Network Organization

Question 1

Identify the numbered components

Answer 1

1- MS

2- BTS

3- BSC

4- MSC

5- VLR

6- EIR

7- HLR

8- AUC

9- GMSC

Question 2

What are the components of the Base Station Subsystem?

Answer 2

The Base Transceiver Station (BTS) and the Base Station Controller (BSC).

Question 3

What are the components of the Network Switching Station?

Answer 3

The MSC, the VLR, the GMSC, the EIR, the HLR, AUC.

Question 4

Identify the the subsets of this identifier

Answer 4

1-

2-

3-

4-

5-

6- GCI

7- CELLID

Question 5

What is a PLMN?

Answer 5

A service provider is identified by its six-digit Public Land Mobile Network ID or “PLMN”

The PLMN consists the Mobile Country Code (MCC) and Mobile Network Code (MNC)

Question 6

What is a LAC?

Answer 6

Mobile networks are divided into geographic service regions called Location Areas identified by a five-digit Location Area Code (LAC).

LACs are serviced by a single MSC that routes all traffic in and out of the LAC.

Information for every user in the LAC is stored in the local VLR.

Adding the LAC to a PLMN creates the Location Area Identity (LAI) which uniquely identifies a LAC from all others in the world.

Question 7

What is a cell in regards to BTS services?

Answer 7

A “Cell” is the area covered by a single ARFCN in GSM.

A single physical BTS can have multiple cells if it is sectorized.

A Cell is identified by a cell ID ranging from 3-5 digits

Question 8

How many digits are in an IMEI and how do they break out?

Answer 8

15 digits.

The first 8 digits are the TAC.

The next 6 digits are the serial number.

The last digit is a check digit and is not passed in traffic.

Question 9

How many digits are in an IMSI and how do they break out?

Answer 9

15 digits.

The first six digits are the Mobile Country Code (MCC) and Mobile Network Code (MNC), which make up the Public Land Mobile Network (PLMN).

The last nine digits identify the user’s Mobile Subscriber Identity Number (MSIN).

Question 10

What is the TMSI?

Answer 10

The TMSI is an eight-digit hexadecimal number that is assigned to a user when the handset is first authenticated.

Question 11

What is a BTS?

Answer 11

The BTS is what is typically called a cell phone tower.

It consists of all the actual radio equipment and antennas.

Responsible only for transmitting, receiving, encrypting, and decrypting radio signals.

Often described by the size of the area they cover.

The largest coverage areas are Macro cells, followed by Micro and Pico cells.

Question 12

What is a macro cell?

Answer 12

Macro Cells: designed to be general purpose cells with a wide range of coverage, usually mounted on a tall (50-150 meters) structure, or on a point of high elevation. Also called Boomers

Question 13

What is a micro cell?

Answer 13

Micro Cells: designed for general use in urban areas or other areas of high subscriber density. Typically, microcells are placed on shorter structures (10-20 meters).

Question 14

What is a pico cell?

Answer 14

Pico Cells: smaller cells designed for use in very high traffic areas, such as large hotels, airports, malls and sporting arenas.

Question 15

What is an umbrella cell?

Answer 15

Umbrella cells: used to keep strain off the normal cells in a LAC, often found along highways or other areas where a high rate of handoffs could occur and potentially degrade system performance. Also known as Cell Tiering, Underlay /Overlay, Hierarchal Cells.

Question 16

What is a cell-on-wheels and how is it used?

Answer 16

Cell-on-wheels: A self-sustained BTS that is used to provide service where there isn’t a currently a GSM tower. These are used to provide service to temporary events like music festivals, or in natural disasters when emergency responders need service.

Question 17

What is sectoring and what kinds are there?

Answer 17

This is done by using directional antennas on a base station, each antenna pointing in a different direction. These typically have a beam width of 40-120 degrees. This is done to increase capacity.

The three kinds are tri-sectoring, bi-sectoring, and omnidirectional.

Question 18

What is a BSC?

Answer 18

A Base Station Controller provides the intelligent management of a group of 10s to 100s of BTS’s.

The BSC makes all the decisions in a radio network such as allocation radio channels, managing handoffs between towers, and power control.

Routes all calls to and from towers under its control.

Question 19

What is the MSC?

Answer 19

The Mobile Switching Center performs all call routing in and out of a Location Area, is identified by its Location Area Code, and performs authentication.

It keeps track of all users in its Location Area with a local database called a Visitor Location Register (VLR).

Question 20

What is the VLR?

Answer 20

The VLR is the local database used by an MSC to track who comes in and out of a LA.

The VLR is responsible for TMSI assignments so no two users in a LA have the same TMSI

Once a user leaves the Location Area, the VLR erases their information.

Question 21

What is the GMSC?

Answer 21

A Gateway Mobile Switching Center (GMSC) is an MSC with the added capability of routing calls to and from external PLMNs (AT&T to T-Mobile for example).

When routing calls to external networks, GMSC’s assign a Mobile Station Routing Number (MSRN) that identifies the user to a foreign network.

Question 22

What is the HLR?

Answer 22

The Home Location Register (HLR) is a network’s master database.

Permanently stores the subscriber information for every single user in the network.

The MSC queries the HLR to authenticate users not yet stored in its local VLR.

Question 23

What is the AUC?

Answer 23

The Authentication Center (AuC) is responsible for storing user’s unique encryption keys and the encryption algorithms.

Also responsible for generating the authentication vectors called “The Triplets.”

Question 24

What is the EIR?

Answer 24

The EIR is responsible for storing the IMEI for the network’s users.

The EIR also groups the IMEIs of its users into three lists: the White, Grey, and Black Lists.

The White list denotes phones with no issues. Not a real list, just the absence of being on the grey or black list

The Grey list denotes handsets with some minor issue (too old for current revision, needing software updates, etc.

The Black list is for phones that have been determined illegal (cloned, missing, stolen, fraudulent, etc.)

The International EIR is in Dublin