southern methodist university fall 2003 eets 8316/ntu cc745-n wireless networks

#1EETS 8316/NTU TC 745, Fall 2003 ENGINEERINGSMU

Southern Methodist University Fall 2003

EETS 8316/NTU CC745-NWireless Networks

Lecture 9: Review

Instructor: Jila Serajemail: [email protected]

http://www.engr.smu.edu/~jseraj/tel: 214-505-6303


Terminology, Cont….

Low-tier cellular (PCS)—Between cellular and cordless

—Very small cells, limited mobility, usually campus range

High tier cellular—Large cells

Protocols


Protocols

—Rules for exchanging data between different entities, Protocol layers

—Concept of dividing (usually complex) protocols into separate functions

—Higher protocol layers build on the functions (“services”) of lower layers

—Each protocol layer can be designed and analyzed separately, if “services” provided to higher protocol layers is unchanged

—Each protocol layer uses separate overhead information (eg, header fields)

—Protocol “entities” in each layer communicate with their “peer entities” in the same layer


OSI protocol reference model

physical

data link

network

transport

session

presentation

application

Host A

physical

data link

network

transport

session

presentation

application

Host B


TCP/IP protocol reference model

network access

internet

transport

application

Host A

network access

internet

transport

application

Host B

Application Layer: user program that generates dataTransport Layer: end-to-end connection management, error recoveryInternet Layer: route IP packets between different networksNetwork Access Layer: any network and physical layer protocols


Hierarchical Network

Access Tandem

Tandem

Local

Subscriber


Voice and Signaling

Signaling is used to transfer information between entities for the purpose of carrying traffic or performing other functions/ services.

Rules governing the signaling between entities are called protocols.

There are many signaling protocols, however Signaling System Number 7 is the most commonly used of all


Voice and Signaling

SCPSS7STP

STP: Signal Transfer PointSCP: Switching Control Point, stores translation Tables


TDMA Network Structure

HLR

PSTN

Base stationBase station Air interface

Base station

GMSC/MSC/VLR

BSC

BSCBSC

Base station Controller

Mobilestation

AUCEIC


Cellular DCCH Structure

DCCH

Reverse Forward

RACH SPACH BCCH SCF Reserved

PCH ARCH SMSCH FBCCH EBCCH SBCCH


Roaming and Registration

When a mobile moves in the network, it is called roaming

When a mobile is powered up, it sends a registration message to BSC.

Registration informs MSC of the presence of the mobile, or that it has changed location

MSC request information about the MS from HLR, which replies with subscriber data


Roaming and Registration

Registration—Power Up/Power Down Registration

—Location Area Update Registration

• The coverage area of MSC is divided into location areas. Location areas are chosen by the network operator to simplify operation and improve performance of the network.

• Every time an MS crosses the boundary between location areas, it re-register with the MSC.

—Periodical Registration


Roaming and Registration, Cont…

Location cancellation, AKA de-registration— MSC triggered

• MSINACT with or without De-registration parameter

• Bulkdereg, remove all mobiles associated with the MSC

— HLR triggered

• Location update in another switch

• Administrative actions

• Data failure in HLR

If registration happens in several MSC, HLR decides which one is valid.

If registration happens in several BS, MSC determines which one is valid


Location Area, MSC border

MSC-1LA-1

MSC-1LA-2

MSC-1LA-3

MSC-2LA-1

MSC-2LA-2


Handoff

Movement into a different cell requires MTSO to automatically transfer call to another base station without interruption

Hard handoff: “break before make”, connection is broken then re-established

Soft handoff: temporarily connected to two or more base stations simultaneously before dropping all but one


Handoff, Cont…

Initiation: Base station detects measured uplink signal strength drops below threshold (first generation), or mobile station reports signal from neighboring base stations and one of them is stronger than current base station (second generation), or the uplink quality is lower than minimum acceptable.

Resource reservation: frequencies are reserved with new base station

Execution: actual handoff of connection

Completion: unneeded resources are cleared


Handoff Challenge

Measured signal strength drop is caused by momentary fading

Handoff must be completed before signal strength drops below a minimum acceptable level

No channels are free at nearby base stations, causing call connection problems, dropped calls.

