Message Transfer Part Functions and Protocol

Message Transfer Part (MTP) provides services for transferring the messages of user parts (such as TUP, ISUP, and SCCP) to destinations through SS7. If SS7 experiences a fault, the messages can be transmitted correctly through the signaling network management function.
The overall objectives of the MTP are provided the means for:

- The reliable transport and delivery "User Part" signaling information across the SS7 network.
- The ability to react to system and network failures that will affect, and take the necessary action to ensure that is achieved.

MTP1 Signaling Data Link Layer
Bidirectional transparent transmission channel (64kbit/s)

MTP2 Signaling Link Layer
MTP2 provide reliable signaling links for transmitting signal messages between two directly connected signaling points.
- It indicates the starting point and the ending point of a signal unit, namely, recognizing signal units from bit streams in a signaling data link.
- It is used to check whether signaling links in service is loss of alignment. Loss of alignment will cause a change in the mode of operation of the signal unit error rate monitor.
- It is used to check whether the bit streams of a signal unit are transmitted incorrectly.
- It is used to re-obtain correct signal units if some errors are found. There are two correction methods: basic method and preventive cyclic re-transmission method.
- The initial alignment procedure is appropriate to both first time initialization (e.g. after "switch-on") and alignment in association with restoration after a link failure.
Signaling Link Error Monitoring
It is used to monitor the errors of a signaling link to ensure high-quality service. It include signal unit error detection and the alignment error rate detection
- It is used to process the congestion problem detected by the second layer to lighten the congestion condition.
- It is used to mark or remove the faulty status of a processor.

MTP3 Functional Layer of Signaling Network
A normal signaling link sends an SLTM message every minute. The signaling point that receives the SLTM message sends an SLTA message. The value of test code in the SLTA message must be same to that in the SLTM message. Also, the number of SLTA messages must be equal to that of SLTM messages.

After MTP2 is aligned successfully, MTP3 sends an SLTM message and starts a six-second timer. If an SLTA message is received before the timer is time out, the link service is activated. If an SLTA message is not received, the SLTM message is transmitted again after the timer is timeout. If the SLTA message is not received within six seconds, the link service cannot be activated.

If a signaling link is unavailable, the changeover is performed. During the changeover process, signaling services are transferred from an unavailable signaling link to one or more replaced link and avoiding message loss, duplication or mis-sequencing. Figure 1-9 shows the changeover procedure.

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GSM Location Update Precedure

The location update flow is as follows:

- When a MS finds that better service is provided in a certain location area, this MS notifies the MSC of the MSC/VLR which this new location area belongs to. 

- If the original location area and new location area belong to the same MSC/VLR, the MSC only changes the location information of this MS in the VLR. If the original location area and new location area belong to different MSCs/VLRs, the new MSC/VLR requests the data of this MS from the HLR. 

- The HLR returns the information needed by the new MSC/VLR and at the same time notifies the original MSC/VLR to delete the original registration information of this MS and notifies the new MSC/VLR to register this MS. 

Location Update of MS in the Same VLR

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Electromagnetic Propagation Path

The electromagnetic propagation takes on an energy propagation mode. During the propagation, the electric field is vertical to the magnetic field, both vertical to the propagation direction. Through interaction between the electric field and the magnetic field, the energy is propagated to the distance, 

just like propagation of water waves. 

When the radio wave propagates in the air, the electric field direction changes regularly. This phenomenon is known as polarization of radio wave. The electric field direction of radio wave is known as radio wave polarization direction. 

- If the electric field direction of radio wave is vertical to the ground, the radio wave is vertical polarization wave. 

- If the electric field direction of radio wave is parallel with the ground, the radio wave is horizontal polarization wave.

Radio wave can be propagated from the transmitting antenna to the receiving antenna in many ways: perpendicular incidence wave or ground refraction wave, diffraction wave, troposphere reflection wave, ionosphere reflection wave, as shown in the diagram. As for radio wave, the most simple propagation mode between the transmitter and the receiver is free space propagation.  One is perpendicular incidence wave; the other is ground reflection wave. The result of overlaying  the perpendicular incidence wave and the reflection wave may strengthen the signal, or weaken the signal, which is known as multi-path effect. Diffraction wave is the main radio wave signal source for shadow areas such building interior. The strength of the diffraction wave is much dependent of the propagation environment. 

