1. Networks 5 Mobile Communications Dr John Cowell
phones off (please)
CTEC1414 Lecture 20
Networks 5
Mobile Communications
Dr John Cowell

2. Overview Developmental stages Overall architecture Common applications
Analogue
High-power transmitter based networks
Low-power transmitter based networks (cellular)
Digital
GSM & WAP 3G
Overall architecture
Cellular telephony
Hands-off protocol
Common applications
SMS, network broadcast, voice mail, etc

3. Basic Principles There are some mobile transmitters, i.e. phones
There are some stationary bases called mobile base stations
They communicate through radio frequencies, typically in the band Mhz
The radio frequencies are divided into channels, typically 30KHz each
A conversation uses 2 channels, i.e. duplex communication (send & receive)
Concurrent calls must be on different pairs of channels

4. The early days

5. Analogue mobile networks
High-power transmitters
Very much like terrestrial TV
One transmitter covers 100s of sq. miles
Small number of transmitters
Each transmitter has a dedicated number of channels
Limited number of calls
(( )*1000/30)/21600
And that’s over an area of 100s of sq. miles

6. Cellular networks
A collection of small, low-power transmitters carpet an area
The signal is still analogue and each transmitter works in the same frequency band
But
the number of calls is again 1600, but over a cell, i.e. much smaller area!
therefore, the number of calls increases over the whole area of a country

7. Cellular networks However
frequencies from close cells can interfere between each other
a notion of cluster is introduced
no two adjacent cells use the same frequency band
this reduces the number of calls in a cell by a factor of 7 on a hexagonal network,
but it allows reuse of frequencies

8. Capacity of cellular networks
Within a cluster, a frequency can’t be re-used
Over the whole range of Mhz, there are 1600 calls
This range is split between 7 cells in a cluster
Therefore each cell can handle 200 calls
Multiply by the number of cells in the whole country

9. Capacity of cellular networks
The number of calls a network can handle depends on the number of cells available
If increased capacity is required in a particular area (say in Leicester city centre), the number of cells will have to be increased
This means that the size of each cell will get smaller
Hence a cell will have to be split
create a cluster within a cell

10. Capacity of cellular networks

11. Digital Mobile Telephony

12. Digital Mobile Telephony
When the signals are encoded digitally, we have digital mobile telephony
The benefits are several
time sharing can be used
i.e. one channel is used by 3-6 users
the capacity is increased 3-6 times
quality of transmission can be improved by increasing noise resistance
security can be improved by scrambling the parts in the timeshared slots

13. Components of Digital Mobile Telephony
SIM
Subscriber Identification Module
smart card
address book
details of transmitted/received calls
network specific encoding
Handset
search for closest network base station
radio transmission/reception for calls, messages, etc
user interface to network services
Cell System
communicate with handset
transparent re-connection to a new cell whilst user is in motion
call routing
all other applications

14. Digital mobile telephony
Two types of digital standards
TDMA (Time Division Multiple Access)
used in GSM (the most popular standard)
the channel is shared in time between 3-6 users
capacity increases up to 15 times by using on-the-fly compression of the quiet pauses in a conversation
CDMA (Code Division Multiple Access)
each call has an identifier (code)
this allows many users to use one and the same frequency
there is no interference between different cells
the cluster has just one cell in it

15. Digital mobile telephony
GSM
(Global Standard for Mobile telecom) uses TDMA
Used by over 3 billion people over 212 countries
Easier roaming.
Standard emergency number (112)
2G (SMS) Short Message Service. – Text messaging
2.5G (WAP)
3G Wideband CDMA
(WCDMA)
144Kbps
Video
Internet access

16. Code Division Multiple Access
Allows everybody in a cell to use the same frequency
Separates calls by encoding each one uniquely
analogy:
international cocktail party with many people talking in different languages
if you hear a voice speaking in your language, your brain filters the background noise and locks onto to that one person
however, every other conversation is background noise!
CDMA base station controls the mobile phone’s power output which saves battery life
adjusts signal strength according to distance from base station

17. GSM: overall architecture

18. Top level view The GSM network is divided into three major systems:
the switching system (SS)
the base station system (BSS)
the operation and support system (OSS)

19. The Switching System
The switching system (SS) is responsible for performing call processing and subscriber-related functions
The SS includes the following functional units:
home location register (HLR)
database used to store and manage subscriptions
mobile services switching centre (MSC)
telephone switch
visitor location register (VLR)
database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers
authentication centre (AUC)
authentication and encryption of users
equipment identity register (EIR)
database with the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile phones
each handset has a unique IMEI number

20. The Base Station System (BSS)
All radio-related functions are performed in the BSS
consists of base station controllers (BSC) and the base stations (BS)
BSC - controls functions and physical links between the mobiles and BS
BS - handles the radio interface to the mobile station
antenna
Tower pictures from

21. The Operation and Support System
The OSS is the functional entity from which the network operator monitors and controls the system
cost-effective support for centralized, regional, and local operational and maintenance activities that are required for a GSM network
call routing, bill charging, administration, etc
provides a network overview and support the maintenance activities of different operation and maintenance organizations
eg turn off a cell for maintenance

22. Hands-off Protocol

23. Problem Statement
All is well until a mobile moves away from a base station
the signal between the mobile and the base station weakens
Moving away from one base station results (in most cases) in getting closer to another
Different base stations use different frequencies
a call must either be dropped or transferred to a different frequency
but dropping a call is unacceptable!

24. How is it done
The (home) base continuously monitors the strength of a mobile’s signal
The adjacent bases also monitor the signal
If it becomes weak, the base requests (to the switching system) a frequency transfer
The switching system finds out (from the neighbouring bases) which cell the mobile is closest to
The call is transferred to a new pair of frequencies without the user noticing

25. Applications SMS Network broadcast Voicemail
Short Message Service is a globally accepted
Allows 160 characters and transfer between mobile, , voic and paging
Uses a subsystem called Short Message Service Centre (SMSC) as a part of the Switching System
SMSC is a “store-and-forward” service; sometimes with “advice of delivery”
Network broadcast
sends messages to all mobiles in a cell
used for traffic, emergencies and weather updates
Voic
“store-and-retrieve” service, part of the Switching System
Fax, , notification, etc

26. Push-To-Talk In the USA text messaging is not widely used
alternative voice system – PtT
Uses half-duplex communication.
Real-time direct one-to-one and one-to-many voice communication
Active call group - ‘always on’ connection – permanently listening. Usually between people on the same network.
Extra button on handset
push to talk (release to listen)

27. Push-To-Talk
Based on half-duplex Voice over IP (VoIP) technology over the 2nd generation GSM/GPRS network
Uses cellular access and radio resources more efficiently than circuit-switched cellular services
network resources reserved only for duration of talk spurts
instead of for an entire call session
Read

28. And Finally…
A payphone on Lake Victoria in Uganda using GSM Technology and Solar Power
From

29. Summary Developmental stages Overall architecture Common applications
Analogue
High-power transmitter based networks
Low-power transmitter based networks (cellular)
Digital
GSM & WAP 3G
Overall architecture
Cellular telephony
Hands-off protocol
Common applications
SMS, network broadcast, voice mail, etc