Download Review, Performance Metrics, Cont…

Southern Methodist University Fall 2003
EETS 8316/NTU CC745-N
Wireless Networks
Lecture 6: Mobie Data
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
Session Outline
Review of last week
Network Performance Discussion
Primer on Aloha
Mobile Data
—Mobites
—ARDIS
—CDPD
—GPRS
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#2
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#3
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#4
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#5
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#6
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.
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#7
Review, IS-95 CDMA Channels
Types of channels
Forward channels
Pilot
Synchronization
Paging
Traffic
Reverse channels
Application
System mon.
Sync.
Signaling
Voice/data
Access
Signaling
Traffic
Voice/data
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#8
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#9
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#10
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#11
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#12
Review, Performance Metrics
Two types of performance metrics
―Customer perceived.
―Operator Defined.
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#13
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#14
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.
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#15
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
Pr{blocking}=
C
C
A
C!
k
A
k!
K=0
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#16
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 >0 ] =
C
A + C!
C-1
(
1- A
C
)
k
A
k!
K=0
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#17
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#18
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#19
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#20
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.
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#21
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#22
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#23
Review, More on Counters
Typical counters in BSC/MSC
# of measured RXQUAL=n
# of dropped calls
# of call attempts
Duration of call (average)
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#24
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.
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#25
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.
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#26
Review, Primer to switching systems
Two basic types of Switching
—Circuit switched
—Packet switched
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#27
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#28
Review, Circuit Switched Connection
Traffic can be concentrated for better use of
resources
Channel 1
:
:
1
1
:
K
:
M
Channel N
Concentrator
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#29
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.
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#30
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.
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#31
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)
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#32
Network Performance Discussion
 What is the target of performance
monitoring, or the level
Connection type
Function/feature
 Counters
Last weeks assignment !
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#33
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#34
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%
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#35
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#36
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
- collision if starts in interval (-T,0)
frame B
frame C
-T
0
EETS 8316/NTU TC 745, Fall 2003
- collision if starts in interval (0,T)
time
T
SMU
ENGINEERING
#37
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#38
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#39
Primer: Slotted ALOHA
“Vulnerable interval” is reduced by factor of
2 to just T
-T
frame A
- transmission attempt at time 0
frame B
- collision if frame B was ready in
interval (-T,0)
0
EETS 8316/NTU TC 745, Fall 2003
time
T
SMU
ENGINEERING
#40
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#41
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#42
Primer: (CSMA)
Carrier Sense Multiple Access =
CSMA
Sense the presence of carrier, sense the
channel is free, send data, wait for Ack, resend if timed-out, if busy back off and try
again. Max throughput 60%
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#43
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#44
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, RequestTo-Send (RTS) and Clear-To-Send (CTS) .
— Method of choice for wireless LAN
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#45
Primer: CSMA/CD (cont)
3 alternating states: (1) transmission (2)
contention (3) idle
time
frame
transmission
EETS 8316/NTU TC 745, Fall 2003
frame
contention:
series of time
slots for collisions
SMU
frame
idle
ENGINEERING
#46
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#47
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)
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#48
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/mobitex/mobitex.shtml)
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#49
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#50
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#51
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
#52
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
#53
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
#54
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#55
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.
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#56
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
#57
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#58
Mobitex - Network Layer , Cont...
MPAK header Contains
—Identification of application that generated
packet
—Class (significance)
—Type (priority)
—Whether can be stored in recipient’s mailbox
(temporary storage) if cannot be delivered
immediately
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#59
Mobitex - Data Link Layer
Data link layer protocol is MASC (Mobitex
Asynchronous Communication)
MPAK delivers user packets plus
addressing and network data to MASC
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#60
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#61
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#62
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.
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#63
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
Block+CRC Block+CRC
header
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
• • • Block+CRC
#64
Mobitex - Radio Interface , Cont...
—Receiver checks each frame for bit errors in
blocks
—Correct frames are Ack’ed, 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)
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#65
Mobitex - Radio Interface , Cont...
• Successful if frame is Ack’ed 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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#66
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#67
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
#68
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
#69
ARDIS - Network Architecture
X.25 = public ITU
standardized connectionoriented packet switching
protocol
X.25 network
Message
switch
RNC
Message switches route
messages, keep subscriber
info, accounting
Message
switch
Radio network Controllers
manage RF resources mostly proprietary protocols
RNC
Cell areas overlap to increase
probability of receiving a
message at least at one BS
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#70
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)
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#71
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.
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#72
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/pr23_09_1998-01.shtml
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#73
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
#74
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
#75
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
#76
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
#77
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
#78
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#79
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)
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#80
CDPD - Network Architecture , Cont...
Mobile data intermediate system (MD-IS)
—Mobility management: location tracking,
registration, authentication, encryption
• Exchange location information by CDPDspecific mobile network location protocol
(MNLP)
• “Mobile home function” (MHF) in home
network maintains current location info
for a mobile end system and forwards
packets
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#81
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)
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#82
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#83
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#84
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#85
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#86
CDPD , Cont...
CDPD network layer
—Internet protocol (IP and mobile IP) and
connectionless network protocol (CLNP,
OSI’s 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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#87
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
#88
GPRS , Cont...
GSM Release’97 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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#89
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
#90
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
#91
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
#92
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
#93
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
#94
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
#95
GPRS, Terminal Attach
BTS
1
2
4
BSC/PCU
1
2
MSC/VLR
3
3
4
2
SGSN
EETS 8316/NTU TC 745, Fall 2003
3
SMU
ENGINEERING
HLR
#96
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.
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#97
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
EETS 8316/NTU TC 745, Fall 2003
SMU
ENGINEERING
#98