Wireless Networks and Protocols
Ram Dantu
This material is compiled from various sources including
several industrial presentations, university lecture notes etc.,
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
•
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.
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).
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
•
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 -> a spoke-and-wheel network configuration
Non-centralized 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.
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.
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)
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
Satellite Based PCS
Services or Features: Voice paging and messaging.
Coverage Area: Almost anyplace on earth.
Limitations: COST!
Examples: Iridium, Teledesic.
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
FAMA-TDMA Uplink
FAMA-TDMA Downlink
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.
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.
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.
Coverage Area: Relatively small terrain.
Limitations: Very limited range, scalability problems (under certain
circumstances).
Examples: Defense and civilian applications. Monitor inhospitable
or inaccessible terrain.
A Simple Wireless Network
Mobile Data Set
Base Station
Controller
(BSC)
Mobile
Switching
Center (MSC)
PSTN
Mobile
Voice Unit
Packet
Network
Base Transceiver
System (BTS)
Packet InterWorking Function
Challenge is to keep connection and not loose any data during handoff operation
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 mobile’s supplementary services. Maintains a register of
visitors operating within the coverage area of the MSC’s connected
BTSs.
• PDSN: Packet data service node is basically a packet router.
Current Wireless Network Architecture
MSC
PDSN
TDM
channels
Packets
BSC
(SDU)
BSC
(SDU)
BSC
BSC
(SDU)
TDM
channels
in the central office
BTS
24xDs0
in T1
- 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
BTS
BTS
BTS
BTS
BTS
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 )
-85.2
34
33.9
highway
Rural
Urban
33.8 Typi
US c
33.7 BTS
30x3
33.6
mile
33.5
33.4
33.3
33.2
33.1
-85
-84.8
-84.6
-84.4
-84.2
-84
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
BTS
BTS
BTS
BTS
BTS
BTS
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
SDU
-2
WR-B
WR-A
BTS-2
SDU
-2
BTS-1
SDU
SDU-1
-1
BTS-3
What happens when a cellular user turns on their phone?
Terminal scans control channels and locks on to strongest one.
If can’t find a strong enough signal, “no service”
With receiver turned to strongest control channel, terminal
extracts important information from broadcast channel.
Strongest
Signal
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.
Page for User with MIN X
Paging Channel
(one of broadcast
channels)
User with Mobile Identification Number (MIN) X
Power consumed by radio receiver while it waits for paging
message has strong influence on standby time of terminal’s
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.
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
BS
HLR
VLR
BS
MSC
BS
Private, Public,
Residential
Networks
HLR contains subscription information of a set of terminals and also
records terminal’s current location and status. Information about
each terminal is accessed using mobile identification number.
A call request for a terminal arrives at terminal’s 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 MSC’s.
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 user’s presence at VLR.
• VLR notifies terminal’s HLR of terminal’s location.
• 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.
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.
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.
Access Techniques
TIME
TIME
User 3
1
2
3
User 2
User 1
FREQUENCY
FREQUENCY
TDMA
FDMA
CODE
TIME
User 3
User 2
User 1
FREQUENCY
CDMA
o r “Spread Spectrum”
Spatial Diversity
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
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
Modeling radiation pattern
R
R
R
• Which one is the best model?
R
R
Desirable Topologies
cluster size (N = 3)
Desirable Topologies
(N = 4)
Desirable Topologies
(N = 5)
Desirable Topologies
N=7