If mobile station moves to another cellular system (controlled by different MTSO), an intersystem handoff is required - more complicated


Handoff, Cont….

There are three type of handoffs—MS controlled handoff

—Network controlled handoff

—Mobile assisted handoff (MAHO)

D-AMPS and CDMA use MAHO, AMPS uses network controlled handoff.

Capabilities required for the MS are taken into account.


Path Optimization Process

BS

BS

Anchor MSC Target MSC

Serving MSC

BS

PSTN

1

3

2

4

6

5

7

10

89


Path Optimization Process, Cont…

BS

BS

Anchor MSC

MSC

BS

PSTN

Call Path after path minimization process

New Serving MSC


Paging, Cont..

Gateway MSC has now sufficient information to connect to the visiting MSC.

Gateway MSC send call set up request to the visiting MSC, which sets up the call

What happens when more than one MSC report to GMC that the mobile is its coverage area?


Paging, Cont..

When HLR receives more than one response, it chooses the MSC with strongest signal.

It send the address of the chosen VMSC to the gateway MSC and informs other MSC that the call is off.

How does HLR know it has received response from all MSCs? Internal timer


North American Numbering Plan

North American Numbering Plan consists of 10 digits, NPA-NXX-XXXX

All phone numbers follow the same structure.

NPA is the area code

NXX is the switch identifier

XXXX indicates the subscriber in the switch


North American Numbering Plan, Cont

Due to this structure, there is no way for a switch to identify that a number belongs to a mobile subscriber, nor can it identify the network provider.

Mobile network provider “buy” a certain number series in each area for their users.

Therefore we can not bill a caller to a mobile user for the air usage. They do it in other countries!


GSM Network Structure

BSS

MSC

VLR

HLR

EIR

AUC

MTTE

MS

Um

A

PSTN

BSC

BTS BTS

OMC

NMC

ADC

OSS

BSS

MS

GSM Public land mobilenetwork (PLMN)

OSS: operation subsystemBSS: base station subsystemMS: mobile station


GSM Interfaces, cont..

RF

LAPD

MM

RRM

CM

RRM

RF

LAPD LAPD

RF

RRM

RF

LAPD LAPD

RF

SCCP

RF

LAPD

MM

RRM

CM

SCCP

Air InterfaceUm Abis A


GSM Logical Channel Structure

CCH

TCH/F TCH/H

BCH CCCH DCCH

FCCH SCH BCCH PCH AGCH RACH

TCH CBCH

ACCH SDCCH

FACCHSACCH


GSM Numbers

IMEI = International mobile station equipment identity.

IMEI= TAC + FAC + SNR + SP— TAC = Type Approval Code, 6 decimals

— FAC = Final Assembly Code, 6 decimals, assigned by manufacturer

— SNR = Serial Number, 6 decimals, assigned by manufacturer

— SP = Spare, 1 decimal place

EIR = Equipment Identity Register, has while, black and optionally grey list


GSM Numbers, Cont…

IMSI = International mobile Subscriber Identity, is stored on the SIM (Subscriber Identity Module) card. IMSI is obtained at the time of subscription. IMSI is not made public.

IMSI = MCC + MNC + MSIN

MCC = Mobile Country Code, 3 decimals

MNC = Mobile Network Code, 2 decimals



MSIN = Mobile Subscriber Identification Number, maximum 10 decimal digits

MSISDN = Mobile Station ISDN number, is the real phone number of the subscriber. Stored in HLR and on SIM card

MSISDN = CC + NDC + SN

<=3d 2-3d <= 10d



Mobile Station Roaming Number (MSRN), same format as MSISDN. A temporary location dependent ISDN number. Is assigned at call set up.

Location Area Identity (LAI). Regularly sent on BCCH LAI = CC + MNC + LAC, LAC = Location Area Code, max 5 decimals

Temporary Mobile Subscriber Identity (TMSI). Stored only in the VLR and SIM card. Consists of 4*8 bits excluding value FFFF FFFFhex



TMSI has only local meaning and can be defined according to operator’s specifications.

