Southern%20Methodist%20University%20Fall%202003

1
Southern Methodist University Fall 2003 EETS
8316/NTU CC745-N Wireless Networks
Lecture 6 Mobie Data
Instructor Jila Seraj email jseraj_at_engr.smu.edu
http//www.engr.smu.edu/jseraj/ tel
214-505-6303
2
Session Outline

• Review of last week
• Network Performance Discussion
• Primer on Aloha
• Mobile Data
• Mobites
• ARDIS
• CDPD
• GPRS

3
Review, IS-95 CDMA

• 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

4
Review, IS-95 CDMA - Radio Aspects

• 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

5
Review, IS-95 CDMA , 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

6
Review, IS-95 CDMA Features

• 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

7
Review, IS-95 CDMA, Features

• Near-far problem if power of multiple users are
  unequal, strongest received mobile signal will
  capture demodulator at base station
• Power Control to ensure that each mobile within
  coverage area provides same signal level to base
  station receiver
• 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.

8
Review, IS-95 CDMA Channels

• Types of channels

9
Review, 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

10
Review, Handoff in CDMA

• Two types of handoffs
• hard handoff
• Soft handoff, requires synchronization
• 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

11
Review, Mobility Management in CDMA

• Five type of registration
• Periodic
• Power up
• Power down
• Zone change
• Distance. When the distance between the current
  base station and the previously registered base
  station exceeds a certain limit. Requires GPS in
  all base stations

12
Review, Performance Metrics

• Performance metrics are defined to measure the
  behavior of network objectively
• Availability
• Retainability
• Integrity
• Delays Dial tone delay, post dialing delay,
  through connection delay

13
Review, Performance Metrics

• Two types of performance metrics
• Customer perceived.
• Operator Defined.

14
Review, Performance Metrics

• Specific metrics defined for different signaling
  systems and nodes
• Standards and reference models are defined
• Mostly on the wire line side, government Control

15
Review, Performance Metrics

• One of the challenges of a network performance is
  to predict the capacity of the system (also
  called dimensioning resources)
• Erlang is the unit used when dealing with
  traffic. One Erlang is one call held for one
  hour.

16
Review, Performance Metrics

• Erlang-B formula provided blocking probability,
  I.e. the probability of an incoming call can not
  find an idle device.
• C Number of devices
• A Offered traffic in Erlang

C A
C!
Prblocking
C
k A
k!
K0
17
Review, Performance Metrics

• Delay within switching network is of concern,
  timers that are defined in intermediate switches.
• Erlang C formula is used to calculate the
  probability of delay in a wait system.

C A
Pr Delay gt0
18
Review, Performance Metrics

• Performance metrics in wireless and wire line are
  similar, but not identical
• Accessibility Ability to make and
  receive calls
• Retainability Ability to maintain a call
• Voice quality Voice quality during the call

19
Review, Performance Metrics

• How do we calculate these metrics?
• No common standards are defined. Only operator
  defined standards.
• Measurements are collected from network elements
• Formulas are developed per vendor product
• Performance metrics can be
• Theoretically calculated
• Measured using counters produced at each node
• Verified by drive testing

20
Review, Performance Metrics

• Theoretical estimation
• Normally used during the network design.
• Queuing theory, traffic forecast, statistical
  estimates and product specification are the
  corner stone of this work
• Result are good for this phase but are not always
  correct

21
Review, Performance Metrics, Cont

• Calculating metrics using counters
• All switching nodes produce counters.
• These counters represent events in the network
• Using these counters metrics are defined.
• Each vendor has its own counters, thus the
  formula for deriving performance metric varies
  for each vendor.
• Counters are produced on different levels, MSC
  level, BSC level, cell level, etc.

22
Review, Performance Metrics, Cont

• Example of counters are
• MSC level of page attempts
• BSC level of intra-BSC handoff
• Cell level page responses
• VLR level of visiting mobiles
• HLR level Length of mobile activity

23
Review, More on Counters

• Typical counters in MSC/BSC
• of page request from HLR
• of page response after one attempt
• of pager response after second attempt
• of page with no response

24
Review, More on Counters

• Typical counters in BSC/MSC
• of measured RXQUALn
• of dropped calls
• of call attempts
• Duration of call (average)

25
Review, Verification, Drive Test

• A commonly used method
• Expensive and time consuming
• Good tool for trouble shooting
• Mobile handset is connected to a computer. All
  communication between the mobile handset and the
  base station is recorded. For example, layer
  three messages, layer 2 messages, measures signal
  strength, quality, etc.

