Mobile Computing. Basic Call Calling terminal Network Called terminal 10/25/14. Public Switched Telephone Network - PSTN. CSE 40814/60814 Fall 2014

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Mobile Computing

CSE 40814/60814

Fall 2014

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Public Switched Telephone

Network - PSTN

Local

switch Local switch

Transit switch Outgoing call Incoming call Transit switch Transit switch

Long distance network

- Transfer mode: circuit switching

- 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 Dial tone Dialing

Ring indication Alert signal Off hook Remove ring indication

Bi-directional channel On hook Billing On hook signal Resource allocation Translation + routing Conversation

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Cellular Network Basics

•  Cellular network/telephony is a radio-‐based technology; radio waves are electromagneGc 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 staGons transmit to and receive from mobiles at the assigned spectrum

– MulGple base staGons use the same spectrum (spectral reuse) •  The service area of each base staGon is called a cell •  Each mobile terminal is typically served by the ‘closest’ base

staGons

– Handoﬀ when terminals move

Architecture of Cellular Networks

External

Network

Cellular Network Mobile

Station _Base

Station Mobile Switching Center Server

(e.g., Home Location Register)

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6

7

Registration

Tune on the strongest signal Nr: 079/4154678 8

Service Request

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

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9

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

Channel Assignment

Channel 47 Channel 47 Channel 68 Channel 68

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12

Conversation

13

Handoff (or Handover)

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Message Sequence Chart

Caller Base _Station Switch Base

Station Callee

Periodic registration Periodic registration

Service request _{Service request}

Ring indication Ring indication

Page request Page request

Paging broadcast _{Paging broadcast}

Paging response Paging response

Assign Ch. 47

Tune to Ch.47 Assign Ch. 68 Tune to Ch. 68 Alert tone User response User response

Stop ring indication Stop ring indication

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Cellular Network Genera3ons

• It is useful to think of a cellular network in terms of

genera+ons:

– 0G: Briefcase-‐size mobile radio telephones – 1G: Analog cellular telephony

– 2G: Digital cellular telephony

– 3G: High-‐speed digital cellular telephony (including video

telephony)

– 4G: IP-‐based “anyGme, anywhere” voice, data, and mulGmedia telephony at faster data rates than 3G (being deployed now)

Evolu3on of Cellular Networks

1G 2G 2.5G 3G 4G

The Mul3ple Access Problem

• The base staGons need to serve many mobile

terminals at the same Gme (both downlink

and uplink)

• All mobiles in the cell need to transmit to the

base staGon

• Interference among diﬀerent senders and

receivers

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Mul3ple Access Schemes

•  Frequency Division MulGple Access (FDMA) •  Time Division MulGple Access (TDMA) •  Code Division MulGple Access (CDMA)

3 orthogonal Schemes:

Frequency Division Mul3ple Access

•  Each mobile is assigned a separate frequency channel for the duraGon 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

– cogniGve radio research

frequency

Time Division Mul3ple Access

•  Time is divided into slots and only one mobile terminal

transmits during each slot

•  Each user is given a speciﬁc slot. No compeGGon in

cellular network

– Unlike Carrier Sensing MulGple Access (CSMA) in WiFi

Guard Gme – signal transmihed by mobile terminals at diﬀerent locaGons do not arrive at the base staGon at the same Gme

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Code Division Mul3ple Access

•  Use of orthogonal codes to separate diﬀerent transmissions •  Each symbol of bit is transmihed as a larger number of bits

using a user-‐speciﬁc code – Spreading

– Bandwidth occupied by the signal is much larger than the informaGon transmission rate

– But all users use the same frequency band together

Orthogonal among users

GSM (2G)

• AbbreviaGon for Global System for Mobile

CommunicaGons

• 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 staGons – mulG-‐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” Internet access and the MulGmedia

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

•  Physical Channel: Each Gmeslot on a carrier is referred to as a physical channel

•  Logical Channel: Variety of informaGon is transmihed between the MS and BTS. Diﬀerent types of logical channels:

– Traﬃc channel – Control Channel Downlink Uplink

Channels

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 kHz

1710 MHz 1785 MHz 1805 MHz 1880 MHz

UPLINK FREQUENCIES DOWNLINK FREQUENCIES

UPLINK AND DOWNLINK FREQUENCY SEPARATED BY 95MHZ

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Mobile Sta3on (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 • EncrypGon • SMS

– Equipment IMEI number

• Subscriber IdenGty Module

Subscriber Iden3ty Module

•  A small smart card

•  EncrypGon codes needed to idenGfy the subscriber •  Subscriber IMSI number

•  Subscriber’s own informaGon (telephone directory) •  Third party applicaGons (banking, etc.)

•  Can also be used in other systems besides GSM, e.g., some WLAN access points accept SIM based user authenGcaGon

Base Sta3on Subsystem

•  Transcoding Rate and AdaptaGon Unit (TRAU)

– Performs coding between the 64kbps PCM coding used in the backbone network and the 13kbps coding used for the Mobile StaGon (MS)

•  Base StaGon Controller (BSC)

– Controls the channel (Gme slot) allocaGon implemented by the BTSes

– Manages the handovers within BSS area

– Knows which mobile staGons are within the cell and informs the MSC/VLR about this

•  Base Transceiver System (BTS) – Controls several transmihers

– Each transmiher has 8 Gme 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 addiGonal cellular network capabiliGes

•  Mobile Switching Center (MSC)

– A typical telephony exchange (ISDN exchange) which supports mobile communicaGons

– Visitor LocaGon Register (VLR)