The higher the frequency is, the weaker the diffraction signal will be. The troposphere reflection wave derives from the troposphere. The heterogeneous media in the troposphere changes from time to time for weather reasons. Its reflectance decreases with the increase of height. This slowly changing reflectance causes the radio wave to curve. The troposphere mode is applicable to the wireless communication where the wavelength is less than 10m.

Ionosphere reflection propagation: When the wavelength of the radio wave is less than 1m (frequency is greater than 300MHz), the ionosphere is the reflector. There may be one or multiple hops in the radio wave reflected from the ionosphere, so this propagation is applicable to long-distance communication. Like the troposphere, the ionosphere also presents the continuous fluctuation feature.

In a typical cellular mobile communication environment, a mobile station is always far shorter than a BTS. The direct path between the transmitter and the receiver is blocked by buildings or other objects. Therefore, the communication between the cellular BTS and the mobile station is performed via many other paths than the direct path. In the UHF band, the electromagnetic wave from the transmitter to the receiver is primarily propagated by means of scattering, namely, the electromagnetic wave is reflected from the building plane or refracted from the man-made or natural objects. 

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The Signaling Process of MS Originated USSD Call

When the MS makes originate the USSD operation according to the input of the subscriber, the MS first sets up a connection to the network, and then sends a request to the MSC and waits for the response. Once the MS receives the response from the network side, the MS displays the information in the response message on its screen to the subscriber. During the waiting process, the MS may receive the USSD REQUEST or USSD NOTIFY message originated by the network. At this time, the MS must handle these messages immediately and continuously wait for the response of the original USSD operation from the MSC.

After the MSC/VLR receives the MS originated USSD operation: 

- If the Service Code in this operation indicates that this operation must be handled by the HPLMN, the MSC/VLR tries to set up the conversation with the network. If such a conversation cannot be set up, the MSC/VLR returns error information to the MS. Otherwise, the MSC/VLR forwards this operation to the network side. The MSC/VLR transfers the subsequent USSD operations transparently between the MS and network. 

- If the Service Code in this operation does not indicate that this operation must be handled by the HPLMN, the MSC/VLR decides how to handle this USSD operation according to the configuration of the USSD Control table, that is, handles this USSD operation by itself or forwards it to other network element. 

- If the MSC/VLR does not support the characters in the MS originated USSD operation, the MSC/VLR must forwards this USSD operation to the network side. If the network side does not support the characters in the USSD operation, the network side must notify the MS and release the conversation. 

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The Architecture of Signaling Number 7 (SS7)

From TUP, ISUP, and SCCP points of view, MTP is only a message transfer point that is used to transfer the messages of user parts to the user parts of destination signaling points.
In brief, SS7 can be regarded as a TCP/IP network

MTP Functions
Message Transfer Part (MTP) provides services for transferring the messages of user parts (such as TUP, ISUP, and SCCP) to destinations through SS7. If SS7 experiences a fault, the messages can be transmitted correctly through the signaling network management function. The overall objectives of the MTP are provided the means for:

- The reliable transport and delivery "User Part" signaling information across the SS7 network.
- The ability to react to system and network failures that will affect, and take the necessary action to ensure that is achieved.

MTP has the following three functional levels:
- Signaling data link
- Signaling link function
- Signaling network function

Signaling Data Link Layer (MTP1):
- Bidirectional transparent transmission channel (64kbit/s)
- Switching function
- Signaling data link layer switches an SS7 terminal to a trunk device through semi-permanent

Signaling Link Layer (MTP2):
MTP 1 and MTP2 provide reliable signaling links for transmitting signal messages between two directly connected signaling points. MTP2 has the following functions:

Signal Unit Delimitation
It indicates the starting point and the ending point of a signal unit, namely, recognizing signal units from bit streams in a signaling data link.

Signal Unit Alignment 
It is used to check whether signaling links in service is loss of alignment. Loss of alignment will cause a change in the mode of operation of the signal unit error rate monitor.

Signal Unit Error Detection 
It is used to check whether the bit streams of a signal unit are transmitted incorrectly.

Error Correction 
It is used to re-obtain correct signal units if some errors are found. There are two correction methods: basic method and preventive cyclic retransmission method.

Initial Alignment 
The initial alignment procedure is appropriate to both first time initialization (e.g. after "switch-on") and alignment in association with restoration after a link failure.

Signaling Link Error Monitoring
It is used to monitor the errors of a signaling link to ensure high-quality service. It include signal unit error detection and the alignment error rate detection

Flow Control
It is used to process the congestion problem detected by the second layer to lighten the congestion condition.

Processor Outage
It is used to mark or remove the faulty status of a processor.
Three types of signal unit are described as follows:

Fill-In Signal Unit (FISU) 
If no signal unit is transmitted on a SS7 signaling link, FISU (The length identity (LI) is 0) is sent to the peer end for notifying that the MTP2 of local office is normal. There is no information field in FISU.

Link Status Signal Unit (LSSU) 
The LSSU provides the information of link status to achieve the connection or recovery of signal link. Information field obtained in LSSU is called Status Field (SF). The status indicator represents the status of the MTP2 of the signaling point. The length of SF is 1 or 2.

Message Signal Unit (MSU)
It has a longer information field, which has two sections. The first section is a service information octet (SIO) that consists of eight bits. The second section is a signaling information field (SIF) that consists of multiple eight bits, that is, MTP signaling messages and MTP user part messages, such as TUP and ISUP (The length identity (LI) is greater than 2). All the signals transmitted on a SS7 link belong to one of the three signal units

Signaling link alignment process 
Initial alignment is a control program used for initializing or recovering a signaling link. There are two initial alignment procedures: normal initial alignment and emergent initial alignment. The decision to apply either the "normal" or "emergency" procedures is made unilaterally at MTP3. The proving period of normal initial alignment is different from that of emergent initial alignment. For a 64 kbit/s link, the nominal value of normal initial alignment stipulated in the protocol is 8.2 s, whereas the nominal value of emergent initial alignment is 0.5 s. During the proving period, the alignment error rate monitoring (AERM) function is used to check whether the error rate of a link is within the acceptable scope.

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Basic Core Network Feature and Technologies

Core Network is devided into CS domain and PS domain. CS domain is based on original GSM network. PS domain is based on original GPRS network. CS domain is used to provide Circuit-switched service. Network mode can support TDM, ATM and IP. Physical entities include switching equipment (such as MSC/VLR, GMSCs), and inter-working equipment (IWF). PS domain is used to provide Packet-switched service. Network mode is IP. Physical entities include SGSN, GGSN, CG , BG etc.

Common Function of CS domain and PS

HLR: to realize mobile subscriber management and location information management ;

VLR: to deal with all kinds of data information of current mobile subscriber ;

AUC: to store authentication information of mobile subscriber ;

EIR: to store IMEI data of mobile subscriber; 

SMS: Short Message Center. 

Function entity of CS domain

MSC: to realize switching and signaling control function of circuit service. 

GMSC: to realize addressing function for different network. GMSC and MSC can be integrated together or separated independently. 

IWF: integrated together with MSC, to realize inter-working with PLMN and ISDN, PSTN , PDN (Mainly complete signaling transfer function), its functions can be defined based on specified services and network types. 

Function entity of PS domain

SGSN: to realize packet switching function, signaling control function  and route function of packet service. 

GGSN: to realize inter-working with PS domain and external data network. 

CG: to realize billing function of packet service. 

BG: to realize inter-working of two GPRS network and guarantee the security of network.

Interface between CN and RAN/BSS

A Interface: to realize mobile station management, base station management, mobility management and call processing, etc. 

Gb interface: to realize packet data transmit and mobility management.

Iu-CS interface: to realize RNS management, mobility management and call control function. 

Iu-PS interface: its basic function is similar to Iu-CS interface.

CS internal interface

B interface (MSC-VLR): A private interface between VLR and MSC. It is used for the MSC to query the current location information of a Mobile Station (MS) from the Visit Location Register (VLR), or request the VLR to update the current location information of the MS or is used for the operations of supplementary services. 

