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Southern Methodist University Fall 2003 EETS 8316/NTU CC745-N Wireless Networks

Southern Methodist University Fall 2003 EETS 8316/NTU CC745-N Wireless Networks. Lecture 9: Review. Instructor : Jila Seraj email : jseraj@engr.smu.edu http://www.engr.smu.edu/~jseraj/ tel: 214-505-6303. Terminology, Cont…. Low-tier cellular (PCS) Between cellular and cordless

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Southern Methodist University Fall 2003 EETS 8316/NTU CC745-N Wireless Networks

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  1. Southern Methodist University Fall 2003 EETS 8316/NTU CC745-N Wireless Networks Lecture 9: Review Instructor: Jila Seraj email: jseraj@engr.smu.edu http://www.engr.smu.edu/~jseraj/ tel: 214-505-6303

  2. Terminology, Cont…. • Low-tier cellular (PCS) • Between cellular and cordless • Very small cells, limited mobility, usually campus range • High tier cellular • Large cells • Protocols

  3. 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

  4. Host A Host B application application presentation presentation session session transport transport network network data link data link physical physical OSI protocol reference model

  5. Host A Host B application application transport transport internet internet network access network access TCP/IP protocol reference model 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

  6. Hierarchical Network • Access Tandem • Tandem • Local • Subscriber

  7. 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

  8. SCP SS7 STP Voice and Signaling STP: Signal Transfer Point SCP: Switching Control Point, stores translation Tables

  9. TDMA Network Structure PSTN AUC EIC GMSC/MSC /VLR HLR Base station Controller BSC BSC BSC Air interface Base station Mobile station Base station Base station

  10. DCCH Reverse Forward SPACH RACH BCCH SCF Reserved PCH ARCH SMSCH FBCCH EBCCH SBCCH Cellular DCCH Structure

  11. 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

  12. 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

  13. 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

  14. Location Area, MSC border MSC-1 LA-1 MSC-1 LA-2 MSC-1 LA-3 MSC-2 LA-1 MSC-2 LA-2

  15. 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

  16. 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

  17. 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

  18. 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.

  19. Path Optimization Process 6 BS 7 BS BS Serving MSC PSTN Anchor MSC 4 Target MSC 1 2 3 5 8 9 10

  20. BS BS BS MSC PSTN Anchor MSC New Serving MSC Call Path after path minimization process Path Optimization Process, Cont…

  21. 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?

  22. 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

  23. 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

  24. 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!

  25. PSTN OMC GSM Public land mobile network (PLMN) VLR MSC AUC HLR NMC A EIR BSS BSS BSC ADC BTS BTS OSS Um OSS: operation subsystem BSS: base station subsystem MS: mobile station MS MS MT TE GSM Network Structure

  26. CM MM CM RRM MM RRM RRM SCCP LAPD LAPD LAPD LAPD RF RF RF RF GSM Interfaces, cont.. Air Interface Um Abis A RRM SCCP LAPD LAPD RF RF

  27. GSM Logical Channel Structure CBCH CCH TCH TCH/F TCH/H BCH CCCH DCCH AGCH PCH FCCH SCH RACH SDCCH BCCH ACCH FACCH SACCH

  28. 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

  29. 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

  30. 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 2-3d <=3d <= 10d

  31. 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

  32. 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

  33. 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

  34. 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

  35. 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

  36. 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

  37. 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

  38. 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.

  39. 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

  40. 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

  41. 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.

  42. Soft handoff (cont) • This requires synchronization of the base stations. • It also requires that the mobile dedicates one correlator for searching other pilot channels.

  43. Soft handoff (cont) Candidate BS Current BS Mobile MSC Conversation Neighbor pilot can be a candidate Measure the strength of pilot New Active Set, handoff direction Conversation Measurements Conversation Measurements Handoff Handoff Conversation

  44. Mobitex - Architecture NCC NCC: network Control center Main exchange Local switch covers a service area, each with 10-30 frequency pairs Regional switch Regional switch Local switch Local switch Base stations use 1-4 frequencies each 8 kb/s FEP

  45. Applications MPAK MASC RS232 Applications MPAK X.25 X.21 MPAK MASC ROSI RS232 GMSK MPAK HDLC X.25 X.21 X.21 MPAK ROSI HDLC GMSK X.21 Mobitex, protocol architecture 4-7 3 2 1 Server Local switch Base Station Mobile Radio modem

  46. 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

  47. 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.

  48. Mobitex - common functions • Requires subscription • individual • groups of terminals • host computer • groups of host computers • Security • Password based • ESN • CUG (Closed User Group)

  49. 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.

  50. CDPD - Network Architecture Internet or other networks IS Intermediate systems = generic packet switches in backbone network IS IS Mobile data intermediate systems = packet switches with mobility management capabilities MD-IS MD-IS Mobile data base station = base station

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