Mobile Network Architecture and Applications

Sisteme mobile in Internet
(Arhitectura si aplicatiile retelelor mobile)

Nicolae Tomai
FSEGA

nicolae.tomai@econ.ubbcluj.ro
http://www.econ.ubbcluj.ro/~nicolae.tomai
449

Cuprins
 Introducere
 Wireless LANs: IEEE 802.11
 Rutarea IP mobila
 TCP in retele fara fir
 Retele GSM
 Arhitectura retelelor GPRS
 WAP(Wireless application protocol)
 Agenti mobili(Mobile agents)
 Retele mobile si peer-to-peer(MANET-Mobile
ad hoc networks)
2

References
 J. Schiller, “Mobile Communications”, Addison Wesley, 2000
 802.11 Wireless LAN, IEEE standards, www.ieee.org
 Mobile IP, RFC 2002, RFC 334, www.ietf.org
 TCP over wireless, RFC 3150, RFC 3155, RFC 3449
 A. Mehrotra, “GSM System Engineering”, Artech House, 1997
 Bettstetter, Vogel and Eberspacher, “GPRS: Architecture, Protocols
and Air Interface”, IEEE Communications Survey 1999, 3(3).
 M.v.d. Heijden, M. Taylor. “Understanding WAP”, Artech House, 2000
 Mobile Ad hoc networks, RFC 2501

 Site-uri web:
– www.palowireless.com
– www.gsmworld.com; www.wapforum.org
– www.etsi.org; www.3gtoday.com
3

Retele fara fir
 Ofera servicii de acces la calcul/comunicare in miscare

 Retele celulare
– Sisteme cu infastrucura bazata pe statii de baza

 Wireless LANs
– Retele locale in topologie infrastructura(cu AP)
– Foarte flexibile in zona de receptie
– Banda de transmisie destul de buna(>1 Mbit/s….)

 Ad hoc Networks
– Nu folosesc topologia infrastructura
– Sunt folosite pentru aplicatii militare, de salvare, acasa, etc.
4

Dispozitive mobile

Tablets
Palm-sized
Clamshell handhelds

Laptop computers Net–enabled mobile phones

Sisteme de operare pentru disozitive mobile

 Symbian-promovat ca OS(open source) de un
consortiu: Nokia, Motorola, etc.
 Windows Mobile-Microsoft
 Windows CE
 Windows mobile 7.0
 iPhone
 RIM BlackBerry
 Linux-cu varianta Android de la Google
 Palm OS

6

Evolutia telefoanelor mobile

7

Spectrul alocat pentru telefoanele mobile

Sursa: (S60 Programming A Tutorial Guide Paul Coulton,
8
Reuben Edwards With Helen Clemson)

Elemente componente ale unui sistem de telefonie
mobila

9

FDMA(Frecvency Division Multiple Access)(1G)

10

TDMA(Time Division Multiple
Access)(2G)

11

CDMA(Code Division Multiple Access)

 Permite utilizatorilor sa imparta atit timpul cit si frecventa
in acelasi timp, prin alocarea unui numar unic de
identificare
 Acest numar de identificare permite sistemului sa
separe un apel de altul, daca acestea erau facute in
acelasi timp
 E o tehnica de baza pentru telefoanele de generatia a
treia (3G) si permite viteze mari de transfer pentru
fiecare utilizator

12

CDMA(Code Division Multiple Access)

13

Sistemul GPRS

•SGSN(Serving GPRS Support Node) controlează transmiterea pachetelor
de date prin întreaga reţea şi
•GGSN(Gateway GPRS Support Node) care are rolul de a conecta reţeaua
de telefonie mobilă la infrastructura Internetului.
14

Alocarea frecventelor in 3G

15

Alocarea frecventelor in 3G
 Europa şi Japonia au optat pentru banda largă CDMA (W- CDMA) folosind
diviziunea frecvenţei(FDD) în două perechi de benzi ale spectrului de
frecvenţă.
 USA a optat pentru CdmaOne care foloseşte benzi multiple ale sistemului
pentru a realiza aşa numita undă purtătoare CDMA prin care se permitea
accesul mai multor utilizatori în acelaşi timp.
 Un alt sistem 3G, care e mai degrabă o extensie a GRPS -ului a sporit
transferul de date spre evoluţia GSM(EDGE), care modifică legăturile fără
fir între telefoanele mobile şi staţia de bază a sistemului GSM/GPRS pentru
îmbunătăţirea ratei de transfer a datelor, standard care a fost dezvoltat de
3GPP.(3G Partnership Project)care a fost creat în urma asocierii a două
categorii de organizaţii: organisme de standardizare şi reprezentaţi
comerciali. Organismele de standardizare participante sunt ETSI (Europa),
ARIB/TTC (Japonia), ANSI T1 (SUA) şi TTA (Coreea). Reprezentaţii pieţei
de telecomunicaţii sunt UMTS Forum, GSA şi GSM Association. Cele trei
reprezintă grupări majore de producători, operatori, companii de
consultanţă, etc., care susţin interese comerciale proprii legate de evoluţia
sistemului GSM
16

W-CDMA in sistemul GSM/GPRS

17

Evolutia spre 4G

18

Evolutia sistemelor celulare

19

Limitari ale sistemelor mobile
 Limitări datorate retelelor fara fir
 Limitari ale largimii benzii de comunicatie
 Deconectari frecvente
 Eterogeneitatea si fragmentarea retelelor

 Limitări datorate mobilitatii
 rute defecte(intrerupte)
 Lipsa facilitatilor privind mobilitatea a
sistemelor/aplicatiilor

 Limitări datorate dispozitivelor mobile
 Timp scurt de viata al bateriei
 Capacitati limitate(privind memoria, procesarea, etc.)
20

Comparatie intre retelele fara fir si cele cu fir
 Reglemetari ala frecventelor
– Limitarea disponibilitatii si necesitatea coordonarii
– Frecventele utilizate sunt deseori ocupate de alte aplicatii
 Latimea benzii si intirzierile
– Rate de transmisie relativ mici
• De la cativa Kbits/s la Mbit/s.
– Intirzieri mari
• Sute de milisecunde
– Rata mare a pierderilor
• susceptibile la interferenta, de ex, cu masini elecrice, siste de
iluminat, etc.
 Partajeaza intotdeauna un mediu comun
– Securitate scazuta, simplu de atacat
– Interferente radio
– Staţii de bază false poate atrage apelurile de pe telefoanele
mobile
– Necesita mecanisme de acces securizat
21

Siteme celulare: ideea de baza

 Conectivitate fara fir cu un singur salt(hop)
– Spatiul este divizat in celule
– O statie de baza este responsabila cu comunicarea
cu hosturile in celula ei
– Hosturile mobile pot schimba celulele in timpul
comunicarii
– Operatia de hand-off apare atunci când o gazdă
mobila începe comunicarea prin intermediul unei noi
staţii de bază
 Factorii ce determina dimensiunea celulei
– Numarul de utilizatori ce vor fi suportati
– Multiplexarea si tehnologiile de transmisie
22

Conceptul celular
 Numarul limitat de frecvente => limiteaza numarul canalelor
 Puterea de emisie a antenei => limiteaza numarul
utilizatorilor
 Celule mai mici => posibilitatea reutilizarii frecventelor =>
mai multi utilizatori
 Statie de baza (BS): implementeaza multiplexarea diviziunii
spatiului
– Cluster: group de BS apropiate care impreuna utilizeaza toate
canalele apropiate
 Statiile mobile comunica numai printr-o statie de baza
– FDMA, TDMA, CDMA pot fi utilizate intr-o celula
 O cerere de crestere se face (mai multe canale sunt
necesare)
– Numarul statiilor de baza este crecut
– Puterea de transmisie este redusa(descrescuta) corespunzator
pentru a reduce interferentele
23

Arhitectura sistemelor celulare
 Fiecare celula este deservita de o statie de baza(BS-Base Station)
 Fiecare sitem (BSS-Base Station Sistem) compus din statia de
baza si dispozitivele “legate” la ea este conectat la un centru de
comutare mobila (mobile switching center -MSC) prin legaturi fixe
 Fiecare MSC este conectat la alte MSC-uri si PSTN(Public
Switched Telephone Network)

