2. Communication Paradigms & Mobile Telecommunications Infrastructures

Question 1

2 major paradigms for data transmission in communication

Answer 1

CIRCUIT-SWITCHED=communication line is used exclusively for communication parties (network resources used even if no data transferred; duration of connection used for billing)
— PACKET-ORIENTED=communication is divided into several packets, which get addressed & transferred using a shared transmission medium (network only used when data is transmitted; amount of transferred data used for billing)

Question 2

Cellular Network

Answer 2

radio network consisting of several transmitters/base stations which cover a certain area (=cell)

Question 3

Advantages of CBC

Answer 3

++higher capacity (e.g. by multiplexing*): more users can use infrastructure
++reduced transmission power: reduced power consumption for mobile devices
++robustness: failure of one base station doesn’t affect complete infrastructure
++better coverage: better availability of infrastructure

Question 4

Disadvantages of CBC

Answer 4

• -required infrastructure to link base stations is complex & costly
• -handover needed when changing from one cell to another (complex)
• -careful frequency planning to minimize interference (LTE 800 & LTE 700 = digital dividends)

Question 5

Multiplexing

Answer 5

describes how several users can share medium (e.g. mobile network) with minimum or no interference

Question 6

1st mobile radio network in Germany: “A-Network”/1G

Answer 6

Manual switching by an operator & for a call to a mobile; Operator needed to know the location area of the callee

Question 7

2nd mobile radio network in Germany: “B-Network”/2G

Answer 7

Automatic dial switching by area code, but caller needed to know that code of callee –> GSM = mobile communication standard for fully digital mobile networks (made cross-border mobile communication possible)

Question 8

3rd mobile radio network in Germany: “C-Network”/3G

Answer 8

First cell based mobile radio system in Germany;
Change of cells happens automatically by distance measuring to the nearest base station;
Network can automatically detect place of the call partner by use of HLR; telephone number ≠ allocated to the terminal but to a magnetic stripe card & later a chip card

• -> UMTS = Global Uniform Standard - Successor of GSM for mobile multimedia incl. video & audio transmissions
• -> HSPA/HSPA+ = High Speed Packet Access/Evolved HSPA

Question 9

LTE (Long Term Evolution)/4G

Answer 9

first all-IP mobile network technology -> higher data rates, capacity, etc.

Question 10

5G

Answer 10

fifth generation of cellular network technology – high bandwidth, low latency, high number of devices connected, …. –> use cases: autonomous driving, virtual reality
– View 1: hyper-connected vision – View 2: next-generation radio access technology

Question 11

Definition: GSM

Answer 11

(Global System for Mobile Communications) = mobile communication standard for fully digital mobile networks (worldwide roaming among different mobile network operators became possible)

Question 12

Services of GSM

Answer 12

• Carrier services: Services to transfer signals over GSM network (focus on voice services)
• Telecommunication services: Supports mobile communication among users
• Supplementary services: Specific to network operators -> e.g. caller ID, call redirect, closed user groups, Teleconference

Question 13

Architecture of GSM (3 main parts)

Answer 13

Network & Switching System (NSS), Operation Subsystem (OSS), Radio Subsystem (RSS) —> Process: PSTN - GMSC - HLR - VLR - MSC - BSC - BTS

Question 14

Network & Switching System (NSS)

Answer 14

COMPONENTS:
Mobile Switching Center (MSC): Switching Center for initiation, termination & handover –
Home Location Register (HLR): Central Data Base with subscribers’ data (mobile number, location) –
Visitor Location Register (VLR): Data Base assigned to every MSC with data of active subscribers in MSC’s range

Question 15

Operation Subsystem (OSS)

Answer 15

COMPONENTS:
Operation & Maintenance Center (OMC): supervises each network component & creates status reports –
Authentication Center (AuC):protects identity of participants –
Equipment Identity Register (EIR): data list with identification list for devices

Question 16

Radio Subsystem (RSS)

