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Networks 5 Mobile Communications Dr John Cowell

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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

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