26
Review, Verification, Drive Test

• Normally, the test calls are done towards a test
  number that sends a tone for verification of
  voice quality
• The quality of the test equipment influences the
  result
• Reliability of the test software is a key.

27
Review, Primer to switching systems

• Two basic types of Switching
• Circuit switched
• Packet switched

28
Review, Circuit Switched Connection

• Connection has three phase setup, transmission,
  disconnection.
• Bandwidth is reserved end-to-end for duration of
  connection
• Congestion and delay in the setup phase
• Only propagation delay during transmission
• Well suited for real-time, Continuous traffic,
  e.g., speech

29
Review, Circuit Switched Connection

• Traffic can be concentrated for better use of
  resources

Channel 1
1
1



K
M
Channel N
Concentrator
30
Review, Packet Switch Connection

• Information is packetized, i.e. segmented and
  padded with header and trailer information.
• Contents of header and trailer information is
  determined by the protocol governing the packet
  switched network, origination and destination of
  the information and other services invoked.

31
Review, Packet Switch Connection, Cont

• No resources/trunks are reserved.
• All network resources are shared by all users.
• Delay is variable based on the load level in the
  network.

32
Review, Packet Switch Connection

• Well suited for non-real-time, bursty traffic
• 2 types of packet switching, connectionless and
  connection oriented
• Connectionless each packet is routed
  independently
• Packets can arrive out of order
• Example Internet protocol (IP)

33
Network Performance Discussion

• What is the target of performance monitoring, or
  the level
• Connection type
• Function/feature
• Counters
• Last weeks assignment !

34
Primer Aloha

• Aloha is a wireless network designed in Hawaii
  and thus the name Aloha
• It was experimented in many way to find a good
  solution for wireless communication
• The system consisted of wireless devices
  communicating together using a communication
  sattelite

35
Primer Aloha

• Aloha
• Stations starts sending when they have something
  to send
• Pure Aloha, no contention resolution, relies on
  timed-out acks, max throughput approximately 18
• Slotted Aloha, no contention resolution, relies
  on timed-out acks, only can start sending in the
  beginning of a slot, max through put
  approximately 36

36
Primer Pure ALOHA

• Throughput
• Assume infinite population of stations generating
  frames at random times
• Each frame is transmitted in fixed time T
• Assume average number of transmission attempts is
  S in any interval T
• Number of new transmission attempts in any
  interval t has Poisson probability distribution
• Pr(k transmissions in interval t ) (St)ke- St
  /k!
• Let G offered load new transmissions and
  retransmissions

37
Primer Pure ALOHA

• In equilibrium, throughput (rate of successfully
  transmitted frames) rate of new transmissions,
  S
• S GP0
• where P0 probability of successful
  transmission (no collision)
• P0 depends on vulnerable interval for frame, 2T

- transmission attempt at time 0
frame A
frame B
- collision if starts in interval (-T,0)
- collision if starts in interval (0,T)
frame C
time
-T
0
T
38
Primer Pure ALOHA

• P0 Pr(no other frame in 2T interval)
• Assume total number of frames in any interval t
  is also Poisson distributed, with average G
• Pr(k transmissions in t) (Gt)ke-Gt/k!
• then P0 e-2G
• By substitution, throughput is
• S GP0 Ge-2G
• This is maximum at G 0.5, where S 1/2e
  0.184 (frames per interval T)
• Pure ALOHA achieves low throughput

39
Primer Slotted ALOHA

• Slotted ALOHA is a modification to increase
  efficiency
• Time is divided into time slots transmission
  time of a frame, T
• All stations are synchronized (eg, by periodic
  synchronization pulse)
• Any station with data must wait until next time
  slot to transmit
• Any time slot with two or more frames results in
  a collision and loss of all frames
  retransmitted after a random time

40
Primer Slotted ALOHA

• Vulnerable interval is reduced by factor of 2
  to just T

- transmission attempt at time 0
frame A
frame B
- collision if frame B was ready in interval
(-T,0)
time
-T
0
T
41
Primer Slotted ALOHA

• Throughput
• P0 Pr(no frames ready in previous time slot)
  e-G
• Now throughput is
• S GP0 Ge-G
• This is maximum at G 1, where S 1/e 0.368
  (frames per interval T)
• Slotted ALOHA doubles throughput of pure ALOHA