Answer 16

COMPONENTS:
Mobile Station (MB): System of mobile terminal (device) & SIM –
Base Transceiver Station (BTS): Radio facility for signal transfer –
Base Station Controller (BSC): Administrates affiliated BTS & supervises e.g. frequency allocation & connection handover between cells

Question 17

GSM Security Services

Answer 17

ACCESS CONTROL & AUTHENTICATION: authentication of the subscriber to the SIM by input of a PIN & to the GSM network by challenge-response procedure (idea: nobody should make use of your phone or your SIM card without password) –
CONFIDENTIALITY: data & voice transferred between mobile station & BTS = encrypted, but no end-to-end encryption –
PARTIAL ANONYMITY: no transfer of data which can identify the subscriber via radio, instead temporary identification; temporary mobile subscriber ID, TMSI; only the operator, but none of your neighbors should identify your mobile phone due to the data sent

Question 18

Security in GSM (2G): SIM based Subscriber Authentication

Answer 18

Key K(i) & Algorithm A3 are stored on SIM and deposited in the AuC.

1. AuC creates random number
2. AuC ecyrypts rand and K(i) via A3 (SRes*)
3. AuC transfers rand & SRes* to VLR
4. VLR transfers exclusively rand to SIM
5. SIM computers with “own” K(i) and A3 response SRes
6. SRes computed by SIM is transmitted to VLR & compares with SRes*
7. If SRes* = SRes the subscriber is authenticated successfully

Question 19

Security in GSM (2G): Encryption

Answer 19

1. AuC creates random number
2. AuC generates K(c) for encryption of transferred data via rand, K(i) & A8
3. VLR transfers only rand to SIM
4. SIM computed K(c) using A8, the rand received & local K(i)
5. Mobile station & mobile radio network use generated K(c) and A5 for encryption & decryption of sent and received data

Question 20

GSM (2G) Security Model: Shortcomings

Answer 20

Network doesn’t authenticate itself (only authentication of terminal/subscriber): assumption that network is trustworthy per se

• • Subscriber localization is almost exclusively controlled by network: centralized movement tracking is possible
• • Model bases partly on secret encryption algorithms (A3 & A8 published without authentication)
• • No encryption from terminal to terminal but only over air interface (deactivation by network possible, without notification of users)
• • Encryption comparatively “weak” because of key length

Question 21

Definition: UMTS

Answer 21

(Universal Mobile Telecommunication System) = Problem with 2G: different standards in different continents which avoid worldwide roaming -> demand for 3G: globally uniform standard -> Common approach: worldwide reservation of frequencies in 2GHz range

Question 22

UMTS Architecture

Answer 22

• UMTS Terrestrial Radio Access Network (UTRAN)
• Radio Network Subsystem (RNS) (=RSS)
• Radio Network Controller (RNC): controls the Node Bs (=MSC)
• Node B: UMTS base stations (equivalent to BTS in GSM): not so different as in GSM
• UMTS Core network: corresponds to Network- &Switching Subsystems (NSS) in GSM

Question 23

UMTS Security Model

Answer 23

• UMTS complements the security mechanisms known by GSM:
• -> enhanced participant authentication (EMSI)
• -> network authentication
• -> integrity protection of data traffic
• -> transferred security keys are also encrypted in the fixed network (e.g. HLR-VLR)
• -> increased key length
• -> end-to-end encryption = possible

Question 24

LTE

Answer 24

Long Term Evolution -> LTE Advanced (4G) makes use of frequency spectrum more efficiently, resulting in higher data rates (towards 1Gbit/s) & lower latency

Question 25

5G

Answer 25

Two views of 5G: 1. Hyper-connected vision (everything will be connected) - 2. Next-generation radio access technology (download/upload faster)

Potential Use Cases: autonomous driving, wireless-cloudbased office, machine-2-machine connectivity, AR

Question 26

Roaming

Answer 26

Denotes a change of network access.
• Change of GSM network operator
• Change between different network systems (UMTS, GSM, WLAN, …)
• Cell change within GSM system (handover): GSM manages handover with a short connection interruption -> reason: update of VLR