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

This lecture covers the essentials of CDMA, performance metrics, switching systems, and GSM numbering and interfaces in wireless networks.

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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 5: CDMA, Mobile Data Instructor: Jila Seraj email: jseraj@engr.smu.edu http://www.engr.smu.edu/~jseraj/ tel: 214-505-6303

  2. Session Outline • Review of the last week • CDMA essentials • Primer to performance metric • Primer to switching systems • Midterm exam, Oct 26, One hour at the end of the class. Covers first 6 sessions

  3. Review, GSM General Architecture PSTN OMC GSM Public land mobile network (PLMN) VLR MSC AUC HLR NMC A EIR BSS BSS BSC ADC Abis BTS BTS OSS Um OSS: operation subsystem BSS: base station subsystem MS: mobile station MS MS MT TE

  4. Review, GSM Logical and Physical Channels • Um interface: various logical channels are mapped to physical channels • A physical channel is a timeslot with timeslot number in a sequence of TDMA frames • 8 physical channels mapped onto 8 timeslots within TDMA frame per frequency carrier

  5. Review, GSM Physical Channels TDMA frame = 4.615 ms Timeslot 1 2 3 4 5 6 7 8 Frequency 1 Ch 1 Ch 2 Ch 3 Ch 4 Ch 5 Ch 6 Ch 7 Ch 8 Frequency 2 Ch 1 Ch 2 Ch 3 Ch 4 Ch 5 Ch 6 Ch 7 Ch 8 : : : : Frequency 124 Ch 1 Ch 2 Ch 3 Ch 4 Ch 5 Ch 6 Ch 7 Ch 8

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

  7. OMAP Layer 4-7 INAP MAP ISUP TCAP MUP Review, OSI Model for SS7 SCCP Layer 3 MTP Layer 1-2 Signaling link function, Signaling link physical requirement

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

  9. Review, GSM Protocol Layers • RF : Physical Layer • LAPD: Link Layer, ISDN protocol based • SCCP: Signal Connection Control Layer, part of link layer • RR: Radio Resource • MM: Mobility Management • CC: Call Control

  10. Switching System VLR AUC MAP MAP HLR VLR EIR MAP MAP MSC BSSAP Base Station Subsystem BSS Review, GSM Interfaces

  11. Switching System BSSAP Base Station Subsystem BSS BSC LAPD BTS MS Review, GSM Interfaces, cont…

  12. Review, GSM Numbers • International mobile station equipment identity (IMEI). 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 has while, black and optionally grey list.

  13. Review, GSM Numbers • International mobile station equipment identity (IMEI). 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 has while, black and optionally grey list

  14. Review, GSM Numbers, cont… • International mobile Subscriber Identity (IMSI). 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

  15. Review, GSM Numbers, cont… • MSIN = Mobile Subscriber Identification Number, maximum 10 decimal digits • Mobile Station ISDN number (MSISDN), is the real phone number of the subscriber. Stored in HLR and on SIM card • MSISDN = CC + NDC + SN • CC = Country Code, up to 3 decimals

  16. Review, GSM Numbers, cont… • NDC = National Destination Code, typically 2-3 decimals • SN = Subscriber Number, maximum 10 decimals. • Mobile Station Roaming Number (MSRN), same format as MSISDN. A temporary location dependent ISDN number. • Is assigned in two cases, at registration or at call set up.

  17. Review, GSM Numbers, cont… • Location Area Identity (LAI). Regularly sent on BCCH LAI = CC + MNC + LAC, • LAC = Location Area Code, max 5 decimals (<FFFFhex). • Temporary Mobile Subscriber Identity (TMSI). Stored only in the VLR and SIM card. Consists of 4*8 bits excluding value FFFF FFFFhex

  18. Review, GSM Numbers, cont… • TMSI has only local meaning and can be defines according to operator’s specifications. • LAI + TMSI uniquely identifies the user, I.e. IMSI is no longer needed for ongoing communication

  19. Review, GSM Numbers, cont.. • Local Mobile Subscriber Identity (LMSI). 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.

  20. Review, GSM Numbers, cont… • Global Cell Id = LAI + CI • CI = Cell id, unique id within the LAI. Maximum 2*8 bits • Base Transceiver Station Identity Code (BSIC) = NCC + BCC

  21. 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

  22. Review, GSM Roaming From Another PLMN • VLR registers users roaming in its area • Recognizes mobile station is from another PLMN • If roaming is allowed, VLR finds the mobile’s HLR in its home PLMN • VLR constructs a global title from IMSI to allow signaling from VLR to mobile’s HLR via public telephone network

  23. Review, GSM Roaming, cont... • VLR registers users roaming in its area • VLR generates a mobile subscriber roaming number (MSRN) used to route incoming calls to mobile station • MSRN is sent to mobile’s HLR

  24. Review, GSM Roaming, cont… • VLR contains • MSRN • TMSI • Location area where mobile station has registered • Info for supplementary services (if any) • IMSI • HLR or global title • Local identity for mobile station (if any)

  25. Review, GSM Security • 3 security problems: unauthorized access, privacy from eavesdropping, protection of subscriber identity/location • Unauthorized (fraudulent) access • GSM handsets must be presented with a subscriber identity module (SIM) • SIM must be validated with personal identification number (PIN)

