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Mobile Computing

CSE 40814/60814 Fall 2012

Public Switched Telephone Network - PSTN! Transit switch Transit Transit switch Long distance

Local Local switch switch

Outgoing Incoming call call

- Transfer mode: - All the network (except part of the access network) is digital - Each voice channel is usually 64kb/s

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Basic Call! Calling terminal Network Called terminal

Off-hook

Resource allocation Dial tone

Dialing

Translation + routing Ring indication Alert signal

Remove ring indication Off hook

Bi-directional channel Conversation

On hook On hook signal

Billing

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Cellular Network Basics • /telephony is a radio-based technology; radio waves are electromagnec waves that antennas propagate • Most signals are in the 850 MHz, 900 MHz, 1800 MHz, and 1900 MHz frequency bands

Cell phones operate in this frequency range (note the logarithmic scale)

Cellular Network

• Base staons transmit to and receive from mobiles at the assigned spectrum – Mulple base staons use the same spectrum (spectral reuse) • The service area of each base staon is called a cell • Each mobile terminal is typically served by the ‘closest’ base staons – Handoff when terminals move

Architecture of Cellular Networks!

Server (e.g., Home Location Register)

External Mobile Network Station Base Switching Center

Cellular Network

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Registration!

Nr: 079/4154678

Tune on the strongest signal

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Service Request!

079/4154678 079/8132627 079/4154678 079/8132627

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Paging Broadcast!

079/8132627? 079/8132627?

079/8132627?

079/8132627?

Note: paging makes sense only over a small area

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Response!

079/8132627

079/8132627

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Channel Assignment!

Channel Channel 47 Channel 47 68

Channel 68

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Conversation!

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Handoff (or Handover)!

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Message Sequence Chart!

Base Base Caller Switch Callee Station Station

Periodic registration Periodic registration

Service request Service request

Page request Page request Paging broadcast Paging broadcast

Paging response Paging response

Assign Ch. 47 Assign Ch. 68 Tune to Ch.47 Tune to Ch. 68

Ring indication Ring indication Alert tone User response User response Stop ring indication Stop ring indication

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Cellular Network Generaons

• It is useful to think of a cellular network in terms of generaons: – 0G: Briefcase-size mobile radio telephones – : Analog cellular telephony – : Digital cellular telephony – : High-speed digital cellular telephony (including video telephony) – : IP-based “anyme, anywhere” voice, data, and mulmedia telephony at faster data rates than 3G (being deployed now)

Evoluon of Cellular Networks

1G 2G 2. 3G 4G

The Mulple Access Problem • The base staons need to serve many mobile terminals at the same me (both downlink and uplink) • All mobiles in the cell need to transmit to the base staon • Interference among different senders and receivers • So we need mulple access scheme

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Mulple Access Schemes

3 orthogonal Schemes: • Frequency Division Mulple Access (FDMA) • Time Division Mulple Access (TDMA) • Code Division Mulple Access (CDMA)

Frequency Division Mulple Access

frequency • Each mobile is assigned a separate frequency channel for the duraon of the call • Sufficient guard band is required to prevent adjacent channel interference • Usually, mobile terminals will have one downlink frequency band and one uplink frequency band • Different cellular network protocols use different frequencies • Frequency is a precious and scare resource. We are running out of it – cognive radio research

Time Division Mulple Access

Guard me – signal transmied by mobile terminals at different locaons do not arrive at the base staon at the same me • Time is divided into slots and only one mobile terminal transmits during each slot • Each user is given a specific slot. No compeon in cellular network – Unlike Carrier Sensing Mulple Access (CSMA) in WiFi

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Code Division Mulple Access • Use of orthogonal codes to separate different transmissions • Each symbol of bit is transmied as a larger number of bits using a user-specific code – Spreading – occupied by the signal is much larger than the informaon transmission rate – But all users use the same frequency band together

Orthogonal among users

GSM (2G) • Abbreviaon for Global System for Mobile Communicaons • Concurrent development in USA and Europe in the 1980s • The European system was called GSM and deployed in the early 1990s

GSM Services • Voice, 3.1 kHz • Short Message Service (SMS) – 1985 GSM standard that allows messages of at most 160 chars. (incl. spaces) to be sent between handsets and other staons – mul-billion $ industry • General Packet Radio Service (GPRS) – GSM upgrade that provides IP-based packet data transmission up to 114 kbps – Users can “simultaneously” make calls and send data – GPRS provides “always on” access and the Mulmedia Messaging Service (MMS) whereby users can send rich text, audio, video messages to each other – Performance degrades as number of users increase – GPRS is an example of 2.5G telephony – 2G service similar to 3G

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GSM Channels

Downlink Channels

Uplink

• Physical Channel: Each meslot on a carrier is referred to as a physical channel • Logical Channel: Variety of informaon is transmied between the MS and BTS. Different types of logical channels: – Traffic channel – Control Channel