MSC MSC

HLR HLR
La alte
VLR MSC-uri VLR

PSTN PSTN
24

Apel de configurare pentru iesirea in
retea-la apel(Outgoing call setup)
 Apel de configurare la iesire:
– Se introduce numarul şi se trimite
– Trnsmisiile mobile necesita o cerere de acces pe un
canal ascendent(uplink)de semnalizare
– Dacă reţeaua poate procesa apelul, BS trimite un
mesaj de alocare a canalului
– Reteaua procedeaza la setarea conexiunii(si
realizeaza incasarea)
 Activitatea retelei:
– MSC determina locatia curenta a tintei mobile
utilizind HLR, VLR si prin comunicarea cu alte MSC-
uri
– MSC-ul sursa initiaza un mesaj apel de configurare
la MSC-ul care acoperă zona ţintă 25

Apel de configurare la intrarea in
retea-la primire(Incoming call setup)
 Apel de configurare la iesire:
– MSC-ul tinta (ce acopera locatia curenta a mobilului)
initiaza un mesaj de paginare
– BS trimite mai departe(forward) mesajul de paginare
pe canalul de aducere(downlink) in aria de acoperire
– Daca mobilul este activat(monitorizand canalul de
semnalizare), el raspunde la BS
– BS trimite un mesaj de alocare a canalului si
informeaza MSC-ul
 Activitatea retelei:
– Reţeaua completează cele două jumătăţi ale
conexiunii
26

Termenul de hand-off(predare –preluare) se referă la procesul de transfer al unui
apel sau sesiuni de date de la un canal conectat la reţeaua de bază pentru un altul

 Initierea BS-ului:
– Parasirea unei celule si trecerea la una noua (hand-off) apare în
cazul în care nivelul semnalului de telefonie mobilă scade sub un
prag minim
– Creste incarcarea pe BS
• Semnalul de monitorizare a fiecarui mobil
• Determinarea tintei BS pentru predare-preluare(hand-off)
 Asistarea mobilului:
– Fiecare BS transmite periodic un semnal de prezenta/far(beacon)
– Mobiul la receptionarea unui semnal de prezenta/far puternic de
la un BS nou, iniţiază un proces de trecere(predare-primire)
 Intersistem:
– Se mută mobilele peste zone controlate de către diferite MSC-uri
– Gestionarea similara cu cazul mobilelor asistate prin
suplimentarea unui efort aditional al HLR/VLR
27

Efectul mobilitatii asupra stivei de
protocoale
 Aplicatie
– Aplicatii noi si adaptari
 Transport
– Controlul congestiei si al fluxului
 Retea
– Adresarea si rutarea
 Link
– Accesul la mediu si trecerea de la o celula la alta
(hand-off)
 Fizic
– Transmisia, erorile si interferenta
28

Aplicatii mobile(1)

 Vehicule
– Transmisia de noutati, conditii de drum, etc.
– Retele ad-hoc cu vehicule apropiate pentru
prevenirea accidentelor
 Urgente
– Transmiterea rapidă la spital a datelor pacienţilor
– Retele ad-hoc in caz de cutremure sau dezastre
naturale
– militare ...

29

Aplicatii mobile(2)
 Agenti de vinzari mobili
– Acces direct la baza de date centrala cu clientii
– Baze de date consistente pentru toţi agenţii
 Acces la Web
– Acces la Web dinafara companiei(de pe teren)
– Ghid turistic inteligent cu informaţii actualizate si
dependente de locatie
 Localizarea serviciilor
– Gasirea serviciilor in mediul local

30

Aplicatii mobile(3)

 Servicii de informare
– Cotatii bursiere, etc.
– Vremea

 Operatii deconectate
– Agenti mobili, cumparaturi, etc.

 Divertisment
– Retele ad-hoc pentru jocuri multi-utilizator
 Mesagerie

31

Aplicatii mobile in industrie

 Wireless access: (phone.com) openwave
 Alerting services: myalert.com
 Location services: (airflash) webraska.com
 Intranet applications: (imedeon) viryanet.com
 Banking services: macalla.com
 Mobile agents: tryllian.com
 ….

32

Latimea de banda si aplicatiile
UMTS
EDGE
GPRS, CDMA 2000
CDMA 2.5G
2G
Speed, kbps 9.6 14.4 28 64 144 384 2000

Transaction Processing
Messaging/Text Apps
Voice/SMS
Location Services
Still Image Transfers
Internet/VPN Access
Database Access
Document Transfer
Low Quality Video
High Quality Video

33

Evolutia retelelor celulare
 First-generation: Analog cellular systems (450-900 MHz)
– Frequency shift keying; FDMA for spectrum sharing
– NMT (Europe), AMPS (US)
 Second-generation: Digital cellular systems (900, 1800 MHz)
– TDMA/CDMA for spectrum sharing; Circuit switching
– GSM (Europe), IS-136 (US), PDC (Japan)
– <9.6kbps data rates
 2.5G: Packet switching extensions
– Digital: GSM to GPRS; Analog: AMPS to CDPD
– <115kbps data rates
 3G: Full-fledged data services
– High speed, data and Internet services
– IMT-2000, UMTS
– <2Mbps data rates
 4G 34

GSM to GPRS
 Resursele radio sunt alocate numai pentru unul sau mai
multe(câteva) pachete la un moment dat, aşa ca GPRS
permite:
– Ca mai multi utilizatori sa partajaeze resursele radio
şi transportul eficient de pachete
– conectivitate la reţele externe de date orientate spre
pachete
– Tarifarea bazata pe volumul de trafic

 Rata datelor mai mare (pana la 171 kbps in cazul ideal)
 GPRS transmite SMS-urile pe canalele de date si nu pe
cele de semnalizare ca GSM

35

UMTS: Universal Mobile Telecomm. (standard)

 Global seamless operation in multi-cell environment
(SAT, macro, micro, pico)
 Global roaming: multi-mode, multi-band, low-cost
terminal, portable services & QoS

 High data rates at different mobile speeds: 144kbps at
vehicular speed (80km/h), 384 kbps at pedestrian
speed, and 2Mbps indoor (office/home)
 Multimedia interface to the internet
 Based on core GSM, conforms to IMT-2000
 W-CDMA as the air-interface

36

Evolution to 3G Technologies

2G 3G
IS-95B
cdma2000
CDMA

FDD
GSM W-CDMA

TDD
GPRS
EDGE & 136
HS outdoor

IS-136 136 HS
UWC-136
TDMA indoor
37

Tehnologii fara fir

802.11n
>150 Mbps 802.11n

70 Mbps
802.16(WiMax)
54 Mbps 802.11{a,b}
5-11 Mbps 802.11b .11 p-to-p link
1-2 Mbps
Bluetooth
802.11 µwave p-to-p links

4G

3G
384 Kbps WCDMA, CDMA2000
2G
56 Kbps IS-95, GSM, CDMA

Interior Exterior Exterior Exterior pe Distanta
pe dist medie dist. mare lunga

10 – 30m 50 – 200m 200m – 4Km 5Km – 20Km 20m – 50Km

38

Comparatie intre tehnologii

Covearge
10
3G -HSPA
WiFi
8
LTE
6
WiMAX
4
QoS Data rate
2

0

Mobility Cost effectiveness per bit

LTE (Long Term Evolution)-4G
39

Arhitectura retelei 3G

Core Network
Wireless
Telephone
Access Network
Programmable Network
Gateway
Mobile Access Softswitch
Router
Application
IP Intranet Server

Access (HLR)
IP Intranet
IP Point User Profiles &
Base Stations Authentication
802.11

802.11
3G Air Wired Access
Internet
Interface
Access
Point
40

Retele fara fir locale WLAN

 Advantage
– Foarte flexibile in aria(zona) de receptie
– Posibilitatea de realizare topologii ad-hoc
– Legare usoara la retelele cablate
 Dezavataje
– Banda relativ joasa comparativ cu retelele cablate
– Multe solutii proprietar

 Topologie infrastructura sau ad-hoc (802.11)

41

Topologiile retelelor fara fir(infrastructura si
Adhoc)
infrastructure
network
AP: Access Point
AP

AP wired network
AP

ad-hoc network

42
Source: Schiller

Difference Between Wired and
Wireless
Ethernet LAN Wireless LAN
B
A B C
A C

 If both A and C sense the channel to be idle at the
same time, they send at the same time.
 Collision can be detected at sender in Ethernet.
 Half-duplex radios in wireless cannot detect collision
at sender.
43

Hidden Terminal Problem

A B C
– A and C cannot hear each other.
– A sends to B, C cannot receive A.
– C wants to send to B, C senses a “free” medium
(CS fails)
– Collision occurs at B.
– A cannot receive the collision (CD fails).
– A is “hidden” for C.