• A database, part of the MSC

• Contains the locaGon of the acGve Mobile StaGons

•  Gateway Mobile Switching Center (GMSC) – Links the system to PSTN and other operators •  Home LocaGon Register (HLR)

– Contain subscriber informaGon, including authenGcaGon informaGon in AuthenGcaGon Center (AuC)

•  Equipment IdenGty Register (EIR)

– InternaGonal Mobile StaGon Equipment IdenGty (IMEI) codes for e.g., blacklisGng stolen phones

Home Loca3on Register

•  One database per operator

•  Contains all the permanent subscriber informaGon – MSISDN (Mobile Subscriber ISDN number) is the telephone

number of the subscriber

– InternaGonal Mobile Subscriber IdenGty (IMSI) is a 15 digit code used to idenGfy 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 IdenGty Module)

– Charging informaGon – Services available to the customer

•  Also the subscriber’s present LocaGon Area Code, which refers to the MSC, which can connect to the MS.

Other Systems

•  OperaGons 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

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Loca3on Updates

• The cells overlap and usually a mobile staGon

can ‘see’ several transceivers (BTSes)

• The MS monitors the idenGﬁer for the BSC

controlling the cells

• When the mobile staGon reaches a new BSC’s

area, it requests a locaGon update

• The update is forwarded to the MSC, entered

into the VLR, the old BSC is noGﬁed and an

acknowledgement is passed back

Handoﬀ (Handover)

• When a call is in process, the changes in locaGon

need special processing

• Within a BSS, the BSC, which knows the current

radio link conﬁguraGon (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 son handoﬀ, the MS is connected to two BTSes

simultaneously

4 types of handover

MSC MSC BSC BSC BSC BTS BTS BTS BTS MS MS MS MS 1 ₂ ₃ ₄

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Handover decision

receive level BTSold receive level BTSold MS MS HO_MARGIN BTSold BTSnew

Handover procedure

HO access BTSold BSCnew measurement result BSCold Link establishment MSC MS measurement report HO decision HO required BTSnew HO request resource allocaGon ch. acGvaGon ch. acGvaGon ack HO request ack HO command HO command HO command 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 idenGﬁed by the informaGon in the SIM

card and the idenGﬁcaGon request is forwarded

to the home operator

– The home HLR is updated to reﬂect the MS’s current

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Evolu3on from 2G

IS-95 GSM- IS-136 & PDC

EDGE GPRS HSCSD IS-95B Cdma2000-1xRTT Cdma2000-1xEV,DV,DO Cdma2000-3xRTT W-CDMA EDGE TD-SCDMA 2G 3G 2.5G 3GPP 3GPP2

Service Roadmap

Improved performance, decreasing cost of delivery

Typical average bit rates (peak rates higher) WEB browsing Corporate data access Streaming audio/video

Voice & SMS Presence/location xHTML browsing Application downloading E-mail

MMS picture / video Multitasking 3G-specific services take advantage of higher bandwidth

and/or real-time QoS A number of mobile

services are bearer independent in nature HSDPA 1-10 Mbps WCDMA 2 Mbps EGPRS 473 kbps GPRS 171 kbps GSM 9.6 kbps Push-to-talk Broadband in wide area Video sharing Video telephony Real-time IP multimedia and games Multicasting C D MA 2000- EVD O C D MA 2000- EVD V C D MA 2000 1x

GSM Evolu3on to 3G

GSM 9.6kbps (one timeslot) GSM Data Also called CSD GSM

General Packet Radio Services Data rates up to ~ 115 kbps 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)

GPRS HSCSD High Speed Circuit Switched Data

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)

EDGE Enhanced Data Rates for Global Evolution Uses 8PSK modulation

3x improvement in data rate on short distances Can fall back to GMSK for greater distances Combine with GPRS (EGPRS) ~ 384 kbps Can also be combined with HSCSD

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UMTS

• Universal Mobile TelecommunicaGons System

(UMTS)

• UMTS is an upgrade from GSM via GPRS or EDGE

• The standardizaGon work for UMTS is carried out

by Third GeneraGon 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.

UMTS Architecture

SD Mobile Station MSC/ VLR Base Station Subsystem GMSC Network Subsystem AUC EIR HLR Other Networks

Note: Interfaces have been omitted for clarity purposes.

GGSN SGSN BTS BSC Node B RNC RNS UTRAN SIM ME USIM ME + PSTN PLMN Internet

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UMTS Network Architecture

• UMTS network architecture consists of three

domains

– Core Network (CN): Provide switching, rouGng and

transit for user traﬃc

– UMTS Terrestrial Radio Access Network (UTRAN):

Provides the air interface access method for user equipment.

– User Equipment (UE): Terminals work as air interface

counterpart for base staGons. The various idenGGes are: IMSI, TMSI, P-‐TMSI, TLLI, MSISDN, IMEI, IMEISV

UTRAN

•  Wide band CDMA technology is selected for UTRAN air interface

– WCDMA

– TD-‐SCDMA

•  Base staGons are referred to as Node-‐B and control equipment for Node-‐B is called as Radio Network Controller (RNC).

– FuncGons of Node-‐B are • Air Interface Tx/Rx • ModulaGon/DemodulaGon – FuncGons of RNC are:

• Radio Resource Control • Channel AllocaGon • Power Control Seqngs • Handover Control • Ciphering

• SegmentaGon and reassembly

3.5G (HSPA)

High Speed Packet Access (HSPA) is an amalgamaGon of two mobile telephony protocols, High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), that extends and improves the performance of exisGng 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: