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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. Session Outline. Review of the last week CDMA essentials
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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
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
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
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
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
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
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
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
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
Switching System VLR AUC MAP MAP HLR VLR EIR MAP MAP MSC BSSAP Base Station Subsystem BSS Review, GSM Interfaces
Switching System BSSAP Base Station Subsystem BSS BSC LAPD BTS MS Review, GSM Interfaces, cont…
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.
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
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
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
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.
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
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
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.
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
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
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
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
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)
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)
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
Review, GSM Security, cont… • Privacy from eavesdropping • Temporary encryption key is used for privacy of data, signaling, and voice • Info is encrypted before transmission
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
Review, GSM Security, cont… • Anonymity of users • Only network and mobile station know true identity • New TMSI is assigned when roam into new area
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
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
Review, GSM Security, cont.. Visited system Location update HLR Acknowledge Registration cancel Old VLR
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
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
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
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
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
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
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
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
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
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
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 • 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
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
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.
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.
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
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
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