44

IEEE 802.11

 Acknowledgements for reliability
 Signaling packets for collision avoidance
– RTS (request to send)
– CTS (clear to send)
 Signaling (RTS/CTS) packets contain
– sender address
– receiver address
– duration (packet size + ACK)
 Power-save mode

45

Spectrum War: Status today
Enterprise 802.11 Wireless Carrier Public 802.11
Network

46
Source: Pravin Bhagwat

Spectrum War: Evolution
Network

 Market consolidation
 Entry of Wireless Carriers
 Entry of new players
 Footprint growth

47

Spectrum War: Steady State
Network

Virtual Carrier

 Emergence of virtual
carriers
 Roaming agreements

48

Routarea si mobilitatea

 Gasirea unei cai de la o sursa la o destinatie
 Probleme
– Schimbarea frecventa a rutelor
– Schimbarea rutei poate fi in legatura cu miscarea
hostului
– Latimea de banda relativ mica a legaturilor
 Scopul protocoalelor de rutare
– Micsorarea rutarii in ce priveste cimpurile
aditionale(overhead)
– Gasirea celor mai scurte rute
– Gasirea rutelor “stabile”(despite mobility)

49

IP-ul mobil: Ideea de baza

MN Router
S
3

Home
agent

Router Router
1 2

50
Source: Vaidya

IP mobil: ideea de baza

miscare

Router
S MN
3

Foreign agent

Home agent

Router Router Pachetele sunt tunelate
utilizind IP in IP
1 2

51
Source: Vaidya

Protocoalele TCP si UDP in cazul retelelor fara
fir
 TCP asigură:
– Livrarea sigura si ordonata a pachetepor(utilizeaza
retransmisiile, daca este necesar)
– ACK-uri cumulative(un raspuns ACK-acknowledges
pentru date primite contiguu-contiguously received data)
– ACK-uri duplicat (ori de cite ori este receptionat un
segment “neasteptat”-cu numar de secventa incorect-
out-of-order)
– Semantici cap la cap-end-to-end(receptorul trimite ACK
dupa ce data a ajuns)
– Implementeaza evitarea congestiei si utilizeaza
controlul de tip fereasta de congestie- congestion
window 52

TCP in retele fără fir

 Factorii ce afecteaza protocolul TCP in retele fara fir:
– Erorile de transmisie in mediul fara fir
• Pot cauza retransmiterea rapida- fast retransmit, ceea ce duce la
diminuarea dimensiunii ferestrei de congestie
• Reducerea ferestrei de congestie ca raspuns la erori nu este
necesara
– Rute cu multe salturi(multi-hop routes) in mediul fara fir
partajat
• Conexiunile “lungi”(rute cu multe hopuri) sunt mai dezavantajoase
decit cele “scurte” pentru ca trebuie sa contina accesul la mediul
fara fir in fiecare hop
– Defectarea rutelor datorita mobilitatii

53

Indirect TCP (I-TCP)
 I-TCP splits the TCP connection
– no changes to the TCP protocol for wired hosts
– TCP connection is split at the foreign agent
– hosts in wired network do not notice
characteristics of wireless part
– no real end-to-end connection any longer

mobile host
access point
(foreign agent) „wired“ Internet

„wireless“ TCP standard TCP

54
Source: Schiller

TCP mobil (M-TCP)

 Gestioneaza deconectari frecvente si interminabile
 M-TCP spliteaza ca si I-TCP dar,
– Nu modifica TCP-ul pentru reteaua fixa la agentul
strain(FA-foreign agent)
– optimizeaza TCP pentru FA to MH
 Agentul strain(FA-Foreign Agent)
– Monitorizeaza toate pachetele si daca detecteaza o
deconectare atunci:
• Seteaza fereastra emitatorului la 0
• Emitatorul(sender) trece automat in modul repetat
– Fara caching, fara retransmisii
55

Adaptarea aplicatiilor pentru mobilitate
 Probleme de proiectare
 Sistem transparent sau sistem netransparent/adecvat/
care cunoaste faptul ca va lucra intr-o retea fara fir
 Aplicatie transparenta sau aplicatie netransparenta
/adecvata/care cunoaste faptul ca va lucra intr-o retea
fara fir
 Modele
 Model conventional de tip client/server
 Model client/proxy/server
 Mode caching/cu pre-incarcare
 Model cu agenti mobili
56

World Wide Web-ul si mobilitatea
 Caracteristicile protocolului HTTP
– A fost proiectat pentru banda larga si intirziere mica
– E de tip client/server, iar comunicarea este de tip
cerere/raspuns
– E orientat conexiune, o conexiune pe cerere
– Utilizeaza protocolul TCP intr-un dialog in trei pasi,
foloseste de asemenea protocolul DNS
 Caracteristicile HTML
– Proiectat pentru calculatoare cu performante
ridicate, afisaje color de mare definitie, mose, hard
disk, etc.
– De obicei paginile Web sunt optimizate pentru
proiectare nu pentru comunicare, ignorind
caracteristicile sistemelor clientului(end-system). 57

Sisteme suport pentru WWW mobil
 Browsere cu facilitati adaugate/sporite
– Suport client adecvat pentru mobilitate
 Proxi
– Client proxi: cu pre incarcare, memorare
temporara, utilizare off-line
– Retea proxi: transformarea adaptiva a continutului
pentru conexiuni
– Proxi pentru client si retea
 Servere cu facilitati sporite
– Servere cu suport adecavat pentru mobilitate
– Furnizarea continutului in multiple moduri in
functie de capabilitatile clientului
 Protocoale/limbaje noi
– WAP/WML 58

Modelul client/proxy/server
 Functiuni proxi atit pentru un client cit si pentru
serverul retelei fixe
 Functiuni proxi pentru server adecvate mobilitatii
la clientul mobil
 Proxi-ul poate fi plasat in hostul mobil(Coda), sau
in reteaua fixa sau la ambele (WebExpress)
 Permite proiectarea de de clienti
“slabi”(smart/thin client) in cazul unor dispozitive
cu resurse reduse(aplicatia se lanseaza din
browser si poate rula numai conectata si nu
autonom fat client)
59

Proxi Web in WebExpress

The WebExpress Intercept Model

60
Source: Helal

WAP(Wireless Application Protocol)
 Navigator-browser
– “Micro browser”, similar navigatoarelor existente
 Limbajul de script
– Similar limbajului Javascript, adaptat la dispozitivele mobile
 Poarta-Gateway
– Transitie de la sistemele fara fir la cele cu fir
 Serverul
– “Serverul WAP/ origine-Wap/Origin server”, similar
serverelor Web existente
 Nivelele protocolului
– Nivelul transport, securitate, sesiune, etc.
 Interfata cu aplicatia de telefonie
– Functii de accces la telefonie
61

WAP: elementele componente
fixed network wireless network

HTML WML WAP Binary WML
Internet
filter proxy

HTML WML
HTML
filter/ Binary WML
WAP
web HTML proxy
server

WTA Binary WML
server
PSTN

Binary WML: binary file format for clients

62
Source: Schiller

WAP: modelul de referinta
Internet A-SAP WAP

HTML, Java Application Layer (WAE) Servicii aditionale
si aplicatii
S-SAP
Session Layer (WSP)
HTTP TR-SAP
Transaction Layer (WTP)
SEC-SAP
SSL/TLS Security Layer (WTLS)
T-SAP

TCP/IP, Transport Layer (WDP) WCMP
UDP/IP,
media Purtatoarele (GSM, CDPD, GPRS ...)

