of the phase path result in a comparably wide spectrum. To reduce the bandwidth requirement by means of improved attenuation of the side bands a low-pass prefilter with the pass through characteristic of a Gaussian bell-shaped curve is used. The filter used with bandwidth B8 has the following impulse answer (Rec. 05.04):

H t T e with B T

H f e

t T

B f

( ) / * / *

( )

/

(ln / )*

= ==

1 2 2 2

2 2 2

2 2

2

2 2

The data signals Gaussian filtered here have "softer" transitions thus affecting the phase path. A frequency or phase change without jumps (continuous) results.

1.1.3 Time Organization (Framing) The transmission of the control and user information (speech/data) takes place in physical channels. A time slot is available in the TDMA frame every 4.615 ms. The information is transmitted according to specific time schemas, i.e. certain contents are repeated at specific time intervals. This process, i.e. the periodical repeat of the TDMA frame is called "framing".

80.3 was chosen as a standardized filter bandwith for the Gauss filter, i.e. bandwidth B * bit duration T 0.3; given B = 81.25 kHz


22


RFC1

RFC2

7

6

5

4

3

2

1

RFC3

0

TDMAFrame

FDMA

Time

t

Frequency

0123456

.

.

.

.

4647484950

0123456

.

.

.

.

2425

BCCH-Multiframe

TCH-Multiframe

RFC174

.

.

.

.

Fig. 10 Physical channels / traffic and control channel multiframe

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23


Multiframe TDMA frames belonging together in a first framing level are called multiframes. There are 26 TDMA frames in a user channel9 multiframe, in a control or signaling channel multiframe there are 51 TDMA frames. The control channel multiframes are transmitted as a rule in time slot 0 of one of the radio channels (RFCs) from a BTSE, the remaining time slots are available for user channel multiframes.

9Note: Not only subscriber information (speech/data) can be transmitted in a traffic channel. If the signaling requirement increases, signaling can also be transmitted via a traffic channel. A change between subscriber information and signaling is indicated by the so-called stealing flags in the normal burst.


24


0 1 2 49 50

0 1 2 3 4 5 6 7

8 Time slot = 1 TDMA frame4,615 ms

1 Time slot

BURST = Contents of a time slot

156,25 bit = 576,88 s(1 bit = 3,692 s)

0 1 2 24 25

1 Multiframe for Speech/Data 2)26 TDMA frame = 120 ms

1) Signaling channels

2) User channels andassociated signalingchannels

Time organization of the air interface

1 Multiframe for signaling 1)51 TDMA frame = 235,38 ms

Fig. 11Multiframe

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25


The multiframe for user information (full or half rate transmission) will be used at this point as an example of multiframe time organization. The diagram shows the periodical repeat of the fixed structure of certain "logical contents" for a user channel with half and full rate transmission. All 26 TDMA frames repeat specific contents. This is necessary because not only user information (data, speech) is transmitted in the traffic channel connection (called TCH here) but also further specific signaling information (SACCH) has to be transmitted repeatedly at fixed intervals in a traffic channel multiframe. The information (user information signaling) is transmitted between MS and BTS as burst in physical channels. To differentiate the contents a division into "logical" channels is useful. These "logical channels" specify therefore certain contents of the transmission over the air interface. In particularly for signaling different contents and therefore "logical channels" are relevant. They are repeated in the signaling channel multiframe every 51 TDMA frames.


26


26 TDMA frame 120 ms

A full rate TCH

T/t TDMA frame for TCHA/a = TDMA frame for SACCH/T

Two full rate TCH

UPLINK / DOWNLINK: Traffic Channel (TCH)

T T T T T T T T T T T T A T T T T T T T T T T T T -

T t T t T A T t T at T tt T t T t T t t T t T t T

Fig. 12 Multiframe for a user channel

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Superframe / Hyperframe The data packets from the time slots are compiled in even higher frame structures in addition to this first framing level of the merging of the TDMA frames to multiframes. These are called superframes and hyperframes. A superframe comprises 26 x 51 = 1326 frames and is thus 6.12 s long. The superframe is the smallest common multiple of traffic channel multiframes (26 frames) and control or signaling channel multiframes (51 frames). The time window of a superframe is the shortest cycle in which the organization of all channels is repeated. Some characteristics of the channel organization are excluded from this repeat. These are contained in the hyperframe. The hyperframe is the numbering period. It comprises 2048 superframes and is thus exactly 12,533.760 s or 3 h 28 min 56.76 s long. It is a multiple of all cycles described up to now and determines all transmission cycles or periods on the air interface in practice.


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Hyperframe = 2048 Superframe

Superframe = 26 x 51 (Multi-) Frames

TCH Multiframe

TDMA Frame

BCH Multiframe

0 1 3 24 25 0 1 2 3 4 49 50

0 1 2 3 4 5 6 7

2

Fig. 13 Time organization: multiframe, superframe and hyperframe

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29


1.1.4 Adaptive Frame Alignment

Adaptive frame alignment is the time condition between uplink (UL) and downlink (DL). The TDMA frames (UL) running from the mobile station (MS) to the base station (BS) or especially the transmission and reception station (BTSE) are delayed by 3 time slots (burst periods) compared with the (DL) TDMA frame running in the opposite direction. The BTSE sets the frame (master), the MS has to adapt itself to the presetting. The delay of 3 time slots (= 1.73 ms) is GSM-defined. This GSM convention is set up in such a way that the numbering of the time slots can be identical both in the UL and the DL direction. The time delay allows the mobile station to avoid sending and receiving at the same time. This means substantially simpler technical implementation as the MS receiver does not have to be protected against the transmitter from the same MS ("signal isolation"). The so-called "combining" of antennas is thus not necessary.

Timing advance

There is a problem, however, when implementing this convention. If the distance between the BTSE and the MS is greater, the delay due to the run time of the signals must be taken into consideration. Even at the speed of light (3 x 108 m/s) the radio signals also require a specific time to bridge the path between the BTSE and the MS. With a maximum cell radius (GSM900) of approx. 35 km this means a delay of approx. 0.1 ms for the path BTSE - MS or of approx. 0.2 ms for a "round path". This delay in the run time must be taken into consideration when the signal is sent from the MS because it is absolutely necessary that the BTSE receives the signals (bursts) from the different MS in the correct time range. Bursts can ot

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