42
Primer Slotted ALOHA

• Note that throughput is never very high
• Also, at high loads, throughput goes to 0 a
  general characteristic of networks with shared
  resources
• Number of empty time slots and successful slots
  decrease, number of collisions increase
• Average number of retransmissions per frame
  increases
• Average delay (from first transmission attempt to
  successful transmission) increases

43
Primer (CSMA)

• Carrier Sense Multiple Access CSMA
• Sense the presence of carrier, sense the channel
  is free, send data, wait for Ack, re-send if
  timed-out, if busy back off and try again. Max
  throughput 60

44
Primer CSMA

• Family of CSMA protocols defined by rules for
  backing off with varying degrees of persistence
• 1-persistent CSMA stations are most persistent
• P-persistent CSMA persistence increases with
  value of p
• Non-persistent CSMA stations are not that
  persistent

45
Primer CSMA (Cont)

• Carrier Sense Multiple Access-Collision Detection
  (CSMA-CD)
• Send when carrier is free, listen to detect
  collision.
• CSMA-CA is the method of choice
• Carrier Sense Multiple Access-Collision Avoidance
  (CSMA-CA)
• Uses two messages before transmission,
  Request-To-Send (RTS) and Clear-To-Send (CTS) .
• Method of choice for wireless LAN

46
Primer CSMA/CD (cont)

• 3 alternating states (1) transmission (2)
  contention (3) idle

time
frame
frame
frame
transmission
idle
contention series of time slots for collisions
47
Primer CSMA/CD (cont)

• Performance depends on time to detect collision
  (assume transmissions can be aborted immediately)
• If D is worst-case propagation delay between any
  two stations, then collision detection time is 2D

A begins transmit
A detects collision after 2D
station A
signal
time
station B
B begins transmit just before signal reaches B
48
Mobile Data

• 2 main options for wireless packet data
• High speed wireless LANs (eg, 802.11)
• Low speed wide area services
• Mobitex/RAM Mobile Data
• CDPD (cellular digital packet data)
• GPRS (general packet radio service)
• ARDIS (advanced radio data information services)

49
Mobile Data , Cont...

• ERMES (European Radio Message System) was
  standardized by ETSI early 1980.
• Originated by Swedish Telecom (now Telia Mobitel)
  as private mobile alarm system for field
  personnel
• Development Continued by MOA (Mobitex Operators
  Association) and Ericsson Mobile Communications
• http//www.ericsson.com/wireless/products/mobsys/m
  obitex/mobitex.shtml)

50
Mobitex/RAM Mobile Data

• Mobitex - widely accepted de facto standard for
  wireless packet data
• Developed by Swedish Telecom (now Telia Mobitel)
  as private mobile alarm system for field
  personnel
• Development Continued by MOA (Mobitex Operators
  Association) and Ericsson
• 1986 Commercial operation in Sweden
• Now widely deployed in Europe, US, Australia

51
Mobitex , Cont...

• 1986 Commercial operation in Sweden
• Now widely deployed in Europe, US, Australia
• In US, RAM Mobile Data, a joint venture between
  RAM broadcasting and Cingular.
• http//www.cingular.com/business/mobitex_map

52
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

53
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.

54
Mobitex - Architecture
NCC
NCC network Control center
Main exchange
Regional switch
Regional switch
Local switch covers a service area, each with
10-30 frequency pairs
Local switch
Local switch
Base stations use 1-4 frequencies each 8 kb/s
FEP
55
Mobitex - Architecture , Cont...

• Network Control Center (NCC), provides network
  management functions
• Main Exchange and Regional Switch have basically
  the same function, but they reside on different
  level of network hierarchy.
• Packet switching
• Protocol handling (X.25 and HDLC)
• Subscriber data for nodes below
• Multiple connection to other switches
• Alternate routing

56
Mobitex - Architecture , Cont...

• Local Switches, similar to regional switches.
  Also handles
• Communication with base stations
• Connection to host computers via FEP
  (Front-End-Processor)
• FEP provides
• Protocol conversion to hosts supporting X.25,
  TCP/IP, and SNA
• Convert connectionless protocol to connection
  oriented protocol.