WAE comprises WML (Wireless Markup Language), WML Script, WTAI etc.

63
Source: Schiller

Stiva de protocoale WAP
 WDP
– Functionalitati similare cu UDP in retele IP
 WTLS
– functionalitati similare cu SSL/TLS (optimizat pentru retele fara fir)
 WTP
– Clasa 0: analog cu UDP
– Clasa 1: analog cu TCP (fara setarea privind overhead-ul conexiunii)
– Clasa 2: analog cu RPC (optimizat pentru retele fara fir)
– features of “user acknowledgement”, “hold on”
 WSP
– WSP/B: analog cu http 1.1 (cu facilitati de suspendare/reluare)
– metoda: analoaga cu RPC/RMI
– Caracteristici de invocare asincrona (confirmate/neconfirmate)
64

Modelul cu agenti mobili
 Agentul mobil primeste cererea clientului si
 Agentul mobil se muta in reteaua fixa

 Agentul mobil actioneza la server ca si un client
 Agentul mobil realizeaza transformarile si filtrarea

 Agentul mobil se intoarce inapoi la platforma mobila
atunci cind clientul este conectat

65

Mobile Agents: Exemplu

66

Cuprins
 Introducere
 Retele GSM
ad hoc networks)
67

How Wireless LANs are different

 Destination address does not equal destination
location
 The media impact the design
– wireless LANs intended to cover reasonable
geographic distances must be built from basic
coverage blocks
 Impact of handling mobile (and portable)
stations
– Propagation effects
– Mobility management
– power management

68

Wireless Media
 Physical layers in wireless networks
– Use a medium that has neither absolute nor readily
observable boundaries outside which stations are unable to
receive frames
– Are unprotected from outside signals
– Communicate over a medium significantly less reliable than
wired PHYs
– Have dynamic topologies
– Lack full connectivity and therefore the assumption normally
made that every station (STA) can hear every other STA in
invalid (I.e., STAs may be “hidden” from each other)
– Have time varying and asymmetric propagation properties

69

802.11: Motivation
 Can we apply media access methods from fixed networks
 Example CSMA/CD
– Carrier Sense Multiple Access with Collision Detection
– send as soon as the medium is free, listen into the medium if a
collision occurs (original method in IEEE 802.3)
 Medium access problems in wireless networks
– signal strength decreases proportional to the square of the
distance
– sender would apply CS and CD, but the collisions happen at the
receiver
– sender may not “hear” the collision, i.e., CD does not work
– CS might not work, e.g. if a terminal is “hidden”
 Hidden and exposed terminals
70

Solution for Hidden/Exposed Terminals
 A first sends a Request-to-Send (RTS) to B
 On receiving RTS, B responds Clear-to-Send (CTS)
 Hidden node C overhears CTS and keeps quiet
– Transfer duration is included in both RTS and CTS
 Exposed node overhears a RTS but not the CTS
– D‟s transmission cannot interfere at B

RTS RTS
D A B C
CTS CTS
DATA
71

IEEE 802.11
 Wireless LAN standard defined in the unlicensed
spectrum (2.4 GHz and 5 GHz U-NII bands)

 Standards covers the MAC sublayer and PHY layers
 Three different physical layers in the 2.4 GHz band
– FHSS, DSSS and IR
 OFDM based PHY layer in the 5 GHz band

72

Components of IEEE 802.11
architecture
 The basic service set (BSS) is the basic building
block of an IEEE 802.11 LAN
 The ovals can be thought of as the coverage area
within which member stations can directly
communicate
 The Independent BSS (IBSS) is the simplest LAN. It
may consist of as few as two stations
ad-hoc network BSS1 BSS2

73

802.11 - ad-hoc network (DCF)
802.11 LAN

STA1  Direct communication
BSS1 STA3
within a limited range
– Station (STA):
terminal with access
STA2
mechanisms to the
wireless medium
– Basic Service Set (BSS):
BSS2 group of stations using the
same radio frequency
STA5

STA4 802.11 LAN

74
Source: Schiller

802.11 - infrastructure network (PCF)
Station (STA)
802.11 LAN – terminal with access
802.x LAN
mechanisms to the wireless
medium and radio contact to
STA1 the access point
BSS1 Basic Service Set (BSS)
Portal
Access – group of stations using the
Point same radio frequency
Distribution System Access Point
Access – station integrated into the
ESS Point wireless LAN and the
distribution system
BSS2 Portal
– bridge to other (wired)
networks
Distribution System
STA2 STA3
802.11 LAN – interconnection network to
form one logical network (EES:
Extended Service Set) based
on several BSS 75
Source: Schiller

Distribution System (DS) concepts
 The Distribution system interconnects multiple BSSs
 802.11 standard logically separates the wireless
medium from the distribution system – it does not
preclude, nor demand, that the multiple media be
same or different
 An Access Point (AP) is a STA that provides access
to the DS by providing DS services in addition to
acting as a STA.
 Data moves between BSS and the DS via an AP
 The DS and BSSs allow 802.11 to create a wireless
network of arbitrary size and complexity called the
Extended Service Set network (ESS)

76

802.11- in the TCP/IP stack
fixed terminal
mobile terminal

server

infrastructure network

access point

application application
TCP TCP
IP IP
LLC LLC LLC
802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC
802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY

77

802.11 - Layers and functions

 MAC  PLCP Physical Layer Convergence
Protocol
– access mechanisms,
fragmentation, encryption – clear channel assessment
signal (carrier sense)
 MAC Management
– synchronization, roaming,
 PMD Physical Medium Dependent
MIB, power management – modulation, coding
 PHY Management

Station Management
– channel selection, MIB
LLC  Station Management
DLC

MAC MAC Management – coordination of all
management functions
PLCP
PHY

PHY Management
PMD

7.8.1 78

802.11 - Physical layer
 3 versions: 2 radio (typically 2.4 GHz), 1 IR
– data rates 1, 2, or 11 Mbit/s
 FHSS (Frequency Hopping Spread Spectrum)
– spreading, despreading, signal strength, typically 1 Mbit/s
– min. 2.5 frequency hops/s (USA), two-level GFSK modulation
 DSSS (Direct Sequence Spread Spectrum)
– DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying),
DQPSK for 2 Mbit/s (Differential Quadrature PSK)
– preamble and header of a frame is always transmitted with 1 Mbit/s
– chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code)
– max. radiated power 1 W (USA), 100 mW (EU), min. 1mW
 Infrared
– 850-950 nm, diffuse light, typ. 10 m range
– carrier detection, energy detection, synchonization

79

Spread-spectrum communications

80
Source: Intersil

DSSS Barker Code modulation

81
Source: Intersil

DSSS properties

82
Source: Intersil

802.11 - MAC layer

 Traffic services
– Asynchronous Data Service (mandatory) – DCF
– Time-Bounded Service (optional) - PCF

 Access methods
– DCF CSMA/CA (mandatory)
• collision avoidance via randomized back-off mechanism
• ACK packet for acknowledgements (not for broadcasts)
– DCF w/ RTS/CTS (optional)
• avoids hidden terminal problem
– PCF (optional)
• access point polls terminals according to a list

83

802.11 - Carrier Sensing
 In IEEE 802.11, carrier sensing is performed
– at the air interface (physical carrier sensing), and
– at the MAC layer (virtual carrier sensing)
 Physical carrier sensing
– detects presence of other users by analyzing all detected
packets
– Detects activity in the channel via relative signal strength
from other sources
 Virtual carrier sensing is done by sending MPDU duration
information in the header of RTS/CTS and data frames
 Channel is busy if either mechanisms indicate it to be
– Duration field indicates the amount of time (in microseconds)
required to complete frame transmission
– Stations in the BSS use the information in the duration field to
adjust their network allocation vector (NAV)

84

802.11 - Reliability
 Use of acknowledgements
– When B receives DATA from A, B sends an ACK
– If A fails to receive an ACK, A retransmits the DATA
– Both C and D remain quiet until ACK (to prevent collision of
ACK)
– Expected duration of transmission+ACK is included in
RTS/CTS packets

