Wireless Networks and Protocols

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Wireless Networks and Protocols

• Ram Dantu
• This material is compiled from various sources
  including several industrial presentations,
  university lecture notes etc.,

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• Why Wireless?
• Reduced cost
• More flexibility for locating equipment
  (deployment ease)
• User mobility
• Reduced space needed for cabling
• Provision for location information/location-based
  applications
• More suitable for broadcasting
• Limitations of Wireless
• Capacity reduction reduced data rates,
  increased blocking
• Spotty coverage
• Variability in network performance
• Energy constraints

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The Wireless Media Signal paths between a
transmitter and a receiver can contain physical
obstructions. The presence of these multiple
paths and/or physical obstructions implies
fading.
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The Wireless Media (continued) Furthermore,
fading can vary in time, frequency, and space
(for example, as users move). In addition to
fading, wireless transmissions are susceptible to
interference from other users and/or
systems. Consequently, the wireless media can
significantly impact the transmission and
reception of signals (much more so
than transmissions on wired networks).
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• Implications
• Thus, network protocols designed assuming wired
  channel
• characteristics may not perform well in wireless
  scenarios.
• Wireless networks should be designed and
  studied for the wireless
• channel.
• Considerations in Design and Study
• How are networks used?
• What are the capabilities of the network
  elements?
• What information is available to network
  elements?
• How to jointly exploit the information and
  capabilities of elements?
• And so on

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Centralized vs Non-centralized Centralized
wireless networks have two types of
transceivers access points (AP's) and user
terminals. AP's have wired connections to
other AP's and/or larger wired networks. This
implies control is centralized at the AP's, i.e.,
user terminals communicate only to AP's -gt a
spoke-and-wheel network configuration Non-central
ized networks can also contain both AP's and user
terminals but communication is not limited
between AP and user terminals only. Ad hoc,
peer-to-peer connections are possible, as is
dynamic routing of information from source to
destination.
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Cellular Services or Features Voice and
data through handheld phones. Coverage Area
Continuous coverage. At times limited to
metropolitan regions. Limitations Available
bandwidth is very low for most data intensive
applications. Examples AMPS, IS-136, IS-95,
GSM, WCDMA, cdma2000, etc.
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Wireless Local Area Networks (WLAN's) Services
or Features Traditional LAN extended with
wireless interface Coverage Area Used only in
local environments Limitations Limited
range. Examples IEEE 802.11 suite (a,b,g)
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Global Positioning System (GPS) Services or
Features Helps to determine the
three-dimensional position, velocity, and
time. Coverage Area Anyplace on
earth. Limitations Cost. Examples GNSS,
NAVSTAR, GLONASS
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Satellite Based PCS Services or Features
Voice paging and messaging. Coverage Area
Almost anyplace on earth. Limitations
COST! Examples Iridium, Teledesic.
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TDMA Operation

• Fixed-assisgnment multiple access (FAMA) This
  assignment of capacity within the overall
  satellite channel is distributed in a fixed
  manner among multiple stations.
• Transmission in the form of repetitive sequence
  of frames
• Each frame is divided into a number of time slots
• Each slot is dedicated to a particular
  transmitter
• Earth stations take turns using uplink channel
• Sends data in assigned time slot
• Satellite repeats incoming transmissions
• Broadcast to all stations
• Stations must know which slot to use for
  transmission and which to use for reception

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FAMA-TDMA Uplink
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FAMA-TDMA Downlink
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Home Networking Services or Features To
connect different PC's in the house to share
files and devices such as printers. Also to
connect different appliances in the
house. Coverage Area Anywhere in the
house. Limitations Limited to a
home. Examples HomeRF. This can also be
categorized as a local area network.
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Ad Hoc Networks Services or Features
Group of users share data with each other for a
short period of time. Coverage Area Equal to
that of a LAN, but without fixed
infrastructure. Limitations Limited
range. Examples Bluetooth, defense
applications. Battlefields, disaster locations.
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Sensor Networks Services or Features A
large number of tiny (cheap, low-power) sensors
with wireless capabilities. Collected data is
transmitted back to a central processor. Coverag
e Area Relatively small terrain. Limitations
Very limited range, scalability problems (under
certain circumstances). Examples Defense and
civilian applications. Monitor inhospitable or
inaccessible terrain.
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A Simple Wireless Network
Mobile Switching Center (MSC)
Mobile Data Set
Base Station Controller (BSC)
PSTN
MobileVoice Unit
Packet Inter-Working Function
Base Transceiver System (BTS)
Challenge is to keep connection and not loose any
data during handoff operation
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The Components

• BTS
• BTS consists of one or more transceivers placed
  at a single location. The BTS terminates the
  radio path on the network side.
• BSC
• Provides allocation and management of radio
  resources.
• SDU Selection and distribution unit. Also
  responsible for handoff coordination
• MSC
• Provides and controls mobile access to the PSTN.
  Interprets the dialed number, routes and switches
  call to destination number. Also manages mobiles
  supplementary services. Maintains a register of
  visitors operating within the coverage area of
  the MSCs connected BTSs.
• PDSN Packet data service node is basically a
  packet router.