C interface (MSC-HLR): to get MSRN,  the IN service related subscriber status and location information. 

D interface (VLR-HLR): used to exchange the location information of the MS and the subscriber management information. 

E interface ( between MSC): to be used for handover process between two MSCs. 

F interface (MSC-EIR) : to be used for EIR to verify IMEI status information of mobile equipment.

G interface (between VLR): to exchange IMSI and authentication information when MS moves from one VLR to another VLR. 

PS internal interface

Gr interface (SGSN-HLR): to exchange mobile location information and manage subscriber signed information. It is similar to C interface.

Gn, Gp interface (SGSN-GGSN): GTP is adopted, used for establish tunnel and transmit data packet between GSN equipment. 

Gc interface (GGSN-HLR) : optional.

Gf interface (SGSN-EIR) .

Gi interface (GGSN – external network).

Common interfaces of CS and PS

Gs interface (MSC/VLR-SGSN): used for combined location update between MSC and SGSN) .

H interface (HLR-AUC): used for authentication and ciphering. It is a private  interface.

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Characteristics of WCDMA FDD

The DS-CDMA system with a bandwidth of approximately 1MHz, i.e. IS-95, is generally named as narrowband CDMA system. But WCDMA owns a chip rate of 3.84Mcps, bringing approximately 5MHz carrier bandwidth.This feature enables the system to support higher bit rate, and at the same time brings other benefits, for example, increasing of multi-path diversity.

Gold sequence is used as scrambling code in WCDMA

In IS-95, only coherence detection is used in the downlink, but in WCDMA, coherence detection based on pilot symbol or common pilot is adopted for both the uplink and the downlink, increasing the coverage scope and the capacity of the uplink. IS-95 only uses closed loop power control in the uplink, while WCDMA uses this in both of the uplink and the downlink.With the closed loop power control used in the downlink, link performance and downlink capacity is improved.
IS-95 system mainly aims at macro cell. Because BTS synchronization is necessary, BTS is generally placed on the roof, etc. for the sake of receiving GPS signal. In this case, a global time reference can be used. But this application is difficult to carry out in the places where it is hard to receive the GPS signals. WCDMA system supports asynchronous BTS operation, and it may not use the global reference, thus it is different with the IS-95 system requiring BTS synchronization operating mode. Thus, the application of indoor cell and microcell is much simpler. This makes that the handover of WCDMA is slightly different with that of IS-95.

- Channel bandwidth: 5MHz

- Chip rate: 3.84Mcps

- Frame length: 10ms

- Voice coding: AMR (Adaptive Multi-Rate)

- Uplink and downlink modulation: QPSK/QPSK 

- Coherence demodulation aided with pilot

- Fast closed loop power control:  1500Hz

- Handover: soft/hard handover

- Support synchronous and asynchronous NodeB operation 

- Satisfy the minimum performance requirement of IMT2000

- Compatible with GSM-MAP core network

- Comparatively steady version R99 has been released

- Support open loop and closed loop transmit diversity mode

- Support Common Packet Channel(CPCH) and Downlink Share Channel, adapt to Internet data access mode

- Support macro diversity, selection diversity of NodeB location

- Support different fast power control algorithms and open loop, out loop power control

- Fully support UE locating services

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Basic Concept of Signaling SS7

Message Transfer Part (MTP) is mainly responsible for transmitting "User Part" signaling messages across the SS No. 7 network. If a system or signaling network is faulty, MTP can take measures to prevent messages from losing, and repeatedly or disorderly transmitting. MTP consists of signaling data links, signaling link functions, and signaling network functions.

Here are the following exchange in SS7

Signaling point (SP):
A signaling point is a node of SS7. It can be identified by signaling point code (SPC). Source signaling point refers to a signaling point that generates signaling messages. Destination signaling point refers to a signaling point that receives signaling messages. In general, international SPC is 14-bit and national SPC is 24-bit.

Signaling link: 
A signaling link is a physical link that connects signaling points and transmits signaling messages.

Signaling network:
Logically, a signaling network is separated from a communication network. It is used to transmit signaling messages. A signaling network mainly consists of signaling points and signaling links.