GSM Frequencies

• Originally designed on 900MHz range, now also available on 800MHz, 1800MHz and 1900 MHz ranges. • Separate Uplink and Downlink frequencies – One example channel on the 1800 MHz frequency band, where RF carriers are spaced every 200 MHz UPLINK FREQUENCIES DOWNLINK FREQUENCIES

1710 MHz 1785 MHz 1805 MHz 1880 MHz

UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 95MHZ

GSM Architecture

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Mobile Staon (MS) • MS is the user’s handset and has two parts • Mobile Equipment – Radio equipment – User interface – Processing capability and memory required for various tasks • Call signalling • Encrypon • SMS – Equipment IMEI number • Subscriber Identy Module

Subscriber Identy Module

• A small smart card • Encrypon codes needed to fy the subscriber • Subscriber IMSI number • Subscriber’s own informaon (telephone directory) • Third party applicaons (banking, etc.) • Can also be used in other systems besides GSM, e.g., some WLAN access points accept SIM based user authencaon

Base Staon Subsystem • Transcoding Rate and Adaptaon Unit (TRAU) – Performs coding between the 64kbps PCM coding used in the backbone network and the 13kbps coding used for the Mobile Staon (MS) • Base Staon Controller (BSC) – Controls the channel (me slot) allocaon implemented by the BTSes – Manages the handovers within BSS area – Knows which mobile staons are within the cell and informs the MSC/VLR about this • Base Transceiver System (BTS) – Controls several transmiers – Each transmier has 8 me slots, some used for signaling, on a specific frequency

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Network and Switching Subsystem

• The backbone of a GSM network is a telephone network with addional cellular network capabilies • Mobile Switching Center (MSC) – A typical telephony exchange (ISDN exchange) which supports mobile communicaons – Visitor Locaon Register (VLR) • A database, part of the MSC • Contains the locaon of the acve Mobile Staons • Gateway Mobile Switching Center (GMSC) – Links the system to PSTN and other operators • Home Locaon Register (HLR) – Contain subscriber informaon, including authencaon informaon in Authencaon Center (AuC) • Equipment Identy Register (EIR) – Internaonal Mobile Staon Equipment Identy (IMEI) codes for e.g., blacklisng stolen phones

Home Locaon Register • One database per operator • Contains all the permanent subscriber informaon – MSISDN (Mobile Subscriber ISDN number) is the telephone number of the subscriber – Internaonal Mobile Subscriber Identy (IMSI) is a 15 digit code used to idenfy the subscriber • It incorporates a country code and operator code – IMSI code is used to link the MSISDN number to the subscriber’s SIM (Subscriber Identy Module) – Charging informaon – Services available to the customer • Also the subscriber’s present Locaon Area Code, which refers to the MSC, which can connect to the MS.

Other Systems • Operaons Support System – The management network for the whole GSM network – Usually vendor dependent – Very loosely specified in the GSM standards • Value added services – Voice mail – Call forwarding – Group calls • Short Message Service Center – Stores and forwards the SMS messages – Like an E-mail server – Required to operate the SMS services

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Locaon Updates • The cells overlap and usually a mobile staon can ‘see’ several transceivers (BTSes) • The MS monitors the idenfier for the BSC controlling the cells • When the mobile staon reaches a new BSC’s area, it requests a locaon update • The update is forwarded to the MSC, entered into the VLR, the old BSC is nofied and an acknowledgement is passed back

Handoff (Handover) • When a call is in process, the changes in locaon need special processing • Within a BSS, the BSC, which knows the current radio link configuraon (including feedbacks from the MS), prepares an available channel in the new BTS • The MS is told to switch over to the new BTS • This is called a hard handoff – In a so handoff, the MS is connected to two BTSes simultaneously

4 types of handover

1 2 3 4 MS MS MS MS

BTS BTS BTS BTS

BSC BSC BSC

MSC MSC

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Handover decision

receive level receive level

BTSold BTSold

HO_MARGIN

MS MS

BTSold BTSnew

Handover procedure

MS BTSold BSCold MSC BSCnew BTSnew measurement measurement report result

HO decision HO required HO request resource allocaon ch. acvaon ch. acvaon ack HO command HO request ack HO command HO command HO access Link establishment

HO complete HO complete clear command clear command

clear complete clear complete

Roaming • When a MS enters another operators network, it can be allowed to use the services of this operator – Operator to operator agreements and contracts – Higher billing • The MS is idenfied by the informaon in the SIM card and the idenficaon request is forwarded to the home operator – The home HLR is updated to reflect the MS’s current locaon

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3G Overview • 3G is created by ITU-T and is called IMT-2000