RTS RTS
D A B C
CTS CTS
DATA

ACK

85

802.11 - Priorities

 defined through different inter frame spaces – mandatory idle time
intervals between the transmission of frames
 SIFS (Short Inter Frame Spacing)
– highest priority, for ACK, CTS, polling response
– SIFSTime and SlotTime are fixed per PHY layer
– (10 s and 20 s respectively in DSSS)
 PIFS (PCF IFS)
– medium priority, for time-bounded service using PCF
– PIFSTime = SIFSTime + SlotTime
 DIFS (DCF IFS)
– lowest priority, for asynchronous data service
– DCF-IFS (DIFS): DIFSTime = SIFSTime + 2xSlotTime

86

802.11 - CSMA/CA
contention window
DIFS DIFS (randomized back-off
mechanism)

medium busy next frame

direct access if t
medium is free  DIFS slot time

– station ready to send starts sensing the medium (Carrier Sense
based on CCA, Clear Channel Assessment)
– if the medium is free for the duration of an Inter-Frame Space
(IFS), the station can start sending (IFS depends on service
type)
– if the medium is busy, the station has to wait for a free IFS, then
the station must additionally wait a random back-off time
(collision avoidance, multiple of slot-time)
– if another station occupies the medium during the back-off time
of the station, the back-off timer stops (fairness)
87

802.11 –CSMA/CA example
DIFS DIFS DIFS DIFS
boe bor boe bor boe busy
station1

boe busy
station2

busy
station3

boe busy boe bor
station4

boe bor boe busy boe bor
station5
t

busy medium not idle (frame, ack etc.) boe elapsed backoff time

packet arrival at MAC bor residual backoff time

88

802.11 - Collision Avoidance
 Collision avoidance: Once channel becomes idle, the
node waits for a randomly chosen duration before
attempting to transmit
 DCF
– When transmitting a packet, choose a backoff interval in the
range [0,cw]; cw is contention window
– Count down the backoff interval when medium is idle
– Count-down is suspended if medium becomes busy
– When backoff interval reaches 0, transmit RTS
 Time spent counting down backoff intervals is part of
MAC overhead

89

DCF Example

B1 = 25 B1 = 5
wait data

data wait
B2 = 20 B2 = 15 B2 = 10

B1 and B2 are backoff intervals
cw = 31 at nodes 1 and 2

90

802.11 - Congestion Control

 Contention window (cw) in DCF: Congestion
control achieved by dynamically choosing cw
 large cw leads to larger backoff intervals
 small cw leads to larger number of collisions

 Binary Exponential Backoff in DCF:
– When a node fails to receive CTS in response to
its RTS, it increases the contention window
• cw is doubled (up to a bound CWmax)
– Upon successful completion data transfer, restore
cw to CWmin
91

802.11 - CSMA/CA II
 station has to wait for DIFS before sending data
 receivers acknowledge at once (after waiting for SIFS) if the
packet was received correctly (CRC)
 automatic retransmission of data packets in case of
transmission errors

DIFS
data
sender
SIFS
ACK
receiver
DIFS
other data
stations t
waiting time contention

92

802.11 –RTS/CTS
 station can send RTS with reservation parameter after waiting for DIFS
(reservation determines amount of time the data packet needs the medium)
 acknowledgement via CTS after SIFS by receiver (if ready to receive)
 sender can now send data at once, acknowledgement via ACK
 other stations store medium reservations distributed via RTS and CTS

DIFS
RTS data
sender
SIFS SIFS
CTS SIFS ACK
receiver

NAV (RTS) DIFS
other NAV (CTS) data
stations t
defer access contention

93

Fragmentation

DIFS
RTS frag1 frag2
sender
SIFS SIFS SIFS
CTS SIFS ACK1 SIFS ACK2
receiver

NAV (RTS)
NAV (CTS)
NAV (frag1) DIFS
other NAV (ACK1) data
stations t
contention

94

802.11 - Point Coordination Function

95

802.11 - PCF I

t0 t1
SuperFrame

medium busy PIFS SIFS SIFS
D1 D2
point
coordinator SIFS SIFS
U1 U2
wireless
stations
stations„ NAV
NAV

96

802.11 - PCF II

t2 t3 t4

PIFS SIFS
D3 D4 CFend
point
coordinator SIFS
U4
wireless
stations
stations„ NAV
NAV contention free period contention t
period

97

CFP structure and Timing

98

PCF- Data transmission

99

Polling Mechanisms

 With DCF, there is no mechanism to guarantee
minimum delay for time-bound services
 PCF wastes bandwidth (control overhead) when
network load is light, but delays are bounded
 With Round Robin (RR) polling, 11% of time was
used for polling
 This values drops to 4 % when optimized polling is
used
 Implicit signaling mechanism for STAs to indicate
when they have data to send improves performance

100

Coexistence of PCF and DCF
 PC controls frame transfers during a Contention Free
Period (CFP).
– CF-Poll control frame is used by the PC to invite a station to
send data
– CF-End is used to signal the end of the CFP
 The CFP alternates with a CP, when DCF controls
frame transfers
– The CP must be large enough to send at least one
maximum-sized MPDU including RTS/CTS/ACK
 CFPs are generated at the CFP repetition rate and
each CFP begins with a beacon frame

101

802.11 - Frame format
 Types
– control frames, management frames, data frames
 Sequence numbers
– important against duplicated frames due to lost ACKs
 Addresses
– receiver, transmitter (physical), BSS identifier, sender (logical)
 Miscellaneous
– sending time, checksum, frame control, data
bytes 2 2 6 6 6 2 6 0-2312 4
Frame Duration Address Address Address Sequence Address
Data CRC
Control ID 1 2 3 Control 4

version, type, fragmentation, security, ...

102

Frame Control Field

103

Types of Frames

 Control Frames
– RTS/CTS/ACK
– CF-Poll/CF-End
 Management Frames
– Beacons
– Probe Request/Response
– Association Request/Response
– Dissociation/Reassociation
– Authentication/Deauthentication
– ATIM
 Data Frames

104

MAC address format
scenario to DS from address 1 address 2 address 3 address 4
DS
ad-hoc network 0 0 DA SA BSSID -
infrastructure 0 1 DA BSSID SA -
network, from AP
infrastructure 1 0 BSSID SA DA -
network, to AP
infrastructure 1 1 RA TA DA SA
network, within DS

DS: Distribution System
AP: Access Point
DA: Destination Address
SA: Source Address
BSSID: Basic Service Set Identifier
RA: Receiver Address
TA: Transmitter Address

105

802.11 - MAC management
 Synchronization
– try to find a LAN, try to stay within a LAN
– timer etc.
 Power management
– sleep-mode without missing a message
– periodic sleep, frame buffering, traffic measurements
 Association/Reassociation
– integration into a LAN
– roaming, i.e. change networks by changing access points
– scanning, i.e. active search for a network
 MIB - Management Information Base
– managing, read, write

106

802.11 - Synchronization

 All STAs within a BSS are synchronized to a common
clock
– PCF mode: AP is the timing master
• periodically transmits Beacon frames containing Timing
Synchronization function (TSF)
• Receiving stations accepts the timestamp value in TSF
– DCF mode: TSF implements a distributed algorithm
• Each station adopts the timing received from any beacon that has
TSF value later than its own TSF timer
 This mechanism keeps the synchronization of the TSF
timers in a BSS to within 4 s plus the maximum
propagation delay of the PHY layer

107

Synchronization using a Beacon
(infrastructure)

beacon interval

B B B B
access
point
busy busy busy busy
medium
t
value of the timestamp B beacon frame

108

Synchronization using a Beacon (ad-
hoc)

beacon interval

B1 B1
station1

B2 B2
station2

busy busy busy busy
medium
t
value of the timestamp B beacon frame random delay

109

802.11 - Power management
 Idea: switch the transceiver off if not needed
– States of a station: sleep and awake
 Timing Synchronization Function (TSF)
– stations wake up at the same time
 Infrastructure
– Traffic Indication Map (TIM)
• list of unicast receivers transmitted by AP
– Delivery Traffic Indication Map (DTIM)
• list of broadcast/multicast receivers transmitted by AP
 Ad-hoc
– Ad-hoc Traffic Indication Map (ATIM)
• announcement of receivers by stations buffering frames
• more complicated - no central AP
• collision of ATIMs possible (scalability?)