LAI + TMSI uniquely identifies the user, I.e. IMSI is no longer needed for ongoing communication


GSM Numbers, cont..

LMSI = Local Mobile Subscriber Identity. Created in VLR and stored in HLR. Like TMSI is operator defined. Used in communication with VLR to speed the search for mobile records.

Speed is essential to achieve short call setup times.

GCI = Global Cell Id = LAI + CI. CI = Cell id, unique id within the LAI. Maximum 2*8 bits.

BSIC = Base Transceiver Station Identity Code. BSIC= NCC + BCC


GSM Numbers, cont…

BSIC is broadcast periodically by the base station on the synchronization channel.

NCC = Network Color Code, 3 bits

BCC = Base Station Color Code, 3 bits


GSM Handoffs

3 types of handoffs

—Intra-BSS: if old and new BTSs are attached to same base station

•MSC is not involved

—Intra-MSC: if old and new BTSs are attached to different base stations but within same MSC

—Inter-MSC: if MSCs are changed


IS-95 CDMA

D-AMPS increased capacity of AMPS by factor 3

CDMA claimed to increase capacity by factor 20

Spread spectrum techniques adapted from military (used since 1950)

—Narrowband signal is multiplied by very large bandwidth signal (spreading signal)

—All users, each with own pseudorandom codeword approximately orthogonal to all other codewords, can transmit simultaneously with same carrier frequency


IS-95 CDMA - Radio Aspects (cont)

—Receiver performs a time correlation operation to detect only desired codeword

—All other codewords appear as noise due to decorrelation

—Receiver needs to know only codeword used by transmitter

—In other words, users are separated by their codes rather than frequency and time slot


IS-95 CDMA Interesting Features

Multiple users can share same frequency

Spatial diversity provides soft handoff: MSC monitors signal of a user from multiple base stations and chooses best version of signal at any time

Multipath fading is reduced by signal spreading

CDMA is dual mode like TDMA.

The system can move a call from digital to analog when the call enters the coverage area of a cell that does not have CDMA capability. The opposite does not work.


IS-95 CDMA Interesting Features (cont)

Soft capacity limit: more users raises noise floor linearly, no absolute limit on number of users - performance degrades gradually for all users

Self-jamming is a problem: because spreading sequences of different users are not exactly orthogonal

—When despreading, other users can contribute significantly to receiver decision statistic


IS-95 CDMA Interesting Features (cont)

Near-far problem: if power of multiple users are unequal, strongest received mobile signal will capture demodulator at base station

—Base stations must implement power control to ensure that each mobile within coverage area provides same signal level to base station receiver


Soft handoff

Two base stations receive signals from the mobile. The signals are sent to the MSC that decides which one has lowest bit error rate. Vocoder in CDMA is in the switch.

Mobile receives signals from two base stations and combine them before decoding. Uses rake receiver. Each tunes to one base station.


Soft handoff (cont)

This requires synchronization of the base stations.

It also requires that the mobile dedicates one correlator for searching other pilot channels.


Soft handoff (cont)

MSCCurrent

BSMobile Candidate

BS

Conversation

Neighbor pilot can be a candidate

Measure the strength of pilot

New Active Set, handoff direction

Conversation

ConversationMeasurements

Measurements

Handoff Handoff

Conversation


Mobitex - Architecture

Mainexchange

NCC NCC: network Control center

Regionalswitch

Localswitch

Regionalswitch

Localswitch

Base stations use 1-4 frequencies each 8 kb/s

Local switch covers a servicearea, each with 10-30 frequency pairs

FEP


Applications

MPAK

MASC

RS232

Mobitex, protocol architecture

Applications

MPAK

X.25

X.21

MPAK

MASC ROSI

RS232 GMSK

MPAK

ROSI HDLC

GMSK X.21

MPAK

HDLC X.25

X.21 X.21

Mobile Radio modem

Base Station

Localswitch

Server

4-7

3 2 1


Mobitex, Major features, Cont...