Signaling link set:
A signaling link set is a set of parallel signaling links that directly connects two signaling points.

Signaling route:
A signaling route is a route that transmits signaling messages from source signaling point to destination signaling point.

Signaling transfer point (STP):
A signaling transfer point is neither a source signaling point, nor a destination signaling point. It is a signaling point that transfers signaling messages. If a signaling transfer point can serve as a signaling point, it is called an integrated STP. If a signaling transfer point cannot serve as a signaling point, it is called an individual STP.

There are the following two working modes:

Associated: 
Three signaling points (VMSC, GMSC, and VMSC) cannot serve as an STP. The signaling messages between VMSC and GMSC are transmitted through a directly connected signaling link. GMSC can serve as a voice transfer path; however, GMSC cannot serve as a signaling transfer point.If SS7 signaling messages are transmitted through two or more signaling links with one or more STPs, it is called quasi-direct connection. Figure 1-2 shows the quasi-direct connection.

Quasi-associated: 

There is a direct voice path between VMSC and VMSC; however, there is no direct signaling link between VMSC and VMSC. Signaling messages between VMSC and VMSC are transferred by GMSC. GMSC serves as an integrated STP.            

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Multiple Access Technology

There are 3 types of multiple access technology

1.       Time division multiple access (TDMA)

2.       Frequency division multiple access (FDMA)

3.       Code division multiple access (CDMA)

Time Division Multiple Access means that the wireless carrier of one bandwidth is divided into multiple time division channels in terms of time (or called timeslot). Each user occupies a timeslot and receives/transmits signals within this specified timeslot. Therefore, it is called time division multiple access. This multiple access mode is adopted in both a digital cellular system and a GSM. TDMA is a complex architecture and the simplest case is that a single channel carrier is divided into many different timeslots, each of which transmits one-way burst-oriented information. The key part in TDMA is the user part, in which each user is allocated with one timeslot (allocated when a call begins). The user communicates with a base station in a synchronous mode and counts the timeslot. When his own timeslot comes, the mobile station starts a receiving and demodulation circuit to decode the burst-oriented information sent from the base station. Likewise, when a user wants to send any information, he should first cache the information and waits for his timeslot to come. After a timeslot begins, the information is transmitted at a double rate and next burst-oriented transmission begins to be accumulated.

Frequency Division Multiple Access:  frequency division, sometimes called channelization, means dividing the whole available spectrum into many single radio channels (transmit/receive carrier pair). Each channel can transmit one-way voice or control information. Under the control of the system, any user can be accessed to any of these channels. Analog cellular system is a typical example of FDMA structure. Similarly, FDMA can also be used in a digital cellular system, except that pure frequency division is not adopted. For example, FDMA is adopted in GSM and CDMA.

CDMA is a multiple access mode implemented by Spread Spectrum Modulation. Unlike FDMA and TDMA, both of which separate the user information in terms of time and frequency, CDMA can transmit the information of multiple users on a channel at the same time. That is to say,mutual interference between users is permitted. The key is that every information before transmission should be modulated by different Spread Spectrum Code-Sequence to broadband signal, then all the signals should be mixed and send. The mixed signal would be demodulated by different Spread Spectrum Code-Sequence at the different receiver.Because all the Spread Spectrum Code-Sequence is orthogonal,only the information that was be demodulated by same Spread Spectrum Code-Sequence can be reverted in mixed signal.  

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GSM Location Update Flow

The procedure of location update is as follows:

• When a MS finds that better service is provided in a certain location area, this MS notifies the MSC of the MSC/VLR which this new location area belongs to.
• If the original location area and new location area belong to the same MSC/VLR, the MSC only changes the location information of this MS in the VLR. If the original location area and new location area belong to different MSCs/VLRs, the new MSC/VLR requests the data of this MS from the HLR.
• The HLR returns the information needed by the new MSC/VLR and at the same time notifies the original MSC/VLR to delete the original registration information of this MS and notifies the new MSC/VLR to register this MS.

Location Update When the MS Roams Inside the Same VLR Area

Location Update When the MS Roams In the Areas of Different VLRs  

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