Authentication Center (AUC) - Discount Coupons Center (AUC) ... bandwidth being wasted across both the Abis (BTS-BSC) and A (BSC-MSC) interface. Case 2, TRAU at BSC: If the TRAU …

Authentication Center (AUC)

AUC is always integrated with HLR for the purpose of the authentication. At subscription time, the Subscriber Authentication Key (Ki) is allocated to the subscriber, together with the IMSI. The Ki is stored in the AUC and used to provide the triplets, same Ki is also stored in the SIM.

AUC stores the following information for each subscriber

1. The IMSI number,

2. The individual authentication key Ki,

3. A version of A3 and A8 algorithm.

Continued…..

Section 3 – NSS Topology

Authentication is required at each registration, at each call setup

attempt (mobile originated or terminated), at the time of location updating, before supplementary service activation, de-activation , registration.

HLR uses the IMSI to communicate with AUC, triplets are requested in sets of five.

Continued…..


In AUC following steps are used to produce one triplet:

1. A non- predictable random number, RAND, is produced

2. RAND & Ki are used to calculate the Signed Response (SRES) and the Ciphering Key (Kc)

3. RAND, SRES and Kc are delivered together to HLR as one triplet.

HLR delivers these triplets to MSC/VLR on request in such a way that VLR always has at least one triplet.


Authentication Procedure:

The MSC/VLR transmits the RAND (128 bits) to the mobile. The MS computes the SRES (32 bits) using RAND, subscriber authentication key Ki (128 bits) and algorithm A3. MS sends back this SERS to AUC and is tested for validity.


MS BTS MSC/VLR HLR AUC OMC

A4

A4 IMSI Ki

A3 A8 Triplets

Generation

Ki RAND

RAND SERS

Kc

A2

Triplets

Ciphering Function

A5

Kc

RAND

=? SERS

IMSI

Ki

A3

A8

Ciphering Function

A5

Kc

SIM Card


Ciphering

The user data and signaling data passes over the radio interface are ciphered to prevent intrusion. The ciphered key (Kc) previously computed by the AUC is sent from the VLR to the BSS after the mobile has been authenticated. The Kc is also computed in the MS and in the way both ends of the radio link (MS and BSS) possess the same key.


Procedure:

For the authentication procedure, when SRES is being calculated, the Ciphering Key (Kc), is calculating too, using the algorithm A8.

The Kc is used by the MS and the BTS in order to cipher and decipher the bit stream that is sent on the radio path.


AUC

Ki

A3

A8 A8

Ciphering/Deciphering

Speech, data,sig

A5

Kc

A3

Choice of random no RAND (128 bits)

=?

OK

SIM

Ki

A3

A8

A5 Speech, data,sig

A8

Kc (64 bits)

A3

SERS

RAND

SERS

Ciphering Command

Ciphered Data


Subscriber Confidentiality

The subscriber identity (IMSI), since is considered sensitive information, is not normally transmitted on the radio channel. A local, temporary identity is used for all interchanges. The identity (TMSI) is assigned after each change of authenticated location. For other cases:

• Call set-up

• Use of supplementary services

• Use of SMS

Continued…..


A TMSI is allocated when the one supplied by the MS is considered out of

date or when the MS does not provide the TMSI.

Transmission of the TMSI over the traffic channel is ciphered.


Equipment Identification Register (EIR)

Purpose of this feature is to make sure that no stolen or unauthorized mobile equipment is used in the network.

EIR is a database that stores a unique International Mobile Equipment Identity (IMEI) number for each item of mobile equipment.


Procedure:

• The MSC/VLR requests the IMEI from the MS and sends it to a EIR.

• On request of IMEI, the EIR makes use of three possible defined lists:

- A white list: containing all number of all equipment identities that have been allocated in the different participating countries.

- A black list: containing all equipment identities that are considered to be barred.

- A grey list: containing (operator’s decision) faulty or non-approved mobile equipment.

• Result is sent to MSC/VLR and influences the decision about access to the system.


EIR MSC/VLR MS

Storage of all number series mobile equipment that have been allocated in the different GSM -

countries

Storage of all grey/black – listed mobile equipment

Storage of the equipment identity

IMEI

Call Setup

IMEI Request

Sends IMEI

Check IMEI

Access/ barring info

Continues/Stops call setup procedure


Echo Canceller

In order to eliminate echo effects (noticeable by the mobile subscribers while in conversation with PSTN subscribers) caused by the time delay due to coding and decoding of signal processing, group of echo cancellers are installed even for local calls.

This is rarely a problem when communicating between two MSs. However, when connecting to a PSTN telephone, the signal must pass through a 4-wire to 2-wire hybrid transformer.

Continued…..


The function of this transformer is - some of the energy at the 4-

wire receive side from the mobile is coupled back to the 4-wire transmit side and thus speech is retransmitted back to the mobile.

As a result, all calls on to the PSTN must pass through an echo canceller to remove what would otherwise be a noticeable and annoying echo.

Continued…..


The process of canceling echo involves two steps:

• First, as the call is set up, the echo canceller employs a digital adaptive filter to set up a model or characterization of the voice signal and echo passing through the echo canceller. As a voice path passes back through the cancellation system, the echo canceller compares the signal and the model to dynamically cancel existing echo. It removes more than 80 to 90 percent of the echo across the network.

• The second process utilizes a non-linear processor (NLP) to eliminate the remaining residual echo by attenuating the signal below the noise floor.


Transcoder and Rate Adaptor Unit (TRAU)

The primary function of the TRAU is to convert 16kps (inc signaling) GSM speech channels to 64kbps PCM channels in the uplink direction and the reverse in the downlink direction. The reason this process is necessary is because MSCs only switch at the 64kbps channel level.