110

802.11 - Energy conservation

 Power Saving in IEEE 802.11 (Infrastructure
Mode)
– An Access Point periodically transmits a beacon
indicating which nodes have packets waiting for them
– Each power saving (PS) node wakes up periodically
to receive the beacon
– If a node has a packet waiting, then it sends a PS-
Poll
• After waiting for a backoff interval in [0,CWmin]
– Access Point sends the data in response to PS-poll

111

Power saving with wake-up patterns
(infrastructure)
TIM interval DTIM interval

D B T T d D B
access
point
busy busy busy busy
medium

p d
station
t
T TIM D DTIM awake

B broadcast/multicast p PS poll d data transmission
to/from the station

112

Power saving with wake-up patterns
(ad-hoc)
ATIM
window beacon interval

B1 A D B1
station1

B2 B2 a d
station2

t
B beacon frame random delay A transmit ATIM D transmit data

awake a acknowledge ATIM d acknowledge data

113

802.11 - Roaming
 No or bad connection in PCF mode? Then perform:
 Scanning
– scan the environment, i.e., listen into the medium for beacon
signals or send probes into the medium and wait for an
answer
 Reassociation Request
– station sends a request to one or several AP(s)
 Reassociation Response
– success: AP has answered, station can now participate
– failure: continue scanning
 AP accepts Reassociation Request
– signal the new station to the distribution system
– the distribution system updates its data base (i.e., location
information)
– typically, the distribution system now informs the old AP so it
can release resources 114

Hardware
 Original WaveLAN card (NCR)
– 914 MHz Radio Frequency
– Transmit power 281.8 mW
– Transmission Range ~250 m (outdoors) at 2Mbps
– SNRT 10 dB (capture)
 WaveLAN II (Lucent)
– 2.4 GHz radio frequency range
– Transmit Power 30mW
– Transmission range 376 m (outdoors) at 2 Mbps (60m
indoors)
– Receive Threshold = –81dBm
– Carrier Sense Threshold = -111dBm

115

802.11 current status

802.11i LLC
security
WEP MAC
802.11f MAC Mgmt
Inter Access Point Protocol

802.11e MIB
QoS enhancements
PHY
DSSS FH IR

OFDM
802.11b
5,11 Mbps
802.11a
6,9,12,18,24
802.11g 36,48,54 Mbps
20+ Mbps

116

IEEE 802.11 Summary

 Infrastructure (PCF) and adhoc (DCF) modes

 Signaling packets for collision avoidance
– Medium is reserved for the duration of the transmission
– Beacons in PCF
– RTS-CTS in DCF

 Acknowledgements for reliability
 Binary exponential backoff for congestion control
 Power save mode for energy conservation

117

Cuprins
 Introducere
 Retele GSM
ad hoc networks)
118

Rutarea traditională
 Un protocol de rutare populeaza tabela de
rutare a unui router

 Un protocol de rutare se bazeaza pe
algoritmii Distance-Vector sau Link-State 119

Routarea si mobilitatea

 Gasirea unei cai de la o sursa la o destinatie
 Probleme
– Schimbarea frecventa a rutelor
– Schimbarea rutei poate fi in legatura cu miscarea
hostului
– Latimea de banda relativ mica a legaturilor
 Scopul protocoalelor de rutare
– Micsorarea rutarii in ce priveste cimpurile
aditionale(overhead)
– Gasirea celor mai scurte rute
– Gasirea rutelor “stabile”(despite mobility)

120

IP mobil (RFC 3220): motivarea
 Rutarea traditionala
– Bazata pe adrese IP; prefixul retelei determina subreteaua
– Schimbarea fizica a subretelei implica
• Schimbarea adresei IP (dupa noua subretea), sau
• O tabela de rutare cu intrari speciale pentru transmitrea
pacheteor la noua subretea
 Schimbarea adeselor IP
– Actualizarea DNS necesita un timp mare
– Conexiunile TCP se opresc
 Schimbarea intrarilor in tabelele de rutare
– Nu exista o evidenta cu numarul hosturilor mobile si
schimbarile frecvente a locatiilor lor
– Probleme de securitate
 Cerintele solutiei
– Folosirea aceleiasi adrese IP, utilizarea acelorasi protocoale
– Autentificarea mesajelor, … 121

IP-ul mobil: Ideea de baza

MN(mobile Router
S(sender) Node) 3

Home
agent

Router Router
1 2

122
Source: Vaidya

IP mobil: ideea de baza

miscare

Router
S MN
3

Foreign agent

Home agent

Router Router Pachetele sunt tunelate
utilizind IP in IP
1 2

123
Source: Vaidya

IP mobil : terminologia
 Nod Mobil(Mobile Node-MN)
– Nod care se muta prin retea fara a-si schimba adresa IP
 Agent de acasa/local(Home Agent-HA)
– Host din reteaua de-acasa/proprie a nodului mobil( MN), de obicei
un router
– Inregistreaza locatia nodului MN, tuneleaza pachetele IP la COA
 Agent strain( Foreign Agent -FA)
– Host din reteaua curenta/straina, unde se gaseste momentan MN,
de obicei un router
– Forwardeaza pachetele tunelate la MN, de obicei ruterul implicit al
lui MN din reteaua de acasa/proprie
 “Ingrijitorul/gestionarul” de adrese(Care-of Address -COA)
– Adreseaza punctele de capat ale tunelului curent( tunnel end-
point) de la MN( la FA sau MN)
– Acualizeaza locatia MN-ului din punctul de vedere al IP
 Nodul corespondent(Correspondent Node (CN)
– Hostul cu care MN doreste sa “corespondeze” ( conexiunea TCP )
124

Transferul datelor la sistemul mobil
HA
2
MN

Reteaua proprie 3 Receptor
(home network) Internet (receiver)

FA foreign
network

1 1. Emitatorul trimite la adresa IP a nodului
CN mobil MN, iar HA intercepteaza
pachetele (proxy ARP)
2. HA tuneleaza pachetele la COA, aici FA,
Emitator(sender)
prin incapsulare
3. FA trimite pachetele mai departe la MN
125
Soursa: Schiller

Transferul datelor de la sistemul mobil
Agent propriu
HA-home agent 1 MN

Retea proprie Emitator
(home network) Internet (sender)

FA Retea straina
(foreign
network)

1. Emitatorul trimite la adresa IP a
CN a receptorului;de obicei FA
lucreaza ca si un router implicit
Receptor(receiver)

126
Source: Schiller

IP-ul mobil: Operatia de bază
 Agentul de averizare
– Periodic HA/FA trimit messaje de avertizare in subreteaua lor
fizica
– MN asculta mesajele si detecteaza daca acestea sunt din
reteaua proprie sau straină
– MN citeste o/un COA din mesajele de averizare a FA
 Inregistrarea MN
– MN semnaleaza COA la HA prin FA
– HA raspundela MN prin FA
– Timpul de viata este limitat, necesar sa fie securizat dupa
autentificare
 Proxi-ul HA
– HA avertizeaza asupra adresei IP a lui MN (ca si pentru
sistemele fixe)
– Pachetele pentru MN sunt trimise la HA
– Schimbari in COA/FA
 Tunelarea pachetelor
– HA la MN prin FA 127

Agentul de averizare
0 7 8 15 16 23 24 31
type code checksum
#addresses addr. size lifetime
router address 1
preference level 1
router address 2
preference level 2
...

type length sequence number
registration lifetime R B H F M G V reserved
COA 1
COA 2
...

128

Inregistrarea(registration)
MN FA HA MN HA

t

t

129

Cererea de inregistrare(Registration request)

0 7 8 15 16 23 24 31
type S B DMG V rsv lifetime
home address
home agent
COA
identification

extensions . . .