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Current Wireless Network Architecture
MSC
PDSN
TDM channels
Packets
BSC
BSC (SDU)
BSC (SDU)
BSC (SDU)

• - Backhaul cost is by /mile
• 10-100 miles between BTS and BSC
• Voice or data use one DS0 channel at a time
• BTSs are located in the tower
• BSC and MSCs are located
• in the central office

24xDs0 in T1
TDM channels
BTS
BTS
BTS
BTS
BTS
BTS
 
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Soft Handoff between two BTS
Handoff A handoff mechanism is needed to
maintain connectivity as devices move, while
minimizing disruptions to ongoing calls. This
mechanism should exhibit low latency, incur
little or no data loss, and scale to a large
network.
Handoffs ( Hard Soft )
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highway
Typical US city BTS Map 30x30 miles
Rural
Urban
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2G/3G RAN Network (Traditional)
Interoffice distance (costs per mile) cost
Fixed Cost
CO
CO
CO
CO
Channel Termination Cost
Channel Termination Cost
BSC
MSC

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SDU and soft handoff

• 3 to 6 BTSs involved in soft handoff
• SDU changeover due to weak signal
• from primary BTS
• BTS forwards even corrupted
• radio frames to the SDU for selection

 
 
 
 
 
 
WR-A
 
BTS-2
 
 
 
 



SDU
-
1
SDU-1
-
BTS-1
BTS-3
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What happens when a cellular user turns on their
phone? Terminal scans control channels and locks
on to strongest one. If cant find a strong
enough signal, no service With receiver turned
to strongest control channel, terminal extracts
important information from broadcast channel.
Strongest Signal
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On interpreting this broadcast information,
terminal turns on roaming sign, determines DCC,
paging channels, etc. Once this initialization is
complete, mobile enters idle mode. When no call
in progress, terminal monitors paging messages in
order to detect arrival of a new call.
Paging Channel (one of broadcast channels)
Page for User with MIN X
User with Mobile Identification Number (MIN) X
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Power consumed by radio receiver while it waits
for paging message has strong influence on
standby time of terminals battery. Terminals
can operate in sleep mode when no call in
progress. In this mode, terminal turns off its
receiver for significant fraction of time. Wakes
up for short period of time. If there is paging
message for terminal, BS schedules message
to arrive during brief wake-up interval. This is
synchronized by using a hyperframe
counter. Paging messages arrive in SPACH blocks
of superframe. Indicates assigned traffic
channels, etc.
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• How does system deal with roaming users?
• Aside from connecting radio system to
  public/private wired networks,
• MSC also connects to two types of databases
  (DBs)
• Home Location Registers (HLRs)
• Visitor Location Registers (VLRs)

BS
MSC
Private, Public, Residential Networks
BS
VLR
HLR
BS
MSC
BS
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HLR contains subscription information of a set of
terminals and also records terminals current
location and status. Information about each
terminal is accessed using mobile identification
number. A call request for a terminal arrives at
terminals home MSC. Home MSC interrogates HLR in
order to determine location of terminal. MSC
then coordinates actions to page terminal in
visited system. VLR stores information about
terminals currently in service area. This
information is used to setup calls initiated by
cellular users And to deliver calls directed to
cellular users. Service area of VLR Spans
coverage areas of one or more MSCs.
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• Mobility Management
• Mobility management procedures begin when
  system detects
• visting terminal.
• When mobile is in coverage area of base, it
  will at some point send
• either a registration message or origination
  message (when making
• a call) to BS.
• BS will inform MSC which will register users
  presence at VLR.
• VLR notifies terminals HLR of terminals
  location.

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• Whenever terminal that does not have a call in
  progress enters
• a new location area, it sends a registration
  message to local
• base station.
• Location areas are clusters of contiguous
  cells.
• When a call arrives for a terminal, system
  pages terminal only
• in the location area where it last registered.
• Registration tradeoff if terminal registers
  often, it is easier to
• locate the terminal but it uses up resources.

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How are Handoffs Coordinated?
Mobile Assisted Handoff (MAHO) A terminal tuned
to a digital traffic channel relies on MAHO to
move Its call to a new BS. With MAHO terminal
with call in progress monitors quality of Signal
on active traffic channel. During intervals in
each frame when it is not required to
transmit/ Receive information on active traffic
channel, terminal measures Strength of signals
received from surrounding BS. Terminal reports
measurements to its own BS on SACC.
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MSC and BS control procedure by transmitting
measurement Order and stop measurement order
messages to terminals With calls in
progress. Measurement order messages identify
active channels in Surrounding cells. Terminal
tunes to these channels and Observes signal
strengths. Terminal measures bit error rate
(BER) and received signal Strength indication
(RSSI). MAHO monitors quality of signal received
at terminal as well as At BS. Can react promptly
to signal-quality problems. When signal at
another BS exceeds current for some period Of
time, MSC coordinates between two BS to switch
user to New BS. Switch requires terminal to tune
into new channel.
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Access Techniques
Spatial Diversity
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Cellular Systems

• Early Mobile Radio Systems
• single high powered transmitters
• good coverage, but impossible to reuse same
  frequency (e.g., Bell Mobile System 70 -- max 12
  calls over thousand sq. Miles)
• it was impossible for the FCC to allocate new
  spectrum with the growing demand

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Cellular Systems

• Cellular Concept
• technique of using a fixed a number of channels
  to serve an arbitrary large number of subscribers
  by reusing channels throughout the coverage area
• high system capacity in a limited spectrum
• many low powered transmitters (small cells)
• each base station allocated a portion of the
  spectrum
• neighboring base stations assigned different
  groups of channels

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Modeling radiation pattern
R
R
R
R
R

• Which one is the best model?

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Desirable Topologies
cluster size (N 3)
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Desirable Topologies
(N 4)
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Desirable Topologies
(N 5)
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Desirable Topologies
N 7