57
Mobitex, protocol architecture
4-7 3 2 1
Server
Local switch
Base Station
Mobile
Radio modem
58
Mobitex - Network Layer

• Network layer packet MPAK (Mobitex PAcKet)
• User data, segmented into packets of maximum 512
  bytes
• Alert messages (high priority)
• Network layer signaling, e.g., login/logout
  requests

59
Mobitex - Network Layer , Cont...

• MPAK header Contains
• Identification of application that generated
  packet
• Class (significance)
• Type (priority)
• Whether can be stored in recipients mailbox
  (temporary storage) if cannot be delivered
  immediately

60
Mobitex - Data Link Layer

• Data link layer protocol is MASC (Mobitex
  Asynchronous Communication)
• MPAK delivers user packets plus addressing and
  network data to MASC

61
Mobitex - Data Link Layer, Cont

• Data link layer functions
• Selection of most suitable base station
• Retransmissions of frames lost on the radio
  channel (stop-and-wait ARQ)
• Channel access procedure - variation of slotted
  ALOHA

62
Mobitex - Data Link Layer , Cont...

• Base station initiates a Contention cycle by
  sending a FREE frame downlink
• Mobile stations can Contend for number of free
  timeslots by choosing a random slot and
  transmitting during that slot
• If mobile has more data than fits in a time slot,
  it can start by sending a short access request
  message
• Base station grants access to requesting mobile
  stations after a free cycle

63
Mobitex - Data Link Layer , Cont...

• ROSI (Radio SIgnalling), takes care of
  transmission towards Mobitex infrastructure
• GMSK (Gaussian Minimum Shift Keying)
• X.21 and X.25 are the packet data communication
  protocols used for many years. Good for
  connectionless short bursts of data.

64
Mobitex - Radio Interface

• Data link layer ROSI (RadiO SIgnaling)
• Function of radio modem in mobile terminal
  communicating with base station
• Data link frames set of 20-byte blocks of data
  from network layer 16-bit CRC per block for
  error detection frame header

Data link frame
Frame header
BlockCRC
BlockCRC
BlockCRC
 
65
Mobitex - Radio Interface , Cont...

• Receiver checks each frame for bit errors in
  blocks
• Correct frames are Acked, or errored blocks are
  retransmitted selectively until frame is correct
  (selective ARQ at block level)
• Previous frame must be correct before
  transmitting next frame (stop-and-wait ARQ at
  frame level)

66
Mobitex - Radio Interface , Cont...

• Successful if frame is Acked by base, otherwise
  it Continues to Contend for channel in free
  cycles
• Mobile terminal can transmit long frames (longer
  than timeslot) by first sending short Access
  Request message to base station

67
Mobitex - Radio Interface , Cont...

• Data link also handles channel access procedure -
  variation of slotted ALOHA
• Base station broadcasts a FREE frame indicating a
  free cycle, including number and length of time
  slots in the free cycle
• Mobile terminal chooses a random timeslot in next
  free cycle and transmits its frame then

68
Mobitex - common functions

• Requires subscription
• individual
• groups of terminals
• host computer
• groups of host computers
• Security
• Password based
• ESN
• CUG (Closed User Group)

69
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.

70
ARDIS - Network Architecture
X.25 public ITU standardized connection-oriented
packet switching protocol
X.25 network
Message switches route messages, keep subscriber
info, accounting
Message switch
Message switch
Radio network Controllers manage RF resources -
mostly proprietary protocols
RNC
RNC
Cell areas overlap to increase probability of
receiving a message at least at one BS
71
ARDIS

• Advanced Radio Data Information Services (ARDIS)
• Joint venture by IBM and Motorola, 1995 owned
  entirely by Motorola, 1998 merged with American
  Mobile Satellite Corp
• Covers 90 urban business areas, 1400 base
  stations, more than 40,000 users
• 2 proprietary Motorola air interface protocols
• 4,800-b/s MDC-4800 (most common)
• 19,200-b/s RD-LAP (few major areas)

72
ARDIS , Cont...

• Not a true cellular system.
• Does not have handoff or reuse
• Main goal is to have deep in-building coverage.
• A terminal may receive signals from mor than one
  base station, guaranteeing good coverage.

73
ARDIS , Cont...

• Several base stations may receive from a
  terminal. Radio network Controller decides which
  one has least errors.
• Closed system. Not much info available.
• http//www.rim.net/news/partner/1998/pr-23_09_1998
  -01.shtml

74
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

75
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.

76
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

77
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

78
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
79
CDPD - Network Architecture , Cont...

