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Southern Methodist University Fall 2003 EETS 8316/NTU CC745-N Wireless Networks Lecture 9: Review Instructor: Jila Seraj email: [email protected] http://www.engr.smu.edu/~jseraj/ tel: 214-505-6303 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #1 Terminology, Cont…. Low-tier cellular (PCS) — Between cellular and cordless — Very small cells, limited mobility, usually campus range High tier cellular — Large cells Protocols EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #2 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #3 OSI protocol reference model Host A Host B application presentation session transport network data link physical EETS 8316/NTU TC 745, Fall 2003 application presentation session transport network data link physical SMU ENGINEERING #4 TCP/IP protocol reference model Host A Host B application application transport transport internet internet network access network access Application Layer: user program that generates data Transport Layer: end-to-end connection management, error recovery Internet Layer: route IP packets between different networks Network Access Layer: any network and physical layer protocols EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #5 Hierarchical Network Access Tandem Tandem Local Subscriber EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #6 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #7 Voice and Signaling STP: Signal Transfer Point SCP: Switching Control Point, stores translation Tables SCP EETS 8316/NTU TC 745, Fall 2003 STP SMU ENGINEERING SS7 #8 TDMA Network Structure PSTN AUC EIC GMSC/MSC /VLR HLR BSC BSC Base station Controller BSC Base station Base station Air interface Mobile station Base station EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #9 Cellular DCCH Structure DCCH Reverse RACH PCH ARCH EETS 8316/NTU TC 745, Fall 2003 Forward SPACH BCCH SMSCH FBCCH SMU ENGINEERING SCF EBCCH Reserved SBCCH #10 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #11 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #12 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #13 Location Area, MSC border MSC-1 LA-1 MSC-1 LA-2 MSC-1 LA-3 MSC-2 LA-1 MSC-2 LA-2 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #14 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #15 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #16 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #17 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. EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #18 Path Optimization Process 6 BS 7 BS BS Serving MSC PSTN Anchor MSC 4 Target MSC 1 2 3 5 8 9 10 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #19 Path Optimization Process, Cont… BS BS BS MSC PSTN Anchor MSC New Serving MSC Call Path after path minimization process EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #20 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? EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #21 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #22 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #23 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! EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #24 GSM Network Structure PSTN VLR GSM Public land mobile network (PLMN) MSC HLR OMC AUC A EIR BSC BSS NMC BSS ADC BTS BTS OSS Um MT TE MS EETS 8316/NTU TC 745, Fall 2003 OSS: operation subsystem BSS: base station subsystem MS: mobile station MS SMU ENGINEERING #25 GSM Interfaces, cont.. Air Interface Um Abis A CM CM MM MM LAPD RF RRM RRM RRM LAPD LAPD RF EETS 8316/NTU TC 745, Fall 2003 RF SCCP SCCP LAPD LAPD LAPD RF SMU RRM ENGINEERING RF RF #26 GSM Logical Channel Structure TCH/F TCH/H BCH FCCH SCH CBCH CCH TCH CCCH BCCH DCCH PCH AGCH RACH SACCH EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING ACCH SDCCH FACCH #27 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #28 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #29 GSM Numbers, Cont… 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 EETS 8316/NTU TC 745, Fall 2003 2-3d SMU <= 10d ENGINEERING #30 GSM Numbers, Cont… 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #31 GSM Numbers, Cont… 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #32 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #33 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #34 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #35 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #36 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #37 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. EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #38 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #39 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #40 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. EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #41 Soft handoff (cont) This requires synchronization of the base stations. It also requires that the mobile dedicates one correlator for searching other pilot channels. EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #42 Soft handoff (cont) Mobile Current BS Conversation MSC Candidate BS Neighbor pilot can be a candidate Measure the strength of pilot New Active Set, handoff direction Conversation Measurements Conversation Measurements Handoff Handoff Conversation EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #43 Mobitex - Architecture NCC NCC: network Control center Main exchange Regional switch Local switch Regional switch Local switch Base stations use 1-4 frequencies each 8 kb/s FEP EETS 8316/NTU TC 745, Fall 2003 Local switch covers a service area, each with 10-30 frequency pairs SMU ENGINEERING #44 Mobitex, protocol architecture Applications 4-7 Applications 3 2 1 MPAK MPAK MPAK MASC MASC RS232 RS232 GMSK Mobile EETS 8316/NTU TC 745, Fall 2003 ROSI ROSI HDLC GMSK X.21 Base Station Radio modem SMU ENGINEERING MPAK HDLC X.25 X.21 X.21 Local switch MPAK X.25 X.21 Server #45 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #46 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. EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #47 Mobitex - common functions Requires subscription —individual —groups of terminals —host computer —groups of host computers Security —Password based —ESN —CUG (Closed User Group) EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #48 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. EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #49 CDPD - Network Architecture Internet or other networks IS IS MD-IS Intermediate systems = generic packet switches in backbone network IS Mobile data intermediate systems = packet switches with mobility management capabilities MD-IS Mobile data base station = base station EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #50 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #51 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. EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #52 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #53 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #54 GPRS - Network Architecture Internet or other networks HLR SGSN MSC/ VLR GGSN Gateway GSN = packet switch interworks with other networks SGSN Serving GPRS support node = packet switch with mobility management capabilities BSC/PCU GPRS makes use of existing GSM base stations EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #55 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 SMSMC EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #56 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. EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #57 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. EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #58 GPRS , Cont... For mobility management a new concept is defined, Routing Area RAI = MCC +MNC + LAC + RAC EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #59 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #60 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 EETS 8316/NTU TC 745, Fall 2003 SMU ENGINEERING #61