130

Incapsularea IP-in-IP
 Incapsularea IP-in-IP- (obligatorie in RFC 2003)
– tunel intre HA si COA

ver. IHL TOS length
IP identification flags fragment offset
TTL IP-in-IP IP checksum
IP address of HA
Care-of address COA
ver. IHL TOS length
IP identification flags fragment offset
TTL lay. 4 prot. IP checksum
IP address of CN
IP address of MN
TCP/UDP/ ... payload

131

IP-ul mobil: Alte probleme
 Tunelarea inversa
– Firewall-urile permit numai adresari topologice
“topological correct“
– Un pachet de la MN incapsulat de FA este corect din
punct de vedere topologic(topological correct)

 Optimizari
– Rutarea triunghiulara
• HA informeaza emitatorul privitor la locatia curenta a lui MN
– Schimbarea lui FA
• noul FA informeaza vechiul FA sa evite pachetul pierdut, iar
vechiul FA forvardeaza pachetele ramase la noul FA.
132

IP-ul mobil -recapitulare
 Nodul mobil se muta la noua locatie
 Agent de avertisment de agentul strain
 Inregistrarea nodului mobil cu agentul de acasa
 Realizarea proxi-ului de agentul de-acasa
pentru nodul mobil
 Incapsularea pachetelor
 Tunnelarea agentului de-acasa la nodul mobil
prin nodul strain

 Tunelarea inversa
 Optimizarea pentru rutarea triunghiulara(in
bucla) 133

Cuprins
 Introducere
 Retele GSM
ad hoc networks)
134

Transmission Control Protocol (TCP)
 Livrarea sigura si ordonata
– Prin pachete de raspuns si retransmisii
 Dialog de lucru cap la cap(end-to-end
semantics)
– Raspunsurile trimise la emitator confirma livrarea
datelor primite de receptor
– Ack este trimis numai dupa ce data a ajuns la
receptor
– Ack cumulativ(pentru mai multe segmente)
 Implementeaza evitarea congestiei si controlul
de flux

135

Controlul fluxului bazat pe ferestre

 Protocolul de transmisie cu fereastră glisantă
 Dimensiunea ferestrei este minimul din
– Fereastra de averizare a receptorului- determinata
de spatiul disponibil in bufferul(memoria tampon) a
receptorului
– Fereastra de congestie – determinata de emitator
pe baza reactiei retelei
Fereastra emitatorului

1 2 3 4 5 6 7 8 9 10 11 12 13

Ack-urile primite Ne transmise

136

Comportamentul de baza TCP
14 Evitarea congestiei
Congestion Window size

12
10
(segments)

8 Nivelul startului
6Startul incet incet

4
2
0
0 1 2 3 4 5 6 7 8
Time (round trips)

Exemplul presupune ca ACK-urile nu sunt intirziate
137

TCP: detectarea pachetelor pierdute

 Timeout-ul de retransmisie
– Initiaza startul incet

 Raspunsuri duplicate
– Initiaza retransmiterea rapida

 Presupunerea ca toate pachetele sunt pierdute
datorita congestiei

138

TCP dupa timeout
Dupa timeout
Fereastra de
Congestion window (segments)

25 congestie(cwnd) =20
20

15

10
Nivelul startului
Nivelul startului incet
5 Incet ssthresh = 10
ssthresh = 8
0 12

15

20

22

25
0

3

6

9

Time (round trips)

139

TCP dupa retransmisia rapida
Dupa recuperarea rapida

10
Window size (segments)

Fereastra initiata de receptor
8
6
4
2
0
0 2 4 6 8 10 12 14
Time (round trips)

Dupa retransmisia rapida si recuperarea rapida
dimensiunea ferestrei este redusa la jumatate.
140

Impactul erorilor de transmisie

 Canalele fara fir pot avea erori aleatoare in
avalansa
 Erorile in avalansa pot cauza timeout
 Erorile aleatoare pot cauza retransmisii rapide
 TCP nu poate face distinctia intre pachetele
pierdute datorita congestiei si cele pierdute
datorita erorilor de transmisie
 Nu totdeauna este necesara reducerea ferestrei
de congestie la erori (multe fiind datorate inrautatirii
transmisiei prin mediu)

141

Splitarea conexiunii
 Conexiunea TCP capat la capat(end-to-end) este
Impartita/”sparta” intr-o conexiune pe partea
cablata a rutei si una pe partea fara fir a rutei
 Conexiunea intre hostul fara fir MH si hostul fix FH
trece prin statia de baza BS
 FH-MH = FH-BS + BS-MH

FH BS MH

Host fix Statia de baza Hostul mobil

142

I-TCP: Consideratii privind splitarea
conexiunii
Starea conexiunii prin-TCP

Conexiunea TCP Conexiunea TCP

application application application
rxmt
transport transport transport
network network network
link link link
physical physical physical

Fara fir(wireless) 143

Protocolul snoop
 Pachetele de date sunt memorate(bufferate) in statia se
baza BS
– Se permite astfel nivelului legatura de date retransmisia
lor
 Cind raspunsurile duplicat sunt primite de BS de la MH
– Se retransmit pe legatura fara fir, daca pachetul este
prezent in buffer
– Se arunca raspunsul duplicat(drop dupack)
 Se previne retransmisia rapida TCP de emitatorul FH(fix
host)
FH BS MH

144

Protocol snoop

Starea conexiunii prin TCP

TCP connection

application application application
transport transport transport
network network network
rxmt
link link link
physical physical physical

FH BS MH
wireless
145

Impactul trecerii de la un nod la altul( la alt BS)(Hand-offs)

 Splitarea conexiunii
– Starea “hard” a conex. din statia de baza trebuie sa fie mutata la
noua statie de baza
 Protocolul Snoop
– Starea “soft” a conex. nu e nevoie sa fie mutata
– In timp ce noua statie de baza construieste noua stare, pachetele
pierdute nu pot fi recuperate local

 Trecerile frcvente de la un nod la altul constituie o
problema pentru schemele care realizeaza o cantitate
semnificativa a starilor de conexiune la statiile de baza
– Starea” hard” a conex. nu se pierde
– Starea “soft” a conex. tebuie sa fie recreata pentru a obtine o
performanta buna

146

M-TCP(mobile TCP)
 Similar cu splitarea conexiunii, M-TCP spliteaza o
conexiune TCP in doua parti logice
– Cele doua parti au control de flux independent ca si
in I-TCP
 BS nu trimite un ACK la MH, pina cind BS a primit un
ACK de la MH
– Pastreaza semanticile(modul de lucru) capat la capat
 BS cu mentinerea ack pentru ACK-ul ultimului octet al
lui MH(?)
Ack 999 Ack 1000

FH BS MH

147

M-TCP

 Cind este receptionat un nou ACK impreuna cu
un avertisment al receptorului de tipul
Window=0, emitatorul intra in modul “continuare”
 Emitatorul nu trimite nici o data in modul
– Cu exceptia cazului in care modul “continuare” este
anulat
 Cind este primit un avertisment de tip fereastra
pozitiva, emitatorul iese din modul “continuare”
 La iesirea din modul “continuare” , valorile pentru
RTO si cwnd sunt aceleasi ca si inaintea modului
“continuare”
148

BlocareaTCP
 M-TCP are nevoie de ajutor de la statia de
baza(base station)
– Statia de baza mentine ack pentru un octet(?)
– Statia de baza utilizeaza acest ack sa trimita o
fereastra de avertisment egala cu zero cind un host
mobil se muta la alta celula
 Blocarea TCP cere receptorului sa trimita o
fereastra de avertizare egala cu zero (ZWA)
Mobile
TCP receiver

FH BS MH

149

TCP in medii fara fir-recapitulare
 Presupunerea ca pachetele pierdute implica o
congestie nu este adevarata in mediile fara fir
 Nu este adecvata invocarea controlului
congestiei ca raspuns la pachetele pierdute

 Citeava propuneri de adaptare a TCP in mediile
fara fir
 Modificări la:
– Nodul fix(FH)
– Statia de baza(BS)
– Nodul mobil(MH)
150

Cuprins
 Introducere
 Retele GSM
ad hoc networks)
151

GSM: Arhitectura

PSTN-Public Switched Telephone Network,
ISDN-Integrated Services Digital Network,
PDN-Packet Data Network 152