• Mobile end system (MES) may be handheld PDA to
  laptop to terminal
• Stationary or mobile, but treated as potentially
  mobile
• Network Continually tracks location to ensure
  that packets are delivered even if physical
  location changes
• May sleep - messages are then queued in network

80
CDPD - Network Architecture , Cont...

• Mobile data base station (MDBS) mobile data link
  relay
• Supports CDPD MAC and data link protocols across
  radio interface
• Handles radio channel allocation, interoperation
  of channels between CDPD and voice calls, tracks
  busy/idle status of channels
• Often co-located with AMPS base stations (shares
  AMPS antenna)

81
CDPD - Network Architecture , Cont...

• Mobile data intermediate system (MD-IS)
• Mobility management location tracking,
  registration, authentication, encryption
• Exchange location information by CDPD-specific
  mobile network location protocol (MNLP)
• Mobile home function (MHF) in home network
  maintains current location info for a mobile end
  system and forwards packets

82
CDPD - Network Architecture , Cont...

• Mobile data intermediate system (MD-IS)
• Mobile serving function (MSF) in visited
  network maintains info for visiting mobile end
  systems in its area (through registration
  process)
• Accounting and billing (based on usage)

83
CDPD - Radio Interface

• Mobile end systems connected to same MDBS share a
  common reverse channel (to the MDBS)
• MDBS uses a common forward channel to broadcast
  data to mobile end systems but no Contention
• Reverse channel MAC protocol is slotted
  non-persistent digital sense multiple access with
  collision detection (DSMA/CD), similar to CSMA/CD
• Collision detection is done differently though

84
CDPD - Radio Interface , Cont...

• In forward channel, a 5-bit busy/idle 1-bit of
  7 decode status flag is repeated once every 60
  bits - indicates whether reverse channel is busy
  or idle
• Decode status flag indicates whether the
  tranmission has been successful or not.
• Mobile end system with data ready will sense
  busy/idle flag

85
CDPD - Radio Interface , Cont...

• If reverse channel is busy, will defer for random
  number of timeslots and then sense again
  (nonpersistent because sensing is not Continuous)
• When reverse channel is seized, the mobile can
  send up to 64 blocks in a burst until finished or
  decoded status flag indicate unsuccessful
  transmission

86
CDPD - Radio Interface , Cont...

• On the forward direction, one block Contains 378
  bits encoded data 42 bits Control data.
• On the reverse direction, one block Contains 378
  bits encoded data 58 bits Control data
• The transmission capability is 19.2 kb/s

87
CDPD , Cont...

• CDPD network layer
• Internet protocol (IP and mobile IP) and
  connectionless network protocol (CLNP, OSIs
  equivalent of IP) are supported
• Backbone network of intermediate systems (ISs)
  provides connectionless packet routing
• ISs can be off-the-shelf IP or CLNP routers

88
GPRS - Network Architecture
Internet or other networks
MSC/ VLR
GGSN
HLR
Gateway GSN packet switch interworks with other
networks
SGSN
SGSN
Serving GPRS support node packet switch with
mobility management capabilities
BSC/PCU
GPRS makes use of existing GSM base stations
89
GPRS , Cont...

• GSM Release97 introduced general packet radio
  service (GPRS) for bursty data
• Make use of existing GSM network equipment and
  functions
• In Contrast to CDPD, it is integrated into GSM,
  ie dedicated Control channel and data channel.
• Requires two new network element, GGSN and SGSN

90
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

91
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.

92
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.

93
GPRS , Cont...

• For mobility management a new concept is defined,
  Routing Area
• RAI MCC MNC LAC RAC

94
GPRS - Radio Interface

• Mobile station must register and establish a
  temporary logical link identity (TLLI) with its
  serving GSN
• Mobile stations 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

95
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

96
GPRS, Terminal Attach
BTS
BSC/PCU
MSC/VLR
3
2
1
3
4
2
HLR
SGSN
3
97
GPRS Attach , Cont...

• 1. Mobile termianl request to be attached to the
  network. The request is sent to the SGSN
  (indicates multi slot capability, ciphering and
  type of service)
• 2. Authentication between HLR and the terminal
• 3. Subscriber data is inserted into MSC/VLR and
  SGSN
• 4. SGSN informs the terminal that it is attached.

98
Packet Switch Connection, Cont

• Connection-oriented packets follow same route
  along virtual circuit
• Packets arrive in same order
• 3 phase connection setup, transmission,
  termination
• Examples ATM, frame relay, X.25