CTEC1414 Lecture 20

1. phones off(please) CTEC1414Lecture 20 Networks 5 Mobile Communications Dr John Cowell

2. Overview • Developmental stages • 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 800-900Mhz • 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 • 1G • 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 • ((900-800)*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 800-900Mhz, 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 http://www.telstra.com.au/newsroom/photob.htm#towers

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 • Short Message Service is a globally accepted • Allows 160 characters and transfer between mobile, e-mail, voicemail 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 • Voicemail • “store-and-retrieve” service, part of the Switching System • Fax, e-mail, 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 http://en.wikipedia.org/wiki/Push_to_talk

28. And Finally… • A payphone on Lake Victoria in Uganda using GSM Technology and Solar Power • From http://www.payphone-project.com/payphones/photos/africa/

29. Summary • Developmental stages • 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