TRAU Locations

TRAU can be physically located in the BTS, BSC or MSC and hence leads to a variety of installation configurations.


Advantages of Different Configurations

Case 1, TRAU at BTS: If the TRAU is installed at the BTS, each 16kbps GSM channel would need to be mapped to its own 64kbps PCM channel. This results in 75% of the transmission bandwidth being wasted across both the Abis (BTS-BSC) and A (BSC-MSC) interface.

Case 2, TRAU at BSC: If the TRAU is installed at the BSC, 16kbps GSM channel mapped to 64kbps at the A (BSC-MSC) interface, which increases the requirement of the Transmission trunks.


Case 3, TRAU at MSC: If the TRAU is placed at the MSC, as is generally

the case in current networks, a multiplexer can be placed at the BTS which enables 4 x 16kbps GSM channels to be multiplexed onto one 64kbps PCM channel, using 4 x 16kbps ISDN D-channels. In this configuration, only at arrival at the MSC is the 16-64kbps channel conversion necessary, thereby maximizing the efficient usage of the transmission medium by increasing the GSM channel throughput per PCM 2048 bearer from 30 to 120 channels.


Operation And Maintenance Center (OMC)

The OMC centralizes all operations and maintenance activities for the MSCs and BSSs using remote software control. It provides remote testing, operations, and maintenance capabilities for the entire system from one central location. Each BSS, MSC, HLR, VLR, EIR, and AUC can be monitored and controlled from the OMC.


OMC Functional Architecture

Operating System

Communications Handler

Database

MMI

Event/ Alarm Management Security

Management

Fault Management

Performance Management

Configuration Management


The OMC supports the following network management functions:

• Event Management - General functions of the OMC include operator input and output messages, application input commands, and application output reports.

• Fault Management - The OMC provides fault management such as diagnostics and alarms for the MSC and BSS. It provides the means to isolate and minimize the effects of faults in the network thereby enabling the network to operate in efficient manner.

Continued…..


• Security Management – It provides an extensive range of features to ensure that access to the OMC functions is restricted to relevant personnel.

The security features are as follows:

Password Authentication of OMC operator

Logging of OMC access attempt

Configurable user access restrictions

Automatic logoff


• Configuration Management - Configuration management for the

BSS consists of generic download, non-volatile memory download, database administration, and translations download. For the MSC, software release updates, database administration (route analysis, IMSI analysis table), and subscriber administration (connect/disconnect) are supported.

• Performance Management - Performance management supports data collection (such as traffic data, handovers, statistics, plant measurements, and volume data) and basic reporting.


Billing Center

Charging analysis is the process of analysing the Charging Case and then ultimately generating the TT (Toll Ticketing) record so that an itemised bill can be produced and then sent on to the customer.

The tariff structure consists of two parts:

• The network access component

• The network utilization component


The network utilization component is registered on a per call basis.

Charging starts at the moment the subscriber answers, or on connection to an answering machine internally in the network.

The main elements are:

• Use of GSM PLMNs

• Use of national / international PSTNs

• Use of connection between different networks

• Use of the signaling system no.7


Depending on the type of call, one or more call tickets can be generated:

• Outgoing call to fixed network: a call tickets is generated by the originating MSC.

• Incoming call from the fixed network: two call tickets are created: one in the GMSC and another in the destination MSC. If a call forwarding supplementary service is in operation, other call tickets are generated in the MSC and the GMSC.

Continued…..


• Outgoing call from a mobile subscriber to another mobile

subscriber belonging to same PLMN: three call tickets are created: one in the originating MSC, one in the GMSC (which is in this case is the originating MSC) and another in the destination MSC.

Call tickets mainly register the following information:

1. IMSI

2. Identity (MSISDN) and type (MSC or GMSC)

3. Mobile subscriber location identity


4. Other party’s identity

5. Call type (incoming, outgoing, forwarded etc)

6. Call status

7. Teleservices and bearer service

8. Date and time

9. Call duration


Call Detail Record (CDRs)

• Each call within the PLMN creates one or more call records

• These records is generated by the MSC/GMSC originating the call

• The records are known as a ‘Call Detail Records’ (CDRs) • CDRs contain the following information:

- Subscriber Identity

- Number called

- Call Length

- Route of call

• Often referred to as ‘Toll Tickets’


Call Charge Procedure

• Network supplies originating MS with CAI details

• MS calculates AOC record using CAI details

• This record acts as a ‘toll ticket’ which tracks the call on its route through various networks

• Each call component can generate a separate CDR

• The record passes along the backbone to the home network

• Billing computer generates bills based on cumulative CDRs

• HPLMN collects the charges

• HPLM reimburses VPLMN using TAPs in accordance with roaming agreement



The Transferred Account Procedure (TAP) is the mechanism by which operators exchange roaming billing information. This is how roaming partners are able to bill each other for the use of networks and services through a standard process.


Gateway MSC (GMSC)

Gateway MSC (GMSC) connects the PLMN with other networks and the entry point for the mobile subscriber calls having the interrogation facility. It has the function to obtain the information from the HLR about the subscriber’s current location and reroute the calls accordingly.

In case of the network having only on MSC, the same MSC work as the GMSC, while in the case having more than one MSC, one dedicated MSC works as GMSC.


Documents

Authentication Center (AUC) - Discount Coupons Center (AUC) ... bandwidth being wasted across both the Abis (BTS-BSC) and A (BSC-MSC) interface. Case 2, TRAU at BSC: If the TRAU …