Evoluon from 2G

2G IS-95 GSM- IS-136 & PDC

GPRS IS-95B 2.5G HSCSD EDGE

Cdma2000-1xRTT W-CDMA 3G Cdma2000-1xEV,DV,DO EDGE TD-SCDMA Cdma2000-3xRTT 3GPP2 3GPP

Service Roadmap

Improved performance, decreasing cost of delivery

Broadband 3G-specific services take in wide area advantage of higher bandwidth and/or real-time QoS Video sharing Video telephony A number of mobile Real-time IP services are bearer Multitasking multimedia and games independent in nature WEB browsing Multicasting Corporate data access Streaming audio/video MMS picture / video xHTML browsing Application downloading E-mail Voice & SMS Presence/location Push-to-talk Typical average bit GSM GPRS EGPRS WCDMA HSDPA rates 9.6 171 473 2 (peak rates 1-10 higher) kbps kbps kbps Mbps Mbps 2000- 2000- EVDV EVDV EVDO EVDO CDMA CDMA CDMA CDMA CDMA 2000 1x 2000 1x

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GSM Evoluon to 3G High Speed Dedicate up to 4 timeslots for data connection ~ 50 kbps Good for real-time applications c.w. GPRS Inefficient -> ties up resources, even when nothing sent Not as popular as GPRS (many skipping HSCSD)

Enhanced Data Rates for Global Evolution GSM HSCSD Uses 8PSK modulation 9.6kbps (one timeslot) 3x improvement in data rate on short distances GSM Data Can fall back to GMSK for greater distances Also called CSD Combine with GPRS (EGPRS) ~ 384 kbps Can also be combined with HSCSD GSM GPRS WCDMA General Packet Radio Services Data rates up to ~ 115 kbps EDGE Max: 8 timeslots used as any one time Packet switched; resources not tied up all the time Contention based. Efficient, but variable delays GSM / GPRS core network re-used by WCDMA (3G)

UMTS • Universal Mobile Telecommunicaons System (UMTS) • UMTS is an upgrade from GSM via GPRS or EDGE • The standardizaon work for UMTS is carried out by Third Generaon Partnership Project (3GPP) • Data rates of UMTS are: – 144 kbps for rural – 384 kbps for urban outdoor – 2048 kbps for indoor and low range outdoor • Virtual Home Environment (VHE)

UMTS Frequency Spectrum

• UMTS Band – 1900-2025 MHz and 2110-2200 MHz for 3G transmission – In the US, 1710–1755 MHz and 2110–2155 MHz will be used instead, as the 1900 MHz band was already used.

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UMTS Architecture Base Station Network Subsystem Mobile Station Subsystem Other Networks

MSC/ GMSC BSC VLR PSTN BTS ME SIM

EIR HLR AUC PLMN

RNS GGSN SGSN RNC Internet ME B USIM

SD + UTRAN

Note: Interfaces have been omitted for clarity purposes.

UMTS Network Architecture

• UMTS network architecture consists of three domains – Core Network (CN): Provide switching, roung and transit for user traffic – UMTS Terrestrial Radio Access Network (UTRAN): Provides the air interface access method for . – User Equipment (UE): Terminals work as air interface counterpart for base staons. The various idenes are: IMSI, TMSI, P-TMSI, TLLI, MSISDN, IMEI, IMEISV

UTRAN • Wide band CDMA technology is selected for UTRAN air interface – WCDMA – TD-SCDMA • Base staons are referred to as Node-B and control equipment for Node-B is called as Controller (RNC). – Funcons of Node-B are • Air Interface Tx/Rx • Modulaon/Demodulaon – Funcons of RNC are: • Radio Resource Control • Channel Allocaon • Power Control Sengs • Handover Control • Ciphering • Segmentaon and reassembly

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3.5G (HSPA)

High Speed Packet Access (HSPA) is an amalgamaon of two protocols, High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), that extends and improves the performance of exisng WCDMA protocols 3.5G introduces many new features that will enhance the UMTS technology in future. 1xEV-DV already supports most of the features that will be provided in 3.5G. These include: - Adapve Modulaon and Coding - Fast Scheduling - Backward compability with 3G - Enhanced Air Interface

4G (LTE) • LTE stands for Long Term Evoluon • Next Generaon technology • Promises data transfer rates of 100 Mbps • Based on UMTS 3G technology • Opmized for All-IP traffic

Advantages of LTE

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Comparison of LTE Speed

Major LTE Radio Technogies • Uses Orthogonal Frequency Division Mulplexing (OFDM) for downlink • Uses Single Carrier Frequency Division Mulple Access (SC-FDMA) for uplink • Uses Mul-input Mul-output(MIMO) for enhanced throughput • Reduced power consumpon • Higher RF power amplifier efficiency (less baery power used by handsets)

LTE Architecture

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LTE vs UMTS • Funconal changes compared to the current UMTS architecture

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