Major features

—Seamless roaming

—Store and forward of messages

—Dependability above 99.99%

—Interoperability and many connectivity options

—Capacity to support millions of subscribers

—Security against eavesdropping


Mobitex, Major features, Cont...

Major features

—Packet switching occurs at lowest level of system hierarchy - relieves backbone traffic

—Packet multicasting (to multiple recipients) is handled by network

—Closed User Group (CUG) feature

—Frequency depends of the country, 900 MHZ in US and 450 in most others.


Mobitex - common functions

Requires subscription

—individual

—groups of terminals

—host computer

—groups of host computers

Security

—Password based

—ESN

—CUG (Closed User Group)


Mobitex - Mobility

Mobiles monitor and evaluate signals from other base stations

At power-up, mobile tries to resgister with the last base station in its memory, if possible

Base station provides necessary information, such as acceptable signal strength, neighbour list,etc periodically.


CDPD - Network Architecture

MD-IS MD-IS

Mobile data base station = base station

Mobile data intermediate systems = packet switches with mobility management capabilities

Intermediate systems = generic packet switches in backbone networkIS IS

Internet or other networks

IS


CDPD

Cellular digital packet data (CDPD): connectionless packet-switched data designed to work with an analog cellular system (eg, AMPS)

—Originated by IBM as packet-switching overlay to analog cellular system, early 1990s developed by CDPD Forum, now developed by Wireless Data Forum

—Overlay system uses unused bandwidth in cellular system and existing AMPS functions and capabilities


CDPD , Cont...

CDPD is a value added system. Other users do not need to be aware of its presence in the network. This has implicaitons:

CDPD transmission must not interfere with transmission of other services

No dedicated bandwith, uses only idle time between users, channel-hop

No dedicated Control channel, all Control is in-band.


CDPD , Cont...

CDPD is transparent to voice system

—To avoid collisions with voice calls, CDPD uses channel hopping when antenna detects a power ramp-up (indicating initiation of voice traffic)

—Base station closes current transmission channel within 40 msec and new idle channel is chosen to hop to


CDPD , Cont...

CDPD is transparent to voice system

—New channel may or may not be announced before old channel closed

• If not announced, mobile terminal must hunt around set of potential CDPD channels to find new one


GPRS - Network Architecture

GPRS makes use of existing GSM base stations

Serving GPRS support node = packet switch with mobility management capabilities

Gateway GSN = packet switch interworks with other networks

Internet or other networks

GGSNMSC/VLR

SGSN SGSN

HLR

BSC/PCU


GPRS , Cont...

SGSN = Serving GPRS Support Node

—Ciphering

—Authentication, IMEI check

—Mobility Management

—Logical Link Management towards mobile station

—Packet routing and transfer

—Connection to HLR, MSC, BSC and SMS-MC


GPRS , Cont...

GGSN = Gateway GPRS Support Node

— External interfaces

— Routing

GPRS register maintains GPRS subscriber data and routing information. Normally it is integrated in GSM HLR

PCU (Packet Control Unti) is collocated with BSC.


GPRS , Cont...

SGSN communicates with MSC/VLR with SS7 based protocol based on BSSAP.

Three class of mobile terminals

—Class A: Operates GPRS and Circuit switched service simultaneously

—Class B: Monitors the Control channels of GPRS and GSM simulataneously but can opeate one set of services at a time

—Class C: Only CS or GPRS capable.


GPRS , Cont...

For mobility management a new concept is defined, Routing Area

RAI = MCC +MNC + LAC + RAC


GPRS - Radio Interface

Mobile station must register and establish a temporary logical link identity (TLLI) with its serving GSN—Mobile station’s HLR is queried for access

privileges

Data is transmitted over a number of GSM physical channels that network provider dedicates to GPRS (packet data channels or PDCHs)—Each PDCH = one physical timeslot in

TDMA frame


GPRS - Radio Interface , Cont...

Mobile station with data ready sends a short random access message to BTS on packet random access channel (PRACH) requesting a number of GPRS slots

—When BSC grants slots, mobile station can transmit

Packets for mobile stations use paging channels to locate MS and reserve timeslots

southern methodist university fall 2003 eets 8316/ntu cc745-n wireless networks

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