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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.,. Why Wireless? Reduced cost More flexibility for locating equipment (deployment ease) User mobility

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Wireless networks and protocols

Wireless Networks and Protocols

Ram Dantu

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


Wireless networks and protocols

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


Wireless networks and protocols

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.


Wireless networks and protocols

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).


Wireless networks and protocols

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


Wireless networks and protocols

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.


Wireless networks and protocols

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 networks and protocols

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)


Wireless networks and protocols

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


Wireless networks and protocols

Satellite Based PCS

Services or Features: Voice paging and messaging.

Coverage Area: Almost anyplace on earth.

Limitations: COST!

Examples: Iridium, Teledesic.


Tdma operation
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




Wireless networks and protocols

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.


Wireless networks and protocols

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.


Wireless networks and protocols

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

PacketNetwork

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


The components
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.


Wireless networks and protocols

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


Soft handoff between two 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 )


Wireless networks and protocols

highway

Rural

Typical US city BTS Map

30x30 miles

Urban


Wireless networks and protocols

BTS

BTS

BTS

BTS

BTS

BTS

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


Sdu and soft handoff

WR-B

SDU

SDU

-

-

-

-

2

2

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


Wireless networks and protocols

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


Wireless networks and protocols

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


Wireless networks and protocols

Power consumed by radio receiver while it waits for paging on “roaming” sign, determines DCC, paging channels, etc.

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.


Wireless networks and protocols

  • How does system deal with roaming users? on “roaming” sign, determines DCC, paging channels, etc.

  • 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

HLR

VLR

BS

MSC

BS


Wireless networks and protocols

HLR on “roaming” sign, determines DCC, paging channels, etc. 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.


Wireless networks and protocols

  • Mobility Management on “roaming” sign, determines DCC, paging channels, etc.

  • 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.


Wireless networks and protocols

  • 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.


Wireless networks and protocols

How are Handoffs Coordinated? enters

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.


Wireless networks and protocols

MSC and BS control procedure by transmitting “measurement enters

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.


Wireless networks and protocols

TIME enters

TIME

User 3

1

2

3

User 2

User 1

FREQUENCY

FREQUENCY

FDMA

TDMA

CODE

TIME

User 3

User 2

User 1

FREQUENCY

CDMAo r “Spread Spectrum”

Access Techniques

Spatial Diversity


Cellular systems
Cellular Systems enters

  • 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 systems1
Cellular Systems enters

  • 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


Modeling radiation pattern
Modeling radiation pattern enters

  • Which one is the best model?

R

R

R

R

R


Desirable topologies
Desirable Topologies enters

cluster size (N = 3)


Desirable topologies1
Desirable Topologies enters

(N = 4)


Desirable topologies2
Desirable Topologies enters

(N = 5)


Desirable topologies3
Desirable Topologies enters

N = 7