Base Transceiver Station (BTS)
 Una pe celula
 E compusa dintr-un transmitator si un receptor de mare
viteza
 Functiile statiei BTS
– Are doua canale
Un canal de semnalizare si unul pentru date(Signalling
and Data Channel)
Programarea mesajelor
Detectarea accesului aleatoriu
– Realizeaza codificarea pentru protectia la erori a
canalului radio
• Adaptarea vitezei in functie de erori, conditii de propagare,
etc.
 Identificarea BTS prin codul de identitate (BtS Identity
153
Code-BSIC)

Base Station Controller (BSC)
 Controleaza mai multe BTS-uri
 Consta dintr-o entitatea de control si din una
pentru un protocol inteligent
 Functiile BSC
– Asigura managementul resursei radio
– Asigneaza si elibereaza frecvente si sloturi de timp pentru
toate MS-urile din aria sa de acivitate/actiune
– Realocarea de frecvente pentru toate celulele
– Realizeaza protocolul de predare primire a unei MS
– Semnale de sincronizare a timpului si frecventei la
BTS-uri
– Masurarea timpului de intirziere si notificarea unui
MS la BTS
– Mangementul puterii la BTS si MS 154

Mobile Switching Center (MSC)

 Comuta nodul la un PLMN(Public/Private
Land Mobile Network)
 Aloca resursa radio (RR)
– Realizeaza primirea predarea unei MS ublic
 Mobilitatea subscrierii
– Inregistrarea locatiei de subscriere
 Pot fi citeva MSC pentru un PLMN

155

Gateway MSC (GMSC)

 Conecteaza reteaua mobila la reteaua fixa
– Punct de intrare la un PLMN
 Usual unul pentru PLMN
 Cere informatia de rutare de la HLR si ruteaza
conexiunea la MSC-ul local

156

Canalul fizic
 Legatura ascendenta/descendenta
(Uplink/Downlink) la 25MHz
– 890 -915 MHz pentru legatura ascendenta
– 935 - 960 MHz pentru legatura descendenta
 Combinatie de FDMA si TDMA
– FDMA
– 124 canale a 200 kHz
– 200 kHz banda de garda
– TDMA
– Sit de biti/Avalansa(Burst)
 Modulatia utilizata
Gaussian Minimum Shift Keying (GMSK)

157

Bursts

 Building unit of physical channel

 Types of bursts
– Normal
– Synchronization
– Frequency Correction
– Dummy
– Access

159

Normal Burst

 Normal Burst
– 2*(3 head bit + 57 data bits + 1 signaling bit) + 26
training sequence bit + 8.25 guard bit

– Used for all except RACH, FSCH & SCH

160

Air Interface: Logical Channel
 Traffic Channel (TCH)

 Signaling Channel
– Broadcast Channel (BCH)
– Common Control Channel (CCH)
– Dedicated/Associated Control Channel
(DCCH/ACCH)

161

Traffic Channel

 Transfer either encoded speech or user data
 Bidirectional

 Full Rate TCH
– Rate 22.4kbps
– Bm interface

 Half Rate TCH
– Rate 11.2 kbps
– Lm interface

163

Full Rate Speech Coding
 Speech Coding for 20ms segments
– 260 bits at the output
– Effective data rate 13kbps
 Unequal error protection
– 182 bits are protected
• 50 + 132 bits = 182 bits
– 78 bits unprotected
 Channel Encoding
– Codes 260 bits into (8 x 57 bit blocks) 456 bits
 Interleaving
– 2 blocks of different set interleaved on a normal
burst (save damages by error bursts)
164

Speech 20 ms 20 ms

Speech Coder Speech Coder

260 260

Channel Encoding Channel Encoding
456 bit 456 bit

Interleaving

1 2 3 4 5 6 7 8

NORMAL BURST
3 57 1 26 1 57 3 8.25
Out of first 20 ms 165
Out of second 20ms

Traffic Channel Structure for Full Rate
Coding
Slots 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2

Bursts for Users allocated in Slot
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 26
T
T T T T T T T T T T T T S T T T T I

T = Traffic
S = Signal( contains information about the signal
strength in neighboring cells)

166

Slots 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2

Burst for one users
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 26
T T T T T T S T T

Bursts for another users allocated in alternate
T
1 2 Slots4 5 6 T
3 7 8 9 10 11 12 13 14 15 16 17 26
T T T T T T T T S
=
T
Traffic Channel Structure for Half Rate
Coding 167

BCCH

 Broadcast Control Channel (BCCH)
– BTS to MS
– Radio channel configuration
– Current cell + Neighbouring cells
– Synchronizing information
– Frequencies + frame numbering
– Registration Identifiers
– LA + Cell Identification (CI) + Base Station Identity Code
(BSIC)

168

FCCH & SCH

 Frequency Correction Channel
– Repeated broadcast of FB

 Synchronization Channel
– Repeated broadcast of SB
– Message format of SCH
PLMN color BS color T1 Superframe T2 multiframe T3 block frame
3 bits 3 bits index 11 bits index 11 bits index 3bits
BSIC 6 bits"
FN 19bits

169

RACH & SDCCH

 Random Access Channel (RACH)
– MS to BTS
– Slotted Aloha
– Request for dedicated SDCCH

 Standalone Dedicated Control Channel
(SDCCH)
– MS  BTS
– Standalone; Independent of TCH

170

AGCH & PCH

Access Grant Channel (AGCH)
– BTS to MS
– Assign an SDCCH/TCH to MS

 Paging Channel (PCH)
– BTS to MS
– Page MS

171

SACCH & FACCH
 Slow Associated Control Channel (SACCH)
– MS  BTS
– Always associated with either TCH or SDCCH
– Information
– Optimal radio operation; Commands for synchronization
– Transmitter power control; Channel measurement
– Should always be active; as proof of existence of
physical radio connection
 Fast Associated Control Channel (FACCH)
– MS  BTS
– Handover
– Pre-emptive multiplexing on a TCH, Stealing Flag (SF)

172

Example: Incoming Call Setup
MS  BSS/MSC ------ Paging request (PCH)
MS  BSS/MSC ------ Channel request (RACH)
MS  BSS/MSC ------ Immediate Assignment (AGCH)
MS  BSS/MSC ------ Paging Response (SDCCH)
MS  BSS/MSC ------ Authentication Request (SDCCH)
MS  BSS/MSC ------ Authentication Response (SDCCH)
MS  BSS/MSC ------ Cipher Mode Command (SDCCH)
MS  BSS/MSC ------ Cipher Mode Compl. (SDCCH)
MS  BSS/MSC ------ Setup (SDCCH)
MS  BSS/MSC ------ Call Confirmation (SDCCH)
MS  BSS/MSC ------ Assignment Command (SDCCH)
MS  BSS/MSC ------ Assignment Compl. (FACCH)
MS  BSS/MSC ------ Alert (FACCH)
MS  BSS/MSC ------ Connect (FACCH)
MS  BSS/MSC ------ Connect Acknowledge (FACCH)
MS BSS/MSC ------ Data (TCH)

173

Select the channel with
Power On Scan Channels, highest RF level among
monitor RF levels the control channels

Scan the channel for the
FCCH

Select the channel with NO
next highest Rf level Is
from FCCH detected?
the control list. YES

Scan channel for SCH

NO
Is
SCH detected?
YES

Read data from BCCH
and determine is it BCCH?

From the channel data NO Is
update the control the current BCCH
channel list channel included? YES

Camp on BCCH and
start decoding 174

Adaptive Frame Synchronization

 Timing Advance
 Advance in Tx time corresponding to
propagation delay

 6 bit number used; hence 63 steps
 63 bit period = 233 micro seconds (round trip
time)
– 35 Kms

175

GSM: Channel Mapping Summary
 Logical channels
– Traffic Channels; Control Channels
 Physical Channel
– Time Slot Number; TDMA frame; RF Channel Sequence

 Mapping in frequency
– 124 channels, 200KHz spacing
 Mapping in time
– TDMA Frame, Multi Frame, Super Frame, Channel
– Two kinds of multiframe:
– 26-frame multiframe; usage -Speech and Data
– 51-frame multiframe; usage -Signalling

177

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