  26. Review, GSM Security, cont… • Unauthorized (fraudulent) access • SIM also stores subscriber authentication key, authentication algorithm, cipher key generation algorithm, encryption algorithm • During registration (when roaming), mobile station receives “challenge” and uses authentication key and authentication algorithm to generate “challenge response” to verify user’s identity

  27. Review, GSM Security, cont… • Privacy from eavesdropping • Temporary encryption key is used for privacy of data, signaling, and voice • Info is encrypted before transmission

  28. Review, GSM Security, cont… • Anonymity of users • Supported by temporary mobile subscriber ID (TMSI) • When registered, mobile station sends globally-unique international mobile subscriber ID (IMSI) to network • Network assigns TMSI for use during call - IMSI is not sent over radio link

  29. Review, GSM Security, cont… • Anonymity of users • Only network and mobile station know true identity • New TMSI is assigned when roam into new area

  30. Review, GSM Security, cont.. • Fetched triplets are stored in VLR • Every call uses up one triplet (discarded) • Another set must be fetched when exhausted Visited system Registration request IMSI/TMSI identifies user, LAI points to old VLR, requests data to authenticate user IMSI/TMSI + LAI Subscriber data Old VLR

  31. Review, GSM Security, cont.. Visited system Requests triplets from home system, chooses a triplet Challenge Calculates response by authen- tication algorithm Challenge response Compares to stored response in triplet, registration successful if matches New TMSI Assigns new TMSI Acknowledge

  32. Review, GSM Security, cont.. Visited system Location update HLR Acknowledge Registration cancel Old VLR

  33. Review, 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

  34. Review, GSM Intra-MSC Handoff • Mobile station monitors signal quality and determines handoff is required, sends signal measurements to serving BSS • Serving BSS sends handoff request to MSC with ranked list of qualified target BSSs • MSC determines that best candidate BSS is under its control (assumed here) • MSC reserves a trunk to target BSS

  35. Review, GSM Intra-MSC Handoff, cont.. • Target BSS selects and reserves radio channels for new connection, sends Ack to MSC • MSC notifies serving BSS to begin handoff, including new radio channel assignment • Serving BSS forwards new radio channel assignment to mobile station • Mobile station re-tunes to new radio channel, notifies target BSS on new channel

  36. Review, GSM Intra-MSC Handoff, cont.. • Target BSS notifies MSC that handoff is detected • Target BSS and mobile station exchange messages to synchronize transmission in proper timeslot • MSC switches voice connection to target BSS, which responds when handoff is complete • MSC notifies serving BSS to release old radio traffic channel

  37. Review, GSM Inter-MSC Handoff • Mobile station monitors signal quality and determines handoff is required, sends signal measurements to serving BSS • Serving BSS sends handoff request to MSC with ranked list of qualified target BSSs • Serving MSC determines that best candidate BSS is under control of a target MSC (assumed here) and calls target MSC through PSTN

  38. Review, GSM Inter-MSC Handoff, cont.. • Target MSC notifies its VLR to assign a TMSI • Target VLR returns TMSI • Target MSC reserves a trunk to target BSS • Target BSS selects and reserves radio channels for new connection, sends Ack to target MSC • Target MSC notifies serving MSC that it is ready for handoff

  39. Review, GSM Inter-MSC Handoff, cont.. • Serving MSC notifies serving BSS to begin handoff, including new radio channel assignment • Serving BSS forwards new radio channel assignment to mobile station • Mobile station re-tunes to new radio channel, notifies target BSS on new channel • Target BSS notifies target MSC that handoff is detected

  40. Review, GSM Inter-MSC Handoff, cont.. • Target BSS and mobile station exchange messages to synchronize transmission in proper timeslot • Voice connection is switched to target BSS, which responds when handoff is complete • Target MSC notifies serving MSC • Old network resources are released

  41. IS-95 CDMA • Digital AMPS increased capacity of AMPS by factor 3 • First code division multiple access (CDMA) cellular system was developed 1990, claimed to increase capacity by factor 20

  42. IS-95 CDMA - Radio Aspects • 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

  43. 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

  44. IS-95 CDMA Interesting Features • Multiple users can share same frequency • Soft capacity limit: more users raises noise floor linearly, no absolute limit on number of users - performance degrades gradually for all users • Multipath fading is reduced by signal spreading

  45. IS-95 CDMA Interesting Features (cont) • Spatial diversity provides soft handoff: MSC monitors signal of a user from multiple base stations and chooses best version of signal at any time • 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

  46. 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 • CDMA is dual mode like TDMA.

  47. IS-95 CDMA Interesting Features (cont) • 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.

  48. Forward channels Application Pilot System mon. Synchronization Sync. Paging Signaling Traffic Voice/data Reverse channels Access Signaling Traffic Voice/data IS-95 CDMA Channels • Types of channels

  49. IS-95 CDMA Channels (cont) • Traffic channel on the forward direction has three components • user data • power control (puncturing convolutional code) • signaling message • Traffic channel on the reverse direction has two components • user data • signaling message

  50. Handoff in CDMA • Two types of handoffs • hard handoff • Soft handoff • Hard handoff is needed when the call is moved from one frequency to another and when the mobile moves the coverage area of another MSC

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